Speed of gravity. Whats the problem.

  • #1

Main Question or Discussion Point

"Speed" of gravity. Whats the problem.

I keep finding stuff on the net about the speed of gravity and how some people contest it should be c, the speed of light.

Now I lean more towards an infinite "speed" of gravity, but I really cant see why gravity should be restricted by c. People say that nothing can travel faster than the speed of light. But objects appear to expierience gravitational force direction changes instantly.

So what's the problem. Nothing has travelled faster than the speed of light. Two object have just experienced accellerations at the same time. Relativity says that masses cannot travel at the speed of light relative to one another, but why should this apply to forces.

No Thing has travelled faster than light.
 

Answers and Replies

  • #2
Janus
Staff Emeritus
Science Advisor
Insights Author
Gold Member
3,514
1,261


Originally posted by ObsessiveMathsFreak
I keep finding stuff on the net about the speed of gravity and how some people contest it should be c, the speed of light.

Now I lean more towards an infinite "speed" of gravity, but I really cant see why gravity should be restricted by c. People say that nothing can travel faster than the speed of light. But objects appear to expierience gravitational force direction changes instantly.

So what's the problem. Nothing has travelled faster than the speed of light. Two object have just experienced accellerations at the same time. Relativity says that masses cannot travel at the speed of light relative to one another, but why should this apply to forces.

No Thing has travelled faster than light.
SR holds that no information can travel faster than c. An infinite propagation speed for gravity would violate that. GR shows that gravity can have a propagation speed of c without causes the problems that some people seen to think that it would cause.
 
  • #3
841
1


Originally posted by ObsessiveMathsFreak
Now I lean more towards an infinite "speed" of gravity, but I really cant see why gravity should be restricted by c.
Because experiments indicate that the laws of relativity are correct.


People say that nothing can travel faster than the speed of light. But objects appear to expierience gravitational force direction changes instantly.
No, they don't. Nobody has measured the propagation speed of changes in the gravitational field directly. But we have indirect measurements: the Taylor-Hulse binary pulsar system. The orbits of the neutron stars decay in exactly the way predicted if they were radiating gravitational waves. The rate of decay depends on how quickly the waves travel, so we can infer the speed of gravity from the decay rate. It come out to be c, to within a few percent (if I recall correctly).


So what's the problem. Nothing has travelled faster than the speed of light. Two object have just experienced accellerations at the same time.
No non-tachyonic field theory consistent with relativity can have field disturbances propagate faster than the speed of light.


Relativity says that masses cannot travel at the speed of light relative to one another, but why should this apply to forces.
Imaginary masses (tachyons) can propagate faster than the speed of light; likewise, tachyonic fields can propagate disturbances faster than light. But we don't have any experimental evidence of tachyonic fields, just like we don't have any evidence of tachyons. Non-tachyonic particles and fields have a light-speed limit in relativity.
 
  • #4
But what exactly is wrong with information travelling faster than the speed of light.

Special relativity shows that masses cannot travel faster than light but why should this hold so for information. the theory says nothing about information.


What postulates of special relativity does an infinite speed for gravity violate. It doesn't violate any.

If gravity wasn't infinite then wouldn't that lead to a contridiction. If a large mass was placed in the center of a long rocket, the force it would exert on the front and back of the rocket would be the same according to someone on the rocket, but different according to some one moving relative to the rocket. Wouldn't this lead to a contridiction?

That aside, why can't information travel faster than light? It doesn't have mass so what is the problem?
 
  • #5
841
1
Originally posted by ObsessiveMathsFreak
But what exactly is wrong with information travelling faster than the speed of light.
You can write down relativstic theories in which information travels faster than light. (Maxwellian electromagnetism is not one of those theories.) They are called tachyonic. However, they have, among other things, problems with causality: any influence that propagates from an event A to an event B in one frame will propagate backward in time from B to A in some other frame.


Special relativity shows that masses cannot travel faster than light but why should this hold so for information. the theory says nothing about information.
Special relativity, in the form of the light cone structure of Lorentz geometry, places a constraint on all causal influences, in particle or field theories. It doesn't just constrain particles with mass.


What postulates of special relativity does an infinite speed for gravity violate. It doesn't violate any.
You can write down tachyonic theories consistent with the postulates of SR, but they aren't consistent with reality.


