I was just coming in to say that. I was disappointed as it sounded fantastically interesting.
(Relativistic physicist: an ordinary physicist accelerated to nearly the speed of light)
Seriously, though, the thing that I don’t get about the speed of gravity is the lack of orbital decay in a two-body system.
Say, for instance, that you have two bodies of equal mass that are orbiting around the point at their midpoint. Now, for a body rotating around a central point, the centripetal acceleration a[sub]c[/sub] = v[sup]2[/sup]/r. The bodies’ pull on one another a[sub]g[/sub] = G[sup].[/sup]m/4r[sup]2[/sup]. For a stable orbit, a[sub]c[/sub] = a[sub]g[/sub] (actually, F[sub]c[/sub] = F[sub]c[/sub], but the masses are identical), so we can solve for v® = 1/2[sup].[/sup]sqrt(G[sup].[/sup]m/r).
But can we? The gravitational pull from a given object, as your report stated, travels at the speed of light. The first mass’ gravitational acceleration vector is not directly opposite of the centripetal, it’s from a bit of time before that, however long it took to the gravitational wave to get there. And that means that, in such a case, that the orbit will decay.
Now, if the masses are less evenly matched, then the effects will be smaller, but will still exist. So, for example, the sun’s mass is a bit over 1000 times the mass of Jupiter, so the point they orbit about is significantly closer to the sun. But it’s still not at the sun’s center of mass, so there is some peturbation that, over millions and millions of years, would have caused Jupiter to fall in the sun.
Since this has not happened, my reasoning above is somehow flawed, so I was hoping someone with a better understanding of relativity could explain what I’m missing. The fact that the two bodies are exactly opposite to each other is, of course, true only for the observer who is stationary relative to their center of mass. Am I incorrect about the orbital decay; will it not happen? Is there something about the Lorentz contraction of time/distance that makes the gravitational pull from the opposite mass get there at just the right time? Or is it something else entirely?
Punoqllads, former relativistic physicist here. I’ve slowed down so much, I’m just a programmer too. And, I frequently make mistakes because I’m trying to write between the interruptions my boss calls work.
General Relativity does in fact predict that orbits decay. One of GR’s predicitions is that two orbiting bodies radiate gravitational waves, which travel at the speed of light. Those waves contain energy. That energy comes from somewhere. Chronos mentioned that pulsars provide indirect proof of gravitational waves. That proof is related to the orbital decay. Pulsars are incredibly accurate clocks. So accurate, in fact, that changes in the rate at which they “beat” can indicate orbital decay. All of which answers chunda21’s question, I believe.
And Bucky, I’d be careful being wise with chunda21. I suspect that theologian knows more physics, than you do theology. One thing I learned in graduate school, was that all of those seemingly absurd theological and philosophical types were generally pretty damn sharp.
For some time, my operating hypothesis regarding gravity has been that it is at base a local effect, pulling “space” (whatever that is) into “gravity wells” (whatever they are). The idea that gravity radiates out in particles to pull things in is dubious at best. I think your “waves” of gravity are going to turn out to be the suction in space itself, thus they will travel at whatever speed gravitational acceleration is at that point.
;)The fact that trained physicists don’t see this is, of course, Einstein’s fault. I don’t know how, but always always blame Einstein.
i find it amusing that everyone failed to mention that gravity itself has no speed only acceleration.
anyone remember basic physics.
i believe the number was around 9.8m/s squared.
In General Relativity, gravitational waves are distortions in spacetime. They are called waves because the mathematical description of them looks like waves. They move things in and out just like waves. Complicated, tensor waves, but waves. Is that what you mean by “the suction of space itself”, foolsguinea? It is the quantum mechanics who picture gravity waves as composed of particles.
I believe (it has been 14 years!!) that it is possible to show that any metric theory of gravity predicts waves. (The term “metric theory” is technical, but it basically means that the distance from a to b is the same as the distance from b to a.)
