# Speed of gravity related questions.

• May 26th, 2008, 10:48 AM
Cat1981(England)
Speed of gravity related questions.
From what i have been reading, the speed of gravity is the same as the speed of light. I have two question related to this.

First, light can be slowed down when it passes through a non-vacuum transparent medium such as air or glass. Is there a theoretical or practical way to slow gravity down?

Secondly, do the other three fundamental forces travel (or have their affects felt) at the same speed?

Thank you.
• May 26th, 2008, 11:04 AM
Tenacity
I was under the impression that gravity didn't have a speed only a steepness. If we use the idea that gravity is a sort of well in space-time it should be apparent that you can't change the 'speed' as you put it unless we actually change the mass of the object causing the gravity.
• May 26th, 2008, 11:32 AM
PritishKamat
Quote:

Originally Posted by Tenacity
I was under the impression that gravity didn't have a speed only a steepness. If we use the idea that gravity is a sort of well in space-time it should be apparent that you can't change the 'speed' as you put it unless we actually change the mass of the object causing the gravity.

Actually, if you consider gauge theory, gravity does have speed. It is conveyed by means of bosons called Gravitons and these have mass.So, by special relativity, particles with mass other than zero, cannot travel with the speed of light. So, speed of gravity has to be lesser than the speed of light.
• May 26th, 2008, 12:28 PM
SuperNatendo
As far as current theories and relativity is concerned, SR and GR, Gravity has no speed since there are no gravity particles. Gravity is the effect caused by space-time being curved around bodies of large mass or high energy. The speed at which this effect propagates is more than likely the same as the speed of light, Think of gravity kind of like sound, sound needs matter to propagate just as gravity needs space-time. There is no such thing as sound particles, but rather it is the effect of the vibration of particles of air, there are no such things as gravity particles, just the effect of the warping of space-time.
• May 26th, 2008, 12:33 PM
Cat1981(England)
Well so far it looks like what i have read is wrong.

Third question. If i was to instantly replace the sun with a giant black hole would i feel the affect of it......

1)Instantly.
2)8 minutes later.
3)Just after the last light hits the earth?
• May 26th, 2008, 12:41 PM
SuperNatendo
Here is where I think, the problem is. Since the speed at which gravity propagates has never been measured, it is theorized it is the speed of light. Since no particles or energy can go faster than light. However, is space-time a substance or a dimension? and, can a dimension have a speed? If space-time is a dimension and a dimension cannot have a speed, it is entirely possible that you feel the effects instantaneously.

Well, since gravity is theorized to propagate at the speed of light, the gravity fluctuations would only reach you at the same as the light, this is the current theory
• May 26th, 2008, 02:08 PM
Dishmaster
Here is a link to the first measurement of the speed of gravity.
http://www.nrao.edu/pr/2003/gravity/

The accuracy was pretty poor, but nevertheless ...
• May 26th, 2008, 02:10 PM
SuperNatendo
Quote:

Originally Posted by PritishKamat

Actually, if you consider gauge theory, gravity does have speed. It is conveyed by means of bosons called Gravitons and these have mass.So, by special relativity, particles with mass other than zero, cannot travel with the speed of light. So, speed of gravity has to be lesser than the speed of light.

A photon is a boson and it doesn't have mass, so therefore, a graviton, if such a thing existed, could likely not have mass either, and as a result could theoretically travel as fast as light.

The problem comes in with Newtonian Physics, according to the maths behind Newtonian physics, the gravity has to propagate instantaneously for the effects we have seen concerning gravity to work.

This is what has led me to believe both the current theory that gravity is the result of the warping of the space-time dimension, and that our maths concerning gravity needs updated to fit what we observe without requiring an invisible undetectable unobservable matter (i.e. Dark Matter).
• May 26th, 2008, 02:16 PM
SuperNatendo
Interesting discovery Dishmaster!

So, if space time is restricted to being warped at a speed close or equal to that of light, would this mean that space-time is made up of a substance? Or is there some truth to guage theory and the existence of a graviton?

"we can confidently exclude any speed for gravity that is over twice that of light"

That is still a very large generalization :(
• May 26th, 2008, 04:35 PM
kojax
The speed of gravity works very much like the speed of electricity/electrical interactions. They've never measured gravity itself with total accuracy or anything, but they've got ball park estimates. There's very little reason to think the communication would work differently than it does in electrical circuits.

Say, in a DC circuit where the voltage source (say a battery) is 200 meters of copper wire away from the drain (some kind of ground). Suppose there's a switch right by the ground, and someone flips that switch instantly.

The speed of light in copper is 200,000,000 meters per second, so it will take a full micro-second before the battery realizes you flipped the switch and starts moving positive electrons toward the ground.

The electrons themselves move much slower than light, but the rate of communication between one part of the circuit and any other part of the circuit is the speed of light. It's just a weird rule.

This is one reason we might not want to see light as a particle. It might be that light is simply raw information, and that matter and information are different things.