# Quantum Mechanics after the detection of Gravitational Waves

Of course everyone knows by now of the detection of Gravitational waves

But, since General Relativity and Quantum Mechanics don't get along, can we say now that this detection proves that Quantum Mechanics doesn't actually apply and that General Relativity did prevail?

Another question: how can we identify the ripple's origin (let's say whether it's a result of the big bang or another big event)?

EDIT 16-2-2016

I was reading an article today and I thought I'd share it here; It's basically saying that without a third detector we can't triangulate the signal. Some scientists tried ways to observe the light of the event directly after the observations of the wave but they couldn't detect the merger simply because it's too far away or too faint to be observed with our current technology.

• It was a black hole merger, not from the big bang. Primordial gravitational waves have an even longer wavelength, probably too long for LIGO, Feb 12 '16 at 8:34
• Quantum physics and Relativity are NOT competing theories. They are complimentary theories , with relativity about what happens at massive scales, and quantum talking about really tiny scales. The controversy is nobody really knows how to unify these two fiels. What physicists want is a theory that in one comlete swoop describes how everything works. Maybe an elegant equasion or a set of simple rules. We're not even sure such a thing actually exists, but it'd sure be nice if it did, because that theory would be the pinacle of human scientific achievement. Problem is, nobody really knows how. Feb 16 '16 at 2:12

No more than the observation of light waves disproves quantum mechanics.

Light has properties of both a particle and a wave. At low energies, the particle nature of light is hard to detect: radio waves are made of photons, but individual radio wave photons are pretty hard to detect. I'm not sure that we have directly detected individual photons with energies below the infrared band.

Gravitational waves (probably) also have both a wave and a particle nature. The gravitational field is probably quantised. But at the frequencies and sensitivity at which LIGO operates, individual quanta cannot be measured. So this detection does not prove the ascendency of GR over QM.

If anything, understanding extreme events like black hole mergers might lead to a theoretical understanding of the quantum nature of gravity.

• Thank you for your answer it really helped me understand the idea.. I'll mark it as the answer in a couple of hours to give a little more time for other answers as well Feb 12 '16 at 11:46
• @Odin: waiting a couple (or rather something like 5, or 7) days seems better than just a couple hours, as experts are not always behind their screen... Feb 12 '16 at 15:38
• There are probably no reasonable experiments that can detect an individual graviton. Here reasonable means things like "is not large enough to collapse to a black hole", and "detects at least one graviton per age of the universe". arxiv.org/abs/gr-qc/0601043 And this event really isn't close to where you would expect quantum gravitation. For black holes of 30 solar masses, the Schwarzschild radius is something like $10^5$ m, but the Planck length is something like $10^{-35}$ m. Feb 12 '16 at 16:44
• Of course compared to something like the solar system, this is extreme: a distance of $1$ AU from the sun (i.e., here on earth), the curvature radius is on the order of $10^{12}$ m, at the surface of the sun some $5\cdot 10^8$ m. But gravitation is seriously weak so you are still many many orders of magnitude from quantum gravitation. (Note that large curvature radius = small curvature. A large sphere is less curved than a small one.) Feb 12 '16 at 16:49
• By the way, if anyone knows the energies of the lowest energy photons that have been directly or indirectly observed, I'd be interested. Feb 13 '16 at 22:23

The impact of this measurement on the status of quantum gravitation is exactly zero.

The proper statement of the incompatibility of general relativity and quantum mechanics is that the quantum field theory of general relativity is not renormalizable. Renormalizability essentially means that the theory is well-defined at all energy scales, which seems like a reasonable demand on a proposed fundamental theory.

So what we know is that taking classical general relativity and quantizing it, we do not get a fundamental theory of quantum gravitation. This does nothing to rule out other proposed quantum theories of gravitation, for example, LQG or string theory.

Furthermore, the way physics works is that new theories must reduce to old ones in the domains of applicability of the old theories. Whatever the correct quantum theory of gravitation, its low-energy limit should be quantized general relativity, and the classical limit of that is classical general relativity. It's just not true that you have to choose between general relativity or quantum mechanics.

So this measurement of a prediction of classical general relativity does absolutely nothing to show that no quantum mechanical model of gravitation exists. It couldn't, because we already have a quantum mechanical model of gravitation: quantized general relativity. It's not as "nice" as we would like, but that really only rules it out as the fundamental theory.

• This site attracts quite high quality answers. I upvoted the whole lot (and I do not do that .. just about ever..) Feb 13 '16 at 21:07

Another question, how can we identify the ripple's origin (let's say that if it's the result from the big bang or another big event)?

LIGO have produced an image which shows their best estimate of where these two black holes were:

All they can say is, somewhere in the southern sky. In the future a network of more detectors will allow such events to be pinpointed much more precisely.

• That's truly amazing.. Thanks for sharing this Feb 12 '16 at 13:37
• Just one more detector coming online will make a huge difference. The two LIGO detectors were only able to localize this event to a 600 square degree region. During the press conference once of the scientists stated that after the Virgo detector comes online later this year they should be able to narrow it to a single digit number of square degrees. That's a small enough area of space for fast response optical scopes to survey for the afterglow expected from the merger of neutron stars (last paragraph of conclusion). Feb 13 '16 at 23:34
• If you want a little more new details about that part of the question, check the edit on 16-2-2016 @Andy :) Feb 16 '16 at 6:17
• The ability to locate sources should get another huge improvement in a couple of years now that LIGO India has been approved by the Indian government. Feb 18 '16 at 13:03

At the announcement press conference (2/11/2016), Kip Thorne said that the detection puts an upper limit on the rest mass of the graviton. They determined this limit by looking at distortions of the detected signal waveform compared to the idealized signal produced by computer simulations. The upper limit from the publication is $m_{graviton} < 1.2 × 10^{−22} \frac{eV}{c^2}$ or $1.9 × 10^{−41} kg$.

• A little too short maybe. Reference? Feb 14 '16 at 11:46

Though the twin discovery of Gravitational Waves and Black Hole merger might not affect directly the status of QM it might indirectly bring new "surprises" For example, in this link: http://news.discovery.com/space/weve-detected-gravitational-waves-so-what-160213.htm They comment that: "For some reason, the final spin of the black hole is slower than expected, indicating that the two black holes collided at a low speed, or they were in a collision configuration that caused their combined angular momentum to counteract each other. “That is very curious; why would nature do that?” said Lehner." And the final comment is: "This early puzzle could be down to some basic physics that hasn’t been considered, but more excitingly it could reveal some “new” or exotic physics that is interfering with the predictions of general relativity". Wow! "Interfering with general relativity" is a polite way of suggesting that it might be wrong. So maybe QM might come to the rescue of Gen.Relativity rather than the other way around.