Re: Riding on a photo
antimatter33_at_yahoo.com
Date: 12/19/04
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Date: Sun, 19 Dec 2004 12:48:17 +0000 (UTC)
mike.james wrote:
> Although I know SR I still feel confused about what it tells us about
> what the universe would look like from the point of view of a photon.
> While arguments based on limits help they don't really give final
> answers (to my mind) when v=c.
First, realize that you can't "ride a photon". A photon is not
localizable (has no rest frame) and represents the interaction of the
EM field with some apparatus. But, it is possible to ask what the world
looks like at a very high speed. See here:
http://www.anu.edu.au/Physics/Searle/
To summarize:
1) Aberration crams the world into an ever-shrinking forward cone,
according to the formula
tan 1/2 A' = sqrt(1-v/1+v) tan 1/2 A
where A' is the aberrated angle. As v->1 all the world is directly
ahead.
2) Light coming off objects crammed into the forward area by aberration
is intensely blueshifted as v->1. The frequency has an angular
dependence.
3) The cramming is conformal on a small scale (that is, a small patch
of the celestial sphere goes over into a similar small patch), but on a
large scale, objects tend to look curved into arcs around the forward
direction.
All the above is simple kinematics.
The next most important visualation to understand concerns a moving
sphere. *A moving sphere always looks spherical, regardless of its
speed*. A naive interpretation of the Lorentz transformation would lead
you to imagine that a moving sphere would be flattened into a prolate
ellipsoid. Not so, and one must keep this fact uppermost in mind when
trying to imagine how the world looks at high speed. Another strange
effect - the *surface features* of a moving sphere seem to crawl around
on the sphere (Terrell rotation) as the speed changes.
Now to your questions:
> 1) Presumably an observer on a photon sees all other photons as
moving at c?
The photon cannot be localized. But, an observer moving so fast that
the world amounts to an intensely blue-shifted dot directly ahead,
still measures the speed of light at C.
> 2) What does the EM field look like if you are travelling with it?
The photon cannot be localized. The free EM field looks like E and B
standing perpendicular to each other, and to the direction in which it
propagates. E and B are the same length in natural units.
> 3) Is proper time stopped for a photon?
The photon cannot be localized and so has no proper time.
> 4) If a photon is emitted and absorbed what is its proper time for
the
> interval between these events?
No observer can watch a photon get emitted and subsequently absorbed,
since this requires a timelike separation of events. This is consistent
with the non-existence of a rest frame for the photon.
-drl
- Next message: Mike Helland: "How many things can happen in a single instant?"
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