Speed of gravity

[Erasmus00]

Van Flandern doesn't try to explain GR in terms of gravity speed and I don't think it is ever presented that way.

 

No, but the fact remains that built into GR is a propagation speed for gravity. Otherwise, you could signal by wiggling a gravitational object and letting the instant travel time carry the distortion to someone light years away. The fact is, if for some strange reason the Sun disappeared, it would take 8 minutes before the light stopped, and 8 minutes before gravity gave out.

 

And Van Flandern is a proponent of gravity's transit time being infinite, which he feels disqualifies the geometric interpretation in favor of the optical interpretation.

-Will

[CraigD]

With very precise measurement of the Earth/Sun system, we should see that the Earth is orbiting around a point that is eight minutes ahead of where the visible sun is, eight minutes ahead in the Sun's path around the galactic centre. This should be if gravity is a curvature in space. If gravity is propagated by gravitons at c, then the one focus of the orbit should be exactly where the visible sun is. Have this test been done, or can't we make such fine observations yet?

I read something along these lines in an Alternative View column of Analog magazine. It argued that this negated both the graviton and conventional GR.

 

Because the sun and all objects in the solar system can be considered in the same inertial frame, and because the difference in the effects of extra-solar gravity on all of them are very small, I think the effect to be observed would be even smaller, due only to perturbations of the sun's position by the planets and other such objects. Given that the sun’s center of mass is so vaguely defined by its observable disk, and it’s interior density likely to be complexly varied, I doubt that it’ll be possible to measure such an effect.

 

I think the inclusion of the graviton in the standard model has very grave difficulties ahead of it, matching the highly-verified predictions of GR for the orbit of the planets high among them. The other bosons had something of a free ride, since their domain is so physically small that quantum exclusion locks the fermions with which they interact - quarks and electrons via photons - into very un-GR-like orbits, or even smaller, beyond the realm where a continuous geometric theory is at all useful, in the nucleus with its quarks and gluons.

[Jay-qu]

Given that the sun’s center of mass is so vaguely defined by its observable disk, and it’s interior density likely to be complexly varied, I doubt that it’ll be possible to measure such an effect.

 

I also think that you couldnt measure it, but that doesnt mean that its not happening

 

Because the sun and all objects in the solar system can be considered in the same inertial frame...

 

are you trying to say that relativity does not apply to gravity as it does with light and it could travel faster than the speed of light if its source has a velocity? because that is obviously incorrect...

 

I like what your trying to get at boerseun, so do we orbit around the image of our sun or the true position of it?

[Boerseun]

I like what your trying to get at boerseun, so do we orbit around the image of our sun or the true position of it?

That's a fine way of putting it. I think the result of such a measurement should have fundamental consequences either way - wether we orbit the image or the true position.

 

So - has anybody attempted such a measurement yet?

[coldcreation]

No, but the fact remains that built into GR is a propagation speed for gravity. Otherwise, you could signal by wiggling a gravitational object and letting the instant travel time carry the distortion to someone light years away. The fact is, if for some strange reason the Sun disappeared, it would take 8 minutes before the light stopped, and 8 minutes before gravity gave out.

 

And Van Flandern is a proponent of gravity's transit time being infinite, which he feels disqualifies the geometric interpretation in favor of the optical interpretation.

-Will

 

Tom Van Flandern in a paper titled The Speed of Gravity – What the Experiments Say (1998), has found experimental and observational evidence confirming that if gravitation is taken as a propagating force of nature in flat spacetime, its speed is substantially faster than the speed of light. “This is because gravity, in contrast to light, has no detectable aberration or propagation delay for its action…General relativity explains these features by suggesting that gravitation (unlike electromagnetic forces) is a pure geometric effect of curved space-time, not a force that propagates…Although faster-than-light force propagation speeds do violate Einstein special relativity (SR), they are in accord with Lorentzian relativity, which has never been experimentally distinguished from SR-at least, not in favor of SR.” (Flandern, 1998, abstract)

 

Indeed, Eddington had shown (1920) that orbits are unstable if gravitational force propagates with finite speed. While GR is well know for having reduced Newtonian gravity to the low-velocity, weak-field limit, the speed of gravity in Newton’s Universal Law is categorically infinite, “action at a distance.” As Van Flandern explains further:

 

“If gravity were a simple force that propagated outward from the Sun at the speed of light, as radiation pressure does…It would act continuously, but would tend to speed the Earth up rather than slow it down because gravity is attractive and radiation pressure is repulsive. Nonetheless, the net effect of such a force would be to double the Earth’s distance from the Sun in 1200 years. There can be no doubt from astronomical observations that no such force is acting. The computation using the instantaneous positions of Sun and Earth is the correct one. The computation using retarded positions is in conflict with observations.” (Flandern, 1998)

 

Coldcreation

[CraigD]

Because the sun and all objects in the solar system can be considered in the same inertial frame...

are you trying to say that relativity does not apply to gravity as it does with light and it could travel faster than the speed of light if its source has a velocity?...

