Is it possible that speed of a gravitational waves are greater than c?

Drakkith

Gravity does not define spacetime. Gravity is merely a property of energy and mass occupying an area of spacetime that causes an effect we call "curvature". We measure distance and time using measuring devices, such as a meter stick and a clock and have defined a specific distance and a specific interval on the clock as one meter and one second. There is no known reason that the speed of light be the limiting speed at which information can travel through the universe. It is only known that this limit exists.

GTOM

"We measure distance and time using measuring devices, such as a meter stick and a clock and have defined a specific distance and a specific interval on the clock as one meter and one second. "


But as far as i understand, a clock will tick with a different rate at the bottom and at the top of a gravity well. Locally of course it cant be noticed, just like you cant notice Earth's orbit and motion locally, but you can measure time dilation, when connecting different time zones.

So if Sun would suddenly disappear, time and space would also change nearby, as theese things are flexible like rubber IMHO. So even if gravity waves proceed with speed of light, is it obligatory, that they have to reach Earth in 8,33 minutes? /Normally one astronomical unit is 8,33 light-minute./ Again, spacetime itself would be rearranged, rulers stretch, clocks will tick faster without strong gravity.


(I dont see the casuality violation just bacause someone could get an information FTL, if he cant CHANGE the past, although he can see the sender's past, when he gets that information. )

Of course, theese are theoretical questions, i waiting for corrections again.

Whovian

GTOM, since gravity is transmitted by gravitational waves, it'll take 8 min 33 sec (?) for the disturbance to reach Earth.

And the causality violation? Basically:

Observer A and B are moving away from each other (so we get time dilation effects.)

Observer A sends a message to observer B faster than c.

Observer B, as soon as (s)he receives it, sends it back to observer A faster than c.

Depending on the speeds of observers A and B, and how much faster than c the signal's transmitted, it's possible for observer A to receive the message before (s)he sends it. So say (s)he decides to send a simple true/false and sends the opposite of what they received. What then?

GTOM

And what if A and B sends radio waves, then they arrive instantly? /If there is any possibility, that FTL messages arrive before they were sent. I'm pretty sure, one can see, that they arrived before they were sent./
There is a different time zone on a mountaintop, if somebody put a clock up to a top, and to a valley, eventually, they would show significantly different times. But it mean no time travel and casuality violation to go up and down.


Otherwise, yes if an object falls into a black hole, it arrives from the outside, but what if the black hole itself is moving? The gravity field has to be updated outside the event horizont.

Also i wonder, what is exactly a gravity wave? It can be also a small oscillation remained after the update of a gravity field, like sound remains after a gunshot. /The blast itself is supersonic/

Whovian

The key is

a) They're moving at a significant relative velocity away from each other

b) The messages are quite significantly faster than light

You can perform the calculations if you like to show that it'll be received before it's sent.

And the case of another clock on top of a mountaintop, that is not the same case in any way. In the case I just mentioned, one of the significant details is there's an absolute symmetry between observer A's reference frame and observer B's.

mrspeedybob

Suppose a pulse of light falls into a massive object. As the light falls down the gravity well it gains momentum by blue-shifting. Conservation of momentum demands that the object also gains momentum in the opposite direction. If the object does accelerate it would mean that the pulse of light is having an effect on the object prior to its arrival.

I don't mean this as an argument for FTL gravity, it's just scenario I don't understand. Does conservation of momentum not hold in GR or is it conserved in this scenario in some way I don't see?

Drakkith

Interesting. Given that a change in a field such as gravity propagates at c, as does light, that almost seems to make light constantly in a "gravity shockwave" as it moves. Unless I'm totally mistaken, which is likely.

GTOM

"You can perform the calculations if you like to show that it'll be received before it's sent."

My calculations : t = s/v shows you need infinite speed just to get immediate response.
Okay, lets complicate things with time dilation. So their clocks will show, they didnt get the reply in 10 hours, just one hour.
One minute if message is sent with 60c.

So Observer B gets message from sender A and he can see, it has arrived before it was sent.
Than, he resends it, so A can notice the message was arrived before it was sent by B, and B already saw it was arrived before it was sent.
The problem is, what they see, is information depreacted by hours.

Drakkith

GTOM this has nothing to do with time dilation. If you perform a lorentz transformation between two relativistic frames sending signals at a greater than c velocity relative to each other (The exact values vary with observer and signal velocity), then according to the math one of the frames WILL receive a signal before it sent it's first. You cannot calculate this without using the lorentz transformation.

GTOM

t=s/v

How should i perform the calculations to get a negative value instead of positive or imaginary value at worst?

What arrives instantly, if FTL message can possibly arrive before it was sent?

Whovian

a) Observer A sends an FTL message, wait until Observer B receives the message in Observer A's reference frame. Note that Observer B's moving away from the message, so that must be taken into account.

b) Now calculate how much time's passed for Observer B, and now we're in Observer B's reference frame. As Observer B receives it, calculate how much time's passed for Observer A in their reference frame since the beginning of the experiment.

c) Observer B now sends the message back to Observer A, and take into account the fact that observer A's also moving away from the message. Using the same logic, it should arrive before it was sent in some cases.

(Note that this depends on how fast Observer A and B are moving relative to each other and how much faster than light the message is going.)

GTOM

I guess i am a retard, but i get lost at the points : WAIT UNTIL Observer B receives the message. Calculate how much time's PASSED.
Maybe if i could see the exact equations at least.

Even if a clock will show it has arrived before it was sent, if you add the time dilation caused by the DISTANCE, it will be still a positive value.

Whovian

The time dilation caused by the distance. Do you mean for Observer A? In this case, both observers A and B measure the same distance between them, no length contraction is present.

EDIT: Forget that. It is present. It just means Observer B appears contracted from Observer A's point of view and vice versa.

GTOM

Ok, would you be so kind to write down the equations, how do you get the negative value of time passed?
I can get only a complex value if the gravity propagation also affected by Lorentz transformations, not just electromagnetic waves.

Also : how can the gravity field of black hole updated outside the event horizont if the black hole moves?

Drakkith

Perhaps this will help.
http://en.wikipedia.org/wiki/Special...transformation

justwondering

"So if I tug on a string of space-time, it's not individual atoms moving, it's a continuous string so as I pull on my end, the other end moves simultaneously."

Can't be simultaneous across our universe! The 'ends' are in question also.

Drakkith

Are you quoting someone in the thread? If so, you can just hit the "Quote" button on their post.