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Can someone explain to me how photons are directed from one place to another without interference with (i.e., "observation" by) other atoms? This seems impossib

Can someone explain to me how photons are directed from one place to another without interference with (i.e., "observation" by) other atoms? This seems impossible, but then it pops up the question: why is this interference (which seems to qualify as "observation") not a problem?

hugh4 11y ago
The short answer is: because glass. Glass (especially the stuff they use in optical fibres) is pretty good at not interacting with photons, which is why it looks like glass. Why doesn't glass interact with photons? Because it has a large band gap. There are no excitations to be had at optical wavelengths, so there's no way for the photon's energy to be transferred to anything else. Some fraction of the photons used in this experiment will nonetheless have been absorbed along the way, but that's not a problem, we only count ones where we have a pair.
- wfunction 11y ago
  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...)
    - 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.
- MichaelApproved 11y ago
  I have a question that's slightly off-topic from what OP asked. Is it possible to know which particles are entangled? If I have a pile of particles, can I say that a select group of them were still entangled while others are not? Another question along the same lines: If I give my friend one of my entangled particles, can I tell if he has measured the spin by examining my half of the pair of entangled particles? *edit missed a word
  - ars 11y ago
    No to both questions. Both would allow you to violate the speed of light if it was possible. Once you measure a particle it is no longer entangled, so if you could tell if a particle was entangled or not then you can violate the speed of light by instantly knowing an action taken on the other particle. Read also: https://en.wikipedia.org/wiki/No-cloning_theorem https://en.wikipedia.org/wiki/No-cloning_theorem
    - MichaelApproved 11y ago
      So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? This outside observer would simply observe that the particle has spin "up" or "down" but would never know that it was entangled earlier by someone else. This observer would never know it was in a superposition unless they had knowledge that someone had previously done this. Only the person who entangled the particle can clue the observer in on the fact that they were entangled. I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. You only know about its current state. There is nothing in your observation that shows "hey the particle just changed from superposition to spin up" Thanks for the replies! I watch "word science festival" and similar videos and I am fascinated by the quantum world but I have wondered these questions for a while and didn't know anyone who could answer them. I saw this great Veritasium video about measuring quantum particles but it still left me with the questions above https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c
tim333 11y ago
Interaction in general with the photons doesn't effect their polarisation and so the polarisation state remains unobserved. It's only when you check their polarisation that it is 'observed'. (It's not so much observation as a reduction in the possible states. When the photon is heading down the fiber it could be in any state and has a probability amplitude for being in various states. After it goes through a polariser and into a detector it's definitely in one particular state so you don't get the multitude of probability amplitudes).
- deleted 11y ago
  [deleted]

hugh4 11y ago

The short answer is: because glass. Glass (especially the stuff they use in optical fibres) is pretty good at not interacting with photons, which is why it looks like glass. Why doesn't glass interact with photons? Because it has a large band gap. There are no excitations to be had at optical wavelengths, so there's no way for the photon's energy to be transferred to anything else. Some fraction of the photons used in this experiment will nonetheless have been absorbed along the way, but that's not a problem, we only count ones where we have a pair.

wfunction 11y ago
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...)
  - 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.
MichaelApproved 11y ago
I have a question that's slightly off-topic from what OP asked. Is it possible to know which particles are entangled? If I have a pile of particles, can I say that a select group of them were still entangled while others are not? Another question along the same lines: If I give my friend one of my entangled particles, can I tell if he has measured the spin by examining my half of the pair of entangled particles? *edit missed a word
- ars 11y ago
  No to both questions. Both would allow you to violate the speed of light if it was possible. Once you measure a particle it is no longer entangled, so if you could tell if a particle was entangled or not then you can violate the speed of light by instantly knowing an action taken on the other particle. Read also: https://en.wikipedia.org/wiki/No-cloning_theorem https://en.wikipedia.org/wiki/No-cloning_theorem
  - MichaelApproved 11y ago
    So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? This outside observer would simply observe that the particle has spin "up" or "down" but would never know that it was entangled earlier by someone else. This observer would never know it was in a superposition unless they had knowledge that someone had previously done this. Only the person who entangled the particle can clue the observer in on the fact that they were entangled. I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. You only know about its current state. There is nothing in your observation that shows "hey the particle just changed from superposition to spin up" Thanks for the replies! I watch "word science festival" and similar videos and I am fascinated by the quantum world but I have wondered these questions for a while and didn't know anyone who could answer them. I saw this great Veritasium video about measuring quantum particles but it still left me with the questions above https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c

tim333 11y ago

Interaction in general with the photons doesn't effect their polarisation and so the polarisation state remains unobserved. It's only when you check their polarisation that it is 'observed'. (It's not so much observation as a reduction in the possible states. When the photon is heading down the fiber it could be in any state and has a probability amplitude for being in various states. After it goes through a polariser and into a detector it's definitely in one particular state so you don't get the multitude of probability amplitudes).

deleted 11y ago
[deleted]