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SOME 384,400 kilometres from Earth, the moon glides silently through space. Actually, we can do better than that: we know exactly where the moon is now, and what path it will take around us, to within a millimetre. To be any more accurate, we would have to consider details of Earth鈥檚 mass distribution as subtle as the mass of leaves growing in the spring in one hemisphere as they wither in the other.

The laws of gravity govern the moon鈥檚 trajectory, and the precision with which we understand them is thanks to the equations of general relativity, finalised by Albert Einstein just over a century ago. Those same equations apply across the solar system and beyond. Our understanding of gravity is one of the most impressive achievements of our species.

But there is a problem. General relativity doesn鈥檛 align with quantum theory, our most successful attempt at understanding reality so far. And the universe is expanding at an accelerating pace that doesn鈥檛 tally with gravity鈥檚 attractive nature. Our existing picture may allow us to predict the motion of the moon, but when it comes to explaining the wider cosmos, we鈥檙e missing something.

My attempts to find out what have led me to ask whether gravity itself has a mass. Physicists have argued about this for decades. Now my colleagues and I have stumbled on an intriguing answer that may lead us to a clearer picture of gravity. If we are right, then this most enigmatic of forces not only does have a mass, but the evidence that proves it is painted across the cosmos.

One of the most remarkable things about gravity is that, in contrast to other forces like electromagnetism, its effects are universal. It was Galileo who first grasped this in about 1590. Whether he really dropped objects with different masses from the Leaning Tower of Pisa or not, he had the beautiful insight that Earth鈥檚 gravity is felt the same way by everyone and everything. Indeed, if it weren鈥檛 for air resistance, a hammer and a feather dropped simultaneously would hit the ground at the same time. We have where there is no air resistance and shown that Galileo was right.

A century later, this ubiquity was at the centre of Isaac Newton鈥檚 formulation of the laws of universal gravitation. It is essentially these laws that enable us to predict the motion of the moon and the planets with such incredible accuracy. Newton鈥檚 laws get the moon鈥檚 orbit right to within 10 centimetres, and corrections introduced by Einstein make it better still.

But there is one conceptual aspect behind Newton鈥檚 laws that is a source of deep concern. It is the instantaneous way in which Newton assumed that gravity acts. To see why it is so troubling, imagine that the sun disappears. According to Newton, we would feel the lack of the sun鈥檚 gravitational tug instantly, so that even as Earth careered off its usual orbit, we would see our now-vanished star shining benignly above us for 8 minutes and 20 seconds or so. The result of this thought experiment offends our understanding that nothing can travel faster than light, not even gravity.

This maxim originated with Einstein. He showed that Newton鈥檚 laws explain gravity only when the motion of objects is slow compared with that of light. He then formulated general relativity as a more complete picture of gravity. It remains our best effort to this day.

So how does gravity work? While Newton鈥檚 theory barely engaged with this question, Einstein gave us an answer that transformed how we think about reality. General relativity paints space and time as a unified entity called space-time that flexes and adapts to everything within it. The reason that objects move as they do in response to gravity can be explained by the bulges in this flexible canvas of reality.

If that is tricky to visualise, here is an analogy. If you have ever flown long-distance, you might have looked at the maps of your flight path that are sometimes displayed on the entertainment screens. It always looks like an arc on the map, not a straight line, say from east to west if you are crossing from Europe to the US. But that is an illusion. The pilot will fly as straight as an arrow; it is only because Earth鈥檚 surface itself is curved that the flight path looks that way too when displayed in two dimensions. General relativity explains the dance of the planets in a similar way. They seem to follow elliptical orbits, but are actually moving efficiently in straight lines through curved space-time.

This strikingly strange theory was the first of two 20th century revolutions in physics. The second, quantum theory, was, if anything, even more profound. Quantum theory deals with the smallest aspects of nature and shows us that this world is nothing like the one in which we spend our days. Down there, it doesn鈥檛 make sense to speak of particles occupying particular places, they exist only as a nebulous cloud of probabilities. It sounds odd, but we know this idea to be as right as anything in science can be.

鈥淭hese 鈥榞host鈥� particles would wreak havoc, quickly erasing all order in the universe鈥�

For that reason, it is worth taking seriously what quantum theory says about forces. At its heart are quantum fields that can form waves and propagate through space. It turns out that it is equally valid to think of these waves as particles. This applies to light itself. When physicists talk about light, they sometimes talk about waves in the electromagnetic field and at other times they talk about photons, the particle, or boson, that carries the electromagnetic force.

Quantum theory in fact says that each fundamental force has its quantum field and one or more bosons (see 鈥�Boson bonanza鈥�). It also says that the mass of a boson is inversely proportional to the range of the force. This is why light, carried by photons that we believe to be massless, has an infinite range. That鈥檚 why we can see stars on the other side of the universe. If a force has heftier bosons, its reach is more limited.

