Proposals to measure speed of gravity?
upt...@my-deja.com
theory is that gravity propagates through space at the speed of light,
although there are some theories (mostly crackpot, from what I've seen,
but a few are plausible) that still have gravity acting
instantaneously. I haven't heard of any other proposed velocities, but
that's not the point.
The point is, that although I have been thinking of c as the speed of
gravity's propagation through space, I still think it needs to be
checked, which, to my knowledge, it hasn't. As they used to say here in
the U.S. about dealing with the Soviet Union, "Trust, but verify".
I've been thinking about this for some time but have yet to come up
with a suitable experiment. Naturally, the task is made difficult by
the weakness of gravity as a force, and I'm not sure we yet have
sensitive enough instruments to do the test in a laboratory. But what
of astronomy? Can we, for instance, determine the gravitational
attraction of Jupiter on the Earth to sufficient precision to see if
its direction is the same direction we see it in? After all, we see
Jupiter not where it is, but where it was a few minutes ago, when the
light we're seeing now left old Jove. If gravity travels at c, where we
see Jupiter and the direction of gravitational attraction should line
up. If not, physicists will have something new to keep them busy.
Anyway, if anyone has any ideas on how to measure gravity's
propagational speed, or comments, please respond. Thanks.
Regards to all
"Upthink"
Sent via Deja.com http://www.deja.com/
Before you buy.
Nathan Urban
> After all, we see
> Jupiter not where it is, but where it was a few minutes ago, when the
> light we're seeing now left old Jove. If gravity travels at c, where we
> see Jupiter and the direction of gravitational attraction should line
> up.
Actually, they shouldn't; the "force of gravity" should line up with the
extrapolated position ("where it is now") and not the retarded position
("where it was at time x/c ago") even if gravitational effects propagate
at c. See the FAQ:
http://www.corepower.com/~relfaq/grav_speed.html
As to your question, no, the speed of gravity has not been measured
directly. It's hard to do (partially for the above reason). There is
indirect evidence though, also mentioned in the FAQ.
gamemaster
nur...@vt.edu wrote:
> In article <7uk4je$t9i$1...@nnrp1.deja.com>, upt...@my-deja.com wrote:
>
> > After all, we see
> > Jupiter not where it is, but where it was a few minutes ago, when
the
---snip---
>
> As to your question, no, the speed of gravity has not been measured
> directly. It's hard to do (partially for the above reason). There is
> indirect evidence though, also mentioned in the FAQ.
>
What about the Walker-Dual experiments? I understand that they are
controversial (OF COURSE!) but they claim that these experiments
demonstarate FTL propagation of "changes in the gravitational force"
(as opposed to "gravity waves", which are a different phenomenon).
Does anyone know of any plans to duplicate these experiments?
Nathan Urban
> and here I always thought graviton particles were convergencies of neutrons...
Why did you think that?
DanielTCat
convergencies of neutrons....I will research this for you...
Charles Francis
>Newton thought that gravity acted instantaneously.
Newton was never so rash. He thought:
That one body may act upon another at a distance through a vacuum
without the mediation of any thing else, by and through which their
action and force may be conveyed from one to the other, is to me so
great an absurdity, that I believe no man, who has in philosophical
matters a competent faculty of thinking, can ever fall into it.
If Einstein did not like the Copenhagen interpretation of quantum
mechanics, one dreads to think of the contempt in which Newton would
have held it.
--
Charles Francis
cha...@clef.demon.co.uk
Joe Fischer
: Newton thought that gravity acted instantaneously. Nowadays, accepted
: theory is that gravity propagates through space at the speed of light,
: although there are some theories (mostly crackpot, from what I've seen,
: but a few are plausible) that still have gravity acting
: instantaneously. I haven't heard of any other proposed velocities, but
: that's not the point.
There is little thought that gravity propagates
at all, it is only gravitational radiation that moves at c.
The problem is in people assuming things that are
not true.
: The point is, that although I have been thinking of c as the speed of
: gravity's propagation through space, I still think it needs to be
: checked, which, to my knowledge, it hasn't. As they used to say here in
: the U.S. about dealing with the Soviet Union, "Trust, but verify".
What is to propagate? Use Newtonian gravitation,
but don't believe it is correct.
: I've been thinking about this for some time but have yet to come up
: with a suitable experiment. Naturally, the task is made difficult by
: the weakness of gravity as a force,
Which it is not, inertial motion is freefall
and non accelerated, and forces are not needed.
