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Let's say I'm over here I'm going to do two scenarios.

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I'm an observer over here, this is me.

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And then, even better, maybe I should just draw my eyeball.

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Because we're going to be observing light.

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So I'm just going to draw my eyeball. This is me in

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the first scenario, and this is one of my eyeballs.

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And this is one of my eyeballs in the second scenario.

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Now in the first scenario, (let me draw it), so in both scenarios we are going to have an object.

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We're going to have some type of source of light

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But in the first scenario (relative to me), the source of light

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will not be moving. While in the second scenario

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the source of light, just for the sake of discussion,

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just for fun, will be moving at half the speed of light.

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Unimaginably fast speed, but let's just assume that it is.

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So it has a velocity of one half the speed of light...

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One half light speed, away from me.

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Away from me, who is the observer.

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Now let's just imagine what would happen. They're both emitting

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light. And they're both going to start emitting

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light at the exact same time. And when they

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start emitting light, they are both the exact same

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distance from my eye. The only difference is that

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this is stationary relative to me,

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while this is moving away from me at

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Half the speed of light.

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So lets say after some period of time, that light wave

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from this source reaches my eye,

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and it looks something like this, I'll try my best

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to draw it. I'm going to draw a couple of wavelengths

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here, here's half a wavelength, that's a full wavelength

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that's another half, a full wavelength, another half,

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full wavelength, and then a half, and then a full

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wavelength. So let me see if I can draw that.

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So it would look like: full wavelength, full wavelength

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full wavelength, (this is not easy to do), and then

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you've got something like that, in the actual waveform.

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The front of the waveform is just getting to my eye

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then as the wave forms keep going past my eye

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my eye will perceive some type of a wavelength

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or frequency, and perceive it to be some type of color

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assuming that we are in some visible part

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of the Electromagnetic Spectrum.

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Now think about what's going to happen with this

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source. First thing is, that the front of the

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waveform is going to reach me at the exact

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same time. One of the amazing things

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about light traveling, in general or especially in a vacuum,

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is that it doesn't matter that this is moving away

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from me at half the speed of light. The light will still

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move towards me at the speed of light.

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It's absolute, it doesn't matter if this is going away at 0.9

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the speed of light. The light will still travel

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to me at the speed of light.

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It's very un-intuitive because in our every day sense

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if I'm moving away from you at half the speed of

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a bullet, and I shoot a bullet, the bullet will only move

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towards you (half of its velocity will be subtracted) at half

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of it's normal velocity relative to whether it was stationary. That is not the case with light.

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Let's think about what the waveform would look like.

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So by the time the light reached here, actually let

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me redraw this over here... redraw this eyeball...

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This is me again. So by the time the light

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reaches my eye, this guy has traveled half this distance.

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If it took light a certain amount of time to get this far,

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this guy will get half as far in the same amount

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of time. So by the time the light reaches my eye,

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this guy will have traveled about half that distance.

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So he would have traveled about.. that far.

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But they started emitting light at the same time.

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So that very first photon (if you view light as a particle)

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will reach my eye at the very same time as the very first

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photon from this guy.

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So the wave form is going to essentially be stretched.

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So we are still going to have, one, two, three, four

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full wavelengths, but they'll now be stretched.

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So let me see if I can draw four full wavelengths.

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Half over here, let me cut each of those in half,

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so each of these are going to be a full wavelength

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and then they're going to have a half wavelength, in between.

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And so the waveform is going to look like this...

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I'll try my best to draw it... this the hardest part

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of drawing the stretched out waveform.

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And there you go, it's going to look like this.

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And so when it get's to my eye, my eye will perceive it

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as having a longer wavelength. Even though from the

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perspective of each of these objects, if you with each of them,

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the frequency and the wavelength of the light emitted

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is the same. The only difference is, this guy

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is moving away from me, or I'm moving away from it,

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depending on how you want to view it, while I am stationary,

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or it is stationary, where in this first case, the observer

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and the object are both stationary. Now in this situation,

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what's my eye going to say? Well my eye will get

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each of these successive pulses, or each of these successive

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wave trains, and will say, "Hey, there's a longer perceived

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wavelength here, and also a perceived lower frequency."

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So what would that do to the perception of the light?

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Let's say that this is green light. If we are stationary

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with the observer it would be green light.

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So let's look at the electromagnetic spectrum. (I got this from Wikipedia)

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So if I was stationary to the observer, we would

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be in the green light part of the spectrum. So a

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500nm wavelength. But if all of a sudden, because the object

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is moving away from me at this huge velocity, the

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perceived wavelength becomes wider. So from my perception it's

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going to have a wider wavelength. And you can see

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what's happening, it will look redder. It will move

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towards the red part of the spectrum. And this

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phenomenon is called Red Shift.

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This is RED SHIFT.

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And I have done a bunch of videos in the physics playlist

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on the Doppler Effect, and over there I talk about sound

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waves, and the perceived frequency of sound as something

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travels towards you versus away from you.

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That's the exact same idea. This is the Doppler Effect applied

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to light. And the reason why the Doppler Effect works for light

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traveling through space, AND for sound traveling through

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air, is because a sound wave in air, regardless of whether

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the source is moving away or towards you,

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the sound wave is going to be moving at the speed

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of sound in air at a certain pressure and all of that.

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And light is the same thing! But in a vacuum, regardless

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of what the source is doing, the actual light wave

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itself will always travel at the same velocity. The only

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difference is that it's perceived frequency and wavelength

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will change. And now the whole reason

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why I'm talking about this, is you can use this

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property of light (that it gets Red Shift), to see whether

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things are traveling away or towards you!

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And people talk about Red Shift because frankly

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because most things are traveling away from us, and that

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is one of the reasons we tend to believe in the Big Bang.

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The opposite, if something is traveling towards me at

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super high velocities, then we would have something called

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violet shift. So the frequency would increase, and it would look

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more blue or purple. Now the other thing I want to highlight

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is this Red Shift phenomenon, this idea, it doesn't apply

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only to visible light. So it could even apply to things

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that we can't even see. So it would become redder,

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(but it's not like you could even see it), it could even be

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applied to things even more red then red!

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So maybe it's a microwave that is being emitted but because

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the source is moving away from us so fast, it can be

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perceived as an actual radio-wave.

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And actually (I should have talked about this in the video

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on the microwave background radiation) is that we're

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perceiving it as microwaves, but the sources were moving

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away from us. They were being Red Shift. So they were

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not actually emitting microwave radiation. Just what WE

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observe, (this would be predicted by the Big Bang) is actual

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microwave radiation. So anyway, hopefully that gives you a

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sense of what Red Shift is, and now we can use this tool

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to explain why we think many many things are moving away

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from us. And now let me just make sure you get that idea.

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If I have two objects, let's say that these are both suns,

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(or both galaxies, either way) and because of other

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properties, (and I won't talk about them right now) they are

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probably emitting light of the same color. Because we know

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other properties of that star, or galaxy.

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Now if what we actually perceive is that this one looks

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redder to us, than this one, then we know that it is traveling

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away from us. And the redder it looks, the more it's wavelength

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is spread out, relative to this other star, the faster we know

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it is moving away from us.