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Q & A: distance seen by light?

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Most recent answer: 04/08/2014
Q:
THANK YOU.SIR I REPEAT MY QUESTION WHEN I TRAVEL NEAR SPEED OF LIGIT ,SPACE CONTRACTION OCCUR, SO TRAVEL DISTANCE FOR ME LESS THAN 10 LIGHT YEAR, HOW MUCH DISTANCE FOR PHOTON TO TRAVEL ?
- HITESH (age 15)
SURAT,GUJARAT,INDIA
A:
That's a subtle question. From the point of view of someone traveling just below c along the same spatial path, the distance is extremely small. In the limit as the speed approaches c, the distance becomes zero. However, there is no reference frame, no point of view, corresponding to that limit.

Mike W.

(published on 12/13/2010)

Follow-Up #1: viewpoint of photon?

Q:
Two questions on the topic of light and light speed. 1. Using the universe as a (very loose) frame of reference, it's known that photons travel at velocity 'c'. If we were to change our frame of reference to the photon, would this not make the universe now be travelling at velocity 'c'? If so, doesn't this then show that a mass (the universe) can travel at velocity 'c'? 2. From our frame of reference, photons that are emitted from a LASER travel in one direction, and in parallel to each other. Moving our frame of reference to one of those photons, and keeping the assumption that the speed of light is constant in all frames of reference, this would mean that a second photon from that LASER would be moving away from the first at velocity 'c'; but those photons, from our frame of reference, are all travelling "together." How could these photons be both moving "together" AND away from each other, at the same time?
- Martin Towell (age 38)
Sydney, NSW, Australia
A:

There is no reference frame in which the photon is at rest. As a youth, Einstein tried to imagine such a frame, but his relativity didn't end up having anything like that in it.

On another question, when you suggest using the universe as a reference frame, to make that idea concrete you need to specify somethig or other in  the universe. The average motion of all the stuff in some big volume works pretty well- that's called the "co-moving" reference frame. It's also the frame in which the cosmic microwave background radiation comes with almost equal intensity from all directions.

Mike W.


(published on 04/08/2014)

Follow-up on this answer.