Speed of Gravity Experiment Challenged

An anonymous reader writes "The previous hoopla over the discovery of the speed of gravity has an opponent from the Lawrence Berkeley National Laboratory. Read about the latest calculations."

Re:I, too

[KDan]

The truly interesting question is whether the different lines are orthogonal according to the Sturm-Liouville theory, and what partial differential equation they are the solution of. I suspect you could get a Nobel prize for finding this out. Once you've done that, the fact that they change with time becomes easy to deal with thanks to perturbation theory.

Daniel

Re:I, too

[Kiriwas]

That is not what they are trying to prove. The speed of gravity they are referring to is the speed of propigation of a change in gravity. If I am a giant mass which you are being influenced by gravitationally, and I move to a different location, does the force you feel change instantaneously? Or does it change after some set amount of time? Is that amount of time dependent on the distance you are from me? If so, then there is a speed of gravity that is not infinite. These scientists think that is is c, others do not. THAT is the controversy.

Which one does general relativity predict?

[A55M0NKEY]

'Spose I were rooting for Einstein, then do I want an instantaneous gravity or one that takes time to propagate?

Re:Which one does general relativity predict?

[harrkev]

'Spose I were rooting for Einstein, then do I want an instantaneous gravity or one that takes time to propagate?

You would want gravity that moves at the speed of light. This is what most reasonable scientists expect, and probably what they assume.

All sorts of strange things can happen if it is instantaneous. According to Einstein, two people can disagree about what happens first if they are moving. A person at rest can see that event A happens at the same time as B. A person moving one direction will argue that A happens before B, while a person moving the opposite direction will argue that B happens before A. The strange thing is that everybody would be right!

Let's assume that gravity can travel at faster-than-light speed, and can be used for communication. Now, a person who is moving can see A happening, and call the operator at "B" and tell them to stop event "B from happening. The person moving in the opposite direction can see B happening and tell the operator at "A" to stop event A from happening. Who is right? Clearly, they both cannot be right!

It is possible that I am missing something here. Does anybody with more experience in this stuff have more insight?

Faster than light doesn't mean backwards in time.

[ka9dgx]

Contrary to popular opinion, the ability to travel or send information faster than light does not allow one to travel backwards in time. If you leave earth, get to Alpha Centauri and come back, and it takes you only one day to go each way, you still show up 2 days after you left.

--Mike--

IANAP

Re:Faster than light doesn't mean backwards in tim

[Jackazz]

Two days from whose frame of reference?

Re:Faster than light doesn't mean backwards in tim

[mmontour]

Contrary to popular opinion, the ability to travel or send information faster than light does not allow one to travel backwards in time.

Maybe not "travel", but you can send a message backwards in time. This assumes that you can send faster-than-light messages in two different reference frames that are moving with a high relative velocity - you bounce the message back and forth between the reference frames, and the net result is that it arrives at its point of origin before it was sent.

Re:Faster than light doesn't mean backwards in tim

[Anonymous Coward]

Wouldn't basic calculus seem to indicate that there should be a speed between "delay, slower than instantaneous" and "time travel, faster than instantaneous"? If at some message speed X that may be greater than C, communication between two points can be not only instantaneous, but as you say can allow the sender to receive a response before they send, then shouldn't there be a speed between X and C where communication between two points is "merely" instantaneous? Clearly, this is not true of C, where ther

Re:Faster than light doesn't mean backwards in tim

[chrisleonard]

This assumes that you can send faster-than-light messages in two different reference frames that are moving with a high relative velocity - you bounce the message back and forth between the reference frames, and the net result is that it arrives at its point of origin before it was sent.


But doesn't this argument assume that the special theory of relativity applies? Because if you look at the general theory of relativity, you have to account for (speaking from the point of view of one reference frame on

Re:Faster than light doesn't mean backwards in tim

[mmontour]

But doesn't this argument assume that the special theory of relativity applies?

Yes, this scenario assumes SR.

Because if you look at the general theory of relativity, you have to account for (speaking from the point of view of one reference frame only) the massively negative acceleration as the message is stopped and bounced back.