: and I'm not sure we yet have
: sensitive enough instruments to do the test in a laboratory.
You can't measure what doesn't exist.
: But what
: of astronomy? Can we, for instance, determine the gravitational
: attraction of Jupiter on the Earth to sufficient precision to see if
: its direction is the same direction we see it in?
Yes, and it isn't.
: After all, we see
: Jupiter not where it is, but where it was a few minutes ago, when the
: light we're seeing now left old Jove. If gravity travels at c, where we
: see Jupiter and the direction of gravitational attraction should line
: up. If not, physicists will have something new to keep them busy.
Apparently the problem is only in your mind.
: Anyway, if anyone has any ideas on how to measure gravity's
: propagational speed, or comments, please respond. Thanks.
Nothing propagates in gravity, so find some
other confusing problem to solve.
Joe Fischer
Tom Roberts
> What about the Walker-Dual experiments?
Have you a reference for their _RESULTS_? I have only a preprint which
is essentially an experimental proposal.
I suspect that a careful computation in GR will predict values consistent
with their experiment. As is well known, in GR the aberration of the
"gravitational force" is essentially zero, because of velocity- and
acceleration- dependent terms (it is not exactly zero due to
gravitational radiation, but in a terrestrial experiment that will be
unmeasurably small).
Interpretaing measurements of aberration as "propagation speed" depends
_strongly_ on one's theoretical assumptions. Remember that in classical
electrodynamics a uniformly-moving charge has no aberration in the force
it exerts on a charge at rest, due to velocity-dependent terms. For
gravitation in GR there are also acceleration-dependent terms. Only
if one assumes a _purely_central_ force with no velocity- or
acceleration-dependent terms does lack of aberration imply "infinite
propagation speed"; such an assumption gives the WRONG ANSWER for both
E&M (Maxwell's equations) and gravitation (GR).
Steve Carlip has recently posted several articles on this to
sci.physics.relativity. He also has a recent preprint about it:
http://xxx.lanl.gov/abs/gr-qc/9909087
Tom Roberts tjro...@lucent.com
z@z
| upthink wrote:
| > Newton thought that gravity acted instantaneously.
|
| Newton was never so rash. He thought:
|
| That one body may act upon another at a distance through a vacuum
| without the mediation of any thing else, by and through which their
| action and force may be conveyed from one to the other, is to me so
| great an absurdity, that I believe no man, who has in philosophical
| matters a competent faculty of thinking, can ever fall into it.
This prejudice of Newton is responsible for the fact that actions
at a distance were forcefully eliminated from physics at the end
of the last and at the beginning of this century.
Already Occam, the first consequent advocate of Galilean relativity
I know of, had developed three centuries before Newton a much sounder
and more modern epistemology according to which actions at a distance
are a completely reasonable assumption.
We must judge hypotheses and theories only by verifiable consequences
and predictions and never by metaphysical claims such as Newton's
prejudice concerning actions at a distance !!!
Kepler accepted instantanous effects over huge distances. This
allowed him not only to assume that the moon affects the tides
but also to postulate a universal gravitational force. Newton's
precursor Galilei ridiculed Kepler's notion that the moon affects
the tides, because the assumption that particles from the moon
come to the earth and affect the tides is really a bit strange.
One should also take into account that (the successful part of)
Newton's theory of gravitation is based on Kepler and that the
assumption that gravity propagates at c would have completely
spoiled the theory.
The faq concerning the problem of finite propagation speed
of gravitity shows once more very elegantly that modern physics
is completely unfalsifiable:
"This cancellation may seem less strange if one notes that a
similar effect occurs in electromagnetism. If a charged particle
is moving at a constant velocity, it exerts a force that points
toward its present position, not its retarded position, even
though electromagnetic interactions certainly move at the speed
of light. HERE, AS IN GENERAL RELATIVITY, SUBTLETIES IN THE
NATURE OF THE INTERACTION "CONSPIRE" TO DISGUISE THE EFFECT OF
PROGAGATION DELAY. ... [emphasis mine]
Since this point can be confusing, it's worth exploring a little
further, in a slightly more technical manner. Consider two bodies
-- call them A and B -- held in orbit by either electrical or
gravitational attraction. As long as the force on A points
directly towards B and vice versa, a stable orbit is possible. If
the force on A points instead towards the retarded (propagation-
time-delayed) position of B, on the other hand, the effect is to
add a new component of force in the direction of A's motion,
causing instability of the orbit. This instability, in turn,
leads to a change in the mechanical angular momentum of the A-B
system. But total angular momentum is conserved, so this change
can only occur if some of the angular momentum of the A-B system
is carried away by electromagnetic or gravitational radiation."
http://www.corepower.com/~relfaq/grav_speed.html
These reasonings can be considered either a refutation of the
the theory or a proof of gravitational radiation which is
assumed to exactly compensate the effects resulting from the
finite propagation speed of gravity, so that the result is the
same as in the case of instantanous propagation.
