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PROFESSOR: So I want to begin
by reviewing a little bit what

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I said last time in terms
of this overview lecture.

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00:00:31,920 --> 00:00:35,580
And then we'll finish
the overview lecture.

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So summary of last lecture
is actually on five slides.

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00:00:38,240 --> 00:00:41,359
It's not all on this one slide.

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We started by talking about the
standard Big Bang, by which I

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00:00:43,900 --> 00:00:47,590
mean the Big Bang without
thinking about inflation.

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And I pointed out that
it really describes

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00:00:49,340 --> 00:00:51,730
only the aftermath of a bang.

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00:00:51,730 --> 00:00:54,890
It begins with the
description of the universe

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00:00:54,890 --> 00:00:59,870
as a hot, dense soup of
particles which more or less

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00:00:59,870 --> 00:01:02,500
uniformly fills the
entire available space,

20
00:01:02,500 --> 00:01:07,150
and the entire system
is already expanding.

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00:01:07,150 --> 00:01:13,140
Cosmic inflation is a prequel
to the conventional Big Bang

22
00:01:13,140 --> 00:01:14,770
story.

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00:01:14,770 --> 00:01:18,250
It describes how
repulsive gravity, which

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00:01:18,250 --> 00:01:19,870
in the context of
general relativity,

25
00:01:19,870 --> 00:01:24,136
can happen as a consequence
of negative pressure.

26
00:01:24,136 --> 00:01:25,510
This repulsive
gravity could have

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00:01:25,510 --> 00:01:28,310
driven a tiny patch
of the early universe

28
00:01:28,310 --> 00:01:32,800
into a gigantic burst of
exponential expansion.

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00:01:32,800 --> 00:01:34,750
And our visible
universe would then

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00:01:34,750 --> 00:01:38,930
be the aftermath of that event.

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00:01:38,930 --> 00:01:42,530
As this happened, the
total energy of this patch

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00:01:42,530 --> 00:01:45,810
would be very small and
could even be identically 0.

33
00:01:45,810 --> 00:01:49,870
And the way that's possible
is caused by the fact

34
00:01:49,870 --> 00:01:53,380
that the gravitational
field that fills the space

35
00:01:53,380 --> 00:01:55,580
has a negative
contribution to the energy.

36
00:01:55,580 --> 00:01:57,710
And as far as we can tell
in our real universe,

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00:01:57,710 --> 00:01:59,209
there are about
equal to each other.

38
00:01:59,209 --> 00:02:02,250
They could cancel each other
exactly as far as we can tell.

39
00:02:02,250 --> 00:02:03,790
So the total energy
could in fact

40
00:02:03,790 --> 00:02:06,000
be exactly zero,
which is what allows

41
00:02:06,000 --> 00:02:07,950
one to build a huge
universe starting

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00:02:07,950 --> 00:02:10,130
from either nothing
or almost nothing.

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00:02:13,330 --> 00:02:14,040
Inflation.

44
00:02:14,040 --> 00:02:16,660
The next item is
evidence for inflation.

45
00:02:16,660 --> 00:02:18,700
Why do we think there's
at least a good chance

46
00:02:18,700 --> 00:02:20,700
that our universe
underwent inflation?

47
00:02:20,700 --> 00:02:23,290
And I pointed out three items.

48
00:02:23,290 --> 00:02:26,270
The first was that inflation
could explain the large scale

49
00:02:26,270 --> 00:02:29,180
uniformity that we
observe in the universe

50
00:02:29,180 --> 00:02:31,760
and that large
scale uniformity is

51
00:02:31,760 --> 00:02:35,230
seen most clearly in the
cosmic microwave background

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radiation, which is observed
to be uniform to one

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00:02:38,700 --> 00:02:42,280
part in $100,000, that is same
intensity all across the sky

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00:02:42,280 --> 00:02:44,490
no matter what direction
you look, once you account

55
00:02:44,490 --> 00:02:48,085
for the Earth's motion,
to an accuracy of one part

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00:02:48,085 --> 00:02:50,300
in 100,000.

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00:02:50,300 --> 00:02:53,920
Secondly, inflation can explain
a rather remarkable fact

58
00:02:53,920 --> 00:02:56,860
about this quantity
omega, where omega

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is defined as the actual
mass density of the universe

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rho divided by rho critical,
the critical mass density which

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is the density that would make
the universe precisely flat.

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00:03:08,330 --> 00:03:11,520
The statement that that
ratio is equal to 1 we know

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is accurate to about 15
decimal places at one second

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after the Big Bang.

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And prior to inflation,
we didn't really

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have any explanation
for that at all.

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But inflation
drives omega to one

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and gives us an explanation
for why, therefore,

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started out so
extraordinarily close to 1.

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And in fact, it
makes a prediction.

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We'd expect that if
inflation is right,

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omega should still be one today.

73
00:03:35,160 --> 00:03:39,900
And we now have measured omega
to be 1.0010 plus or minus

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00:03:39,900 --> 00:03:43,730
0.0065, which I
think is fabulous.

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Finally, inflation
gives an explanation

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00:03:48,110 --> 00:03:52,440
for the inhomogeneities
that we see in the universe.

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It explains them as
quantum fluctuations which

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happened during the inflationary
process and, most importantly

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00:03:57,860 --> 00:04:01,690
really, as inflation was
ending, the quantum fluctuations

80
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cause inflation to go
on for a little bit

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00:04:03,810 --> 00:04:06,110
longer in some
regions than others.

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And that sets up
these inhomogeneities.

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Today, we can see
these inhomogeneities

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most accurately.

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00:04:14,996 --> 00:04:17,579
inhomogeneities, of course, are
huge at the level of galaxies,

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so they're obvious.

87
00:04:18,470 --> 00:04:21,339
But it's hard to connect
them to the early universe.

88
00:04:21,339 --> 00:04:24,620
So we can make our most
precise comparison between what

89
00:04:24,620 --> 00:04:27,870
we observe and theories
of the early universe

90
00:04:27,870 --> 00:04:30,350
by making careful observations
of the cosmic background

91
00:04:30,350 --> 00:04:33,890
radiation itself, which has
these ripples in the intensity,

92
00:04:33,890 --> 00:04:36,450
It is not quite uniform.

93
00:04:36,450 --> 00:04:39,830
There really are ripples at the
level of one part in 100,000,

94
00:04:39,830 --> 00:04:42,030
which can now be observed.

95
00:04:42,030 --> 00:04:44,110
And inflation makes
a clear prediction

96
00:04:44,110 --> 00:04:46,310
for the spectrum
of those ripples,

97
00:04:46,310 --> 00:04:49,590
how the intensity should
vary with wavelength.

98
00:04:49,590 --> 00:04:53,540
And I showed you this graph last
time from the Planck satellite.

99
00:04:53,540 --> 00:04:56,300
The agreement between the
prediction and the theory

100
00:04:56,300 --> 00:04:59,080
is really marvelous.

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So we'll be coming back to that
near the end of the course.

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Finally, in the last
lecture, I began

103
00:05:06,960 --> 00:05:11,970
to talk about inflation and
the possible implications

104
00:05:11,970 --> 00:05:17,650
for a multiverse, the idea that
our universe might be embedded

105
00:05:17,650 --> 00:05:19,470
in a much larger
thing consisting

106
00:05:19,470 --> 00:05:23,920
of many universes, which
we call a multiverse.

107
00:05:23,920 --> 00:05:26,810
And the key point is that
most inflation models

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00:05:26,810 --> 00:05:28,440
tend to become eternal.

109
00:05:28,440 --> 00:05:32,000
And that is once inflation
starts, it never stops.

110
00:05:32,000 --> 00:05:33,650
And the reason for
that, basically,

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is that the metastable
material, this repulsive gravity

112
00:05:37,500 --> 00:05:42,780
material that's causing
the inflation, decays,

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00:05:42,780 --> 00:05:45,480
but it also
exponentially expands.

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00:05:45,480 --> 00:05:48,970
And for typical models,
the exponential expansion

115
00:05:48,970 --> 00:05:51,060
completely overpowers the decay.

116
00:05:51,060 --> 00:05:54,590
So even though it's an
unstable material that decays,

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00:05:54,590 --> 00:05:57,290
the total volume of it actually
increases exponentially

118
00:05:57,290 --> 00:06:01,030
with time rather than decreases.

119
00:06:01,030 --> 00:06:03,540
Decays happen however,
and wherever decay happen,

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00:06:03,540 --> 00:06:06,100
it forms what we call
a pocket universe.

121
00:06:06,100 --> 00:06:10,100
We would be living in one
of those pocket universes.

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00:06:10,100 --> 00:06:12,690
And the number of
pocket universes

123
00:06:12,690 --> 00:06:16,040
grows exponentially with time as
the whole system grows and goes

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00:06:16,040 --> 00:06:18,240
on, as far as we
can tell, forever.

125
00:06:18,240 --> 00:06:21,060
And that is the picture
of the multiverse

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00:06:21,060 --> 00:06:23,550
that inflation tends to lead to.

127
00:06:27,830 --> 00:06:29,630
Finally, this is my
last summary slide

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00:06:29,630 --> 00:06:31,880
and then we'll
start new material.

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00:06:31,880 --> 00:06:33,620
At the very end of
lecture, I talked

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00:06:33,620 --> 00:06:38,460
about a problem, which is very
important in our present day

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00:06:38,460 --> 00:06:41,130
thinking about
physics and cosmology,

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00:06:41,130 --> 00:06:45,650
and that is the nightmare that
this discovery of dark energy

133
00:06:45,650 --> 00:06:46,250
leads to.

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00:06:49,300 --> 00:06:52,050
What was discovered
at about 1998

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00:06:52,050 --> 00:06:53,820
is that the expansion
of the universe

136
00:06:53,820 --> 00:06:56,450
is not slowing down under
the influence of gravity

137
00:06:56,450 --> 00:06:59,090
as one might expect,
but Instead, it's

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00:06:59,090 --> 00:07:00,380
actually accelerating.

139
00:07:00,380 --> 00:07:03,520
The universe is expanding
faster and faster.

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00:07:03,520 --> 00:07:05,430
And that indicates
that space today

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00:07:05,430 --> 00:07:09,190
is filled with some repulsive
gravity material, which

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00:07:09,190 --> 00:07:11,380
we call the dark energy.

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00:07:11,380 --> 00:07:14,170
And the simplest interpretation
of the dark energy

144
00:07:14,170 --> 00:07:18,570
is that it simply vacuum energy,
the energy of empty space.

145
00:07:18,570 --> 00:07:20,600
Space does have an
energy density that

146
00:07:20,600 --> 00:07:22,630
have exactly the
properties that we observe,

147
00:07:22,630 --> 00:07:24,495
so it seems natural to
draw that connection.

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00:07:27,070 --> 00:07:29,300
Vacuum energy, at first,
might seem surprising.

149
00:07:29,300 --> 00:07:33,030
If a vacuum's empty, why should
it have any mass density?

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00:07:33,030 --> 00:07:36,584
But in a quantum field theory,
it's really not surprising

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00:07:36,584 --> 00:07:38,000
because in a quantum
field theory,

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00:07:38,000 --> 00:07:39,974
the vacuum is really not empty.

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00:07:39,974 --> 00:07:41,390
In a quantum field
theory, there's

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00:07:41,390 --> 00:07:44,330
no such thing as
actual emptiness.

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00:07:44,330 --> 00:07:48,440
Instead, in the vacuum, one has
constant quantum fluctuations

156
00:07:48,440 --> 00:07:49,580
of fields.

157
00:07:49,580 --> 00:07:54,160
And in our current theory
of particle physics,

158
00:07:54,160 --> 00:07:56,040
the standard model
of particle physics,

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00:07:56,040 --> 00:07:58,660
there's even one particular
field called the Higgs field,

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00:07:58,660 --> 00:08:01,510
which has a non-zero
mean value in the vacuum

161
00:08:01,510 --> 00:08:04,160
besides fluctuations.

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00:08:04,160 --> 00:08:06,010
So the vacuum is a
very complicated state.

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00:08:06,010 --> 00:08:07,720
What makes it the
vacuum is simply

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00:08:07,720 --> 00:08:10,620
that it's alleged to be the
state of lowest possible energy

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00:08:10,620 --> 00:08:12,474
density, but that
doesn't have to be zero

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00:08:12,474 --> 00:08:13,890
and doesn't even
look like there's

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00:08:13,890 --> 00:08:16,210
any reason why it
should be zero.

168
00:08:16,210 --> 00:08:19,550
So there's no problem
buying the fact

169
00:08:19,550 --> 00:08:23,040
that maybe the vacuum does
have a non-zero energy density.

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00:08:23,040 --> 00:08:25,290
The problem comes
about though when

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00:08:25,290 --> 00:08:28,230
we try to understand the
magnitude of this vacuum

172
00:08:28,230 --> 00:08:29,250
energy.

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00:08:29,250 --> 00:08:31,250
If it was going to have
a vacuum energy density,

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00:08:31,250 --> 00:08:34,190
we'd expect it to be
vastly larger than what

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00:08:34,190 --> 00:08:37,090
is observed in the
form of the expansion

176
00:08:37,090 --> 00:08:39,929
acceleration of the universe.

177
00:08:39,929 --> 00:08:42,610
So a typical order of magnitude
in the particle physics

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00:08:42,610 --> 00:08:45,400
model for the vacuum
energy is, in fact,

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00:08:45,400 --> 00:08:47,980
about a full 120
orders of magnitude

180
00:08:47,980 --> 00:08:51,260
larger than the
number that's implied

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00:08:51,260 --> 00:08:53,590
by the acceleration
of the universe.

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00:08:53,590 --> 00:08:56,990
So that is a big problem.

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00:08:56,990 --> 00:08:59,940
I began to talk about a possible
resolution to that problem.

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00:08:59,940 --> 00:09:01,330
It's only a possible resolution.

185
00:09:01,330 --> 00:09:03,765
Nobody has really
settled on this.

186
00:09:03,765 --> 00:09:05,140
But there's a
possible resolution

187
00:09:05,140 --> 00:09:07,080
which comes out
of String theory,

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00:09:07,080 --> 00:09:09,685
and in particular
from this idea, which

189
00:09:09,685 --> 00:09:12,510
is called the landscape
of String theory.

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00:09:12,510 --> 00:09:15,970
Most String theorists believe
that String theory has

191
00:09:15,970 --> 00:09:19,140
no unique vacuum,
but instead, there's

192
00:09:19,140 --> 00:09:23,650
a colossal number, perhaps
something like 10 to the 500,

193
00:09:23,650 --> 00:09:26,990
different metastable
states, which

194
00:09:26,990 --> 00:09:30,230
even though they are metastable,
are incredibly long-lived,

195
00:09:30,230 --> 00:09:34,120
long-lived compared to the age
of the universe as we know it.

196
00:09:34,120 --> 00:09:37,790
So any one of these 10 to
the 500 different states

197
00:09:37,790 --> 00:09:41,390
could serve as effectively
the vacuum for one

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00:09:41,390 --> 00:09:42,774
of these pocket universes.

199
00:09:42,774 --> 00:09:44,190
And the different
pocket universes

200
00:09:44,190 --> 00:09:51,310
would presumably fill the
whole set of possible vacua

201
00:09:51,310 --> 00:09:55,880
in the landscape, giving reality
to all these possibilities

202
00:09:55,880 --> 00:09:58,080
that come about
in String theory.

203
00:09:58,080 --> 00:10:03,050
And in particular, each
different type of vacuum

204
00:10:03,050 --> 00:10:05,910
would have its own
vacuum energy density.

205
00:10:05,910 --> 00:10:08,050
And because there are
both positive and negative

206
00:10:08,050 --> 00:10:10,330
contributions-- I think I
didn't read that out loud--

207
00:10:10,330 --> 00:10:12,579
but there are both positive
and negative contributions

208
00:10:12,579 --> 00:10:14,700
that arise in quantum
field theories.

209
00:10:14,700 --> 00:10:16,830
So the vacuum energy
of a typical state

210
00:10:16,830 --> 00:10:18,750
could be either
positive or negative.

211
00:10:18,750 --> 00:10:20,910
And what we would
expect of these 10

212
00:10:20,910 --> 00:10:23,380
to the 500 different
vacua is that they

213
00:10:23,380 --> 00:10:26,010
would have a range
of energy densities

214
00:10:26,010 --> 00:10:29,610
that would range from something
like minus 10 to the 120

215
00:10:29,610 --> 00:10:34,264
to plus 10 to the 120
times the observed value.

216
00:10:34,264 --> 00:10:35,930
So the observed value
would be in there,

217
00:10:35,930 --> 00:10:38,700
but would be an incredibly
small fraction of the universes.

218
00:10:38,700 --> 00:10:39,200
Yes?

219
00:10:39,200 --> 00:10:42,518
AUDIENCE: Does this mean that
so many pocket universes could

220
00:10:42,518 --> 00:10:46,073
be closed and opened as well
in terms of their geometry?

