Nobody understands whether gravity is truly quantum in nature at a fundamental level. It's strongly suggested by a novel experiment.
Everything in our Universe is made up of individual
quanta, each of which has both wave and particle attributes at the same time,
if you reduce it down to its tiniest and most fundamental subatomic constituents.
If you pass one of these quantum particles through a double-slit and don't pay
attention to which slit it goes through, it will behave like a wave,
interfering with itself on its path and leaving us with just a probabilistic
set of outcomes to define its ultimate route. Only by seeing it can we pinpoint
its exact location at any given time
For three of our fundamental forces, the
electromagnetic force, as well as the strong and weak nuclear forces, this
strange, undefined behaviour has been thoroughly observed, researched, and
characterised. It has never been tested for gravitation, which is the only
surviving force that has only a classical description in Einstein's general
relativity. Despite numerous sophisticated experiments attempting to determine
if a quantum description of gravity is required to account for the behaviour of
these fundamental particles, none has ever been conclusively performed.
However, the Aharonov-Bohm effect, a well-studied
quantum phenomena, has recently been revealed to occur in gravity as well as
electromagnetism. It could be our first sign that gravity is actually quantum
in nature, and it has gone mostly unnoticed.
(Credit: SLAC National Accelerator Laboratory) |
The quantum question:
Few experiments in quantum physics are more
illustrative of reality's strange character than the double-slit experiment.
Shining light through two thin, closely spaced slits resulted in an
interference pattern rather than two illuminated images on the screen behind
the slits when conducted with photons more than 200 years ago. Before reaching
the screen, the light that passed through each of the two slits must have
interacted, resulting in a pattern that demonstrates light's intrinsic
wave-like nature.
Later, it was demonstrated that the same interference
pattern could be formed with electrons as well as photons; for single photons,
even when they were passed through the slits one at a time; and for single
electrons, even when they were sent through the slits one at a time. The
wave-like behaviour is plainly seen as long as you don't measure which slit the
quantum particles pass through. It demonstrates the system's paradoxical, yet
very real, quantum mechanical nature: In a sense, a single quantum is capable
of passing through "two slits at once," where it must interfere with
itself.
Despite this, there is no interference pattern when
you measure which slit these quanta pass through. Instead, two
"clumps" appear on the far side of the screen, corresponding to the
set of quanta that passed through slit #1 and slit #2, respectively.
This is a bizarre result that gets to the heart of
what makes quantum physics so strange, but also so powerful. In a classical,
pre-quantum description of such quantities, you can't just assign definite
values like "position" and "momentum" to each particle.
Position and momentum must instead be seen as quantum mechanical operators:
mathematical functions that "operate" (or act) on a quantum
wavefunction.
You obtain a probabilistic set of outcomes for what is
possible to observe when you "act" on a wavefunction. When you make
that crucial observation — that is, when you make the quantum you're
"observing" interact with another quantum whose consequences you then
detect - you only get one value back.
Let's pretend you're conducting this experiment with
electrons, which have a fundamental, negative electric charge, and you're
sending them through these slits one by one. It's simple to describe the
electric field generated by an electron as it passes through a slit if you know
which one it passes through. Even if you don't make that essential observation
— even if the electron passes through both slits at the same time — the
electric field it generates can still be described. You can do this because
quantum nature encompasses not only individual particles or waves, but also the
physical fields that pervade all of space they obey the rules of quantum field
theory.
We've confirmed and validated quantum field theory
predictions for the electromagnetic interaction, as well as the strong and weak
nuclear interactions, several times. The concordance between theoretical
predictions and experimental, measurement, and observation results is
astounding, with many examples agreeing to better than one part in a billion
precision.
However, if you pose a question like "what
happens to an electron's gravitational field when it passes through a double
slit," you will almost certainly be disappointed. We can't make a reliable
prediction without a working quantum theory of gravity, and detecting such an
effect empirically is much beyond our current capabilities. No experiment or
observation has been able to produce such a vital measurement, hence we don't
know if gravity is an intrinsically quantum force or not.
What about gravity?
When it comes to experimenting with the gravitational
force, the major issue is that gravitational effects are so insignificant.
Despite the fact that individuals have been designing studies to identify this
effect for decades, a major breakthrough was made in 2012. A group of experts
lead by Michael Hohensee devised a plan for an experiment that might be carried
out with existing technology.
The concept was that by pulsing a laser beam into an
area where the gravitational potential — but not the field — differs from other
locations, you might produce ultra-cold atoms and control their motion. The
non-zero potential could have an influence even in locations where the
gravitational force is zero, which can be achieved with careful setup. You may
see an interference pattern by splitting a single atom into two matter waves,
moving them into locations with differing potentials, and then bringing them
back together, measuring their phase and thereby quantifying the gravitational
Aharonov-Bohm effect.
We anticipate a purely quantum phenomenon. However,
for the first time, it is solely reliant on gravity rather than any other
interaction.
It was accomplished a decade later by a team led by
Chris Overstreet. The scientists used numerous ultra-cold rubidium atoms in
quantum superpositions with one another and forced them to trace two
alternative pathways inside a vertical vacuum chamber, according to a studypublished in Science on January 13, 2022. Because there was a large mass at the
top of the chamber, but it was axially symmetric and fully outside of the
chamber, it simply modified the gravitational potential of the atoms, with the
atoms on a higher trajectory experiencing a larger change in potential.
The atoms are then brought back together, and a phase shift emerges from the resulting interference pattern. The amount of phase shift to be measured should be equivalent to:
- The distance between the two atoms,
- The distance between them and the top of the chamber,
- And whether or not the external mass that modifies the gravitational potential is present.
Overstreet's team was able to measure the phase shifts
of these atoms for the first time and compare them to theoretical predictions
for the gravitational Aharonov-Bohm effect by repeating the experiment with a
variety of settings. Not only has it been found, but the match is also perfect.
This is an outstanding achievement. However, the
analysis might be extended to any quantum or classical force or field that can
be derived from a potential. It's a huge step forward for quantum mechanics
under the effect of gravity, but it's not nearly enough to show that gravity is
quantum. Maybe one day we'll get there. Meanwhile, the search for a better
understanding of gravitation continues.
References:
Observation
of a gravitational Aharonov-Bohm effect
-
CHRIS OVERSTREET, PETER ASENBAUM, JOSEPH CURTI, MINJEONG KIM AND MARK A. KASEVICH DOI: 10.1126/science.abl7152
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