This is the third of three articles posted to sci.physics.relativity:
[1] Subject: Theories Equivalent to SR
[2] Subject: Why the Ether is Unobservable
[3] Subject: Why the Ether is Not Part of Modern Physics
These articles should be read in order, as a set; they do not stand
alone from each other.
In the first two articles of this series I have shown that there is an
infinite class of ether theories each of which is equivalent to SR, in
the sense that they are all experimentally indistinguishable from SR.
Why then, are these theories not given equal weight with SR in the
teaching of modern physics? Why are these ether theories, and indeed
this entire equivalence class of theories, not well known and familiar
to most physicists? What justification is there for the mainstream of
physics to completely ignore these theories which are validated by
experiments to _exactly_ the same extent as is SR?
The answer to all these questions can be summed up in a single word:
Symmetry. This article is devoted to discussing why the symmetry aspects
of SR completely overshadow all of these ether theories in modern physics.
Symmetry in SR is a rich and varied topic. The basic symmetry of SR is
Lorentz invariance, and the essence of SR is encapsulated in the
statement that the laws of physics are locally Lorentz invariant (i.e.
unchanged under the operation of any member of the Lorentz group). This
is an instance of the modern approach to symmetries: a symmetry principle
states that something remains unchanged when a specific type of operation
is performed. Note that Einstein's original two postulates for SR are
both symmetry principles.
Einstein was instrumental in bringing the importance of symmetries to
the forefront of modern physics, and SR is an excellent example of the
power of symmetry groups in determining the possible structure of
physical laws: considerations of group theory alone plus the simple
observation that pion beams exist are sufficient to derive the
equations of SR. In addition, an assumption of Lorentz symmetry and
the guess that electrodynamics is the simplest possible gauge theory
is enough to derive the Maxwell's equations. Symmetry principles are a
very powerful (nay indispensable) tool in modern theoretical physics.
And none of the ether theories contain such a symmetry as a fundamental
part of the theory (LET has an "accidental" Lorentz symmetry, but it is
not a principle of the theory). It is highly doubtful that any of the
modern theories of physics would have been discovered without the
symmetry principles of SR leading the way -- modern gauge theories are
direct descendants of the geometrical description of SR; this includes
both GR and the Standard Model. Such a geometrical description is not
possible in any ether theory (geometry is inherently coordinate
independent, but the ether is not).
There are several auxiliary reasons why the ether is not part of modern
physics. These are essentially confirmations of the basic reason due to
the lack of symmetry in ether theories -- in most cases these are direct
consequences of that lack.
1) In every viable ether theory, the ether itself is unobservable [2].
Quantum theory and the experiments related to it have repeatedly
pounded home the lesson that one must not attempt to describe things
which have not been observed or measured. This is a second powerful
argument against the believability of ether theories.
2) This unobservability of the ether frame borders on a reductio ad
absurdum in math, as the ether is assumed to be unique, but its
unobservability makes it merely one member of the equivalence class
of inertial frames -- _any_ inertial frame can be assumed to be the
ether frame without changing the predictions of any viable ether
theory. And since the ether frame is intrinsically unobservable,
assumptions and guesses are all the ether advocate has.
3) The assumption of a unique ether frame is directly analogous to the
assumption that there is a preferred frame in a Euclidean space. It
is absurd to claim that there _is_ a preferred frame in Euclidean
space but it is unobservable. But that's essentially what the viable
ether theories do.
4) In every viable ether theory one's measurement tools must change in
an unobservable manner if one is moving wrt the ether. This seems
both counterintuitive and strange -- it's as if these effects were
diabolically constructed simply to make the viable ether theories
indistinguishable from SR. As ether theories are a clear attempt to
preserve an older, seemingly "common sense" approach to physics, it
seems unreasonable to have tools change, because there is no
precedent for such behavior in our everyday lives, or in older
physical theories -- this is very much not "common sense".
5) In every viable ether theory except LET, the one-way speed of light
differs from c, but is unmeasurable. It seems strange that slow clock
transport does not give any method to measure the one-way speed of
light (such approaches always measure c, not the "true" one-way speed
of light). Indeed, _ALL_ one-way speeds are subject to this, and
rulers and clocks cannot measure the "true" one-way speed of
_anything_ in a natural way; this includes police radar guns and your
automobile on the highway; the redshifts of distant galaxies do not
correspond to their "true" velocities; etc. This is decidedly not
"common sense".
6) Ether theories require a new postulate for every new phenomenon that
is discovered, which basically states that the ether applies to it
in the same ways the ether applies to elecromagnetism. SR's symmetry
principles automatically apply to new phenomena, so SR has more
explanatory power than ether theory in this regard. Note that all
phenomena discovered since 1905 do indeed exhibit the local Lorentz
invariance of SR -- what is happenstance in ether theory was directly
predicted by SR. It seems surprising that the "gravitational ether"
has exactly the same underlying properties as the "lumeniferous
ether" and the "weak interaction ether" and the "strong interaction
ether", when these interactions differ so enormously in strength
(by a factor of more than 10^40), and differ so wildly in their
properties (e.g. gravitation and electrodynamics lack the rich
spectrum of particle and resonances characteristic of the others).
But SR naturally and correctly predicts the local Lorentz invariance
of these vastly different phenomena.
In summary, there are good reasons for the ether to be absent from modern
physics; virtually all modern physicists consider these reasons both
cogent and sufficient (at least those modern physicists who have actively
considered the issue), and no ether theory is part of modern physics.
While the viable ether theories are equivalent to SR in the sense that
they are experimentally indistinguishable, they are most definitely NOT
equivalent to SR in either mathematical elegance, explanatory power, or
suitability as a starting point for further theories. But it is these
latter properties which are most important for the basic theories of
physics.
Tom Roberts tjrobe...@lucent.com
