4 ms·

> 2. “The Speed of Light is constant.” Well, actually, the speed of light is constant, it just takes time to ping-pong around a lattice/gas. Photons hit partic

> 2. “The Speed of Light is constant.” Well, actually, the speed of light is constant, it just takes time to ping-pong around a lattice/gas. Photons hit particles, get absorbed, and re-emitted, which causes an effective slowdown at a larger scale. A gas is just vacuum with particles floating around in it. When photons travel between those particles, they are traveling at c. The effective slowdown is determined by the lattice/gas, and we denote this slowdown with the refractive index... > 7. “Light is both a particle and a wave.” Well, actually, it’s neither. Light, as everything else, is described by a wave-function in quantum mechanics. A wave-function is a mathematical object, that can both be sharply focused and look pretty much like a particle. This is a great example of how quantum mechanical analogies don't work if over-analyzed. A great professor of mine once told me that you can't look too far into classical analogies for quantum mechanics -- you will end up coming to the wrong conclusion. At the end of the day, understand the math. As someone else in the comments said, most of these are pedantic gotcha's. I thought the one about water being blue was the only one that actually surprised me as something people thought was true.

wruza 6y ago
Pin-ball theory is a common misconception, afaik. https://qr.ae/pNSdj9 https://qr.ae/pNSdj9 (quora answer with sixty symbols video link in it; explains reasons for every debunked version of why)
Bjartr 6y ago
> it just takes time to ping-pong around a lattice/gas. Photons hit particles, get absorbed, and re-emitted Well, actually the speed of light, specifically the rate of propagation or phase velocity, is not slower in mediums because of absorption and re-emittance. Instead, a photon is affected by feedback loops between its oscillating electric field and the charges of atoms it is passing near, without being actually absorbed by the atoms. Paraphrased as best I could from: https://en.wikipedia.org/wiki/Refractive_index#Microscopic_explanation https://en.wikipedia.org/wiki/Refractive_index#Microscopic_e...
- randallsquared 6y ago
  Okay, but I would expect the main takeaway is that, for the purposes of things like tachyons and black holes and CTCs, the "speed of light" is still c, not whatever speed one measures in various non-vacuums. The only assertion of which I'm aware that the apparent speed of light in a medium is what really matters for relativity was Ron Mallet's work: https://www.phys.uconn.edu/~mallett/main/papers.htm https://www.phys.uconn.edu/~mallett/main/papers.htm . I don't believe there's been any experimental confirmation of it, though.
- marcosdumay 6y ago
  Whatever way you cut it, the photons themselves can not run in any speed that is not c. Besides, electromagnetic interaction is modeled as emission and absorption of photons on a small enough scale. You are just using a population simplification to dismiss the individual based analysis (what would be pretty reasonable because the later one is meaningless when you can't observe individuals, except that the entire point is pedantic, and pedantism can't be subject to dismissal by meaninglessness).
  - cygx 6y ago
    It doesn't work the way you imagine. Cf. http://dx.doi.org/10.1103/PhysRevLett.68.2421 http://dx.doi.org/10.1103/PhysRevLett.68.2421
gugagore 6y ago
For a video to address this misconception of the speed of light being constant, and it just appears to slow down due to ping-pong, I recommend "Why does light slow down in water?" by Fermilab: https://www.youtube.com/watch?v=CUjt36SD3h8 https://www.youtube.com/watch?v=CUjt36SD3h8
- air7 6y ago
  I still don't get it. Why do the two waves add up? Why can't they just coexist? And if they do add up, doesn't that mean that the atoms are piggyback riding on the light wave and essentially slowing it down which shouldn't be possible?
  - cygx 6y ago
    Why do the two waves add up? Because the electric field only has a single value at any point in space. The equations that describe this are linear (ignoring quantum stuff irerelevant at these scales), so the waves combine by simple addition. Why can't they just coexist? In a way, they do continue to coexist (again due to the linearity): For example, if two waves pass each other by, you get back the original waves once you leave the region of interference. And if they do add up, doesn't that mean that the atoms are piggyback riding on the light wave and essentially slowing it down which shouldn't be possible? Rather, the electrons around the atoms start wiggling around, making new waves. I wonder if one could set up a mechanical analogue by putting some floating balls in a pool and attaching them to the bottom so that they breach the surface if a wave passes through, creating new waves...
  - l33tman 6y ago
    I'm not sure that video helps anything. At the very least it confuses the QFT level description of light and electrons with the EM classical description. The "waves" you mention add up because each photon sent in through the glass takes all the possible paths, and you need to add up the contributions from all these paths to see where this single photon wants to go. This looks like wave propagation mathematically. The reason the waves add up is because they are all the same photon. If you send in two unrelated photons, they get their own waves and in that case they do coexist without interference, for all practical field strengths (there are light-on-light scattering effects that come into play at immense field strengths, just now starting to get probed).
deleted 6y ago
[deleted]
vecter 6y ago
> Well, actually, the speed of light is constant, it just takes time to ping-pong around a lattice/gas. Photons hit particles, get absorbed, and re-emitted, which causes an effective slowdown at a larger scale. A gas is just vacuum with particles floating around in it. When photons travel between those particles, they are traveling at c. The effective slowdown is determined by the lattice/gas, and we denote this slowdown with the refractive index... This is wrong. Two sources [0][1]. [0] https://www.youtube.com/watch?v=CUjt36SD3h8 https://www.youtube.com/watch?v=CUjt36SD3h8 [1] https://www.youtube.com/watch?v=CiHN0ZWE5bk https://www.youtube.com/watch?v=CiHN0ZWE5bk
- anonytrary 6y ago
  You're right, and it's a great example of how a classical analogy (lattice ping-pong) while "makes sense" is not what is physically happening. As I said, classical analogies cannot be taken literally, and this one is definitely a blunder on my part.
  - andi999 6y ago
    So what is happening physically?
    - vecter 6y ago
      Did you watch the linked videos? They explain what is happening physically. It's not as simple as a macroscopic analogy to particles ping-ponging around. It relies on the wave properties of atoms and subatomic particles, mostly electrons.
      - anonytrary 6y ago
        But they don't, it's more just "here's the math, it's all about wave interference". It's quantum mechanics. You will fail if you try to explain it "physically" as I did above haha.
- StavrosK 6y ago
  I realized now that I hate videos. It would take me ten minutes to actually find the reason from each of your sources, while I could skim writing and get to the answer in seconds. Now I want to know why that's wrong but don't want to spend ten minutes getting refraction explained to me beforehand...
aemreunal 6y ago
Veritasium recently published a video [1] on why the speed of light can't be measured one way, only two ways (i.e. the speed it travels from point A to point B then back to point A). While I'm not privy to all the physics details of it, I found the video very compelling even as a thought experiment. [1]: https://www.youtube.com/watch?v=pTn6Ewhb27k https://www.youtube.com/watch?v=pTn6Ewhb27k
- cygx 6y ago
  We never discussed this in our SR lectures - instead, I stumbled upon it in a creationist paper where someone tried to finagle a young earth out of it. For example, you can use a convention where the speed of light is infinite for infalling light rays, and twice as slow than normal for outgoing ones. It's a bit weird (when you look into the mirror, light from the sun reaches you instantaneously, 'slowly' moves towards the mirror, and then jumps back to your retina instantaneously), but predictions will work out the same. Of course, it'll affect more than just travel time of light, you'll also mess with anything that inludes a gamma factor, eg making inertia anisotropic (the force you need will depend on the direction you push in). Ultimately, it doesn't help the creationists, because things like time passed since the big bang for an observer following the Hubble flow does not depend on such conventions. All you'll achive is changing the big bang from an instantaneous thing (which in itself is already a bit of a dicy concept in relativity) that happened everywhere all at one into a continuous thing that started 13.8 billion years ago and has been going on ever since.

