Speed of Gravity

[Josh The Curious]

Hi, I'm new to this whole forum thing, but I've had a question that has been really bothering me. Alright, suppose the sun just disappeared. I know that isn't feasible, but I was wondering if the Earth would continue on a straight path from where it was before the light last emitted by the sun reached the Earth. If so, then how is it that gravitational waves (or lack thereof) affect us faster than light reaches us? If not, then why do gravity's affects appear instantaneous, even moving faster than light? I hope that isn't a stupid question, but I'll be grateful if anyone can answer it for me.

[antoniseb]

Originally posted by Josh The Curious@Jan 6 2005, 03:15 AM
suppose the sun just disappeared... I was wondering if the Earth would continue on a straight path from where it was before the light last emitted by the sun reached the Earth.

Welcome to the forum Josh.

That is the current theory, but we haven't been able to test it yet.

Concerning gravity waves, this would be a simple case that would demonstrate a gravity wave. There are some complicated aspects to gravity waves, so it isn't really that simple. They transmit energy, so in the case of the Sun just disappearing, where did that energy come from?

At the moment, it seems unlikely that a change in gravity [say when two massive objects collide, or one massive object explodes] could be detected faster than the speed of light. It is possible that it goes slower than the speed of light.

[StarLab]

As the speed of gravity has not yet been detected, it is assumed that gravity travels either at C or just beneath it, so either way the absence of the sun's light and gravity would be noted at about the same time.

[RobWolfe]

Here is an interesting link on this subject.

The Speed of Gravity - What the Experiments Say

[Svemir]

I read this just few days ago. I think Van Flandern has a strong case here but I can't wait for results from :
http://einstein.stanford.edu/

[Nereid]

Originally posted by Svemir@Jan 7 2005, 12:19 PM
I read this just few days ago. I think Van Flandern has a strong case here but I can't wait for results from :
http://einstein.stanford.edu/

Why do you think there is a strong case to answer?

For the avoidance of doubt, having read the stuff on Mr Van Flandern's website, my impression is it's Tom that has (naively? perniciously??) misunderstood both the theory and the experiments.

That gravity 'travels at c' has considerable indirect support: in GR (General Relativity), gravity travels at c, and GR has passed all good experimental and observational tests to date (despite Herr Van Flandern's misunderstandings).

[wjwduke]

1st; thanks Starlab for bringing this topiic back to life.
2nd; thanks RobWolfe for the URL which was referred to in another "gravity" topic.
3rd; thanks Svemir for responding to it.
I may be wrong but, reading back through past threads, I did not feel that their was any real “valid” arguments against Dr. Van Flandern's position that the propagation speed of gravity exceeds that of light (2x10^10). I only hear that GR says it can’t!
Now, Neried strongly disputes this for the same reason. Quote: "That gravity 'travels at c' has considerable indirect support: in GR (General Relativity), gravity travels at c, and GR has passed all good experimental and observational tests to date (despite Herr Van Flandern's misunderstandings).”
“Indirect”? “Good”? GR may have passed some experimental (observations) but, if I’m not mistaken, Einstein went to his grave trying to include gravity within his field equations.
From Dr. Flandern’s site, Quote: "How then does the direction of Earth's acceleration compare with the direction of the visible Sun? By direct calculation from geometric ephemerides fitted to such observations, such as those published by the U.S. Naval Observatory or the Development Ephemerides of the Jet Propulsion Laboratory, the Earth accelerates toward a point 20 arc seconds in front of the visible Sun, where the Sun will appear to be in 8.3 minutes. In other words, the acceleration now is toward the true, "instantaneous" direction of the Sun now, and is not parallel to the direction of the arriving solar photons now. This is additional evidence that forces from electromagnetic radiation pressure and from gravity do not have the same propagation speed.”
Now how can you argue with such esteemed institutions mentioned in the quote? Unless of course they were wrong and have since changed their position; I haven’t heard. Hard to find specifics at JPL's web site.

