First of all does gravitational pull has speed? If our sun disappear all in a sudden, would the gravitational pull stops at the exact instant? If not how long does it take?

Second question, is gravity considered a field or a force?

Have to admit I've never seen a calculation for the speed of gravity. <b>James R</b> probably knows more than I on that score. It is generally assumed that the affects of gravity propagate at light speed. So if the sun where to magically disappear it would take a little over 8 minutes before the Earth noticed and took off on a tangential path to it's orbit.

I want to see the results of an experiment involving one of those gravity detector thingies and the sun. Point it at sun, see if it lines up with the known position of the sun given that what we see is actually 8.3 minutes late. See if they are the same. Shouldn't be too hard, I guess.

According to general relativity, gravitational waves travel at the speed of light. This suggests that most changes to the configuration of spacetime propagate at the speed of light. Therefore, if the sun disappeared now, we wouldn't know about it for 8 minutes. This also explains why the direction of the pull of the sun appears to come from where we see the sun now rather than from where we'll see it in 8 minutes time (as it would if gravity was instantaneous).

Some changes to spacetime can travel faster than light, however. For example, in the inflationary period the early universe expanded faster than the speed of light.

Originally posted by Joeman
First of all does gravitational pull has speed? If our sun disappear all in a sudden, would the gravitational pull stops at the exact instant? If not how long does it take?

Second question, is gravity considered a field or a force?

Is that you Fah Que??

Originally posted by empennage


Is that you Fah Que??

No but I live with him unfortunately :D I found this forum and I am trying to get him to post here. When he does he can have his avatar back.

Originally posted by Joeman


No but I live with him unfortunately :D I found this forum and I am trying to get him to post here. When he does he can have his avatar back.

LOL that's funny stuff:D

Despite what some people say about g being instantanious, according to albert Einstein gravity travles at C, the speed of light. There's your answer short and sweet.

-----------------------
"This also explains why the direction of the pull of the sun appears to come from where we see the sun now rather than from where we'll see it in 8 minutes time (as it would if gravity was instantaneous). "
--------------------------------------


JAMESR......

Was curious, has there been an experiment that measures the local direction of the G-field that points to a different location than the suns current position?? seems to me an impossible measurement. The Earth moves very slightly in a basically circular arc in 8 minutes. If it was completely circular such a measurement would always point to the same place regardless of the amount of time. The Earth's path deviates from circular in 8 mintues by an extremely miniscule amount. I cant imagine this being detectable. Also, the G-field should point to the center of mass of the sun which is not neccesarily the center of the sun. The suns differential rotation and our poor understanding of the distribution of solar material internally would certainly thwart our efforts I would think.

But, if the sun sudden disappeared, the space-time diagram for a large mass would no longer exist at all. Say that it still takes the same amount of time for the fold to completely disappear, allowing the earth to travel off at a tangent: what would happen if the sun were twice as massive? Would the time be slowed, due to the deeper effect in space-time, or would it be the same?

Strange as it seems we have not (yet <grin>) measured the speed of gravity. However there are multiple experiments going on to try and detect Gravity Waves. These will acording to theory only exist if gravity has a speed.

Spinning black holes give the best evidence to date. There have been some results (not accurate enough yet to be sure) that support the idea that these black holes are spining/dragging space around them, which again depends on gravity having a speed.

By the way the space-time curvature says that the curve will propogate at the speed of light. So even though you may think the curavture would be instantaneous, the propogation of changes in the curve are not.

the truth is gravity has never been proven (nor disproven) to have velocity, no one actually knows yet.

it's possible that gravity is instantaneous, so if the sun were to implode suddenly, we would instantly know about it...

even though we wouldn't see it for 500 seconds.

How about combining the question of the speed of gravity with the question about the mass of light (elsewhere here on the forum)?
IF light has a mass AND gravity moves at lightspeed, every photon would seem to me to have a gravitational equivalent of the sonic boom :confused: Also every photon will have no forward directed gravity :rolleyes:
that probably won't be it then :o

IF light has no mass AND gravity has the speed of light, there will be no problems with gravitational booms or whatever, as the 2 don't interact.

IF light has got a mass AND gravity moves instantanuous, there also would be no problem, although it would mean that space itself is one big 'whirlpool' of small gravitational fields 'created by' photons.. It actually doesn't seem likely to me either.

of course there are more options, but it seems to me that thinking about it this way shows light has no mass and gravity most likely travels at the speed of light.

