Re: Basic Unification Question

>> > I know that the electromagnetic, strong, and weak forces have been
>> > unified. Is it true that all "forces" except gravity fall under this
>> > theory?
>>
>> Let us not be hasty. The common wisdom is that the electromagnetic and
>> weak forces have been unified into the so-called electroweak theory.
>> This is usually said because when electroweak theory is written in a
>> standard form (the Yang-Mills form), the the individual components of
>> the electroweak force field do not correspond separately to the
>> electromagnetic and weak forces, these are mixed together. However, in
>> this sense, the strong force still stands apart.
>>
>> There are attempts to construct grand unified theories (GUTs) that
>> unify the electroweak and strong forces. However, AFAIK, none of them
>> have been completely successful.
>
> Let me play devil's advocate and ask what is the motivation that the
> fundamental forces (strong, weak, electromagnetism and gravity) MUST be
> unified? Yes, there is history: electricity and magnetism were unified
> to electromagnetism, and electromagnetism was unified with the weak
> force by Salam, Weinberg and Glashow. However, this doesn't necessarily
> imply that ALL forces must be unified via a similar scheme. (The
> unification of electricity and magnetism is also quite different than
> the unification of electromagnetism with the weak force.)

Phillip raises a question which is at the heart of scientific theory.
History gives us experiential reasons to be hopeful that all of physics may
in fact be unified, but there is no denying that there is also a
philosophical and if I might add even a religious conviction in the unity of
nature which drives the quest for unification. Of course, science validates
in hindsight with rigorous demands, but it is fundamentally a belief in the
unity of nature which drive the scientists who attempt unification toward
their quarry, and you either have that belief or you don't.

Unifications have taken many forms over history, and I will second what
Phillip says by saying that a good way to end up on the wrong track pursuing
future unifications is to assume that they will look like the past ones.

For example, Newton unified terrestrial gravitation with the motions of
planetary bodies by concluding, correctly in today's estimation, that the
same phenomenon of nature (which he then called action at a distance) which
were responsible for the dropping of matter to earth was also responsible
for the movement of planets. Einstein firmed up the connection by relating
both to geometry.

Benjamin Franklin, who was the senior founding father and gave credibility
in Europe to his younger protégés such as Washington, Adams and Jefferson
during the American revolution, had around 1750 gained international acclaim
by unifying many different manifestations of electricity, into a single
phenomenon. He hypothesized that the same static electricity which was
observed terrestrially was also responsible for lightning. And, based on
his terrestrial observations that static electricity was better drawn to a
sharp metallic surface than a blunt one, proposed that lightning rods would
attract lightning in lieu of the lightening striking other things. People
followed his recommendation and gave up the superstitious habit of clanging
big metallic church bells beneath tall steeples to ward off lightning which
had killed hundreds of bell-ringers throughout Europe, in favor of
installing lightning rods which ended the foolish epidemic of electrocution
and made him an overnight worldwide sensation. He also made a statement
that remains one of history's most amusing and profound wrong conclusions,
fretting that electricity was a curious thing but that it would never be of
any use for anything.

Maxwell of course, engineered perhaps the most audacious unification to his
time, when he put together the separate works of Faraday, Gauss and Ampere
into his famous equations. That may have been the first time that people
started to really think about "unification qua unification" in a serious
way.

Special relativity unifies space with time in relation to electric /
magnetic unification of Maxwell. More to the point, Einstein found that
neither space nor time was absolute but rather that they could be
transformed into one another following Lorentz. In this way, it became
understood that the transformation of electric fields into magnetic fields
was merely a consequence of relative motion and could be related to
transformations between spatial measurements and time measurements.

Electroweak unity, as I mentioned the other day, and as Igor pointed out in
different words, entails a mixing of interactions via the weak mixing angle.
But, it is a partial unification on its own terms, and that which makes it
partial on its own terms is probably the fact that it does exclude strong
interactions. That is, one would be wise to suspect, based on the features
if not the specific details of e.g., SU(5), that once strong interactions
are included, those things which make electroweak theory only a unification
"to a point" will make it complete. And, as electroweak interactions do in
some sense unify isospin up with isospin down phenomenology, that once
strong interactions are included, there will be a further unification of
quark and lepton phenomenology. Of course, as soon as quarks and leptons
are related, then one will expect some form of proton decay. Obviously, the
10^15 GeV energies predicted by SU(5) for this are still too low and have
been experimentally ruled out. One might be wise to suspect that the Planck
mass itself ~ 10^19 may even turn out to be the energy of this unification.
To my knowledge, proton decay tied to this higher energy level has not been
ruled out to date. If I were to place my bets, I'd bet that the theory
which turns out to correctly describe nature will predict an electroweak /
strong and quark / lepton unification energy that just so happens to be the
Planck mass itself, which is to say, it will predict the Newton coupling out
of the strong, weak and electromagnetic interactions and the quarks and
leptons which it attempts to unify at the particle level. And in so doing,
it will bring us to the back door of gravitation by predicting the ratio of
the Newton to the Fermi coupling constant from particle physics. Such
prediction of this ratio, of course, would be another form of unification,
because then only one, not both of these constants would then be needed as
matter of a first principles.

