Ask Ethan: Do gravitons need to exist?

If you examine the Universe extremely closely, by probing the fundamental entities with it on the smallest possible scales, you’ll discover that reality is fundamentally quantum in nature. Matter itself is made up of indivisible, uncuttable quantum entities: particles like quarks, leptons, and bosons. These quanta have charges (color charge, electric charge, weak isospin and weak hypercharge, and “mass/energy” as a gravitational charge), and it’s the exchange of quanta between these charged particles (gluons, photons, W-and-Z bosons, etc.) that mediate these forces. There’s even a Higgs force as well.

However, one type of quantum that’s never been detected is the graviton: the theoretical particle that mediates the gravitational force. Even though it’s predicted to exist (and to have a spin of 2, unique among all particles) and, just like light is composed of photons, gravitational waves should be composed of gravitons, those predictions rely on an unproven assumption: that gravity is fundamentally a quantum force in nature. Is that assumption necessarily true? It isn’t in Einstein’s general relativity, and that prompted this week’s question from Eddy from Canada, who asks:

“If space-time is geometry, then why is there a need for gravitons? I just can’t understand this simple question.”

It’s not only a great question, it’s a deep one that goes to the very nature of what gravity, fundamentally, actually is. Let’s explore.

When a gravitational wave passes through a location in space, it causes an expansion and a compression at alternate times in alternate directions, causing laser arm-lengths to change in mutually perpendicular orientations. Exploiting this physical change is how we developed successful gravitational wave detectors such as LIGO and Virgo. However, unlike this illustration, the gravitational waves do not simply propagate in a “tube,” but rather spread out through all of three-dimensional space.

The first thing we need to think about is the difference between a “real” particle and a “virtual” particle. Real particles are things we’re familiar with: we can observe, measure, and interact with them directly. They carry energy, and can be absorbed or emitted by other particles. In many regards, the first real quantum particle we ever discovered was the photon: the quantum of light. Every time you see something, that’s a result of a photon exciting a molecule in the rods or cones present in the retinas of your eyes, which then stimulates an electrical signal to your brain, which interprets the set of data coming in and constructs an image of what you observed. Those are all real photons, such as photons emitted by the Sun (or other light sources) and reflected off of the objects around us.

This makes the act of seeing an inherently quantum act, with each photon carrying a specific amount of energy that either will or won’t be absorbed by particular molecules. Although the photoelectric effect, first described by Einstein, was what directly demonstrated the quantum nature of light, it’s important to recognize that all forms of light, from the lowest-energy radio waves to the highest-energy gamma-rays and everything in between, is quantum in nature. Any light signal that’s real and carries energy is inevitably composed of a finite number of real photons: photons that are detectable both in principle and in practice.

The photoelectric effect details how electrons can be ionized by photons based on the wavelength of individual photons, not on light intensity or total energy or any other property. If a quantum of light comes in with enough energy, it can interact with and ionize an electron, kicking it out of the material and leading to a detectable signal.

However, when there’s an electromagnetic force in nature — attraction or repulsion between electrically charged particles, magnetic bending as a charged particle moves in the presence of a magnetic field, or electric/magnetic fields generated in response to a changing, time-dependent magnetic/electric field — it’s also the photon that mediates that force. However, in this case, it isn’t “real” photons that are being exchanged to mediate a force, but “virtual” photons. These virtual particles (in the case of electromagnetism, virtual photons) provide us with a method for calculating the strength and direction of electric and magnetic fields at all locations at any moment in time: one of the key advances of quantum electrodynamics (specifically for electromagnetism) in this specific instance, and of quantum field theory (for any quantum force) in general.

We can draw similar pictures, and similar analogies, for the other quantum forces in nature. When protons collide at the LHC, for example, real gluons often smash into one another (or into quarks) during the collision, but the strong nuclear force is mediated by virtual gluons. When we smash electrons and positrons together with just the right energies, they can create real W (and/or Z) bosons, whereas when neutrons decay into protons, they do so through the emission of a virtual W-boson. There’s even a Higgs force, mediated by (virtual) Higgs bosons, in addition to the real Higgs bosons we’ve successfully created at the LHC.

