Light can travel backward in time (sort of)

Reversing time would seem to fundamentally clash with one of the most sacred tenets of physics, the second law of thermodynamics. Yet, in certain carefully controlled scenarios within relatively simple systems, researchers have managed to turn back time.

Can we turn back time? Ask a savvy physicist, and the answer will be “it depends.”

Schemes for retrograde time travel abound but usually involve irreconcilable paradoxes and rely on outlandish theoretical constructs such as wormholes (which may not actually exist). Yet when it comes to simply turning back the clock—akin to stirring a scrambled raw egg and seeing the yolk and white reseparate—a rich and growing subfield of wave physics shows that such “time reversal” is possible.

Reversing time would seem to fundamentally clash with one of the most sacred tenets of physics, the second law of thermodynamics, which essentially states that disorder—more specifically “entropy”—is always increasing, as humbly demonstrated in the incessant work needed to keep things tidy. This inexorable slide toward mess and decay is what tends to make unscrambling eggs impossibly difficult—and what propels time’s arrow on a one-way trip through our day-to-day experiences. And although so far there’s no way to unscramble an egg, in certain carefully controlled scenarios within relatively simple systems, researchers have managed to turn back time.

The trick is to create a certain kind of reflection. First, imagine a regular spatial reflection, like one you see in a silver-backed glass mirror. Here reflection occurs because for a ray of light, silver is a very different transmission medium than air; the sudden change in optical properties causes the light to bounce back, like a Ping-Pong ball hitting a wall. Now imagine that instead of changing at particular points in space, the optical properties all along the ray’s path change sharply at a specific moment in time. Rather than recoiling in space, the light would recoil in time, precisely retracing its tracks, like the Ping-Pong ball returning to the player who last hit it. This is a “time reflection.”

Time reflections have fascinated theorists for decades but have proved devilishly tricky to pull off in practice because rapidly and sufficiently changing a material’s optical properties is no small task. Now, however, researchers at the City University of New York have demonstrated a breakthrough: the creation of light-based time reflections.

To do so, physicist Andrea Alù and his colleagues devised a “metamaterial” with adjustable optical properties that they could tweak within fractions of a nanosecond to halve or double how quickly light passes through. Metamaterials have properties determined by their structures; many are composed of arrays of microscopic rods or rings that can be tuned to interact with and manipulate light in ways that no natural material can. Bringing their power to bear on time reflections, Alù says, revealed some surprises. “Now we are realizing that [time reflections] can be much richer than we thought because of the way that we implement them,” he adds.

Such structural properties are also found in nature—for example, in the radiant iridescence of a butterfly’s wing. Picking up where nature left off, however, researchers studying metamaterials have engineered structures that can render objects invisible, and applications range from better antennas and earthquake protection to building light-based computers. Now scientists are trading in spatial dimensions of these structural features for temporal ones. “We design metamaterials to do unusual things, and this is one of those unusual things,” says Nader Engheta, a professor at the University of Pennsylvania and a pioneer in metamaterial-modulated wave physics.

Waves gone weird

The device Alù and his collaborators developed is essentially a waveguide that channels microwave-frequency light. A densely spaced array of switches along the waveguide connects it to capacitor circuits, which can dynamically add or remove material for the light to encounter. This can radically shift the waveguide’s effective properties, such as how easily it allows light to pass through. “We are not changing the material; we are adding or subtracting material,” Alù says. “That is why the process can be so fast.”

Time reflections come with a range of counterintuitive effects that have been theoretically predicted but never demonstrated with light. For instance, what is at the beginning of the original signal will be at the end of the reflected signal—a situation akin to looking at yourself in a mirror and seeing the back of your head. In addition, whereas a standard reflection alters how light traverses space, a time reflection alters light’s temporal components—that is, its frequencies. As a result, in a time-reflected view, the back of your head is also a different color. Alù and his colleagues observed both of these effects in the team’s device. Together they hold promise for fueling further advances in signal processing and communications—two domains that are vital for the function of, say, your smartphone, which relies on effects such as shifting frequencies.

