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Recently, we looked at whether nature
has a hard rule against measuring the
graviton, the quantum particle of
gravity. By some approaches, directly
glimpsing this building block of
spaceime seems fundamentally impossible.
But we also saw that there are other
methods that are just so stupidly
difficult they might as well be
impossible. They seem to involve using
planet-siz graviton detectors and even
stellar corpses as graviton sources.
Sounds disheartening. Well, maybe we
have to wait millennia until we can
build our astroengineering
superructures. Or maybe we can just get
cleverer today. In recent years,
theoretical and experimental
developments have made it possible to
conjure up new ideas for detectors. One
such possibility is the use of quantum
sensing outlined in a paper by Turbar,
Manacandon, Badel, and Pikovsky
submitted to Nature in 2024.
In the previous episode, we talked about
a family of experiments that work by
treating the graviton as any other
particle. So rather than trying to
detect its infinite decimal
gravitational field, we instead try to
spot a more direct interaction with
another particle, similar to how we do
with particle colliders. When we account
for the extreme low probability of such
an event, we found that we needed these
planet-siz detectors and stellar
graviton sources to even have a remote
chance at it. In order to claim that we
saw a quantum of gravity, these sorts of
detectors require that the graviton
interact with another quantum particle.
Now, elementary quantum particles are
tiny and gravity is incredibly weak.
Combine these two and the graviton has
only a minuscule chance of having a
sufficiently head-on collision with such
a particle to produce the sort of
interactions that we need. Now, we can't
do much about the weakness of gravity,
but what if we can make a quantum
particle bigger? What if we can make one
that's human scale, that's macroscopic?
That's what this new paper proposes with
its resonant mass detector. It would
work like this. Cool. a metal cylinder
to as close as possible to absolute
zero. In that state, there's almost no
motion in the individual atoms in the
crystal latice. There's also almost no
energy in the global oscillations of the
metal in its vibrational modes. At
sufficiently low temperature, those
vibrational modes are quantum states.
They increase in discrete steps, quantum
jumps, just as an electron jumps up in
discrete energy levels in an atom. These
individual quantum vibrational modes are
called phonons. They're like a quantum
of sound. Now, these are going to be our
macroscopic quantum particles, these
phonons. And they will have a vastly
larger cross-section or probability for
interaction with the graviton than does
a single electron. So, when a graviton
passes by, there's a tiny but no longer
unthinkably tiny chance that it'll
excite a phonon. Then using quantum
sensing techniques, this phonon can in
principle be detected. And that could be
considered the detection of a graviton.
There are of course one or two
complications. To even get our cylinder
into its vibrational ground state, it
needs to be cooled to a small fraction
of a single Kelvin. And I'll come back
to the exact number and the plausibility
later. But even having achieved that,
most of the phonons generated in the
detector will not be due to gravitons.
The excitation energy of a single phonon
is so small that any source of noise can
produce one. That includes the thermal
fluctuations that we are trying to
minimize, but also seismic noise, cosmic
rays, electromagnetic interference,
internal material defects, and feedback
from the device that we're using to
measure these phonons. try as hard as we
might. It's not feasible to reduce noise
to the level that gravitons are the main
source of detected phonons. But what if
somehow we had an independent way to
know that a sudden flood of gravitons
had passed by our detector at the very
instant that a graviton was detected.
And we knew that those gravitons were of
the exact frequency as our excited
phonon. Well, assuming that we got our
noise down low enough, that would
probably give us a lot more confidence
that we'd actually succeeded in
detecting a graviton. And this is the
other brilliant part of this proposal.
We actually have a way of doing just
that. In 2015, we detected our first
gravitational wave from merging black
holes with LIGO. Since then, we've
detected hundreds. If gravitons are
real, then a gravitational wave really
is just a coherent flood of many
gravitons, similar to how a laser is a
coherent beam of photons. A
gravitational wave from a black hole
merger might contain 10 ^ of 36
gravitons. And those gravitons will
share the same well- definfined
frequency as the gravitational wave that
they're part of. Now, this is starting
to all come together. We build a
cylinder with mass and dimensions to
support a vibrational mode with
frequency equal to an expected
gravitational wave frequency. For a
neutron star neutron star merger like
the ones we've seen with LIGO, a
possible candidate would be a burillium
bar of about 15 kg. Alternatively, to
detect a graviton from a black hole
black hole merger, a neobium bar
weighing around 10 tons would have the
right properties to be excited by a
graviton with the right frequency around
175 hertz. The bar would then have to be
cooled to 1 millke. Now, this is colder
than what we are currently able to
achieve. At best, we can manage a few
hundred millichel.
proposed experiments get us a bit
closer. So, this might be viable in the
near future. Once the cylinder is
attached to our phonon detector, we'll
start to get frequent clicks of
detection. Now, almost all of these will
be due to the various sources of noise,
but eventually an excitation will occur
at the exact same time that LIGO detects
a gravitational wave of the same
frequency. And as long as the noisy
excitations are rare enough that such a
coincident detection is unlikely, then
we can hope that we've actually detected
a graviton. And this brings me to the
other main hurdle, continually measuring
what would be an extremely delicate
system without ruining the precision of
the measurement or the quantum state.
Since we don't know when a gravitational
wave is going to hit us, the experiment
needs to run continuously and it needs
to very monitor the cylinder checking
for an excitation.
We're also not there yet with the
quantum sensing technology needed to do
the delicate continuous monitoring, but
it's something that's maybe just decades
away rather than millennia. Okay, so we
build the detector, we detect the bar
shift into a higher frequency level, and
this coincides with a LIGO detection at
the same frequency. Does that mean we've
finally definitively detected a
graviton?