The first robust, 5-sigma detection of the Higgs boson was announced a few years ago by both the CMS and ATLAS collaborations. But the Higgs boson doesn’t make a single ‘spike’ in the data, but rather a spread-out bump, due to its inherent uncertainty in mass. Its mass of 125 GeV/c² is a puzzle for theoretical physics, but experimentalists need not worry: it exists, we can create it, and now we can measure and study its properties as well. Direct detection was absolutely necessary in order for us to be able to definitively say that.

Which brings us to the big question: what about gravity?

This is something where we can’t be certain, as gravitation remains the only known force for which we don’t have a full quantum description. Instead, we have Einstein’s general relativity as our theory of gravity, which relies on a purely classical (i.e., non-quantum) formalism for describing it. According to Einstein, spacetime behaves as a four-dimensional fabric, and it’s the curvature and evolution of that fabric that determines how matter-and-energy move through it. Similarly it’s the presence and distribution of matter-and-energy that determine the curvature and evolution of spacetime itself: the two notions are linked together in an inextricable way.

Now, over on the quantum side, our other fundamental forces and interactions have both a quantum description for particles and a quantum description for the fields themselves. All calculations performed within all quantum field theories are calculated within spacetime, and while most of the calculations we perform are undertaken with the assumption that the underlying background of spacetime is flat and uncurved, we can also insert more complex spacetime backgrounds where necessary. It was such a calculation, for example, that led Stephen Hawking to predict the emission of the radiation that bears his name from black holes: Hawking radiation. Combining quantum field theory (in that case, for electromagnetism) with the background of curved spacetime inevitably leads to such a prediction.

The event horizon of a black hole is a spherical or spheroidal region from which nothing, not even light, can escape. But outside the event horizon, the black hole is predicted to emit radiation, dependent on the curvature of space at each location outside of the horizon itself. Hawking’s 1974 work was the first to demonstrate this, but that work has also led to paradoxes that have yet to be resolved.

Einstein’s general relativity brings along with it a prediction that’s completely absent from its predecessor’s (Newton’s) conception of gravity: the idea that there’s a fundamental form of radiation that’s purely gravitational in nature. In Einstein’s theory, these “gravitational waves” are ripples in the fabric of spacetime itself, and they both carry energy and travel at a finite speed: the speed of light.

Just as a charged particle moving through an electromagnetic field will emit electromagnetic waves (in the form of photons), a mass that moves through a region of curved spacetime (i.e., the analogue of a gravitational field) will emit gravitational radiation, or gravitational waves.

Although the LIGO (and later, Virgo and now KAGRA) detectors famously began detecting these waves directly in 2015, we had known about their existence for many years prior to that. Systems of binary pulsars — where two neutron stars orbit one another and at least one of the neutron stars is regularly “pulsing” from our perspective — exactly represent that scenario: where a mass moves through a region of space where the spacetime curvature is changing. As a result, the orbits of these binary pulsars slowly decay, leading to a gradual change in the orbital time, which shows up as an observable in the timing of the electromagnetic pulses emitted by the pulsar(s) in question.

As two neutron stars orbit one another, the motion of one mass through the curved spacetime generated by the other mass results in the emission of gravitational waves, which carry energy away and cause the orbits to decay. The first binary neutron star system, where at least one neutron star is a pulsar, was discovered in 1974. Even as early as 1982, as the inset diagram shows, the orbit could be observed decaying, in agreement with general relativity’s predictions.

