Why do people keep saying photons are timeless?

  • #27
vanhees71
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Erm... I wasn't planning on a dissertation on how light behaves in a medium...
If wiki is wrong, please enlighten us. I don't mind updating the wiki article as soon as I know what's wrong with it...
Ok, I admit that I wasn't able to write my long planned Insights article about photons (lack of time), but it should be clear that I cannot write a textbook on QED (including many-body QED) in the form of forum postings or Insights articles. Also I think one should first understand classical electrodynamics in its right form, i.e., as a relativistic field theory. That's why I gave hints, where to find the physics in good textbooks and original research articles (admittedly over 100 years old, but obviously the people haven't sufficiently taken care to read these articles in the last 100 years, so it doesn't hurt to quote them).
 
  • #28
Wiki explains that an electromagnetic pulse has a front speed, a phase speed, and a group speed (link).
And the front speed (under certain assumptions) is always equal to c, the speed of light in vacuum.
The phase speed and group speed deviate because the electromagnetic wave interacts with the particles in the medium.

To be honest, I still don't quite get this, since it seems to suggest that whether we shoot a laser through a vacuum or through a medium, that the first effect of the waves arrive on the other side at the same time.
Hmm...Light in a vacuum can move 'without' being hindered..it accomplishes it's task by getting to the other side of the medium in a set time..which would be it's velocity.
I light wave travelling in a medium..is being hindered in it's travels..which means it will take 'longer' to accomplish the task of getting to the other side of the medium. That's considered a 'slower speed'.
I believe I read once that nature 'conspires' such though..that it will get to the other side of the medium in the 'same set time'. If it doesn't..you will simply not 'see' the light...the medium is opaque.
 

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