Btw, hia'all!

anyone

Generally from what I remember Gravity can not go beyond the threshold of light speed.
This means that you can continue accelerating until either:
A: You reach light speed
B: You impact into some surface.

If you had a bottomless pit then you would stop accelerating at lightspeed.

There is also an equation for the acceleration, of course this can be different depending on atmospheric pressure. I know it exists, otherwise parachute jumpers wouldn't say 1000-2000-3000 Check while jumping to make sure they are safely away.

(Namely working out the distance of acceleration away from the craft they just left to know the distance tha they can open their chute.)

I'm sure you would find this if you were to look at Galileo and his law of uniformed acceleration conducted at the leaning tower of Pisa.

""For example, in the inflationary period the early universe expanded faster than the speed of light.""

but since that is just a hypothese ...

Originally posted by c'est moi
""For example, in the inflationary period the early universe expanded faster than the speed of light.""

but since that is just a hypothese ...

But a very important hypothesis. (http://hyperphysics.phy-astr.gsu.edu/hbase/astro/cosmo.html#c5)

yes, but what do you call 'important'? it is a luxury to be able to develop theories about the universe and spent millions of dollars on this

and btw, it is still just a hypothesis, it does not even merit the title of theory in my eyes (a small detail: background radiation has been predicted by others BEFORE the bigbangers, something that many don't seem to know, and they were almost right about the temperature)

it really irritates me hearing people talkin' 'bout these things like somehow they were "facts", they aren't

Almost right does not cut it.

As you can not see the importance of this allow me to explain. The original models of the Big Bang based on General Realtivity made a number of testable predictions. These where tested and found to be sound. Hence competing theories where ditched. On further analysis this model was found to have serious shortcomings. One of which is the Horizon Problem listed in the link I posted above. The other problems are just as important.

To fix all these problems, Guth et al proposed the Inflationary model of the Big Bang as a fix. Again it makes testable predictions.

If experiments fail to verify these tests it means the Inflationary 'fix' to the Big Bang is flawed. Which brings us back to the original problems. Which means the whole model is flawed. Which means it is wrong. Dead flat wrong. A pile of dingo's kidneys. So wrong it is as wrong as a wrong thing. Which also casts serious doubt on Relativity as that spawned the model.

Is that important enough for you?

Oh, and yes, I did say it was a hypothesis, read my post again. Definitely has the phrase "important hypothesis" and not "important fact" or "important theory".

Gravity has never been shown to have a velocity

lightspeed or otherwise

none has been detected at all

so to say that gravity cannot travel faster than light is like saying
everything einstein says is true, so we don't need to test it.

that's not science.

Gravity may have a velocity, or it may not..

.. no one knows

ultravioletten,

Heres a way to prove or disprove gravity having the capacity to accelerate mass.

If you are to drop two balls from two different heights at the same time, one being twice the height from the ground as the other, do they fall at the same rate? Yes....

But when one ball hits the ground the other is still falling, has it's rate changed?

[The following was Previously written but I had a re-think and changed what my explaination contained, but I decided to keep the old explaination between the equals signs]
=======================================
I'm sure it will have, although you would reall need to experiment in timing this, as the ball that hits the ground first has less distance to travel. If it's falling at a constant rate you should be able to take the time it takes, multiply it by 2 and then have the time the other ball takes to hit the ground if it's a constant.

Alternatively you could drop the ball with the longer distance to travel first, and when it reaches the same height as the other ball drop that ball, and see which hits the ground first.

If there is velocity then that ball will hit the ground before that time period is up.
========================================

If you stand on a chair and jump off, you can land (make sure you bend you knees when landing) without injuring yourself.
If you were to jump out of a plane at 5000 feet, you wouldn't stand a chance of surviving without a parchute.

Simply put you accelerate over distance, although Acceleration is uniformed like Galileo produced with his results, in the fact that if two objects are the same mass and let go at the same time they will accelerate at the same speed for the duration of the freefall distance.

hey Thed

""The original models of the Big Bang based on General Realtivity made a number of testable predictions. These where tested and found to be sound.""

wasn't the most important of them all the background radiation? do you wish to have some more info about those who predicted it well before the big bang adherents?

as for the fact-part, wasn't speaking to you directly, just in general

for the record, we just disagree about what is important and what not

believe me, it is a luxury to be able to think about these things and to do research in it, pure luxury

It seems to me that if General Relativity is valid then it must be assumed as well that gravity propogates at greater than light speed or instantaneously (no velocity). Otherwise those outside the event horizon of a black hole wouldn't notice a change in gravity when something fell into the hole.