If gravity wasn't infinite then wouldn't that lead to a contridiction. If a large mass was placed in the center of a long rocket, the force it would exert on the front and back of the rocket would be the same according to someone on the rocket, but different according to some one moving relative to the rocket.
No, it wouldn't. (Nor is this true in electromagnetism, replacing masses with charges and gravitational forces with electric forces.)
 
  • #6
hanserd


Originally posted by Ambitwistor
Because experiments indicate that the laws of relativity are correct.



No, they don't. Nobody has measured the propagation speed of changes in the gravitational field directly. But we have indirect measurements: the Taylor-Hulse binary pulsar system. The orbits of the neutron stars decay in exactly the way predicted if they were radiating gravitational waves. The rate of decay depends on how quickly the waves travel, so we can infer the speed of gravity from the decay rate. It come out to be c, to within a few percent (if I recall correctly).



No non-tachyonic field theory consistent with relativity can have field disturbances propagate faster than the speed of light.



Imaginary masses (tachyons) can propagate faster than the speed of light; likewise, tachyonic fields can propagate disturbances faster than light. But we don't have any experimental evidence of tachyonic fields, just like we don't have any evidence of tachyons. Non-tachyonic particles and fields have a light-speed limit in relativity.

I agree, the speed of gravity, is creation. It has NO SPEED.
When we get this right we have the interstellar and Galaxy travel we dream about. Can anyone say "DUNE"

lets use the MAGNETISM we know about to find the answer. It was done on the ocean once..."other story"
what is Magnetism.....same as gravity, OR bubbles in a sink, surface tension....same stuff. LARGE scale....

work it.
Hans B.
 
  • #7
hanserd
awares of the nova explosion?

I happened to find an news report about 8 to 12 years ago of a super nova testing its speed....
It was millions of times faster than the speed of light.
for about some millions of nano seconds.
but it happened!

the news and the article vanished. in 1 hour as I want ed to print it...figures. no one would like me anyway.

cant find it anyplace.
anyideas?

It haunts me to this day, GUYS, GALS, keep looking for the gravity wave.
 
  • #8
27
0
<c

Ambitwister, hello again.

I think what I was after in the way of an answer in one of our discussions was this;

if we measure speed and change by spectral analysis or by optical verification (ie:using our eyes) then we have limited our observations to the use of visible light and those other spectrums of light which we are able to observe.

Therefore it would seem correct to say that we cannot observe events that are faster than light and will never be able to until we find another method of observation and verification than optical methods. This other method could not include simple hypothetical mathematical formuli because they only prove that we can postulate the conditions for speeds at <c on paper but never actually prove they exist through optical observation.

This does not bode well for those who wish to prove that information or any other format of events can travel <c.

Once again, in the spirit of this thread, I'll ask; is it the effect of gravity (caused by the presence of a mass) that travels outward or is it gravity that travels at whatever speed it travels?
 
Last edited:
  • #9
841
1


Originally posted by Unkaspam
Therefore it would seem correct to say that we cannot observe events that are faster than light and will never be able to until we find another method of observation and verification than optical methods.
Why? There isn't any law that says that we have to probe an object's speed with something that travels faster than that object.


Once again, in the spirit of this thread, I'll ask; is it the effect of gravity (caused by the presence of a mass) that travels outward or is it gravity that travels at whatever speed it travels?
Once again, in order for me to answer that question, you need to precisely define what you mean by "the effect of gravity", as opposed to simply "gravity". To be physically meaningful, you have to define it operationally, in terms of an experiment: describe how to carry out two experiments, one of which measures "the speed of gravity" and one which measures "the speed of the effect of gravity". Then I can tell you what our theories of gravity say the outcomes of those experiments will be.
 
  • #10
russ_watters
Mentor
19,599
5,868
Example: planetary motion. If the "speed" of gravity is infinite, the "force" holding the planet in orbit will be directly on line with the sun. If the speed is finite, there will be a lag between the force and the position relative to the sun. And the effect of this lag would be...
 
  • #11
27
0


Originally posted by Ambitwistor
Once again, in order for me to answer that question, you need to precisely define what you mean by "the effect of gravity", as opposed to simply "gravity". To be physically meaningful, you have to define it operationally, in terms of an experiment: describe how to carry out two experiments, one of which measures "the speed of gravity" and one which measures "the speed of the effect of gravity". Then I can tell you what our theories of gravity say the outcomes of those experiments will be.
Great, thanks again.

Experiment #1: measuring the speed of gravity.

