# Gravitational Waves

1. Jun 24, 2011

### Drakkith

Staff Emeritus
Would gravitational waves act just like other types of waves? IE can they interfere, diffract, etc? I would assume no to some of those, as I don't think there is anything that would cause them to diffract as they aren't EM waves or waves through a materiel. But I don't know really. Any thoughts?

2. Jun 24, 2011

### WannabeNewton

In the linear perturbation of gravity the EFE's reduce to wave equations with gravitational wave solutions that superpose linearly so yeah I would assume they could superpose constructively or destructively. In the full treatment, however, when gravitational waves interact they scatter each other and leave behind a curved region that, according to some observer, turns into a space - like singularity in finite proper time so they don't simply interfere like EM waves.

3. Sep 10, 2011

### debojoti

okh....can u tell me the nature of this waves......like can these be diffracted and be captured in some flurosent screen from which these wave patterns could be obtained...

4. Sep 10, 2011

### Drakkith

Staff Emeritus
They should be able to be detected with interferometers if our equipment is sensitive enough and the waves are strong enough. So far we have yet to detect any waves.

5. Sep 12, 2011

### Naty1

something is definitly different about them because while we can block the other "forces" no one knows how to block or counteract gravity.

Gravitational waves are variations in spacetime energy.

Last edited: Sep 12, 2011
6. Sep 12, 2011

### Naty1

7. Sep 14, 2011

### RayYates

As I understand Gravitational waves, (pardon my newbe language) the wave would alternate between (compressed space + slow time) and (stretch space + faster time)

Wouldn't any detector within the wave experience the distortion rendering the wave undetectable?

An observer sees two scientist holding yardsticks at a distance.
As a gravitational wave passes through one yardstick. The observer sees it alternate from shorter to longer, as compared to the other.
However, the scientist sees no change because he too is within the wave.

8. Sep 14, 2011

### profgemelli

In the early seventies Weber believed to have detected GWs but this proved to have been an illusion. Nowadays cilindrical resonant detectors and even spherical resonant detectors have been working all over the world for many years, while interferometers are also been working for some ten or more years, but with no claimed results. Why? Is there anything out there? Any posible answer?

I have my favorite answer: Carmeli's cosmological relativity. Such theory incorporates Hubble's law in a geometrical way. See e.g.

https://www.amazon.com/Cosmological-Relativity-Theories-Structure-Universe/dp/9812700757

https://www.amazon.com/Relativity-Modern-Large-scale-Spacetime-Structure/dp/9812813756

Now, since the complete theory lives in a 5-dimensional spacetime, waves have different features than ordinary 4-dimensional GW, and thus in this theory THEY DISSIPATE FAST AND CANNOT BE DETECTED! See:

http://arxiv.org/abs/gr-qc/0603067

I find this thing very interesting! Do you?

Last edited by a moderator: May 5, 2017
9. Sep 14, 2011

### DrChinese

An update from the LIGO team:

http://arxiv.org/abs/1109.2295

"Around the globe several observatories are seeking the first direct detection of gravitational waves (GWs). These waves are predicted by Einstein's General Theory of Relativity [Einstein, A., Annalen der Physik 49, 769-822 (1916)] and are generated e.g. by black-hole binary systems [Sathyaprakash, B. S. and Schutz, B. F., Living Rev. Relativity 12, 2 (2009)]. Current GW detectors are Michelson-type kilometer-scale laser interferometers measuring the distance changes between in vacuum suspended mirrors. The sensitivity of these detectors at frequencies above several hundred hertz is limited by the vacuum (zero-point) fluctuations of the electromagnetic field. A quantum technology - the injection of squeezed light [Caves, C. M., Phys. Rev. D 23, 1693-1708 (1981)] - offers a solution to this problem. Here we demonstrate the squeezed-light enhancement of GEO600, which will be the GW observatory operated by the LIGO Scientific Collaboration in its search for GWs for the next 3-4 years. GEO600 now operates with its best ever sensitivity, which proves the usefulness of quantum entanglement and the qualification of squeezed light as a key technology for future GW astronomy. "

10. Sep 14, 2011

### Naty1

The reverse is true: Everything is affected by gravity, everything, no exeptions and you cannot be shielded against it. The only way to avoid uniform gravity is to freefall.