Momentum conservation without actions at distance is almost
impossible. A lot of faith is needed for believing that in all
possible situations secundary effects emerge cancelling exactly
out the violations of momentum conservation resulting from the
primary effects.
| If Einstein did not like the Copenhagen interpretation of quantum
| mechanics, one dreads to think of the contempt in which Newton would
| have held it.
That Einstein did not like QM actions at distance is easy to
understand: if actions at a distance are possible, then many
reasonings which have led him and others to relativity lose
their logical necessity.
Einstein wrote in 1920:
"In setting up the special relativity, the following ... idea
about Faraday's electromagnetic induction played a guiding
role. According to Faraday, relative motion of a magnet and
a closed electric circuit induces an electric current in the
latter. Whether the magnet is moved or the conductor doesn't
matter; only the relative motion is significant. ... The
phenomena of electromagnetic induction ... compelled me to
postulate the principle of (special) relativity."
(Collected Papers of A.E., Volume 2, p.262)
If even such (IMO absurd) EPR actions at a distance could exist
then also 'common sense' actions at distance should be possible
and Einstein's above reasoning becomes almost meaningless,
because the explanation of electromagnetic induction by actions
at a distance is much simpler and much more consistent than the
SR explanation.
Wolfgang Gottfied G.
Relationality instead of Relativity:
http://members.lol.li/twostone/E/physics1.html
DJMenCk
>
>Newton thought that gravity acted instantaneously.
Dennis: Untrue. Netwon most certainly did not believe that--and in fact mocked
action-at-a-distance.
bilge
>If even such (IMO absurd) EPR actions at a distance could exist
>then also 'common sense' actions at distance should be possible
>and Einstein's above reasoning becomes almost meaningless,
>because the explanation of electromagnetic induction by actions
>at a distance is much simpler and much more consistent than the
>SR explanation.
>
Here's a way to try and compare the speed of gravity to the
speed of light. Unfortunately, it's also expensive despite
the fact that I dont have a cheaper method and you'll need to
handle a few details. A few years back, CERN reported that
they were able to to track the gravitational pull of the moon
by the amount it stretched the accelerator ring and consequently
the resonance point. You should also be able to track the
position of the moon optically. Wasnt a mirror left there?
At the point of closest approach, if the speed of gravity
were infinite, you should see the accelerator resonance shift
back a bit before the optical path. Obviously, this is a
precision measurement, and only if you can be assured of some
relationship between the optical path length and the tidal
forces. I personally think gravity and light travel at c,
so I leave the feasibility study to the doubters.
DanielTCat
>
>Why did you think that?
>
Cause it sounded good....
JK, Nathan...
Joe Fischer
: Unattributed wrote;
: >Newton thought that gravity acted instantaneously.
:
: Dennis: Untrue. Netwon most certainly did not believe that--and
: in fact mocked action-at-a-distance.
I don't see where "distance" was mentioned.
Gravity can act instantaneously without acting at a distance.
And I see where I owe aetherists an apology,
there is no difference between imaginary mediums and
imaginary particles and waves that are postulated to
pull or push to cause the effects of gravity.
It is only with the knowledge of the success
of the Principle of Equivalence that I can say,
"you are all alike". :-)
The experts that believe in particles or
whatever as the mechanism of gravity, acting at
long range, are no different than the aether kooks. :-)
Joe Fischer
Charles Francis
>Charles Francis wrote:
>| upthink wrote:
>
>| > Newton thought that gravity acted instantaneously.
>|
>| Newton was never so rash. He thought:
>|
>| That one body may act upon another at a distance through a vacuum
>| without the mediation of any thing else, by and through which their
>| action and force may be conveyed from one to the other, is to me so
>| great an absurdity, that I believe no man, who has in philosophical
>| matters a competent faculty of thinking, can ever fall into it.
>
>This prejudice of Newton is responsible for the fact that actions
>at a distance were forcefully eliminated from physics at the end
>of the last and at the beginning of this century.