221
00:10:46,073 --> 00:10:47,769
Or--

222
00:10:47,769 --> 00:10:49,310
PROFESSOR: They're
actually predicted

223
00:10:49,310 --> 00:10:52,130
to be open due to complications
about how they form,

224
00:10:52,130 --> 00:10:54,350
which I'm not going to go into.

225
00:10:54,350 --> 00:10:57,510
But they should all be open, but
very close to flat for the ones

226
00:10:57,510 --> 00:10:59,200
that under a lot of inflation.

227
00:10:59,200 --> 00:11:00,930
So they'd be
indistinguishable from flat,

228
00:11:00,930 --> 00:11:03,710
but technically, they'd be open.

229
00:11:03,710 --> 00:11:04,533
Yes?

230
00:11:04,533 --> 00:11:08,397
AUDIENCE: Is the minus 10 to
the 120 plus 10 to the 120 just

231
00:11:08,397 --> 00:11:10,812
chosen because we're off
520 orders of magnitude,

232
00:11:10,812 --> 00:11:13,612
or is it predicted
somewhere else?

233
00:11:13,612 --> 00:11:15,195
PROFESSOR: Well,
when we say we're off

234
00:11:15,195 --> 00:11:17,940
by 120 orders of magnitude,
the more precise statement

235
00:11:17,940 --> 00:11:21,740
is that the estimate of what
a typical range of the energy

236
00:11:21,740 --> 00:11:26,150
should be is 10 to 120
times the observed value.

237
00:11:26,150 --> 00:11:29,320
So this is basically just
a restatement of that.

238
00:11:29,320 --> 00:11:31,280
And you might
wonder why I didn't

239
00:11:31,280 --> 00:11:35,150
put 5 times 10 to the
120, but in fact, the 120

240
00:11:35,150 --> 00:11:37,230
itself is only accurate
to within a few orders

241
00:11:37,230 --> 00:11:39,220
of magnitude, so 5
times that wouldn't

242
00:11:39,220 --> 00:11:40,850
have made any difference
in the way one

243
00:11:40,850 --> 00:11:42,320
actually interprets
those numbers.

244
00:11:46,490 --> 00:11:49,280
10 to the 123 is probably
slightly more accurate number

245
00:11:49,280 --> 00:11:50,390
actually.

246
00:11:50,390 --> 00:11:52,381
But this is good enough
for our purposes.

247
00:11:52,381 --> 00:11:52,880
Yes?

248
00:11:52,880 --> 00:11:53,852
AUDIENCE: Just a
general question

249
00:11:53,852 --> 00:11:55,310
about inflation properties.

250
00:11:55,310 --> 00:11:58,226
We think of attractive
gravity as driving

251
00:11:58,226 --> 00:12:01,110
the motion of objects
through space.

252
00:12:01,110 --> 00:12:03,372
So why do we think
of repulsive gravity

253
00:12:03,372 --> 00:12:07,150
like to drive the
expanse of space itself?

254
00:12:07,150 --> 00:12:09,150
PROFESSOR: Well, for one
thing, it does actually

255
00:12:09,150 --> 00:12:10,200
behave differently.

256
00:12:10,200 --> 00:12:12,420
Repulsive gravity,
repulsive gravity

257
00:12:12,420 --> 00:12:14,000
that appears in
general relativity,

258
00:12:14,000 --> 00:12:17,170
is not just ordinary gravity
with the opposite sign.

259
00:12:17,170 --> 00:12:18,620
Ordinary gravity
has the property

260
00:12:18,620 --> 00:12:21,950
that if I have two objects to
attract each other with a force

261
00:12:21,950 --> 00:12:25,390
proportional to the
masses of those objects.

262
00:12:25,390 --> 00:12:26,850
This repulsive
gravity is actually

263
00:12:26,850 --> 00:12:31,140
an effect caused by the negative
pressure in the space between.

264
00:12:31,140 --> 00:12:33,020
So if I have two
objects, they will

265
00:12:33,020 --> 00:12:35,990
start to accelerate apart
by the amount that's

266
00:12:35,990 --> 00:12:38,150
totally independent
of the masses.

267
00:12:38,150 --> 00:12:40,680
This is not really the
masses that's causing it.

268
00:12:40,680 --> 00:12:43,450
So the whole force was
completely different,

269
00:12:43,450 --> 00:12:47,010
so we can't really
just compare them.

270
00:12:47,010 --> 00:12:50,280
In either case, when
everything is moving apart,

271
00:12:50,280 --> 00:12:53,080
it's really a
matter of viewpoint

272
00:12:53,080 --> 00:12:55,069
when you think of the
whole space as expanding

273
00:12:55,069 --> 00:12:56,610
or whether you think
of the particles

274
00:12:56,610 --> 00:12:58,620
as moving through space.

275
00:12:58,620 --> 00:13:01,830
In relativity, there's no
way to put a needle on space,

276
00:13:01,830 --> 00:13:05,170
put a pen in it and
say this is stationary.

277
00:13:05,170 --> 00:13:08,520
So we really can't say that
the space is moving or not.

278
00:13:08,520 --> 00:13:11,660
In cosmology, we usually
find that the simpler picture

279
00:13:11,660 --> 00:13:13,850
and the one that we
will generally use

280
00:13:13,850 --> 00:13:15,922
is that space expands
with the matter.

281
00:13:15,922 --> 00:13:17,380
It gives a much
simpler description

282
00:13:17,380 --> 00:13:18,255
of how things behave.

283
00:13:21,480 --> 00:13:22,470
Good question.

284
00:13:22,470 --> 00:13:23,660
Yes?

285
00:13:23,660 --> 00:13:26,380
AUDIENCE: I have a question
going back a few slides.

286
00:13:26,380 --> 00:13:28,440
PROFESSOR: Sure, you
want me to go back.

287
00:13:28,440 --> 00:13:31,410
AUDIENCE: How the energy
of the early universe

288
00:13:31,410 --> 00:13:34,400
seemed to be close to zero.

289
00:13:34,400 --> 00:13:36,750
Are there theoretical
models that would explain

290
00:13:36,750 --> 00:13:39,645
or that would say it
should be exactly 0?

291
00:13:39,645 --> 00:13:40,770
PROFESSOR: Yeah, there are.

292
00:13:40,770 --> 00:13:42,050
I didn't mention it.

293
00:13:42,050 --> 00:13:45,940
But if the universe is closed,
which is a possibility.

294
00:13:45,940 --> 00:13:48,602
Even if it's very nearly flat,
it could still be closed.

295
00:13:48,602 --> 00:13:50,810
If it were closed, it would
have exactly zero energy.

296
00:13:55,220 --> 00:13:56,120
Yes?

297
00:13:56,120 --> 00:13:58,334
AUDIENCE: So the
cosmic background,

298
00:13:58,334 --> 00:14:00,636
microwave background,
picks ups that it's

299
00:14:00,636 --> 00:14:03,480
pretty similar in all
directions once correct for it.

300
00:14:03,480 --> 00:14:06,210
And this leads to the thought
that the cosmological principle

301
00:14:06,210 --> 00:14:09,270
all over the universe
is pretty identical.

302
00:14:09,270 --> 00:14:11,640
Is it possible that
we are actually

303
00:14:11,640 --> 00:14:15,316
located in just a smaller
like circular pathway

304
00:14:15,316 --> 00:14:19,244
and it may be different
than [? allowed. ?]

305
00:14:19,244 --> 00:14:20,660
And there's many
of these patches,

306
00:14:20,660 --> 00:14:24,760
so we-- there's actually
like a speckled form.

307
00:14:24,760 --> 00:14:25,580
PROFESSOR: OK.

308
00:14:25,580 --> 00:14:27,038
So if you didn't
hear the question,

309
00:14:27,038 --> 00:14:29,820
I was asked if it's possible
that the universe is not

310
00:14:29,820 --> 00:14:32,120
really homogeneous on
a very large scales,

311
00:14:32,120 --> 00:14:35,142
but really speckled, just
that speckles are large

312
00:14:35,142 --> 00:14:36,850
and our speckle might
look very different

313
00:14:36,850 --> 00:14:38,580
from other speckles
that are far away.

314
00:14:38,580 --> 00:14:40,030
And that was the question.

315
00:14:40,030 --> 00:14:41,595
And the answer is certainly
if the multiverse picture

316
00:14:41,595 --> 00:14:42,040
is right.

317
00:14:42,040 --> 00:14:44,090
That is exactly the case
that's being predicted.

318
00:14:44,090 --> 00:14:45,770
These other pocket
universes could

319
00:14:45,770 --> 00:14:48,540
be viewed as other
speckles in your language,

320
00:14:48,540 --> 00:14:51,420
and they'd be very different
from what we've observed.

321
00:14:51,420 --> 00:14:55,820
So inflation actually
changes one's attitude

322
00:14:55,820 --> 00:14:57,870
about this particular question.

323
00:14:57,870 --> 00:15:00,840
Back in the old days,
before inflation,

324
00:15:00,840 --> 00:15:04,180
the uniformity of the
universe had no explanation,

325
00:15:04,180 --> 00:15:06,420
so it became a postulate.

326
00:15:06,420 --> 00:15:08,750
And nobody postulates
that something

327
00:15:08,750 --> 00:15:09,792
is uniform on that scale.

328
00:15:09,792 --> 00:15:11,333
If you are going to
make a postulate,

329
00:15:11,333 --> 00:15:14,040
you just postulate that
the universe is uniform.

330
00:15:14,040 --> 00:15:16,950
So that was the postulate
that was in use.

331
00:15:16,950 --> 00:15:19,350
But now that we think of the
homogeneity of the universe

332
00:15:19,350 --> 00:15:24,260
as being generated by a
dynamical process, inflation,

333
00:15:24,260 --> 00:15:26,180
then, it's a natural
question to ask,

334
00:15:26,180 --> 00:15:27,900
what is the scale
of the homogeneity

335
00:15:27,900 --> 00:15:29,950
that that generates.

336
00:15:29,950 --> 00:15:31,470
And it's certainly
a scale that's

337
00:15:31,470 --> 00:15:33,140
much larger than
what we can observe.

338
00:15:33,140 --> 00:15:35,280
So we don't really expect
to see inhomogeneity

339
00:15:35,280 --> 00:15:38,860
as caused by different
pockets of inflation,

340
00:15:38,860 --> 00:15:42,700
but the model seems to
make it very plausible that

341
00:15:42,700 --> 00:15:45,070
is what we would see if
we could see far enough.

342
00:15:49,880 --> 00:15:54,250
Any other questions while
we're on a little break here?

343
00:15:54,250 --> 00:15:55,130
Yes?

344
00:15:55,130 --> 00:15:56,755
AUDIENCE: If the
universe is expanding,

345
00:15:56,755 --> 00:16:00,105
then I think like we are
expanding as well, so how can

346
00:16:00,105 --> 00:16:01,775
we observe the change
from a distance,

347
00:16:01,775 --> 00:16:04,327
in particular everything
is increasing scale?

348
00:16:04,327 --> 00:16:04,910
PROFESSOR: OK.

349
00:16:04,910 --> 00:16:07,020
That's a very good question.

350
00:16:07,020 --> 00:16:09,430
The question was if the
universe is expanding,

351
00:16:09,430 --> 00:16:10,890
then the universe is everything.

352
00:16:10,890 --> 00:16:12,340
So everything is expanding.

353
00:16:12,340 --> 00:16:14,590
And if everything's expanding,
when you compare things

354
00:16:14,590 --> 00:16:16,950
with rulers, they
have the same length.

355
00:16:16,950 --> 00:16:20,590
So how would you even observe
that everything was expanding?

356
00:16:20,590 --> 00:16:22,660
And the answer to
that is that when

357
00:16:22,660 --> 00:16:24,470
we say the universe
is expanding,

358
00:16:24,470 --> 00:16:27,717
we're not really saying that
everything is expanding.

359
00:16:27,717 --> 00:16:29,300
When we say the
universe is expanding,

360
00:16:29,300 --> 00:16:31,716
we really are saying that the
galaxies are getting further

361
00:16:31,716 --> 00:16:34,840
apart from each other,
but individual atoms

362
00:16:34,840 --> 00:16:36,080
are not getting bigger.

363
00:16:36,080 --> 00:16:38,270
The length of a
ruler, determined

364
00:16:38,270 --> 00:16:40,440
by the number of atoms
and how those atoms move

365
00:16:40,440 --> 00:16:43,980
to ground state, does not
expand with the universe.

366
00:16:43,980 --> 00:16:47,040
So the expansion is
now partially driven

367
00:16:47,040 --> 00:16:48,880
by the repulsive
gravity that exists

368
00:16:48,880 --> 00:16:51,360
now, which is causing the
universe to accelerate.

369
00:16:51,360 --> 00:16:54,580
But most of the expansion is
really just a residual velocity

370
00:16:54,580 --> 00:16:57,770
from the Big Bang,
whatever caused it then.

371
00:16:57,770 --> 00:16:59,390
I would assert inflation.

372
00:16:59,390 --> 00:17:01,425
And it's just a matter
of coasting outward, not

373
00:17:01,425 --> 00:17:03,980
being pulled outward,
and that coasting outward

374
00:17:03,980 --> 00:17:07,626
does not cause
atoms to get bigger.

375
00:17:07,626 --> 00:17:08,126
Yes?

376
00:17:08,126 --> 00:17:10,771
AUDIENCE: Is the current
idea that the expansion,

377
00:17:10,771 --> 00:17:13,699
like the acceleration,
is indefinite or are we

378
00:17:13,699 --> 00:17:16,827
going to reach a stop point?

379
00:17:16,827 --> 00:17:17,410
PROFESSOR: OK.

380
00:17:17,410 --> 00:17:20,839
What will be the ultimate
future, I'm being asked here.

381
00:17:20,839 --> 00:17:25,220
And the answer, as you might
guess, is nobody really knows.

382
00:17:25,220 --> 00:17:28,230
But in the context of the kind
of models I'm talking about,

383
00:17:28,230 --> 00:17:30,620
there is a pretty
definite answer,

384
00:17:30,620 --> 00:17:33,782
which is that our
pocket universe--

385
00:17:33,782 --> 00:17:35,740
I'll answer at the level
of our pocket universe

386
00:17:35,740 --> 00:17:38,930
and I'll answer at the level
of the multiverse as a hole.

387
00:17:38,930 --> 00:17:40,970
At the level of our
pocket universe,

388
00:17:40,970 --> 00:17:43,770
our pocket universe
will thin out.

389
00:17:43,770 --> 00:17:45,790
Life will eventually
become impossible

390
00:17:45,790 --> 00:17:48,465
because matter density
will be too low.

391
00:17:50,990 --> 00:17:53,000
It will probably decay.

392
00:17:53,000 --> 00:17:55,530
Our vacuum is probably
not absolutely stable.

393
00:17:55,530 --> 00:17:57,690
Very few things 2
String theory are,

394
00:17:57,690 --> 00:18:00,530
if something like String
theory is the right theory.

395
00:18:00,530 --> 00:18:02,050
But even though it
will be decaying,

396
00:18:02,050 --> 00:18:04,910
it will be expanding still
faster than it decays.

397
00:18:04,910 --> 00:18:07,260
So the decay will cause
holes in our universe.

398
00:18:07,260 --> 00:18:09,256
It will become
like Swiss cheese.

399
00:18:09,256 --> 00:18:10,880
But the universe, as
a whole, will just

400
00:18:10,880 --> 00:18:13,090
go on exponentially
expanding, perhaps

401
00:18:13,090 --> 00:18:15,860
forever, as far as
we can tell, forever.

402
00:18:15,860 --> 00:18:18,710
The multiverse is a
more vibrant object.

403
00:18:18,710 --> 00:18:20,440
The multiverse,
as I always said,

404
00:18:20,440 --> 00:18:24,060
would continue to generate
new pocket universes forever.

405
00:18:24,060 --> 00:18:26,230
So the multiverse
would forever be alive

406
00:18:26,230 --> 00:18:29,620
even though each pocket
universe in the multiverse

407
00:18:29,620 --> 00:18:33,640
would form at some time
and then ultimately die,

408
00:18:33,640 --> 00:18:38,610
die of thinning out
into nothingness.

409
00:18:38,610 --> 00:18:39,473
Yes?

410
00:18:39,473 --> 00:18:41,405
AUDIENCE: Just to add to that.

411
00:18:41,405 --> 00:18:44,786
Do you believe that maybe
it's a cyclic process?

412
00:18:44,786 --> 00:18:49,616
So it expand and decay and
then come back [? yet again ?]

413
00:18:49,616 --> 00:18:51,777
and then happen all over again?

414
00:18:51,777 --> 00:18:52,360
PROFESSOR: OK.

415
00:18:52,360 --> 00:18:54,820
The question is could it
be a cyclic process that

416
00:18:54,820 --> 00:18:57,090
expands, reaches maximum,
comes back and crunches,

417
00:18:57,090 --> 00:18:58,410
and expands again.

418
00:18:58,410 --> 00:19:00,490
That is certainly a
possibility, and there

419
00:19:00,490 --> 00:19:03,240
is some people who
take it very seriously.

