Physics is this close to understanding the entire universe. And what lives in
this gap? Many physicists think it’s the elusive graviton—the quantum particle of
gravity—whose discovery will finally allow us to stitch together our two great theories
of nature into a single master theory. But what is the graviton, and does it even exist?
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The quantum revolution started when we realized that light is made of particles—photons. Max
Planck guessed it and Albert Einstein proved it. Light is a wave in the electromagnetic
field, and so even from the beginning of modern physics we have this idea that a
force—the electromagnetic force—is communicated by particles. As quantum mechanics evolved into
quantum field theory, we also found the particles for the weak and the strong forces. That’s three
of the four fundamental forces, leaving only gravity lacking a mediating particle. So, if
we could just figure out the graviton, we’d have unified all the forces of nature and be on track
to a theory of everything. We’re now 100 years after the birth of quantum mechanics, and much of
the past century of work towards this master that hinges on the existence of the graviton. But does
it exist? If not, much of that work in unifying our theories has been in the wrong direction.
We can describe nearly the entire universe in terms of two theories—quantum mechanics
and general relativity. Yet they look wildly different to each other. taOn
the quantum side we have these fields filling all of space,
whose oscillations give us all known particles of matter and three of the four fundamental forces.
These quantum fields fracture into discrete shards as you zoom in, and whose properties and
particles can never be pinned down precisely. On the relativity side we have space and time
merging into a unified fabric of spacetime; a fabric that can expand and warp. It stretches
in response to matter and energy, leading to what we experience as the force of gravity.
It’s a pretty effective picture. Quantum theory gives us the stuff of the universe,
relativity gives us the container. Matter in spacetime. Matter tells spacetime how to curve and
spacetime tells matter how to move. So, if this picture works so well in describing the universe,
why do we need to try to force gravity to work the same way as the quantum forces.
Well, partly because the two theories are so different. We expect, or at least hope,
that all the complexity we observe in the universe emerges from unified underlying laws. That proved
to be true of all matter and most of the forces of nature—quantum fields all the way down. It
would be weird if the most fundamental layer of reality was actually two unrelated things. And
extra weird if the theories describing those things are profoundly different—fundamentally
discrete and random for the quantum, fundamentally continuous and deterministic for relativity. But
maybe that’s just how it is. Not very elegant, but does the universe care what we find pretty?
There’s another reason besides aesthetics that we need quantum
theory and relativity theory to connect. That’s because at a fundamental level,
they actively contradict each other. Together they describe most of the universe,
but in certain places like the center of a black hole or at the big bang,
they generate contradictions and paradoxes that, it seems, can only be solved by finding
the master theory in which quantum mechanics and general relativity connect seamlessly.
For various reasons, most physicists trying to solve this problem follow a particular
path. The path of particles. They try to quantize gravity, just as they once
quantized electromagnetism. And the search for this theory of quantum gravity implies
a particle that mediates gravity. That’s the graviton. If the graviton exists,
then gravity has to be quantum, and vice versa. Let’s see why that’s the case.
To get a photon, we start with the classical electromagnetic field—the one described by
Maxwell’s equations. A field in that sense is just something that extends through space
and has some value at each point in space. That value can be a simple number. Like,
the density or temperature of the air in the room,
giving a density or temperature field. Or it can be a vector—a number and direction. The EM
field is like that—a strength of the force and a direction at every point in space.
Fields can support waves—oscillations in field strength that move through space. The
density field in air supports sound waves. The EM field supports EM waves, which we know as light.
So, when Planck and Einstein showed that light is made up of indivisible particles, we were set on
the path to quantizing the EM field itself. This process of “quantization” followed the development
of quantum mechanics in the mid 20s and quantum field theory in the following couple of decades.
Through quantization, the EM field is broken down into countless modes of
all different frequencies. In this new description, even the empty or
vacuum-state of the EM field is made of those modes—made of infinite electromagnetic waves
stacked and canceled into nothing. And those “virtual” modes are made of discrete packets,
just as “real” EM waves—so the EM field is made of virtual photons.
In quantum field theory, we describe the quantum electromagnetic force in terms of particles
exchanging these virtual photons. They take momentum from one particle and give it to another,
causing the push and pull that we see as a smooth EM force. Photons mediate electromagnetism. We’ve
talked about whether these virtual photons can be thought of as “real” before. But what IS real
is the fact that the EM field is extremely well described as being made up of quantized chunks.
If we use basically the exact same process to quantize slightly more
complicated fields we get the other quantum forces and their mediating particles—the
gluons for the strong force and the W and Z bosons for the weak.
