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Answer by Richard Muller, Professor of Physics at UC Berkeley, author of Now, The Physics of Time, on Quora.
Is the general theory of relativity the correct
description of gravity and space-time in the strong-field regime, near
objects (such as black holes) in which orbits approach the speed of
light?
I believe we will soon know the answer by 2025
because of the fantastic success of the LIGO experiment in detecting
merging black holes. I had not expected this; indeed, I’m not sure
anyone had. Within months of their operation they discovered an
incredible event: two black holes were observed merging, one with 36x
the mass of the sun, the other with 29x the mass of the sun. They
combined to form an even larger black hole, with 62x the mass of the
sun. Note that 29+36 adds up to 65. What happened to the other 3 solar
masses? Amazing answer: those three solar masses of energy all got
turned into gravitational radiation!
The second LIGO event was just reported a few
months ago, and it is dramatic but much smaller, with black holes of
14.2 and 7.5 solar masses merging into a 20.8 solar mass object. However
the error uncertainties for each number are about 30% so we can’t reach
the strong conclusions that we can for the first LIGO event.
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Some people say we already confirmed strong field
general relativity from the first, since the general relativistic model
fits the data very well. Unfortunately, however, the signal is not
strong enough for us to determine if there were “small” (e.g. 10%)
departures from general relativity. This last weekend my colleague Shaun
Maguire and I posted a scientific paper in which we predict such a
small departure, and shows that the LIGO event is not quite strong
enough to detect it. The paper can be read (for free) at arXiv posting: Now, and the Flow of Time.
This paper is technical and meant for other scientists, in particular,
those who already understand general realtivity; however I encourage you
to look at it even if you are a physics novice. The introduction should
be understandable for a wide audience. The paper is based on ideas I
present in my popular-level book Now: The Physics of Time which is not yet available (it is in press).
In the coming years, certainly before 2025, we
expect there to be many more LIGO events, some significantly stronger
than those already observed. Such events don’t have to involve bigger
black holes; they only have to be closer to the Earth (e.g. less
than a billion light years away) to give a sufficiently strong signal,
one that could indicate departures from standard general relativity. It
will also help that soon some additional LIGO detectors will be
operational, and the detection of the event by more than the two current
detectors will greatly enhance the information that we can obtain.
Yes, it would be great to confirm general
relativity, but the science is always most exciting when we discover
departures from what we thought was true. We have never probed strong
field general relativity prior to LIGO (at least not so clearly), and so
it will offer a remarkable opportunity to see possible departures from
our current theory of general relativity. That prospect is what
scientists find most thrilling.
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