International Day of Light Edition: Beam Me Up, Scotty: How Lasers Can Counteract Gravity

Physics
We’ve all wished for weightlessness at some point in our lives—that fantastical quality that powers the magic of flying broomsticks and fuels our fascination with space travel. Although we’re a long way from floating down the street, physicists have developed ways to mitigate the effect of gravity, from carefully aligning sound waves to mimicking free fall in reduced-gravity aircraft. But Kosuke Shibata and his colleagues at the Gakushuin University say they’ve developed a new tool for levitation: beams of light.

Figure 1. Astronaut David Scott experiences weightlessness in a C-135 aircraft. Image credit: NASA

It might sound rather Star Trek-esque, but the physics is real. Light can be described mathematically as a wave of electric and magnetic fields moving through space, and when a strong light source like a laser shines on an atom, the changing electric field interacts with the electric charges contained within the atom’s protons and neutrons. If done just right, the resulting force can precisely cancel out that of gravity, allowing the atoms to levitate.

While a similar technique using the magnetic fields has been in use for several years, it had never been done with the more widely applicable electric field. The challenge is that the incoming light intensity must be proportional to the concentration of target atoms at any given point, which requires a finely sculpted—not to mention extremely powerful—laser beam, far beyond the limits of current technology.

Instead, the research team tried a different tactic. They showed theoretically that by carefully sweeping a smaller laser beam across a larger cloud of atoms, they could produce the same effect of a larger contoured beam. In fact, they subsequently used the technique to levitate a small cloud of 87-Rubidium atoms.

Unfortunately for us would-be wizards, there’s a catch. “This technique is efficient when the energy in the system is small and gravity is dominant over other [forms of] energy,” Shibata explains. As it happens, atoms in a room-temperature gas zip about almost without noticing gravity, just like a ball thrown with enough energy can travel against the pull of gravity—for a time at least. Even less-energetic solids and liquids contain enough kinetic energy that their constituent molecules vibrate too much for the optical painting technique to work. Instead, this method is limited to ultracold atoms, which are held at just a few tens of millionths of Kelvins.

Nonetheless, Shibata insists that this work was driven by practicality. Ultracold atoms are something of a sweetheart in physics research, featured in experiments that measure the gravitational constant and investigate the future of quantum computing. On Earth, though, gravity has an annoying tendency to distort the shape of ultracold atom clusters, forcing researchers to develop supports that interfere with their precision. Using this new levitation technique, Shibata sees a more cost-effective way to counteract gravity than performing experiments in space or in ballistic flight. So maybe it isn’t as flashy as Captain Kirk’s transport beam—but it’s every bit as exciting!

What happens when several thousand distinguished physicists, researchers, and students descend on the nation’s gambling capital for a conference? The answer is “a bad week for the casino”—but you’d never guess why.
Lexie and Xavier, from Orlando, FL want to know:
“What’s going on in this video? Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!”
Even though it’s been a warm couple of months already, it’s officially summer. A delicious, science-filled way to beat the heat? Making homemade ice cream.

(We’ve since updated this article to include the science behind vegan ice cream. To learn more about ice cream science, check out The Science of Ice Cream, Redux)

Over at Physics@Home there’s an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?

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