420
00:19:03,240 --> 00:19:05,480
I don't see any evidence for it.

421
00:19:05,480 --> 00:19:08,520
And furthermore, there
never really was and still

422
00:19:08,520 --> 00:19:11,720
really isn't a reasonable
theory of the bounce

423
00:19:11,720 --> 00:19:16,165
that would have to be
a part of that theory.

424
00:19:16,165 --> 00:19:16,665
Yeah?

425
00:19:16,665 --> 00:19:18,778
AUDIENCE: But would it be
the expansion overtaking

426
00:19:18,778 --> 00:19:23,640
the decay in our own vacuum
that our universe exists

427
00:19:23,640 --> 00:19:29,350
in, our own little pocket
vacuums of ultimate decay

428
00:19:29,350 --> 00:19:33,246
within our system create more
little pocket universes--

429
00:19:33,246 --> 00:19:34,220
[INTERPOSING VOICES]

430
00:19:34,220 --> 00:19:34,640
PROFESSOR: Within.

431
00:19:34,640 --> 00:19:35,139
Yes.

432
00:19:35,139 --> 00:19:37,670
Yeah, that's correct.

433
00:19:37,670 --> 00:19:42,120
They would not be a big fraction
of the volume of our universe,

434
00:19:42,120 --> 00:19:43,200
but, yes.

435
00:19:43,200 --> 00:19:46,210
The pieces in our universe
that might decay in the future

436
00:19:46,210 --> 00:19:48,442
would produce new
pocket universes.

437
00:19:48,442 --> 00:19:50,900
Most of them would be very low
energy pocket universes that

438
00:19:50,900 --> 00:19:53,930
would presumably not create
life, but some of them

439
00:19:53,930 --> 00:19:57,920
could nonetheless have a high
enough energy to create life.

440
00:19:57,920 --> 00:20:03,080
So we would expect
new, thriving universes

441
00:20:03,080 --> 00:20:06,010
to appear out of our
own pocket universe

442
00:20:06,010 --> 00:20:11,800
as it reaches this
expansion death.

443
00:20:11,800 --> 00:20:12,300
Yes?

444
00:20:12,300 --> 00:20:14,750
AUDIENCE: What does
distinguishes different vacua

445
00:20:14,750 --> 00:20:17,735
besides the
cosmological constant?

446
00:20:17,735 --> 00:20:19,610
PROFESSOR: The question
is what distinguishes

447
00:20:19,610 --> 00:20:23,260
the different vacua besides
the cosmological constant.

448
00:20:23,260 --> 00:20:28,200
And the answer is that they can
distinguish in many, many ways.

449
00:20:28,200 --> 00:20:30,360
What fundamentally
distinguishes them

450
00:20:30,360 --> 00:20:32,910
is the rearrangement
of the innards

451
00:20:32,910 --> 00:20:37,160
within the space, maybe a little
bit more precise without trying

452
00:20:37,160 --> 00:20:38,770
to get into details
which I probably

453
00:20:38,770 --> 00:20:40,510
don't understand either.

454
00:20:40,510 --> 00:20:43,960
But what's going on is that
String theory fundamentally

455
00:20:43,960 --> 00:20:46,500
says that space has nine
dimensions, not the three

456
00:20:46,500 --> 00:20:47,730
that we observe.

457
00:20:47,730 --> 00:20:50,220
And the way the
nine becomes three

458
00:20:50,220 --> 00:20:52,370
is that the extra
dimensions get twisted up

459
00:20:52,370 --> 00:20:56,420
into tiny little knots, so
they occupy too small a length

460
00:20:56,420 --> 00:20:57,665
to ever be seen.

461
00:20:57,665 --> 00:20:59,040
But there are many
different ways

462
00:20:59,040 --> 00:21:00,677
of twisting up those
extra dimensions,

463
00:21:00,677 --> 00:21:03,010
and that's really what leads
to these very large numbers

464
00:21:03,010 --> 00:21:04,730
of possible vacua.

465
00:21:04,730 --> 00:21:08,060
The extra dimensions are
twisted up differently.

466
00:21:08,060 --> 00:21:13,260
So that means that as
far as the low energy

467
00:21:13,260 --> 00:21:14,890
physics in these
different vacua--

468
00:21:14,890 --> 00:21:16,640
practically everything
could be different,

469
00:21:16,640 --> 00:21:18,598
even the dimension of
space could be different.

470
00:21:18,598 --> 00:21:23,300
You could have different numbers
of dimensions compactified.

471
00:21:23,300 --> 00:21:24,970
And the whole particle
spectrum would

472
00:21:24,970 --> 00:21:28,590
be different because what we
view as a particle is really

473
00:21:28,590 --> 00:21:30,890
just the fluctuation of vacuum.

474
00:21:30,890 --> 00:21:33,656
And if you have a different
structure to the vacuum itself,

475
00:21:33,656 --> 00:21:35,280
the kinds of particles
that exist in it

476
00:21:35,280 --> 00:21:37,180
could be totally different.

477
00:21:37,180 --> 00:21:39,919
So the physics inside
these pocket universe

478
00:21:39,919 --> 00:21:41,710
could look tremendously
different from what

479
00:21:41,710 --> 00:21:44,240
we observe even though
that we're predicating

480
00:21:44,240 --> 00:21:46,190
the whole description
on the idea

481
00:21:46,190 --> 00:21:48,160
that, ultimately, it's
the same laws of physics

482
00:21:48,160 --> 00:21:49,076
that apply everywhere.

483
00:21:55,421 --> 00:21:56,900
Other questions?

484
00:21:56,900 --> 00:21:57,400
Yes?

485
00:21:57,400 --> 00:21:58,316
AUDIENCE: [INAUDIBLE]?

486
00:22:38,807 --> 00:22:39,390
PROFESSOR: OK.

487
00:22:39,390 --> 00:22:43,640
I think you're asking about
if we have a small patch, then

488
00:22:43,640 --> 00:22:47,110
that goes inflation and the rest
doesn't, how does the patch end

489
00:22:47,110 --> 00:22:49,740
up dominating because
it started out

490
00:22:49,740 --> 00:22:51,630
with just a small
fraction of the particles.

491
00:22:51,630 --> 00:22:54,088
Doesn't it still have the same
small fraction of particles?

492
00:22:54,088 --> 00:22:57,042
Is that what you're asking?

493
00:22:57,042 --> 00:22:58,042
AUDIENCE: Well, I guess.

494
00:22:58,042 --> 00:23:01,938
If you start out with the
smooth particles being

495
00:23:01,938 --> 00:23:05,347
the excessive matter,
and one of the particles

496
00:23:05,347 --> 00:23:07,782
behaves and the other
two particles [INAUDIBLE]

497
00:23:07,782 --> 00:23:11,200
even if it's still
just two particles?

498
00:23:11,200 --> 00:23:11,990
PROFESSOR: Right.

499
00:23:11,990 --> 00:23:14,346
It isn't the number of
particles conserved, basically,

500
00:23:14,346 --> 00:23:16,470
as all this happens, is I
think what you're asking.

501
00:23:16,470 --> 00:23:18,960
AUDIENCE: Well, even if it
eventually [? is called ?]

502
00:23:18,960 --> 00:23:26,365
expanded wave because the
second part will [INAUDIBLE]

503
00:23:26,365 --> 00:23:27,490
PROFESSOR: Well, let's see.

504
00:23:27,490 --> 00:23:28,780
I'm having a little
trouble hearing you.

505
00:23:28,780 --> 00:23:30,570
But let me make a definite--
let me make a broader statement,

506
00:23:30,570 --> 00:23:32,070
and you can tell
me if I've answered

507
00:23:32,070 --> 00:23:34,520
what you're asking about or not.

508
00:23:34,520 --> 00:23:36,960
When one of these
patches undergoes

509
00:23:36,960 --> 00:23:40,140
the exponential
expansion of inflation,

510
00:23:40,140 --> 00:23:42,490
the energy is really
not very well described

511
00:23:42,490 --> 00:23:43,490
as particles at all.

512
00:23:43,490 --> 00:23:45,700
It's really described
in terms of fields.

513
00:23:45,700 --> 00:23:48,800
And fields sometimes behave
like particles, but not always.

514
00:23:48,800 --> 00:23:52,290
And in this case-- in principle,
there's a particle description

515
00:23:52,290 --> 00:23:54,540
too, but it's not
nearly as obvious

516
00:23:54,540 --> 00:23:56,260
as the field description.

517
00:23:56,260 --> 00:23:59,365
So you have energy stored in
fields and the region grows.

518
00:23:59,365 --> 00:24:00,740
The energy stored
in those fields

519
00:24:00,740 --> 00:24:02,770
actually increases
as the region goes.

520
00:24:02,770 --> 00:24:05,430
The energy density remains
approximately constant.

521
00:24:05,430 --> 00:24:07,130
And that sounds like
it would violate

522
00:24:07,130 --> 00:24:09,600
the conservation of energy,
but we discussed the fact

523
00:24:09,600 --> 00:24:12,410
that what saves
conservation of energy

524
00:24:12,410 --> 00:24:15,680
and allows this to happen in
spite of conservation of energy

525
00:24:15,680 --> 00:24:17,770
is that as the
region expands, it

526
00:24:17,770 --> 00:24:21,420
is filled by a gravitational
field, which is now occupying

527
00:24:21,420 --> 00:24:24,540
a larger and larger volume, and
that gravitational field has

528
00:24:24,540 --> 00:24:26,650
a negative energy density.

529
00:24:26,650 --> 00:24:29,370
So the total energy, which
is what has to be conserved,

530
00:24:29,370 --> 00:24:32,120
remains very small
and perhaps zero,

531
00:24:32,120 --> 00:24:35,100
and the region can
grow without limit

532
00:24:35,100 --> 00:24:39,370
while still having this very
small or zero total energy.

533
00:24:39,370 --> 00:24:41,690
Then, eventually it
decays and when it decays,

534
00:24:41,690 --> 00:24:44,814
it produces new particles,
and the colossal number

535
00:24:44,814 --> 00:24:46,730
of new particles, and
those would be the stuff

536
00:24:46,730 --> 00:24:48,920
that we would be made out of.

537
00:24:48,920 --> 00:24:51,740
And that number is vastly larger
than the number of particles

538
00:24:51,740 --> 00:24:53,210
that may have been
in this region

539
00:24:53,210 --> 00:24:55,491
when the inflation started.

540
00:24:55,491 --> 00:24:55,990
Yes?

541
00:24:55,990 --> 00:24:59,894
AUDIENCE: So does the emergence
of [INAUDIBLE] just purely

542
00:24:59,894 --> 00:25:02,334
a conservation of energy?

543
00:25:02,334 --> 00:25:05,262
Like, what do you need to
make these [? an organism ?],

544
00:25:05,262 --> 00:25:09,574
the negative energy,
zero [INAUDIBLE] I guess.

545
00:25:09,574 --> 00:25:11,740
PROFESSOR: Are you saying
the conservation of energy

546
00:25:11,740 --> 00:25:15,420
maybe controls the whole
show, and that this is really

547
00:25:15,420 --> 00:25:18,540
the only thing consistent
with conservation of energy?

548
00:25:18,540 --> 00:25:21,890
I think that's probably
an exaggeration

549
00:25:21,890 --> 00:25:25,780
because if nothing happened,
that would conserve energy too.

550
00:25:25,780 --> 00:25:29,330
So I think one needs more than
just the conservation of energy

551
00:25:29,330 --> 00:25:33,120
to be able to describe how the
universe is going to evolve.

552
00:25:37,621 --> 00:25:38,120
OK.

553
00:25:38,120 --> 00:25:39,215
Let me continue.

554
00:25:46,284 --> 00:25:49,270
Get back to the beginning
there, back to the end.

555
00:25:49,270 --> 00:25:49,770
OK.

556
00:25:49,770 --> 00:25:52,420
So I just finished talking about
the landscape of String theory

557
00:25:52,420 --> 00:25:54,530
and how it offers all
these possible vacua.

558
00:25:54,530 --> 00:25:58,960
So in particular, and this is
now the new stuff, if there

559
00:25:58,960 --> 00:26:02,340
are 10 to the 500 vacua of
String theory, for example.

560
00:26:02,340 --> 00:26:05,950
We don't really know the number,
but something crazy like that.

561
00:26:05,950 --> 00:26:09,770
And if only one part in
10 to the 120 of them

562
00:26:09,770 --> 00:26:12,680
have this very small energy,
thus the energy densities

563
00:26:12,680 --> 00:26:17,750
are spread from plus
10 to the 120 times

564
00:26:17,750 --> 00:26:22,429
what we observe to minus 10 to
the 120 times what we observe.

565
00:26:22,429 --> 00:26:23,970
That would mean that
what we observed

566
00:26:23,970 --> 00:26:27,990
would be a narrow slice in the
middle there occupying about 10

567
00:26:27,990 --> 00:26:32,382
to the minus 120th of the
length of that spread.

568
00:26:32,382 --> 00:26:34,340
We would then expect--
and all this, of course,

569
00:26:34,340 --> 00:26:36,675
is very crude estimates.

570
00:26:36,675 --> 00:26:38,550
It's not really the
numbers that's important,

571
00:26:38,550 --> 00:26:41,406
it's whether or not
you believe the ideas.

572
00:26:41,406 --> 00:26:44,640
But we'd expect then that
about 10 to the minus 120

573
00:26:44,640 --> 00:26:48,610
of the different vacua would
have an acceptably low vacuum

574
00:26:48,610 --> 00:26:50,486
energy density.

575
00:26:50,486 --> 00:26:52,360
But that's still a
colossal number because 10

576
00:26:52,360 --> 00:26:55,160
to the minus 120
times 10 to the 500--

577
00:26:55,160 --> 00:26:58,540
you add the exponents--
is 10 to the 380.

578
00:26:58,540 --> 00:27:01,000
So we would still predict
that even though they'd

579
00:27:01,000 --> 00:27:05,320
be very rare, there might be
10 to the 380 different kinds

580
00:27:05,320 --> 00:27:08,470
of vacua, all which would
have a vacuum energy density

581
00:27:08,470 --> 00:27:10,580
as well as what we observe.

582
00:27:10,580 --> 00:27:15,290
So there's no problem finding,
in the landscape, vacua

583
00:27:15,290 --> 00:27:18,160
whose energy density is
as low as what we observe.

584
00:27:18,160 --> 00:27:21,780
But then there's the question
if they're so incredibly rare,

585
00:27:21,780 --> 00:27:23,260
wouldn't it take
a miracle for us

586
00:27:23,260 --> 00:27:27,070
to be living in one of these
incredibly unusual vacua

587
00:27:27,070 --> 00:27:30,520
with such an extraordinarily
low vacuum energy density.

588
00:27:30,520 --> 00:27:33,250
That then leads to
what is sometimes

589
00:27:33,250 --> 00:27:38,490
called Anthropic considerations
or perhaps a selection effect.

590
00:27:38,490 --> 00:27:41,920
And to see how that works and
make it sound not as crazy

591
00:27:41,920 --> 00:27:43,890
as it might sound
otherwise, I want

592
00:27:43,890 --> 00:27:46,620
to begin by giving an example
where I think one could really

593
00:27:46,620 --> 00:27:48,560
say that this effect happens.

594
00:27:48,560 --> 00:27:53,820
And that is suppose we just
look at our own position

595
00:27:53,820 --> 00:27:56,770
in our own visible
universe and look at,

596
00:27:56,770 --> 00:27:59,432
for example, the mass density.

597
00:27:59,432 --> 00:28:01,140
Where we're actually
living is incredibly

598
00:28:01,140 --> 00:28:03,723
unusual in many ways, but one
of the ways we could talk about,

599
00:28:03,723 --> 00:28:05,740
which is just simple
and quantitative,

600
00:28:05,740 --> 00:28:07,050
is the mass density.

601
00:28:07,050 --> 00:28:09,990
The mass density of the
things around this room

602
00:28:09,990 --> 00:28:14,200
is on the order of one gram per
centimeter cubed give or take

603
00:28:14,200 --> 00:28:15,217
a factor of 10.

604
00:28:15,217 --> 00:28:16,800
The factor of 10 is
not very important

605
00:28:16,800 --> 00:28:18,144
for I'm talking about here.

606
00:28:18,144 --> 00:28:19,560
The point is that
the average mass

607
00:28:19,560 --> 00:28:22,890
density of the universe, the
visible universe, is about 10

608
00:28:22,890 --> 00:28:25,610
to the minus 30 grams
per centimeter cubed.

609
00:28:25,610 --> 00:28:30,000
It's really unbelievable
how empty the universe is.

610
00:28:30,000 --> 00:28:33,630
It's actually a far lower mass
density than is possible for us

611
00:28:33,630 --> 00:28:36,990
to achieve in laboratories
on Earth with the best vacua

612
00:28:36,990 --> 00:28:40,130
that we can make in
our laboratories.

613
00:28:40,130 --> 00:28:43,720
So where we're living
has a mass density of 10

614
00:28:43,720 --> 00:28:48,360
to the 30 times the average
of the visible universe.