Just a few months after developing the device, Alù and his colleagues observed more surprising behavior when they tried creating a time reflection in that waveguide while shooting two beams of light at each other inside it. Normally colliding beams of light behave as waves, producing interference patterns where their overlapping peaks and troughs add up or cancel out like ripples on water (in “constructive” or “destructive” interference, respectively). But light can, in fact, act as a pointlike projectile, a photon, as well as a wavelike oscillating field—that is, it has “wave-particle duality.” Generally a particular scenario will distinctly elicit just one behavior or the other, however. For instance, colliding beams of light don’t bounce off each other like billiard balls! But according to Alù and his team’s experiments, when a time reflection occurs, it seems that they do.

The researchers achieved this curious effect by controlling whether the colliding waves were interfering constructively or destructively—whether they were adding or subtracting from each other—when the time reflection occurred. By controlling the specific instant when the time reflection took place, the scientists demonstrated that the two waves bounce off each other with the same wave amplitudes that they started with, like colliding billiard balls. Alternatively they could end up with less energy, like recoiling spongy balls, or even gain energy, as would be the case for balls at either end of a stretched spring. “We can make these interactions energy-conserving, energy-supplying or energy-suppressing,” Alù says, highlighting how time reflections could provide a new control knob for applications that involve energy conversion and pulse shaping, in which the shape of a wave is changed to optimize a pulse’s signal.

Unscrambling the physics

Readers who are well versed in the laws of physics can be reassured that Alù’s device does not violate the tenets of thermodynamics. The waveguide does not, for instance, create or destroy energy but simply transforms it efficiently from one form to another—the energy gained or lost by the waves comes from that which is added or subtracted to change the metamaterial’s properties. But what about the inescapable increase of disorder—entropy—over time, as prescribed by thermodynamics? How is a light beam’s time reflection not the equivalent of unscrambling an egg?

As John Pendry, a metamaterial-focused physicist at Imperial College London, explains, however odd reversing a light beam may look, it’s wholly consistent with ironclad thermodynamic principles. The rise of entropy is really a matter of losing information, he says. For instance, line schoolchildren up in alphabetical order, and someone will know exactly where to find each child. But let them loose in the playground, and there’s a vast number of different ways the children could be arranged, which equates to an increase in entropy, and what information you had for locating each child is lost. “If [something is] time-reversible, it means you’re not generating entropy,” Pendry says, even if it looks like you are. Going back to the playground analogy, although the children still run off to play, they know what lines to form to return to class at the bell—so no entropy is generated. “You don’t lose the information,” he says.

Reflection is far from the only optical phenomenon to receive the time-domain treatment. In April Pendry and a team of researchers, including Riccardo Sapienza of Imperial College London, demonstrated a time-domain analogue of a classic experiment from centuries ago that ultimately played a key role in establishing light’s wave-particle duality. First performed by physicist Thomas Young in 1801, the “double-slit experiment” provided such irrefutable evidence of light’s wavelike nature that in the face of subsequent evidence for light acting as a particle, scientists could only conclude that both descriptions applied. Send a wave at a barrier with two slits, and waves fanning out from one slit will interfere with those emanating from the other. With light, this constructive and destructive interference shows up on a screen beyond the double slit as multiple bright stripes, or “fringes.” Sapienza, Pendry and their colleagues used indium tin oxide (ITO), a photoreactive substance that can rapidly change from transparent to opaque, to produce “time slits.” They showed that a beam of light interacting with double time slits would produce a corresponding interference pattern in frequency, which was used as a time analogue—that is, there were bright light fringes at different frequencies.

According to Engheta, what motivates experiments that swap time and space in optical effects are the “exciting and novel features we can find in the physics of light-matter interaction.” And there are plenty. Pendry describes with a chuckle how he and his colleagues’ temporal explorations with metamaterials have revealed “some very strange things,” including what he calls a “photonic compressor.” Pendry’s photonic compressor is a metamaterial that is striped with regions of different optical properties that affect the speed at which light propagates. The stripes are adjustable, forming a sort of “metagrating,” and when this metagrating moves through the metamaterial alongside light, it can act to trap and herd the photons together, effectively compressing them. Further investigation has also revealed that this kind of photonic compressor shares characteristics with black holes, potentially providing a more manageable lab-scale analogue for studying those extreme astronomical objects. Having unfurled a whole new time dimension to metamaterials, photon-compressing black hole analogues are just one avenue of curious phenomena to delve into, and the possibilities are legion.

“It’s really assembling a toolbox,” Pendry says, “and then showing this to the world and saying, ‘What can you do with it?’”