Gravitational Waves: Constant Motion Through Spacetime

[tonyb1969]

Gravitational "waves"

First, I confess that I am not a physicist by training; I work for an RF engineering firm. However, I study theoretical and astrophysics from those who have the extensive math background, and attempt to conceptualize their theories. I am not a conspiratist, and I certainly hope never to initiate a crackpot subject. My only purpose is to share ideas that wake me up at night with those who may be able to steer me in the proper direction. Okay, enough of the disclaimer.

If we are hurtling through space at high rates of speed, would not each object be moving through the fabric of spacetime constantly? Wouldn't this result in constant wakes of gravity waves? I picture putting a sphere into a large pool of water and moving it quickly from one end to the other--there would be swirls of wake following it until the water settled again. Would not the same apply to massive objects that curve space in all dimensions (like a sphere in water) moving rapidly through spacetime? When I think of some descriptions of Einstein's vision (a ball on a trampoline, for example), it fails to capture this movement through space, which according to expansion theory, we are.

In short, if our currently accepted view of mass curving spacetime in all dimensions (resulting in the effect we call gravity), should we not expect wakes of gravity waves/disturbances in the fabric of spacetime that we could constantly monitor?

 

[A.T.]

If we are hurtling through space at high rates of speed, would not each object be moving through the fabric of spacetime constantly?

Every object advances trough spacetime, even if at rest in space. And inertial movement is equivalent to rest anyway.

Wouldn't this result in constant wakes of gravity waves?

No. GR predicts only that accelerated masses produce gravitational waves. Just like accelerated loaded particles produce EM-waves.

I picture putting a sphere into a large pool of water and moving it quickly from one end to the other--there would be swirls of wake following it until the water settled again. Would not the same apply to massive objects that curve space in all dimensions (like a sphere in water) moving rapidly through spacetime? When I think of some descriptions of Einstein's vision (a ball on a trampoline, for example), it fails to capture this movement through space, which according to expansion theory, we are.

This analogies have little to do, with the model of curved space time in GR. And the expansion doesn't imply that we are moving.

In short, if our currently accepted view of mass curving spacetime in all dimensions (resulting in the effect we call gravity), should we not expect wakes of gravity waves/disturbances in the fabric of spacetime that we could constantly monitor?

If there where big accelerated masses nearby, would expect to measure such waves.

 

[tonyb1969]

Many thanks, A.T. I will read up on the sections of GR implied by your statements.

 

[Naty1]

Wikipedia has a decent discussion of gravitational waves at:
http://en.wikipedia.org/wiki/Gravitational_waves

 

[tonyb1969]

Thanks, Naty1. I will read. Also, A.T., it makes perfect conceptual sense that the space fabric would be expanding equivalient to our space in the universe now that I think of it.

 

[Dmitry67]

I have another question about the waves.

Suppose there are 2 neutron star rotating very close to each other emitting huge amounts of GW and spiralling towards each other. They lose energy, hence, they lose mass too, correct? The energy (and mass?) is carried away by the waves

But GW themselves do not gravitate, as far as I know! So before we had gravitation emiting by like 5 solar masses, after the merge there is only 4.9 solar masses left and 0.1 mass in a form of the gravitational waves.

But that 0.1M does not emit the gravitation!

 

[Vanadium 50]

But GW themselves do not gravitate

Yes they do.

 

[Dmitry67]

I read it here:
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

Here Gmu,nu is the Einstein curvature tensor, which encodes information about the curvature of spacetime, and Tmu,nu is the so-called stress-energy tensor, which we will meet again below. Tmu,nu represents the energy due to matter and electromagnetic fields, but includes NO contribution from "gravitational energy". So one can argue that "gravitational energy" does NOT act as a source of gravity. On the other hand, the Einstein field equations are non-linear; this implies that gravitational waves interact with each other (unlike light waves in Maxwell's (linear) theory). So one can argue that "gravitational energy" IS a source of gravity.

Is it wrong?

 

[Vanadium 50]

I think you're misinterpreting that. The first part has to do with the definition of the tensors in the Einstein field equations, and the second part more or less confirms my point.

Look at it this way - gravitational waves travel along null geodesics, right? A gravitational wave and an electromagnetic wave will travel along the same paths. A mass can gravitational deflect a beam of light, and to conserve momentum, it is (ever so slightly) attracted to that beam of light. Since gravitational radiation follows that same path, the same thing happens to it.