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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.