For an experiment designed to reverse time, the apparatus was surprisingly simple: little more than a tank of water. With a puff of air to disturb the surface, Emmanuel Fort created a set of ripples moving outwards in concentric circles. Then, as the waves spread, he gave the tank a practised jolt – at which point they suddenly started travelling inward, refocusing at the point of origin.
Fort’s work in 2016 was the first of what has become a deluge of experiments in which waves are manipulated, controlled and even reversed with unprecedented precision. And these days, we are no longer just playing with water. Researchers have figured out how to create a range of “temporal metamaterials” that can manipulate and rewind electromagnetic waves, including visible light itself, with ever greater precision.
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The key to these bizarre materials isn’t so much the careful engineering of their spatial arrangement of atoms, but how they work in the fourth dimension, time. Like the rapid jolting of the water tank, temporal metamaterials can radically change their properties in the blink of an eye. This creates a kind of boundary that acts like a mirror in time – and it is this that exerts such subtle power over waves.
Not only are these materials a mind-bending invention, they have also become an unexpectedly rich playground in which physicists can test a raft of fundamental ideas at their limits. What happens when you send quantum particles through a slit in time? Does friction really exist in the quantum world? And can we execute an extreme test of special relativity? We are about to find out.
Though we may not give it much thought, we know that materials can manipulate waves. Mirrors reflect waves of light, while lenses can focus them. A straw in a glass of water looks bent, for instance, because the liquid deflects light, distorting the image. All of which has been understood since the 1600s, when Isaac Newton famously used a prism to separate sunlight into a rainbow. But few have considered what happens when the light-bending properties of a material are modified over time.
It began with metamaterials
The story of how that changed begins with metamaterials – that is, materials engineered to have properties not found in nature. The most famous example is the “invisibility cloak” developed in 2006 principally by at Imperial College London. The original version bent microwaves around an object so their trajectories behave as if it isn’t there, and the concept spawned a raft of other metamaterials, including acoustic cloaks made of an ultra-thin sheet that can suppress the loudest bang. These are now being .
These inventions rely on nanoscale patterns etched into the material’s surface. But a temporal metamaterial would instead depend on patterns in time. To be precise, that means changing its properties so fast that it appears to be instantaneous – creating something scientists call a temporal boundary.
This idea first occurred to electrical engineers studying the general properties of waves in the aftermath of the second world war, in which radio waves and radar had become a major research interest. In 1948, Dennis Gabor hypothesised that temporal boundaries could reflect and reverse waves. It went no further at the time. But fast-forward to 2016, and Fort, who is based at the Langevin Institute in Paris, had the idea of testing the hypothesis with water waves, which can be seen with the naked eye.
He placed a water tank on a platform that could be moved quickly. “You give the bath a jolt with a shaker, you just give it a kick,” he says. For a few milliseconds, the acceleration the water experiences changes, creating a temporal boundary. When the waves hit that boundary, they split into two: one part propagates forwards, the other is reflected backwards. The jolt effectively freezes, then releases, the wave. And when it is loosed, it has “forgotten” which way it was travelling, and so goes in both possible directions.
It looks eerily like a video playing backwards. And it was even more uncanny when Fort created complex images in the ripples. He used moulds and stencils to create ripples in the shape of a smiley face and the Eiffel tower (pictured, above). These too would disperse and then reform, as if by magic.
Since , researchers have taken the idea and run with it, creating engineered temporal metamaterials that can manipulate acoustic and electromagnetic waves. One example, based on indium tin oxide, can almost instantly switch from being transparent to reflective.
These materials offer a completely new way of controlling waves of all types, and industry is clearly excited about where this could lead. In everything from microwave ovens to communications cables, the prospect of being able to more keenly control waves could be extremely handy. A research consortium called Meta4D was recently – and companies such as telecoms giant BT are on board as partners.
Beyond the practical applications, these materials are opening up new ways to test ideas in physics that were previously out of reach. For instance, at Imperial College London was interested in putting a new spin on the famous double-slit experiment first performed by Thomas Young in 1801. In the classic experiment, light was shone through two thin slits. The waves passed through each opening and spread out in a process called diffraction, before interfering with each other and creating a pattern of light and dark stripes on a screen. The experiment was taken as evidence that light can behave like a wave.
The double slit experiment, remixed
Sapienza and his team recreated this experiment with slits not in space, but in time. They created that indium tin oxide material, which can change from being transparent to reflective in millionths of a billionth of a second when hit with a laser. In 2023, the researchers shot two laser pulses at the material, creating two short windows during which light could pass through. They saw an interference pattern, but instead of changing the brightness of light, . No one had tried this before.
Changing the frequency of waves is crucial in all sorts of technology, but really challenging. For instance, radar signals are most efficiently sent at higher frequencies, but they need to be converted to lower frequencies when received by antennas. We currently use several pieces of technology for this; temporal metamaterials could offer a simpler alternative. “What really excites us is that we have a way of changing the way we think about how light interacts with a material,” says Sapienza.
As well as remixing light, temporal metamaterials could help us test how it behaves when slowed to a dawdling pace, and so put Albert Einstein’s special theory of relativity under the closest scrutiny yet.
You may have heard that the speed of light is constant, which is true, but only in a vacuum. Light’s velocity depends on the medium it is travelling through, specifically the material’s refractive index, which describes its ability to slow (and bend) light rays. Light travels at around 300,000 kilometres per second in a vacuum, but only about 225,000 kilometres per second in water. The slowest recorded speed of light was clocked by researchers shining it through an ultracold gas in 1999 – , about the speed of a car pulling onto a motorway.