It should not be taken lightly that this stuff works so incredibly well. We take a classical
field, make some symmetry arguments to guess what other fields might exist,
apply quantization rules, and boom, we figure out almost all of the particles and forces that
make up our universe. The standard model of particle physics is arguably the most
successful theory in physics, and perhaps science. It’s not surprising that we’d
want to envelop gravity and spacetime into the explanatory magnificence of quantum field theory.
So, how do we proceed? Well, we need to identify the field that we’re trying
to quantize and then apply the quantization rules just like we did with the other forces.
We do know that gravity behaves like electromagnetism in important ways. It has a
strength at all points in space. It also supports waves—gravitational waves—that also travel at
the speed of light. There are differences, however. According to general relativity,
the gravitational field is not a field on top of spacetime, like electromagnetism,
but rather in a very real sense it’s the fabric of spacetime itself. This tangling of the field
and its .. is going to be a problem as we’ll see, but for now let’s proceed.
Let’s get a little more precise. In the Einstein field equations,
the field we’re interested in is encapsulated in something called the metric tensor—a 10-valued
object made up of scalars and vectors with a value at every point in space. It’s a field,
albeit a very complicated one. Where the EM field can be thought of as a thing lying on top of the
background grid of spacetime, this tensor field of GR in a real sense IS the grid of spacetime.
In electromagnetism, the field changes with respect to the grid. In general relativity,
the grid itself changes. This is going to prove complicated, but nonetheless,
this grid, this spacetime, this tensor field is the thing we need to quantize.
So let’s do that. We’ll start with an approach that hopefully won’t break anything.
We’re going to treat the gravitational field as a
small fluctuation – or perturbation – to an imaginary flat and static background.
We poke flat spacetime very lightly and see what comes out. Hopefully a graviton.
This is a very standard approach in physics. It’s called perturbation theory. Rather than
completely changing our model, we just make a tiny adjustment to what we understand and see
what happens. The result is a good approximation to reality, assuming the perturbation is very
small. Take for example electromagnetism—we can understand its quantum behavior by taking
account of the exchange of different nudges to the electromagnetic field as represented by
the exchange of virtual photons in different ways. For a weak interaction, only the most
obvious exchanges matter—like the exchange of a single virtual photon. Stronger interactions
require us to account for more complex exchanges, but it all works out beautifully.
When we do this to the gravitational field everything is … also fine!
Not only can we do a sort of gentle quantization of a weak gravitational field,
we get a glimpse of the quantum of that field that mediates the gravitational
force—the graviton. What does it look like? Well, it inherits all of its properties in
a pretty non-negotiable way from the field it comes from. It’s a massless,
spin-2 boson. Massless means it travels at the speed of light. Spin 2 means it has a
180 degree rotational symmetry that reflects the sort of stretch-squish action on space,
just like the gravitational waves that should be built from these gravitons. And boson means
you can stack infinite gravitons together, to make gravitational waves. And just as virtual
photons make the quantum EM field, we can build the fabric of spacetime out of virtual gravitons.
Cool cool. In a sense we just succeeded in quantizing gravity. This approach of
perturbative quantum gravity, and it actually works. We can use it to
recover classical gravity. We can even use it to make predictions,
like Stephen Hawking’s prediction of Hawking radiation—leaky black holes. So why don’t we
just stitch this quantum gravity to the rest of quantum field theory and say we have a theory
of everything? Why not add the graviton to the standard model library and move on with our lives?
Not so fast. We want our final theory to explain the whole universe,
but our perturbative quantum gravity only works in the case of very weak gravity. It
doesn’t work for places with extreme spacetime curvature, like the centers of black holes.
We also want to actually verify our quantum gravity theory with experiment,
and our perturbative theory works only in situations where it’s essentially impossible
to differentiate it from regular Einsteinian gravity with a human-buildable experiment.
We really need the full theory. So, to finish the job of quantizing gravity we have to see
if we can generalize this to a theory that works everywhere, including where gravity is extreme.
As gravity gets stronger, our perturbations get bigger and the corrections we need to
make to our classical theory get larger. More technically,
we need higher-order corrections, represented by more complex interactions via virtual gravitons.
Remember how we do this for the quantum electromagnetic interactions. We calculate our
quantum EM force by summing over all the possible virtual interactions that could have occurred.
For weak EM fields it’s enough to include only the most probable such interactions, and that
means the simplest ones. Like, the exchange of a virtual photon or two. For stronger EM fields,
more complex interactions become important. That includes so-called self-energy terms,
which result from the feedback between the different fields.
These higher-order terms involve loops that appear
in the many Feynman diagrams that we sum together to compute a strong interaction.
In a previous episode we talked about the connection between these loops and the
mass of the electron, and how they appear to make that mass infinite.
