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Red Shift

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Red Shift

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Let's say I'm over here I'm going to do two scenarios.
I'm an observer over here, this is me.
And then, even better, maybe I should just draw my eyeball.
Because we're going to be observing light.
So I'm just going to draw my eyeball. This is me in
the first scenario, and this is one of my eyeballs.
And this is one of my eyeballs in the second scenario.
Now in the first scenario, (let me draw it), so in both scenarios we are 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 assume that it is.
So it has a velocity of one half the speed of light...
One half light speed, away from me.
Away from me, who is the observer.
Now let's just imagine what would happen. They're both emitting
light. And they're both going to start emitting
light at the exact same time. And when they
start emitting light, they are both the exact same
distance from my eye. The only difference is that
this is stationary relative to me,
while this is moving away from me at
Half the speed of light.
So lets say after some period of time, that light wave
from this source reaches my eye,
and it looks something like this, I'll try my best
to draw it. I'm going to draw a couple of wavelengths
here, here'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 wavelength, (this is not easy to do), and then
you've got something like that, in the actual waveform.
The front of the waveform is just getting to my eye
then as the wave forms keep going past my eye
my eye will perceive some type of a wavelength
or frequency, and perceive it to be some type of color
assuming that we are in some visible part
of the Electromagnetic Spectrum.
Now think about what's going to happen with this
source. First thing is, that the front of the
waveform is going to reach me at the exact
same time. One of the amazing things
about light traveling, in general or especially in a vacuum,
is that 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.
It's very un-intuitive because in our every day 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 (half of its velocity will be subtracted) at half
of it's normal velocity relative to whether it was stationary. That is not the case with light.
Let's think about what the waveform would look like.
So by the time the light reached here, actually let
me redraw this over here... redraw this eyeball...
This is me again. So by the time the light
reaches my eye, 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 the 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.
But they started emitting 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 we are still going to have, one, two, three, four
full wavelengths, but they'll now be stretched.
So let me see if I can draw four full wavelengths.
Half over here, 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 waveform is going to look like this...
I'll try my best to draw it... this the hardest part
of drawing the stretched out waveform.
And there you go, it's going to look like this.
And so when it get's to my eye, my eye will perceive it
as having a longer wavelength. Even though from the
perspective of each of these objects, if you with each of them,
the frequency and the wavelength of the light emitted
is the same. The only difference is, this guy
is moving away from me, or I'm moving away from it,
depending on how you want to view it, while I am stationary,
or it is stationary, where in this first case, the observer
and the object 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 will say, "Hey, there's a longer perceived
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. If we are stationary
with the observer it would be green light.
So let's look at the electromagnetic spectrum. (I got this from Wikipedia)
So if I was stationary to the observer, we would
be in the green light part of the spectrum. So a
500nm 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's
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 Red Shift.
This is RED SHIFT.
And I have done a bunch of videos in the physics playlist
on the Doppler Effect, and over there I talk about sound
waves, and the perceived frequency of sound as something
travels towards you versus away from you.
That's 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 a sound wave in air, regardless of whether
the source is moving away or towards you,
the sound wave is going to be moving 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, regardless
of what the source is doing, the actual light wave
itself will always travel at the same velocity. The only
difference is that it's 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 Red Shift), to see whether
things are traveling away or towards you!
And people talk about Red Shift because frankly
because most things are traveling away from us, and that
is one of the reasons 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
violet shift. So the frequency would increase, and it would look
more blue or purple. Now the other thing I want to highlight
is this Red Shift phenomenon, this idea, it doesn't apply
only to visible light. So it could even apply to things
that we can't even see. So it would become redder,
(but it's not like you could even see it), it could even be
applied to things even more red then red!
So maybe it's a microwave that is being emitted but because
the source is moving away from us so fast, it can 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 the sources were moving
away from us. They were being Red Shift. So they were
not actually emitting microwave radiation. Just what WE
observe, (this would be predicted by the Big Bang) is actual
microwave radiation. So anyway, hopefully that gives you a
sense of what Red Shift 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 make sure you get that idea.
If I have two objects, 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) they are
probably emitting light of the same color. Because we know
other properties of that star, or 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 it's wavelength
is spread out, relative to this other star, the faster we know
it is moving away from us.

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Red Shift

Red Shift