# B Transmitting information using gravitational waves

1. Apr 26, 2017

### Agliomby

I have a question only loosely associated with any of the above, but that I hope may interest the minds behind this discussion.
Does the apparent discovery and confirmation of gravity waves give us a source of information transmission that does not necessarily depend on light (of any wavelength)? Could we perhaps be on the cusp of a discovery even greater than the understanding of electromagnetism?

Last edited by a moderator: Apr 26, 2017
2. Apr 26, 2017

### Ibix

You mean gravitational waves. Gravity waves are a type of surface waves on water.

Generating gravitational waves is very difficult. As I recall, the gravitational radiation emitted by the Earth moving around the Sun is about the same power level as a lightbulb. We've no way to manipulate the quantity of mass and energy that we would need to throw around to generate measurable gravitational waves.

In principle you can use gravitational waves to communicate, in practice it's far, far beyond our ability to do so. And I can't immediately see a situation where it would be more practical to use gravitational waves than radio or light.

3. Apr 26, 2017

### Grinkle

Would gravitational waves tend to retain SNR over distance more robustly than EM waves due to (I think) relatively fewer naturally occurring emitters to contaminate?

Creating a transmitter does seem a daunting task.

4. Apr 26, 2017

### PAllen

Equally, or more, daunting is the receiver. A radio burst from a billion light years away that had the energy equivalent of 3 solar masses would be enormously easier to detect than the GW were.

5. Apr 26, 2017

### Simon Peach

Can't quite see why we would bother when we have light that travels just as fast and are a lot easier to propagate and detect

6. Apr 26, 2017

### rootone

Gravitational waves (suggested by Albert), are limited by the speed of light, as is light itself.
Not therefore very useful as an alternative mode of transmitting information.
However some think that we may eventually be able to detect G waves emitted pretty much instantly after the big bang.
That is not possible for light, but whether those waves could reveal anything that could be called information is guesswork at best.

7. Apr 26, 2017

### FactChecker

The gravity wave that was so hard to detect by LIGO was caused by the merging of two objects 36 and 29 times more massive than the Sun which briefly (1/10 second) created more energy than all of the stars in all of the galaxies. That is not an efficient way to transmit information.

8. Apr 26, 2017

### PAllen

More power. As noted, previously, it created GW with the energy equivalent of 3 solar masses in that 1/10 second.

9. Apr 26, 2017

### FactChecker

That brings up a question that puzzled me. Was this event detected in any other way? Why wasn't it really obvious in light and radio waves?

10. Apr 26, 2017

### PAllen

Merging BH are not expected to produce any EM radiation if they are 'bare'. To the extent they have accretion disks, the interaction of these would produce EM (but not the merger of the BH per se). Attempts were made to detect a corresponding event, hindered by the very poor localization of the GW direction. One group claimed a possible match, other groups disputed this. Presumably, when a third detector is brought online, direction resolution will be much improved and possible coincident EM signal might be detected.

11. Apr 26, 2017

### Paul Colby

Basically, $16\pi G/c^4=4.15\times 10^{-43}$ seems/is 43 powers of nope as far as human produced GWs is concerned. However, I do question one precept that seems to be common and I'm not convinced is true in general. It is the space components of $T_{\mu \nu}$ or the mechanical stress which radiates GWs. Technically mass need not move to be a gravitational radiation source term. Example of this would be a piezoelectric material electrically driven at a frequency well above or away from resonance. A stress field is generated with essentially no mass movement. This will radiate and by reciprocity will receive.

12. Apr 26, 2017

### PAllen

I'd be interested in studying a source for this. Everything I've read relates the radiated power to the third derivative of quadrupole moment, which means (for all practical purposes) mass must move.

13. Apr 26, 2017

### Paul Colby

It's a pretty deeply ingrained bias. I've likely read many of the same sources so I got nothing in print. The space and time components of $T_{\mu \nu}$ are related by the conservations laws so it is always possible to blame the time components. However, GW are transverse and only have space components. So in a weak field radiation integral the space components are the ones to radiate just like in EM. That said, the quadrupole formula[1] is correct for what it is applied to.

[1] Another thought. It's like saying the dipole radiation formula for atoms is the end all and be all in radio antenna design. That's just not the case.

14. Apr 27, 2017

### Staff: Mentor

This is equally true of the Einstein tensor, i.e., of spacetime curvature, which is what GWs are made of. In fact, the reason the stress-energy tensor obeys these laws is that the Einstein tensor does because of the Bianchi identities, which forces the SET to as well because of the Einstein Field Equation.

This is only true in a particular coordinate chart (more precisely a restricted set of possible charts, the transverse traceless gauge).

15. Apr 27, 2017

### Staff: Mentor

In the case of a binary pulsar, for example, yes, it's the third time derivative of the quadrupole moment of the mass distribution, because that's all that's significant. But consider the case of a black hole merger: there is no mass present (the holes are both vacuum), so what drives the radiated power?

16. Apr 27, 2017

### Paul Colby

This is the confusing bit. For piezoelectric materials the stress and strain are related through a constitutive relation. It appears to be possible for GW to generate a strain in a material with essentially no motion[1]. This will (in principle) generate an electrical signal. This electric signal can't be gauge dependent. If there some some way to write the material constitutive relations in terms of curvature of course my conceptual issue goes away but I don't see a clear way to do this.

[1] the material displacement is unchanging since the divergence of the stress field is zero in the bulk. Off resonance one may neglect the forces on the material boundary.

17. Apr 27, 2017

### Staff: Mentor

Essentially no average motion of the object as a whole. But individual atoms in the object are certainly moving: that is what "strain" means.

Yes, but the components of the GW are not the same as the electrical signal. It's the same thing as transforming an EM field between frames: for example, a pure E field in one frame will be a combined E and B field in other frames, so the EM field components are frame dependent, but the voltage measured by a particular voltmeter is not frame dependent.

18. Apr 27, 2017

### Paul Colby

My point is the GW need not impart vibrational energy to the crystal for it to generate signal no more than the suspended mirrors in LIGO gain vibrational energy from a passing GW.

19. Apr 27, 2017

### Staff: Mentor

They do if the vibrational energy is what generates the signal, which seems to be what you are describing (the strain within the crystal is what generates the signal). Detection of anything involves transfer of some amount of energy.

Not vibrational energy internal to the mirrors, but the mirrors do gain energy from the GW, because the GW makes them move when they weren't moving before (and their motion is what generates the signal).

20. Apr 27, 2017

### Paul Colby

My view is that the changing distance between the mirrors does work on the laser beam bouncing between them. Radiation pressure does work on the beam (shifting the frequency of the light) and, by Newton, also does work on the suspended mirrors. No light, no work, no energy transfer.