AS FAR as dead ideas go, the luminiferous aether is among the deadest. Over a century ago, it picked a fight with Einstein’s theory of relativity and lost. Few victories in modern physics have been so total. Today, relativity offers us our best picture of the large-scale structure of the universe. It is a byword for human achievement and scientific progress. The aether, if it gets mentioned at all, is an embarrassing footnote in its rise to glory.
But relativity has run into difficulties of its own. Its failure to explain the behaviour of the universe at the smallest scales suggests that some more fundamental theory is waiting to take its place. Einstein’s universe is also plagued by dark forces that his theory cannot cast out.
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In an astonishing twist of fate, the key to relativity’s salvation could lie in the aether. Since the early 2000s, a small group of researchers have claimed that this invisible, space-filling substance could have the power to unify physics. Then, in late 2018, two independent groups suggested that the similarity between the aether and the shadowy powers that populate our cosmos may not be mere coincidence. For one team, the aether is a dead ringer for dark matter. For another, it could explain away dark energy. For others still, it might even be both.
The hunt is now on to see if it really is out there. The biggest laughing stock in physics may yet have the last laugh.
The best way to conceive of the aether (or ether, as it is now more usually called) is as a sort of faint, universe-spanning jelly, through which all the stars, planets and galaxies slowly wade. This is not what makes it absurd. After all, vast invisible entities are widespread in contemporary physics – from the Higgs field that endows some fundamental particles with mass, to the mysterious pull of dark matter and dark energy that make up 95 per cent of the known universe.
There were also good reasons to believe in the aether. For centuries, one of the most hotly debated questions in physics concerned the true nature of light. Were beams of light made up of particles, firing one after the other like bullets out of a gun, or waves, lapping on each other’s heels like a rising tide? Today, we know that light is capable of existing in either form, but in the 17th century such a compromise seemed impossible. Only one school of thought could be right.
Many ardent believers in the wave picture, such as Dutch luminary Christiaan Huygens, found one particular aspect of light’s behaviour bothersome. “They thought it was like a sound wave,” says , a philosopher of physics at Northern Illinois University. Just as sound waves cannot propagate without a medium to travel through, light waves needed a medium too. And given that we see the stars at night, this medium must also be present in the apparent emptiness of outer space. This space-filling feature gave it another important role: it could set a standard frame of reference with respect to which all things moved. Huygens called this the luminiferous aether, a sort of invisible, all-pervading mist devoted to ensuring the smooth passage of light and defining absolute motion.
The aether rapidly became a mainstay of physical thought, even though no evidence for this mysterious medium ever emerged. As the 19th century drew to a close, though, there was less and less reason to believe it existed. First, the newly developed electromagnetic theory identified light waves as vibrations of the intensity of the electromagnetic field. This was a new kind of wave, unlike sound, that had the freedom to travel without the aid of a separate medium. That kicked away the aether’s strongest theoretical strut.
Second, experiments designed to directly measure the aether’s existence found no effect. The most famous, conducted by physicists Albert Michelson and Edward Morley in 1887, sought to see whether light travelling through the aether was in any way slowed down by it. It wasn’t. “People reacted in different ways,” says Allori. They invented all sorts of theoretical reasons why the aether could still be out there. Then Albert Einstein got involved.
Einstein’s theory of special relativity, published in 1905, dealt the aether its final blow. While the theory probably didn’t arise as a reaction to Michelson and Morley’s results, it was published at the ideal moment to capitalise on the concept’s dwindling popularity.
Relatively successful
The theory of relativity made a remarkable prediction about the world. It posited that the laws of physics should be the same to all observers who see themselves as being at rest, even if they are moving at a constant velocity relative to one another. On a gut level, this makes good sense. After all, our own constant motion relative to the ground, on a train or plane journey for instance, doesn’t interfere with our physical movements. It isn’t as if we need to compensate for the plane’s speed when walking down the aisle.
But there is a twist. If two observers can both claim to be perfectly still despite each appearing to be in motion to the other, the notion of anything ever being absolutely “at rest” is a fiction. That meant that the aether – a universe-spanning reference frame that all observers were supposed to see as at rest – would be incompatible with relativity.
From there, the aether rapidly disappeared from serious physics. Nowadays, it is often a symbol for discredited ideas that hang around long past their expiry date. That’s not really a fair judgement, says Allori: Einstein didn’t prove the aether couldn’t exist, he merely showed there was no need for it. His explanation was simpler for contemporary physicists to accept than the existence of the aether, she says. “All I have to do is change how I think about space and time.”
Within a decade, Einstein had enshrined the predictions of special relativity into his general theory of relativity, which remains the foundation of our modern theory of gravity and how we understand the workings of the universe on its largest scales. Yet there are a number of signs that general relativity isn’t the definitive picture of reality either. For one thing, it is hopeless at describing the universe at its smallest scales. Instead, physicists turn to quantum theory to deal with particles and their interactions. These two pictures of reality work brilliantly in their own domains, but have so far resisted being combined into a theory of everything.
One theory to rule them all
Many attempts, including string theory and loop quantum gravity, have been made over the past century to combine relativity with the quantum world. None has so far succeeded. One difficulty comes from the two theories having such differing world views. In general relativity, the fabric of space-time has to be continuous. From the perspective of quantum theory, however, many fundamental quantities need to be discrete. Energy, for example, has to come in small bundles rather than in a constant flow. The same goes for electric charge, momentum and a host of other properties. Including, potentially, the fabric of space-time itself.
