Imagine a spaceship, coasting silently through the dusty void of our solar system, outward-bound on a journey away from both our sun and the pale blue dot that is Earth. Slowly, with mechanical precision and a slight whirr that’s inaudible anywhere but inside the ship, telescoping arms deploy from their hatches positioned around the circumference of the craft. Each close to a mile long, they give the impression of a shining asterisk gliding away in the endless night, or a very leggy spider.
Not for long, though—soon, a series of motors engages a winch, a metal cable grows taut and begins to wind around the winch’s spool. As it does, a silvery sheet unfolds from between the ship’s “arms”—masts, it can now be seen, as the reflective sails are hoisted on all sides to form an enormous frill around the ship. As photons from the sun strike the mirrored surface and bounce off, they transfer momentum, and the ship starts to accelerate, borne out to distant stars on a wind made of light.
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| An artist’s rendition of Japan’s IKAROS satellite, the first functional spacecraft designed to take advantage of solar radiation pressure. Image Credit: Andrzej Mirecki, via Wikipedia CC BY-SA 3.0 |
That’s the dream, anyway—solar sail technology is still in its infancy. The concept originated in the early 1900s with Albert Einstein, as an imaginative exercise to help explore the implications of general relativity with its constant, universal speed of light: although photons have no mass, they have momentum. As a result, light reflecting off a mirror creates a slight pressure on the mirror, pushing it away from the light’s source. It’s once the mirror is moving, though, that things get interesting.
Imagine bouncing a tennis ball off a brick wall. If it hits the wall at 20mph, it’s going to come back to you at 20mph, or a little bit less after losing energy to friction, sound, and air drag. But now say you bounce the ball off the back of a car that’s moving away from you at 5mph. The vehicle only “sees” the ball moving at 15mph, thanks to their relative velocities, so when it reflects back at you, it’s going to be moving at 15mph instead of the 20 that would have resulted had the car had been stationary.
But this isn’t how things work for light—photons always move at c, 2.99*108 m/s. If you shine a laser at a mirror that’s moving away from you, the photons will come back at c, but—just like the tennis ball, they’ll come back lower in energy. This manifests as a redshift, a lengthening of the photons’ wavelength. The closer the mirror gets to c, the longer (and lower-energy) the incoming photons appear to it, which reduces their accelerating effect on the mirror. The faster the mirror goes, essentially, the harder it is to transfer energy to it using light.
This is useful as a tool to help understand some of relativity’s complexities, but in more recent years scientists have started running preliminary tests to see if such technologies might find practical applications here on Earth. With the advent of high-intensity lasers capable of delivering several watts of power, it’s theoretically possible to take a tiny piece of reflective foil and accelerate it very quickly to a significant fraction of light speed!
The key word there is theoretically, though. Simulations of the process don’t quite go as expected—in silico tests of the technique have ended with the foil’s surface developing mysterious instabilities and rapidly disintegrating. For nearly a decade now, scientists have struggled to understand the source of these instabilities, but a breakthrough may be on the horizon thanks to new work from a collaboration of Chinese, Portuguese, and American scientists, recently accepted for publication by the American Physical Society. To understand why the foils are failing, the researchers had to dig into the physics at the core of reflection.
Photons are electromagnetic waves, generated by the motion of charged particles. It’s similar to the way that water waves can be generated by a buoy dragged through the water, or pushed down into the water and allowed to pop back up. Likewise, an ocean wave can toss the buoy around, and electrons can be dragged to and fro by electromagnetic waves. This is how a radio antenna works—electrons are pushed from one end of the receiving antenna to the other, in sync with the electrons in the transmitting antenna, creating a signal in your radio that looks like the one which caused the oscillations in the transmitting antenna.
Photons also have particle-like behavior, though, so when we think of reflection it’s easy to think of them as bouncing off the mirror—like the tennis ball in our earlier analogy—when the reality of the scenario is a little wavier. When a photon hits a surface, the electrons in that surface try to “ride” the wave—oscillating up and down as the wave’s positive and negative components roll through. In non-conductive surfaces, where the electrons are bound to their home atoms, they end up bumping around, absorbing the energy and dispersing it as heat.
In a conductor, on the other hand, the electrons can move freely, pulled toward the positive regions of the electromagnetic field, pushed away by the negative ones. In flowing along with the photon, these electrons generate a wave of their own, equal and opposite in phase to the incoming one—and it’s this secondary wave we see as the reflection of the original photon. (This is also why silver, the most electrically conductive metal, makes one of the best backings for a mirror!)
Knowing this is key to understanding what fouls up the foils in these high-intensity “photon sail” prototype experiments. In an ideal conductor, the electrons would move with zero resistance, responding perfectly to the incident electromagnetic field. In reality, however, these electrons have mass and momentum. Oscillating back and forth a few billion times a second, the electrons become tiny wrecking balls. According to the new paper’s authors, “the ripples on the foil surface are mainly induced by the coupling between the…oscillating electrons and the quasi-static ions, an effect of the non-zero electron mass in the real world.” Essentially, the negative charge of the electrons exerts a pull on the positively-charged nuclei of the metal’s atoms, tearing them out of their ordinarily uniform crystalline lattice, and ultimately causing the foil to disintegrate.
Real-life solar sails used in a spacefaring context probably wouldn’t be subject to the kind of radiation that would tear them apart, but these results still provide an important insight into the physics of relativity and reflection, and may yet lead to new techniques in fields like accelerator technology.
—Stephen Skolnick