Originally posted by zanket
Otherwise those outside the event horiz on of a black hole wouldn't notice a change in gravity when something fell into the hole.

that is incorrect ;) . Photons cannot pass the event horizon because they are attracted so much by a black hole. Gravitational pull isn't affected by the attraction from a black hole, as the pull IS the attraction. That way it can still move at lightspeed and also break through an event horizon.

hope that made sense :D

Gravity propagate at light speed??? I do believe gravity is a FORCE, not a particle that is described in terms of "speed"...

and photons are still attracted to black holes, and can not get out of them. That's why they're "black" holes, because light can not escape from them. I suppose you know that, but just as a reminder to refresh your memories from grade-school astronomy...:D

I do believe gravity is a FORCE, not a particle that is described in terms of "speed"

In quantum mechanics, the force mediated by gravity is the graviton, which propagates at the speed of light.

Maybe a read of Spin Gravity..Ether of Magnetism, may answer some questions.

But basically gravity would have to be transmitted at about 20XC^20 to allow calculations to determine where a body is next going to be, otherwise angular momentum vectors would be constantly causing orbit chaos!

Good luck :)

Originally posted by (Q)
I do believe gravity is a FORCE, not a particle that is described in terms of "speed"

In quantum mechanics, the force mediated by gravity is the graviton, which propagates at the speed of light.

Then does it mean that it takes some time for a body to exert gravitational force to another body, since the gravitons have to get there?? Then if the mass of a body changes, does the other body that is, say, 5 LY off, take 5 years to notice the difference in force? I don't think so...:bugeye:

Zero, this is the aberration problem, with calculations to determine where a body is has to assume no aberration. If there is an aberration the by vector analysis there would have to be an extra angular momentum placed upon the body. This is not see in reality.

I have proposed a new theory of gravity that overcomes this problem. Look in astronomy for Spin Gravity !

I thought gravity was due to the curvature of spacetime.

Well with Newtonian gravity, gravity is the result of mass.

Relativity, it is curved space time.

With Spin Gravity it is due to movement.

Take your pick.....personaly "space-time" is a fantasy :)

spacetime is a model that works...and a scientific theory indicates some model that can plausibly explain the phenomena of the universe. Relativity and the whole spacetime pack works well enough to meet that criterion. Maybe not such a fantasy.

Your choice, but Relativity trashes Newtons first law re inertia, you can have in relativity, falling / rising accelerating inertial frames of reference....to account for gravity acting at a distance, indirectly!!

There is no force at a distance, and inertia is just that, constant velocity or stationary. Two very basic laws of physics are trashed.....well why doesn't this cause concern ?

Spin Gravity has no such problems, nor does it have a problem with the "attraction" aberration problem re speed of gravity that Newtonian and Relativity have.

Basically the classical explanation and action of gravity is fantasy!

Since when were Newtonian laws infallible, and 'basic', as you say?

Well if you refute that a change in parameters of an object can just come about without an external force, then you will not get my support.

I do not bend the integridity of physics, there have been truely great men in history, and I stand shoulder to shoulder with them. :)

zero

Then if the mass of a body changes, does the other body that is, say, 5 LY off, take 5 years to notice the difference in force?

Correct. Both the graviton and the photon take time to travel as they are both moving at the speed of light.

If that is the case Q, stellar bodies would constantly be attracted to places the attracting body isn't and angular momentum changes would create choas, we do not see this.

Zarkov

For objects with a constant velocity, as in the case with the Earth and Sun for example, gravity is propagating outwards from the Sun in all directions at the speed of light. It not only is propagating at the current position of the Earth but also at the Earths 'retardation' point in the Earths orbital path. The effect of propagation delay almost exactly cancels out. NO angular momentum changes and no chaos.

In general Relativity, nothing, not even gravity can go faster than the speed of light.

There are a lot of serious people who believe that gravity has to be instantaneous. But this breaks the GR rules! :)

zarkov

There are a lot of serious people who believe that gravity has to be instantaneous.

