B Can there be time without mass?

[vanhees71]

Photons are no classical point particles. It doesn't make much sense to say they'd travel along worldlines whatsoever. They are not localizable. See also the very interesting discussion going on in the quantum theory forum:

https://www.physicsforums.com/threads/single-photon-states.1048604/

 

[PeterDonis]

But, what happens when photons are slowed down? I'm not referring to the obvious refraction index, but to experiments like the one described in the article (I don't know if it's a reputable source...):

Researchers stop and store light for 60 seconds

"Photons being slowed down" is not a good description of what is happening in these experiments.

Then they are travelling on timelike worldlines and proper can be defined along those worldlines.

This is not a good description of what is happening in the experiments referred to.

They don't seem to travel at all.

https://newatlas.com/stopping-light-inside-crystal/28610/
The photons are converted into atomic spin excitations (or "spin waves"), which can be stored in the crystal until the control beam is fired again and the spin waves are turned back into light, which finally escapes the crystal.

While this ordinary language description still leaves a lot out, at least it acknowledges that the photons have to be "converted" into something else--photons themselves are not being "slowed down" or "stopped".

Further discussion of these experiments belongs in a separate thread in the quantum physics forum. It is off topic here.

 

[guptasuneet]

Thanks everyone for participating in the discussion and for some mind opening replies. What I have understood from the discussion till now is:

1. Spacetime is definable independent of the presence of any particles in it
2. Concept of time cannot be defined for a particle with zero rest mass
3. Interactions between the particles with zero rest mass can be used to define / measure time, even in a spacetime consisting only of such particles with zero rest mass

Not sure if the above summary is proper and request that I may please be corrected.

 

[PeroK]

Thanks everyone for participating in the discussion and for some mind opening replies. What I have understood from the discussion till now is:

1. Spacetime is definable independent of the presence of any particles in it
2. Concept of proper time cannot be defined for a particle with zero rest mass
3. Interactions between the particles with zero rest mass can be used to define / measure time, even in a spacetime consisting only of such particles with zero rest mass

Not sure if the above summary is proper and request that I may please be corrected.

The only thing I'd add is to emphasise that proper time cannot be defined for a particle with zero rest mass.

 

[PeterDonis]

Moderator's note: An off topic subthread has been deleted.