This phenomenon was first observed in the 1980s, providing very strong indirect evidence for gravitational waves. After all, something must have been carrying that orbital energy away, and the suspected culprit (again, according to Einstein’s predictions) was gravitational waves. In the post-LIGO era, we’ve now associated gravitational waves with signals arising from the inspiral, merger, and ringdown phases of:

• black hole-black hole systems,
• black hole-neutron star systems,
• and neutron star-neutron star systems,

with pulsar timing arrays poised to detect individual systems of orbiting supermassive black holes and where future gravitational wave detectors (such as LISA) expect to detect additional classes of gravitational wave-generating systems.

In other words, we’ve robustly demonstrated that gravitational radiation — i.e., a gravitational wave — is indeed a physically real phenomenon, just as light waves (made of photons) are real, and just as gluons and other bosons are real. The big question then becomes, once we’ve convinced ourselves that gravitational waves are real, carry energy, and “exist” the same way these other known entities do, is whether they exhibit wave-particle duality as well?

In other words, just as photons exhibit wave-like properties but also particle-like, quantum properties, is the same thing true for gravitational waves?

The signal from the gravitational wave event GW190521, as seen by all three active gravitational wave detectors at the time: LIGO Hanford, LIGO Livingston, and Virgo. The entire signal duration lasted just ~13 milliseconds, but represents the energy equivalent of 8 solar masses converted to pure energy via Einstein’s E = mc². This is one of the most massive black hole-black hole mergers ever directly observed. The raw data and theoretical predictions, both shown in the top 3 panels, are incredible in how well they match up, clearly showing the presence of a wave-like pattern.

Many physicists assume the answer is “yes.” Just as the wave-like nature of light was demonstrated long before it was known to have quantum-like properties, the wave-like nature of gravitational radiation is much easier to detect and demonstrate than any particle-like quantum properties it may possess. Because we’ve only seen the wave-like part of gravitational radiation, we still aren’t sure about whether a particle-based description of it — a description of it in terms of gravitons — is fundamentally correct or not. The immense difficulty lies in the notion of putting gravitation to the test in order to determine whether gravitational radiation is purely wave-like only, or in whether it has particle-like properties, too.

It’s not too difficult of a concept to visualize, if we want to go down that route.

Consider water waves, for example, which are fundamentally composed of particles (in the form of water molecules), even though that composition isn’t apparent from watching a macroscopic body of water. Imagine that you have a watery surface, such as a still pond, and you throw a bunch of ping pong balls into the water, where they float atop the water’s surface. If you generate waves in that pond, you’ll be able to track the motions of the individual ping pong balls. Individual ping pong balls would move up-and-down, back-and-forth, etc., along the surface of the water, showing you that even though waves are propagating through the water, the individual particles (both the ping pong balls and the underlying water molecules) are only moving in an oscillatory fashion, not “traveling” as the water waves appear to.

A series of particles moving along circular paths can appear to create a macroscopic illusion of waves. Similarly, individual water molecules that move in a particular pattern can produce macroscopic water waves, individual photons make the phenomenon we perceive as light waves, and the gravitational waves we see are likely made out of individual quantum particles that compose them: gravitons.

Could gravitational waves be similar? Just as:

• water waves are fundamentally composed of individual particles (molecules) moving within a medium (the water),
• light waves are fundamentally composed of individual particles (photons) propagating throughout space,
• could gravitational waves be fundamentally composed of individual particles (gravitons) propagating through the fabric of spacetime itself?

Perhaps. We know that gravitational waves carry real, finite, measurable amounts of energy, and we even know how to deposit a tiny amount of that energy into laboratory detectors. We know that gravitational waves propagate at the speed of light: consistent with the speed at which all massless quanta (including photons and gluons) must travel. We know that gravitational waves ought to interfere with any other ripples in space both constructively and destructively, obeying the rules that any other physical wave obeys. And we’ve observed, from the gravitational waves detected by LIGO and similar detectors, that their wavelengths stretch along with the expansion of the Universe, just as photons traveling through the expanding Universe exhibit a redshift.