This is where things get a little muddy.
If gravitons exist, then yes, the click
was probably due to an actual graviton.
But that's not the same thing as a
formal discovery of gravitons, because
there are ways that gravity could have
made the click happen. even if gravitons
don't exist at all. Now, to explain
that, I want to go back to the
phenomenon that started this whole
forces as quantum fields business, the
photoelectric effect, which is often
described as providing proof of the
existence of the photon. It goes like
this. Take a pair of conducting plates
with a gap between them and shine a
laser at one plate. Energy from photons
in the laser can be absorbed by
electrons in the plate, allowing them to
jump between the plates and resulting in
a measurable current. Now apply a
voltage across the gap. So the electrons
need a minimum energy to make the
crossing. Before we knew about photons,
you might think that as long as the
laser was bright enough, it could
provide the needed energy to cause a
current. But instead we find that if the
frequency of light is below a certain
threshold, no electrons will jump the
gap. It doesn't matter how bright you
make the laser or how long you shine it
for. Now this makes sense if we say that
one, the EM field of the laser is made
of individual chunks. Two, those chunks
have a specific energy defined by the
frequency. And three, an electron can
absorb only a single chunk in order to
make its jump between the plates. Low
frequency light, no matter how intense,
just doesn't contain photons with enough
energy to cause a jump. This is the line
of reasoning usually used to explain why
the photoelectric effect constitutes the
discovery of the photon.
This is wrong. The photoelectric effect
is really the discovery of quantum
energy levels in electrons. That's
because there is a way for a purely
classical EM field to deliver its energy
to enable a quantum jump. A classical
field is able to deliver that energy in
arbitrarily small portions. It doesn't
do this by delivering tiny bits of
energy over time, but rather by slowly
increasing the quantum probability of a
jump. It's technical, but the point is
that the photoelectric effect didn't
really constitute the discovery of
photons. It discovered quantized energy
levels in atoms. Okay. So why the
tangent to the photoelectric effect?
Because the same reasoning applies to
our graviton experiment. The way I
described it, a single gravitton with
the right frequency excites a phonon of
the same frequency in the metal
cylinder. But even if the gravitational
field is not made of gravitons, if it's
really classical, it can still excite
that phonon by slowly increasing the
excitation probability as long as it has
the right frequency. Now, this is a
subtle point. If gravitons exist, then
the clicks from a resonant mass detector
coinciding with the right gravitational
wave are almost certainly graviton
detections, which is amazing because we
didn't need a planet-siz detector. But
they don't prove that gravitons exist
because there's another potential
explanation. So, how do we proceed? In
the case of the photoelectric effect, we
were able to prove the existence of
photons with a much more sophisticated
experiment that involved preparing the
EM field so that it would be in a
non-class state. If it is indeed a
non-class field, for example, extremely
low inensity lasers so that the plate is
hit by just very few photons at a time.
then you can tell from the distribution
of energies of excited electrons whether
the EM field is fundamentally quantized
or smooth and classical. And so in
principle this can be done with
gravitons too except that now we're back
to the realm of how the hell do we do
this? There are no no natural source of
non-class gravity and certainly none of
single gravitons. Now, we may still need
to build a graviton source rather than
relying on natural sources. So, are we
back to the drawing board? Not quite.
Our detector did just get an insane
upgrade, and so the burden of the
challenge now seems to lie in the
graviton source, and perhaps some
unexpected result will show up. Or you
could actually go back to the drawing
board and get even cleverer. A recent
paper by Ralph Schutz proposes an
optical Weber bar instead of a solid
resonant mass. In this experiment, you
use laser pulses in an intererometer
like geometry so that a passing
gravitational wave can transfer a tiny
but lasting amount of energy to the
light, which then shows up as a
measurable phase shift. Unlike LIGO,
which reads out a phase change caused by
changes in the arm lengths as the wave
is passing, this scheme aims to convert
the gravitational waves time dependent
modulation into a permanent frequency
and energy shift of the photons and then
amplify the resulting phase difference
over a much longer period of time. In
graviton language that net energy
exchange can be described as stimulated
emission or absorption of gravitons by
the light and analogy of stimulated
processes in quantum optics. So it's
kind of like lasing the gravitational
wave itself although the effect is
observed in the light not in the
gravitational waves. As with the
resonant mats detector, the energy
transfer alone wouldn't be enough to
prove that gravity is quantized. But
this scheme offers a way to at least see
the signature of quantum gravity. If the
light is prepared in a strongly
non-class state, then energy
conservation ties the photon state to
the gravitational field's own energy.
And this could put the gravitational
wave into an actual quantum superp
position which would reveal itself in
aspects of the phase of the
interferometer beam. And if a
gravitational wave could be in a quantum
superp position then it must be quantum.
The baseline optical Weber bar might
actually be feasible with present-day
interferometric tools while the
genuinely quantum gravity sensitive
version demands a much more extreme
quantum state preparation and readout
and is even further away than the
resonant mass detector. So yeah, the
universe wanted us to believe that we
could never hope to glimpse the quantum
nature of gravity. like it's embarrassed
or something to have us peer too deeply
under its kilt to see how the sausage is
made. And while it's going to be a long
time before we can both catch a graviton
and be sure that we caught one, there
are some brilliant experiments that we
could do much sooner, in some cases
nowish, that may let us tease the
quantum out of the classical seeming
fabric of spaceime. Thank you to
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