Special Relativity and the Photon.

[Don Duq]

An answer posted by mibn2cweus on Thurs 29/06/06 at 07:30 has reminded me of a some ancient playing I once did with special relativity:

"... until finally, at reaching the velocity of light, your are traversing the expanse of the universe in an instant! ..."

Now this, I seem to remember, is exactly the result I obtained. Whether it was in the time-frame of the photon or of the observer I cannot remember.

My question is, is this, in fact, the case? Given the time dilation and length contraction effects, does a photon, from some perspective, move from its source to its destination instantaneously?

[fo3nix]

Yes, it does.

Time has no meaning for a photon in its reference frame.

[Don Duq]

Thank you, fo3nix. Concise and clear.

Does this, then, have any consequences for the observer? For example, when we are told that the light from some distant galaxy has taken 2.3 billion years to reach us here on Earth, is that the whole story? Did the light, in fact, arrive at the same moment it left? Or at some other interval?

I suspect that I am now just spouting nonsense.

[dawkins]

The photon has no concept of time because it always travels at the speed of light, therefore everything happens instantaneously for the photon. However, the photon will always be travelling at the speed of light according to the observer.

Please tell me what on earth "ancient playing with special relativity" is?

D

[Don Duq]

Thank you for your answer, dawkins.

"... ancient playing ..." I knew this was a bit (very) unclear when I typed it but I couldn't be bothered to clarify it at the time. It refers to my making various calculations using the equations of special relativity a long time ago just for interest's sake and fun.

All clear now?

:)

To continue with the foolishness: Let us suppose I fire a photon towards the Earth from a point 2.3 billion light years away and, by some impossible means, I simultaneously send an alert to a recipient on Earth who receives the alert at the same moment I fire my photon. The recipient starts a clock and awaits the arrival of my photon.

Now, presumably, when my photon finally arrives his clock will read 2.3 billion years. If not, why not?

Now, let us further suppose that I ride my photon to Earth. From my frame of reference I arrive at the same moment I left. What time does the clock read and why?

I'm feeling rather wooly-headed at the moment, so I've got the idea that the clock will read 0:00:00, but this would mean either that my photon arrives at the same point twice, 2.3 billion years apart, or I'm just being a complete arse.

I tend towards the latter explanation.

[jake-the-peg]

No. If I understand correctly what you're saying is perfectly correct and really a very simple statement of time dilation under special relativity.

If you could travel really close to the speed of light you could cross the distance to the nearest star in what would seem to you to be a matter of moments but y4 years to everybody back on Earth.

If you could travel actually at the speed of light to you it would seem immediate.

One of the best examples of this is cosmic ray muons.

These are formed as cosmic rays hit our upper atmosphere. They have a half-life so short that even at the speed of light we should never detect them at the Earth's surface - but we do.

Time dilation means that time for them is much slower than for us so they survive long enough to make it to earth.

If you feel adventurous you can see them yourself with a large bottle of water and a few electronics as described here:
http://www.sas.org/E-Bulletin/2002-08-16/hands OnPhys/body.html

[mibn2cweus]

Remember at C length contracts to zero therefore the photon in it reference frame is going nowhere fast!