You're right, they are serious. They are also wrong, or delusional, or both. But the issue is that your statement insists gravity has to be instantaneous. Why would it have to be instantaneous ?

btw - Who are these 'serious' people ? Please don't cite well known cranks and crackpots.

But this breaks the GR rules!

It does more than that. The entire theory of relativity would crumble. The speed of light would not be a constant nor would it be the barrier. There would exist an absolute frame of reference. Time and distance would have no meaning. The list goes on.

The theoretical problem with gravityis this :-

to calculate the effect of gravity on another object, it must be assumed that gravity is instantaneous, (that is if the Sun was removed, we can't wait 8 minutes to feel a change), because if that was not the case a vector would be set up and the resultant of the vector interaction would alter the paths of all the Planets, Stars etc by a change in angular momentum.... this is not seen, so gravity has to go faster than the speed of light, so that this vector problem is removed and what we calculate is what we see.

There are many references to this on the web.

zarkov

to calculate the effect of gravity on another object, it must be assumed that gravity is instantaneous, (that is if the Sun was removed, we can't wait 8 minutes to feel a change)

But we would feel the changes 8 minutes later. Gravity propagates at the speed of light. Nothing travels faster than c.

because if that was not the case a vector would be set up and the resultant of the vector interaction would alter the paths of all the Planets, Stars etc by a change in angular momentum.... this is not seen, so gravity has to go faster than the speed of light, so that this vector problem is removed and what we calculate is what we see.

There are many references to this on the web.

Your conclusions are specious. Please cite those references.

http://www.metaresearch.org/cosmology/gravity/speed_limit.asp
We show that aberration has been suppressed in the GR equations of motion through setting gravity's propagation speed to infinity; and that the absence of aberration cannot be explained through some mathematical 'cancellation' because that would cancel tidal forces too. The mere existence of Lorentzian relativity as an experimentally viable model for the relativity of motion nullifies the 'proof' that nothing can propagate faster than light in forward time. Experiments indicate that gravity and electrodynamic forces both propagate far in excess of lightspeed.

http://www.faqs.org/faqs/astronomy/faq/part4/section-6.html.
To begin with, the speed of gravity has not been measured directly in the laboratory---the gravitational interaction is too weak, In the simple Newtonian model, gravity propagates instantaneously: the force exerted by a massive object points directly toward that object's present position.
In general relativity, on the other hand, gravity propagates at the speed of light; that is, the motion of a massive object creates a distortion in the curvature of spacetime that moves outward at light speed.

Consider two bodies---call them A and B---held in orbit by either electrical or gravitational attraction. As long as the force on A poin 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. The orbit of this binary system is gradually decaying, and this behavior is attributed to the loss of energy due to escaping gravitational radiation. But in any field theory, radiation is intimately related to the finite velocity of field propagation, and the orbital changes due to gravitational radiation can equivalently be viewed as damping caused by the finite propagation speed. (In the discussion above, this damping represents a failure of the "retardation" and "non-central, velocity-dependent" effects to completely cancel.)

The rate of this damping can be computed, and one finds that it depends sensitively on the speed of gravity. The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%. (Measurements of at least one other binary pulsar system,
PSR B1534+12, confirm this result although to less precision.)

There are numerous others. But the problem is the angular momentum changes. With the last article it is postulated that angular momentum problem actually causes dampening. This is not seen in the solar system, so then problem with conventional gravity rages, not so with spin gravity :)

Zarkov

The first link is a well known crackpot site which has been debunked on many occasions. The second link has a paragraph at the end of it which states quite clearly:

If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%. (Measurements of at least one other binary pulsar system, PSR B1534+12, confirm this result although to less precision.)

Your statement:

There are numerous others. But the problem is the angular momentum changes. With the last article it is postulated that angular momentum problem actually causes dampening. This is not seen in the solar system

There is no real problem with angular momentum changes, those are cancelled out. It may not be observed in our solar system because of our inability to detect the speed of gravity. That should hopefully change very soon.

so then problem with conventional gravity rages, not so with spin gravity

There is no problem with the current theories of gravity, they have been shown to work. Spin gravity has serious flaws.

Does gravity travel at the speed of light?

--------------------------------------------------------------------------------

Matthew P Wiener <weemba@sagi.wistar.upenn.edu>
Geoffrey A Landis <Geoffrey.Landis@sff.net>

To begin with, the speed of gravity has not been measured directly in
the laboratory---the gravitational interaction is too weak, and such
an experiment is beyond present technological capabilities. The
"speed of gravity" must therefore be deduced from astronomical
observations, and the answer depends on what model of gravity one uses
to describe those observations.