615
00:28:48,360 --> 00:28:51,070
So we're not living in a typical
place in our visible universe.

616
00:28:51,070 --> 00:28:54,470
We're living in an
extraordinarily atypical place

617
00:28:54,470 --> 00:28:56,160
within our visible universe.

618
00:28:56,160 --> 00:28:59,224
And we can ask how
would we explain that.

619
00:28:59,224 --> 00:29:00,890
Is it just a matter
of chance that we're

620
00:29:00,890 --> 00:29:03,550
living in a place that's
such a high mass density?

621
00:29:03,550 --> 00:29:05,805
Doesn't seem very likely
if it's a matter of chance.

622
00:29:05,805 --> 00:29:06,500
Is it luck?

623
00:29:06,500 --> 00:29:08,560
Is it divine
providence, whatever?

624
00:29:08,560 --> 00:29:11,460
I think most of us would admit
that it's probably a selection

625
00:29:11,460 --> 00:29:12,450
effect.

626
00:29:12,450 --> 00:29:15,020
That that's where life happens.

627
00:29:15,020 --> 00:29:18,740
Life doesn't happen throughout
most of the visible universe,

628
00:29:18,740 --> 00:29:20,930
but in these rare
places, like the surface

629
00:29:20,930 --> 00:29:23,280
of our planet, which is
special in more ways than just

630
00:29:23,280 --> 00:29:25,370
the mass density, but
the mass density alone

631
00:29:25,370 --> 00:29:28,950
is enough to make it
extraordinarily special.

632
00:29:28,950 --> 00:29:31,020
We're off by a factor
of 10 to the 30

633
00:29:31,020 --> 00:29:33,890
from the average
of our environment.

634
00:29:33,890 --> 00:29:36,420
So if we're willing
to explain why

635
00:29:36,420 --> 00:29:40,560
we live in such an unusual place
within our visible universe

636
00:29:40,560 --> 00:29:44,000
and explain that as simply
a requirement for life,

637
00:29:44,000 --> 00:29:47,450
then it doesn't seem to be such
a stretch to maybe imagine--

638
00:29:47,450 --> 00:29:51,542
and it was Steve Weinberg who
first emphasized this in 1987.

639
00:29:51,542 --> 00:29:53,250
Certainly not the
first person to say it,

640
00:29:53,250 --> 00:29:56,310
but the first person to
say it and have people

641
00:29:56,310 --> 00:29:58,670
sometimes believe him.

642
00:29:58,670 --> 00:30:01,940
He pointed out that may be the
low energy-- the low vacuum

643
00:30:01,940 --> 00:30:05,190
energy density could be
explained the same way.

644
00:30:05,190 --> 00:30:07,490
If we're not living
in a typical place

645
00:30:07,490 --> 00:30:10,090
within our visible
universe, there's

646
00:30:10,090 --> 00:30:14,410
no reason for similar
ideas to expect

647
00:30:14,410 --> 00:30:16,370
that we should be living
in a typical place

648
00:30:16,370 --> 00:30:18,150
in the multiverse.

649
00:30:18,150 --> 00:30:21,720
Maybe only a small fraction
of these different types

650
00:30:21,720 --> 00:30:24,820
of pocket universe's
can support life.

651
00:30:24,820 --> 00:30:27,290
And maybe the only
way to have life

652
00:30:27,290 --> 00:30:31,170
is to have a very small value
for the vacuum energy density.

653
00:30:31,170 --> 00:30:33,700
And there is some
physics behind that.

654
00:30:33,700 --> 00:30:38,050
Remember this vacuum energy
density drives expansion--

655
00:30:38,050 --> 00:30:41,000
acceleration, I should say.

656
00:30:41,000 --> 00:30:46,210
So if the vacuum energy density
were significantly larger

657
00:30:46,210 --> 00:30:48,070
than what we
observe, the universe

658
00:30:48,070 --> 00:30:49,594
would accelerate
incredibly rapidly

659
00:30:49,594 --> 00:30:51,010
and would fly apart
before there'd

660
00:30:51,010 --> 00:30:53,280
be any time for anything
interesting to happen

661
00:30:53,280 --> 00:30:55,306
like galaxies forming.

662
00:30:55,306 --> 00:30:57,430
Weinberg based his arguments
here on the assumption

663
00:30:57,430 --> 00:31:00,340
that galaxies are a
necessity for life.

664
00:31:00,340 --> 00:31:00,840
Yes?

665
00:31:00,840 --> 00:31:02,820
AUDIENCE: So that's
what I was going to ask.

666
00:31:02,820 --> 00:31:05,112
Why do we assume that our
universe is the only one that

667
00:31:05,112 --> 00:31:07,570
could have like-- why couldn't
just all the multi-universes

668
00:31:07,570 --> 00:31:08,162
have like--

669
00:31:08,162 --> 00:31:08,870
PROFESSOR: Right.

670
00:31:08,870 --> 00:31:09,090
Right.

671
00:31:09,090 --> 00:31:09,800
Well, that's OK.

672
00:31:09,800 --> 00:31:11,430
That is what I am talking about.

673
00:31:11,430 --> 00:31:13,540
I'm trying to answer it.

674
00:31:13,540 --> 00:31:16,430
So if the vacuum energy density
were significantly larger

675
00:31:16,430 --> 00:31:18,424
than what we observe,
the universes

676
00:31:18,424 --> 00:31:20,340
would fly apart so fast
that there could never

677
00:31:20,340 --> 00:31:24,085
be galaxies and therefore
never planets, none of things

678
00:31:24,085 --> 00:31:26,460
that we think of as being
associated with life as we know

679
00:31:26,460 --> 00:31:27,670
it.

680
00:31:27,670 --> 00:31:31,820
Conversely, if the vacuum energy
density were negative, but had

681
00:31:31,820 --> 00:31:34,530
a magnitude large compared
to what we observed,

682
00:31:34,530 --> 00:31:37,550
that would be a large negative
acceleration, an implosion.

683
00:31:37,550 --> 00:31:40,050
And those universes would
just implode, collapse,

684
00:31:40,050 --> 00:31:44,840
in an incredibly short amount
of time, much too fast for life,

685
00:31:44,840 --> 00:31:47,750
of any type that we
know of, to form.

686
00:31:47,750 --> 00:31:51,780
So there is a physical argument
which suggests that life only

687
00:31:51,780 --> 00:31:56,040
forms when the vacuum
energy density is very low.

688
00:31:56,040 --> 00:31:58,392
And Weinberg and his
collaborators-- and this

689
00:31:58,392 --> 00:32:00,892
is the same Steve Weinberg who
wrote the First Three Minutes

690
00:32:00,892 --> 00:32:04,860
that we're reading-- calculated
what the requirements would

691
00:32:04,860 --> 00:32:06,220
be for galaxy formation.

692
00:32:06,220 --> 00:32:09,710
And they decided that, within
about a factor of 5 or so,

693
00:32:09,710 --> 00:32:11,260
the vacuum energy
density would have

694
00:32:11,260 --> 00:32:14,430
to be about the same as
what we observe or less

695
00:32:14,430 --> 00:32:17,300
in order for galaxies to form.

696
00:32:17,300 --> 00:32:19,354
So it seems like a
possible explanation.

697
00:32:19,354 --> 00:32:21,520
It's certainly not a generally
accepted explanation.

698
00:32:21,520 --> 00:32:22,820
These things are very
controversial one.

699
00:32:22,820 --> 00:32:24,278
I guess that's, in
fact, what I was

700
00:32:24,278 --> 00:32:26,940
going to talk about
on my next slide.

701
00:32:26,940 --> 00:32:30,910
Some physicists by this
selection effect idea.

702
00:32:30,910 --> 00:32:33,720
I tend to buy it.

703
00:32:33,720 --> 00:32:35,791
But a number of physicists
regard it as totally

704
00:32:35,791 --> 00:32:37,665
ridiculous, saying you
could explain anything

705
00:32:37,665 --> 00:32:40,680
if you except
arguments like that.

706
00:32:40,680 --> 00:32:42,960
And there's some truth to that.

707
00:32:42,960 --> 00:32:44,730
You can explain a lot
of things if you're

708
00:32:44,730 --> 00:32:46,800
willing to just say,
well, maybe that's needed

709
00:32:46,800 --> 00:32:49,050
for life to happen.

710
00:32:49,050 --> 00:32:52,840
So because of that, I would
say that these selection effect

711
00:32:52,840 --> 00:32:55,530
arguments or anthropic
arguments should always

712
00:32:55,530 --> 00:32:59,200
be viewed as the
arguments of last resort.

713
00:32:59,200 --> 00:33:01,659
That is, unless we
actually understand

714
00:33:01,659 --> 00:33:03,200
the landscape of
String theory, which

715
00:33:03,200 --> 00:33:05,660
we do not in detail,
and once we actually

716
00:33:05,660 --> 00:33:08,640
understand what it
takes to create life,

717
00:33:08,640 --> 00:33:12,510
we really can't do more than
give plausibility arguments

718
00:33:12,510 --> 00:33:17,190
to justify these
anthropic explanations.

719
00:33:17,190 --> 00:33:22,620
But these anthropic
arguments do sound sensible.

720
00:33:22,620 --> 00:33:24,670
I think there's nothing
illogical about them,

721
00:33:24,670 --> 00:33:28,060
and they could very well be the
explanations for some things.

722
00:33:28,060 --> 00:33:31,070
As I pointed out, I think it
is the explanation for why

723
00:33:31,070 --> 00:33:33,270
we are living in
such an unusual place

724
00:33:33,270 --> 00:33:36,580
within our own visible universe.

725
00:33:36,580 --> 00:33:41,310
And it means that these
selection effect arguments

726
00:33:41,310 --> 00:33:44,760
become very attractive when the
search for more deterministic

727
00:33:44,760 --> 00:33:46,800
explanations have failed.

728
00:33:46,800 --> 00:33:50,370
And in the case of trying to
explain the very small vacuum

729
00:33:50,370 --> 00:33:52,770
energy density, I think
other attempts have failed.

730
00:33:52,770 --> 00:33:58,270
We don't have any calculational,
deterministic understanding

731
00:33:58,270 --> 00:34:03,040
for why the vacuum energy
should be so small.

732
00:34:03,040 --> 00:34:07,900
So is it time to accept this
explanation of last resort

733
00:34:07,900 --> 00:34:10,090
that the vacuum energy
density is small

734
00:34:10,090 --> 00:34:13,665
because it has to be
for life to the evolve?

735
00:34:13,665 --> 00:34:15,320
Your guess is as good as mine.

736
00:34:15,320 --> 00:34:17,380
I don't really know.

737
00:34:17,380 --> 00:34:20,520
But I would say that, in the
case of the vacuum energy

738
00:34:20,520 --> 00:34:22,929
density, people have
been trying very, very

739
00:34:22,929 --> 00:34:24,760
hard for quite a
few years now to try

740
00:34:24,760 --> 00:34:28,610
to find a particle physics
explanation for why the vacuum

741
00:34:28,610 --> 00:34:32,872
energy has to be small, and
nobody's really found anything

742
00:34:32,872 --> 00:34:35,205
that anybody has found-- that
any large number of people

743
00:34:35,205 --> 00:34:37,770
have found to be acceptable.

744
00:34:37,770 --> 00:34:39,520
So it is certainly a
very serious problem.

745
00:34:39,520 --> 00:34:41,900
And I think it is
time to take seriously

746
00:34:41,900 --> 00:34:45,770
the argument of last resort,
that maybe it's that way only

747
00:34:45,770 --> 00:34:49,120
because in the parts of the
multiverse where it's not

748
00:34:49,120 --> 00:34:51,139
that way, nobody lives there.

749
00:34:53,576 --> 00:34:55,409
So I would say it's
hard to deny, as of now,

750
00:34:55,409 --> 00:34:58,720
that the selection
effect explanation is

751
00:34:58,720 --> 00:35:00,930
the most plausible of
any explanation that

752
00:35:00,930 --> 00:35:04,440
is known at the present time.

753
00:35:04,440 --> 00:35:06,570
To summarize things--
I'm actually done now,

754
00:35:06,570 --> 00:35:09,410
but let me just
summarize what I said

755
00:35:09,410 --> 00:35:12,856
to remind you where we're at.

756
00:35:12,856 --> 00:35:14,605
I've argued that the
inflationary paradigm

757
00:35:14,605 --> 00:35:16,790
is in great shape.

758
00:35:16,790 --> 00:35:20,007
It explains the large
scale uniformity.

759
00:35:20,007 --> 00:35:21,840
It predicts the mass
density of the universe

760
00:35:21,840 --> 00:35:26,180
to better than about 1% accuracy
and explains the ripples

761
00:35:26,180 --> 00:35:29,260
that we see in the cosmic
background radiation,

762
00:35:29,260 --> 00:35:32,010
explaining them as a result
of quantum fluctuations

763
00:35:32,010 --> 00:35:36,590
that took place in the
very early universe.

764
00:35:36,590 --> 00:35:39,560
The picture leads
to three ideas that

765
00:35:39,560 --> 00:35:42,610
at least point towards
the idea of a multiverse.

766
00:35:42,610 --> 00:35:45,560
It certainly doesn't prove that
we're living in a multiverse.

767
00:35:45,560 --> 00:35:47,800
But the three ideas that
point in that direction

768
00:35:47,800 --> 00:35:50,610
are, first of all, the
statement that almost

769
00:35:50,610 --> 00:35:53,230
all inflationary models
lead to this feature

770
00:35:53,230 --> 00:35:57,030
of eternal inflation, that
the exponential expansion

771
00:35:57,030 --> 00:35:59,630
of the inflating material,
generally speaking,

772
00:35:59,630 --> 00:36:02,530
out runs the decay
of that material

773
00:36:02,530 --> 00:36:07,490
so that the volume grows
exponentially forever.

774
00:36:07,490 --> 00:36:12,030
Second point is that, in 1998,
the astronomers discovered

775
00:36:12,030 --> 00:36:15,290
this rather amazing fact
that the universe is not

776
00:36:15,290 --> 00:36:21,420
slowing down as it expands,
but in fact, is accelerating.

777
00:36:21,420 --> 00:36:23,380
And that indicates
that there has

778
00:36:23,380 --> 00:36:26,560
to be some peculiar material
in the universe other than what

779
00:36:26,560 --> 00:36:29,510
we already knew was here,
and that peculiar material is

780
00:36:29,510 --> 00:36:31,780
called the dark energy.

781
00:36:31,780 --> 00:36:35,410
And we don't have any simple
interpretation of what it is,

782
00:36:35,410 --> 00:36:37,970
but it seems to most
likely be vacuum energy.

783
00:36:37,970 --> 00:36:40,800
And if it is, it
leads immediately

784
00:36:40,800 --> 00:36:43,470
to the important
question of can we

785
00:36:43,470 --> 00:36:47,420
understand why it has
a value that it has.

786
00:36:47,420 --> 00:36:50,810
It seems to be much smaller
than what we would expect.

787
00:36:50,810 --> 00:36:54,370
And then three, the
String theorists

788
00:36:54,370 --> 00:36:57,172
give us an interesting
way out here.

789
00:36:57,172 --> 00:36:59,130
The String theorists tell
us that maybe there's

790
00:36:59,130 --> 00:37:01,780
not unique vacuum to
the laws of physics,

791
00:37:01,780 --> 00:37:03,410
but maybe there's a
huge number, which

792
00:37:03,410 --> 00:37:06,410
seems to be in fact what
String theory predicts.

793
00:37:06,410 --> 00:37:11,910
And if there is, then of
the many different vacua

794
00:37:11,910 --> 00:37:14,070
you expect there to be,
in fact perhaps even

795
00:37:14,070 --> 00:37:17,460
a large number, that would have
this very small vacuum energy

796
00:37:17,460 --> 00:37:22,680
density, a tiny fraction of
the total different vacua,

797
00:37:22,680 --> 00:37:24,820
but nonetheless a
large number of vacua

798
00:37:24,820 --> 00:37:26,740
that would have this property.

799
00:37:26,740 --> 00:37:29,170
And then this
selection effect idea

800
00:37:29,170 --> 00:37:30,620
can provide a
possible explanation

801
00:37:30,620 --> 00:37:35,230
for why we are living in one of
those very unusual vacua which

802
00:37:35,230 --> 00:37:39,630
has this incredibly tiny
vacuum energy density.

803
00:37:39,630 --> 00:37:42,040
So finally, I'd
just like to close

804
00:37:42,040 --> 00:37:45,460
with a little sociological
discussion here.

805
00:37:45,460 --> 00:37:48,130
Do physicists really
take this seriously?

806
00:37:48,130 --> 00:37:50,810
And I'd like to tell you about
a conversation that took place

807
00:37:50,810 --> 00:37:52,670
at a conference a few years ago.

808
00:37:52,670 --> 00:37:56,260
Starting with Martin Rees, who I
don't know if you know the name

809
00:37:56,260 --> 00:37:58,850
or not, but he's an Astronomer
Royal of Great Britain,

810
00:37:58,850 --> 00:38:00,590
former president
of Royal Society,

811
00:38:00,590 --> 00:38:02,810
former master of
Trinity College as well,

812
00:38:02,810 --> 00:38:06,082
a very distinguished person,
nice guy, too, by the way.