[Nereid]

Originally posted by wjwduke@Jan 9 2005, 05:02 AM
1st; thanks Starlab for bringing this topiic back to life.
2nd; thanks RobWolfe for the URL which was referred to in another "gravity" topic.
3rd; thanks Svemir for responding to it.
I may be wrong but, reading back through past threads, I did not feel that their was any real “valid” arguments against Dr. Van Flandern's position that the propagation speed of gravity exceeds that of light (2x10^10). I only hear that GR says it can’t!
Now, Neried strongly disputes this for the same reason. Quote: "That gravity 'travels at c' has considerable indirect support: in GR (General Relativity), gravity travels at c, and GR has passed all good experimental and observational tests to date (despite Herr Van Flandern's misunderstandings).”
“Indirect”? “Good”? GR may have passed some experimental (observations) but, if I’m not mistaken, Einstein went to his grave trying to include gravity within his field equations.
From Dr. Flandern’s site, Quote: "How then does the direction of Earth's acceleration compare with the direction of the visible Sun? By direct calculation from geometric ephemerides fitted to such observations, such as those published by the U.S. Naval Observatory or the Development Ephemerides of the Jet Propulsion Laboratory, the Earth accelerates toward a point 20 arc seconds in front of the visible Sun, where the Sun will appear to be in 8.3 minutes. In other words, the acceleration now is toward the true, "instantaneous" direction of the Sun now, and is not parallel to the direction of the arriving solar photons now. This is additional evidence that forces from electromagnetic radiation pressure and from gravity do not have the same propagation speed.”
Now how can you argue with such esteemed institutions mentioned in the quote? Unless of course they were wrong and have since changed their position; I haven’t heard. Hard to find specifics at JPL's web site.

Whew, where to start?

Whether "Einstein went to his grave trying to include gravity within his field equations" is irrelevant ... GR has passed all the tests it's been subject to to date, with flying colours - what else can you ask of a theory? (Oh, and it was quantum mechanics, not gravity; GR is an extraordinarily successful theory of gravity).

The stuff you quote from Flandern is one of the things I meant when I said that he has misunderstood the theory. While the math is pretty tough, the result is quite clear ... the observed motions of the planets, moons, minor bodies, ... in our solar system are consistent with GR. There are several ways you can talk about Flandern's misunderstanding, perhaps the easiest is to say that he's applying a Newtonian perspective to arrive at a conclusion that is inconsistent with a GR one (and for the avoidance of doubt, what can you say that except 'duh!&#39.

[Ola D.]

suppose the sun just disappeared... I was wondering if the Earth would continue on a straight path from where it was before the light last emitted by the sun reached the Earth.

Re: Original question

If the Sun just suddenly disappears it will cause a gravitational disturbance in the space-time fabric - think of it more like a flexible trampoline surface - and will cause a wave to travel along. The Earth wouldn't feel any change until the ripples reach its orbit. Einstein theoretically calculated that these ripples travel at the speed of light.

This cosmic catastrophe isn't likely to happen, or any else event causing any disturbances.

[Svemir]

The problem is that one can not connect SR and GR.
I don't who said that but "You can't go there(GR) from here (SR)."
As I understood that, Van Flandern does not dispute GR but SR.
GR only needs some modification.
And he is not alone. Recent paper from Rabinowitz was posted on this forum.
He says that GR is good for explaining gravity CLOSE to MASSIVE objects, etc, etc.
Myself, I like to think logically and I have some questions to Einstein and followers:
If you don't need eather to explain gravity, how in earth mass communicate to Space-time (ST), which has no physical significance?

[Nereid]

Originally posted by Svemir@Jan 11 2005, 07:27 AM
The problem is that one can not connect SR and GR.
I don't who said that but "You can't go there(GR) from here (SR)."
As I understood that, Van Flandern does not dispute GR but SR.
GR only needs some modification.
And he is not alone. Recent paper from Rabinowitz was posted on this forum.
He says that GR is good for explaining gravity CLOSE to MASSIVE objects, etc, etc.
Myself, I like to think logically and I have some questions to Einstein and followers:
If you don't need eather to explain gravity, how in earth mass communicate to Space-time (ST), which has no physical significance?

You've lost me Svemir, I always thought that SR 'is' GR in the limit (of zero mass) ... just as GR 'is' Newtonian physics in the limit of small mass and/or low speed.

Are you saying that these 'in the limit' behaviours are incorrect? If so, in what way?

Please post a link to the Rabinowitz paper, if you would be so kind.

I'm also lost as to which regimes Tom feels GR is 'good at explaining' ... AFAIK, it's been tested in 'weak field' conditions (in practice this means everywhere except near black holes), and 'non-dynamic' conditions (in practice this means just about everywhere except in colliding neutron stars, etc), and at Planck scales (where it is well-known that it it is incompatible with QFT). It's passed all the tests it's been subject to in the observed domains ... in which domains does Herr Flandern feel it has failed experimental or observational tests?

