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Light and the luminiferous ether

If waves are disruptions traveling through a medium and light is a wave, what medium does it travel through? A luminiferous ether? 

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  • male robot hal style avatar for user Samarth Pusalkar
    At Sal said that acutually the sun is white. Is it so because I have seen some outer space pictures of sun of NASA that are yellow.
    I am a bit confused.
    (17 votes)
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  • leafers seed style avatar for user Noah Schwartz
    Black Holes are supposed to suck in everything including light. Although, it is also said that if a man was to fall into a Black Hole, his sense of time would move more rapidly until he would see the collapse of the universe before falling into the Black Hole. Another man in space watching his friend fall into the Black Hole, however, would see his friend gradually slow down. His friend will then hit the Event Horizon and appear to be frozen for the point of the man watching. For as long as the universe lasts, that man's friend will never actually fall into the Black Hole.
    That was just an example of using a man to be the one to fall in a Black Hole. However, for as long as Black Holes existed, before the Earth was ever formed, stuff, including light, has always been sucked inside of Black Holes. Although, merging these two concepts together, does that mean nothing has ever actually fallen inside a Black Hole, but are just frozen in the rim of it, through the point of view of everything outside of the Black Hole?
    (19 votes)
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    • male robot hal style avatar for user Charles LaCour
      Observers in different frames of reference do not experience time in the same way. In general relativity there are two concepts of time: coordinate time and proper time.

      Proper time is the space-time distance between two events. Coordinate time is the amount of time measures by a clock with an observer.

      Proper time is the same for all observers but coordinate time is unique to the frame of reference the observer is in.

      For the person falling into the black hole their coordinate time shows the amount of time until encountering the singularity as finite and well within a human lifespan even for a super massive black hole and the space further away from the black hole is moving faster through time. The coordinate time of an observer well outside the black hole will show the person falling in to the black hole as moving slower and slower and freezing in time at the event horizon. These two views are because coordinate time is different between different frames of reference.

      If the observers were to use proper time there for the comparison they would match each other.
      (6 votes)
  • leafers seed style avatar for user Hardy
    I thought mechanical waves need a medium and that electromagnetic waves don't need a medium to travel through, isn't light a electromagnetic wave?
    (10 votes)
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  • leaf orange style avatar for user Jacob Plocher
    In these videos on special relativity, specifically these three, Sal talks about light waves not needing a medium. All of the examples he provided were examples of disturbances through a medium represented in space. Rope, Water, and Air Molecules all exist to be touched within the three dimensions of space; however, we know that spacetime is one continuum of four dimensions:the three dimensions of space and the additional dimension of time. Why are there no waves that are disturbances in the fourth dimension of spacetime?
    Is it possible that Light waves do behave like all other waves? Instead of disturbing a physical medium within the three dimensions of space, they disturb the fourth dimension of time?
    As you continue to watch these videos and learn about these ideas we see time behaving in very interesting ways. Time is warped and changed from different inertial reference frames when light is produced or speeds near light are produced.
    (6 votes)
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  • piceratops sapling style avatar for user Vignesh Ramaraj
    Then how will we calculate the energy of a photon using E=mc^2?
    (4 votes)
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    • male robot hal style avatar for user Charles LaCour
      The m in E=mc^2 really refers to the rest mass of something and photons do not have rest mass. There is a more accurate version of that equation which is: E^2 = (pc)^2 + (mc^2)^2 where p is the momentum of the object. So for lite the m is 0 so you end up with E = pc. The momentum of a photon is related to the frequency of the light. It turns out that for light E=hf where h is Planck's constant and f is the frequency. This relationship is where the value for Planck's constant comes from.
      (8 votes)
  • female robot amelia style avatar for user Melisha Benny
    Hey Sal, is that the Young's double slit experiment you used to explain the dark and bright fringes of light? At you say about dark points of light, i can't get it how light behaves dark. Would be grateful if anyone could explain it out.
    (3 votes)
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    • blobby green style avatar for user Teacher Mackenzie (UK)
      Its a good question... how can light add up to make dark...?

      Ok, so think about light as a wave.
      It has peaks and troughs. (You could say positive and negative...kinda...for now...)

      so now if you have two waves meeting, their positive bit meeting will give extra positive (or brighter)
      but if a trough meets a peak (positive meets negative) then they will cancel out.

      The energy from the lost, dark bits goes into the other (positive + positive ) bits.

      does that make some sense??
      (8 votes)
  • aqualine ultimate style avatar for user Haris Qureshi
    Wait a minute, sun is white? What makes me see the yellow sun or yellow sun rays? Thank you.
    (2 votes)
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    • leaf green style avatar for user Mark Zwald
      The Sun does look yellow when the blue light is scattered more than red/green light. Since blue light is scattered more by the atmosphere, the Sun takes on a yellowish appearance, though as Andrew points out, if you were to eliminate the scattering, the Sun would indeed be white. This increased scattering of blue light by the atmosphere also explains why the sky is blue.