>
>Already Occam, the first consequent advocate of Galilean relativity
>I know of, had developed three centuries before Newton a much sounder
>and more modern epistemology according to which actions at a distance
>are a completely reasonable assumption.
>
>We must judge hypotheses and theories only by verifiable consequences
>and predictions and never by metaphysical claims such as Newton's
>prejudice concerning actions at a distance !!!
It is no prejudice, but clear thinking about what is possible in a
description of matter. It is not only a principle of relativity, but
also a principle of quantum electrodynamics, which does indeed make
verifiable predictions covering every observable physical phenomena bar
strong interactions and gravity. QED already extends to electroweak
theory, and when QED is reformulated as a discrete theory of particle
interactions with no ontological space-time then it encompasses and
explains gravity too. A number of extensions to the strong interactions
are possible, but they all incorporate the same principle.
--
Charles Francis
cha...@clef.demon.co.uk
DJMenCk
>
>DJMenCk (djm...@aol.com) wrote:
>: Unattributed wrote;
>: >Newton thought that gravity acted instantaneously.
>: Dennis: Untrue. Netwon most certainly did not believe that--and
>: in fact mocked action-at-a-distance.
>
> I don't see where "distance" was mentioned.
>Gravity can act instantaneously without acting at a distance.
>
> And I see where I owe aetherists an apology,
>there is no difference between imaginary mediums and
>imaginary particles and waves that are postulated to
>pull or push to cause the effects of gravity.
D: Do you believe that the "imaginary" and invisible medium of the atmosphere
pushes objects toward vacuums?
Or have you not accepted that controversial medium theory either?
upt...@my-deja.com
>
> > After all, we see
> > Jupiter not where it is, but where it was a few minutes ago, when
the
> > light we're seeing now left old Jove. If gravity travels at c,
where we
> > see Jupiter and the direction of gravitational attraction should
line
> > up.
>
the
> extrapolated position ("where it is now") and not the retarded
position
> ("where it was at time x/c ago") even if gravitational effects
propagate
> at c. See the FAQ:
>
> http://www.corepower.com/~relfaq/grav_speed.html
>
> directly. It's hard to do (partially for the above reason). There is
> indirect evidence though, also mentioned in the FAQ.
>
The explanation given in the FAQ references the instability of orbits
if, due to retardation, gravity pointed elsewhere than the current
position of the bodies in question, but it seems to me that the force
of attraction will always point towards the other body in an orbital
situation, since the gravitational field of a body such as, say, the
sun, is essentially spherically symetrical, and is, therefor, always
oriented towards the other body no matter where in its orbit it happens
to be. That's the reason I picked Jupiter - because it and the Earth
are not in orbit around each other. I'll readily admit this example
might not work anyway because the field MAY always point towards the
extrapolated position (the explanation given in the FAQ compares
gravitation and electromagnetism, but I'm not sure analogies drawn
between them can be counted upon) but remember, what I sought in
starting this thread is something that WOULD work.
upt...@my-deja.com
starting this thread is something that WOULD work. A situation where
there is lateral rather than orbital motion does have the feature that
the gravitational interaction varies in magnitude as well as direction
over time, thus leading at least in principle to the possibility of
measurement, but I'm not sure about the limits of astronomical
precision, or if, indeed, there are any suitable astronomical phenomena
for such observations/measurements/calculations. Suggestions?
Regards,
Upthink
upt...@my-deja.com
Charles Francis <cha...@noj.unk> wrote:
> In article <7uk4je$t9i$1...@nnrp1.deja.com>, upt...@my-deja.com writes
> >Newton thought that gravity acted instantaneously.
>
>
> That one body may act upon another at a distance through a vacuum
> without the mediation of any thing else, by and through which their
> action and force may be conveyed from one to the other, is to me so
> great an absurdity, that I believe no man, who has in philosophical
> matters a competent faculty of thinking, can ever fall into it.
>
> mechanics, one dreads to think of the contempt in which Newton would
> have held it.
> Charles Francis
> cha...@clef.demon.co.uk
According to the FAQ, "In the simple Newtonian model, gravity
propagates instantaneously". The mechanism for this propagation was a
mystery, but that's not the point- the point is that a direct
measurement would be a good thing to do - so how do we go about it?
Joe Fischer
: >:
: >: Dennis: Untrue. Netwon most certainly did not believe that--and
: >
: > I don't see where "distance" was mentioned.
: >Gravity can act instantaneously without acting at a distance.
