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Red shift | Scale of the universe | Cosmology & Astronomy | Khan Academy


Let’s say I’m over here. I’m going to do two scenarios. So I’m an observer over here. This is me. And then maybe even better,
I should just draw my eyeball because we’re going
to be observing light. So I’m just going
to draw my eyeball. So this is me in the
first scenario or this is one of my eyeballs. And then this is
one of my eyeballs in the second scenario. Now in the first scenario–
so let me draw it– so in both scenarios, we’re
going to have an object. We’re going to have some
type of source of light. But in the first
scenario, relative to me, the source of light
will not be moving. While in the second
scenario, the source of light– just for the sake
of discussion, just for fun– will be moving at half
the speed of light. Unimaginably fast speed, but
let’s just assuming that it is. So it’s moving at– it
has a velocity of 1/2 the speed of light, 1/2
light speed away from me who is the observer. Now let’s just imagine
what would happen. They’re both emitting light. So and they’re
both going to start emitting light at
the exact same time. And when they start
emitting light, they’re both at the exact
same distance from my eye. The only difference
is is that this is stationary, relative to
me, while this is moving away from me at half
the speed of light. So let’s say that after
some period of time, that the light wave from
this source reaches my eye. And then it looks
something like this. I’ll try my best to draw it. So let’s say I have– I want
to draw a couple of wavelengths here. So let’s say that’s
half a wavelength. That’s a full wavelength. That’s another half, a full
wavelength, another half, full wavelength, and
then a half, and then a full wavelength. So let me see if
I can draw that. So it would look
like full wavelength, full wavelength,
full wave length. This is not easy to do. And then you got
another full wavelength. So it would look something
like that, the actual waveform. And so the front of the wave
form is just getting to my eye. And then as the wave forms
keep going past my eye, my eye will perceive some type
of a wave length or frequency and perceive it to be
some type of color, assuming that we’re
in the visible part of the electromagnetic spectrum. Now let’s think
about what’s going to happen with this source. So the first thing is is that
the front of the wave form is going to reach me
at the exact same time. One of those neat and amazing
things about light travelling in general, or
especially in a vacuum, it doesn’t matter that this
is moving away from me at half the speed of light. The light will still
move towards me at the speed of light. It’s absolute. It doesn’t matter if
this is going away at 0.9 the speed of light. The light will still travel
to me at the speed of light. And it’s very unintuitive. Because in our everyday sense,
if I’m moving away from you at half the speed of a
bullet and I shoot a bullet, the bullet will only
move towards you at– it’ll kind of–
that half of its velocity will be subtracted. And it’ll only move towards me
at half of its normal velocity, relative to whether
it was stationary, but not the case with the light. So with that out of
the way, let’s think about what the wave
form would look like. So by the time the
light reached here, we need to think of– let me
actually redraw this over here. Let me redraw this
eyeball right over here. So this is me again. So by the time the light reaches
my eye– so they both started emitting the light at
the exact same time– this guy has traveled
half this distance. If it took light a certain
amount of time to get this far, this guy will get half as far
in that same amount of time. So by the time the
light reaches my eye, this guy will have traveled
about half that distance. So he would have
traveled about that far. They started emitting the
light at the same time. So that very first photon, if
you view light as a particle, will reach my eye at
the very same time as the very first
photon from this guy. So the wave form is going
to essentially be stretched. So instead of having–
so we’re still going to have one, two,
three, four full wavelengths, but they’ll now be stretched. Let me see if I can draw
four full wavelengths. So let me cut this
in half over here. And let me cut each
of those in half. So each of these are going
to be a full wavelength. And then they’re going to have
a half wavelength in between. And so the wave form is
going to look like this. Let me try my best to draw it. This is the hardest part,
drawing the stretched out wave form. And there you go. It’s going to look like this. And so when it gets
to my eye, my eye is going to perceive it as
having a longer wavelength, even though from the
perspective of each of these objects, if you’re
traveling with each of them, the frequency and the
wavelength of the light emitted is the same. The only difference is this
guy’s moving away from me– or I’m moving away
from it, depending on how you want to view
it– while I am stationary, while in this first case,
the observer and the source are both stationary. Now, in this situation,
what’s my eye going to say? Well, my eye will get each
of these successive pulses, or each of these
successive wave trains. And it’s going to say, hey,
there’s a longer wavelength here, a perceived
longer wavelength– let me write that– perceived
longer wavelength here, and also, a perceived
lower frequency. So what would that do to
the perception of the light? Let’s say that this
is green light. So if you are stationary
with the observer, it would be green light. So let’s look at the
electromagnetic spectrum. I got this off of Wikipedia. So if I were stationary
towards– with the observer, we’d be in the green light
part of the spectrum, so a 500 nanometer wavelength. But if all of a sudden,
because the object is moving away from me
at this huge velocity, the perceived wavelength
becomes wider. So from my perception,
it is going to have a wider wavelength. And you can see
what’s happening. It will look redder. It will move towards the
red part of the spectrum. And this phenomenon
is called redshift. And I’ve done a bunch of
videos of the physics playlist on the Doppler effect. And over there, I talk about
sound waves and the perceived frequency of sound– if
something travels towards you verses away from you–
as the exact same idea. This is the Doppler
effect applied to light. And the reason why
the Doppler effect works for light
traveling through space and for sound
traveling through air is because the sound
wave in air, regardless of whether the
source is moving away or towards you,
the sound wave is going to move at the
speed of sound in air at a certain pressure
and all of that. And light is the same thing. But in a vacuum, It will always,
regardless of the source, regardless of what
the source is doing, the actual light wave
itself will always travel at the same velocity. The only difference is is
that its perceived frequency and wavelength will change. And now the whole reason
why I’m talking about this is you can use this
property of light, that it gets redshift, to see
whether things are traveling away or towards you. And people talk about redshift
because, frankly, most things are traveling away from us. And that’s one of
the reasons why we tend to believe
in the Big Bang. The opposite, if something
is traveling towards me at super high
velocities, then we would have something
called– you don’t hear the word–
it would be violetshift. The frequency would increase. So it would look
bluer or more purple. Now the other thing
I want to highlight is this redshift
phenomenon, this idea, it doesn’t apply only
to visible light. So it could even apply the
things that we can’t even see. So it would only– it
would become redder. But it’s not like
you can even see. It could even be
applied to things that are even more red than red. So maybe it’s a microwave
that is being emitted. But because the source is
moving away from us so fast, it could be perceived
as an actual radio wave. And actually, I should have
talked about this in the video on the microwave
background radiation is that we’re perceiving
it as microwaves. But these sources were
moving away from us. They were being redshift. So they were not actually
emitting microwave radiation. It’s just what we observe–
and this is actually what would be predicted based
on the Big Bang– is actually microwave radiation. So anyway, hopefully,
that gives you a sense of what redshift is. And now we can use
this tool to explain why we think many, many things
are moving away from us. And now let me just actually
make sure you get that idea. If I have two objects, let’s
say that these are suns. Let’s say that these are
both suns, or both galaxies, either way. And because of
other properties– and I won’t talk
about them right now– we know that they
are probably emitting light of the same color. They’re probably emitting
light of the same color because we know other properties
of that star or of that galaxy. Now, if what we
actually perceive is that this one looks
redder to us than this one, then we know that it is
traveling away from us. And the redder it looks,
the more its wavelength is spread out relative to this
other star, the faster we know that this is moving
away from us.

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