Speed of Light
Chuck and Barbara Burger
beginning to question whether the speed of light is indeed a constant, as
Einstein speculated. This has raised several questions for me:
1. Has any new physical evidence led to these doubts?
2. Is the problem about 1) whether the speed of light may have
differed historically, in the early stages of an evolutionary
universe? or 2) whether, instead of being constant, the speed of light
actually varies constantly?
3. If the speed of light DOES vary, what are the implications for the
Special Theory of Relativity? Would the uniformity of the laws of physics
still apply? Would the theory itself need modification so as to
remain valid? COULD it remain valid?
Barbara Burger
Uncle Al
1) Lightspeed can be exceeded in the Scharnhorst effect
http://arXiv.org/abs/gr-qc/0107091
Whether this obtains in fact is unknown. Given the necessary
circumstances, there is no impact upon relativity. The evidence for
the operational validity of both General Relativity and Special
Relativity (GR with the gravitation turned off) is overwhelming - from
atomic (hydrogen-like uranium(91+)) to cosmic scales. No
counterdemonstrations exist to date,
<http://rattler.cameron.edu/EMIS/journals/LRG/Articles/Volume4/2001-4will/index.html>
2) Pull out a CRC Handbook and look at the physical constants
section. If you change lightspeed even a percent you massively impact
the universe as we know it. We can look back in time 10+ billion
years. Gravitationally and spectroscopically, lightspeed has remained
invariant within experimental error. Ditto any contemporary
fluctuations - chemistry would go ape, as would your FM radio and more
than a hundred high energy particle accelerators around the globe.
Almost everything technological would go out of calibration.
3) SR doesn't care what lightspeed is. SR only requires that it be
identical for all inertial observers.
There is no explanation for the persistence of spiral galaxies
throughout the visible history of the universe. Their visible mass
distribution does not allow for their perisistence. The big problem
in cosmology is not change, it is the large scale long time absence of
change. Tap dancing like "dark matter" is only curve fitting. There
is no experimental evidence for its existence. One would imagine an
ad hoc 90% of the universe could not hide from multiple venues of
observation.
--
Uncle Al
http://www.mazepath.com/uncleal/eotvos.htm
(Do something naughty to physics)
[Moderator's note: here's some experimental evidence for the
existence of dark matter:
DAMA collaboration, http://www.lngs.infn.it/lngs/htexts/dama/dama39.html
- jb]
Steve Carlip
> According to several articles I've read recently, a number
> of scientists are beginning to question whether the speed
> of light is indeed a constant
A couple of preliminary comments. First, it doesn't quite
make sense to talk about the speed of light changing,
because c is a dimensionful object (that is, it has units,
in this case meters/second). Such a quantity isn't really
measurable, since the universe isn't filled with standard
meter sticks to measure distances and clocks to measure
time. What's always *really* measured is a dimensionless
ratio -- even when you measure a length, you're really
measuring the ratio of that length to the length of a meter
stick. So, popular press oversimplifications to the contrary,
what some scientists are really speculating about is whether
dimensionless constant that involve the speed of light are
changing.
Such speculation is not new; Dirac, for example, wrote a
couple of famous articles about such ideas in the 1930s.
The ideas come in and out of fashion, for various reasons.
> 1. Has any new physical evidence led to these doubts?
There is an interesting set of recent observations, by a single
group of astronomers, that suggests that the fine structure
constant (a dimensionless constant that involves the speed
of light) was slightly different in the early universe. See
Webb et al., Phys. Rev. Lett. 87 (2001) 091301, preprint
astro-ph/0012539. The general reaction, I think, has been
that it's an interesting claim with no obvious mistakes, but
that it depends on both very hard measurements and very
delicate theoretical calculations, and that it will have to be
independently verified at a higher level of precision before
it's taken too seriously. A very recent preprint by another
group (Bahcall et al., astro-ph/0301507) uses a slightly
different method and sees no variation, so you can expect
the dispute to continue for a while.
There are several theoretical reason that the topic has become
popular, as well. First of all, in string theory many physical
constants are actually ``moduli,'' fields that can vary in space
and time. More generally, in theories with ``extra dimensions''
beyond the usual three spatial dimensions plus time, many
physical constants -- such as the fine structure constant --
actually depend on the sizes of the extra dimensions, and these
can evolve in time.
Second, a number of people have looked at varying speed of
light cosmologies in an effort to find alternatives to the standard
inflationary models of cosmology that don't immediately fail
observational tests. People are working on this not so much
because there's anything obviously wrong with the standard
models, but because the only way to really test a model is to
come up with some alternatives to test it against.
Third, there's been growing interest in the possibility that some
effects of quantum gravity might be observable, now or in the
not-too-distant future. A central idea is that perhaps quantum
fluctuations of spacetime lead to violations of Lorentz invariance,
which might show up, for example, as slightly different speeds
for different light frequencies. There's no really solid theoretical
argument for this, in part because there's no quantum theory
of gravity that's been developed to the point of making reliable
predictions. But there are hand-waving arguments that are at
least plausible, so lquite a few people think it's worth looking.