813
00:38:06,082 --> 00:38:07,540
And he said that
he is sufficiently

814
00:38:07,540 --> 00:38:12,340
confident in the multiverse
to bet his dog's life on it.

815
00:38:12,340 --> 00:38:16,490
Andrei Linde, from Stanford,
a real enthusiastic person

816
00:38:16,490 --> 00:38:21,410
about the multiverse, one of the
founders of inflation as well,

817
00:38:21,410 --> 00:38:23,227
said that he's so
confident that he

818
00:38:23,227 --> 00:38:25,801
would bet his own life on it.

819
00:38:25,801 --> 00:38:27,550
Steve Weinberg was not
at this conference,

820
00:38:27,550 --> 00:38:29,758
but he wrote an article
commenting on this discussion

821
00:38:29,758 --> 00:38:31,467
later which became known.

822
00:38:31,467 --> 00:38:33,050
And I always considered
Steve Weinberg

823
00:38:33,050 --> 00:38:34,827
the voice of reason,
which is why we're

824
00:38:34,827 --> 00:38:36,160
reading the First Three Minutes.

825
00:38:36,160 --> 00:38:38,076
And he said that I have
just enough confidence

826
00:38:38,076 --> 00:38:40,130
in the multiverse to
bet-- guess what's

827
00:38:40,130 --> 00:38:43,556
coming-- the lives of both
Andrei Linde and Martin Rees'

828
00:38:43,556 --> 00:38:44,055
dog.

829
00:38:49,730 --> 00:38:52,870
That's it for the
summary, or the overview.

830
00:38:52,870 --> 00:38:56,000
Anymore overview type
questions before we get back

831
00:38:56,000 --> 00:38:59,140
to the beginning, actual
beginning of the class?

832
00:39:05,810 --> 00:39:07,066
Yes?

833
00:39:07,066 --> 00:39:13,345
AUDIENCE: You said-- so
selection effect argument says

834
00:39:13,345 --> 00:39:19,668
that it's because life
exists within these certain

835
00:39:19,668 --> 00:39:24,150
constraints, omega being one
and low energy larger than it

836
00:39:24,150 --> 00:39:29,130
generally is allowed, that
life could exist in this way.

837
00:39:29,130 --> 00:39:32,118
But we're considering
carbon-based life.

838
00:39:32,118 --> 00:39:36,351
What if there's some
other [INAUDIBLE] life

839
00:39:36,351 --> 00:39:39,588
forms out there that gives
us different energies

840
00:39:39,588 --> 00:39:41,997
and radiation and
stuff like that?

841
00:39:41,997 --> 00:39:43,830
PROFESSOR: Yeah, what
you're pointing toward

842
00:39:43,830 --> 00:39:47,720
is certainly a severe weakness
of these selection effect

843
00:39:47,720 --> 00:39:51,760
arguments, that we really know
about carbon-based life, life

844
00:39:51,760 --> 00:39:54,600
that's like us, and we can
talk about what conditions

845
00:39:54,600 --> 00:39:58,090
are needed to make life
like us, but maybe there's

846
00:39:58,090 --> 00:39:59,860
life that's totally
different from us

847
00:39:59,860 --> 00:40:01,490
that we don't know
anything about that

848
00:40:01,490 --> 00:40:02,970
might be able to
live under totally

849
00:40:02,970 --> 00:40:04,485
different circumstances.

850
00:40:08,160 --> 00:40:10,590
That is a real weakness.

851
00:40:10,590 --> 00:40:14,240
However, I would argue-- and
this is also controversial.

852
00:40:14,240 --> 00:40:17,110
Not everybody would agree
with what I'm about to say.

853
00:40:17,110 --> 00:40:21,830
But I would argue that if
we're willing to explain

854
00:40:21,830 --> 00:40:25,990
the unusual features of
the piece of the universe

855
00:40:25,990 --> 00:40:29,350
that we live in by
selection effect arguments--

856
00:40:29,350 --> 00:40:32,140
the fact I used, the
example is simply

857
00:40:32,140 --> 00:40:34,580
that we're living a place
where the mass density is

858
00:40:34,580 --> 00:40:37,060
10 to the 30 times
larger than the mean.

859
00:40:37,060 --> 00:40:39,790
If we're willing to use the
anthropic arguments to explain

860
00:40:39,790 --> 00:40:43,930
that, then I think all those
same issues arise there also.

861
00:40:43,930 --> 00:40:46,210
If life was really teem--
if the universe was really

862
00:40:46,210 --> 00:40:49,040
teeming with a different
kind of life that

863
00:40:49,040 --> 00:40:51,290
thrived in vacua,
then we'd be much more

864
00:40:51,290 --> 00:40:54,020
likely to be one of them,
extremely unusual creatures

865
00:40:54,020 --> 00:40:56,690
living on the
surfaces of planets.

866
00:40:56,690 --> 00:41:00,010
So I think it's a
possible weakness

867
00:41:00,010 --> 00:41:01,874
that one has to
keep in mind, but I

868
00:41:01,874 --> 00:41:03,540
don't think it should
stop us from using

869
00:41:03,540 --> 00:41:04,720
those arguments completely.

870
00:41:04,720 --> 00:41:07,940
But it is certainly a
cause for skepticism.

871
00:41:07,940 --> 00:41:08,440
Yes?

872
00:41:08,440 --> 00:41:10,606
AUDIENCE: Isn't the point
of the selection principle

873
00:41:10,606 --> 00:41:16,376
just the fact that exist--
the universe selected for us.

874
00:41:16,376 --> 00:41:19,352
Does it matter for the general
of just for like carbon-based

875
00:41:19,352 --> 00:41:21,832
[? organisms? ?] Is the fact
that we exist [INAUDIBLE]

876
00:41:21,832 --> 00:41:24,312
that we've been selected
for [INAUDIBLE]?

877
00:41:28,310 --> 00:41:29,710
PROFESSOR: You're
asking about, I

878
00:41:29,710 --> 00:41:32,750
think, how peculiar
to carbon-based life

879
00:41:32,750 --> 00:41:35,353
should we expect these selection
effect arguments to be.

880
00:41:35,353 --> 00:41:37,728
AUDIENCE: Doesn't the selection
affect where [INAUDIBLE]?

881
00:41:46,262 --> 00:41:47,970
PROFESSOR: Now that's
an important point,

882
00:41:47,970 --> 00:41:51,550
and certainly one that's not
settled among philosophers,

883
00:41:51,550 --> 00:41:54,170
probabilist,
physicists, or anybody.

884
00:41:54,170 --> 00:41:56,680
What you're asking-- if
I'm summarizing it right--

885
00:41:56,680 --> 00:41:59,300
is when we're thinking
of the selection effects,

886
00:41:59,300 --> 00:42:01,554
should we may be only talk
about carbon-based life

887
00:42:01,554 --> 00:42:03,970
because, after all, we know
that we are carbon-based life.

888
00:42:03,970 --> 00:42:06,178
So what does it matter if
there's other kinds of life

889
00:42:06,178 --> 00:42:07,180
out there?

890
00:42:07,180 --> 00:42:09,390
That's one way of
looking at it, certainly.

891
00:42:09,390 --> 00:42:12,420
Or, maybe we should think
about all kinds of life.

892
00:42:12,420 --> 00:42:14,550
That's something
else that people say.

893
00:42:18,560 --> 00:42:21,085
The problem I would--
I tend to be by the way

894
00:42:21,085 --> 00:42:23,460
the kind of person that thinks
that all life is relevant,

895
00:42:23,460 --> 00:42:25,210
not just carbon-based life.

896
00:42:25,210 --> 00:42:26,876
Because we happen
to be carbon-based,

897
00:42:26,876 --> 00:42:28,250
and we happen to
have fingernails

898
00:42:28,250 --> 00:42:29,330
that have a certain
length, and we

899
00:42:29,330 --> 00:42:31,230
happen to have hair
that's a certain length

900
00:42:31,230 --> 00:42:32,990
or a certain
thickness, does that

901
00:42:32,990 --> 00:42:34,930
mean we should only
think of those things

902
00:42:34,930 --> 00:42:36,471
as being relevant
when we're thinking

903
00:42:36,471 --> 00:42:38,066
about selection effects?

904
00:42:38,066 --> 00:42:39,440
And I would say
that they're not.

905
00:42:39,440 --> 00:42:42,100
If our hair had a
different thicknesses,

906
00:42:42,100 --> 00:42:46,870
we would still be able to
make measurements and so on.

907
00:42:46,870 --> 00:42:49,890
So from my point
of view, when one

908
00:42:49,890 --> 00:42:53,680
thinks about these issues
of selection effects,

909
00:42:53,680 --> 00:42:57,350
one should precondition
only on the elements that

910
00:42:57,350 --> 00:43:00,000
are necessary to ask the
question in the first place.

911
00:43:00,000 --> 00:43:03,200
And what I would like to
think-- and as I point out,

912
00:43:03,200 --> 00:43:04,920
this is controversial,
not everybody

913
00:43:04,920 --> 00:43:08,060
agrees with me-- is that
a good theory should

914
00:43:08,060 --> 00:43:11,550
be a theory in which you could
say that most of the people who

915
00:43:11,550 --> 00:43:14,440
ask this particular question
will get the answer that we

916
00:43:14,440 --> 00:43:15,420
say.

917
00:43:15,420 --> 00:43:18,380
If only a tiny fraction of
people who ask that question

918
00:43:18,380 --> 00:43:20,525
will get that answer, but
that same tiny fraction

919
00:43:20,525 --> 00:43:22,150
happens to have hair
of a certain color

920
00:43:22,150 --> 00:43:23,767
and you have hair
of that color, to me

921
00:43:23,767 --> 00:43:26,350
that's still not an explanation
because you don't know why you

922
00:43:26,350 --> 00:43:27,891
have hair of that
color or why you're

923
00:43:27,891 --> 00:43:30,700
living in such an unusual place.

924
00:43:30,700 --> 00:43:33,190
OK that strikes up a
lot of conversations.

925
00:43:33,190 --> 00:43:33,690
Yes?

926
00:43:33,690 --> 00:43:35,065
AUDIENCE: You
mentioned last time

927
00:43:35,065 --> 00:43:36,540
that the different
pocket universes

928
00:43:36,540 --> 00:43:39,460
that comprise the multiverse
are disconnected from each other

929
00:43:39,460 --> 00:43:44,460
though they start out as patches
within the preceding vacua.

930
00:43:44,460 --> 00:43:46,630
What starts to disconnect
them fundamentally

931
00:43:46,630 --> 00:43:50,850
from the vacuum
which they formed?

932
00:43:50,850 --> 00:43:53,540
PROFESSOR: The question is
what is it that separates these

933
00:43:53,540 --> 00:43:55,410
different pocket universe's.

934
00:43:55,410 --> 00:43:57,200
If they start out all
in the same space,

935
00:43:57,200 --> 00:43:59,520
don't they remain all
in the same space?

936
00:43:59,520 --> 00:44:02,250
And the answer is they do, but
the space they started out in

937
00:44:02,250 --> 00:44:04,660
was expanding at
a very rapid rate.

938
00:44:04,660 --> 00:44:08,130
So in most cases,
but not all actually,

939
00:44:08,130 --> 00:44:10,480
two pocket universes will
form far enough apart

940
00:44:10,480 --> 00:44:12,040
from each other
that they will never

941
00:44:12,040 --> 00:44:14,460
touch each other as they grow
because the space in between

942
00:44:14,460 --> 00:44:17,820
will expand to fast to
ever allow them to meet.

943
00:44:17,820 --> 00:44:21,010
However, collisions
of pocket universities

944
00:44:21,010 --> 00:44:24,710
will occur if two pocket
universes form close enough

945
00:44:24,710 --> 00:44:27,240
to each other, the expansion
of space in between

946
00:44:27,240 --> 00:44:31,620
will not be enough to keep them
apart, and they will glide.

947
00:44:31,620 --> 00:44:33,940
How frequent one should
think of that as being

948
00:44:33,940 --> 00:44:36,380
is an incredibly tough
question to which

949
00:44:36,380 --> 00:44:38,440
nobody knows the answer.

950
00:44:38,440 --> 00:44:40,680
There are actually--
at least there

951
00:44:40,680 --> 00:44:42,650
is at least one
astronomical paper

952
00:44:42,650 --> 00:44:44,610
in the literature by
a group of astronomers

953
00:44:44,610 --> 00:44:49,160
who have looked for possible
signs of a collision of bubbles

954
00:44:49,160 --> 00:44:50,410
in our past.

955
00:44:50,410 --> 00:44:53,380
They did not find
anything definitive.

956
00:44:53,380 --> 00:44:55,860
But it is something
to think about,

957
00:44:55,860 --> 00:44:57,882
and it's something people
are thinking about.

958
00:44:57,882 --> 00:44:59,340
There really are
quite a few papers

959
00:44:59,340 --> 00:45:03,760
about collisions of
bubbles in the literature.

960
00:45:03,760 --> 00:45:04,260
Yes?

961
00:45:04,260 --> 00:45:06,260
AUDIENCE: How long
is long-lived?

962
00:45:06,260 --> 00:45:10,760
So if the energy density was
too large and too negative

963
00:45:10,760 --> 00:45:13,092
would that still
be long-lived if it

964
00:45:13,092 --> 00:45:15,777
were to collide upon itself?

965
00:45:15,777 --> 00:45:18,110
PROFESSOR: Talking about the
lifetime of these universes

966
00:45:18,110 --> 00:45:20,250
that I said would
collapse very quickly.

967
00:45:20,250 --> 00:45:21,630
How quickly do I mean?

968
00:45:21,630 --> 00:45:25,429
AUDIENCE: Like the
metastable long-lived.

969
00:45:25,429 --> 00:45:26,970
PROFESSOR: I used
the word long-lived

970
00:45:26,970 --> 00:45:29,095
at least twice in what I've
talked about-- I talked

971
00:45:29,095 --> 00:45:31,280
about the long-lived
metastable vacua.

972
00:45:31,280 --> 00:45:33,600
And there, by long-lived
I mean anything

973
00:45:33,600 --> 00:45:37,250
that's long compared to the age
of our universe since the Big

974
00:45:37,250 --> 00:45:38,080
Bang.

975
00:45:38,080 --> 00:45:40,170
Long means long
compared to 10 to the 10

976
00:45:40,170 --> 00:45:42,980
years in that context.

977
00:45:42,980 --> 00:45:47,640
I also said that if the
vacuum energy of a universe

978
00:45:47,640 --> 00:45:51,220
were large and negative, it
would very rapidly collapse.

979
00:45:51,220 --> 00:45:54,890
That could be as fast as
10 to the minus 20 seconds.

980
00:45:54,890 --> 00:45:57,110
It could be very fast
depending on how large

981
00:45:57,110 --> 00:45:59,860
the cosmological constant was.

982
00:45:59,860 --> 00:46:00,705
Yes?

983
00:46:00,705 --> 00:46:04,330
AUDIENCE: So I have read
that there's an effect such

984
00:46:04,330 --> 00:46:07,180
that if you're vacuum
can be seen differently

985
00:46:07,180 --> 00:46:08,180
by different observers.

986
00:46:08,180 --> 00:46:10,090
For example, inertial--
there's something

987
00:46:10,090 --> 00:46:13,710
that I read in effect it says
that if one inertial observer

988
00:46:13,710 --> 00:46:15,860
sees vacuum, another
observer that's

989
00:46:15,860 --> 00:46:17,930
accelerating with
respect to that observer

990
00:46:17,930 --> 00:46:21,140
would see like a number
of particles [INAUDIBLE]

991
00:46:21,140 --> 00:46:22,140
a warm gas.

992
00:46:22,140 --> 00:46:25,800
So how much of the effect we
observe are due to the fact

993
00:46:25,800 --> 00:46:28,490
that perhaps we believe the
universe is accelerating,

994
00:46:28,490 --> 00:46:31,580
and we're accelerating perhaps
with respect to some vacuum

995
00:46:31,580 --> 00:46:32,830
and we're just observing that.

996
00:46:32,830 --> 00:46:36,115
That's just a fact of our
motion not necessarily the--

997
00:46:36,115 --> 00:46:37,740
PROFESSOR: You're
touching on something

998
00:46:37,740 --> 00:46:40,920
that is in fact very confusing.

999
00:46:43,640 --> 00:46:44,390
What is your name?

1000
00:46:44,390 --> 00:46:45,110
AUDIENCE: Hani.

1001
00:46:45,110 --> 00:46:46,060
PROFESSOR: Hani?

1002
00:46:46,060 --> 00:46:48,920
What Hani said was
that he had heard--

1003
00:46:48,920 --> 00:46:52,810
and this is correct--
that if one had simply

1004
00:46:52,810 --> 00:46:55,602
a region of ordinary
vacuum-- and I am now

1005
00:46:55,602 --> 00:46:57,560
going to talk about
special [INAUDIBLE] vacuum.

