Can anyone else confirm the truth of this statement? If so, how could any model of physics honestly predict it? If a model can just answer the questions of “why does the effect of gravity appear to be instantaneous?” with “because gravity knows where the sun will be in the future and exerts a force based on it’s future location, not it’s present one” I call bullshit.
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The statement is a misunderstanding.
The idea that a body, whilst in orbit, should act according to the time-retarded gravity position of a second body is superficially plausible. However, this neglects a significant amount of canceling out required to preserve the total angular momentum of the system, due to the mechanical instability introduced by the time-retarded gravity. Since a body in orbit under constant acceleration cannot radiate gravity, this additional compensation is in terms of the gravitational focus.
In reality, and demonstrated experimentally, this means that a body acts according to the interpolated time-retarded gravity position of a second body. This phenomenon is why instantaneous Newtonian gravity and speed-of-light General Relativity gravity are in such close agreement.
While there is no practical method for transmitting information faster than the speed of light, I have read of experiments indicating it may be possible.
I’m going by Bill Bryson’s “Short History of …” wherein he relates an experiment with particle pairs having opposing spin. Apparently when you ‘flip’ one of them the partner reacts instaneously.
I don’t know how valid this is, but if true it does imply that some form of information can be transferred faster than the speed of light.
I’ve read about that hypothesis in the past, but as of yet it remains highly suppositional, and thus far we lack even a theory of how to transmit information faster than light, much less an actual method.
I think the phenomenon your refering to is called Quantum Entanglement. I won’t pretend to be able to describe it - but note the following from the Wiki link:
Nice try, but no-communication theorem probably throws a spanner in the works. Long story short, no meaningful transfer of information can occur at faster than light speeds.
I guess we’ll never KNOW until/when it happens. One question that comes to mind is if the sun disappears and it takes 8 minutes for us to know, what is allowing us to revolve around the sun/unsun? And is it true that light travels at 186,000 miles per second? What’s that in metric? Kilometers?
Please PHONE ME before it happens.
If you read the thread above you will learn the answer.
300,000 km/sec is a very good approximation, once again proving that metric is the superior unit system 
(Need more proof? 1 lightyear = 10 trillion kilometers)
It is indeed quantum entanglement, it has been observed in the lab, but it doesn’t allow faster-than-light transmission of information.
It’s not that you “flip” the particle and the other one responds. Rather you measure the spin of one particle and discover that the other one matches.
What’s so weird about that you might ask? Obviously the two particles had their spins synched when they were together so of course it should match when you measure it when they’re apart. This is called the “hidden variable” explanation.
However, an experiment can be set up to prove that no hidden variables exist. Unfortunately, it’s difficult to provide a simple explanation of how the experiment works. Basically you create a situation where you gradually vary how you take one of the measurements. If the correlation were the result the two particles being synched when they were together, then it should drop off linearly as you gradually change how you measure. However, it doesn’t work out that way. Instead of a linear decrease you get a curve – they stay correlated longer than they should, then there’s an abrupt drop off. Somehow both particles “know” what technique will be used to measure them at each end of the experiment and that affects their spin.
But you can’t use the technique to transmit information. At both ends of the experiment all you see are a random sequence of spin measurements. It’s only when you put them next to each other and compare them that you can see that what you did at one end of the apparatus instantly altered what happened at the other end.
As the universe’s expansion is accelerating, there is something acting against gravity. Perhaps it can be collected and put to use, maybe for a gravity shield.
General relativity is a theory that explains the effects that gravity has on masses. In no way does it explain how gravity really “works”, except that the manifestation of it is a property an underlying plenum (which we can’t measure or see) that is influenced by the presence and relative velocity of local mass. Charles Misner, Kip Thorne, and John Archibald Wheeler wrote a book quite simply titled Gravitation which explains the tenants of general relativity and is widely considered the primary reference text on gravitation, and Thorne himself admits that we don’t really understand what the underlying mechanic is, just that we can describe it (in most regimes) in very accurate detail. General relativity is like most non-computer engineers conception of a computer; they can describe it, they can use it for a multitude of tasks, if they’re clever they can even program it, but from the processor down it may as well be all little green men with abacuses as far as they can tell.
