Reading about relativity, one point seems to crop up again and again; the
story of the thought process that led Einstein to his theory. In various
forms, it is said "Einstein pondered what he would see if he could travel at
the speed of light". In a Newtonian Universe he would, for instance, be
unable to see his face in a mirror held in front of him because the light
reflected from it could not race ahead to bounce off the mirror. This is
always presented as so paradoxical and wrong that clearly another theory was
needed.

Much as I try, I cannot see why this would be such an impossible universe.
If we did live in such a universe, strange optical effects would simply be
recognised as a consequence of very high speed travel. Put another way, the
paradox is presented that if one were travelling at the speed of light,
light rays (only those which are travelling parallel to yourself) would have
zero frequency and thus zero energy, thus violating the law of conservation
of energy. But again, surely this would be seen as just an "optical
illusion"? It would be no different to the "paradox" of a surfer finding
that the ocean's waves "disappear" when he is travelling at the same speed
as them. Anyone observing you could clearly understand that you are
experiencing a unique, very special case view of the universe; the light
would still be there even if you could not see it.

I'm not arguing against relativity here at all. It seems to be at least very
close to correct. But I do think this argument is very dubious indeed. It
would be a different universe for very high speed travellers, but surely not
an impossible one.

Ian

On 10/7/2003 8:32 AM, Ian Bland wrote:
Reading about relativity, one point seems to crop up again and again; the
story of the thought process that led Einstein to his theory. In various
forms, it is said "Einstein pondered what he would see if he could travel at
the speed of light".

Correct up to there, but you then diverged.

What Einstein asked himself was really: If I moved at the speed of light
and looked at a comoving light ray, it would presumably be a static
object relative to me. But that violates Maxwell's equations (in the
absence of charge, a spatial variation in E and B requires a variation
in time as well, so the comoving light ray cannot have any structure --
where did the light wave go?). This contradiction led him (eventually)
to SR.

In SR, it is not possible for an observer to travel at the speed of
light. And as one approaches the speed of light while traveling along a
light beam, it gets Doppler-shifted to lower and lower frequencies (and
longer and longer wavelengths), asymptotically to nothing as one
approaches the speed of light. So the light wave does indeed "disappear"
to such an observer.


Tom Roberts

"Tom Roberts" wrote in message
...
On 10/7/2003 8:32 AM, Ian Bland wrote:
Reading about relativity, one point seems to crop up again and again;
the
story of the thought process that led Einstein to his theory. In various
forms, it is said "Einstein pondered what he would see if he could
travel at
the speed of light".

Correct up to there, but you then diverged.

What Einstein asked himself was really: If I moved at the speed of light
and looked at a comoving light ray, it would presumably be a static
object relative to me. But that violates Maxwell's equations (in the
absence of charge, a spatial variation in E and B requires a variation
in time as well, so the comoving light ray cannot have any structure --
where did the light wave go?). This contradiction led him (eventually)
to SR.


So the argument is that for an observer at the speed of light, the electric
and magnetic fields of the comoving light would cease to exist. Why could we
not simply say that they are not observable to this observer? After all, in
relativity we routinely accept that observers see the same light having
wildly different frequencies and thus fields of different frequencies too.

If our observer wishes to find the fields, all he has to do is apply a tiny
amount of acceleration and hey presto, he realises the light is there after
all

A spinning magnet creates, from the POV of a static observer, an oscillating
magnetic field. If however our observer chooses to orbit the magnet at the
same speed as it is rotating, hey presto its magnetic field becomes static
(from his and his POV alone). Where is the difference?

Ian