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I was just wondering if there's a (hypothetical) situation where a photon could accelerate and what the consequences of this might be?

Sean
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  • Photons slow down when entering dense media. So they must accelerate afterwards when they leave. – John Alexiou Aug 07 '15 at 13:55
  • Also related: http://physics.stackexchange.com/q/121421/50583 – ACuriousMind Aug 07 '15 at 14:03
  • @ja72 light does that. Not the individual photons which build it up. – anna v Aug 07 '15 at 14:10
  • Sean: photons decelerate and accelerate in the vector sense in Compton and inverse Compton scattering. There is also a sense wherein light changes speed in a medium, google on photon effective mass. Note that if you trap a photon in a gedanken mirror-box, all of its energy-momentum is exhibited as effective mass. The system is harder to move because the photon is in there. It's being continually accelerated in the vector sense because it's bouncing back and forth, and it's effectively at rest. Check out standing waves in cavities. – John Duffield Aug 07 '15 at 15:44
  • Thanks for the comments and answers everyone and apologies, didn't realise this had been asked before! Also thanks for the info John, great stuff :) – Sean Aug 10 '15 at 14:40

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Photons undergo angular acceleration in very strong gravitational fields, gravitational lensing.. An acceleration can be defined in its change of direction, angular acceleration in radians/second^2, so the answer is positive, yes, light can be accelerated, but its speed will still be c, only the direction relative to the gravitational source changes.

anna v
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  • Wouldn't that be transverse acceleration and not angular acceleration? – John Alexiou Aug 07 '15 at 14:25
  • @ja72 I think not, I do not know what you mean by transverse acceleration, and I have defined what I mean in "angular acceleration" about a strong center of gravity. – anna v Aug 07 '15 at 16:48
  • Transverse acceleration is acceleration perpendicular to the direction of motion causing the path to curve. It is equal to $\frac{c^2}{r}$ where $r$ is the radius of curvature. – John Alexiou Aug 07 '15 at 18:17
  • @ja72 well then we are talking of the same thing with different words. – anna v Aug 08 '15 at 03:59
  • Maybe, but transverse acceleration has units of $m/s^2$ and angular acceleration of $rad/s^2$. – John Alexiou Aug 08 '15 at 14:02