That's not an issue if you're only dealing with messages, rather than people (e.g. the "twins paradox"). To send a message between two reference frames, all you have to do is

Re:Faster than light doesn't mean backwards in tim

[chrisleonard]

I appreciate the reply, and would like to ask a follow up question. Remember, IAJADBG (I am just a database guy).

That's not an issue if you're only dealing with messages, rather than people (e.g. the "twins paradox"). To send a message between two reference frames, all you have to do is send a pulse of light between them.

But this would be communicating with gravity permutations, not light pulses. What would the equivalent of color or spectrum shift be for gravity?

Re:I, too

[Rares Marian]

If there is a difference, the error was already recorded as a specific force constant. Obviously if a body is moving away the average force over a second is going to be less than if it sits still.

So we already recorded the difference without taking the speed of gravity into account. Our values are good to an extent, they fail where our theory is wrong.

Re:I, too

[shadowbearer]

Just as interesting, is does the speed of gravitation propogation (the rate of change in local gravitation measurements) follow Einsteinien physics? As I remember, he was interested in that question also, but there still doesn't seem to be an answer to it.

If gravitational changes propogate faster than c, it could have all kinds of interesting effects on our understanding of physics. Robert Forward started experiments on gravity "waves" back in the 60s, but still, as far as I know, nobody has demonstrated

Highschool physics

[icemax]

Wow. Where to begin. Your applying initial velocity and accelleration to the one ball, thats why it lands first. What does this have to do with the speed of gravity?!

Re:I, too

[confused one]

I hate feeding trolls, but sometimes I have to...

the question could be more fundamental. Does gravity have a speed? Consider this: light is a particle (wave) traveling along a many dimensional space-time membrane. Now, we've defined (more or less) what a photon is; and, how it behaves. We expect it to travel no faster than c. The problem is, we don't really undertand the space-time thing.

We have some theories as to how space is constructed. One of the things physics is trying to do is to create a theory that ties together space, time, gravity, energy, mass, quantum mechanics (basically everything). It's proving to be very difficult and gravity is the problem. Would a gravity wave have to obey all of the "laws of physics" as we know them? maybe not. Not if our theories are wrong. A lack of understanding wrt gravity might be why we need to make claims about "dark matter" and "dark energy" in order to explain the accelerating universe.

You have to remember that our "laws" are based on observation. The rules (like no speed exceeds c) are based on mathematical models created to explain the observations. What if the models are close, but wrong?

Just something to think about.

Re:I, too

[bobbozzo]

One of the things physics is trying to do is to create a theory that ties together space, time, gravity, energy, mass, quantum mechanics (basically everything). It's proving to be very difficult and gravity is the problem.

So what you're trying to say is that the speed of gravity is 42?

Re:I, too

[Ceriel Nosforit]

Switching off my sense of humor for a bit, what he's saying is that 'speed' as we know it won't be a factor in the final equation, but a result of it. Some theories (heard this at CERN in 2001) say that the graviton is a 4D particle/wave. Therefore it would also 'travel' in the time dimension and to call the graviton a tachyon might not even be a silly idea.

Getting off the beaten path of time, velocity and momentum is /essential/ in coming up with the ToE (Theory of Everything). It'll require a new kind of

Re:Black Holes

[Artcfox]

That doesn't mean that the speed of gravity is faster than the speed of light. The more massive an object is, the higher its escape velocity is. A black hole is defined as an object so massive that it's escape velocity happens to be faster than the speed of light, hence the black hole.

Re:Black Holes

[Mr.Sharpy]

Ok so I have no idea what I'm talking about, but if the gravity of the black hole is what determines the escape velocity, then if gravity is a wave wouldn't that mean that the speed of gravity caused by the singularity exceeds the speed of light.

Either that or gravity is instantaneous. Or light doesn't come out because it goes somewhere else.

Gravity

[ArmorFiend]

Gravity: we all know it sucks, we just don't know how fast.

