[Paul25 (OP)]

Is length infinitely contracted for a photon at c?
Or to put another way could a photon have a longer length when travelling as a wave before it collapses into a particle on measurement?

[Halc]

A photon does not represent a valid reference frame.  All valid frames have physics that is no different than any other frame, and this includes light moving at c. You're attempting to describe a frame in which a photon is at rest, which yields meaningless results.
So the best I can answer is that in such a 'frame', length is meaningless, as is time, mass, and just about any other meaningful thing in physics.

[Paul25 (OP)]

A photon does not represent a valid reference frame.  All valid frames have physics that is no different than any other frame, and this includes light moving at c. You're attempting to describe a frame in which a photon is at rest, which yields meaningless results.
So the best I can answer is that in such a 'frame', length is meaningless, as is time, mass, and just about any other meaningful thing in physics.

While I understand and agree with you, understanding this principle of length contraction from differing reference frames might help our interpretations of the double slit exp. and others.

If a photon could be thought of a being capable of infinite length contraction then conversely when travelling its length could be of infinite length but from our frame of reference its the size of a photon.

It would mean there is no reverse causality involved in the quantum eraser exp because the photon is still connected to it's source until the instant it collapses to a particle.

[hamdani yusuf]

A photon does not represent a valid reference frame.  All valid frames have physics that is no different than any other frame, and this includes light moving at c. You're attempting to describe a frame in which a photon is at rest, which yields meaningless results.
So the best I can answer is that in such a 'frame', length is meaningless, as is time, mass, and just about any other meaningful thing in physics.

What about a photon moving in a medium which makes it move at less than c? Could it represent a valid reference frame?

[Halc]

What about a photon moving in a medium which makes it move at less than c? Could it represent a valid reference frame?

It seems so.  You could have water moving through a pipe at about 0.76c and light going through it would make zero net progress.Time passes by normally in such a frame.

[hamdani yusuf]

What about a photon moving in a medium which makes it move at less than c? Could it represent a valid reference frame?

It seems so.  You could have water moving through a pipe at about 0.76c and light going through it would make zero net progress.Time passes by normally in such a frame.

What I recall from relativistic explanation of Fizeau's experiment is that you must use relativistic velocity addition. If the light move against water flow with equal speed then its speed becomes 0 according to a stationary observer.

[yor_on]

If you put a 'photon' through another medium than a vacuum it doesn't as much 'propagate' as it is absorbed and then 're-emitted' by the constituents of that water.

[jeffreyH]

You have to ask, length contraction from whose perspective? It can't be the photon's perspective. The photon isn't an observer. It records no useful information about the universe around it. It CAN be detected by an observer. This observer can obtain useful information FROM the photon.