6 ms·

And how exactly can a photon change direction without interacting with something?

jacquesm 11y ago
A new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original. The few that remain absorbed will slightly warm up the glass which then regains thermal equilibrium by throwing off a photon at a much longer wavelength in a random direction.
- wfunction 11y ago
  > the new one is absolutely indistinguishable from the original This is where I get stuck, because no one explains this. How do we know this is true? Is this because they match in all the parameters that we're aware of (say, momentum, spin, whatever), or is this because it has been proven (through a Bell-like test) that there cannot be any other parameters that we are potentially unaware of, and hence if they match in the known ones, then they are one and the same?
  - sitharus 11y ago
    No, it doesn't mean that there is no magical hidden property that actually distinguishes them, but that as far as we can tell they're indistinguishable. Given that nothing about the photons is different they must be the same right? For more confusing theory, look up the single-electron universe.
  - wfunction 11y ago
    Yup I've heard about the single-electron universe. Okay, so doesn't what you said imply there could very well be a hidden variable that we're unaware of, that affects the two entangled photons as they are separated? What's ruling this out?
    - sanxiyn 11y ago
      Nothing rules that out. What Bell tests, of which this one is the best implementation so far, rule out is local hidden variable. Hidden variable that can propagate faster than light is not ruled out by the test. (For the most people, it is ruled out by being faster than light.)
  - Jach 11y ago
    The reason we know the photons are indistinguishable is because quantum mechanics deals with configurations of particles at locations, and these configurations predict different outcomes depending on whether "photon A here and photon B there" is a different configuration than "photon B here and photon A there". If A and B are distinguishable, the configurations aren't the same, and you'll notice real quick when your experimental results don't line up with expectations based on them being the same. Mathematically it's as simple as the fact that x^2 + y^2 != (x+y)^2 for all x,y. To read a longer discourse on this, you can check out http://lesswrong.com/lw/ph/can_you_prove_two_particles_are_identical/ http://lesswrong.com/lw/ph/can_you_prove_two_particles_are_i... (and the sequence in general... which can help you with the 'change direction' bits)
    - Jach 11y ago
      (Would appreciate if downvoters explained whether they downvoted because my understanding and thus explanation is bunk (hey, I'm not a physicist) or because they're generally against LessWrongthink-peddling...)
  - jacquesm 11y ago
    There could be other parameters but since we're not aware of them to us the photons are indistinguishable. It's in a nutshell why your mirror image still looks remarkably like you. It's all 'brand new' photons moving in a new direction but they carry all the properties of the originals, the only thing that is different is that there are fewer of them because some of the absorbed ones never made it to re-emission because their interaction with the mirror substrate caused them to stay there.
- wfunction 11y ago
  > A new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original I also don't understand this, because: if they are indistinguishable, then how do their directions differ?!
- CamperBob2 11y ago
  A new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original. Isn't this a lossless process of the sort forbidden by thermodynamics? What keeps us from creating a system where "identical" photons perpetually bounce between two electrons?