For . “Ever since I learned about quantum mechanics and relativity, I thought that somehow space and time should be quantised,” he says. The concept of such atoms of space-time, tiny unsplittable pixels of reality, seems unobjectionable enough. Einstein’s relativity, however, posits that the size of a measured object changes depending on the observer. A shortest unit of space sounds sensible in practice, says Jacobson, “but which frame of reference is it shortest in?”
The trouble didn’t end there. In the 1990s, strong hints emerged that string theory and loop quantum gravity, when completed, would feature a preferred frame of reference too. These results led Jacobson and other researchers to ask a forbidden question: what if the aether – or something like it – weren’t dead, but merely slumbering?
In 2000, Jacobson and his colleague David Mattingly , now known as Einstein-aether theory, that allowed an aether-like frame of reference to exist alongside general relativity. It rapidly became a widely used tool, says . Their flexible framework let researchers predict the aether’s impact on the ways in which gravity shows itself, such as the rate at which the universe is expanding.
Checking the predictions against real data didn’t immediately turn up signs of the aether, but Jacobson was unsurprised. At least for tests in the model’s early days, he says, “I always expected that there would be no evidence”. But absence of evidence is not evidence of absence. If experiments left reasonable room for an aether to be hiding within the margins of error, then it would be worth looking closer.
So far, no identifiable departures from relativity have appeared in any actual data, and new experiments are tightening up those margins all the time. When the first gravitational waves were detected in 2015, these vibrations in the fabric of space-time were found to travel at very close to the speed of light. This matches Einstein’s 1915 predictions, making the broader predictions made by Einstein-aether theory seem, once again, like unnecessary additions. If such a substance is there at all, according to the data, it must be so well hidden that it might as well not exist.
At this point, 20 years on, Jacobson is a little pessimistic about the aether’s prospects. But cosmologist . “The fundamental reasons to look at aether are still valid,” he says. He thinks we have been looking for it in the wrong kinds of experiments. So far, most work has tried to uncover the aether’s effect by zooming in to the mooted atoms of space-time, or else by studying higher energies and faster speeds in the hopes that relativity breaks down.
Afshordi believes we need to reverse our approach, looking for anomalies at lower energies and across larger distances. It may be that zooming out will allow us to see the bigger picture. In the context of the aether, that means using experiments that look at interactions with radiation from the moments after the big bang, or with gravitational waves of even lower frequency than those glimpsed so far.
Even if Afshordi is wrong and no such evidence turns up, some physicists think we already have a good enough reason to turn to the aether.
According to the best data we have, 95 per cent of the universe is made of invisible substances whose identity remains unknown. General relativity can describe their behaviour, but offers no answers as to what they might be. With a mystery of that size awaiting resolution, no theoretical model is off the table. But how do we even know that these dark substances are out there?
“The aether could be the very thing we need to help make sense of the universe”
If we observe the way visible matter moves about inside clusters of galaxies, gravity tells us there must be extra, but unseen, dark matter lurking within them. And if we look at the rate of the universe’s expansion, gravity says that something mysterious, dubbed dark energy, is causing that expansion to accelerate. We have lots of models for what they could be, we just lack any conclusive answers.
One tantalising possibility is that the aether could fill in the gaps. If true, we might be able to connect the two biggest challenges to Einstein’s general relativity – quantum gravity and these dark influences – via the very thing that Einstein’s special relativity banished.
Tom Złosnik, a cosmologist at the Czech Academy of Sciences in Prague, is one of those looking to achieve that goal. Like Jacobson, his original intention was to paint a unified picture of quantum gravity that incorporated an aether-like field. But as he and his colleagues got stuck into the mathematics, they made a remarkable discovery. The aether that fit best into their model was one that matched the demands cosmologists made of dark matter. “The result,” says Złosnik, “was general relativity with a dark matter dust.”
They . At about the same time, Richard Battye at the Jodrell Bank Centre for Astrophysics in Manchester, UK, published a paper suggesting the aether could explain dark energy as well. He and his team followed in Złosnik’s footsteps, using an expanded form of Jacobson and Mattingly’s Einstein-aether theory to see how such models tallied with cosmological data.
They found that there is hope for the aether here, but no definitive evidence. “You can construct something that works,” says Battye, “but they are not ruled in.” The problem is that although some Einstein-aether models match the raw data, we don’t yet know how to distinguish them from any other suggestion for dark energy, he says. “It doesn’t come up with a unique prediction: something this predicts that nothing else predicts,” says Battye. “At least not yet.”
Heart of darkness
The most exciting outcome would be for the aether to explain both dark matter and dark energy in one fell swoop. Such a mathematical tour de force remains far off, but .
Whether the aether actually does make up dark matter, dark energy or both, the dark sector may be the best place to look for clues, says Blas. “It opens a window of detection to the aether.” We could check any experiment that probes the properties of dark matter to look for signs of a preferred frame, he says.
If anything does turn up, it would be an irony of truly cosmic proportions. More than a century after its banishment from the realm of respectable science, the aether could be the very thing we need to help make sense of the universe. In the graveyard of failed ideas, something ethereal is stirring.
