Question

Why the excitement about gravitational waves? They confirm the correctness of the formulae, fine. But its the formulae that allow us to ask Qs, and we have the formulae already. What additional possibilities has their empirical confirmation provided?

Answer 1

Gravitational waves were first verified to exist (discovered?) back in 1974 by Joe Taylor and Russ Hulse, emanating from what is now called the "Taylor-Hulse" or "Hulse-Taylor" or "Binary" pulsar. Over the next decade the period of the co-rotatation decreased, very closely matching the change expected from the emission of gravitational waves.

Indeed, if the rate had not been seen to decrease, it would have falsified the prediction. That puts this observation as well as the theory into the highest class of scientific result. According to the eminent philosopher of science, Karl Popper, what makes a theory a scientific theory (versus a speculation or a musing) is that it can be falsified.

The rumors that gravity waves have been detected at LIGO are just that, until they can do the tests to show that the signals that they are (presumably) seeing don't come from one of the many spurious sources that could cause them.

If they do announce that LIGO has detected such waves, it will be a cause for rejoicing in the scientific community. It will open up a new window for observation of distant phenomena. Such a successful detection will not test general relativity, nor will it confirm that general relativity is better than alternative gravity wave theories. What it will mean is that gravity wave detection may be putting us on the threshold of a new era of discovery.

We all hope it is true. If it holds up, special congratulations are due to the founders of the project, Ron Drever and Kip Thorne. (Incidentally, Kip Thorne was one of the producers of the recent movie about gravity and black holes, Interstellar.) And, of course, congratulations to the current team, the "LIGO Scientific Collaboration", with its 941 members in 88 institutions.

Gravitational waves are a new lens through which we can observe the universe

Suppose you live in an apartment building and you are very nosy. So you want to know what your next-door neighbor is doing. How do you do that? Well, you can watch their window or their door to see them come and go and see what they're doing when they're near the window. But that's not a complete picture. You can get additional information from sound. The walls block light, but not sound. So you can put your ear to the wall and listen to your neighbors going about their lives.

Now, imagine you're an astronomer and you want to observe what's going on inside a star as it goes supernova. Unfortunately, stars are made of plasma, which is reflecting. So it takes a long time for light to escape, and it's been distorted by travelling through and bouncinc around the plasma. In other words, the surface of the star is like a wall. There's a window through which information comes out, but you don't get a complete picture.

{Major Optimism follows}:

• The farthest we can look into the past (as of now) is through the cosmic microwave background. But CMB provides information only up to the time when matter became sparse enough to be decoupled from electromagnetic radiation. Since gravity interacts a lot weaker with matter, gravitational waves (if they were produced during the big bang) would have decoupled from matter a lot earlier. So this "gravitational wave background" (if it exists) can be used to probe the origin of the universe.
• The biggest mystery in astrophysics today is dark matter. There are two major theories for dark matter - Massive Halo Objects (MACHOs - small black holes and cold neutron stars and white dwarfs in the halo) and WIMPS (Weakly Interacting Massive Particles). If dark matter is made of MACHOs, some of them will be in inspiralling binaries emitting gravitational waves.
• Supernova explosions are poorly understood events. There is no way to probe the collapsing core using EM radiation because all the light we receive is from the outer envelope of the star. If the core collapse if asymmetric, it is expected to be a source of gravitational waves. The inner mechanisms of supernovae can then be probed using GWs and neutrinos.
• Another mysterious class of objects are violent events known as gamma ray bursts. Little is known about them, but anything as wild as them will (probably) emit significant amount of energy in GWs

All these can be inferred and all these additional possibilities have opened up..