wruza 6y ago

Pin-ball theory is a common misconception, afaik. https://qr.ae/pNSdj9 https://qr.ae/pNSdj9 (quora answer with sixty symbols video link in it; explains reasons for every debunked version of why)

Bjartr 6y ago

> it just takes time to ping-pong around a lattice/gas. Photons hit particles, get absorbed, and re-emitted Well, actually the speed of light, specifically the rate of propagation or phase velocity, is not slower in mediums because of absorption and re-emittance. Instead, a photon is affected by feedback loops between its oscillating electric field and the charges of atoms it is passing near, without being actually absorbed by the atoms. Paraphrased as best I could from: https://en.wikipedia.org/wiki/Refractive_index#Microscopic_explanation https://en.wikipedia.org/wiki/Refractive_index#Microscopic_e...

randallsquared 6y ago
Okay, but I would expect the main takeaway is that, for the purposes of things like tachyons and black holes and CTCs, the "speed of light" is still c, not whatever speed one measures in various non-vacuums. The only assertion of which I'm aware that the apparent speed of light in a medium is what really matters for relativity was Ron Mallet's work: https://www.phys.uconn.edu/~mallett/main/papers.htm https://www.phys.uconn.edu/~mallett/main/papers.htm . I don't believe there's been any experimental confirmation of it, though.
marcosdumay 6y ago
Whatever way you cut it, the photons themselves can not run in any speed that is not c. Besides, electromagnetic interaction is modeled as emission and absorption of photons on a small enough scale. You are just using a population simplification to dismiss the individual based analysis (what would be pretty reasonable because the later one is meaningless when you can't observe individuals, except that the entire point is pedantic, and pedantism can't be subject to dismissal by meaninglessness).
- cygx 6y ago
  It doesn't work the way you imagine. Cf. http://dx.doi.org/10.1103/PhysRevLett.68.2421 http://dx.doi.org/10.1103/PhysRevLett.68.2421

