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PROFESSOR: Write in your notes
the Doppler effect is this--

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the observed wavelength minus
the emitted wavelength divided

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by the emitted wavelength
is equal to the change

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in wavelength divided by the
emitted wavelength, which

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we said was equal to the
velocity that is moving--

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the object is moving--

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divided by the speed
of light, which

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is "Z," which we
call the "redshift."

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The Doppler effect relates
a change in the wavelength--

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or if you're thinking
about photons,

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a change in the energy.

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So this is change in energy
of the photons, and it--

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AUDIENCE: What does
that "C" stand for?

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PROFESSOR: What's that?

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AUDIENCE: What does
that "C" stand for?

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PROFESSOR: "C?"

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Oh, I'm sorry.

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I should make this clear.

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So, "V" stands for the velocity.

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And if it's positive velocity,
that means it's moving away.

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So if velocity greater than
zero, that means moving away.

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Velocity is less than zero,
that means moving toward you.

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And we saw that if
something's moving

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across your line of sight,
we don't get a Doppler shift.

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We don't get any change.

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"C" is the speed of light,
which is equal to 3 times 10

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to the 8th, meters per second.

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So we're saying the amount
of wavelength change

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is equal to the speed
that the object is moving

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divided by the speed of light.

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The speed of light is
like the total amount

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that you could change.

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Nothing can go faster
than the speed of light.

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AUDIENCE: Nothing?

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PROFESSOR: Nothing.

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No information can go faster
than the speed of light.

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Two things can move
apart from each other

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faster than the speed
of light, so long

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as you're not trying to
talk to that other person.

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So the Doppler effect
relates the change

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in the energy of photons,
which is this part

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to the speed of the motion--

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actually, it's speed
and direction of motion.

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So on one side we've got this--
change in energy of photons

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is related to the speed of
the motion and the direction.

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It says nothing about
distance-- nothing.

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What does Hubble's law say--

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or Hubble's observation?

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"Law" is just a fancy word
for super observation.

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AUDIENCE: It has to be
the distance in space.

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PROFESSOR: Hubble's observation.

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And before we get
to that, what did

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we do where we learned
about the Doppler effect?

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What did we use in class here?

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AUDIENCE: [INAUDIBLE]

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PROFESSOR: We used the what?

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The animation, that applet,
that application online

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where we moved the
emitting source around

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and we measured the
different wavelengths?

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But then we looked at
Hubble's observation.

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How do we describe that?

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AUDIENCE: [INAUDIBLE]

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PROFESSOR: Say it again?

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AUDIENCE: [INAUDIBLE]

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PROFESSOR: What did we do?

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What do we look at when we
observed Hubble's observation?

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What were we doing yesterday?

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Peter?

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AUDIENCE: [INAUDIBLE]

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PROFESSOR: OK, that's
when we applied

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the model of the
expanding universe,

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but what did we do before that?

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[? Jalen? ?]

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AUDIENCE: Wait, before
we did the paper?

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PROFESSOR: Before we
did the paper expanding,

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what did we look at?

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AUDIENCE: Oh, we looked at
the distance between the two

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galaxies.

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PROFESSOR: No.

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It's all on your desk.

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AUDIENCE: The spectrum
of the [INAUDIBLE]??

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PROFESSOR: The spectrum--

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AUDIENCE: Of the galaxies?

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PROFESSOR: Of the galaxies.

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What else did we look at?

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AUDIENCE: The
spectrum of hydrogen?

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PROFESSOR: The spectrum
of hydrogen. And what else

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did we look at?

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AUDIENCE: The LAMP.

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PROFESSOR: The what?

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AUDIENCE: The LAMP.

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PROFESSOR: The
spectrum of the LAMP.

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And what was the thing
that we compared to?

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AUDIENCE: Images
of the galaxies.

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PROFESSOR: Images
of the galaxies.

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So think about it.

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This is why I always ask you
how did you learn these things.

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We learned this by
playing with the applet.

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We learned this by playing--
so this was from the applet.

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We learned this by observing
the images and the spectra

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of different galaxies.

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It's in that little plastic
bag that's still on your table.

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We looked at images and spectra.

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And what did we find out?

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AUDIENCE: That the bigger
ones-- though they're all

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the same linear
size, but the one

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that larger angular
sizes were closer.

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PROFESSOR: OK, so
the closer ones.

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What did we find out about
their motion using the Doppler

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effect?

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AUDIENCE: They were [INAUDIBLE].

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PROFESSOR: OK, we found that the
velocity of a receding galaxy--

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and when I say
"receding," that means

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getting further away from us--

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the velocity was equal to
Hubble's constant times

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the distance to the galaxy.

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Because we saw that the
ones that looked big

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when we looked at the images
were also moving away slower.

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So what are we relating here?

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Here we're relating speed--

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well, let's just say speed.

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Keep it simple.

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Here we're relating
speed to distance.

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Velocity of a
receding galaxy equals

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00:07:21,920 --> 00:07:26,310
h naught times the
distance to that galaxy.

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Now, the Doppler effect
happens anywhere.

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Anytime something is moving
away from you or towards you

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00:07:33,060 --> 00:07:35,190
and it's emitting
waves, you're going

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to get a change in the energy
of those waves, a change

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in the energy of those photons.

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Hubble's observation only holds
for galaxies in the expanding

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universe.

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This is more general.

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This is more specific.

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We can't always say that if an
object is farther away from us

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it must be moving faster.

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[? Shakib ?] is farther
away from me than Bianca is,

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so he must be moving faster.

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AUDIENCE: Well, of course.

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PROFESSOR: That
doesn't work, right?

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AUDIENCE: [INAUDIBLE]

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[LAUGHS]

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AUDIENCE: How can you say
that if they're not moving?

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PROFESSOR: Say that again?

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AUDIENCE: How you can say
that if they are not moving?

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PROFESSOR: Well, I'm just saying
that this relationship doesn't

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apply, because [? Shakib ?] is
not a galaxy far away from me.

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But if [? Shakib ?] had a
LAMP and he was running away

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from me, I could take a picture,
take a spectrum of that LAMP,

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and I would see
that the wavelength,

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or the energy of
those photons, would

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have shifted a little bit.

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So that would happen here
in this room, all right?