: >
: > And I see where I owe aetherists an apology,
: >there is no difference between imaginary mediums and
: >imaginary particles and waves that are postulated to
: >pull or push to cause the effects of gravity.
:
: D: Do you believe that the "imaginary" and invisible medium
: of the atmosphere pushes objects toward vacuums?
There isn't anything imaginary about the
atmosphere, it can be liquified and made visible.
: Or have you not accepted that controversial medium theory either?
You are being either very childish, very dull,
or disingenuine, and frankly I resent all postings that
support an aether until and unless there is the first
bit of evidence that one is needed, or that one exists.
If you are suggecting that gravity "pushes"
things "down", then you are both childish and dull. :-)
If anybody thinks there is such a thing as a
graviton or gravity particle or medium or wave that
pushes or pulls things down, then _they_ are also
childish and dull (or never heard of Albert Einstein
or the General Theory of Relativity).
I don't know how long you think people are
going to be interested in reading about aether,
but unless something develops that suggests an
aether exists, I think it would be good to read
a book, get a life, and talk about physics, not
100 year old illusions.
Regards,
Joe Fischer
Nathan Urban
> The explanation given in the FAQ references the instability of orbits
> if, due to retardation, gravity pointed elsewhere than the current
> position of the bodies in question, but it seems to me that the force
> of attraction will always point towards the other body in an orbital
> situation,
i.e., it will be a central force? I think you missed this part of the FAQ:
"For weak fields, though, one can describe the theory in a sort of
Newtonian language. In that case, one finds that the `force' in GR is
not quite central -- it does not point directly towards the source of
the gravitational field -- and that it depends on velocity as well as
position."
You can probably find details either in Ohanian and Ruffini or Ciufolini
and Wheeler.
> I'll readily admit this example
> might not work anyway because the field MAY always point towards the
> extrapolated position (the explanation given in the FAQ compares
> gravitation and electromagnetism, but I'm not sure analogies drawn
> between them can be counted upon)
In the weak-field linearized gravity case, the analogy is fairly strong
-- you get something that looks very similar to the Lorentz force law,
for example. Again, you can find details in the above two books.
> but I'm not sure about the limits of astronomical
> precision, or if, indeed, there are any suitable astronomical phenomena
> for such observations/measurements/calculations. Suggestions?
While I haven't sat down and worked out much of the details in these
kinds of scenarios (you might ask Steve Carlip for references), I
suspect the general results is that the propagation speed of gravity
is undetectable due to cancellations until you get down to the level
of gravitational waves. (And practically speaking, if there were some
suitable astrophysical test of the speed of gravity in the manner you're
suggesting, it probably would already have been done..)
Tom Roberts
> According to the FAQ, "In the simple Newtonian model, gravity
> propagates instantaneously". The mechanism for this propagation was a
> mystery, but that's not the point- the point is that a direct
> measurement would be a good thing to do - so how do we go about it?
There is no way to do it.
There is an equivalence class of theories which are experimentally
indistinguishable from SR. This class includes theories with wildly
differing anisotropic one-way speeds of light, but they all share the
property that the round-trip speed of light is isotropically c in
any inertial frame. Basically the need for clock synchronization
precludes one from measuring the one-way speed of light; all one
can measure is the way one chose to synchronize clocks (which was an
_arbitrary_ choice in the first place).
This applies directly to any attempt to measure the "speed of gravity",
for the same reasons it applies to measuring the "one-way speed of
light". We can measure the round-trip speed of light by using a mirror
-- but there are no "mirrors" for gravity.
Some people claim that measuring the aberration of the "force of
gravity" implies that the "speed of gravity" is infinite. That conclusion
requires that one assume that gravity is a purely central force
independent of the velocity or acceleration of the source -- assumptions
which are completely invalid in GR.
Measuring the speed of gravitational waves can be done in principle
(modulo the equivalence class mentioned above). But to date nobody has
even detected such waves, so measuring their speed is currently out of
the question.
Tom Roberts tjro...@lucent.com
DJMenCk
>
>DJMenCk (djm...@aol.com) wrote:
>: >DJMenCk (djm...@aol.com) wrote:
>: >: Unattributed wrote;
>: >: >Newton thought that gravity acted instantaneously.
>: >:
>: >: Dennis: Untrue. Netwon most certainly did not believe that--and
>: >: in fact mocked action-at-a-distance.
>: >
>: > I don't see where "distance" was mentioned.
>: >Gravity can act instantaneously without acting at a distance.
