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To progress to the next level in understanding reality, we need to combine quantum mechanics
and Einstein’s general relativity. And to do that, most physicists believe we
need a theory of quantum gravity .. which means we need gravitons. But it also seems
like the laws of physics make it impossible to ever detect this quantum particle of gravity.
Almost like the universe is set up to keep the final answer forever out of our reach.
So, can we outsmart the universe, catch a graviton, and finally solve physics?
if there’s anything you had your eye on there’s a link in the description. Now onto the episode
In 2012, the legendary physicist Freeman Dyson gave a talk in which he speculated
on the possibility of ever detecting gravitons. He was not optimistic. In his Poincare prize lecture,
he laid out how the universe seems to conspire to make the detection of the quantum particle
of gravity impossible. In some cases it seems to be impossible for all practical experiments—the
experiments are just too outlandish to ever happen. But in other cases,
graviton detection seems to be in-principle and fundamentally impossible—we’re thwarted by
the appearance of black holes or by the quantum vacuum from ever glimpsing a graviton. So does
the universe really prohibit us from ever seeing the building block of the fabric of spacetime?
As a reminder, the graviton is the quantum particle of gravity,
just as the photon is the quanta of electromagnetism, gluons of the strong force,
etc. But the graviton is more than that—it’s the building block of the fabric of the universe. In
the same way that an electromagnetic field is made of a sea of “virtual” photons,
spacetime is made of gravitons. At least, that’s true if spacetime has a quantum nature, as most
physicists seem to believe. All theories of quantum gravity require them, from string theory
to loop quantum gravity. So, probably worth trying to spot one if we want to verify these theories.
Now in the past we’ve talked about indirect ways to test the quantum nature of gravity,
but what about detecting the graviton itself? Well, there are two broad ways we can think
about graviton detection. On the one hand, we can take what we know about detecting
classical gravitational fields and try to apply it to a quantized field of gravitons—basically,
to detect the gravitational effect of a single graviton. On the other hand,
we can treat gravitons like every other particle and use the same techniques we once used to
squeeze out photons from the electromagnetic field. By tackling both perspectives we can
get a reasonably comprehensive sense of how impossible graviton detection really is.
Let’s start by trying to detect the gravitational effect of a single graviton.
We’ll think about this in terms of the most sensitive gravity detectors
ever built: the Laser Interferometer Gravitational Wave Observatory—LIGO.
This pair of detectors tracks the subtle differences in phase of a pair of laser
beams traveling along 4km perpendicular arms. When those phases fail to line up,
it indicates a relative change in the lengths of those arms—a possible signature of a passing
gravitational wave. We’ve used these observatories to spot gravitational
waves from hundreds of merging black holes and neutron stars since the first detection in 2015.
So what would it take for a laser interferometer along these lines to detect a single graviton?
We’ll follow Dyson’s argument here. LIGO is able to detect gravitational waves with a “strain” of
10^-22—that’s the relative change in length it’s sensitive to. It corresponds to one-one thousandth
the width of a proton for the 300 back and forth reflections along LIGO’s 4km arms. If gravity
is indeed quantum then gravitational waves are made of a coherent superposition of gravitons,
in the same way that our lasers are made of photons. A wave at LIGO’s strain limit
would contain at least 10^36 gravitons. We’re trying to see a single graviton,
so in principle we’d need a detector that’s 10^36 times more sensitive.
The critical question we need to ask is how precisely do we need to measure the
variation in the length of detector arms to pick up a single graviton event? It turns
out that the answer is independent of the type of gravitational wave:
for optimal sensitivity, we need to be able to measure a length difference of order a single
Planck length. This should already raise concerns because the Planck length is the
distance where our current understanding of space breaks down. But let’s proceed.
The limits of measurability of literally everything is defined by the Heisenberg
uncertainty principle. The principal sets an absolute limit on how precisely we can
know complementary pairs of properties simultaneously, for example the position
or momentum of something. Let’s start simple by imagining one LIGO arm as a
pair of free-floating mirrors. A gravitational wave causes the distance between them to change,
and we want to measure that changing distance. That’s effectively measured by bouncing a
photon between them. However that photon also imparts momentum on the mirrors, inducing an
uncertainty in the mirror position. The more precisely we want to measure those positions,
the higher the frequency of light we need, therefore the more momentum we have to impart. The
position-momentum uncertainty relation can be used to relate the limit of the position measurement
to the imparted mirror momentum. We can then convert this relation to one involving time.