In the simple Newtonian model, gravity propagates instantaneously: the
force exerted by a massive object points directly toward that object's
present position. For example, even though the Sun is 500 light
seconds from the Earth, Newtonian gravity describes a force on Earth
directed towards the Sun's position "now," not its position 500
seconds ago. Putting a "light travel delay" (technically called
"retardation") into Newtonian gravity would make orbits unstable,
leading to predictions that clearly contradict Solar System
observations.

In general relativity, on the other hand, gravity propagates at the
speed of light; that is, the motion of a massive object creates a
distortion in the curvature of spacetime that moves outward at light
speed. This might seem to contradict the Solar System observations
described above, but remember that general relativity is conceptually
very different from Newtonian gravity, so a direct comparison is not
so simple. Strictly speaking, gravity is not a "force" in general
relativity, and a description in terms of speed and direction can be
tricky. 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. The net result is that the effect of propagation
delay is almost exactly cancelled, and general relativity very nearly
reproduces the Newtonian result.

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 propagation delay. It should be
emphasized that in both electromagnetism and general relativity, this
effect is not put in _ad hoc_ but comes out of the equations. Also,
the cancellation is nearly exact only for *constant* velocities. If a
charged particle or a gravitating mass suddenly accelerates, the
*change* in the electric or gravitational field propagates outward at
the speed of light.

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.

Now, in electrodynamics, a charge moving at a constant velocity does
not radiate. (Technically, the lowest order radiation is dipole
radiation, which depends on the acceleration.) So to the extent that
that A's motion can be approximated as motion at a constant velocity,
A cannot lose angular momentum. For the theory to be consistent,
there *must* therefore be compensating terms that partially cancel the
instability of the orbit caused by retardation. This is exactly what
happens; a calculation shows that the force on A points not towards
B's retarded position, but towards B's "linearly extrapolated"
retarded position. Similarly, in general relativity, a mass moving at
a constant acceleration does not radiate (the lowest order radiation
is quadrupole), so for consistency, an even more complete cancellation
of the effect of retardation must occur. This is exactly what one
finds when one solves the equations of motion in general relativity.

While current observations do not yet provide a direct
model-independent measurement of the speed of gravity, a test within
the framework of general relativity can be made by observing the
binary pulsar PSR 1913+16. The orbit of this binary system is
gradually decaying, and this behavior is attributed to the loss of
energy due to escaping gravitational radiation. But in any field
theory, radiation is intimately related to the finite velocity of
field propagation, and the orbital changes due to gravitational
radiation can equivalently be viewed as damping caused by the finite
propagation speed. (In the discussion above, this damping represents
a failure of the "retardation" and "non-central, velocity-dependent"
effects to completely cancel.)

The rate of this damping can be computed, and one finds that it
depends sensitively on the speed of gravity. The fact that
gravitational damping is measured at all is a strong indication that
the propagation speed of gravity is not infinite. If the
calculational framework of general relativity is accepted, the damping
can be used to calculate the speed, and the actual measurement
confirms that the speed of gravity is equal to the speed of light to
within 1%. (Measurements of at least one other binary pulsar system,
PSR B1534+12, confirm this result, although so far with less
precision.)

Are there future prospects for a direct measurement of the speed of
gravity? One possibility would involve detection of gravitational
waves from a supernova. The detection of gravitational radiation in
the same time frame as a neutrino burst, followed by a later visual
identification of a supernova, would be considered strong experimental
evidence for the speed of gravity being equal to the speed of light.
However, unless a very nearby supernova occurs soon, it will be some
time before gravitational wave detectors are expected to be sensitive
enough to perform such a test.

References:

There seems to be no nontechnical reference on this subject. For a
technical reference, see

T. Damour, in _Three Hundred Years of Gravitation_, S.W. Hawking and
W. Israel, editors (Cambridge Univ. Press, 1987)

For a good reference to the electromagnetic case, see







Hundreds of years of observation have established the existence of a
universal attraction between physical objects. In 1687, Isaac Newton
quantified this phenomenon in his law of gravity, which states that
every object in the Universe attracts every other object, with a force
between any two bodies that is proportional to the product of their
masses and inversely proportional to the square of the distance between
them. If M and m are the two masses, r is the distance, and G is the
gravitational constant, we can write:
F = GMm/r^2 .
The gravitational constant G can be measured in the laboratory and has a
value of approximately 6.67x10^{-11} m^3/kg sec^2. Newton's law of
gravity was one of the first great "unifications" of physics, explaining
both the force we experience on Earth (the fall of the proverbial apple)
and the force that causes the planets to orbit the Sun with a single,
simple rule.