1006
00:46:57,560 --> 00:46:59,650
You don't even need relativity.

1007
00:46:59,650 --> 00:47:01,810
You don't general relativity,
you just need this.

1008
00:47:01,810 --> 00:47:03,870
If you had an accelerating
observer moving

1009
00:47:03,870 --> 00:47:06,250
through that vacuum, the
accelerating observer

1010
00:47:06,250 --> 00:47:08,380
would not see something
that looked like vacuum,

1011
00:47:08,380 --> 00:47:10,630
but rather would see particles
that in fact would look

1012
00:47:10,630 --> 00:47:12,629
like they had a finite
temperature which you can

1013
00:47:12,629 --> 00:47:16,200
calculate, a temperature that's
determined by the acceleration.

1014
00:47:16,200 --> 00:47:17,750
So the question is,
how much of what

1015
00:47:17,750 --> 00:47:20,590
we see should we think
of as really being there

1016
00:47:20,590 --> 00:47:24,780
and how much might be caused
just by our own motion.

1017
00:47:24,780 --> 00:47:29,175
And there's not a terribly
great answer to that question

1018
00:47:29,175 --> 00:47:34,600
that I know of except that we--
when these questions come up,

1019
00:47:34,600 --> 00:47:38,190
we tend to just
adopt the philosophy

1020
00:47:38,190 --> 00:47:41,060
that an observer
who's freely moving,

1021
00:47:41,060 --> 00:47:44,290
which really means moving
with the gravitational field,

1022
00:47:44,290 --> 00:47:49,280
a geodesic observer as the
word phrase is sometimes used,

1023
00:47:49,280 --> 00:47:51,362
essentially defines what
you might call reality

1024
00:47:51,362 --> 00:47:53,820
and then if you calculate what
accelerating observers might

1025
00:47:53,820 --> 00:47:57,160
see in terms of that reality.

1026
00:47:57,160 --> 00:48:00,620
And we are almost
geodesic observers.

1027
00:48:00,620 --> 00:48:02,620
The Earth is exerting a
force on our feet, which

1028
00:48:02,620 --> 00:48:05,115
violates that a little bit,
but by the overall cosmic scale

1029
00:48:05,115 --> 00:48:06,531
of things where
the speed of light

1030
00:48:06,531 --> 00:48:08,960
is what determines
what's significant,

1031
00:48:08,960 --> 00:48:14,590
we are essentially inertial
or geodesic observers.

1032
00:48:14,590 --> 00:48:17,230
Yes, Aviv?

1033
00:48:17,230 --> 00:48:18,900
Aviv first and then
the one in front.

1034
00:48:18,900 --> 00:48:21,358
AUDIENCE: So I'm wondering
about the philosophical approach

1035
00:48:21,358 --> 00:48:26,232
to this discussion and why
the very-- by the definition,

1036
00:48:26,232 --> 00:48:28,420
we can't possibly
observe another universe.

1037
00:48:28,420 --> 00:48:31,125
And so maybe we
have a theory that

1038
00:48:31,125 --> 00:48:33,125
makes a lot of great
predictions like inflation.

1039
00:48:33,125 --> 00:48:36,618
But it may also make
predictions about multiverse.

1040
00:48:36,618 --> 00:48:39,778
We can't possibly empirically
determine whether that's true

1041
00:48:39,778 --> 00:48:41,770
or not, so a
nonfalsifiable question.

1042
00:48:41,770 --> 00:48:46,740
And so I feel like
[INAUDIBLE] who [INAUDIBLE]

1043
00:48:46,740 --> 00:48:49,728
essentially never
going to be answered.

1044
00:48:49,728 --> 00:48:52,192
And if we're going to
be strict empiricists,

1045
00:48:52,192 --> 00:48:55,720
should we not be concerned
with this question?

1046
00:48:55,720 --> 00:48:58,350
PROFESSOR: The question is if we
could never see another pocket

1047
00:48:58,350 --> 00:49:01,210
universe, is it even
a valid question

1048
00:49:01,210 --> 00:49:03,825
to discuss whether
or not they exist,

1049
00:49:03,825 --> 00:49:06,770
a valid scientific question.

1050
00:49:06,770 --> 00:49:09,210
That is also a question
which is generally

1051
00:49:09,210 --> 00:49:13,420
debated in the community, and
people have taken both sides.

1052
00:49:13,420 --> 00:49:14,970
There certainly is
a point of view,

1053
00:49:14,970 --> 00:49:18,110
which I think I
tend to take, which

1054
00:49:18,110 --> 00:49:21,150
is that we never really
insist that every aspect

1055
00:49:21,150 --> 00:49:23,540
of our theories can be tested.

1056
00:49:23,540 --> 00:49:27,560
If you think about any theory,
even Newtonian gravity,

1057
00:49:27,560 --> 00:49:30,105
you can certainly
imagine implications

1058
00:49:30,105 --> 00:49:31,480
of Newtonian
gravity that you can

1059
00:49:31,480 --> 00:49:34,440
calculate that
nobody's ever measured.

1060
00:49:34,440 --> 00:49:38,686
So I think in practice we
tend to accept theories

1061
00:49:38,686 --> 00:49:41,060
when they have made enough
measurements that we've tested

1062
00:49:41,060 --> 00:49:44,910
so that the theory
becomes persuasive.

1063
00:49:44,910 --> 00:49:47,970
And when that happens, I think
we should, at the same time,

1064
00:49:47,970 --> 00:49:50,420
take seriously whatever
those words mean,

1065
00:49:50,420 --> 00:49:54,670
the implications that the theory
has for things that cannot be

1066
00:49:54,670 --> 00:49:56,270
directly tested.

1067
00:49:56,270 --> 00:49:58,010
As far as the other
pocket universes,

1068
00:49:58,010 --> 00:49:59,385
some people think
it's important,

1069
00:49:59,385 --> 00:50:03,280
and maybe I do too,
that even though it's

1070
00:50:03,280 --> 00:50:06,880
highly unlikely, incredibly
unlikely, unbelievably unlikely

1071
00:50:06,880 --> 00:50:09,800
that we'll ever acquire
direct observational evidence

1072
00:50:09,800 --> 00:50:12,830
for another pocket universe,
it's not really in principle

1073
00:50:12,830 --> 00:50:16,180
impossible because of the fact
that pocket universes can,

1074
00:50:16,180 --> 00:50:18,210
in principle, collide.

1075
00:50:18,210 --> 00:50:21,020
So we could, in principle,
describe with evidence

1076
00:50:21,020 --> 00:50:23,865
that our universe has had
contact with another pocket

1077
00:50:23,865 --> 00:50:24,740
universe in the past.

1078
00:50:27,250 --> 00:50:28,170
Yes.

1079
00:50:28,170 --> 00:50:29,840
AUDIENCE: What
determines the stability

1080
00:50:29,840 --> 00:50:33,190
of a particular vacuum state?

1081
00:50:33,190 --> 00:50:36,785
Is it simply things with higher
vacuum energies are less stable

1082
00:50:36,785 --> 00:50:39,035
and things with lower vacuum
energies are more stable?

1083
00:50:42,000 --> 00:50:43,750
PROFESSOR: The question
is what determines

1084
00:50:43,750 --> 00:50:47,260
the stability of
the different vacua.

1085
00:50:47,260 --> 00:50:48,940
Is it simply that
higher energy ones

1086
00:50:48,940 --> 00:50:51,990
are more unstable and lower
energy ones are more stable

1087
00:50:51,990 --> 00:50:54,630
or is it more
complicated than that?

1088
00:50:54,630 --> 00:50:56,140
And the answer,
as far as I know,

1089
00:50:56,140 --> 00:50:59,450
is that there is a trend
for higher energy ones

1090
00:50:59,450 --> 00:51:02,639
to be more unstable and lower
energy ones to be more stable.

1091
00:51:02,639 --> 00:51:03,930
But it's not as simple as that.

1092
00:51:03,930 --> 00:51:06,450
There are also wide
variations that

1093
00:51:06,450 --> 00:51:08,390
are independent of
the energy density.

1094
00:51:08,390 --> 00:51:10,410
AUDIENCE: If the one
that we're living in

1095
00:51:10,410 --> 00:51:14,800
is incredibly is really
ridiculously close to zero

1096
00:51:14,800 --> 00:51:19,400
in a city that seems to make
it incredibly unlikely that we

1097
00:51:19,400 --> 00:51:25,910
would pay anything else I soon

1098
00:51:25,910 --> 00:51:27,570
PROFESSOR: Right.

1099
00:51:27,570 --> 00:51:29,330
The question is if
our universe has

1100
00:51:29,330 --> 00:51:31,310
such has such a
small energy density

1101
00:51:31,310 --> 00:51:33,460
relative to the average.

1102
00:51:33,460 --> 00:51:35,980
Wouldn't that mean
that we should also

1103
00:51:35,980 --> 00:51:39,040
expect to be much more
long-lived than average?

1104
00:51:39,040 --> 00:51:41,140
And the answer is I guess so.

1105
00:51:41,140 --> 00:51:43,920
But as far as the effect
on the Swiss cheese

1106
00:51:43,920 --> 00:51:46,579
picture that I described
for the ultimate future,

1107
00:51:46,579 --> 00:51:48,245
it doesn't change the
words that I used.

1108
00:51:48,245 --> 00:51:52,320
It just changes how frequent
those decays would be.

1109
00:51:52,320 --> 00:51:55,470
But since the future of
this pocket universe,

1110
00:51:55,470 --> 00:51:58,280
if this picture is
right, will be infinite,

1111
00:51:58,280 --> 00:52:02,280
decays will happen no matter
how small the probability is.

1112
00:52:02,280 --> 00:52:05,550
An infinite number of
decays will happen in fact.

1113
00:52:05,550 --> 00:52:07,050
OK we should probably
go on now even

1114
00:52:07,050 --> 00:52:08,380
if there are more questions.

1115
00:52:08,380 --> 00:52:12,960
We have a whole term to
discuss things like this.

1116
00:52:12,960 --> 00:52:15,120
The next thing I
want to do is handle

1117
00:52:15,120 --> 00:52:17,170
some housekeeping details.

1118
00:52:17,170 --> 00:52:19,700
I'd like to arrange
office hours.

1119
00:52:19,700 --> 00:52:23,220
And the problem sets
are due on Friday,

1120
00:52:23,220 --> 00:52:25,730
so what [? Tsingtao ?]
and I thought

1121
00:52:25,730 --> 00:52:28,220
was that a good time
for office hours

1122
00:52:28,220 --> 00:52:30,740
would be on Wednesdays
and Thursdays.

1123
00:52:30,740 --> 00:52:33,480
One of us on each of those days.

1124
00:52:33,480 --> 00:52:35,470
It turns out that I can't
really do Thursdays,

1125
00:52:35,470 --> 00:52:37,870
so one of us on
each of those days

1126
00:52:37,870 --> 00:52:39,430
ends up meaning
that I'll probably

1127
00:52:39,430 --> 00:52:41,290
be having office
hours on Wednesdays.

1128
00:52:41,290 --> 00:52:42,790
This is all
provisional depending

1129
00:52:42,790 --> 00:52:45,202
on how it works with you folks.

1130
00:52:45,202 --> 00:52:46,660
And [? Tsingtao ?]
will probably be

1131
00:52:46,660 --> 00:52:49,660
having office
hours on Thursdays.

1132
00:52:49,660 --> 00:52:51,784
Generally speaking, if one
wants to have an office

1133
00:52:51,784 --> 00:52:53,200
hour that most
people can come to,

1134
00:52:53,200 --> 00:52:56,520
I think it should be
in the late afternoon.

1135
00:52:56,520 --> 00:52:59,350
So maybe we'll start by
discussing my office hours

1136
00:52:59,350 --> 00:53:01,500
since it comes before
[? Tsingtao's, ?] Wednesday

1137
00:53:01,500 --> 00:53:03,300
versus Thursday.

1138
00:53:03,300 --> 00:53:07,450
So on Wednesday, I
can do an office hour

1139
00:53:07,450 --> 00:53:13,220
in the late, normal afternoon,
which might mean 4:00 to 5:00

1140
00:53:13,220 --> 00:53:16,590
I think after five some
people have sports activities

1141
00:53:16,590 --> 00:53:17,090
and things.

1142
00:53:17,090 --> 00:53:20,224
We're told to try to
avoid those hours.

1143
00:53:20,224 --> 00:53:22,265
So 4:00 to 5:00 would be
a reasonable possibility

1144
00:53:22,265 --> 00:53:24,610
for my office hour on Wednesday.

1145
00:53:24,610 --> 00:53:26,464
If that doesn't
work, I could stay

1146
00:53:26,464 --> 00:53:28,130
and have the office
hour in the evening.

1147
00:53:28,130 --> 00:53:30,290
That's actually what
I did two years ago.

1148
00:53:30,290 --> 00:53:33,140
I had an office hour
from 7:30 to 8:30.

1149
00:53:33,140 --> 00:53:35,030
It was also
Wednesdays-- I forget.

1150
00:53:35,030 --> 00:53:38,130
But it was in the evening,
and that's a possibility.

1151
00:53:38,130 --> 00:53:44,200
So let me ask if I have my
office hour from 4:00 to 5:00

1152
00:53:44,200 --> 00:53:47,490
on Wednesdays, how
many of you who

1153
00:53:47,490 --> 00:53:50,090
might be interested in coming
would not be able to come?

1154
00:53:53,150 --> 00:53:57,500
1, 2, 3, 4, 5, 6,
7, 8, 9, 10, 11, 12.

1155
00:53:57,500 --> 00:54:02,970
A significant number, but
most of you can come at least.

1156
00:54:02,970 --> 00:54:06,760
Let me ask the corresponding
question for the evening.

1157
00:54:06,760 --> 00:54:10,490
Suppose I made the office
hour from 7:30 to 8:30

1158
00:54:10,490 --> 00:54:12,690
in the evening on Wednesdays.

1159
00:54:12,690 --> 00:54:17,120
In that case, how many of
you who might want to come

1160
00:54:17,120 --> 00:54:18,230
would not be able to come?

1161
00:54:21,158 --> 00:54:28,170
1, 2, 3 5, 6, so it's a smaller
number, but not vastly smaller.

1162
00:54:33,100 --> 00:54:35,760
OK I think I'll do it in
the evening for the benefit

1163
00:54:35,760 --> 00:54:37,780
of the difference
between those two groups.

1164
00:54:37,780 --> 00:54:40,480
And the evening also has
the little slight advantage

1165
00:54:40,480 --> 00:54:43,220
that it can be a little more
open-ended if people still

1166
00:54:43,220 --> 00:54:46,270
have questions after
the normal time is over.

1167
00:54:46,270 --> 00:54:56,270
So I will make my office hour on
Wednesday's from 7:30 to 8:30.

1168
00:54:56,270 --> 00:54:58,990
Is that particular hour as good
an hour as any on Wednesday

1169
00:54:58,990 --> 00:54:59,554
evening.

1170
00:54:59,554 --> 00:55:01,470
Would people want to
move it earlier or later?

1171
00:55:05,410 --> 00:55:08,651
Any suggestions for moving
it earlier or later?

1172
00:55:08,651 --> 00:55:11,151
AUDIENCE: I know people have
sports til technically at least

1173
00:55:11,151 --> 00:55:17,354
7:00, but if it's 6:30 to
7:30 might be a little--

1174
00:55:17,354 --> 00:55:18,020
PROFESSOR: 6:30?

1175
00:55:18,020 --> 00:55:21,400
You'll be starting at 6--
starting at 6:30 versus-- 6:30

1176
00:55:21,400 --> 00:55:24,480
to 7:30, starting at 6:30.

1177
00:55:24,480 --> 00:55:26,884
Well, I'd be happy
to do that, but I

1178
00:55:26,884 --> 00:55:28,800
suspect we might run
into problems with people

1179
00:55:28,800 --> 00:55:30,940
who have sport
activities, but let's see.

1180
00:55:30,940 --> 00:55:34,290
How many of you would
be inconvenienced

1181
00:55:34,290 --> 00:55:36,277
if I started at 6:30
instead of 7:30?

1182
00:55:39,079 --> 00:55:42,780
3, 4, 5, 6, a number.

1183
00:55:42,780 --> 00:55:47,440
So I think we'll honor
that and start at 7:30.

1184
00:55:47,440 --> 00:55:49,910
I assume 7 is also a bit of
a problem for those people.

1185
00:55:55,225 --> 00:55:55,850
We'll say 7:30.

1186
00:56:22,287 --> 00:56:24,245
Now, I have to announce
that this week is going

1187
00:56:24,245 --> 00:56:26,661
to unfortunately have to be
an exception because I already

1188
00:56:26,661 --> 00:56:28,670
have plans for Wednesday night.

1189
00:56:28,670 --> 00:56:32,030
So for this week, I
think the best thing--

1190
00:56:32,030 --> 00:56:33,730
the only possible
thing, probably

1191
00:56:33,730 --> 00:56:35,810
the best-- it's almost
the only possible thing

1192
00:56:35,810 --> 00:56:38,700
would be 4:00 to
5:00 on Wednesday.

