*
*

*In his
1905 paper on Special Relativity, and during later years, Albert Einstein worked
out some of the main physical consequences of his theory, like length
contraction, time dilation and the much celebrated formula E=mc ^{2}.
After more than a hundred years from their formulation, these effects have all
been experimentally verified to a high degree of precision, over and over again.
As the theory was developed assuming constancy of the one-way velocity of light
(see main text and Box 1), and given that this is equivalent to setting ε=1/2
in Reichenbach’s synchronization formalism (see Box
1), it would appear that
Special Relativity success is a confirmation that Einstein’s convention is in
fact a true property of nature and that it is perfectly feasible to consider the
one-way velocity of light as a true physical quantity. Work by a renowned group
of philosophers of science, known as the conventionalists, has shown this to be
a misconception. In a paper published in 1970 John Winnie, from the University
of Hawaii, has been able to re-formulate Special Relativity without the one-way
velocity of light assumption, maintaining throughout the theory the ε
synchronization parameter as undetermined, i.e. explicitly avoiding to setting
it to ˝. The author has calculated length contraction and time dilation with
his formulation and has shown them to lead to conclusions experimentally
identical to what Einstein derived in 1905. Here we will shortly described the
time dilation case, because it is the basis for one of the most dramatic
experimental verification of Relativity, particles decay during accelerators
experiments.
*

*
*

*Certain
particles of the sub-atomic world have been observed to decay after a given time
since their creation. This is true for the neutron, for muons and many other
particles. In giant accelerators like those at SLAC in the States or CERN at
Geneva, this decay is observed on a daily basis. Normally the decay is measured
as a distance, rather than as a time interval. Consider a particle whose decay
time is Δt.
If the particle travels with speed v,
it should decay after a length d _{L}=vΔt.
According to Special Relativity, the observed decay time, Δt’, will be
different for somebody watching the particle from a measuring station. More
specifically, the following relation holds:
*

(2.1)

Because
the quantity inside square root is always smaller than 1, *Δt’* is
always greater than *Δt*. This is what is known as time dilation in
Special Relativity; a physical process takes longer to develop if observed by a
moving reference system. Thus, if Special Relativity were not true, the particle
would decay after a length *d _{L}=vΔt*. But, given that the
observer is moving relatively to the particle, the decay distance is given by
the following formula:

(2.2)

Useless to say, the decay distance measured in all experiments performed well agrees with formula (2.2). Let us now turn to the prediction using a value different from ˝ for the synchronization parameter. It can be shown that time dilation, in this case, is not given by formula (2.1) but, rather, by the following one:

(2.3)

If ˝ is
replaced for ε in formula (2.3), Special Relativity time dilation, equation
(2.1), is found, as it should be. The fact that formula (2.3) is so different
from formula (2.1) leads us to think that alternatives clocks synchronizations
are definitely ruled out by experimental results, as the decay distance (2.2)
cannot be derived from formula (2.3). But, if we use a different
synchronization, we should expect that the particle velocity, which is a one-way
velocity, is not going to be *v*, but rather *v _{ε+}*, as
derived in (1.4) (Box 1). Thus, the decay
distance is not

This result is identical with result (2.2). The experimental verification of formula (2.2) for particles decay does not rule out different clocks synchronizations or, alternatively, it is not a confirmation that one-way velocities can be unambiguously measured.

Further readings

The paper by John Winnie mentioned above is:

John A. Winnie, “*Special
Relativity without one-way velocity assumptions*”, Philosophy of Science **37**
(1970): 81-99, 223-238

This is not an easy reading for students and it is rather suggested that teachers read it first, in order to elicit its best approachable parts for their pupils.