There isn鈥檛 yet a complete quantum theory of gravity, but we do have strong evidence that this force must ultimately fit into the quantum mould. That means space-time is a quantum field and that waves in it can also be thought of as a boson. We call this particle the graviton.

We don鈥檛 know for sure that gravitons exist, but all the signs point that way. Take the discovery of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration in 2015. This showed that the gravitational field can vibrate just like the electromagnetic field. As yet, these waves show no trace of quantum behaviour, so they aren鈥檛 direct evidence for gravitons. But they are a highly suggestive hint that they are out there.

We have some knowledge about what gravitons ought to be like. Gravity exerts its influence across cosmic scales just like light, so the graviton must be either massless or very light. But which is it? This seemingly innocent question matters because the answer will reveal how gravity behaves over the largest of distances and how fast gravitational waves move. If gravitons have no mass, then we know from the mathematics that they must travel at the speed of light. However, if they have mass, they can travel at different speeds.

Physicists have jousted over this question for decades, but the argument was silenced when we realised that there was a problem with the very idea of a massive graviton. It turns out that if gravitons have mass, they instantly acquire the ability to come in different varieties, each with different quantum properties. As physicists have tried to describe these various types of gravitons, they have found they couldn鈥檛 avoid one of them coming out as a 鈥済host鈥�: a particle with negative energy.

These ghosts would wreak havoc. There would be nothing to stop them interacting with regular particles in a chain reaction that would quickly erase all order in the universe. The fact that this hasn鈥檛 happened means ghosts can鈥檛 exist, and so gravitons can鈥檛 have mass.

Extra dimensions

That, at least, has long been the argument. Contradicting it was far from my intention when I teamed up with my husband , who works with me at Imperial College London, and at New York University. We were interested in an entirely different problem. The expansion of the universe is accelerating much slower than we anticipate when we combine predictions from quantum field theory with general relativity. Our aim was to explore whether the laws could differ at cosmological scales and so level the balance. There were two ways to do this: invoking extra dimensions or allowing the graviton to have mass. We thought that the extra dimensions route was distinctly more plausible.

We discovered something remarkable. When we played around with models of general relativity that included extra dimensions, it seemed possible to have massive gravitons without any ghosts. This wasn鈥檛 a model that could describe the real universe, but it showed that massive gravity wasn鈥檛 quite as impossible as we had always thought. And switching back to thinking about four-dimensional space, we found a way to describe massive gravitons in such a way that they didn鈥檛 inevitably produce those troublesome ghosts. It is the first consistent, rigorous theory of massive gravity.

If we are right, and the graviton has mass, it would change the way we think gravity operates. The effect would touch all sorts of processes, from the early evolution of the universe to the motion of the planets and the fate of the cosmos. Even the path of the moon would be affected: there would be a tiny extra attraction between it and Earth, changing the lunar orbit so that its position would be different by about a nanometre each month.

鈥淕ravitational rainbows ought to be spreading through the universe all the time鈥�

More profound would be the effect on the speed of gravitational waves. Take a collision between two black holes, one so epic that it creates detectable waves in space-time 鈥� the sort of thing LIGO first detected. If gravitons have mass, it means these waves don鈥檛 have to travel at the speed of light in a vacuum. Instead, you might have a situation similar to what happens when light is refracted by a raindrop, where the different frequencies or colours of light change speed by different amounts and so spread into a rainbow. 鈥淕ravitational rainbows鈥� wouldn鈥檛 be coloured, but if gravitons do have mass, they ought to be spreading through the universe all the time. None of the gravitational waves detected with the LIGO detectors have shown any signs of being part of a rainbow, but it isn鈥檛 out of the question that we might see one in future detectors.

I like to think of gravity as the force that connects us all. Every time we probe it more deeply, a new and unexpected layer of rich and fundamental structure is unveiled. Newton鈥檚 laws are beautiful in their simplicity. Einstein鈥檚 theory rewrote our assumptions about space and time. Now, the search for quantum gravity is leading us towards a new understanding of fundamental particles. In parallel, it is possible that we are getting a first glimpse of what the graviton is like. These advances don鈥檛 invalidate the older theories, of course 鈥� they provide a more fundamental description of nature.

Whether or not we are right about massive gravitons, physics is certainly entering a new era in which we understand gravity better than ever. Just compare what we now know about the seemingly obscure graviton with our knowledge of the photons that constantly stream into our eyes. Measurements of gravitational waves have already provided us with a more accurate limit on the mass of the graviton than that of the photon. Despite the graviton being so mysterious, this is one way in which we already know it better than light itself.

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