Speed of gravity

[Qfwfq]

Curved spacetime doesn't initiate motion. An object must already be moving to be affected by a curved spacetime.

A velocity in space is a direction in space-time. So is rest, it is the direction (locally) parallel to the time axis. Therefore the curvature of space-time can cause a body to change from rest to motion.

 

Further, the above idea does not hold up as soon as you remember that a body at rest in your coordinates is not at rest in the coordinates of anybody that is moving in your coordinates.

 

IMO curved spacetime is a mathematical gimmick to make gravitational fields easier to use.

Yes, it's a mathematical gimmick called differential geometry, applied to the principle of equivalence.

[Bobby]

A velocity in space is a direction in space-time. So is rest, it is the direction (locally) parallel to the time axis. Therefore the curvature of space-time can cause a body to change from rest to motion.

 

 

I don't think this is correct. A particle moving in space at a constant velocity will continue to move at the same velocity unless acted upon by a force. As long as the spacetime geometry remains unchanges the motion of the particle will remain unchanged.

 

Further, the above idea does not hold up as soon as you remember that a body at rest in your coordinates is not at rest in the coordinates of anybody that is moving in your coordinates.

 

 

Hmmm. I thought this is what I said. All motion can be explained as a change in coordinate systems.

 

 

 

Yes, it's a mathematical gimmick called differential geometry, applied to the principle of equivalence.

 

 

Well, I know a little bit about the principle of equivalence and differential geometry. You are right on the differential geometry and wrong on the principle of equivalence.

[Qfwfq]

You appear to have not understood my points at all. Unfortunately I'm very busy and can't sort the matter out at the moment, in the mean time try reading what you replied to again.

 

You are right on the differential geometry and wrong on the principle of equivalence.

You say right and wrong without giving any support. This isn't the way we discuss science on these boards.

[Bobby]

You appear to have not understood my points at all. Unfortunately I'm very busy and can't sort the matter out at the moment, in the mean time try reading what you replied to again.

 

You say right and wrong without giving any support. This isn't the way we discuss science on these boards.

 

 

I post a lot and sometimes I am not clearly understood. When this happens I don't take offense, I try to restate what I said. Who is this WE from this board. As I said above, sometimes I am misunderstood, so let me make this clear ==> Your answer makes me doubt that you know crap about Physics. Perhaps you can discuss this with the WE you mention.

[damocles]

Well, I know crap about physics, but when I'm confused I look at this guy;

 

http://www.nap.edu/books/0309090512/html/R1.html

 

and he helps explain a lot to my muddled mind. About gravity and non-locality;

 

http://www.nap.edu/openbook/0309090512/html/106.html

 

I find to be helpful everyday explanations.

 

Best wishes.

[Bobby]

Well, I know crap about physics, but when I'm confused I look at this guy;

 

http://www.nap.edu/books/0309090512/html/R1.html

 

and he helps explain a lot to my muddled mind. About gravity and non-locality;

 

http://www.nap.edu/openbook/0309090512/html/106.html

 

I find to be helpful everyday explanations.

 

Best wishes.

 

 

OK. so you know a little bit about Physics. When I'm wrong, I'm wrong. The reason I thought the equivalence principle and differential geometry were not connected is as follows. Differential Geomety is the Calculus applied to Geometry. Usually only mass and energy are considered when coming up with a Geometry. It seems to me that adding motion, especially accelerated motion, would add to the mathematical difficulties without adding anything to the solution.

 

 

The links you provided are very interesting.

 

Regards, BP

[Southtown]

Well, I know crap about physics, but when I'm confused I look at this guy;

 

http://www.nap.edu/books/0309090512/html/R1.html

 

and he helps explain a lot to my muddled mind. About gravity and non-locality;

 

http://www.nap.edu/openbook/0309090512/html/106.html

 

I find to be helpful everyday explanations.

 

Best wishes.

THAT is a nice link dude. Thanks!

 

I don't think this is correct. A particle moving in space at a constant velocity will continue to move at the same velocity unless acted upon by a force. As long as the spacetime geometry remains unchanges (sic) the motion of the particle will remain unchanged.

This seems true as well. If the particle is not moving relative to the gravitational field, then its time will not be distorted, and motion cannot result.

[Qfwfq]

I post a lot and sometimes I am not clearly understood.

Quite true, and I don't understand what you mean by WE. What WE did I mention?

 

When this happens I don't take offense, I try to restate what I said.

I did not take offence either Bobby, I said that I couldn't further discuss the matter at the moment but pointed out that you missed my points. I may have been brief when I made them but if you make out that you know more than I do, you should have been able to understand. Actually, you would not have said what you said in the first place.