Hypothetically we find a way to identify "gravitons" or waves that have a constitution that is completely separate from the particles, gases and radiation found in the space surrounding a mass and that show an interaction with those particles, gases and radiation that help us determine that these are, in fact, gravity waves or "gravitons". Then we are able to track these waves with or without relying on observations of their interaction and influence on the surrounding space and its contents. In this experiment we have isolated gravity as a separate wave or sub-atomic force and are able to measure its speed.

Experiment #2: measuring the speed of the effect of gravity.

Hypothetically we find out that gravity has no radiating waves or "gravitons" and that it is only by observing the actions of particles, gases and radiation surrounding a mass in space that we are able to say the mass is generating a gravitational pull. So that, in this experiment, we pick a region of space with close to zero mass in it but does have some incidental particles, gases and radiation. Then (instantaneously) we introduce a large mass and immediately measure the speed of the influence of the mass by observing the reaction of the particles, gases and radiaion surrounding the newly introduced mass. In this experiment we have only isolated the effect of gravity and the speed at which this effect spreads through space (and matter).
 
  • #12
479
0


Originally posted by ObsessiveMathsFreak
People say that nothing can travel faster than the speed of light. But objects appear to expierience gravitational force direction changes instantly.
A water wave is rolling towards the beach at few miles per hour. You hit it at hundred mph in your highspeed boat. Is it that the wave travels faster than light, or is it that you crossed that which was already there?

Crash your plane into the mountain. Ask whether you experienced collisional force direction changes instantly.
 
  • #13
None of this has answered my initial question.

In what way would an infinite speed for gravity break the postulates of relativity. Namely.

All Physical laws are exactly the same in all interial frames.
All observers will measure the same spped for light regardless of their inertial frame.
 
  • #14
selfAdjoint
Staff Emeritus
Gold Member
Dearly Missed
6,786
7
You could posssibly contrive to send a message via gravity. That message would travel instantly and would incur the time-travel consequences of FTL in relativity, which contradict causality.
 
  • #15
But information travelling faster than the speed of light doesn't break any of the postulates.

Causality isn't violated by faster than light trasmission of data.

Just because I know that something is coming towards me, before I can see the light from that object, will not violate either of the postulates.
 
  • #16
Perhaps I'm harping too much on the postulates alone.

Maybe someone could describe a thought experiment which would show a paradox in having an infinite speed for gravity.
 
  • #17
Hurkyl
Staff Emeritus
Science Advisor
Gold Member
14,916
19
As was stated earlier:

any influence that propagates from an event A to an event B in one frame will propagate backward in time from B to A in some other frame.

It doesn't take too much creativity to set up a paradox:

Person A sends a FTL signal to person B someplace far away. Upon receiving A's signal, B sends a signal to person C who is next to A. Upon receiving B's signal, person C kills person A.

Then, if the signals are sent in the right way, they both travel backwards in time in a particular reference frame, thus in all reference frames, person C receives B's signal before person A sends a signal, and we have a paradox.
 
Last edited:
  • #18
I don't see the paradox.
 
  • #19
653
0


Originally posted by Ambitwistor

No non-tachyonic field theory consistent with relativity can have field disturbances propagate faster than the speed of light.



Imaginary masses (tachyons) can propagate faster than the speed of light; likewise, tachyonic fields can propagate disturbances faster than light. But we don't have any experimental evidence of tachyonic fields, just like we don't have any evidence of tachyons. Non-tachyonic particles and fields have a light-speed limit in relativity.
my understanding from the physics USENET faq is that even with tachyonic field theories, no local field disturbance can propagate faster than light.

which is nice, since even though there aren t any tachyonic matter fields in the standard model, there are possible field configurations at unstable points in the potential which act like tachyons in some regimes. even these fields do not have signals propagating faster than light.
 
  • #20
27
0


Originally posted by Ambitwistor
you need to precisely define what you mean by "the effect of gravity", as opposed to simply "gravity". To be physically meaningful, you have to define it operationally, in terms of an experiment: describe how to carry out two experiments, one of which measures "the speed of gravity" and one which measures "the speed of the effect of gravity". Then I can tell you what our theories of gravity say the outcomes of those experiments will be.
Hello Ambit, please see the two experiments I have laid out in response to your request. I am interested in finding out if these are one and the same or not. Thank you.
 
