Researchers are building models of everything from black holes to the big bang in tanks of liquid. Now some claim these surprisingly simple models are showing us where our theories of space-time are wrong.
GERMAIN ROUSSEAUX owns what looks like a very long and very narrow fish tank, minus the fish. At the bottom, in the middle, is a plastic ramp. When he switches on the apparatus, waves sweep along the tank and pass over the ramp, speeding up as they do so. This, he says, is a black hole.
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| A space-time analogue made of liquid at the Gravity Laboratory at the University of Nottingham, UK Leonardo Solidoro/Gravity Laboratory |
Well, not a black hole in the common sense. Not a star-gobbling pit in the fabric of space-time. Rousseaux’s experiment at the Institut Pprime in Poitiers, France, is a physical model of how the immense gravity of black holes can suck in waves – conventionally light waves, but in this case water waves – so they can’t escape.
It is what is known in the trade as a “gravity analogue”, and it is far from the only one. Over the past 15 years, researchers have created dozens of these tabletop models – despite the mutterings of many theorists, who are sceptical that such simple experiments can tell us anything about the universe’s most darkly mysterious objects.
Yet some researchers have begun to simulate more and more aspects of the universe, including even the entire infant cosmos. Now, some of them believe the models are giving us insights into the deepest nature of reality. There is even a suggestion that the speed of light, that hallowed constant of physics, might not be fixed after all. “Applying insights from these models would imply a radical shift in view,” says Rousseaux. But can we really rely on tanks of liquid to solve the mysteries of how the universe works?
One thing is for certain: there are many such mysteries to deal with. We don’t fully understand how the universe began or why it looks the way it does today, with stars and galaxies strewn in every direction with an unlikely evenness. Then there are those two enormous unknowns, dark matter and dark energy. No one knows what these entities are, but the former seems to make up the bulk of the universe’s matter, while the other appears to be driving it to expand at an increasing pace.
General relativity
All these issues are to do with the nature of space and time, and we have Albert Einstein to thank for our modern take on those. In 1905, his special theory of relativity established that the speed of light in a vacuum is a constant, implying that space and time – the way we measure speed – must be flexible, depending on who is watching. Then, in 1915, he went a step further with general relativity, equating the fabric of space-time with gravity. By default, space-time is flat, in the sense that anything passing through it travels undisturbed in a straight line. Introduce matter, though, and space-time warps and things begin to drift down its slope. This, said Einstein, is what we perceive as gravity.
Normally, gravity is a weak force. But pile more and more matter into a small volume and space-time can be bent into the bottomless chasm that is a black hole. Anything that strays too close, past a threshold known as the event horizon, will fall in and never get out. We know that black holes are common, littering our galaxy by the million. Yet they remain poorly understood.
One of the strangest predictions about them came from physicist Stephen Hawking. Back in the 1970s, he was thinking about black holes and the empty vacuum of outer space in the context of quantum mechanics. This theory treats a vacuum as a froth of low-level quantum fields: not quite nothing, not quite something. Hawking showed that a black hole makes this picture even weirder. It compresses the vacuum quantum fields, making them strong enough to manifest as proper matter and radiation. Inside the event horizon, the negative component of the field, which is associated with antiparticles, predominates; outside, the positive component, made of particles, can radiate away. In short, Hawking predicted that black holes aren’t totally black: they glow.
Hawking radiation
Ever since, theorists have been beguiled by this hypothetical glow, called Hawking radiation. No one has ever detected it from a real black hole and it is likely that no one ever will, because it is predicted to be so incredibly faint.
Still, could there be a way to find evidence of it elsewhere? In 1981, physicist Bill Unruh at the University of British Columbia in Vancouver, Canada, came up with the idea of making a laboratory model of a black hole to see if it might produce an analogue of Hawking radiation. After all, space-time is a bit like a fluid and quantum fields are like waves, so it ought to be possible to create waves in a liquid that are analogous to Hawking radiation.
