Re: one way light speed


Martin Hogbin wrote:
<nobody1357@xxxxxxxxxxxxx> wrote in message news:1162712385.097629.136390@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have been told a few times that all measurements of light speed are
two way and I can't think of an experiment setup which can show a
different result if one way speed differed in two directions. So, if
one way speed can never be measured, doesn't this mean it has no effect
on laws of nature and in a sense it doesn't exist like aether?

There are two ways, in principle, of measuring the speed of
light. You can use one clock to measure the two-way speed of light
or two clocks to measure the one-way speed. There are no ways
round this limitation. Both methods have been used.

In the case of a one-clock two-way measurement the, result is
always c (in an inertial frame and in a vacuum). This result has
confirmed many times.

The two-clock one-way case is more difficult to interpret since the
result depends on how the clock are synchronised. As you can
appreciate, this is a vitally important issue. If the clocks are not
synchronised then the speed measurement is meaningless.

There are several ways in which the clocks could be
synchronised. As we know of no way of
instantaneously transmitting a signal from one clock
to another the only way to guarantee the clocks are
synchronised is to have them both in the same place.
They can then be synchronised easily, but to
measure the one-way speed of light we need to
separate them. If we do this slowly (compared to
the speed of light) then the method of clock
synchronisation we have used is referred to as
'slow transport'. When slowy transported clocks
are used to measure the one-way speed of light,
the result is also c (usual conditions). However,
it could be argued that the clocks might have
changed when we moved them. We have no
way to detect this.


There are other ways we might choose to synchronise
our clocks but you are absolutely right ther is no single
way that is forced upon us - so we have to choose one!
We could choose slow transport, but the method chosen
by Einstein was to define (or postulate as it is usually
translated) the one-way speed of light to be constant
and equal to the measured two-way speed. Einstein,
and physicists since, were fully aware of the arbitrary
nature of this choice but it happens to be one which is
practical and useful and ,as you can see, it agrees with
synchronisation by slow transport.


Thanks.


Suppose
I'm saying that speed of light is infinite in this direction and c/2 in
the opposite direction, what can disprove this claim?

Nothing, it is a matter of free choice.

I was expecting an objection, but see below.

If this can be
disproved, one way speed of light can be measured. But it would also
mean there is no speed limit in one direction and c/2 speed limit in
opposite direction, and energy required to change direction of speed of
a particle would be different in two directions. This is not so in
particle accelerators so isn't this indirect proof that one way speed
has to be the same in all directions?

When you think about it in detail you will see that you cannot
measure any one-way speed without some clock synchronisation
convention.

If you can understand the issue of clock synchronisation
you will avoid the confusion that many of the crackpots
on this site have.

Martin Hogbin

Imagine a particle accelerator that is not a circle but a very long
line. This can not accelerate a particle faster than c in any
direction. Clock sychronization does determine how the speed of this
particle is measured, but with an array of clocks sychronized the same
way, it is measured that succesive accelerations do not cause equal
speed increase, so there is a speed limit, which is not infinite. If
this particle is put in a race with light, it arrives just after light,
so this speed limit is the same as one way speed of light. So at least
it is certain that one way speed of light can not be inifinite in any
direction, correct?

.

Relevant Pages

  • Re: one way light speed
    ... detector at each such that the clocks can record the time of emission ... But _interpreting_ the measurement is complicated by ... The fact that rotations come back to the same orientation ... a particle would be different in two directions. ...
    (sci.physics.relativity)
  • Re: one way light speed
    ... detector at each such that the clocks can record the time of emission ... But _interpreting_ the measurement is complicated by ... The fact that rotations come back to the same orientation ... a particle would be different in two directions. ...
    (sci.physics.relativity)
  • Re: one way light speed
    ... detector at each such that the clocks can record the time of emission ... But _interpreting_ the measurement is complicated by ... The fact that rotations come back to the same orientation ... a particle would be different in two directions. ...
    (sci.physics.relativity)
  • Re: one way light speed
    ... Take two clocks separated by some distance, with a light source and detector at each such that the clocks can record the time of emission and arrival of light pulses. ... In testing a theory applied to a one-way measurement, the method of clock synchronization must be included in the analysis, and for all viable physical theories it will cancel out. ... The fact that rotations come back to the same orientation after an angle of 2pi puts rather strong constraints on how it can vary with orientation. ... a particle would be different in two directions. ...
    (sci.physics.relativity)
  • Re: An Interesting Occurrence in Circular Motion
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