      This also explains why the Sun gets redder as it sets, since more and more blue and even green light is scattered away as the light has to travel through a larger distance of the atmosphere before reaching your eyes.
      (8 votes)
  • leafers tree style avatar for user Omar Abdelghani
    Doesn't the electromagnetic field in QFT qualify as a "medium" for light to travel through?
    (3 votes)
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    • blobby green style avatar for user chris.becke
      In the classical view a medium was a substance that waves propagate through by disturbing the molecules in the substance. QFT fields don't describe a substance - they're just equations that describe probability functions of finding particles in a space and they're observer dependent.
      (3 votes)
  • starky tree style avatar for user {Rayeed}^3
    What is the luminiferrous ether made up of ?
    (3 votes)
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  • piceratops sapling style avatar for user Micahtw
    What about light in a vacuum? It has no medium. I've heard about String-Net Liquid as the medium, but is that just an urban myth?
    (2 votes)
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Video transcript

- [Voiceover] So let's think a little bit about waves. The classical notion of what a wave is, is it's a disturbance traveling through a medium. Disturbance traveling through a medium. So what do we mean by disturbance and what do we mean by a medium? We could come up with some fairly simple examples. Let's say that I have a little puddle here. So that's my little puddle. And let's say I were to drop a stone in that puddle. So this is my stone. It gets dropped in the puddle. We've all seen what happens. Right when it goes into the water, that pushes that water down, pushes the water around it out, then that water comes back up. Then you have this traveling disturbance. You have this traveling disturbance. These kind of rings that start to radiate outward from where I actually dropped the pebble. You can actually see them move outwards. You can see the crests of those waves move outwards. We're talking about a disturbance. Well this is that disturbance. The pond was completely flat, but then we agitate it. We pushed it down. It pushed the water outwards. Then that pushed the water next to it up and down, up and down. It traveled outwards from the point of that initial disturbance. We have this disturbance traveling through the medium. What's the medium here? The medium here is the water. The initial disturbance was the rock disturbing that water. But then that water disturbed the water around it, which disturbed the water around it. That disturbance kept traveling through the medium. We can give other examples of this. If I take a really long string and hold it right there. That's my hand. The string is, let's just say, attached to something. Let's say it's a really long string and right now there's some slack in it. It's attached to a wall. Let's say that I were to start moving my hand up and down. So I move my hand up and down. What's going to happen? If I just did it once, what I'm gonna have is this lump of string move from the left to the right. So that's what it's gonna look like at first. Then a few seconds later, it's gonna look like this. It's gonna look something like this. You're gonna have this disturbance, which is this wave, this lump that I just generated. It is going to move to the right. What's the medium? The medium in this case is the string. It is the string. What just happened there? When I jerk the string up, I'm disturbing those string molecules right next to my hand. They're pulling on the string molecules next to them pulling on the sting molecules next to them. Then you have this traveling lump. Once again, these are both examples of waves. Disturbances traveling through a medium. Now I can give other through a medium. I can give other examples. Sound waves. What causes sound waves? You have a bunch of air particles all bouncing around at actually surprisingly fast speeds. But if you disturb them, if you cause something to, say, compress a bunch of air particles right here. These air particles get ultra-compressed. Some type of clap happens, so these get ultra-compressed. Then they're gonna bounce on the ones next to them. Once again, you have this disturbance traveling through the medium. In this case, the medium is the air. The medium is the air. So, now that we've seen some classical notions of waves, let's think about something a little bit more mysterious. That is the notion of light. Light definitely has wave like properties. Wave light can interfere with each other. We can do a whole other videos on light. If I were to take a barrier like that with two small slits in it. If I were to shine some light, one way to think about it is these slits are the only place where the disturbance gets through and then it would cause the light to propagate from each of them like that. Maybe I'm just drawing the crests of the waves, but also come out from here. You can see where the two crests meet. They're going to constructively enhance each other. They're going to constructively interfere. If you were to put some type of a detector right over here, you would see the bright points of the light and you would see the dark points of the light. So light definitely behaves like a wave. I'm just showing you even one example of light behaving like a wave. But if it is a wave, that means it needs to be or if we use our classical logic, you would say, well that means it's a disturbance traveling through a medium. But what is that medium? We have light coming from our sun. We call it sunlight. We have light coming, I keep wanting to make it a - actually our sun is white, we tend to draw it as a yellowish color, but that's just because of its - what happens is it goes through the atmosphere. So you have the sun, you have the earth. This is not drawn to scale. Somehow that light is able to reach us over 93 million miles. What's it going through? Is it a disturbance traveling through a medium? Well, for a long time, people said, well it must be. It has these wave-like properties. It seems to be traveling with a velocity, the speed of light. So there must be medium that it is traveling through. People theorize what this medium is. They called it the luminiferous ether. Let me write that down cause it's a fun word. It's a good name for a band. Luminiferous. Lumine, no I'm not spelling it right. Luminiferous ether. It was this idea that maybe, even in the vacuum of space, you have this substance. This luminiferous ether that the light is traveling through. That it's somehow a disturbance in that luminiferous ether. Now, what we're gonna do in future videos, is we're gonna test that. We're gonna see if that's actually the case because what's interesting about if there is a luminiferous ether, we're not gonna be stationary relative to that luminiferous ether. In fact, we're orbiting around the sun. The sun is orbiting around the center of the galaxy. There's no way that we're gonna be stationary relative to that luminiferous ether. If we're not stationary relative to that luminiferous ether, we should be able to detect how light behaves. If it's going in the direction of ether or if it's going against the direction of the relative movement of the ether relative to us. So anyway, I'll leave you there. This mystery of science. Waves, disturbance through a medium. We saw that with the water, with the air, with the string. But what about light? Is there a luminiferous ether? Put a question mark there.