# Gravitational waves detection

I am aware that the everything with mass produces gravitational waves and the only reason why we don't detect them is because they are incredibly small.

If gravitational waves act like every other wave then they should super impose. Since in the universe there are a large number of massive objects which are moving at all times, shouldn't the gravitational pulses superimpose such that LIGO should always be detecting gravitational waves?

• I would recommend you to send this question by a mail to questions@ligo.org The scientists are very helpful and always answer your queries, as they have done in several cases of mine. Moreover, who can know about their theories better than they themselves? – Wrichik Basu May 17 '17 at 18:23
• You can find more content at www.ligo.org Don't forget to post an answer to your question when you get an answer from LIGO. I'll look forward to that. 😊😊 – Wrichik Basu May 17 '17 at 21:57

Well, yes, ask LIGO, but it's not that hard, and people in PSE can give you some answers.

But, no need to ask, they've published some and there are other papers and more detailed summaries that you can google. For LIGO you can start with https://www.ligo.caltech.edu/page/gw-sources. Or google gravitational wave sources

Yes, of course gravitational waves superimpose, but weak waves superimposed on stronger waves can be unnoticeable. Like electromagnetic waves, in the linear approximation the propagate and decay with distance as 1/r far enough away. The strongest gravitational waves are emitted by very dense and very small object, such as black holes and neutron stars, when they are strongly perturbed such as in collision or merger. There's other strong sources likely from the early days of the universe, before recombination, that we would love to build big enough detectors to see their likely longer wavelengths.

We don't see the sum of all that because we don't have enough sensitivity, and because their frequencies can be different (as with electromagnetic waves they superimpose at the same frequency, the more nonlinear effects are much weaker).

The nonlinear regime for gravitational waves is where things get much more complicated, because gravitation, in General Relativity, essentially interacts with itself -- gravitational effects interfere with each other if strong enough.

Another factor that is interesting for gravitational waves is that they can not be produced by monopoles (single masses), or even dipole distributions, but can by quadrupoles and n-poles, for higher $2^n$. Most (or all) we've detected so far has been from the mergers happening not exactly radially so they have a quadrupole moment (think of a barbell rotating in any axis other than its axis of symmetry). If we get more sensitive detectors we'll see more and effects from other gravitational distortions.

The astrophysical and physics community is looking to build more sensitive and bigger detectors of gravitational waves. They are looking at those as basically being gravitational observatories, a whole new regime to explore the universe. One paper on that is for instance at https://arxiv.org/pdf/1209.0667.pdf. I'm sure there are more recent papers that detail what specifically could be seen for each size of interferometer.

• Thank you very much, all incredibly insightful!!! I was just wondering, I understand that in the future the atrophysics community wants to build new interferometrs in space to notice the smallest perturbations in space-time. If we were able to detect a background gravitational wave as you mentioned, then would we expect it to have the same density discrepacies as the CMB, or would these 2 phenomena be non related? – Francesco May 18 '17 at 21:47
• @Francesco. I s a fair and good question. The answer is almost surely yes. Since most of the CMB fluctuations is caused by density fluctuations at the time that light was able to escape after the recombination allowed it (about 380,000 years after the Big Bang), and those density perturbations (just variations of mass of energy density in different part of the then sky) would act on gravitational waves as would any mass (of the same total amount), it will have some effect. 2-3 years ago we thought we detected some patterns in the CMB due to primordial GW waves. It was determined --- see next – Bob Bee May 19 '17 at 1:05
• that it was a false alarm. Primordial GWs were generated in the early times of the universe, including due to inflation, and propagated. If and when we detect it we should see a real proof of inflation, and probably also more information on the mass inhomogeneities early and up to some time. Whether we see the inhomogeneities in it that CMB shows, or from earlier time periods, there may be some correlations. If we see with enough sensitivity. See the figure in the following wiki article for what we might see of GWs. en.m.wikipedia.org/wiki/Gravitational-wave_observatory – Bob Bee May 19 '17 at 1:14