 

so let me make this clear ==> Your answer makes me doubt that you know crap about Physics. Perhaps you can discuss this with the WE you mention.

Doubt if you like, it doesn't make much difference to me if you think I graduated in physics (including a course in GR) for naught, but that's not the way to respect those you are discussing with.

 

BTW, without the principle of equivalence there would be absolutely no sense at all in describing gravitation as a geometrical effect. If you couldn't get even that much together before following damocles' links, perhaps you should have asked me for clearer explanations of what I had said instead of making such remarks about it.

[maddog]

I once spent two hours argueing that sodium metal could be induced to explode on contact with water. The school science teacher in the group insisted it was impossible, and that it would only burn - and he had done the experiments to prove it!

How long ago are you talking about??? :Waldo: It's an old trick that every chemist knows. I don't know why on Earth that school science teacher insisted it was impossible. :hyper:

As Qfwfq says - old trick. I know it works. I saw it done by a bunch of us bored students

in chemistry in high school waiting for class to start while the teacher was out of the

room. We use about a 1/4 gm into 4 oz distilled water. Quite violent. Definitely more

than just fizz. Some us even got in trouble when it was discovered a lot of our school

supply Sodium missing. ;-) In fact Potassium, I believe is just as violent (maybe not

explosive).

 

maddog

[maddog]

The reason I thought the equivalence principle and differential geometry were not connected is as follows. Differential Geomety is the Calculus applied to Geometry. Usually only mass and energy are considered when coming up with a Geometry. It seems to me that adding motion, especially accelerated motion, would add to the mathematical difficulties without adding anything to the solution.

Principle of equivalence links force of gravity to an accelerated reference frame.

Differential Geometry is a mathematical description of relating the curvature of a space

to an accelerated reference frame (exactly what Qfwfq said). Thus the link.

I believe I started a bit of Diff Geom in Calculus when learned how to compute the

radius of curvature in curvillinear coordinates.

 

maddog

[maddog]

I have noted this thread quite meandering and quite off topic. Since I have not been

able to put my 2-cents in from the beginning, excuse me for digressing....

 

As to the "speed of gravity", I am not sure if there is a current consensus as to whether

gravity moves at the speed of light © or not. I would think it doesn't have infinite

speed either (instantaneous field response) otherwise Quantum Field Theory might be

impossible to do in this case. It was Newton who actually thought gravity did travel

instantly (Action at a Distance). No one questioned Newton until Einstein. Though

Einstein for most part glossed over considering any other speed than c for gravity. He

never specifically stated thus. He did often argue against Newton's philosophy. Read

the "Spinning Bucket" story very well treated by Brian Greene in "Fabric of the Cosmos".

You'll get a great lesson in this notion. Best I've read.

 

I would speculate that maybe there is a causal relation between the spin of a Boson

and the speed with which the associated field propogates. Since Light (Photon) is a

spin of 1 it would naturaly go at the speed of c. And Gravity (Graviton) has a spin of

2 as computed from String Theory and thus maybe could travel at 2*c. Were there

to be a Boson of spin 3, it would then travel at 3*c.

 

Once we can determine the subtle energy levels of detecting gravity waves with LIGO

or its future upgrades, we might be able to verify an actual speed rather than just

theory and speculation.

 

Boson are not Bosons because they are the force carriers and travel at the speed of

c. They are Bosons because they have integral spin. Yes, so far all do carry a force.

Yet for example Z0 Boson has Zero spin, has mass and travel at less than c. This is

a particle used in propogating the Weak force.

 

The last point is because of the wave particle duality prevelant in QM, it has been

thought (and so far corroborated) that all particle have a dual wave nature and visa

versa. Thus no distinction is drawn between the propagation of a wave or the

motion of its dual particle. A field description allows for both.

 

maddog

[Qfwfq]

I disagree with a point or two in your last post Maddog.

 

It was Newton who actually thought gravity did travel instantly (Action at a Distance).

This is often said about Newton but I didn't find it in the Principia, I rather found him saying that many things were still to be deterimined including the nature of the forces. His aim in Principia was to discuss the mathematical descriptions that could explain planet's motions and also falling bodies.

 

He did often argue against Newton's philosophy. Read the "Spinning Bucket" story very well treated by Brian Greene in "Fabric of the Cosmos".

How is that an argument against Newton? I don't see that it is.

 

I would speculate that maybe there is a causal relation between the spin of a Boson

and the speed with which the associated field propogates. Since Light (Photon) is a

spin of 1 it would naturaly go at the speed of c. And Gravity (Graviton) has a spin of

2 as computed from String Theory and thus maybe could travel at 2*c. Were there

to be a Boson of spin 3, it would then travel at 3*c.

