12 mo. ago • 93%

Speediest little fella.

!physics@mander.xyz

61 comments

Photons cannot accelerate

39
- Acceleration and Velocity are vectors. Changes in a velocity vector are an acceleration. Therefore when photons change direction technically it’s a form of acceleration.
  
  23
  
  I thought photons are always moving in straight lines from their perspective, and it's space that's bent. Unless it's through a medium, then they just get absorbed and re-emitted, sort of.
  
  14
  
  Ok but photons don’t change direction either. Treating photon scattering as an individual particle accelerating due to an applied force, well that’s just not a correct description of how perturbative QED models photon interactions.
  
  1
- They do at 0m/s^2.
  
  8
- Not with that attitude.
  
  2
- What about diffraction?
  
  2
  
  Since photons are indistinguishable, it's hard to say too much concretely, but it some sense a diffracted photon is different photon. In order for a photon to interact with say, a diffraction grating, the interaction is done with "virtual photons".
  
  So for a photon to change course, aka accelerate, it does it by absorbing a virtual photon and emitting another. Whether that is the "same photon" after the interaction is kinda more philosophy than physics, at least to me.
  
  Feynman diagrams are surprisingly accessible for how much information they contain. It's one way to think about photon (and other particle) reactions.
  
  4
Does a photon actually accelerate? Sure seems like it always goes at light speed through whatever medium from its creation.

26
- well, if it get reflected and change direction it going to be at light speed, so it can be interpreted (probably incorrectly lol) that it "accelerated instantly to the other direction after the reflection"?
  
  10
  
  This is an interesting question. Instant acceleration is mathematically implausible, but I don't know if there's a better physical interpretation for what happens to a bouncing photon. I'm guessing this is one of those "less particle, more wave" situations where the instantaneous velocity of the photon is undefined.
  
  According to some random internet sources, reflection is the not-quite-instantaneous process of the photon being absorbed and then emitted by the electrons in the mirror.
  
  7
- This is acceleration with no mass and no resistance to medium.
  
  2
  
  Photons are born and die at c. They experience no time and have no frame of reference.
  
  11
  
  https://www.wtamu.edu/~cbaird/sq/mobile/2014/06/26/how-does-a-photon-accelerate-to-light-speed-so-quickly/
  
  1
- Permanently Deleted
  
  0
  
  The fact that light cannot change speed is one of the core axioms of relativity
  
  15
  
  No, they don't. They can get absorbed and re-emitted, and the space they are moving though can compress sideways. But they can't make curves at all.
  
  2
PHOTONS HAVE MASS

ANYONE WHO DOES NOT BELIEVE THEY HAVE MASS IS A COWARD

18
- But every time i put it on a scale, it just flys away. GIVE ME PROOF. I have a kitchen scale to offer.
  
  2
  
  MEASURE FASTER
  
  IT MOVES REAL QUICK
  
  2
- Photons can have little a mass, as a treat.
  
  2
Who is the dude on the right?

9
- A scientist at CERN.
  
  32
  
  Ah, I get the joke now. Classic
  
  11
Because you're so light

6
Without mass how could you do anything else?

5
- Wave!
  
  14
  
  I see you there
  
  6
- Without mass you have to occupy parts of time and possibly gravity.
  
  3
  
  39 years old... Can confirm that time is perceptibly accelerating
  
  2

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