Whether we “believe” in an underlying causal mechanic in the physics of the universe or not is immaterial; the universe will continue to do whatever it pleases. A study of quantum mechanics indicates that while an assumption of causality is at least a valid approximation at most scales in which quantum behavior is decoherent (not distinguishable from normal mechanics with expected classical uncertainty of measurement and perturbation), on the quantum level behavior such as that found in the famous double slit experiment is explained only by non-local connection (entanglement, de Broglie–Bohm and other explicitly non-local hidden variable interpretations), predetermined mechanics (consistent histories), the intersection of multiple consistent permutations (Everrett-DeWitt many worlds), solipsism (Consciousness Causes Collapse), “it all just works at the boundaries” (pilot wave theory), or any other number of theories all of which violate classical causality either in locality or explicit determinism.
However, for mass-energy to suddenly and completely disappear would be a violation of conservation of mass, and this would cause a complete breakdown of physics. The coupling between two masses at any distance is more than just an exchange of information; it actually controls their binding properties to each other and the other masses around them, and for one of those to change with an infinite time rate of change is just outside of the ability of current physical theory to address.
I implore you to please, please, please don’t treat anything that Bryson offers in this book to be anything other than ill-informed regurgitation of vaguely understood factoids.
Stranger
It’s been a year so I have no supporting links at the moment, hopefully another Doper can fill in or at least add to the info so I can research it properly.
I was watching a show about string theory and alternates, mathematical proofs of extra dimensions, general geek stuff. One of the issues they brought up is that gravity isn’t as strong as is should be. Then one of the brainiacs that thinks about these things realized that the numbers for gravity work out if gravity is actually native to a different dimension and all we’re getting is the leakage. This was suggested by a woman researcher if that helps anyone looking it up.
I hadn’t considered until now, but that’s a perfectly valid reason why we wouldn’t be able to block gravity. We would have to erect our barrier in another dimension to keep it from leaking into here. I could also concede that the speed of light isn’t an absolute barrier in the gravity-0 dimension. So removing the local gravity component could mean that the effects don’t have to propogate.
If I’m not mistaken, you might be talking about a couple of things here:
Dark matter - From observing the motion of stars in a galaxy, it appears that the galaxies are spinning faster than the observable amount of mass would allow. Therefore, there must be a significant amount of non-observable mass (75% of the mass in the universe IIRC) whose gravity is holding it together.
Dark energy - By some calculations, the stars and whatnot are continuing to accelerate away from each other faster than their mass should allow. This implies that there is some form of unobserved energy pushing them along.
IIRC there have been some theories around these observations and I believe one of them includes multiple universes or the additional dimensions inherent to string theory.
Dark matter/energy were mentioned, but the primary focus of this episode were the theories of extra dimensions and the possible ramifications. I’m thinking they were talking about something like 11 or 14 total. One interesting note was the disagreement between the string theorists and a new idea that was being pushed.
And then someone pointed out that one of the shortcomings of string theory meshed nicely with the new school. Suddenly everyone is all buddy-buddy again.
Which is a good thing. Theoretical physicists feuding is not something for the squeamish.
If by “works” it is meant works on a sub atomic level then fair enough. For the purposes of the original question (about the speed of propagation of gravitational effects) general relativity suffices.
It is material as the first example that popped into my head about why information (energy) can’t travel faster than light. This is scientific fact regardless of all the weirdness going on at the quantum level.
But to come back to topic. Sorry about that.
If you just mean to flip off the lights and leave the mass of the sun in place, I can’t imagine that we would have any warning before things went dark.
If, on the other hand, you want the sun to completely vanish taking its gravity well with it, then you have two possible paths.
If gravity travels at the speed of light, then you’re still going to wait 8 minutes to get the bad news.
If gravity isn’t bound by the speed of light and is indeed faster, then you would have up to 8 minutes warning. What would happen in that time is anybody’s guess.
Googling speed of propagation of gravity as to the speed of light, the concensus seems to be , “maybe”.
Very little. Earth’s orbital velocity is about 30 km/sec. That means in 8 minutes it only travels about 14,000 km. For comparison, Earth itself is about 12,500 km wide. So the deviation from our normal orbital path during those first 8 minutes would be TINY. I’m not sure we could even detect it if we were trying to.
Hmmm, I would assume the people on the sun side of the earth would be slightly heavier, as the sun’s gravity had been tugging them ever so slightly away from the earth. And correspondingly the people on the dark side should be lighter as the gravity of the two bodies would no longer be additive.
Anybody have the knowledge to calculate the magnitude of the effect? I can’t imagine it would be large enough to notice.