  • #21
841
1


Originally posted by Unkaspam
Hypothetically we find a way to identify "gravitons" or waves that have a constitution that is completely separate from the particles, gases and radiation found in the space surrounding a mass and that show an interaction with those particles, gases and radiation that help us determine that these are, in fact, gravity waves or "gravitons". Then we are able to track these waves with or without relying on observations of their interaction and influence on the surrounding space and its contents. In this experiment we have isolated gravity as a separate wave or sub-atomic force and are able to measure its speed.
General relativity predicts the existence of gravitational waves. Their speed is c. We could measure them with a laser interferometer like LIGO; we could measure their speed by seeing how long it takes them to travel between two detectors.

I can't think of a way of detecting any gravitational effect, wave or otherwise, without observing the interaction of gravity on the surrounding space on its contents, because gravity is the curvature of space itself.

Note that gravitational waves carry changes in the gravitational field; there won't be, for example, any gravitational waves in the field of a static body.


Hypothetically we find out that gravity has no radiating waves or "gravitons" and that it is only by observing the actions of particles, gases and radiation surrounding a mass in space that we are able to say the mass is generating a gravitational pull.
This isn't an either-or issue; one mass can influence another gravitationally without producing any gravitational waves, but that doesn't mean that gravitational waves don't exist or don't influence things gravitationally.

So that, in this experiment, we pick a region of space with close to zero mass in it but does have some incidental particles, gases and radiation. Then (instantaneously) we introduce a large mass and immediately measure the speed of the influence of the mass by observing the reaction of the particles, gases and radiaion surrounding the newly introduced mass. In this experiment we have only isolated the effect of gravity and the speed at which this effect spreads through space (and matter).
In general relativity, the speed of influence of the mass will be c, because the influence is carried by gravitational waves which propagate at c.

(By the way, you can't instantaneously introduce a mass in GR, but you can jiggle it or something and see how long it takes for the news of the movement to reach somewhere else.)
 
  • #22
27
0


Originally posted by Ambitwistor
General relativity predicts the existence of gravitational waves. Their speed is c. We could measure them with a laser interferometer like LIGO; we could measure their speed by seeing how long it takes them to travel between two detectors.

I can't think of a way of detecting any gravitational effect, wave or otherwise, without observing the interaction of gravity on the surrounding space on its contents, because gravity is the curvature of space itself.

Note that gravitational waves carry changes in the gravitational field; there won't be, for example, any gravitational waves in the field of a static body.



This isn't an either-or issue; one mass can influence another gravitationally without producing any gravitational waves, but that doesn't mean that gravitational waves don't exist or don't influence things gravitationally.



In general relativity, the speed of influence of the mass will be c, because the influence is carried by gravitational waves which propagate at c.

(By the way, you can't instantaneously introduce a mass in GR, but you can jiggle it or something and see how long it takes for the news of the movement to reach somewhere else.)
Excellent, thank you, I forgot about the use of laser interferometers like LIGO. Good idea about jiggling a mass to observe the speed of changes in a gravitational wave.

Could a gravitational wave be mistakenly observed as emanating from a source when what is really being observed is space warping in response to the simple influence of a jiggling mass?

Does space change in density or in some other way when in the vicinity of a gravitational mass?
 
  • #23
Gravity as curvature in space

Gravity as quantized gravitrons is only a reflection of the electro-magnetic particle/wave fields. General relativity stated that gravity is the curvature of space itself! If a change in that curvature is created by mass which cannot move faster than light, then the change cannot be faster than light. The effect of gravity, ie. being in a curved space is instantaneous, since objects exist in space in the present time.
 
  • #24
841
1


Originally posted by Unkaspam
Could a gravitational wave be mistakenly observed as emanating from a source when what is really being observed is space warping in response to the simple influence of a jiggling mass?
If you jiggle a mass, then the curvature of space changes; the changing curvature propagates outward from the source, and is called a gravitational wave.

Does space change in density or in some other way when in the vicinity of a gravitational mass?
The geometry at a point of space depends on how close that point is to a mass.
 
Last edited:
  • #25
27
0


Originally posted by Ambitwistor
If you jiggle a mass, then the curvature of space changes; the changing curvature propagates outward in the source, and is called a gravitational wave.



The geometry at a point of space depends on how close that point is to a mass.
Thank you Ambitwister. You've cleared up the current veiw of gravity for me. I guess that, so far, we measure the speed of gravity by measuring the changing, outward propagation of the curvature or geometry of space when it is influenced by a change in the postion of a mass. Thanks again.
 
Top