So in order to accurately measure the separation of the mirrors, the measurement has to happen
fast—faster than the mirrors move out of position due to the act of measuring.
Long story short, we can maximize the precision of our measurement of the mirror separation in
two ways: by reducing the distance between the mirrors—that reduces measurement time. And by
increasing the mass of the mirrors, heavy mirrors move more slowly when bumped by the measuring
photon. If we want a 1-Planck-length position precision due to the passage of a graviton,
the mirrors need to be massive enough and close enough together that they form a black hole. Well,
that sucks. By definition, the formation of an event horizon prevents our distance measurement.
Dyson shows that we get the same issue via a different argument even if we fix
the mirrors in place. In general, any distance measurement on the scale of
the Planck length gives us black holes, which means that direct measurement of the effect
of a single graviton by a LIGO-like device is fundamentally impossible.
Okay, let’s try something else. The gravitational wave approach tries to
measure the actual gravitational effect of a single graviton. But maybe treating gravitons
gravitationally is using a classical hammer to crack a quantum nut. Instead,
maybe we should look for gravitons by the same method as we look for other quantum particles. We
can summarize that method in very general terms: crash particles together and see what happens.
The physics of colliding particles is inherently different to the interactions of classical waves,
so maybe that’s the right approach for spotting a particle that arises from a quantum field.
The most obvious example of the particle collision method is, surprise surprise,
a particle collider. We discovered the Higgs boson by building a collider so large that
its collision energies could generate the massive Higgs particle. And that’s the 27
km diameter large hadron collider. So how large a collider do we need to generate the graviton?
Gravitons are massless, so energy isn’t needed to give them mass like with the Higgs. Rather, energy
is needed to increase the chance of generating one. Gravity is the weakest force by a long,
long way. It’s 24 orders of magnitude weaker than the weakest of the other fundamental forces,
a fact which is itself a conundrum called the hierarchy problem,
which obviously we’ve talked about before.
In quantum terms, that weakness can be expressed as a very small coupling constant between the
graviton and other particles. The probability of generating a graviton in a particle collision
depends on that coupling constant. Despite the word “constant”, the coupling factor actually
increases with the energy of the interaction At around a billion Joules, the coupling for
gravity reaches the ballpark strength of the other forces. So that’s the energy we need to
reach in our collisions to have a fair chance of producing gravitons. How big an accelerator
do we need to reach that energy? Well the LHC collisions reach a whopping millionth of a Joule.
For a fixed magnetic field strength, collision energy scales directly with collider size. To
get to the energy to detect a graviton, a collider with the same magnets as the
27km LHC would need to be around 3 light years in diameter—much bigger than our solar system. That
sounds insanely difficult, but it’s in-principle possible. And once we’ve build a graviton-factory,
we get to the real challenge— it's not creating gravitons but detecting them.
Now massless gravitons are stable, so we can’t rely on looking for their
decay products. We’d need to see one via its gravitational effect,
which we just saw may be impossible, or when it’s absorbed by or scatters another particle.
The OG example of discovering a particle this way is the photoelectric effect, in which an
electron is ejected from a conducting plate by a single photon. Einstein used this effect to
demonstrate the existence of photons as quanta of the electromagnetic field. So can we use a
similar test to detect gravitons? An equivalent gravito-electric effect? Well in principle,
yes. A sufficiently high-frequency graviton will carry enough energy to kick an electron
out of an atom or out of a conducting plate. We could also imagine a graviton equivalent of
the Compton effect, in which electrons are ejected from atomic orbitals. Either way,
we shift the problem from detecting gravitons to detecting electrons, which is pretty easy.
The probability of an electron absorbing a graviton to produce one of these phenomena depends
on the coupling strength of gravity. So just as with graviton creation, it’s super unlikely.
Another way to think about this interaction strength is in terms of the cross-section.
It’s called a cross-section because it tells us how big an electron looks to a graviton—how
close the graviton needs to get to the electron center in order to hit it and get absorbed. And
for the graviton-electron interaction the cross section is proportional to the square
of the Planck length. So even if we can create a graviton in our interstellar-sized collider,
let’s call it the SLC—“stupidly large collider”, we can probably never detect it.