Not at all. If gravity is a boson – the graviton – then like electo-magnetic radiation’s photon, it’s subject to Special Relativity, and will always be measured as traveling at the same speed in a vacuum. I doubt we’ll be able to directly experimentally confirm this the way we did with the photon 120 years ago, but I believe it none the less.

 

The point I was making is that, as a consequence of SR’s equivalency principle, we shouldn’t be able to make any measurement of photon or graviton interaction between the sun and earth that will tell us how we are moving relative to an arbitrary absolute reference frame, such as the galactic center. So, instead of the hefty ~1600 km difference between the actual and the photon/graviton indicated position of the sun Boerseun proposes looking for, we actually need to look for a much smaller difference due to local gravitational influences. I fear that this will difference will be to small to be practically observable any time soon, perhaps ever.

[Erasmus00]

Tom Van Flandern in a paper titled The Speed of Gravity – What the Experiments Say (1998), has found experimental and observational evidence confirming that if gravitation is taken as a propagating force of nature in flat spacetime, its speed is substantially faster than the speed of light. “This is because gravity, in contrast to light, has no detectable aberration or propagation delay for its action…General relativity explains these features by suggesting that gravitation (unlike electromagnetic forces) is a pure geometric effect of curved space-time, not a force that propagates…Although faster-than-light force propagation speeds do violate Einstein special relativity (SR), they are in accord with Lorentzian relativity, which has never been experimentally distinguished from SR-at least, not in favor of SR.” (Flandern, 1998, abstract)

 

I am familiar with Van Flandern's work, though I've never read the Eddington paper he quotes, so I will not talk about it. I want to point out that Van Flandern's work is mathematically unsound. First, there are measurable effects of gravity having a finite speed, and these have been observed. Second, Van Flandern ignores the fact that in GR, as in SR moving bodies set up fields that encode their motion. The object reacts as if it "knows" the sources is moving with a constant velocity (it is accelerations that take time to propagate). The effect is similar to the way moving charges encode their motion in relativistic EM theory. It is only accelerations that produce radiation.

 

One easy way to prove Van Flandern is dead wrong is to set up an orbital simulator encompassing GR as it is currently formulated, and see if the orbits actually do expand as he claims they would, and to see if momentum is non-conserved, as he says it would be. It is not.

-Will

[EWright]

Can someone explain how the theoretical graviton works exactly? Is it considered to be a particle? A 'bundle of energy' like a photon? I have trouble conceiving of either of these types of entities acting with gravitational influence. Light, as we conceive it, works by reflecting off of a surface and then we view the object as it was when the light was reflected. The point here is that it has to return to the observer to be witnessed, but does not have to return to its original source to have any affect. Gravity, however, it tied to the original source. A graviton would have to be sent out, attract the object it is influencing, and then somehow reattract it back to the source (like spiderman snatching an apple from across the room with his webslingers). It seems any force *sent* from an object, be it photons or gravitons, would then have a tendency to give momentum to that object (ie, solar sails), however small that momentum might be. Can someone explain who this is supposed to work with in the framework or relativity or other current theories (with minimal use of formulas please)?

[Erasmus00]

Can someone explain how the theoretical graviton works exactly? Is it considered to be a particle? A 'bundle of energy' like a photon? I have trouble conceiving of either of these types of entities acting with gravitational influence.

 

In quantum electrodynamics (QED) particles called bosons are responsible for the forces between particles(electric and nuclear). The graviton would be one such particles (the photon is as well). In QED, the photon is responsible for all electric interactions (Both attractive and repulsive). It is theorized the graviton would perform the role of the gravitational force.

 

Its worth noting that as of yet, no one has figured out how to formulate a gravitational theory using gravitons.

-Will

[Jay-qu]

these bosons must be really weakly interacting because otherwise there would be a way to sheild against them... so is there still a hope for anti-grav technology?

[questor]

if objects are receeding from us at c, doesn't that mean we are also moving away from other objects at c ? so our galaxy is also part of the expanding universe. where does the additional gravity come from needed to fill this larger universe? where does the additional

dark matter come from? is the universe ''thinning out'' as it expands? what is the propelling power for this expansion?

[Bobby]

according to relativity gravity couldnt go any faster that light, but this does mean it goes at the speed of light... any way the real point i wanted to make is that if there is matter at the other end of the universe receding from us at a rate faster than c than becuase gravity cant go faster than c, the gravity from that matter will never reach us and hence never effect us and pull us in, so then how could the universe ever stop expanding?

 

 

( 1 ) Regardless of how big the Universe is, only a sphere computed by multiplying the diaeter by the speed of light can have any effect. By this I mean pick any number you want as the age of the Universe, perhaps 12 billion years. The radius of a sphere that could have any effect is 12 billion Light Years.

 

( 2 ) Whether or not the Universe will ever stop expanding seems to change every few years. If there is enough gravity to stop the expansion and start a contraction, this is called a closed universe. If there is not enough gravity to stop the expansion, this is called an open universe. If there is exactly enough sothe universe will expand forever and everm but at an ever decreasing rate, this is call a flat universe. Recently some astronomers have come to believe that the expansion rate is acceleration. IMO the universe will someday stop expanding and will begin to contract ending in a "Big Crunch."