>
> Going back further in history, there were several types of "action at a
> distance": electricity, magnetism, gravity but also the sense of smell:
> the steak in the kitchen is burning and I notice it in the attic.
> However, the sense of smell has not been unified with the other forces
> in any real sense. (Of course, it ultimately depends on biochemistry,
> which ultimately depends on electromagnetism, but that is stretching the
> definition of unification.) Perhaps gravity today is what the sense of
> smell was 200 years ago: it really is something qualitatively different
> and might not be capable of unification with the other forces at all.
>
> This is not to say that some sort of quantum gravity is unnecessary; the
> question is whether the route to this is via traditional unification
> schemes.

I think it fair to suspect, without of course knowing the answers that
future generations may know, that the unification of gravitation and quantum
theory will clearly NOT be "via traditional unification schemes," that is,
as I said the other day, gravitation is a whole other animal. In fact, each
of the unifications I pointed out above, as well as those pointed out by
Phillip, has a different cast, and we are probably not going to be fruitful
if we expect the next unification to be analogous to the last one. In fact,
there are so many smart people who have tried to push the known unifications
to new areas and have failed and would have succeeded if success were
destined to be found on that path, that we really do have to try to think
differently.

>
> Something else: the strength of forces is often plotted as a function of
> energy. The idea is that at some unification energy the strengths will
> be the same. However, apart from the expectation of unification, is
> there any reason to expect all the curves to intersect at ONE point?
> If, experimentally (ignoring for now the practicality of such an
> experiment), the curves were found NOT to intersect at one point, what
> would this mean for traditional unification ideas?
>
> Is there any fundamental reason why gravity, say, could not be something
> completely different from the other forces?

Again, gravity is a whole other animal. BUT, now we come to the philosophy
of science. History gives us confidence in the unification programme since
we have the experience of various "pieces" of science having been unified
already. Philosophy and religion give us the conviction that at the end of
the road all of the pieces will be shown to be unified. History pushes, and
philosophy and religion pull, our scientific progress. One cannot prove the
article of faith that all of science can be unified except in the retrospect
of success; nor can one ever unify all of science in a provable way without
having and being driven by that article of faith to begin with.

One question is this: Is it possible that nature really boils down to two
distinct and totally separate phenomena -- gravitation and everything
else -- unconnected to one another except where aggregates are concerned
(i.e., via correspondence principles)? Or, will we one day see that quantum
mechanics and gravitation are in some way consistent, and even more, that in
some way we don't yet understand, one actually implies the other? I'd put
my bets on the latter.

>
> Since the electroweak unification, has any GUT or similar effort at more
> unification made a testable prediction which has turned out to be true?
> (As Igor noted, some have turned out to be false, like proton decay.)

Not to my knowledge. At least, nothing yet recognized.

The oldest unanswered question in all of physics is WHY we observe electric
but not magnetic charges at low energies, i.e., why electric / magnetic
duality symmetry appears to be broken in nature. In different. more modern
terms, the oldest question in physics is whether electric / magnetic duality
exists at high energies and comes to be broken only at low energies, and if
so, how that happens. This became a question as soon as Maxwell's equations
were formulated. It has still not been answered more than 130 years after
Maxwell. It predates all of 20th century physics, and was more or less
dropped from the mainstream scientific agenda in the rush of events that
began in the first decade of the 20th century with Planck and Einstein. And
it remains as a question to this date. It may well be that this
long-ignored question must finally be answered for unification of the
non-gravitational interactions to proceed to the next step.

My paper at http://arxiv.org/abs/hep-ph/0508257 tackles precisely this
question, and lays the foundation for then showing in
http://arxiv.org/abs/hep-ph/0509223 that the NuTeV anomaly may be the first
experimental evidence we have of the existence of magnetic charges and
duality symmetry in nature, but only at very high energies. In the next few
years, with the TeV accelerators ramping up, it WILL become possible for
nature herself to confirm or disprove what is in these two papers. Let
nature be the arbiter!

Jay.

.

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