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I'm a non-scientist. Today's announcement of LIGO's detection of GW waves has been heralded as "huge" –on the same level as the Higgs Boson detection. I realize this is the last remaining part of Einstein's theory of general relativity to be directly observed. Is that why it's such a big deal? Does this mean the theory is now on pace to become the law of general relativity?

Side note: I had the privilege to visit and take a general tour LIGO in WA in 2000 or so and found it quite fascinating from a layman's pov. I've been waiting for this day ever since.

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The way I see it, there are three reasons it's such a big deal.

The first, as you say, is the (further) confirmation of Einstein's theory of gravity. Newtonian gravity doesn't have gravitational waves. Their existence was already quite established by Hulse and Taylor's discovery and analysis of the binary pulsar PSR B1913+16. The system's orbit is decaying almost exactly as predicted by General Relativity. (Note that in the classic figure, the curve is not a fit: that is the prediction!) The direct detection of gravitational waves, however, better confirms that the waves behave as we expect they would, in the sense that the observations match the so-called "chirp" that we expected.

The second reason is because this is now a new way of measuring things in the Universe. Note that the announcement told us (roughly) the masses of the two black holes. It's actually very difficult to weigh black holes! For example, this 2010 paper discusses constraints on 23 stellar mass black holes, all from the fact that they're in binary systems, and the motion of the partner can be observed. Even then those are mostly mass functions that still depend on the inclination of the orbits, which is often unknown. So the gravitational waves give us a nice measurement, not just confirmation that the phenomenon exists.

With the method proven, there are some fascinating potential applications. One of my favourites is that LIGO (or similar) could potentially detect the formation of a black hole in a core-collapse supernova. Just like the detection of neutrinos from SN1987A confirmed some of our basic ideas about these events, detecting the gravitational waves from the birth of a black hole would inform our understanding further still. And it might even tell us, for example, how massive the black hole is at birth. There's no end to the novel ways that our theories could be tested!

The final reason is simply the astonishing technical achievement. They measured motions of at most a few thousandths of a femtometer over the 4km arms. That's mind-bogglingly precise. Hopefully this will give more impetus to other gravitational wave detectors. Here's hoping that eLISA will fly!

The parallel with the Higgs boson is quite apt. In both cases, we built a big machine to find something that we were pretty sure should be there. (Though the LHC was a tad more expensive...) Now it's a case of using the new tool to observe the Universe, and seeing if the waves behave as we think they should. And who knows, maybe some completely unexpected signal will appear to confound us completely.

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    $\begingroup$ Turns out that black holes must actually exist, too! $\endgroup$ Feb 12 '16 at 16:14
  • $\begingroup$ There is also the fact that their detection opens up an entirely new way of observing the universe. Previously ALL of our observations of the universe and gravity were through its influence on matter and light. With sufficiently sensitive detection apparatus (eLISA looks promising) it would enable the direct observation of distant gravitational events invisible through light alone, including events very close to the beginning of the universe. $\endgroup$ Aug 14 '16 at 5:16
  • $\begingroup$ You could think of it as adding a topographical layer to a geographical map. $\endgroup$ Aug 14 '16 at 5:25
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I can't think of any direct applications of this detection, but it's a huge achievement for fundamental research. As you said, it was the last bit of Einstein's GR to experimentally confirm

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    $\begingroup$ Get more detectors to localize the signals better, and we might see unexpected effects of mass or distance on ring time and amplitude. That'd be new physics. $\endgroup$ Feb 12 '16 at 16:17
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I realize this is the last remaining part of Einstein's theory of general relativity to be directly observed. Is that why it's such a big deal?

Correct: though gravity waves were predicted for a long time, their final direct measurement adds a bit more reinforcement to relativity theory, the behaviour of spacetime, etc.

The important thing is, we know there must be more "out there" than just relativity and quantum mechanics - there has to be something happening at the crossover point between the two (Quantum gravity and stuff like that).

The next stage may be to look in more detail at gravity wave events using the growing LIGO network and its more sensitive successors, and see if gravitational waves behave exactly as predicted, or if there are any gaps, which may point to new physics...

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