Re:Gravity

[KDan]

Scanning for not-so-astonishingly-bad-that-it-makes-you-want-to -kill-yourself-very-painfully-right-now-joke... scanning... still scanning. Nothing found, captain.

;-)

Daniel

Luckily...

[hummassa]

Vulcans don't have a sense of humour, this one would make mass suicide the only logical thing to do in a galaxy that produces such kind of joke (the parent and the grandparent posts, I mean)

Re:Luckily...

[KDan]

You wouldn't believe it, but I've been told a bad sense of humour is the most dangerous thing that can happen to a galaxy. Where do you think all those giant active galactic nuclei (which are in fact black holes) came from? They're all the result of astonishingly bad jokes being left out of check, causing the implosion of a galaxy. My invisible friend also mentioned that ours is due for a collapse any time soon, if the quality of posting here keeps on getting worse.

Daniel

Re:Luckily...

[shadowbearer]

IMHO, this whole thread deserves a +Funny mod :-)

It's an in-joke, ok?

SB

Re:Gravity

[bobbozzo]

But does it swallow?

Re:Gravity

[mikiN]

But does it swallow?

Depends on whether unladen [armory.com] or not. If unladen, it will be different for Africa and Europe.

I sig therefore I am.

Repurcussions

[geek42]

What do you suppose the repurcussions would be if it could be shown that gravity was instantaneous, rather than propagating at the speed of light? Could we use that to transmit information instantaneously? Would that violate causality?

Re:Repurcussions

[confused one]

short answers: Ugly, no, yes.

How's that?

[JMZero]

If gravity was instantaneous, how is it that we couldn't use it to transmit information? Surely it would be difficult in practice, but it seems pretty simple in theory.

"When we're ready, I'll start moving this ball'o'mass towards and away from you."

What's impossible? Moving the mass around, detecting the mass in motion, or detecting it instantaneously?

Re:How's that?

[geek42]

I'm with you on this one. Measuring and manipulating gravity are both easy enough - they're both just a matter of, well, matter! I think this conclusion, along with the agreement that instantaneously transmitting information would violate causality, is a pretty good hint that gravity must go at the speed of light (or slower - why not?!).

Too bad it looks like the experiment was bunk... it'd be nice to know for sure. And what if it did go slower than the speed of light....? ;\

Re:Repurcussions

[shadowbearer]

It's been shown (reference, help! :-) that no matter how fast the propogation medium, information propogation is still limited to c. I believe the latest was in the quantum experiments with electrons which were simultaneously created yet still "knew" what the others' state was.

God, it's late, and I'm tired, someone help me out on this one? I know I've read it over and over :-( just don't remember the specifics.

SB

Re:Repurcussions

[masterkool]

Quantum entanglement [stanford.edu] would be a possible way to have relativity hold true, yet allow superluminal information transport. Essentially, the states of two similar particles become, for a lack of a more definate word: "entangled" and changes made to one of the particles occurs instantaneously with the second.

Re:Repurcussions

[KDan]

I'm sorry, but your post shows remarkable lack of understanding for what relativity means when it says "nothing can move faster than light". The movement of anything faster than light is mathematically equivalent to it moving backwards in time. That holds true for information as well.

Do not try to understand this by imagining a universe with an absolute time frame. That is the very understandable mistake that led you to the above post in the first place. The point of relativity is that there is no such fr

Re:Repurcussions

[shadowbearer]

Thank you, that was well put. I like your dice analogy, have to remember that one.

As I understand it, the effective transmission bandwidth(and therefore the amount of info that can be exchanged / function of time) is constrained in some way by the speed of light.
It's been a long time since I read it, and I don't really pretend to understand the math involved (although I do understand GR and SR from a conceptual viewpoint; higher calculus has never been my strongest subject :-).

But I did remember the

Re:Repurcussions

[drinkypoo]

It's been shown (reference, help! :-) that no matter how fast the propogation medium, information propogation is still limited to c. I believe the latest was in the quantum experiments with electrons which were simultaneously created yet still "knew" what the others' state was.

