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| Artist’s illustration of Lense-Thirring frame-dragging resulting from a rotating white dwarf in the PSR J1141-6545 binary star system. (Image credit: Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)) |
Most experts think we have to tweak general relativity to fit with quantum
theory. Physicist Jonathan Oppenheim isn't so sure, which is why he’s made a
5000:1 bet that gravity isn’t a quantum force.
JONATHAN OPPENHEIM likes the occasional flutter, but the object of his
interest is a little more rarefied than horse racing or the one-armed
bandit. A quantum physicist at University College London, Oppenheim likes to
make bets on the fundamental nature of reality – and his latest concerns
space-time itself.
The two great theories of physics are fundamentally at odds. In one corner,
you have general relativity, which says that gravity is the result of mass
warping space-time, envisaged as a kind of stretchy sheet. In the other,
there is quantum theory, which explains the subatomic world and holds that
all matter and energy comes in tiny, discrete chunks. Put them together and
you could describe much of reality. The only problem is that you can’t put
them together: the grainy mathematics of quantum theory and the smooth
description of space-time don’t mesh.
Most physicists reckon the solution is to “quantise” gravity, or to show how
space-time comes in tiny quanta, like the three other forces of nature. In
effect, that means tweaking general relativity so it fits into the quantum
mould, a task that has occupied researchers for almost a century already.
But Oppenheim wonders if this assumption might be mistaken, which is why he
made a 5000:1 bet that space-time isn’t ultimately quantum.
New Scientist caught up with him to find out what makes him think
conventional wisdom might be misguided here, how the question might be
resolved with experiments – and why physicists love a good wager.
Joshua Howgego: Is it fair to say that most physicists think the best route to uniting general relativity and quantum theory is to fiddle with the former?
Jonathan Oppenheim: The smart money is on general relativity being
ultimately a quantum theory. But there is a divide between people who study
quantum theory and just want to quantise everything, and a smaller number of
others. For example, in the relativity community, they think an awful lot
about time, and because of this there is more uncertainty. If you try to
think about quantising time, you get very confused. So there’s a bit more
doubt there.
My perspective is: I don’t really know! I think it’s quite possible that a
theory that can, in some sense, describe the subatomic realm, and space and
time too, might not be anything like either quantum or classical physics.
Then the question is: Will our next theory of gravity be closer to a quantum
theory of gravity or a modified classical theory? I think we ought to be
more cautious. We could be making a big mistake by putting all our eggs in
one basket.
Why is time in particular a sticking point?
We think of quantum theory as describing events in the subatomic world that
evolve through time. The theory treats time as a kind of constant background
structure, and quantum systems change with respect to this background. The
trouble is that in general relativity, space-time itself becomes dynamical:
it can warp. If we quantise the speed at which time flows, then we lose that
crucial background structure that quantum theory relies on. It is difficult
to even talk about an instant of time, because I can’t even say with
certainty which “chunks” of space-time lie in the future and which in the
past.
It might be possible to get rid of this background structure from quantum
theory, but it is very hard. People don’t really know what to do with time.
How did the idea that we need to quantise gravity become dogma?
I think it really solidified in the 1980s, when there was a lot of debate
about whether gravity had to be quantised. At the time, people decided that
it was inconsistent to keep gravity classical. But it may date back even
further. In the late 1950s there was already a lot of debate about the
subject. I recently went back and read the proceedings of the Chapel Hill
conference, an important meeting in 1957 for which we have a full historical
record. There were these debates between luminaries of physics, people like
Richard Feynman and John Wheeler, where they debated this question. It makes
for a really interesting read. At that conference, I get the impression that
many researchers decided that gravity had to be made quantum, mostly based
on arguments from Feynman.
But when you revisit the debates, well, our understanding of quantum theory
has evolved. We now better understand the role of entanglement, where two
particles separated by some distance appear to share information, and the
similarity between classical probability distributions and quantum
wave-functions, which give you odds on what the properties of a quantum
object will be when it is measured. So we now know that it could be
consistent not to quantise gravity. However, a certain viewpoint has already
been baked in.
There are lots of big questions in physics. How important is this one about quantum gravity?
It’s a pretty big deal. Any questions about cosmology, the standard model of
particle physics, dark matter – they are questions about our particular
universe. Our universe consists of various particles and forces, but all of
these are governed by quantum theories. So quantum theory should be thought
of as the framework we use to understand our universe. So, the question of
whether the laws of physics are fully quantum, some hybrid or something else
is a different order of question. It’s about the framework of natural laws.
It’s almost metaphysics.
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