3 ms·

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

by no_gravity 11y ago

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.
- kps 11y ago
  What he said. Remember that ‘classical’ waves (water, sound, strings…) are all just large-scale assemblies of tiny particles interacting with each other locally.

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.

kps 11y ago
What he said. Remember that ‘classical’ waves (water, sound, strings…) are all just large-scale assemblies of tiny particles interacting with each other locally.