However, all of these properties would still hold true whether gravitational waves were fundamentally wave-like and purely classical, as they are in Einstein’s theory, or whether they’re fundamentally quantum in nature, as they would be in a quantum theory of gravity where they were fundamentally composed of gravitons.

Beginning with a low-power laser pulse, you can stretch it, reducing its power, then amplify it, without destroying your amplifier, and then compress it again, creating a higher-power, shorter-period pulse than would otherwise be possible. With a fast enough, precise enough set of lasers working across an array of gravitational wave detectors, we could detect departures from general relativity during the merger of two black hole singularities.

Gravitational waves, however, because of the tensor-like nature of general relativity (as opposed to the vector-like nature of electromagnetic and the nuclear forces, or the scalar-like nature of the Higgs), are a little different in detail than the other waves we’re used to: they’re not scalar waves like water waves, nor are they even vector waves like light, where you have in-phase, oscillating electric and magnetic fields. Instead, these must be tensor waves, which causes space to contract and rarify in mutually perpendicular directions as the wave passes through that area. That means, if there’s a quantum analogue (gravitons) description of them, they cannot be scalar (with spin=0) or vector (with spin=±1) in nature, but must be tensorial (with spin=±2) instead.

However, if we want to demonstrate that gravity is fundamentally quantum in nature — which is what’s required to demonstrate the real existence of gravitons — we have to look for indications of some effect that goes beyond what Einstein predicts. Example questions include:

• Are there departures from purely Einsteinian predictions when two black hole singularities merge? (Perhaps a sensitive enough array of gravitational wave detectors could tell.)
• What happens to the gravitational field of an electron as it passes through a double slit? (Perhaps a sensitive enough force probe could tell.)
• Are there, and can we detect, fundamental B-mode polarization arising from gravitational wave production during inflation? (This would show that gravity is quantum in nature, but wouldn’t directly demonstrate the existence of gravitons.)
• And is there some way we can show that the energy levels of a quantum system are dependent on the system’s gravitational self-energy? (There are schemes to do this that, so far, have not delivered positive results.)

These would be steps along the path to demonstrating that gravity is quantum in nature, and hence, steps toward demonstrating the physical reality of gravitons.

The energy levels of a nanogram-scale disk of osmium, and how the effect of self-gravitation will (right) or won’t (left) affect the specific values of those energy levels. The disk’s wavefunction, and how it’s affected by gravitation, may lead to the first experimental test of whether gravity is truly a quantum force, and whether it obeys predictions that go beyond Einstein’s general relativity.

All of these are still a long way off, however, at least as far as current technology is concerned. The most “quantum” thing we’ve ever discovered about gravity, at least as far as I’m aware of it, is that the Aharanov-Bohm effect appears not only in the case of an electromagnetic field, but in a gravitational field as well. It shows that a phase shift for atoms can be induced by a gravitational potential alone: showing that it isn’t just the gravitational force or field that’s real, but that the gravitational potential itself has measurable, quantifiable effects on the quantum mechanical properties of a system. But that doesn’t prove that gravity itself is quantum; only that quantum mechanical effects are altered by a gravitational potential. It says nothing about whether that potential is quantum or classical in nature.

We’ve come remarkably far in our quest to understand the nature of the Universe, and we’ve thus far demonstrated that every quantum force, particle, and interaction predicted by the Standard Model is, in fact, borne out experimentally in our reality. The same is true for Einstein’s general relativity: wherever we’ve been able to test the predictions of the theory against alternatives, it’s emerged victorious. As to the question of whether gravity is truly quantum in nature, however — which is the real question we need to be asking concerning the existence or non-existence of gravitons — we still have neither confirmation nor refutation of the idea. Most of us, professionally, strongly suspect that gravity will turn out to be fundamentally quantum, and that gravitons will exist. Until some sort of experimental confirmation comes our way, though, we have no choice but to be honest, and declare, “We haven’t proven it, yet.”

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