# A quickly oscillating mass produces a gravitationnal wave

1. Sep 24, 2004

### Gonzolo

Whether they have been detected or not (someone might want to update on this, LIGO etc.?), it seems undeniable that gravitationnal waves exist. It seems sufficient to wave my hand to create one.

My question is the following : if an oscillating charge produces a linearly polarized E-wave, there is a corresponding, perpendicular B-wave, but if an oscillating mass produces a linearly polarized G-wave, is there a corresponding "GB-wave"? Is there somekind of a gravitation "magnetic-like" field totally distinct from conventionnal magnetism? I would expect it to be quite miniscule, but what do the theorists say?

Mixing Coulomb's Law with special relativity gives rise to regular magnetism, but what happens if you mix F = GMm/r^2 with special relativity?

2. Sep 24, 2004

### pervect

Staff Emeritus
Gravitomagnetism is one of the things that's going to be tested by Gravity probe B. Other names for it are frame-dragging, and the Lense-Thirring effect.

It's fairly similar to magnetism, with a few differences - linear parallel currents attract in electromagnetism, they repel in gravitomagnetism.

In electromagnetism, dipole moments cause radiation - for gravity waves, it's the quadropole moment, one reason gravity waves are so weak (the other is that the force itself is a lot weaker than elctromagnetism).

The equations for gravity waves are fairly similar to Maxwell's equations for electromagnetism, but there is another index. Instead of a magnetic vector potential Au like one has with electromagnetism, one has the linearized symmetrized metric coefficients $$\bar h_{uv}$$

WIkipedia has an article on gravitomagnetism, but it's not terribly detailed.

http://en.wikipedia.org/wiki/Frame_dragging

3. Sep 24, 2004

### Creator

Yes, Gonzolo. There is a type of gravity predicted by GR that is the direct analog of the magnetic field in electromagnitism. It is generally referred to as the 'gravito-magnetic' field since arises from the motion of mass in direct analogy with charge currents in EM.

However, gravity waves are a result of acceleration of non-uniform mass distribution and results in quadrupolar (not linear) polarized emissions. That is, GW energy will not radiate from the source unless there is an accelerated quadrupolar mass moment.

BTW, to my knowledge, positive signatures of GW's have yet to be detected by LIGO.

Creator

--"Gravity always brings me down"--

Last edited: Sep 24, 2004
4. Sep 25, 2004

### Chronos

Wave functions are very well behaved. They behave like waves. It is very logical to assume gravity obeys the wave form equations. There is too much indirect evidence to say it does not.

5. Sep 25, 2004

### Garth

Agreed, but should we have detected gravitational waves by now?
Garth