11. Sep 14, 2011

### Drakkith

Staff Emeritus
As space is altered within the detector itself (it is very large, on the scale of kilometers), the photodetector will register a shift in the interference from the lasers beams.
See here: http://en.wikipedia.org/wiki/Interferometry
And here: http://en.wikipedia.org/wiki/Gravitational_wave_detector

12. Sep 14, 2011

### WannabeNewton

I assume you are talking in the context of linearized GR. No, that is not the case because when one measures the effect of a gravitational wave one is using proper distance which is frame - invariant so all observers must agree on it. Interestingly though, if you consider the coordinates of the object then, due to the nature of the coordinate system defined by the transverse - traceless gauge, for a particle initially at rest $\frac{\mathrm{d} ^{2}x^{\alpha }}{\mathrm{d} \tau ^{2}} = -\Gamma ^{\alpha }_{\mu \nu }u^{\mu }u^{\nu } = 0$ so the coordinates of the object remain fixed when the gravitational wave passes by. Of course this has no physical meaning unlike proper distance.

13. Sep 15, 2011

### RayYates

What gravitational wavelength would this be able to detect?

14. Sep 15, 2011

### Drakkith

Staff Emeritus
I have no idea on the specifications of these detectors. But even with these kilometer long beasts we have yet to detect any waves.

15. Sep 15, 2011

### Nabeshin

Depends on the size of the interferometer! Contrary to popular belief, a larger detector is not necessarily more sensitive -- just sensitive to a different frequency range!

For the ground based LIGO detectors, the frequency range is something like 10-2000Hz, while the proposed space-based mission LISA (Not LISA anymore) is sensitive in the range 1-100mHz.

The canonical figure is here: http://upload.wikimedia.org/wikipedia/commons/e/eb/LIGO-LISA.jpg

16. Sep 16, 2011

### RayYates

If gravitational waves lengths were generated by two orbiting black holes, wouldn't they have a wave length on the order of AU as they orbit each other? Should they be looking for ultra low frequency waves instead of ultra high?

17. Sep 16, 2011

### jimgraber

Yes and some people are very seriously looking for ultra low frequency gravitational waves. Google "pulsar timing networks" for some interesting links.
best,
Jim Graber

18. Sep 16, 2011

### Nabeshin

In general, when we consider binary black hole systems, the signals we are looking for are at the very end of the orbit, as the two are right about to merge. The signals at these times are much higher frequencies, and thus have wavelengths which are much shorter than AU. The reason we focus on this portion of the spectrum is that most of the power is released in these final milliseconds of the binary black hole merger, thus the signals are the easiest to detect here.

The proposed space detector, on the other hand, would primarily be sensitive to things like binary white dwarf systems as they slowly inspiral, which is why the relevant frequencies are much lower than in the binary black hole case.

19. Sep 16, 2011

### Nabeshin

It is a little premature to make this kind of claim. Based on our best population synthesis models, we are in fine agreement with our experimental results -- namely, we didn't expect to see anything anyways! Once aLIGO completes a few science runs over the course of half a decade or so, we expect to have detected at least a few events. If we have not, then that will be the time to start seriously entertaining alternate notions.

(Note: Nothing against alternate theories of gravity! They're great and all, but it's a mistake to say that LIGO or any gravitational wave detector has provided any evidence against GR up to this point.)

Last edited by a moderator: May 5, 2017
20. Sep 16, 2011

### RayYates

I see. In the last moments of a black hole merger the frequency would be increasing; so any tuned detector (like a laser beam) might see a momentary "blip" as the wave resonated at the same frequency as the detector. That's a big challenge.

Does space-time offer any "resistance" to the propagation of the wave? In other words, would the wave energy get converted to heat and dissipate?