I wouldn't speculate that at all, certainly not about speeds greater than c. String theory isn't necessary to predict spin 2 for gravity, GR predicts it.

 

Boson are not Bosons because they are the force carriers and travel at the speed of c. They are Bosons because they have integral spin.

Bosons are bosons because they act according to Bose-Einstein statistics. Fermions are Fermions because they act according to Fermi-Dirac statistics, of which Pauli's exclusion principle is a consequence. Bosons could hardly be the parts of matter and fermions would be ill suitable for mediating forces. These two possible types of statistics are consequence of whether the amplitudes for more than one identical particle are symmetrical or anti-symmetrical for exchange.

 

The explanation of why bosons ar of integral spin and fermions of semi-integral spin is very complicated, it's a result of field theory.

[maddog]

This is often said about Newton but I didn't find it in the Principia, I rather found him saying that many things were still to be deterimined including the nature of the forces. His aim in Principia was to discuss the mathematical descriptions that could explain planet's motions and also falling bodies.

Have I fallen prey to an Urban Legend ? I may have misinterpreted my Freshman

Physics text, Halliday & Resnick. You intrigue me again to look Principia. ;-)

How is that an argument against Newton? I don't see that it is.

I didn't mean that as an arguement agains Newton. In Green's book, it was used as a

discussion in the development of the philosophy behind Relativity (even before Einstein).

I wouldn't speculate that at all, certainly not about speeds greater than c. String theory isn't necessary to predict spin 2 for gravity, GR predicts it.

That is a derivation I would be insterested. Having only partially gone through

Gravitation by Misner, Thorne, Wheeler (I think), I wasn't aware of that.

Bosons are bosons because they act according to Bose-Einstein statistics. Fermions are Fermions because they act according to Fermi-Dirac statistics, of which Pauli's exclusion principle is a consequence. Bosons could hardly be the parts of matter and fermions would be ill suitable for mediating forces. These two possible types of statistics are consequence of whether the amplitudes for more than one identical particle are symmetrical or anti-symmetrical for exchange.

No dispute.

The explanation of why bosons ar of integral spin and fermions of semi-integral spin is very complicated, it's a result of field theory.

I am reading QFT in Nutshell by A. Zee. So maybe I will find that result. ;-)

 

maddog

[SockCymbal]

Have I fallen prey to an Urban Legend ? I may have misinterpreted my Freshman

Physics text, Halliday & Resnick. You intrigue me again to look Principia. ;-)

 

I didn't mean that as an arguement agains Newton. In Green's book, it was used as a

discussion in the development of the philosophy behind Relativity (even before Einstein).

 

That is a derivation I would be insterested. Having only partially gone through

Gravitation by Misner, Thorne, Wheeler (I think), I wasn't aware of that.

 

No dispute.

 

I am reading QFT in Nutshell by A. Zee. So maybe I will find that result. ;-)

 

maddog

Additionally In Green's "The Elegant Universe," he points out that it was in fact initially Newton who thought gravity to be an instantaneaous force. That is, if the sun were just simply vanish right now, the earth, and everything else in the universe would be affected immediately. Then when Einstein came along with relativity, he claimed gravity to travel as a wave, at exactly c. Therefore, if the sun were to vanish, we would not be affected for ~8 mins. If you say Green's Nova special, they do a very good job of visually animating this, as well as many other things.

[Bobby]

Additionally In Green's "The Elegant Universe," he points out that it was in fact initially Newton who thought gravity to be an instantaneaous force. That is, if the sun were just simply vanish right now, the earth, and everything else in the universe would be affected immediately. Then when Einstein came along with relativity, he claimed gravity to travel as a wave, at exactly c. Therefore, if the sun were to vanish, we would not be affected for ~8 mins. If you say Green's Nova special, they do a very good job of visually animating this, as well as many other things.

 

 

I believe you will find that Newton stated that he did not know what the force which controls the planets was and he made no assumptions.

[Turtle]

___If you talk about gravity without talking about Fuller then you don't know all the facts. Whether you care to attack him or get on board, it's best to know what he said.

http://hypography.com/forums/showthread.php?p=63668#post63668

:hyper:

[Bobby]

___If you talk about gravity without talking about Fuller then you don't know all the facts. Whether you care to attack him or get on board, it's best to know what he said.

http://hypography.com/forums/showthread.php?p=63668#post63668

:hyper:

 

Hmmm. Why would I attach this Fuller person when I never heard of him. For that matter, I don't recall attacking anyone on this board.