ZenMonkey, you are remembering the acceleration of an object on Earth due to gravity, not “the acceleration of gravity”. Neither gravity nor light ever accelerate. If they exist, they are moving at the speed of light.
[whoosh]ZenMonkey[/whoosh]
Oh geez, I hope chunda21 knows that I was just teasing him a little. And I certainly don’t think theological and philosophical types are absurd. Those topics might be absurd but then quantum physics is pretty counterintuitive to an outsider, too.
I’m a big buff of the history of science and technology (and Feynman) and have been observing how the scientific community responded as gravity wave research went from being ‘little science’ to ‘big science’. The original GW researchers and their ‘low-tech’ equipment are treated somewhat like a family reacts to their slightly wierd relative who shows up at gatherings but are reluctant to be associated with them. (I know EXACTLY how that goes…)
At the same time, the scientific community doing GW work has encouraged a diverse set of opinions to be expressed in the scientific journals. They should be given credit for their inclusive attitude. That’s why in my initial post, I started out with position that might be considered a ‘crank’ point of view. That was diliberate, just to generate some discussion.
And slowmindthinking, you are correct about chunda21 knowing more physics than I do theology. That’s easy because I know nothing about either topic really. I’m just an old geek who just really wishes he could do computational astronomy or physics.
best regards,
buck
“I was born not knowing and have only had a little time to change that here and there.”
- Richard Feynman
Wait a minute, it takes energy to generate light or other waves or particles, right? Nothing can generate waves or particles forever, right? If gravity is a wave or particle, does that mean the sun will some day run out of gravity? Does its mass decrease over time due to the emission of gravity waves or particles? I’m sure its mass decreases because its emitting lots and lots of energy and particles, but does it also decrease because it’s “emitting” gravity?
Or, am I wrong about needing energy to generate waves/particles? Are there other examples of things the can generate that stuff indefinitely?
Yes, an electron has a charge, but it never runs out. The charge is sent out into space by the photon. This is a pretty close analogy to gravity. No energy is lost.
Tuco/White Lightning give the LIGO site another shot looks like its back.
But, doesn’t an electron only send a photon out if it changes energy states? So, if it is excited to a higher energy state (from some outside energy source), then falls back down, it emits a photon. Or, if it is shoved up and down inside a wire, it emits photons. It doesn’t just sit there and emit photons.
Is that also true for gravity waves? In other words, do you have to wiggle a mass to have it emit gravity waves or particles? Does a lump of iron just sitting there not emit those things? Maybe that’s (among) my point(s) of confusion.
I have heard of this effect before, and some people call it the ‘aberration of Gravity’ as an analogy to the aberration of starlight.
Not sure SlowMindThinking fully answered the question- as I understand it, this effect would cause orbits to decay in a few millions of years, rather than billions- but unfortunately I don’t know the real solution to the problem, although I did have a link to it which no longer works.
From memory it had to do with inertial frames of reference and a compensating force in the other direction,
which allows gravity to be propagated at lightspeed.
as we know, anything which could be used to transmit information
can not be allowed to propagate faster than light, or very bad things will happen.
Sci fi worldbuilding at
http://www.orionsarm.com/main.html
It is a confusing topic, when you’re talking quantum effects. Consider the case of an electron in space by itself, and a proton in space by itself. They’re not emitting photons, right? But now start to bring them closer together. There will be some attractive force on the two, and the photon is the medium of this force. As they started to get closer, how did the electron know that the photon was there, so it could exchange a photon with it? Is it sending out photons all the time in every direction, seeing what photons it can interact with? No, that would take energy.
The answer I’ve heard is that it’s continuously emitting “virtual photons,” pretend ones that only become real when something is there to interact with.
I don’t understand this, but once you accept it, the idea of how quantum gravity might work is a straightforward extension.
Rittersport and CurtC, the solution to your conundrum is that static fields doing nothing require no energy to maintain. Consider an infinite universe containing but an electron. The static electric field of that electron is detectable everywhere in that universe, assuming that the electron has always existed. Yes, the electric field is carried by photons, but the electric field is static, which means it doesn’t change with time. Since it doesn’t change with time, it’s frequency is zero (and wavelength infinite). Since the frequency is zero, the wave requires no energy.
