When an electron changes its orbit energy is released or obsorbed. My question is that does the variatiions in the radius of an orbit cause a gravitational system to release energy? If so, could this not or does it justify the GRBs say if two supermasses have a rapidly changing orbital radius?

Absolutely, for short-duration GRBs thought to be merging neutron stars. It is also the energy of long-duration GRBs and supernovae-- the gravitational collapse of a stellar core.

As Ken said, the gravitational energy (from a Newtonian POV) is indeed released when systems collapse. The things Ken mentioned are "regular" ways energy can be released -- light, heat, shrapnel, etc.

However, General Relativity predicts orbiting systems can shed energy via gravitational radiation as well. Normally that is miniscule, save for some very exotic, highly relativistic cases like two inspiralling black holes, where an enormous amount of gravitational radiation is predicted.

What fascinates me there is in such cases, the radiation is not isotropic and has a net linear momentum, giving the merged result a big radiation recoil kick. That kick can apparently be large, on the order of 100km/s or tow relative to original center of mass. They think it can well kick it out of the host galaxy and are looking for signatures of that. That is definitely a non Newtonian result!


-Richard

It looks that even in an atom that radiates a photon, the photon carries with itself part of atom's mass equal the photon's energy (divided by c^2). It must be case of gravitational radiation and one may call the photon a "graviton" if one is interested only in the loss of mass by the atom.

You don't even need black holes for that.A binary neutron star is sufficient and many of them were already discovered.

A question I always had, a body in a perfectly
circular orbit around a dense star. How does
the advance of perihelion effect manifest
itself? A slightly faster orbit time than
Newtonian or by degrading to an eliptical
orbit?

No, it must not be gravitational radiation, it is electromagnetic radiation. And no, it's not a graviton, it's a photon. You have the quantum numbers wrong. If any particle with mass-energy was a graviton, then all particles would be gravitons. They're not.

A binary neutron star can make gravitational radiation, but it won't produce the significant "kicks" that publius is talking about. That must happen from a very sudden and very powerful pulse at the moment highly compact objects merge, or from an integrated asymmetry over time in a very strong gravitational wave source (very compact fast orbits). I don't know which dominates but I would suspect the former.

I would venture that in large part it would depend on the shape of the "body" doing the perfect circling and the number mass and distance of any other bodies. But in a static space two bodied system with no interference, there is high statistical probability that the "body" would continue in a perfectly circular orbit and perihelion would be meaningless, if not I would venture heizenburg and/or radomness.

I do have another question related to this. If a system absorbs mass does the background radiation lose or gain heat from the process or have I jumped too far? Also, as masses move closer - grav radiation is released; is the inverse true as well? meaning if the masses are moving apart do the bodies absorb more gravitational radiation?

Yes, it's a slightly faster circular orbital speed than Newton. While radial Schwarzschild geodesics are fairly simple, going to 2D and orbital geodesics gets more complex. If you're familiar with the basic Newtonian orbital equations in polar coordinates, those get modified with additional terms. IIRC, you get two circular orbital solutions (that converge to the single classic Newtonian circular orbit in the weak field limit), one stable and the other unstable. In the unstable case, the slightest perturbation sends it flying wild.

In Newton, circular orbital speed is always less than radial free fall speed at a given radius. In Schwarzschild, circular orbital speed exceeds free fall speed at some point, and becomes light speed at 1.5 Schwarszschild radii. (and this is unstable -- the last stable circular orbit is at 3 radii).

-Richard

I worry about the quantum numbers too however if gravitons are emited from the atom at the same time when a photon is (when the atom loses its h*nu energy) with is the same as its loss of gravitational energy) then at least there is a strong suspicion that it is the same particle. And so the same kind of radiation, namely electromagnetic. Since otherwise where do you see the difference between the two? There must be such places if those two radiations are different.

That is a logical fallacy-- that if two things involve the loss of mass from the object, the two things must be the same. If you take your clothes off to go to bed, are you "emitting gravitational radiation", or just taking your clothes off? There's a reason that science defines its terms more carefully than that.
Exactly. There are (the quantum numbers of the particle, and its attributes like charge, rest mass, etc.). These are basically the same kind of things that distinguish clothes from gravitational waves.

And it's a good thing, too.

Forum rules prevent further digressions. However, I am e-mailing this post to my significant other, to see if she laughs hard enough to spray coffee on the screen.

Seriously, Ken, your explanations are excellent as always.

Thanks, John M.

Thanks Richard. It could have been interesting
to have unstable states in GR. And I am
reminded of something I read that this part
of GR is because of an abberation effect
whereby the fast moving body senses the
shift in position of the star. Which strongly
suggests gravity being caused by a flux at
lightspeed. Probably too simple an explanation.
And anyway Think of two equal sized bodies
orbiting each other. Again faster than simple
Newtonian I expect.

Ken,

Don't hold me to it exactly because I'm not sure of the details, but I think the kick does indeed occur during the last moments when they "hit" each other.

These things are a complex mess to say the least and can only be tackled numerically. And numerical GR solutions are something else. IIRC, it took some ridiculous amount of total CPU time on some big parallel processor farm specially designed to solve them just for the last orbit before merger.

I've forgotten the details, but I remember reading a description of just how difficult and numerically back-breaking the thing was. A big problem was sanity checks. They were going off in uncharted waters -- no analytical solutions or other simplified solutions to compare with and keep things between the ditches. It would be bad to waste wads of CPU time after some numerical hiccup through the solution into la-la land.

Another interesting thing I remember is the final "kick" depends strongly on the spin of the merging black holes (the mind boggles at the complexity of two merging Kerr holes!). You've apparently some highly complex "spin-orbit" interactions going on that contribute to the final linear kick.

-Richard

Both of course but what is your point? I didn't they MUST be the same they just MIGHT.

Both photons and gravitons have zero charge and zero rest mass. Why they can't have the same spin?

The point was to keep you from confusing those who thought there was a difference between a photon and a graviton. Saying they might be the same is like saying a chicken might be the same as a poodle. Yes, they might, but you can see how the claim could be confusing.
Because they don't. A photon has spin 1, a graviton has spin 2. The former is experimentally established, the latter comes from the theory of gravity waves. More importantly, photons are very easy to detect, gravitons very difficult, so such a difference in property could not be from the same thing.

Is a theory, in which graviton had spin 1, possible? Otherwise there has to be a graviton (or several) coupled to each photon. How is this problem handled in the theory of gravitatinal waves?

One can only go with the current best understanding of the data compiled so far, and that understanding says that gravity waves have nothing directly to do wth photons. In string theory, there is some deeper connection between everything, but so far no such connection is useful or predictive. At the level that most problems are treated, it is important to distinguish a graviton from a photon, and there is not a need to "couple" them. I have no idea what the science of gravitons and photons will look like a thousand years from now, but I do know that precise language is required to use science effectively.

Thanks for your answer Bigsplit sorry I did
not see it yesterday. Should have scrolled up
more. Dont think we need Heisenberg here in
classical territory. I think I read somewhere
that gravitational waves do not carry energy
so no absorption would seem possible. But then
what the hell are they

The fast speed of neutron stars I understood
to be the result of unsymmetrical collapse
of supernovae. After all only a manmade gadget
achieves true uniform implosion because it is
made to.(you know what!) A supernova must
start at one point of the final burning shell
and the infalling spread out from this point
around the shell. Must be in the models that
people try. And spin comes in somewhere I
suppose.