  - claudius 11y ago
    > Isn't this a lossless process of the sort forbidden by thermodynamics? It is lossless on the single-particle level, but not in large thermodynamic ensembles of many, many particles. There, only most photons interact losslessly, while some get absorbed nonetheless, heat up the glass and give you thermodynamics. How exactly unitary quantum mechanical time evolution transforms into ergodic, classical and thermodynamic time evolution in large systems is (relatively) open question.
  - Natanael_L 11y ago
    Thermodynamics is statistical. It don't exactly apply to individual particle pairs interacting. It apply to the larger system they're a part of. Also: http://www.livescience.com/18580-perpetual-motion-time-crystals.html http://www.livescience.com/18580-perpetual-motion-time-cryst... The thing is that you can have essentially static periodic systems, where in entropy do not go down but simply don't increase. Overall in the universe entropy will however increase on average.
- jacquesm 11y ago
  Replying to myself, more awake. That had an error in it, it should have been 'after absorption of the old one'. Remember to pay more attention when posting tired.
hugh4 11y ago
Waveishly.
no_gravity 11y ago
Does a photon have a direction? I would think it expands like a wave in all directions at the same time?
- teekert 11y ago
  My laser pen seems to disagree...
  - wfunction 11y ago
    This doesn't seem very convincing considering lasers expand too.
    - teekert 11y ago
      But not backwards. The photons have a direction. Perhaps we need physicist to comment :)
    - tinco 11y ago
      Isn't this just because not all of the laser is perfectly parallel? The household laser is created by exciting atoms to emit photons in a cylinder with mirror caps. One of the caps is 'perfect', the other is 99% reflective. The photons go in random directions, but will bounce around within the cylinder until they get absorbed (and re-emitted in a random direction). Photons that are (near) parallel to the cylinder have a much higher chance of going through the imperfect mirror. Thus what shoots out the end is a (near) parallel stream of photons.
- MichaelApproved 11y ago
  You might be thinking about a probability wave. This video shows how a particle can travel like a wave, except when it doesn't https://www.youtube.com/watch?v=YoQYnhHQ95U https://www.youtube.com/watch?v=YoQYnhHQ95U
- Jach 11y ago
  You're kind of there, but I think it's helpful to drop the notion of wave-particle duality you have since it's confusing you. Watch Feynman's videos about it (or just read his book called QED) if you want clarification, but I think it's best to for the most part think of the particles described in particle physics as particles with special rules and behaviors that wouldn't make sense if they were just tiny spheres, avoid thinking in terms of "waves", and learn about those special rules as you can. Anyway one way you can think of the direction is that it's on the most probable path based on the weighted sum of all possible (even absurd) paths through the local area, see https://en.wikipedia.org/wiki/Path_integral_formulation https://en.wikipedia.org/wiki/Path_integral_formulation. At the classical level, absurd paths cancel each other out, and in general the path a photon takes from A to B will be along the curve representing the least amount of time needed. (Hence the "bend" of light as it enters water since photons propagate through water at a different speed.) Whether one actually observes the photon going from A to B depends on that configuration's probability.
analog31 11y ago
A photon in glass is a quantum state of the glass, and its path of propagation is the axis of the fiber.