1193
00:56:38,700 --> 00:56:40,040
Wednesday's bit tomorrow.

1194
00:56:40,040 --> 00:56:42,412
I'll send you all an email
when I find a room for that.

1195
00:56:42,412 --> 00:56:44,370
I think I'll probably
not have it in my office,

1196
00:56:44,370 --> 00:56:46,850
but maybe it will
be in my office.

1197
00:56:46,850 --> 00:56:47,680
Comment up there?

1198
00:56:47,680 --> 00:56:49,930
AUDIENCE: Oh, I was just
going to ask where, but you--

1199
00:56:49,930 --> 00:56:51,132
PROFESSOR: Where?

1200
00:56:51,132 --> 00:56:53,090
OK, I guess then the
fourth option's my office.

1201
00:56:53,090 --> 00:56:54,714
I was hoping to put
a sign of my office

1202
00:56:54,714 --> 00:56:56,950
if we're someplace
other than my office.

1203
00:56:56,950 --> 00:57:00,260
So should put this
on the board too.

1204
00:57:00,260 --> 00:57:07,110
Tomorrow 4:00 to 5:00 PM.

1205
00:57:15,790 --> 00:57:17,770
So be at my office
or I'll send email.

1206
00:57:27,470 --> 00:57:28,416
Yes?

1207
00:57:28,416 --> 00:57:32,280
AUDIENCE: How will we be turning
in the Thursday problem set?

1208
00:57:32,280 --> 00:57:35,910
PROFESSOR: We're going
to talk about that now.

1209
00:57:35,910 --> 00:57:38,819
For Thursday, for
[? Tsingtao, ?] I remember

1210
00:57:38,819 --> 00:57:39,860
you had some constraints.

1211
00:57:39,860 --> 00:57:41,026
So what was possible?

1212
00:57:41,026 --> 00:57:41,760
TSINGTAO: Yeah.

1213
00:57:41,760 --> 00:57:44,215
So I usually leave
around 7:00 PM,

1214
00:57:44,215 --> 00:57:47,652
so I have appointment
[? meeting ?],

1215
00:57:47,652 --> 00:57:52,562
and today is probably
not very good at 4:00 PM.

1216
00:57:56,090 --> 00:57:58,800
PROFESSOR: So 4:00 to 5:00 is a
possibility for [? Tsingtao ?]

1217
00:57:58,800 --> 00:58:02,290
on Thursday, and I
guess later than that.

1218
00:58:02,290 --> 00:58:04,422
But should be over by
5:00-- by 7:00 either

1219
00:58:04,422 --> 00:58:06,130
or do you want it to
be over before then.

1220
00:58:06,130 --> 00:58:08,380
TSINGTAO: Oh, 6:00 to 7:002
Oh, 6:00 to 7:002 I guess.

1221
00:58:08,380 --> 00:58:10,412
PROFESSOR: 6:00 to
7:00 would be OK?

1222
00:58:10,412 --> 00:58:11,495
TSINGTAO: Yeah, that's OK.

1223
00:58:17,540 --> 00:58:20,587
PROFESSOR: OK, so let's
start with 4:00 to 5:00.

1224
00:58:20,587 --> 00:58:23,170
If [? Tsingtao ?] was to have
an office hour from 4:00 to 5:00

1225
00:58:23,170 --> 00:58:26,990
on Thursdays, how many of you
think you might want to go

1226
00:58:26,990 --> 00:58:29,640
would be unable to?

1227
00:58:29,640 --> 00:58:31,100
Wow, tons!

1228
00:58:31,100 --> 00:58:34,750
OK, that seems more than
half of you I think.

1229
00:58:34,750 --> 00:58:36,400
So I guess we try to avoid that.

1230
00:58:36,400 --> 00:58:40,235
This impact's probably
an athletic region,

1231
00:58:40,235 --> 00:58:44,250
but maybe we'll have to do that
for lack of an alternative.

1232
00:58:44,250 --> 00:58:46,240
Suppose it were 5:00 to 6:00.

1233
00:58:46,240 --> 00:58:48,907
How many of you who would be
interesting in coming-- who

1234
00:58:48,907 --> 00:58:50,740
might be interesting
in coming, I should say

1235
00:58:50,740 --> 00:58:52,180
I guess because it'll
vary from week to week--

1236
00:58:52,180 --> 00:58:53,190
but how many of you
who think you might

1237
00:58:53,190 --> 00:58:54,780
be interested in
coming would not

1238
00:58:54,780 --> 00:58:58,620
be able to come from 5:00
to 6:00 on Thursdays.

1239
00:58:58,620 --> 00:58:59,660
OK, a small group.

1240
00:58:59,660 --> 00:59:03,872
1, 2, 3, 4, 5, 6, 7.

1241
00:59:03,872 --> 00:59:06,810
Looks to me like 7.

1242
00:59:06,810 --> 00:59:10,090
And let's say, I
said 4:00-- That

1243
00:59:10,090 --> 00:59:13,850
was at 6:00-- that
was 5:00 to 6:00.

1244
00:59:13,850 --> 00:59:17,170
So maybe we should next try 5:30
to 6:30 in smaller increments

1245
00:59:17,170 --> 00:59:17,850
here.

1246
00:59:17,850 --> 00:59:20,100
If we're 5:30 to
6:30, how many of you

1247
00:59:20,100 --> 00:59:21,210
would not be able to come?

1248
00:59:24,745 --> 00:59:27,800
Looks like pretty
much the same people.

1249
00:59:27,800 --> 00:59:30,330
And if it were 6:00 to
7:00, how many of you

1250
00:59:30,330 --> 00:59:33,050
would not be able to come?

1251
00:59:33,050 --> 00:59:36,810
Same people, I think it is
literally the same people.

1252
00:59:36,810 --> 00:59:37,680
OK.

1253
00:59:37,680 --> 00:59:40,450
So it looks like 4:00
to 5:00 is very bad.

1254
00:59:40,450 --> 00:59:44,200
And all other times
are about equivalent.

1255
00:59:44,200 --> 00:59:46,695
So I think if all other
times are bad equivalently,

1256
00:59:46,695 --> 00:59:50,930
we probably might as well
make it 5:00 to 6:00.

1257
00:59:50,930 --> 00:59:53,313
And that way
[? Tsingtao ?] can get off

1258
00:59:53,313 --> 00:59:57,670
to an earliest possible start
to wherever he's going at 7:00,

1259
00:59:57,670 --> 01:00:00,620
and it also means a little
more flexibility in the end

1260
01:00:00,620 --> 01:00:02,005
if there are more questions.

1261
01:00:34,969 --> 01:00:36,752
AUDIENCE: Where is that located?

1262
01:00:36,752 --> 01:00:38,210
PROFESSOR: That
also, I think, will

1263
01:00:38,210 --> 01:00:40,685
require us to get a room
which will be announced.

1264
01:00:54,860 --> 01:00:56,980
So I will try to
arrange rooms tomorrow

1265
01:00:56,980 --> 01:00:59,390
morning and send
it by email, and I

1266
01:00:59,390 --> 01:01:01,140
guess I'll post it on
the website as well.

1267
01:01:04,360 --> 01:01:07,900
Any other organizational--
and questions

1268
01:01:07,900 --> 01:01:10,080
limited to organizational
questions now?

1269
01:01:10,080 --> 01:01:12,130
Get back to physics later.

1270
01:01:12,130 --> 01:01:13,880
Any organizational
questions before we

1271
01:01:13,880 --> 01:01:16,340
start on Doppler shifts?

1272
01:01:16,340 --> 01:01:17,612
Yes?

1273
01:01:17,612 --> 01:01:20,067
AUDIENCE: If I can't make
a single office hour,

1274
01:01:20,067 --> 01:01:23,020
how should I field questions
when I have questions?

1275
01:01:23,020 --> 01:01:25,409
PROFESSOR: A good question.

1276
01:01:25,409 --> 01:01:27,700
Yeah, there may be some
people, and apparently there is

1277
01:01:27,700 --> 01:01:29,825
at least one who cannot
make either of these times,

1278
01:01:29,825 --> 01:01:31,700
even though we tried
to optimize things.

1279
01:01:31,700 --> 01:01:34,582
So by all means,
don't feel like you

1280
01:01:34,582 --> 01:01:36,040
don't have a channel
for questions.

1281
01:01:36,040 --> 01:01:37,715
If you have a question,
send an email to

1282
01:01:37,715 --> 01:01:39,250
either me, or
[? Tsingtao, ?] or both.

1283
01:01:39,250 --> 01:01:40,930
And we'll either answer
it together with you

1284
01:01:40,930 --> 01:01:43,360
or answer you by email depending
on what the question is

1285
01:01:43,360 --> 01:01:46,484
and what seems useful.

1286
01:01:46,484 --> 01:01:47,650
And that goes for everybody.

1287
01:01:54,720 --> 01:01:57,600
In that case, if
everybody's on board,

1288
01:01:57,600 --> 01:02:02,060
we will now start the actual
material for the term.

1289
01:02:02,060 --> 01:02:03,599
Well, the overview
is an overview

1290
01:02:03,599 --> 01:02:06,015
of the material for the term,
but not at the standard pace

1291
01:02:06,015 --> 01:02:08,880
and the standard
level of detail.

1292
01:02:08,880 --> 01:02:11,200
So what I want to
talk about this week--

1293
01:02:11,200 --> 01:02:13,430
and I guess I'll only get
to start today and finish

1294
01:02:13,430 --> 01:02:15,106
on Thursday-- I had
planned to tell you

1295
01:02:15,106 --> 01:02:17,480
everything you need to know
for the problem set by today,

1296
01:02:17,480 --> 01:02:20,160
but that's not going to happen.

1297
01:02:20,160 --> 01:02:24,240
So I don't-- if people complain,
we could consider postponing

1298
01:02:24,240 --> 01:02:28,321
the due date of the problem
set, so consider that an option.

1299
01:02:28,321 --> 01:02:30,320
But probably you could
do the problem set anyway

1300
01:02:30,320 --> 01:02:32,970
because it is all
described in lecture notes.

1301
01:02:32,970 --> 01:02:35,670
But if any of you have
difficulties meeting

1302
01:02:35,670 --> 01:02:38,360
that deadline, it will be a
somewhat flexible deadline

1303
01:02:38,360 --> 01:02:40,110
this week because of
the fact that I'm not

1304
01:02:40,110 --> 01:02:43,270
covering the material
today as I had planned.

1305
01:02:43,270 --> 01:02:47,080
And I'll admit that's not
necessarily a good thing

1306
01:02:47,080 --> 01:02:50,680
to do in terms of problem set.

1307
01:02:50,680 --> 01:02:54,420
So we're going to begin
the course, in principle,

1308
01:02:54,420 --> 01:02:58,490
by talking about Hubble's law,
although Hubble's law will

1309
01:02:58,490 --> 01:03:01,050
rapidly lead us to the
question of the Doppler shift,

1310
01:03:01,050 --> 01:03:03,150
which is what I'll
mainly be talking

1311
01:03:03,150 --> 01:03:07,650
about for the rest of today
and for most of Thursday.

1312
01:03:07,650 --> 01:03:17,920
Hubble's law itself
is a simple equation

1313
01:03:17,920 --> 01:03:24,810
that v is equal to h r,
where v is the recession

1314
01:03:24,810 --> 01:03:33,365
velocity of any typical galaxy.

1315
01:03:38,126 --> 01:03:39,500
Hubble's law is
not an exact law,

1316
01:03:39,500 --> 01:03:42,754
so individual galaxies will
deviate from Hubble's law.

1317
01:03:42,754 --> 01:03:44,420
But in principle,
Hubble's law tells you

1318
01:03:44,420 --> 01:03:48,000
what the recession velocity
is of a galaxy, at least

1319
01:03:48,000 --> 01:03:50,160
to reasonable accuracy.

1320
01:03:50,160 --> 01:04:00,880
Where h is what is often
called Hubble's constant.

1321
01:04:00,880 --> 01:04:04,290
Sometimes, it is called
the Hubble parameter.

1322
01:04:04,290 --> 01:04:08,410
I like actually-- it's called
the Hubble expansion rate.

1323
01:04:08,410 --> 01:04:10,720
The problem with calling
Hubble's constant

1324
01:04:10,720 --> 01:04:12,310
is that it's not
really a constant

1325
01:04:12,310 --> 01:04:13,970
over the lifetime
of the universe.

1326
01:04:13,970 --> 01:04:17,202
It's a constant over the
lifetime of an astronomer,

1327
01:04:17,202 --> 01:04:19,410
but not a constant over the
lifetime of the universe?

1328
01:04:19,410 --> 01:04:21,720
And we'll be talking about
universes, not astronomers,

1329
01:04:21,720 --> 01:04:23,550
at least for the most part.

1330
01:04:23,550 --> 01:04:26,302
And even over history,
it's not a constant

1331
01:04:26,302 --> 01:04:28,010
because the estimate
of Hubble's constant

1332
01:04:28,010 --> 01:04:31,160
has actually changed by a
factor of about 10 or so

1333
01:04:31,160 --> 01:04:33,730
since Hubble's
original estimate.

1334
01:04:33,730 --> 01:04:37,500
And the r that appears here
is the distance to the galaxy.

1335
01:04:48,360 --> 01:04:51,250
And if you look at the lecture
notes from two years ago,

1336
01:04:51,250 --> 01:04:53,230
they start out by saying
that Hubble's law was

1337
01:04:53,230 --> 01:04:56,150
discovered by Hubble in 1929.

1338
01:04:56,150 --> 01:04:59,430
When I looked at that
first sentence in my notes,

1339
01:04:59,430 --> 01:05:01,430
and when I started to
revise them for this year,

1340
01:05:01,430 --> 01:05:03,980
I realized that I heard
that that statement has

1341
01:05:03,980 --> 01:05:05,525
become controversial.

1342
01:05:05,525 --> 01:05:07,525
Almost everything in
cosmology is controversial,

1343
01:05:07,525 --> 01:05:09,720
so even that statement
is controversial.

1344
01:05:09,720 --> 01:05:12,650
There are claims
that Lemaitre really

1345
01:05:12,650 --> 01:05:15,280
deserves credit for Hubble's
law rather than Hubble.

1346
01:05:15,280 --> 01:05:17,412
And there's some
validity to that claim.

1347
01:05:17,412 --> 01:05:19,370
There's also some
[? intrigued ?] that happens,

1348
01:05:19,370 --> 01:05:21,320
if you want to read about this.

1349
01:05:21,320 --> 01:05:23,794
It was discovered by
several of-- I think

1350
01:05:23,794 --> 01:05:26,210
amateur historians I think is
what they are often referred

1351
01:05:26,210 --> 01:05:32,277
to in the press-- that we know
mainly of Lemaitre's work-- we

1352
01:05:32,277 --> 01:05:34,610
being the Western speaking,
the Western English speaking

1353
01:05:34,610 --> 01:05:37,490
world-- know mainly
of a Lemaitre's work

1354
01:05:37,490 --> 01:05:41,830
through a 1931 translation
in a 1927 paper

1355
01:05:41,830 --> 01:05:45,270
he wrote about the
foundations of cosmology.

1356
01:05:45,270 --> 01:05:50,140
And it turned out that several
significant seeming paragraphs

1357
01:05:50,140 --> 01:05:52,950
in the 1927 French
article somehow

1358
01:05:52,950 --> 01:05:57,090
didn't make it to the 1931
English translation, paragraphs

1359
01:05:57,090 --> 01:06:00,970
about the Hubble constant.

1360
01:06:00,970 --> 01:06:03,739
And for a while, that
seemed like dirty play

1361
01:06:03,739 --> 01:06:06,280
and there were accusations that
Hubble, or friends of Hubble,

1362
01:06:06,280 --> 01:06:09,260
had suppressed those
paragraphs when

1363
01:06:09,260 --> 01:06:11,700
the article was translated.

1364
01:06:11,700 --> 01:06:15,330
The truth finally was
discovered a couple years ago

1365
01:06:15,330 --> 01:06:19,710
by a physicist named Mario Livio
who actually was on the Daily

1366
01:06:19,710 --> 01:06:21,210
Show a couple nights
ago by the way.

1367
01:06:21,210 --> 01:06:25,540
He has a book out now, not about
this, but about other things.

1368
01:06:25,540 --> 01:06:26,990
But anyway, he
discovered by going

1369
01:06:26,990 --> 01:06:34,470
through the archives of the
monthly notices of astronomy,

1370
01:06:34,470 --> 01:06:36,660
which is where the article
was published in English.

1371
01:06:36,660 --> 01:06:38,243
And turned out it
was Lemaitre himself

1372
01:06:38,243 --> 01:06:40,590
he removed those paragraphs.

1373
01:06:40,590 --> 01:06:42,880
The paragraphs basically
gave a numerical estimate

1374
01:06:42,880 --> 01:06:47,250
of the Hubble constant, but by
1931 Hubble's papered already

1375
01:06:47,250 --> 01:06:50,110
been published, so Lemaitre
felt that it was only

1376
01:06:50,110 --> 01:06:52,900
a less accurate estimate
of the same quantity

1377
01:06:52,900 --> 01:06:58,240
that Hubble had estimated, so he
cut it out of his translation.