>: >
>: > And I see where I owe aetherists an apology,
>: >there is no difference between imaginary mediums and
>: >imaginary particles and waves that are postulated to
>: >pull or push to cause the effects of gravity.
>:
>: D: Do you believe that the "imaginary" and invisible medium
>: of the atmosphere pushes objects toward vacuums?
Fischer:
> There isn't anything imaginary about the
>atmosphere, it can be liquified and made visible.
Dennis: 1) Whooa, given your mindset, that liquid could be appearing from
complete empty space--just like many modern day physicists argue about various
particles and virtual particles they create in a lab. Creations of matter from
light is an example of solidifying the ether.
2) Ahhh, so you wouldn't have believed in the atmosphere until we could
liquify it? There were others arguing that it obviously existed 2000 years
ago--and these materialists really became vociferous in the 16th and 17th
century.
It seems some people are little more scientifically intuitive than others.
D: >: Or have you not accepted that controversial medium theory either?
>
Fischer: You are being either very childish, very dull,
>or disingenuine, and frankly I resent all postings that
>support an aether until and unless there is the first
>bit of evidence that one is needed, or that one exists.
Dennis: Well, if you believe due to the success of all other sciences that
material effects need material local causes, then, well, an ether is needed.
This is the same argument that people made for the atmosphere as well.
Yet, other sorts would deny such claims and refused to believe it existed
until they could directly observe it. ;-)
Fisher: > If you are suggecting that gravity "pushes"
>things "down", then you are both childish and dull. :-)
Dennis: Well, then so was Newton, Euler, Le Sage, Herapath, Maxwell, etc...
Fischer: > If anybody thinks there is such a thing as a
>graviton or gravity particle or medium or wave that
>pushes or pulls things down, then _they_ are also
>childish and dull (or never heard of Albert Einstein
>or the General Theory of Relativity).
>
> I don't know how long you think people are
>going to be interested in reading about aether,
>but unless something develops that suggests an
>aether exists,
Dennis: Well, since obvious observed effects of a medium (like interference,
refraction, reflection, pressure, Doppler, Sagnac, etc..) does not convince
you--and since the solidifying of ether also does not convince you, then what
evidence would?
And please tell us what evidence you would have accepted for the atmosphere in
the 18th century.
Fischer: I think it would be good to read
>a book, get a life, and talk about physics, not
>100 year old illusions.
Dennis: Molecules and atoms were 2000 year old illusions according to
positivists like Mach even in the 19th century. Seems sort of naive now,
doesn't it?
David Jonsson
<upt...@my-deja.com> wrote in message news:7uk4je$t9i$1...@nnrp1.deja.com...
> Newton thought that gravity acted instantaneously. Nowadays, accepted
> theory is that gravity propagates through space at the speed of light,
> although there are some theories (mostly crackpot, from what I've seen,
> but a few are plausible) that still have gravity acting
> that's not the point.
Gerber measured this with the same algebraic expression that Einstein
derived with general relativity 17 years later. Gerber came to the
conclusion 305 000 km/s. He used retarded potential on the orbit of Mercury.
See http://www.users.globalnet.co.uk/~haspdn/le02app.htm
Gerbers calculus has been verified with a different method by Petr Beckman
in his book Einsten Plus Two.
If using retarded potential in gravity what will the effect of rotating
bodies lead to?
David
Steve Carlip
> Gerber measured this with the same algebraic expression
> that Einstein derived with general relativity 17 years later.
> Gerber came to the conclusion 305 000 km/s. He used
> retarded potential on the orbit of Mercury.
The way this is phrased, it seems to imply that Gerber merely
took Newtonian gravity and added retardation. That's certainly
not right. Retarded Newtonian gravity is strongly ruled out by
observation---it leads to predictions of drastic instability of
Solar System orbits on time scales on the order of 100 years.
What Gerber did was to put in retardation *and* an extra
dependence of the gravitational field on the velocity of the
source. There are an infinite number of ways to do this,
and it's not clear how Gerber chose the particular velocity
dependence he did, but in any case, he ended up with an
effective potential that looks like a certain weak-field limit
of general relativity.
Gerber's theory does not, of course, explain a whole host of
other observations that are successfully explained by general
relativity (bending of light, Shapiro time delay, gravitational
red shift, de Sitter precession, Nordtvedt effect, orbital decay
of binary pulsars, etc.). In fact, it gives a badly incorrect
prediction for the deflection of light.
For a nice history, see Roseveare's book _Mercury's Perihelion
from Le Verrier to Einstein_.
Steve Carlip