Gravity is actually an extremely weak force. The electrical repulsion
between two electrons, for example, is some 10^40 times stronger than
their gravitational attraction. Nevertheless, gravity is the dominant
force on the large scales of interest in astronomy. There are two
reasons for this. First, gravity is a "long range" force---the strong
nuclear interactions, for instance, fall off with distance much faster
than gravity's inverse square law. Second, gravity is additive.
Planets and stars are very nearly electrically neutral, so the forces
exerted by positive and negative charges tend to cancel out. As far as
we know, however, there is no such thing as negative mass, and no such
cancellation of gravitational attraction. (Gravity may sometimes feel
strong, but remember that you have the entire 6x10^24 kg of the Earth
pulling on you.)

For most purposes, Newton's law of gravity is extremely accurate.
Newtonian theory has important limits, though, both observational (small
anomalies in Mercury's orbit, for example) and theoretical
(incompatibility with the special theory of relativity). These limits
led Einstein to propose a revised theory of gravity, the general theory
of relativity ("GR" for short), which states (roughly) that gravity is a
consequence of the curvature of spacetime.

Einstein's starting point was the principle of equivalence, the
observation that any two objects in the same gravitational field that
start with the same initial velocities will follow exactly the same
path, regardless of their mass and internal composition. This means
that a theory of gravity is really a theory of paths (strictly
speaking, paths in spacetime), which picks out a "preferred" path
between any two points in space and time. Such a description sounds
vaguely like geometry, and Einstein proposed that it *was*
geometry---that a body acting under the influence of gravity moves in
the "straightest possible line" in a curved spacetime.

As an analogy, imagine two ships starting at different points on the
equator and sailing due north. Although the ships do not steer
towards each other, they will find themselves drawn together, as if a
mysterious force were pulling them towards each other, until they
eventually meet at the North Pole. We know why, of course---the
"straightest possible lines" on the curved surface of the Earth are
great circles, which converge. According to general relativity,
objects in gravitational fields similarly move in the "straightest
possible lines" (technically, "geodesics") in a curved spacetime,
whose curvature is in turn determined by the presence of mass or
energy. In John Wheeler's words, "Spacetime tells matter how to move;
matter tells spacetime how to curve."

Despite their very different conceptual starting points, Newtonian
gravity and general relativity give nearly identical predictions. In
the few cases that they differ measurably, observations support GR. The
three "classical tests" of GR are anomalies in the orbits of the inner
planets (particularly Mercury), bending of light rays in the Sun's
gravitational field, and the gravitational red shift of spectral lines.
In the past few years, more tests have been added, including the
gravitational time delay of radar and the observed motion of binary
pulsar systems. Further tests planned for the future include the
construction of gravitational wave observatories (see D.05) and the
planned launch of Gravity Probe B, a satellite that will use sensitive
gyroscopes to search for "frame dragging," a relativistic effect in
which the Earth "drags" the surrounding space along with it as it
rotates.

References:

For introductions to general relativity, try:
K.S. Thorne, _Black Holes and Time Warps_ (W.W. Norton, 1994)
R.M. Wald, _Space, Time, and Gravity_ (Univ. of Chicago Press, 1977)
J.A. Wheeler, _A Journey into Gravity and Spacetime_ (Scientific
American Library, 1990)

For experimental evidence, see:
C.M. Will, _Was Einstein Right?_ (Basic Books, 1986)
or, for a more technical source,
C.M. Will, _Theory and Experiment in Gravitational Physics, revised
edition (Cambridge Univ. Press, 1993)

You can find out about Gravity Probe B at
<URL:http://einstein.stanford.edu/> and
<URL:http://www.nap.edu/readingroom/books/gpb/>.
R.P. Feynman, R.B. Leighton, and M. Sands, _The Feynman Lectures on
Physics_, chapter II-21 (Addison-Wesley, 1989)



D.05 What are gravitational waves?