But what happens when light shines through a moving medium? Augustin-Jean Fresnel wrestled with this question in 1818. He predicted that if light were passed through a material moving in the opposite direction, it would experience drag and slow down – think of it like a plane being pushed back by headwind. And he was right. Three decades later, the French physicist Hippolyte Fizeau split the sun’s rays into two beams that he sent through streams of water flowing in opposite directions. The two beams were then recombined and the resulting interference pattern revealed that the light travelling against the current had indeed slackened in speed, albeit by a tiny amount.
Einstein later showed that you could explain this “Fresnel drag” using the framework of special relativity. When the water moves with the light, the light’s energy is added to the motion of the water, making the light appear to move faster. Conversely, if the water moves against the direction of the light, it subtracts from the light’s motion, making it appear slower. But here’s the thing: we have only ever tested this at fairly slow flow speeds – what if the light were travelling through a medium that was itself moving at close to the speed of light?
In 2019, Paloma Huidobro, then working with Pendry at Imperial College London, outlined . To get your head around this, think of a metamaterial’s surface like a crowd doing a Mexican wave. The individual people are only moving up and down, but the collective action produces the illusion of sideways motion as the wave sweeps along.
Testing special relativity
In a temporal metamaterial, the things moving up and down at each point are properties such as permittivity, which determines how a material affects electric fields, and permeability, or how the material affects magnetic fields. As these properties vary, they create patterns that can ripple through the material’s surface at extreme speeds. If a ray of light is then shone through the material, these ripples push back at approaching the speed of light. “Almost nothing can catch a light beam, the fastest object in physics, but there are ways of sneaking up on it,” says Pendry. Sapienza says he has been working on making this “synthetic motion” work practically in the lab, and he is preparing to publish his results.
The applications of temporal metamaterials get even more far out. One of the strangest predictions of quantum mechanics is that a vacuum – that is, empty space – is never totally empty. Instead, it buzzes with “virtual particles” that flit in and out of existence. These are so short-lived that they are generally impossible to measure – but they do have effects.
In 1948, the Dutch physicist Hendrik Casimir imagined placing two flat surfaces close together in a vacuum. Those two plates, he thought, should be ever-so-slightly attracted to each other, even if they aren’t electrically charged. This is because virtual particles are constrained from appearing and disappearing freely between the plates due to the limited space, creating a situation where there are more virtual particles outside the plates than between them. This results in a pressure between the plates, known as the Casimir effect.
Quantum friction
The Casimir effect is real. But Pendry goes further. Years ago, he proposed that the pressure on the plates should , even though they aren’t touching – quantum friction. However, the idea has long been deeply controversial. at the Weizmann Institute of Science in Israel, for one, gives it short shrift. He claims that, if it were true, it would be possible to extract limitless useful energy from the chaos of those virtual particles within the vacuum.
So, quantum friction: fact or fiction? Metamaterials present a way to settle the debate, says M´rio Silveirinha at the University of Lisbon in Portugal. Quantum friction should result in the emission of particles of light, but these photons would only be produced in detectable amounts if the plates were “rubbed” at incredible speeds. So Silveirinha’s plan is, again, to employ synthetic motion. Imagine two metamaterials next to each other in a vacuum, with ripples running along them at a sizzling pace. It would be as if the plates were moving with respect to each other. If photons are given off, that would be strong evidence in favour of Pendry’s hypothesis.
“This is a very promising way to test quantum friction,” says Silveirinha, who is now working with experimentalist colleagues to try it. He points out that this experiment isn’t just to settle an academic curiosity. If quantum friction exists, it has powerful applications: he imagines a laser powered by synthetic motion. The photons that would be created by quantum friction would usually just be reabsorbed back into the vacuum whence they came. However, if the relative velocity between our two metamaterial surfaces is high enough, we could create so many that they would splurge outwards. These photons would be emitted at frequencies in the terahertz range, says Silveirinha, which straddles the point between infrared radiation and microwaves. Lasers that operate in this region are currently extremely difficult to make.
For the moment, though, many of the ideas around time-varying metamaterials are in their infancy. While physicists have developed a suite of analytical tools to help them understand wave behaviours in different spatial configurations, time presents a fresh challenge. Theorists are rushing to meet this, struggling to overcome mental blocks concerning time boundaries. “Time is a fundamentally different dimension to space because of causality,” says theorist at the University of Exeter, UK, who is among those grappling with these issues. “You can’t have processes travelling backwards in time, so this is the headache of understanding a lot of the effects in these materials.”
Even in Sapienza’s “slits in time” experiment, there are questions over how to interpret what is going on. The material that his team used was able to change its properties much faster than expected, and the researchers don’t fully understand why. “It’s not so intuitive, I think,” says Sapienza. “If I tell you light interacts with a material in space, you can see with your eyes that it goes in different directions. But when I tell you that light interacts with a blip in time, it’s very hard to have an intuition.”
With all that in mind, we are in for an exciting period in the world of temporal metamaterials. Fort says that the response to his 2016 experiment with the water tank was huge. Even though the idea of reversing waves had been around for decades, seeing the water ripples rewind was profound and beautiful, he says. “Even theoreticians see it and go: ‘Wow. It’s true, it’s all really true.'”
Jacklin Kwan is a science writer based in London