(There is also a mild observational anomaly that may be relevant
here, the observation of very high energy cosmic rays. The issue
is that high energy protons can interact with the cosmic microwave
background. Above the GZK (Greisen-Zatsepin-Kuzmin) cutoff
of about 5x10^19 GeV, such interactions cause protons to emit
pions and rapidly lose energy. Unless the sources of cosmic rays
are quite nearby---within the local supercluster of galaxies---we
should not observe protons above this cutoff. But we (probably) do.
There are a number of proposals for explaining these observations,
some of them fairly mundane, but one possible explanation is a
violation of Lorentz invariance.)
> 2. Is the problem about 1) whether the speed of light may have
> differed historically, in the early stages of an evolutionary
> universe? or 2) whether, instead of being constant, the speed
> of light actually varies constantly?
Both possibilities have been subjects of speculation.
> 3. If the speed of light DOES vary, what are the implications for
> the Special Theory of Relativity? [...] Would the theory itself need
> modification so as to remain valid? COULD it remain valid?
Special relativity -- and, for that matter, general relativity -- would
need modification. The modification could be very mild (for example,
the basic structure of the theory could be kept the same but the
electromagnetic interaction could be changed slightly) or quite
drastic. An enormous range of possibilities is under consideration.
At the moment, there's no clearly preferred candidate; you'd be
hard-pressed to find any physicist who advocates *any* model very
strongly.
Steve Carlip
Steve Carlip
> of scientists are beginning to question whether the speed
A couple of preliminary comments. First, it doesn't quite
make sense to talk about the speed of light changing,
because c is a dimensionful object (that is, it has units,
in this case meters/second). Such a quantity isn't really
measurable, since the universe isn't filled with standard
meter sticks to measure distances and clocks to measure
time. What's always *really* measured is a dimensionless
ratio -- even when you measure a length, you're really
measuring the ratio of that length to a meter stick. So,
popular press oversimplifications to the contrary, what
some scientists are realy speculating about is whether
dimensionless constant that involve the speed of light are
changing.
Such speculation is not new; Dirac, for example, wrote a
couple of famous articles about such ideas in the 1930s.
The ideas come in and out of fashion for various reasons.
> 1. Has any new physical evidence led to these doubts?
There is an interesting set of recent observations, by a single
group of astronomers, that suggests that the fine structure
constant (a dimensionless constant that involves the speed
of light) was slightly different in the early universe. (See
Webb et al., Phys. Rev. Lett. 87 (2001) 091301, preprint
astro-ph/0012539.) The general reaction, I think, has been
that it's an interesting claim with no obvious mistakes, but
that it depends on both very hard measurements and very
delicate theoretical calculations, and that it will have to be
independently verified at a higher level of precision before
it's taken too seriously. A very recent preprint by another
group (Bahcall et al., astro-ph/0301507) uses a different
method and sees no variation, so you can expect continued
dispute.
John Devers
news:<b209r0$i18$1...@panther.uwo.ca>...
> According to several articles I've read recently, a number of scientists are
> beginning to question whether the speed of light is indeed a constant,
Scientists are always questioning the models their work has produced.
Current observations tell us that the speed of light is constant in
all directions.
Lorentz Violations? Not Yet
http://www.aip.org/enews/physnews/2003/split/623-2.html
Number 623 #2, February 5,2003 by Phil Schewe, James Riordon, and Ben
Stein
Lorentz invariance, the idea that the result of a physics experiment
should stay the same whether the apparatus is motionless or traveling
at some great constant speed relative to a reference point, is taken
for granted in the theory of special relativity. Yet in recent years
some scientists have come to question this pillar of physics, and to
suggest theoretical models (called "standard model extensions," or
SMEs ) incorporating Lorentz violations and experimental ways of
settling the matter (see Update 578
http://www.aip.org/enews/physnews/2002/split/578-2.html, ).
In these models, the speed of light is not universal but will have
extra terms dependent on the speed or orientation of the apparatus
(see http://media4.physics.indiana.edu/~kostelec/faq.html
http://media4.physics.indiana.edu/%7Ekostelec/faq.html).
Even before the advent of Einstein's relativity, the Michelson-Morley
experiment tried to perceive (unsuccessfully) a difference in the
speed of light when the Earth was traveling in two different
directions in space while on opposite sides of its orbit around the
sun. Now scientists have to be more subtle in their approach.
In one new laboratory experiment, just completed by Stanford
physicists (John Lipa ,) microwaves in two resonant cavities (one
oriented east-west, the other pointing vertically) are monitored as
the Earth sweeps around the sun. Any orientation- or speed-dependent
changes in the speed of light would alter the resonant conditions of
the cavities in a measurable way. The geometry of the experiment gives
it optimal sensitivity to a number of coefficients in a generalized
SME. The Stanford group sees no such anisotropy at the level of 10-13
for velocity-independent terms, and at the 10-9 level for
velocity-dependent terms. (Lipa et al., Physical Review Letters,
upcoming article)
K. Wiersema
> According to several articles I've read recently, a number of scientists are
> beginning to question whether the speed of light is indeed a constant, as
> Einstein speculated. This has raised several questions for me:
>
> 1. Has any new physical evidence led to these doubts?