--------------------------------------------------------------------------------


General Relativity has a set of equations that give results for how a
lump of mass-energy changes the space-time around it. (See D.03.) One
of the solutions to these equations is the infamous black hole, another
solution is the results used in modern cosmology, and the third common
solution is one that leads to gravitational waves.

Over a hundred years ago Maxwell realized that a solution to the
equations governing electricity and magnetism would create waves.
These waves move at the same speed that light does, and, hence, he
realized that light is an electro-magnetic wave. In general,
electromagnetic waves are created whenever a charge is accelerated,
that is, whenever its velocity changes.

Gravitational waves are analogous. However, instead of being
disturbances in electric and magnetic fields, they are disturbances in
spacetime. As such, they affect things like the distance between two
points or the amount of time perceived to pass by an observer.
Moreover, since there is no "negative mass," and momentum is
conserved, any acceleration of mass is balanced by an equal and
opposite change of momentum of some other mass. This implies that the
lowest order gravitational wave is quadrupole, and gravitational waves
are produced when an acceleration changes.

Because gravitational waves are waves, they should exhibit many other
properties of waves. For example, gravitational waves can, in
principle, be scattered or exhibit a redshift. (But see the next
question on the difficulty of testing this prediction.)

[Note, *gravitational* waves...gravity waves are something else
entirely (they occur in a medium when gravity is the restoring force)
and are commonly seen in the atmosphere and oceans.]

D.06 Can gravitational waves be detected?

--------------------------------------------------------------------------------

Steve Willner <swillner@cfa.harvard.edu>

The effects of gravitational waves are ridiculously weak, and direct
evidence for their existence has (probably) not been found with the
detectors built to date. However, no known type of source would emit
gravitational waves strong enough for detection, so no one is worried.

In the 60's and early 70's, Joe Weber at the University of Maryland
attempted to detect gravitational waves using large aluminum bars,
which would vibrate if a gravitational wave came by. Because local
causes also created vibrations, the technique was to look for
coincidences between two or more detectors some distance apart. Weber
claimed to see more coincidences than expected statistically and even
to see a correlation with sidereal time. Unfortunately, other groups
have used far more sensitive detectors operating on the same
principles and found nothing.

Two new experiments, far more sensitive than those using metal bars, are
being built now. These are LIGO in the US and Virgo in Italy. They
will work by detecting displacements between two elements separated by
several kilometers.

An indirect measurement of gravitational waves has been made, however.
Gravitational waves are formed when a mass undergoes change of
acceleration. They are stronger if the mass is dense and the
acceleration changes rapidly. One place where this might happen would
be two pulsars circling each other. A couple of systems like this
exist, and one has been studied actively over the past 20 years or so.
Pulsars make good clocks so you can time the orbital period of the
pulsars quite easily. As the pulsars circle, they emit gravitational
waves, and these waves remove energy (and angular momentum) from the
system. The energy released has to come from somewhere, and that
somewhere is the orbital energy of the pulsars themselves. This leads
to the pulsars becoming closer and closer over time. A formula was
worked out for this effect, and the observed pulsars match it amazingly
well. So well, in fact, that if you plot the data on top of the
prediction, there is no apparent deviation. (It's actually rather
disgusting, none of my results ever come out that well.) Anyway, Joe
Taylor of Princeton and a student of his, Russell Hulse, shared the
Nobel Prize in Physics for, in part, this work.

Useful references are given in section D.03.

V. M. Kaspi discusses pulsar timing in 1995 April Sky & Telescope, p. 18.

The conference proceedings volume _General Relativity and Gravitation
1989_, eds. Ashby, Bartlett, & Wyss, (Cambridge U. Press 1990) contains
a summary of the aluminum bar results.

_General Relativity and Gravitation 1992_, eds. Gleiser, Kozameh, &
Moreschi (IOP Publishing 1993) contains an article by Joe Taylor
summarizing the pulsar results.

An example of recent pulsar research is the article by Kaspi, Taylor,
and Ryba, 1994 ApJ 428, 713, who give instructions for obtaining their
archival timing data via Internet.

Some references to Weber's work are:
1969 Phys. Rev. Lett. 22, 1320.
1970 Phys. Rev. Lett. 24, 276.
1971 Nuovo Cimento 4B, 199.




thanks for your time...

bye!

You might want to check out the latest attempt at measuring

the speed of gravity:

http://www.nature.com/nsu/020902/020902-13.html

Take care ;)