As to the communicate question: isn't this a question about how one interprets GR? (and not what the equations are, how they can be applied, etc).

To my way of thinking, GR is a great deal easier to get your mind around (outside the equations) than QFT! Yet, as I'm sure you know, QED is THE most accurately tested theory in modern physics.

[Note to readers unfamiliar with QED: you can think of it as 'putting Einstein's Special Relativity into Quantum Mechanics'; among other things, it is the foundation for the development of the semiconductors which your computer uses to do its magic for you).

[Guest]

I am sorry Neried but, I have never heard of an association of SR and QED. Maybe Intel can provide thier research into this connection. Do you have any references from Intel, Motorola, T.I. etc. ?

[astromark]

The question.. as I see it. has not been answered. The sun wont disapear. or go out or explode or .. well not for a while yet any way. But if it did just vanish. yes the earth would stop orbiting it at the same moment the light stopped reaching us... and yes we would freez. Unless you live very near geothermal heat sorce, or active volcano. we is stuffed. we would not be to worried about the direction of the earths travel. Needless to say it would be very nearly a strait line.

[Svemir]

Rabinowitz:
http://arxiv.org/ftp/astro-ph/papers/0412/0412101.pdf
How many interpretations GR has?
My naive interpretation is that mass warps Space-Time, gravity is just geometry of ST.
Van Flandern goes in for Lorentz Relativity instead of SR. Implication on GR is, while Einstein will say: "Speed of gravity is c", Lorentz will say: "Not necessary, it may be higher, as high as 10^10m/s".
(On the other hand Newton will say: "It's infinite" (being very unhappy about that)).

These are basically 2 (3&#33 answers to question about disaperance of Sun and implications on Earth.

[Nereid]

Originally posted by Guest@Jan 12 2005, 04:39 AM
I am sorry Neried but, I have never heard of an association of SR and QED. Maybe Intel can provide thier research into this connection. Do you have any references from Intel, Motorola, T.I. etc. ?

This page gives a simple account.

[Nereid]

Originally posted by Svemir@Jan 12 2005, 12:33 PM
Rabinowitz:
http://arxiv.org/ftp/astro-ph/papers/0412/0412101.pdf
How many interpretations GR has?
My naive interpretation is that mass warps Space-Time, gravity is just geometry of ST.
Van Flandern goes in for Lorentz Relativity instead of SR. Implication on GR is, while Einstein will say: "Speed of gravity is c", Lorentz will say: "Not necessary, it may be higher, as high as 10^10m/s".
(On the other hand Newton will say: "It's infinite" (being very unhappy about that)).

These are basically 2 (3&#33 answers to question about disaperance of Sun and implications on Earth.

Thanks for the link.

What other implication(s) does 'Lorentz Relativity' have for GR?

What specific, concrete experiments or observations could be performed to see whether the real world is more consistent with LR than SR? Of these, which have already been conducted? What were the results?

Other than the speed of gravity, does Van Flandern propose there are differences between GR and his idea? If so, which of the numerous tests of GR done so far are also tests of VF's idea? How well has VF's idea fared wrt these tests?

[jamerz3294]

Has anyone considered that the velocity of gravity can vary?
This entire discussion has become a debate of differing points of view, without actually trying to answer the original question. Or, in other words, for us simpletons, please use the KISS method, and speak directly.
Thank you for your support

[rahuldandekar]

KISS method? What is that?

[Ola D.]

Originally posted by rahuldandekar@Jan 23 2005, 07:04 AM
KISS method? What is that?

Keep It Simple Stupid!

No offense though :blink:

[Ola D.]

Hi Jamerz,

Einstein theoritically calculated the speed of gravity to be equal to the speed of light "c".
If the Sun disappears the Earth will travel in a straight path. However, we'll not sense any change until the gravitational disturbances reach our orbit.

This is only a theoritical model of the Sun disappearing as such a catastrophe isn't likely to happen in real life.

[GOURDHEAD]

If the Sun disappears the Earth will travel in a straight path. However, we'll not sense any change until the gravitational disturbances reach our orbit.

Extrapolating to the ridiculous, should the constituents of the sun remember the big bang and emulate their behavior of that time by expanding into a shell with inner radius of 2 light years in less than planck time leaving the location (with respect to Earth) of the center of mass of the system unchanged, what would happen to the Earth?