>
> 2. Is the problem about 1) whether the speed of light may have
> differed historically, in the early stages of an evolutionary
> universe? or 2) whether, instead of being constant, the speed of light
> actually varies constantly?
There is a (very persistent) group of people that uses some kind of a
variable lightspeed model to combine the Bible's creation-story with
cosmology. Setterfield is the most prominent of them. They (initially)
used a series of measurements of the speed of light from the 18th
century up to now, and claimed that the speed of light was
signifacantly decreasing in that period. In their "model", the age of
the universe is 10.000 years, which corresponds nicely with the Bible,
see e.g. http://www.setterfield.org/scipubl.html They do no take into
account the measurements of the last 50 years or so (that show no
significant change). Measurements of SN1987A also show that the speed
of light has not changed during the last 170.000 years or so. I
suppose their crackpot-index score is enormous. There is also a
serious debate on the variability of the speed of light, and the
possible implications, but I'm not an expert on that.
Klaas Wiersema
"What is the purpose of meaning?
It's quite easy once you understand
that space-time is cu-u-urved"
-- DeeDee, Dexters Laboratory
[Moderator's note: there is also recent interest in variable
speeds of light over much longer timescales, on the part of
people with much lower crackpot indices; let's focus on that. - jb]
Eugen Winkler
A causality chain requires an information gradient.
Information transmission speed experiments should take care
that there is definitely an information gradient. In this case the
distance per wavelength ratio is quite important.
Otherwise you could get fooled by the nearfield without
information gradient. I would suggest that in the quantum
range the gradient should be at least one bit per distance,
this could be i.e. the information carried by a photon to flip
a quantum spin. Regarding the necessary photon momentum
it's Heisenbergs condition.
The wavelength dependency has partially been discussed in
http://arXiv.org/abs/gr-qc/0107091, where it is shown that
"Low frequency waves seem to traverse such barriers in zero
time".
Eugen Winkler
Stephen Speicher
On 7 Feb 2003, Chuck and Barbara Burger wrote:
> Is the problem about 1) whether the speed of light may have
> differed historically, in the early stages of an evolutionary
> universe? or 2) whether, instead of being constant, the speed of light
> actually varies constantly?
Actually, both 1 and 2.
As to 1): Several years ago there were experimental observations
which were taken to suggest that the fine structure constant may
be varying over time. (Note that such data remains rather
controversial.) Since the fine structure constant is proportional
to the square of the electric charge, and inversely proportional
to the speed of light, it was surmised that these two constants
were candidates for the variance, rather than the remaining one,
Planck's constant.
In a paper last summer, P.A.W. Davies et al., "Black holes
constrain varying constants," _Nature_, V. 418, pp. 602-603, 8
August 2002, the authors made an argument suggesting that black
hole thermodynamics may provide a criterion to constrain these
candidates as "varying" constants. In particular, the argument
they made favored a change in the speed of light, rather than a
change in the electric charge. However, in a recent short note,
S. Carlip and S. Vaidya, "Do black holes constrain varying
constants," _Nature_, V. 421, p. 498, 30 January 2003, the
authors show that "when the entire thermal environment of a black
hole is considered, no such conclusion [decreasing c] can be
drawn. Basically the authors show that, contrary to the Davies et
al. analysis, such black hole thermodynamics places no
restriction against an increasing electric charge.
So, as a summary for "1)", I would say that there is highly
controversial evidence as to the variability of the fine
structure constant, and, though it was initially thought that
such evidence would favor a varying speed of light, such a
conclusion is no longer warranted.
As to 2): Most all varying speed of light theories have been
motivated for solution of cosmological problems, and in the
typical theory both covariance and Lorentz invariance is broken.
Lorentz invariance follows directly from the principle of
relativity and the constancy of the speed of light. One can
maintain the indistinguishability of inertial frames, but a
varying speed of light leads to what is known as Fock-Lorentz
symmetry. See Fock's original work in "The Theory of Space-Time
and Gravitation," _Pergamon_, 1964, or more recently Bruce A.
Bassett et al., "Geometrodynamics of variable-speed-of-light
cosmologies," _Physical Review D_, Volume 62, 103518, 15 November
2000.
It is this breaking of Lorentz invariance which makes these varying
speed of light theories so difficult to apply to problems such as
black holes. It is interesting to note Bassett et al. develop a
Lorentz symmetry which is broken in a "soft" manner, analogous to
spontaneous symmetry breaking in particle physics. However,
there is some recent work by J. Magueijo who has developed a
varying speed of light approach in which it is claimed to be both
generally covariant and locally Lorentz invariant, and he does
apply this new concept to black holes. See J. Magueijo, "Stars
and black holes in varying speed of light theories," _Physical
Review D_, Volume 63, 043502, 18 January 2001.
Note that Magueijo has recently written an interesting but
somewhat irrevant book geared towards a general audience, on
exactly this subject. Joao Magueijo, "Faster Than the Speed of
Light: The Story of a Scientific Speculation," _Perseus
Publishing_, 2003.
--=20
Stephen
s...@speicher.com
Ignorance is just a placeholder for knowledge.
Printed using 100% recycled electrons.
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