Think of a phone cord. It is easy to create a standing wave in the cord; just wiggle it up and down. The less bumps in your standing wave, the longer the wavelength. It takes less energy to create a one bump standing wave than a two bump standing wave. Imagine stretching the phone cord out to infinity, so that you can make really long waves. It still takes less energy to create longer waves than short ones, and obviously it takes no energy to not create a wave - the infinite wavelength limit.
So, an isolated electron can sit around forever, and not lose it’s charge. Now imagine the same universe with both an electron and a positron. When one of the electron’s virtual photons, which carries that static field, comes in contact with the positron, it is absorbed. Suddenly, that photon has ceased to exist - therefore the field is not static. The energy must come from somewhere, and it comes from the potential energy resulting from the fact that the electron and the positron are in two different places. Reducing the distance between them reduces the potential energy, and supplies the energy for the not static field. This picture presumably applies to gravity also, but without a quantum theory that includes gravity, one should withhold judgement.
However, the gravitational waves that LIGO is designed to detect are different. If you wiggle an electron, it radiates electromagnetic waves. If a negative charge orbits a positive charge, they radiate electromagnetic waves. If you wiggle a mass, it radiates gravitational waves. If one mass orbits another mass, they radiate waves. These waves are both described nicely with classical physics and no quantum is required. (Although the second electromagnetic example what led to Bohr’s first quantum model. Classically, an atom out to radiate all the orbital energy away in some mindboggling short time. It takes quantum mechanics to explain an atom.)
Doesn’t the EPR paradox draw doubt on the impossibility of instantanaeity? Did someone in the thread already say this? What exactly is that attractive shiny object on the ground there?
A more likely possibility for FTL communication comes from GR itself. GR permits solutions in which traveling back in time is possible. Travelling back in time implies FTL, as you could move, go back in time, move, go back in time, etc, and end up someplace more quickly than light could get there. A negative energy density is required, which is only possible with quantum mechanics, and sending matter through destroys the negative energy “gate”. But I haven’t seen anything that precludes sending a photon back in time, or at least a virtual photon. And if you can do that, it might mean a LeGuinn like “ansible” is possible.
New Scientist magazine had an article about measuring the speed of gravity in January. Here is a link to the article
http://www.newscientist.com/news/news.jsp?id=ns99993232
And the results are that gravity travels at the speed of light.
As for orbital decays: The effect to which Punoqllads is referring is not the same effect as orbital decay in GR. This happens a lot, that Newtonian physics will predict a result which, by coincidence, seems to resemble a result from GR (as another example, you can “calculate” the Schwartzschild radius of an object by setting the escape speed at the surface to c. You get the right answer, but for the wrong reasons). If you assume Newtonian gravity, then, in order to explain the continued existance of the Solar System you need to suppose that gravity propagates instantly, or at least, insanely faster than light. GR fixes this problem (please don’t ask me how), but also introduces an orbital decay effect of its own, which is much smaller and perfectly consistent with the Solar System.
And SlowMindThinking, I wouldn’t say that “a negative energy density is only possible with quantum mechanics”. It does not seem to be possible with classical physics, to be sure, but we really don’t know about the quantum level. Oh, you can get a sort of very localized negative energy density, but in turn, it’s not known if that will work for wormholes or any of the other GR “shortcuts”. So far as we know, you can produce a wormhole, but if you try to send anything with energy through it, even just a photon, it’ll collapse. And while a virtual photon is allowed to have zero energy, you can’t transmit any information with a virtual photon.
Yeah, but the advantage of this method over EPR type effects is that most important phrase “So far as we know…”. Perhaps it will be possible to communicate merely by opening and closing wormholes. Actually, I’m sure the uncertainty relations preclude detecting wormholes opening and closing, but given enough clever people and enough time. We can still dream…