Unruh’s proposal inspired many physicists to make such models, including Rousseaux. Start with a fluid, which represents space-time – for Rousseaux, this is water pumped steadily along his tank. Then modify the flow by, for example, inserting an obstacle. The effect is like warping space-time. If the change in current is strong enough, one side of the obstacle acts as a black hole, so that water waves travelling towards it can’t travel fast enough to rebound backwards again. Meanwhile, on the other side, waves can’t get past the obstacle in the first place. Here, the obstacle acts like the theoretical opposite of a black hole, a white hole, into which nothing can enter.
Except, that is, for the gaze of the experimenter – and herein lies an important distinction between analogue black holes and the real thing. Analogue black holes have horizons – points of no return for water waves and other things whose maximum speed is low – but not event horizons, which are impassable for absolutely everything, including the fastest thing of all, light. “Like God,” says Rousseaux, “we can observe the inside and the outside.”
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| Jeff Steinhauer’s black hole analogue involves supercooled rubidium Prof. Jeff Steinhauer |
When Rousseaux and his colleagues created one of their first analogues in 2008, they did see a hint of something akin to Hawking radiation. They focused on the white hole horizon because, for technical reasons, it is easier to study water that is slowing down than speeding up. When those waves approached the white hole’s horizon, they didn’t crawl to a stop, as you would expect. Instead, some bounced back, inverted. In these reflections, the propagating crests and troughs had switched places; mathematically speaking, their relative frequency seemed to turn from positive to negative. As Hawking’s equations predicted, the outside of the white hole’s horizon had generated a negative field – in this case, one made of water waves.
Rousseaux did worry that what he saw was a false signal, a type of harmonic wave that big enough waves can generate by themselves. But within a year or so, several other groups were working on analogue black and white holes. By 2016, Rousseaux had built a refined set-up that generated waves just a couple of millimetres tall, small enough to avoid harmonics, and had seen the same Hawking radiation-like effects.
Meanwhile, Ulf Leonhardt, an early collaborator of Rousseaux’s who is now at the Weizmann Institute of Science in Rehovot, Israel, had been pioneering space-time analogues in a seemingly very different medium: optical fibres. Here, space-time is represented by light travelling through the fibres, and of course light is composed of electromagnetic waves. The “obstacle” in this case is a separate laser pulse that warps the properties of the fibres, making the light slow. In 2019, Leonhardt’s team used this approach to create an optical white hole horizon clearly flanked by positive and negative light fields.
Not far away, at the Technion – Israel Institute of Technology in Haifa, Jeff Steinhauer has developed yet another approach. He takes a microscopic drop of rubidium and cools it to almost absolute zero, whereupon it becomes a superfluid that has strange quantum properties. At such low temperatures, vibrations almost completely cease, making observations much easier. The blast of a laser punctures the drop like a black hole; the atoms fall so fast that they exceed the speed of sound, emitting positive and negative waves in either direction. In 2016, Steinhauer patiently recorded the correlations between these waves, until he convinced himself they were just like Hawking radiation.
These days, the field of gravity analogues is highly competitive and the nuances of what the experiments really show are often questioned by rival researchers. What everyone in the field agrees on, however, is that Hawking radiation is far more general than Hawking himself realised. In any medium that can host waves and have them pass through a horizon, you should see new waves, positive and negative, emitted from either side of the horizon. “Hawking radiation has the same derivation in any system – it’s universal,” says David Bermudez, a theorist at the Center for Research and Advanced Studies of the National Polytechnic Institute in Mexico City.
Buoyed by this, researchers are now being bolder and making analogues of other extreme aspects of space-time. Markus Oberthaler at Heidelberg University in Germany, for example, has been looking at how the universe began. Modern cosmology predicts that, in its first moments, the universe saw a faster-than-light expansion known as inflation. This would have stretched out primordial quantum fields so that their tiny fluctuations seeded the humongous structures, such as galaxies, that we see today.