IIRC they managed to created linked particles and by examining the signal log could tell which bits of the movement of the particle were the data but only after the fact, in other words they could not transmit data that way. In

Re:Repurcussions

[shadowbearer]

More or less my understanding of it is that while the linked particle exchange happened more or less instantaneously, any data exchanged would have a an effective bandwidth (ok, amount of info / second) limited to a value determined to be under/at the speed of light. As I recall that was tied to GR theory.

I may be wrong, tho, it's been a while since I read it.

More stuff to reread. Sigh.

SB

Re:Repurcussions

[drinkypoo]

You are mixing things up, though. The bandwidth is not determined by the speed of light, the propagation time is. Now maybe the latency is related to some wavelength of light, and that's what you're thinking, or something else totally different...

Re:Repurcussions

[shadowbearer]

There seems to be a lot of difference of opinion on this.

See here [ernet.in]

Quote:


There are two conservative ways, and, third, a non-conservative way, to respond to EPR:

accede that nonlocal superluminal influences are possible. This violates Einstein locality ("state vector collapse should occur instantaneously at all point in configuration space") but does not violate signal locality ("no usable signal can be communicated faster than light's speed").
Another is that |Y> is not an intrinsic property of the qu

Re:Repurcussions

[jericho4.0]

Ain't gonna happen. But if gravity was shown to be instantaneous, the repurcussions would be a whole lot of people going back to the drawing board. We could probably figure out some way to use that to transmit information instantaneously, and yes, that would certainly violate causality, which is ugly.

Re:Repurcussions

[Izago909]

What about quantum intertwining? Create 2 nanoscopic black holes at opposite ends of the earth. Then find ways to alter the âspinâ(TM) of one, and the other should instantly alter to match. There are all sorts of weird and spooky things that we know little or nothing about.

Teleportation is simple, in theory. Just get 2 massive superconducting plates (a metal, for example) and place them very close together, just close enough that you can still walk between them. Then repeat at opposite end of t

Re:Repurcussions

[KDan]

Umm, where the hell did you pull that second paragraph out of? It seems like complete and utter nonsense. I don't see what superconducting plates have to do with teleportation (except in bad science fiction).

Daniel

Re:Repurcussions

[Izago909]

It's rather complex, but I was over-simplifying. So to make myself clearer, I will simplify some more. Keep in mind this is all being stated in a very analogous and rather unscientific way in order to demonstrate a currently unproven theory.

If it were possible to look so deeply into space and matter, so far that you could not âoezoomâ in any further, it might resemble something like a bubbling liquid. Now, assume you could charge 2 separate planes (most any sort of matter), which are in near pr

Re:Repurcussions

[banal avenger]

Discover how to find the square root of a negative and I'll make sure you become the most famous person in human history.

Imaginary numbers? That's the way to solve everything: When in doubt, make it up.

Re:Repurcussions

[Izago909]

It worked for Einstein, Rosen, and many others. How many times has an equation been balanced by i? It is what separates science from magick.

Re:Repurcussions

[KDan]

Well mate, I'm sorry to say, but I did a Masters in Physics and what you're saying still sounds like complete mumbo-jumbo. Maybe you want to go into a short mathematical explanation for me because I still don't get what you're saying. All you would get by charging two separate planes (assuming you charge them with opposite charges) that are very close together is a very steep E-field gradient, no matter the scale (if it gets too small and there isn't a perfect void in between you'll get a discharge, but tha

Re:Repurcussions

[Gamasta]

If that violated causality, we wouldn't have to take a piss after drinking too much wine... but that would cut off the opportunity to meet a nice blond who just had orgasm-dissert.

Speed of levity.

[A55M0NKEY]

The time it takes to 'get' a joke.

There will be a quiz on this later.

[rossy]

Teacher:
So as you can see, young Mr Kopeikin, the ability to measure gravity waves is dependant on your frame of reference. As Stuart (Look at what I can do) Samual points out, when you take into account the position of Jupiter and rotate your frame of reference, you cannot accurately measure the propigation effects of gravity with todays measurement instruments.

Mr Kopeikin:

Why do we have to learn this? We aren't really going to need this stuff outside school right? Like I'm really going to g