A new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original. The few that remain absorbed will slightly warm up the glass which then regains thermal equilibrium by throwing off a photon at a much longer wavelength in a random direction.

wfunction 11y ago
> the new one is absolutely indistinguishable from the original This is where I get stuck, because no one explains this. How do we know this is true? Is this because they match in all the parameters that we're aware of (say, momentum, spin, whatever), or is this because it has been proven (through a Bell-like test) that there cannot be any other parameters that we are potentially unaware of, and hence if they match in the known ones, then they are one and the same?
- sitharus 11y ago
  No, it doesn't mean that there is no magical hidden property that actually distinguishes them, but that as far as we can tell they're indistinguishable. Given that nothing about the photons is different they must be the same right? For more confusing theory, look up the single-electron universe.
- wfunction 11y ago
  Yup I've heard about the single-electron universe. Okay, so doesn't what you said imply there could very well be a hidden variable that we're unaware of, that affects the two entangled photons as they are separated? What's ruling this out?
  - sanxiyn 11y ago
    Nothing rules that out. What Bell tests, of which this one is the best implementation so far, rule out is local hidden variable. Hidden variable that can propagate faster than light is not ruled out by the test. (For the most people, it is ruled out by being faster than light.)
- Jach 11y ago
  The reason we know the photons are indistinguishable is because quantum mechanics deals with configurations of particles at locations, and these configurations predict different outcomes depending on whether "photon A here and photon B there" is a different configuration than "photon B here and photon A there". If A and B are distinguishable, the configurations aren't the same, and you'll notice real quick when your experimental results don't line up with expectations based on them being the same. Mathematically it's as simple as the fact that x^2 + y^2 != (x+y)^2 for all x,y. To read a longer discourse on this, you can check out http://lesswrong.com/lw/ph/can_you_prove_two_particles_are_identical/ http://lesswrong.com/lw/ph/can_you_prove_two_particles_are_i... (and the sequence in general... which can help you with the 'change direction' bits)
  - Jach 11y ago
    (Would appreciate if downvoters explained whether they downvoted because my understanding and thus explanation is bunk (hey, I'm not a physicist) or because they're generally against LessWrongthink-peddling...)
- jacquesm 11y ago
  There could be other parameters but since we're not aware of them to us the photons are indistinguishable. It's in a nutshell why your mirror image still looks remarkably like you. It's all 'brand new' photons moving in a new direction but they carry all the properties of the originals, the only thing that is different is that there are fewer of them because some of the absorbed ones never made it to re-emission because their interaction with the mirror substrate caused them to stay there.
wfunction 11y ago
> A new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original I also don't understand this, because: if they are indistinguishable, then how do their directions differ?!
CamperBob2 11y ago
A new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original. Isn't this a lossless process of the sort forbidden by thermodynamics? What keeps us from creating a system where "identical" photons perpetually bounce between two electrons?
- claudius 11y ago
  > Isn't this a lossless process of the sort forbidden by thermodynamics? It is lossless on the single-particle level, but not in large thermodynamic ensembles of many, many particles. There, only most photons interact losslessly, while some get absorbed nonetheless, heat up the glass and give you thermodynamics. How exactly unitary quantum mechanical time evolution transforms into ergodic, classical and thermodynamic time evolution in large systems is (relatively) open question.
- Natanael_L 11y ago
  Thermodynamics is statistical. It don't exactly apply to individual particle pairs interacting. It apply to the larger system they're a part of. Also: http://www.livescience.com/18580-perpetual-motion-time-crystals.html http://www.livescience.com/18580-perpetual-motion-time-cryst... The thing is that you can have essentially static periodic systems, where in entropy do not go down but simply don't increase. Overall in the universe entropy will however increase on average.
jacquesm 11y ago
Replying to myself, more awake. That had an error in it, it should have been 'after absorption of the old one'. Remember to pay more attention when posting tired.

hugh4 11y ago

Waveishly.

no_gravity 11y ago

Does a photon have a direction? I would think it expands like a wave in all directions at the same time?

teekert 11y ago
My laser pen seems to disagree...
- wfunction 11y ago
  This doesn't seem very convincing considering lasers expand too.
  - teekert 11y ago
    But not backwards. The photons have a direction. Perhaps we need physicist to comment :)
  - tinco 11y ago
    Isn't this just because not all of the laser is perfectly parallel? The household laser is created by exciting atoms to emit photons in a cylinder with mirror caps. One of the caps is 'perfect', the other is 99% reflective. The photons go in random directions, but will bounce around within the cylinder until they get absorbed (and re-emitted in a random direction). Photons that are (near) parallel to the cylinder have a much higher chance of going through the imperfect mirror. Thus what shoots out the end is a (near) parallel stream of photons.
MichaelApproved 11y ago
You might be thinking about a probability wave. This video shows how a particle can travel like a wave, except when it doesn't https://www.youtube.com/watch?v=YoQYnhHQ95U https://www.youtube.com/watch?v=YoQYnhHQ95U
Jach 11y ago
You're kind of there, but I think it's helpful to drop the notion of wave-particle duality you have since it's confusing you. Watch Feynman's videos about it (or just read his book called QED) if you want clarification, but I think it's best to for the most part think of the particles described in particle physics as particles with special rules and behaviors that wouldn't make sense if they were just tiny spheres, avoid thinking in terms of "waves", and learn about those special rules as you can. Anyway one way you can think of the direction is that it's on the most probable path based on the weighted sum of all possible (even absurd) paths through the local area, see https://en.wikipedia.org/wiki/Path_integral_formulation https://en.wikipedia.org/wiki/Path_integral_formulation. At the classical level, absurd paths cancel each other out, and in general the path a photon takes from A to B will be along the curve representing the least amount of time needed. (Hence the "bend" of light as it enters water since photons propagate through water at a different speed.) Whether one actually observes the photon going from A to B depends on that configuration's probability.

analog31 11y ago

A photon in glass is a quantum state of the glass, and its path of propagation is the axis of the fiber.