The other way we can increase the likelihood of a gravito-electric
interaction is by increasing the number of gravitons in the right frequency range. So
maybe we amp up the power of our SLC, or we find a natural source of gravitons.
The theoretical physicist Stephen Weinberg calculated that there is potentially a huge
quantity of high-frequency gravitons being emitted by the Sun.These are produced by electron-graviton
interactions in the hot, dense core of our star. At the energy we need, the sun should produce
about 10^24 per second. This amounts to about 4 per meter squared per second passing through
the Earth’s surface. You probably don’t notice the several that passed through you during this
sentence alone. The interaction cross-section for this is so small that these solar gravitons only
interact with a particle of matter roughly once every billion years or so across the entire volume
of the Earth. Now we could build a star-sized detector and maybe get a detection every 1000
years rather than every billion, but that still feels like an unsatisfying yield.
We probably actually need a stronger source of gravitons than a mere entire star. The best
option might be the collapsed core of a dead star. Robert Gould estimates that a hot white
dwarf or a neutron star would emit, respectively, 100 and 100,000 times more gravitons than the Sun,
though over a shorter timescale. So placing a planet or star-sized detector near such
a stellar remnant might net us a single graviton on human timescales. Of course,
the path to building such an experiment is far beyond human timescales if it’s possible at all.
And if we were somehow able to pull this off, we’d face an even more basic issue:
neutrinos. These ghostly particles are notoriously hard to spot. Most of the Sun’s neutrinos pass
straight through the earth, and we’ve built detectors out of the Antarctic ice cap to
catch the occasional rare interaction. But by comparison to the shy graviton, the neutrino is
a party animal. For any of our graviton sources, regardless of whether it’s our Sun, a white dwarf,
or a neutron star, our detector will interact with 10^34 neutrinos per single graviton.
And distinguishing that graviton from the neutrino noise seems practically impossible.
Let’s try one more method that on the surface looks promising. In the 1960s Mikhail
Gertsenshtein showed that electromagnetic and gravitational waves, in a strong magnetic field,
can couple together. This is more promising than looking for interactions between gravitons and,
say, electrons. EM and gravitational waves, which both travel at the speed of light,
can experience wave resonance that allows energy to transfer between them at much lower coupling
strength than is needed for the absorption of a graviton by matter. This means a photon may be
transformed into a graviton and vice versa. So, if we shoot a graviton through a magnetic field, it
will oscillate between graviton states and photon states. This is called the Gertsenshtein effect.
Now to do this, we need a long hollow tube threaded by a strong magnetic field passing
and pointed towards a source of gravitons. The hope is that some of those gravitons will turn
into photons that we can detect. And, as you may have guessed, the universe again conspires
against us. The magnetic field needs to be quite strong for the Gertsenschtein effect to work. How
strong? Strong enough that we have spontaneous creation of matter-antimatter pairs inside the
tube. This vacuum polarization resulting from this limits the coherence of EM waves in the tube,
preventing resonance with gravitational waves and shutting down the Gertsenschtein effect.
So we’re at strike three for detecting gravitons. It seems the universe really
doesn’t want us to see its deepest structure. But is it really impossible,
or just nearly impossible? In a couple of the cases we looked out the answer is …
impossible. The barrier to graviton detection seems fundamental. The formation of black
holes or the breakdown of the vacuum tells us that the laws of physics forbid graviton
detection by these methods. But in other cases it’s just astonishingly difficult—we
need star-sized graviton sources and detectors and some yet-unknown way to filter neutrinos.
But next to impossible is still possible. I’m saying there’s a chance. And maybe
the problem is actually just that we haven’t come up with a smart enough experiment yet.
Or maybe we have. Since Dyson’s 2012 lecture, we’ve detected gravitational
waves directly and we’ve made major advancements in quantum technology.
This has inspired a new generation of physicists to come up with a new generation of proposals.
For example, we may be able to combine the two broad approaches I mentioned:
using a LIGO-like interferometer combined with absorption of gravitons, this time by a detector
with novel quantum properties. How does that work? Well that’s for an episode coming soon,
when we’ll find out how we really may be able to detect the particle at the heart of spacetime.
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