One might think that after centuries of studying light, we know just about everything about it. It’s true that we’ve had breakthrough after breakthrough in using it, from enlightenment to communication, from examining the micro and macro universes to scanning our own bodies. We understand that light is an electromagnetic wave, thanks to James Clerk Maxwell, whose equations established this in 1865; and which also appears as quantum packets of electromagnetic energy called photons, as Albert Einstein recognized in 1905. But the further we look into the light, the more we see, and the more we learn. The classical view of light as a wave still yields new science as light waves interact with man-made metamaterials; and we are still exploring light as a quantum particle. Both approaches provide ways to manipulate light that were once just science fiction. Here are five recent wonders.
1. Curving light for invisibility
The magical rings and invisibility cloaks featured in fantasy stories reflect the ancient human dream of hiding things and people from sight. Invisibility also appears in science fiction, like Star Trek, where hostile Romulan starships lurk with a cloaking device. This uses an idea from relativity, that heavily distorted spacetime causes light to curve around the spacecraft as if it didn’t exist.
Physicists still don’t know how to do this, but the classical optics of light waves and light rays point to another solution. We see an object as it interacts with incoming light. In principle, an invisibility cloak could intercept those incoming rays and bend or refract them back on itself so that they travel within the cloak and emerge along their original paths. An observer, seeing what appears to be undisturbed light, would think there is nothing, just as gently flowing water splits around a rock and then recombines gives no indication downstream from the rock. But for light to follow this complex path, the cloak must be made from a metamaterial.
The researchers first tested this idea in 2006 with a rigid metamaterial mantle, a hollow cylinder whose wall contained thousands of tiny structures that caused microwaves to travel through suitable paths within the wall. Placed around an opaque metallic object, the cloak caused the object to almost completely vanish under microwave radiation. Since then, researchers have made small inanimate objects and a fish, cat and hand vanish under normal visible light, but only when viewed from a narrow field angle. Others have developed a flexible cloak that wraps around a small object to make it vanish, but only at one wavelength. Science cannot yet create a cloak that completely hides a person from ordinary light; but invisibility research is thriving and we are getting closer to the wonderful Harry Potter cloak.
2. Light pushes AND Roll things
Like thrown stones, photons carry the momentum they transfer to an object upon impact. This radiation pressure is why sunlight pushes comet tails away from the sun and why it can propel a spacecraft. In 2010, the Japan Aerospace Exploration Agency (JAXA) launched IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun, in honor of Icarus who flew close to the sun in myth). Its thin, tennis-court-sized polymer sail collected solar photons, which collectively exerted a small force that steadily accelerated IKAROS. Six months and 300 million miles later, it arrived at the near-Venus target without using any fuel for propulsion. Now JAXA and other space agencies are considering longer missions using larger, more effective solar sails.
We are approaching the wonderful Harry Potter cape.
Surprisingly, a light source can also attract an object towards itself, in the opposite direction of light propagation. Physicists have shown that within a specially shaped laser beam, the forward push of photons on a particle is dominated by a backward force due to the electromagnetic response of the particles themselves. The effect is strong enough to pull a microscopic object such as a biological cell back towards the laser. In 2023, however, a related experiment showed that a low-power laser could pull a relatively large, 0.2-inch x 0.1-inch, macroscopic object. This is hardly a powerful sci-fi tractor beam that can wrap around an entire spacecraft, but it could provide a new way to remotely sample the atmosphere on Earth and other planets and phenomena like comet tails.
3. Ghost images: images in the dark
Suppose we wanted to form an image of something like a living cell that could be changed or damaged by the light energy illuminating it. Ghost imaging uses the phenomenon of photon entanglement to produce an excellent image of a barely illuminated object. The entangled pairs of photons, which are formed by certain optical processes, are quantum related, such that measuring the properties of one immediately reveals the properties of the other, no matter how distant.
In ghost imaging, one of each of a swarm of entangled photon pairs interacts with the object and encounters a detector that simply registers its arrival. A second beam from the matching entangled partners never touches the object but goes directly to a sensitive multi-pixel detector. Computer analysis of the correlations between the results of the two detectors creates a high-quality image of the object, even in dim illumination. This approach has uses such as converting images taken surreptitiously by invisible infrared light into visible images taken by a high-resolution camera; or obtaining good quality radiographic images from a patient exposed to a low and relatively safe dose of X-rays.
4. Quantum Fissures in Time
In the famous double-slit experiment, first conducted in 1801, a ray of light splits as it passes through two narrow slits in an opaque barrier. On the flip side, the rays scatter and overlap to form a pattern of light and dark areas on a screen, showing that light is made up of waves that can interfere with each other. But a modern version of the experiment in which only one photon is aimed at the slits at a time still produces a wave-like interference pattern. According to Richard Feynman, this astonishing yet unexplained example of wave-particle duality holds the heart of quantum mechanics and contains the only mystery.
The light was slowed down to 38 mph, which a fit rider could match.
Now physicists have reproduced this experiment with cracks in time instead of space. They used a thin film of indium tin oxide (ITO), which is transparent to infrared light but rapidly becomes reflective within 10-15 sec when excited by a laser. In the experiment, the researchers shot infrared light at the ITO. When the ITO became a mirror for a short time, the reflected infrared light remained in its original form. But when the ITO mirror was very briefly turned on and off twice in rapid succession, the reflected infrared light definitely showed that it had interfered with itself as a result of passing through not one but two time portals or fissures.
One observer commented that this work could become as classic as the original double slit experiment. By extending it in time rather than space, the quest offers a new way to explore the unique mystery. The work also shows the feasibility of using metamaterials such as ITO to control light in optical systems and ultrafast-speed quantum computers.
5. Passing light on a bicycle
If there’s one physical fact that people know, it’s that light is the fastest thing in the universe, traveling at 186,000 miles/sec in a vacuum. The speed is reduced somewhat as light interacts with ordinary matter, dropping to, for example, 124,000 miles/sec in fiber optics and simple glass. This is still fast enough to go around the Earth in a fraction of a second; and so it was big news in 1999 when Harvard researcher Lene Hau dramatically slowed light to the human-scale speed of 38 mph, which a fit cyclist could match. This was accomplished in an exotic medium, a sodium-dense gas cooled to near absolute zero. The result was a quantum medium called a Bose-Einstein condensate. Light interacts with this more strongly than with any ordinary medium and has therefore been greatly slowed down. Later, Hau overcame this accomplishment by bringing the light to an abrupt stop, then retrieving it and sending it her way.
These results are breakthroughs in fundamental physics and could even be useful, except for the need to work at temperatures close to absolute zero. But since the original work, other researchers have slowed light in gases and solids at room temperature, making it possible to use slowed and still light in practical devices. These are currently being developed, for example, to synchronize signals in fiber optic networks and to store digital data in computers. Both applications are important steps towards the development of advanced telecommunications networks and quantum computers based entirely on lightweight rather than conventional electronic chips.