But these sorts of infinites can’t be real or we wouldn’t have a universe. In regular
quantum field theory we have a way to remove them called renormalization. It lets us sort of
divide them out by grounding them in measured values, for example by measuring the true
mass of the electron. This is possible with the regular quantum fields because the perturbative
expansion—the sum of Feynman diagrams—”only” generate a finite number of infinities. It’s
still possible to absorb these into a finite number of additional terms in our equations.
Even if “absorbing infinities” sounds a bit hokey, it works incredibly well. By
grounding our otherwise-infinite predictions in a few finite measurements of the real universe,
quantum field theory is able to make stunningly precise predictions of many, many things. Not
a bad trade-off. Depicting the EM field as being made of virtual photons is incredibly successful,
which forces us to take seriously this picture of a quantized field. At some
level we believe that the EM and other quantum forces are mediated by particles.
If we can do for gravity what we just did for electromagnetism
then we can make the same argument for the reality of the graviton.
So, we’re ramping up the strength of our quantum gravity, increasing the complexity
of our perturbative approximation, and hoping we can renormalize any infinities to get a sensible
theory. And this is where everything goes catastrophically wrong. The problem is that
gravitons interact gravitationally. They interact with themselves. The photon, on the other hand,
does not interact electromagnetically—at least not directly. It interacts with, say,
the electron field, which feeds back on the EM field. That extra
step limits the complexity of the photon self-interaction and limits
the number of self-energy loops, making renormalization possible.
But a graviton can directly spawn other virtual gravitons which can spawn more virtual gravitons,
ad infinitum. Instead of a finite number of self-energy loops we get an infinite number.
An infinite number of terms in our perturbative approximation that can’t
be renormalized—canceled out—with a finite number of real measurements. In cases of strong gravity,
our quantum gravity theory becomes meaningless. We say that perturbative quantum gravity is
non-renormalizable. We can also think of this in terms of the strength of the gravitational
coupling—the gravitational constant—getting stronger as the field getting stronger,
ultimately sending the field strength to infinity.
This straightforward approach at quantizing gravity leads to nonsense. So maybe it follows
that we can’t build the gravitational field out of gravitons after all. Does this mean that the
graviton doesn’t exist? Not really. What it means is that this simple picture of quantum gravity
that we get by applying the same quantization methods as the other forces isn’t right. There
may be a theory of quantum gravity that just can’t be got to by the simple perturbative approach.
Interestingly, it’s the graviton itself that may lead us to the final answer. Even though
the standard approach didn’t get us all the way, the picture of the graviton we got from it is
almost certainly right—if the graviton exists at all, it has to be a massless, spin-2 boson.
Nothing else can generate the Einstein field equations in the classical limit.
And that’s why, when we find a theory that predicts a massless,
spin-2 boson our ears prick up and we wonder if we’ve found a potential for quantum gravity.
And that’s happened once before, giving us string theory. This was originally meant
to be a theory for quark interactions, but a graviton-like particle popped out of the
math in such a profound way that it sparked a many-decade quest to find a stringy theory of
everything. You can’t avoid the graviton in string theory, and in string theory you also
avoid the rampant infinities because. At the smallest scales where gravity can be strongest,
the graviton field is smeared out over the vibrating strings at the theory’s core.
There are other approaches to quantum gravity, like loop quantum gravity and others. All
of them have a graviton, because without the graviton there’s no path to classical gravity.
It’s again sounding like the graviton is inevitable. But again, no. It’s only
inevitable if gravity is fundamentally quantum in the first place. If the final theory looks
more like quantum mechanics than general relativity. What’s the alternative? Well,
it’s that the fabric of spacetime doesn’t have discrete chunks—it’s not made of gravitons. Maybe
a smooth, continuous spacetime is fundamental in some way. A number theories favour this option.
Even Roger Penrose is a proponent, encouraging us to “gravitize the quantum” rather than
quantizing gravity. Fit quantum mechanics into general relativity rather than the way around.
Ultimately, finding evidence of the actual existence of the graviton will be critical to
confirming the quantum nature of gravity and even which quantum gravity we have. As I hinted, that’s
going to be really, really hard to do directly because we’d need to build solar-system-sized
particle accelerators. Fortunately there are brilliant methods to observe the indirect effects
of the interactions of a quantum graviton that we can do now. For example, spotting the mediation
of quantum entanglement by a gravitational field or a Cavendish experiment, but with exceptionally
tiny masses. One of these methods may pan out very soon, or we may have to start astroengineering our
mega-collider. Nature has set a last gigantic hurdle to understanding her deepest laws. We
need to understand and then find the graviton, and then we’ll hold the building block of spacetime.