A radical theory that consistently unifies gravity and quantum mechanics while preserving Einstein's classical concept of spacetime has been announced in two papers published simultaneously by UCL (University College London) physicists.
Modern physics is founded upon two pillars: quantum theory
on the one hand, which governs the smallest particles in the universe, and
Einstein's theory of general relativity on the other, which explains gravity
through the bending of spacetime. But these two theories are in contradiction
with each other and a reconciliation has remained elusive for over a century.
The prevailing assumption has been that Einstein's theory of
gravity must be modified, or "quantized," in order to fit within
quantum theory. This is the approach of two leading candidates for a quantum
theory of gravity, string theory and loop quantum gravity.
But a new theory, developed by Professor Jonathan Oppenheim
(UCL Physics & Astronomy) and laid out in a paper in Physical Review X,
challenges that consensus and takes an alternative approach by suggesting that
spacetime may be classical—that is, not governed by quantum theory at all.
Instead of modifying spacetime, the theory—dubbed a
"postquantum theory of classical gravity"—modifies quantum theory and
predicts an intrinsic breakdown in predictability that is mediated by spacetime
itself. This results in random and violent fluctuations in spacetime that are
larger than envisaged under quantum theory, rendering the apparent weight of
objects unpredictable if measured precisely enough.
A second paper, published simultaneously in Nature
Communications and led by Professor Oppenheim's former Ph.D. students, looks at
some of the consequences of the theory, and proposes an experiment to test it:
to measure a mass very precisely to see if its weight appears to fluctuate over
time.
For example, the International Bureau of Weights and
Measures in France routinely weigh a 1kg mass which used to be the 1kg
standard. If the fluctuations in measurements of this 1kg mass are smaller than
required for mathematical consistency, the theory can be ruled out.
The outcome of the experiment, or other evidence emerging
that would confirm the quantum vs. classical nature of spacetime, is the
subject of a 5000:1 odds bet between Professor Oppenheim and Professor Carlo
Rovelli and Dr. Geoff Penington—leading proponents of quantum loop gravity and
string theory respectively.
For the past five years, the UCL research group has been
stress-testing the theory, and exploring its consequences.
Professor Oppenheim said, "Quantum theory and
Einstein's theory of general relativity are mathematically incompatible with
each other, so it's important to understand how this contradiction is resolved.
Should spacetime be quantized, or should we modify quantum theory, or is it
something else entirely? Now that we have a consistent fundamental theory in
which spacetime does not get quantized, it's anybody's guess."
Co-author Zach Weller-Davies, who as a Ph.D. student at UCL
helped develop the experimental proposal and made key contributions to the
theory itself, said, "This discovery challenges our understanding of the
fundamental nature of gravity but also offers avenues to probe its potential
quantum nature.
"We have shown that if spacetime doesn't have a quantum
nature, then there must be random fluctuations in the curvature of spacetime
which have a particular signature that can be verified experimentally.
"In both quantum gravity and classical gravity,
spacetime must be undergoing violent and random fluctuations all around us, but
on a scale which we haven't yet been able to detect. But if spacetime is
classical, the fluctuations have to be larger than a certain scale, and this
scale can be determined by another experiment where we test how long we can put
a heavy atom in superposition of being in two different locations."
Co-authors Dr. Carlo Sparaciari and Dr. Barbara Šoda, whose
analytical and numerical calculations helped guide the project, expressed hope
that these experiments could determine whether the pursuit of a quantum theory
of gravity is the right approach.
 |
The weighing of a mass—an experiment proposed by the UCL
group which constrain any theory where spacetime is treated classically.
Credit: Isaac Young |
Dr. Šoda (formerly UCL Physics & Astronomy, now at the
Perimeter Institute of Theoretical Physics, Canada) said, "Because gravity
is made manifest through the bending of space and time, we can think of the
question in terms of whether the rate at which time flows has a quantum nature,
or classical nature.
"And testing this is almost as simple as testing
whether the weight of a mass is constant, or appears to fluctuate in a
particular way."
Dr. Sparaciari (UCL Physics & Astronomy) said,
"While the experimental concept is simple, the weighing of the object
needs to be carried out with extreme precision.
"But what I find exciting is that starting from very
general assumptions, we can prove a clear relationship between two measurable
quantities—the scale of the spacetime fluctuations, and how long objects like
atoms or apples can be put in quantum superposition of two different locations.
We can then determine these two quantities experimentally."
Weller-Davies added, "A delicate interplay must exist
if quantum particles such as atoms are able to bend classical spacetime. There
must be a fundamental trade-off between the wave nature of atoms, and how large
the random fluctuations in spacetime need to be."
The proposal to test whether spacetime is classical by
looking for random fluctuations in mass is complementary to another
experimental proposal that aims to verify the quantum nature of spacetime by
looking for something called "gravitationally mediated entanglement."
Professor Sougato Bose (UCL Physics & Astronomy), who
was not involved with the announcement today, but was among those to first
propose the entanglement experiment, said, "Experiments to test the nature
of spacetime will take a large-scale effort, but they're of huge importance
from the perspective of understanding the fundamental laws of nature. I believe
these experiments are within reach—these things are difficult to predict, but
perhaps we'll know the answer within the next 20 years."
The postquantum theory has implications beyond gravity. The
infamous and problematic "measurement postulate" of quantum theory is
not needed, since quantum superpositions necessarily localize through their
interaction with classical spacetime.
The theory was motivated by Professor Oppenheim's attempt to
resolve the black hole information problem. According to standard quantum
theory, an object going into a black hole should be radiated back out in some
way as information cannot be destroyed, but this violates general relativity,
which says you can never know about objects that cross the black hole's event
horizon. The new theory allows for information to be destroyed, due to a
fundamental breakdown in predictability.