[EWright]

( 1 ) Regardless of how big the Universe is, only a sphere computed by multiplying the diaeter by the speed of light can have any effect. By this I mean pick any number you want as the age of the Universe, perhaps 12 billion years. The radius of a sphere that could have any effect is 12 billion Light Years.

 

( 2 ) Whether or not the Universe will ever stop expanding seems to change every few years. If there is enough gravity to stop the expansion and start a contraction, this is called a closed universe. If there is not enough gravity to stop the expansion, this is called an open universe. If there is exactly enough sothe universe will expand forever and everm but at an ever decreasing rate, this is call a flat universe. Recently some astronomers have come to believe that the expansion rate is acceleration. IMO the universe will someday stop expanding and will begin to contract ending in a "Big Crunch."

 

And what mechanism will stop the current expansion rate and result in a big crunch? Current predictions say the expansion rate will continue to increase and will eventually affect even the space between infividual attoms and and particles until the universe is stretched to virtual nothingness.

 

Also, it is not know that gravity can or can not exceed the speed of light. It is theorized that gravity can have an instantaneous effect, though the most common predictions indicate it is equal to light speed. I find it highly interesting that science assigns c to both the speed of light and gravity, without identifying the limiting factor that causes them to travel at the same rate.

 

Also keep in mind that light speed does not account for the phenomenon in quantum mechanics in which particles, no matter how distant from each other, transfer information that affects the spin of its counterpart that is transmitted instantaneously, and thus faster than light speed.

[CraigD]

And what mechanism will stop the current expansion rate and result in a big crunch? Current predictions say the expansion rate will continue to increase and will eventually affect even the space between infividual attoms and and particles until the universe is stretched to virtual nothingness.

You're correct - the most recent, most popular theory (Expansion) make this.

 

It's worth noting that Expansion, and the majority of other cosmological theories, are empirical theories. They attempt to explain observed data, but mostly make little attempt to reconcile with fundimental theories such as Particle Physics. For many people, myself included, this gives them a shaky, ad-hock feel, and makes their predictions untrustworthy.

Also, it is not know that gravity can or can not exceed the speed of light. It is theorized that gravity can have an instantaneous effect, though the most common predictions indicate it is equal to light speed. I find it highly interesting that science assigns c to both the speed of light and gravity, without identifying the limiting factor that causes them to travel at the same rate.

No main-stream theory of which I’m aware theorizes instantaneous effect at a distance. If you believe in the graviton exists, the Standard Model demands it move at c – compliance with Special Relativity requires this. If you believe that General Relativity is correct and irreconcilable with the Standard Model, you believe its prediction of gravitational field change propagating at c. (Also due to SR, thought the formalism for this has been over my head for a decade or so – what you don’t use, you lose!)

 

My personal feeling is that Holographic Principle is correct, and that instantaneous action at a distance is forbidden because it allows the amount of information in a given volume to violate the Beckenstein Bound.

Also keep in mind that light speed does not account for the phenomenon in quantum mechanics in which particles, no matter how distant from each other, transfer information that affects the spin of its counterpart that is transmitted instantaneously, and thus faster than light speed.

This is incorrect. Measuring one of an entangled pair of particles (usually but not limited to photons, or even to fundamental particles) allows you to know the state of the other regardless of how far they have been separated since their entanglement, but this can’t be used to transfer information – the pair are essentially a read-only system. Though semi-hard Science Fiction often takes liberties with this, the hard reality is that you can’t transfer information faster than c using quantum entanglement.

[EWright]

Measuring one of an entangled pair of particles (usually but not limited to photons, or even to fundamental particles) allows you to know the state of the other regardless of how far they have been separated since their entanglement, but this can’t be used to transfer information – the pair are essentially a read-only system. Though semi-hard Science Fiction often takes liberties with this, the hard reality is that you can’t transfer information faster than c using quantum entanglement.

 

I did not mean to imply that we can send information faster than c using this method. However, it is my understanding (and i may be mistaken, and i'm not sure i'd agree with it) that quantum entaglement does require the two particles to have relayed the information required to define their relationship. The theory says that the pairs are not "ready made" as having opposite spins... but that only when you measure the spin of one particle, do you affect the spin of the other. They are not preprogramed, but directly affected by our influence in measuring the first particle and that the corresponding particle then instantly takes on the opposite spin regardless of distance. Theoretically, this distance could be billions of light years, and the affect would still be instantaneous. I have a hard time accepting this, but this is how it's been explained to me and how it is explain in Brian Greene's "The Fabric of the Cosmos" very explicitly.

[Bobby]

Because it will be blue-shifted?

 

 

Can you tell me how to start a new thread?

[EWright]

Can you tell me how to start a new thread?

 

just go to the page where the threads for the particular subject are listed (ie, go to cosmology) and at the top of the list there a button to start new thread, just like you can click to reply to thread (when you don't just reply by quoting or quick reply).