Slashdot Mirror
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."
← Back to Stories (view on slashdot.org)
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.
Gravity: we all know it sucks, we just don't know how fast.
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
Carpe Diem
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?
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?
"-1 Troll" is the apparently the same as "-1 I disagree with you."
--Mike--
IANAP
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.
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?
Let's not stir that bag of worms...
So what you're trying to say is that the speed of gravity is 42?
Nothing to see here; Move along.
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.
The message on the other side of this sig is false.
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.
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 whether or not gravity follows EMR laws, or whether it has it's own laws.
I'm way out of the physics world nowadays, but I'd love to learn more.
SB
It's old. The more humans I meet, the more I like my cats. At least they are honest.
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.
/essential/ in coming up with the ToE (Theory of Everything). It'll require a new kind of thinking in the proportions the ToR was to Classic physics. The ToE will take the form of mutual dependency between variables where interaction is not a process, but a function.
Getting off the beaten path of time, velocity and momentum is
All rites reversed 2010
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 send a pulse of light between them. The light may be a different color when it's received, but the information content will be the same.
I'm sure there's a better description somewhere, but the basic concept is:
Two pairs of spaceships: A,B ; X,Y.
A and B are at rest relative to each other, and are some horizontal distance L apart (in their reference frame).
X and Y are at rest relative to each other, and are some horizontal distance L apart (in their reference frame).
The XY pair is moving horizontally past the AB pair. When A and X are next to each other, A sends a message to X (using "normal" methods). X then relays the information to Y using some faster-than-light channel. Some time after Y receives the message (from Y's perspective), Y and B are next to each other. Y relays the message to B (with normal methods), then B relays it to A using a faster-than-light channel.
For appropriate numbers, using the standard coordinate transformations, it will turn out that the time of arrival at A is earlier than the time at which the message left A. This is pure SR (except for the faster-than-light channel), in flat space, with no acceleration required.