gugagore 6y ago

For a video to address this misconception of the speed of light being constant, and it just appears to slow down due to ping-pong, I recommend "Why does light slow down in water?" by Fermilab: https://www.youtube.com/watch?v=CUjt36SD3h8 https://www.youtube.com/watch?v=CUjt36SD3h8

air7 6y ago
I still don't get it. Why do the two waves add up? Why can't they just coexist? And if they do add up, doesn't that mean that the atoms are piggyback riding on the light wave and essentially slowing it down which shouldn't be possible?
- cygx 6y ago
  Why do the two waves add up? Because the electric field only has a single value at any point in space. The equations that describe this are linear (ignoring quantum stuff irerelevant at these scales), so the waves combine by simple addition. Why can't they just coexist? In a way, they do continue to coexist (again due to the linearity): For example, if two waves pass each other by, you get back the original waves once you leave the region of interference. And if they do add up, doesn't that mean that the atoms are piggyback riding on the light wave and essentially slowing it down which shouldn't be possible? Rather, the electrons around the atoms start wiggling around, making new waves. I wonder if one could set up a mechanical analogue by putting some floating balls in a pool and attaching them to the bottom so that they breach the surface if a wave passes through, creating new waves...
- l33tman 6y ago
  I'm not sure that video helps anything. At the very least it confuses the QFT level description of light and electrons with the EM classical description. The "waves" you mention add up because each photon sent in through the glass takes all the possible paths, and you need to add up the contributions from all these paths to see where this single photon wants to go. This looks like wave propagation mathematically. The reason the waves add up is because they are all the same photon. If you send in two unrelated photons, they get their own waves and in that case they do coexist without interference, for all practical field strengths (there are light-on-light scattering effects that come into play at immense field strengths, just now starting to get probed).

deleted 6y ago

[deleted]

vecter 6y ago

> Well, actually, the speed of light is constant, it just takes time to ping-pong around a lattice/gas. Photons hit particles, get absorbed, and re-emitted, which causes an effective slowdown at a larger scale. A gas is just vacuum with particles floating around in it. When photons travel between those particles, they are traveling at c. The effective slowdown is determined by the lattice/gas, and we denote this slowdown with the refractive index... This is wrong. Two sources [0][1]. [0] https://www.youtube.com/watch?v=CUjt36SD3h8 https://www.youtube.com/watch?v=CUjt36SD3h8 [1] https://www.youtube.com/watch?v=CiHN0ZWE5bk https://www.youtube.com/watch?v=CiHN0ZWE5bk

anonytrary 6y ago
You're right, and it's a great example of how a classical analogy (lattice ping-pong) while "makes sense" is not what is physically happening. As I said, classical analogies cannot be taken literally, and this one is definitely a blunder on my part.
- andi999 6y ago
  So what is happening physically?
  - vecter 6y ago
    Did you watch the linked videos? They explain what is happening physically. It's not as simple as a macroscopic analogy to particles ping-ponging around. It relies on the wave properties of atoms and subatomic particles, mostly electrons.
    - anonytrary 6y ago
      But they don't, it's more just "here's the math, it's all about wave interference". It's quantum mechanics. You will fail if you try to explain it "physically" as I did above haha.
StavrosK 6y ago
I realized now that I hate videos. It would take me ten minutes to actually find the reason from each of your sources, while I could skim writing and get to the answer in seconds. Now I want to know why that's wrong but don't want to spend ten minutes getting refraction explained to me beforehand...

aemreunal 6y ago

Veritasium recently published a video [1] on why the speed of light can't be measured one way, only two ways (i.e. the speed it travels from point A to point B then back to point A). While I'm not privy to all the physics details of it, I found the video very compelling even as a thought experiment. [1]: https://www.youtube.com/watch?v=pTn6Ewhb27k https://www.youtube.com/watch?v=pTn6Ewhb27k

cygx 6y ago
We never discussed this in our SR lectures - instead, I stumbled upon it in a creationist paper where someone tried to finagle a young earth out of it. For example, you can use a convention where the speed of light is infinite for infalling light rays, and twice as slow than normal for outgoing ones. It's a bit weird (when you look into the mirror, light from the sun reaches you instantaneously, 'slowly' moves towards the mirror, and then jumps back to your retina instantaneously), but predictions will work out the same. Of course, it'll affect more than just travel time of light, you'll also mess with anything that inludes a gamma factor, eg making inertia anisotropic (the force you need will depend on the direction you push in). Ultimately, it doesn't help the creationists, because things like time passed since the big bang for an observer following the Hubble flow does not depend on such conventions. All you'll achive is changing the big bang from an instantaneous thing (which in itself is already a bit of a dicy concept in relativity) that happened everywhere all at one into a continuous thing that started 13.8 billion years ago and has been going on ever since.