Oui, ça me paraît évident, ça.Envoyé par Franc84
Peut-être mais Poincaré a dit clairement qu'une longueur n'était plus autre chose que la durée mise par la lumière à la parcourir (alors qu'Einstein s'est accroché à ses barres rigides).
Voici pourquoi la vitesse one-way ne peut être mesurée qu'après avoir établi arbitrairement une synchronisation :
http://www.spacetimecentre.org/vpetk...%20Reality.pdf
To see why the one-way velocity of light cannot be determined experimentally
assume that we are trying to do just that—to measure the velocity of light from
a point A to another point B. To do that we obviously need to know the distance
between A and B and the time for which light propagates from A to B. In order to
measure that time the clocks at A and B should show the same readings simultaneously,
i.e. they should be synchronized. But how can that be done? One can use
two methods to synchronize the clocks at A and B. The first is to send a light (or
any other) signal from A to B whose one-way velocity is known. Hence we arrive
at a vicious circle—to determine the one-way velocity of light propagating from
A to B the clocks at these points should be synchronized, but to synchronize the
clocks the one-velocity of light should be known beforehand.
The second method to synchronize the clocks at A and B is the so called slow
transport of a third clock C from A to B—the C-clock is initially synchronized with
the A-clock and then slowly transported to point B where the B-clock is synchronized
with the third clock. It is called “slow transport” to imply that the time
dilation that the C-clock undergoes should be neglected. However, neglecting it
would mean missing the whole point in the synchronization of distant clocks by
a third clock. No matter how small the time dilation might be, if we attempt to
calculate it we arrive at the same vicious circle as in the case of the first method:
to determine the magnitude of time dilation in order to synchronize the A and B
clocks we should know the one-way velocity of the C-clock, but to measure that
velocity the A and B clocks should be synchronized in advance.
One might object that in the case of the second method of synchronizing two
distant clocks the vicious circle can be avoided if an observer at rest in clock C’s
reference frame1 uses the C-clock itself to measure the time of its journey from A
to B, not the clocks at A and B. Then by knowing the distance between A and B
one can calculate the one-way velocity of C. I believe the problem with this objection
is obvious—the distance between A and B is relativistically contracted for the
observer in C’s reference frame. In order to determine the magnitude of the length
contraction the one-way velocity of C should be known and we again arrive at the
vicious circle.
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