1378
01:06:58,240 --> 01:07:00,500
What certainly is
true is that Lemaitre

1379
01:07:00,500 --> 01:07:02,880
knew about Hubble's law
on theoretical grounds.

1380
01:07:02,880 --> 01:07:06,465
Lemaitre was building a model
of an expanding universe.

1381
01:07:06,465 --> 01:07:08,340
I don't know if he is
really the first person

1382
01:07:08,340 --> 01:07:11,330
to know that an expanding
universe model gave rise

1383
01:07:11,330 --> 01:07:15,870
to a linear relationship
between velocity and distance,

1384
01:07:15,870 --> 01:07:20,930
but he certainly did know about
it and understood Hubble's law

1385
01:07:20,930 --> 01:07:24,350
and give an estimate
of it based on data.

1386
01:07:24,350 --> 01:07:26,440
What he did not
do, however, is try

1387
01:07:26,440 --> 01:07:28,150
to use data to actually
show that there

1388
01:07:28,150 --> 01:07:29,850
was a linear relationship.

1389
01:07:29,850 --> 01:07:32,670
What Lemaitre did,
in those paragraphs

1390
01:07:32,670 --> 01:07:35,010
that were not
translated, was simply

1391
01:07:35,010 --> 01:07:38,690
to look at a large
group of galaxies,

1392
01:07:38,690 --> 01:07:41,840
figure an average value for
v and an average value of r

1393
01:07:41,840 --> 01:07:44,704
and determine h from
dividing those two averages.

1394
01:07:44,704 --> 01:07:46,120
And he admitted
that there was not

1395
01:07:46,120 --> 01:07:48,450
really good enough data to tell
if the relationship is linear

1396
01:07:48,450 --> 01:07:49,050
or not.

1397
01:07:52,430 --> 01:07:54,880
I think it is definitely
fair to say that Hubble

1398
01:07:54,880 --> 01:08:00,380
is the person who deserves
credit for arguing first really

1399
01:08:00,380 --> 01:08:03,620
with a fairly weak argument,
but then got stronger over time,

1400
01:08:03,620 --> 01:08:05,510
that there really is
astronomical evidence

1401
01:08:05,510 --> 01:08:12,689
for this linear relationship
between velocity and distance.

1402
01:08:12,689 --> 01:08:14,980
So probably it will continue
to be called Hubble's law.

1403
01:08:14,980 --> 01:08:16,563
If you look in
Wikipedia, it tells you

1404
01:08:16,563 --> 01:08:19,170
either one is acceptable
at the moment,

1405
01:08:19,170 --> 01:08:20,810
but Wikipedia articles
change rapidly,

1406
01:08:20,810 --> 01:08:25,170
so we'll see what
it says next year.

1407
01:08:25,170 --> 01:08:27,140
It's also mentioned
that we should probably

1408
01:08:27,140 --> 01:08:30,170
root for Lemaitre since
Lemaitre, it turns out-- well,

1409
01:08:30,170 --> 01:08:32,210
he was a Belgian priest,
it was often described,

1410
01:08:32,210 --> 01:08:35,569
but he was also an MIT
student, had a Ph.D. for MIT,

1411
01:08:35,569 --> 01:08:37,060
which he received in 1927.

1412
01:08:37,060 --> 01:08:38,765
You can actually
read his thesis.

1413
01:08:38,765 --> 01:08:40,389
When I was writing
my [INAUDIBLE] book,

1414
01:08:40,389 --> 01:08:42,189
I remember going
to the MIT archives

1415
01:08:42,189 --> 01:08:44,939
and actually picking up
his thesis and reading it.

1416
01:08:44,939 --> 01:08:47,355
It's not that well-written
actually, but it's interesting.

1417
01:08:53,540 --> 01:08:55,559
Although he got
his Ph.D. from MIT,

1418
01:08:55,559 --> 01:08:57,979
it also turns out that
he did most of his work

1419
01:08:57,979 --> 01:09:02,859
down Mass Ave at the
Harvard College Observatory,

1420
01:09:02,859 --> 01:09:04,340
but the Harvard
College Observatory

1421
01:09:04,340 --> 01:09:06,130
did not give degrees
in those days.

1422
01:09:06,130 --> 01:09:07,550
It was just an observatory.

1423
01:09:07,550 --> 01:09:09,540
So he wanted to get a
degree, so he signed up

1424
01:09:09,540 --> 01:09:13,670
at MIT for the Ph.D.
Program and wrote a thesis,

1425
01:09:13,670 --> 01:09:17,060
received a Ph.D.

1426
01:09:17,060 --> 01:09:20,029
Onward, what I really
want to talk about

1427
01:09:20,029 --> 01:09:23,140
is, after mentioning
Hubble's law--

1428
01:09:23,140 --> 01:09:26,220
so Hubble's law as an indication
that the universe is expanding.

1429
01:09:26,220 --> 01:09:29,312
And we'll talk more about the
history of all this later,

1430
01:09:29,312 --> 01:09:31,270
and it actually is very
well-described in Steve

1431
01:09:31,270 --> 01:09:33,729
Weinberg's book.

1432
01:09:33,729 --> 01:09:36,597
But initially, Einstein
proposed a model

1433
01:09:36,597 --> 01:09:38,680
of the universe that was
static, and it was really

1434
01:09:38,680 --> 01:09:42,210
Hubble who convinced
Einstein that observationally

1435
01:09:42,210 --> 01:09:46,350
the universe does not appear
to be static, but does

1436
01:09:46,350 --> 01:09:50,399
appear instead to obey
this expansion law.

1437
01:09:50,399 --> 01:09:54,060
So that gave rise to the theory
of the expanding universe.

1438
01:09:54,060 --> 01:09:57,040
But what I want to
talk about today

1439
01:09:57,040 --> 01:10:03,666
is how one measures the
v that appears here.

1440
01:10:03,666 --> 01:10:05,290
There's also a big
discussion about how

1441
01:10:05,290 --> 01:10:07,890
one measures r, the distance.

1442
01:10:07,890 --> 01:10:11,460
And that is, I think, rather
well-done in Steve Weinberg's

1443
01:10:11,460 --> 01:10:15,070
book, and I'm going to
pretty much leave it

1444
01:10:15,070 --> 01:10:17,350
to your reading of
Steve Weinberg's book

1445
01:10:17,350 --> 01:10:22,340
to learn about how distances to
distant galaxies are estimated.

1446
01:10:22,340 --> 01:10:24,610
Roughly-speaking,
I might just say

1447
01:10:24,610 --> 01:10:27,600
that they are estimated
by finding objects

1448
01:10:27,600 --> 01:10:30,580
in those distant galaxies
whose brightnesses you think

1449
01:10:30,580 --> 01:10:32,520
you know, by one
means or another.

1450
01:10:32,520 --> 01:10:35,110
And a complicated story
is what objects are there

1451
01:10:35,110 --> 01:10:37,320
in brightnesses
we think we know.

1452
01:10:37,320 --> 01:10:39,230
But once you find an
object whose brightness

1453
01:10:39,230 --> 01:10:42,130
you think you know, those
go by the general name

1454
01:10:42,130 --> 01:10:45,490
of standard candles,
a standard candle

1455
01:10:45,490 --> 01:10:48,450
being an object whose
brightness you know,

1456
01:10:48,450 --> 01:10:50,460
then you can tell
how far the object is

1457
01:10:50,460 --> 01:10:51,720
by how bright it appears.

1458
01:10:51,720 --> 01:10:54,120
And that becomes a very
straightforward way

1459
01:10:54,120 --> 01:10:57,370
of estimating distances,
and that is the only way

1460
01:10:57,370 --> 01:11:01,569
we really have of estimating
distances of distant galaxies.

1461
01:11:01,569 --> 01:11:04,110
So it's a much longer story than
what I just said, and you'll

1462
01:11:04,110 --> 01:11:07,000
read about it in
Weinberg's book.

1463
01:11:07,000 --> 01:11:09,540
The velocity is measured
by the Doppler shift,

1464
01:11:09,540 --> 01:11:11,950
and that's what lecture
notes one are mainly about,

1465
01:11:11,950 --> 01:11:14,910
and that's what I'll be
talking about for the remaining

1466
01:11:14,910 --> 01:11:19,120
few minutes of today's class.

1467
01:11:19,120 --> 01:11:24,560
And what we want to do in the
course of this set of lecture

1468
01:11:24,560 --> 01:11:27,560
notes, this week of
class I guess it will be,

1469
01:11:27,560 --> 01:11:30,980
is understand how to
calculate the Doppler

1470
01:11:30,980 --> 01:11:35,690
shift both non-relativistically
and relativistically,

1471
01:11:35,690 --> 01:11:40,440
and we'll just work out the
primary cases of observer

1472
01:11:40,440 --> 01:11:44,690
stationary source moving, source
stationary observer moving,

1473
01:11:44,690 --> 01:11:48,380
and all in a line, for
both the relativistic and

1474
01:11:48,380 --> 01:11:51,590
non-relativistic cases.

1475
01:11:51,590 --> 01:11:54,960
So I think I'll launch into
the first calculation, which

1476
01:11:54,960 --> 01:11:57,790
you might even have
time to finish.

1477
01:11:57,790 --> 01:11:59,600
I'd like to consider
a case where

1478
01:11:59,600 --> 01:12:02,610
the observer is stationary
and the source is moving,

1479
01:12:02,610 --> 01:12:05,357
which is normally how we
think of the distant galaxies.

1480
01:12:05,357 --> 01:12:07,690
We work in our own reference
frame, so we're stationary,

1481
01:12:07,690 --> 01:12:09,180
the galaxy is moving.

1482
01:12:09,180 --> 01:12:11,280
How do we calculate
this redshift

1483
01:12:11,280 --> 01:12:13,420
I should say at the
asset here, however--

1484
01:12:13,420 --> 01:12:15,420
I don't know if I said
it in the lecture notes--

1485
01:12:15,420 --> 01:12:18,090
that the cosmological
redshift is actually

1486
01:12:18,090 --> 01:12:21,170
a little bit different from what
we're calculating this week.

1487
01:12:21,170 --> 01:12:26,830
This week, we're calculating
the special relativity redshift.

1488
01:12:26,830 --> 01:12:31,020
But cosmology is not controlled
by special relativity

1489
01:12:31,020 --> 01:12:34,040
because special relativity
does not describe gravity,

1490
01:12:34,040 --> 01:12:38,910
and gravity plays a
major role in cosmology.

1491
01:12:38,910 --> 01:12:41,990
So the cosmological redshift,
we will talk about a little

1492
01:12:41,990 --> 01:12:44,730
later in the course,
in a more precise way.

1493
01:12:44,730 --> 01:12:48,700
But for now, we, like Hubble--
Hubble didn't know any better--

1494
01:12:48,700 --> 01:12:52,779
are ignoring gravity, which
is OK for the nearby stars,

1495
01:12:52,779 --> 01:12:54,820
and the further away they
are, the more important

1496
01:12:54,820 --> 01:12:58,810
these gravitational influences
are, and ignoring gravity one

1497
01:12:58,810 --> 01:13:03,400
could just use special
relativity or even

1498
01:13:03,400 --> 01:13:07,750
Newtonian kinematics
to calculate

1499
01:13:07,750 --> 01:13:12,020
the relationship between
v and the redshift.

1500
01:13:12,020 --> 01:13:15,309
And that's what we'll
be talking about.

1501
01:13:15,309 --> 01:13:17,350
So the first problem that
we want to talk about--

1502
01:13:17,350 --> 01:13:22,220
and I guess I'll just set it up
and that's as far as we get--

1503
01:13:22,220 --> 01:13:31,970
will be a problem where there's
a source of radiation, which

1504
01:13:31,970 --> 01:13:37,910
is moving to the right in our
diagram with a velocity, v,

1505
01:13:37,910 --> 01:13:46,115
and an observer
who is stationary.

1506
01:13:49,180 --> 01:13:51,960
Now of course, all these are
frame dependent statements,

1507
01:13:51,960 --> 01:13:53,410
but we're working
in a frame where

1508
01:13:53,410 --> 01:13:55,540
the observer is stationary.

1509
01:13:55,540 --> 01:13:58,420
And we're also going to assume
for the non-relativistic case,

1510
01:13:58,420 --> 01:14:02,385
that the air-- we'll be talking
about sound waves-- but the air

1511
01:14:02,385 --> 01:14:03,970
is stationary in this frame.

1512
01:14:03,970 --> 01:14:06,940
So the frame of backboard is not
only the frame of the observer,

1513
01:14:06,940 --> 01:14:09,040
but it's also the
frame of the air

1514
01:14:09,040 --> 01:14:12,740
when we're talking about the
non-relativistic sound wave

1515
01:14:12,740 --> 01:14:14,890
calculation.

1516
01:14:14,890 --> 01:14:16,920
So to define our
notation, we're going

1517
01:14:16,920 --> 01:14:22,580
to let u be equal the
velocity of the sound wave.

1518
01:14:26,580 --> 01:14:29,317
And that would normally be
measured relative to the air,

1519
01:14:29,317 --> 01:14:31,150
but the air will be at
rest in this picture,

1520
01:14:31,150 --> 01:14:33,200
so u will be the velocity
of the sound wave

1521
01:14:33,200 --> 01:14:35,870
relative to the diagram.

1522
01:14:35,870 --> 01:14:39,420
v is the velocity of the
source already shown.

1523
01:14:46,020 --> 01:14:48,930
And we'll be interested
in two time periods,

1524
01:14:48,930 --> 01:14:53,720
delta t sub s where
s stands for source,

1525
01:14:53,720 --> 01:15:07,115
which will be the period of
the wave at the source, which

1526
01:15:07,115 --> 01:15:09,240
is the same as talking
about the period of the wave

1527
01:15:09,240 --> 01:15:12,660
as it would be
measured by the source.

1528
01:15:12,660 --> 01:15:18,710
And delta t sub
O-- that's supposed

1529
01:15:18,710 --> 01:15:21,700
to be a capital O, not a zero.

1530
01:15:21,700 --> 01:15:34,605
It is the period of the wave
at the observer or as observed.

1531
01:15:40,260 --> 01:15:44,470
And the important point, which
is maybe obvious qualitatively,

1532
01:15:44,470 --> 01:15:48,010
is that these two times,
or time intervals,

1533
01:15:48,010 --> 01:15:50,040
will not be equal to each other.

1534
01:15:50,040 --> 01:15:54,170
And the reason, basically,
is that because the source is

1535
01:15:54,170 --> 01:15:57,080
moving-- and I've defined
positive v the way

1536
01:15:57,080 --> 01:16:01,170
astronomers would as moving away
from us-- because the source is

1537
01:16:01,170 --> 01:16:04,270
moving away from us,
each successive wave that

1538
01:16:04,270 --> 01:16:07,690
goes from the source to us has
to travel a little bit further.

1539
01:16:07,690 --> 01:16:10,272
And that means that each wave
crest is slightly delayed

1540
01:16:10,272 --> 01:16:11,730
from when it would
have gotten here

1541
01:16:11,730 --> 01:16:13,650
if everything were stationary.

1542
01:16:13,650 --> 01:16:15,230
And if you delay
each wave crest,

1543
01:16:15,230 --> 01:16:18,130
it means the time
between crests is larger.

1544
01:16:18,130 --> 01:16:20,880
And that means
that we expect here

1545
01:16:20,880 --> 01:16:25,390
that delta t sub O will be
larger than delta t sub s

1546
01:16:25,390 --> 01:16:28,160
because of this extra distance
that each wave crest has

1547
01:16:28,160 --> 01:16:29,310
to travel.

1548
01:16:29,310 --> 01:16:31,120
And what we'll be
doing next time--

1549
01:16:31,120 --> 01:16:32,870
I think I will just
leave the calculations

1550
01:16:32,870 --> 01:16:34,820
for next time-- is
calculating that.

1551
01:16:34,820 --> 01:16:36,820
And then doing the same
thing for the case where

1552
01:16:36,820 --> 01:16:38,903
the observers moving and
the source is stationary,

1553
01:16:38,903 --> 01:16:41,200
and then talking a little
bit about special relativity,

1554
01:16:41,200 --> 01:16:43,050
and then repeating
both calculations

1555
01:16:43,050 --> 01:16:44,980
with a special relativity
situation where

1556
01:16:44,980 --> 01:16:48,150
we'll be talking about light
rays and velocities that

1557
01:16:48,150 --> 01:16:50,380
might be comparable
to the speed of light.

1558
01:16:50,380 --> 01:16:53,189
So see you folks on
Thursday, but maybe I'll

1559
01:16:53,189 --> 01:16:54,980
see some of you at my
office hour tomorrow.

1560
01:16:54,980 --> 01:16:58,035
And I will send an email about
where exactly that office

1561
01:16:58,035 --> 01:17:00,030
hour will take place.