Red Shift Red Shift
⇐ Use this menu to view and help create subtitles for this video in many different languages. You'll probably want to hide YouTube's captions if using these subtitles.
- 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
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
Have something that's not a question about this content?
This discussion area is not meant for answering homework questions.
Share a tip
When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
Have something that's not a tip or feedback about this content?
This discussion area is not meant for answering homework questions.
Discuss the site
For general discussions about Khan Academy, visit our Reddit discussion page.
Flag inappropriate posts
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
- disrespectful or offensive
- an advertisement
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
- a tip or feedback in Questions
- a question in Tips & Feedback
- an answer that should be its own question
about the site