To model this, Oberthaler and his team took a tiny puddle of potassium superfluid and used magnetic fields to tweak the speed with which waves rippled outwards from the centre. By tweaking them just so, the waves slowed until they never reached the outside at all – a curious effect that mimicked what happened during inflation. Oberthaler’s experiment doesn’t prove inflation really happened, of course. But Silke Weinfurtner at the University of Nottingham in the UK calls analogues such as these a “first glimpse” of how lab experiments could provide insights into the nature of the very early universe, which, until now, has been the domain of theory alone.
Weinfurtner is already exploring further. In her “gravity laboratory”, she is surrounded by futuristic glass cauldrons of liquids, illuminated orange and green by lasers that measure the waves within them. In one experiment, she is looking at another prediction of modern cosmology. After inflation, primordial quantum fields are thought to have fed off one another to create “spikes” that manifested as real matter. Last year, Weinfurtner and her colleagues mounted a vessel on a shaking platform to simulate this cosmic period and found that the waves interacted with one another to the same degree as predicted by theory.
For Weinfurtner, the question isn’t whether universe analogues correctly mimic the real universe in all respects, but whether they can reveal nuances beyond the scope of theory. After all, theories tend to reduce the complexity of the universe to a fixed number of parameters. There is a chance that analogue experiments will give physicists a better idea of what to look for in the real, messy world to confirm their hypotheses. “That the equations governing our analogues are not quite the same as the real thing disturbs a lot of people, but actually it makes it really interesting,” she says. “Can we simulate beyond what we can calculate?”
One area where this might play out connects back to Hawking radiation. It is predicted to appear to us as faint radio waves, but that is after it has undergone stretching to escape the black hole’s gravity. When first emitted, it must have had an infinitesimally small wavelength. But the rules of quantum mechanics don’t permit such boundless reduction. There is no accepted solution to this paradox.
In analogue black holes, there are none of these impossible wavelength shifts to fret over. In the vacuum of space, light is supposed to always travel at the same speed. But in nearly all materials – including the water in Rousseaux’s tanks – the speed of light depends on its wavelength. In practice, then, a wavelength can never be zero because the speed of the ray would change to prevent that.
This could be seen as a deficiency of analogue gravities. Or, says Leonhardt, it could be that the theory describing real space is what is at fault. Maybe the speed of light in a vacuum isn’t always a constant. Maybe, at the smallest quantum scales, it changes – keeping the wavelength of astrophysical Hawking radiation within quantum bounds.
Is there a risk of reading too much into these analogues? Perhaps. This isn’t the first time, of course, that researchers have used models of some sort to study things that are too small, too big or too complex to study directly. But these are typically computer simulations or scaled-down physical models. The difference with analogue gravity experiments is that the model is physical, but made of something rather different to the thing being studied. Still, in models of all kinds, there is always an assumption that the correspondence between the model and the real thing is valid – which makes any conclusions provisional.
Since no one has ever directly observed a real black hole, ultimately we can never be sure that analogue black holes are a good guide to the real thing. But for Leonhardt, the fact that gravitational phenomena can be seen in fluids, optical fibres and other media is no accident. Maybe the reverse is also true: maybe space-time behaves more like an everyday material than we usually care to think.
One phenomenon that occurs in everyday materials is the Casimir effect, a subtle force that exists between objects when they are separated by exceptionally small distances. Named after physicist Hendrik Casimir, who first predicted it in the late 1940s, it arises because objects in close proximity limit the ways in which quantum fields can fluctuate between them; as a result, they are pushed together.
In January this year, Leonhardt argued that the same theory that describes the Casimir effect in materials can be applied to the space-time of the universe as a whole. It was a leap of faith. But, in doing so, he has calculated that quantum fluctuations can drive the expansion of the universe at just the rate we observe as a result of dark energy. “It was absolutely thrilling to see that the correct order of magnitude came out naturally,” he says. Arguably, it shows how analogue experiments can provide new ideas.
For decades, many cosmologists have placed their hope in ever more abstract theories and colossal experiments. The idea that progress could instead come from humble apparatus like tanks of water is, for many, a stretch. But, as Einstein showed, sometimes progress requires abandoning deeply held convictions. “As scientists, we have a choice,” says Rousseaux. “To be revolutionary – or not.”