Physicists extract light from seeming emptiness
Nov. 18, 2011
Courtesy of Chalmers University of Technology and World Science staff
Physicists in Sweden say they have managed to create light from vacuum, the closest thing to empty space known to exist.
In findings published this week in the research journal Nature, the scientists said they verified an effect predicted over 40 years ago by capturing some of the particles of light, or photons, that constantly appear and disappear in the vacuum.
A diagram illustrating how virtual photons bounce off a “mirror” that vibrates at a speed approaching that of light. The round mirror in the picture is a symbol, and under that is the quantum electronic component (referred to as a SQUID), which acts as a mirror. This makes real photons appear in pairs, physicists say. (Credit: Philip Krantz, Chalmers U.)
A vacuum is a space devoid of atoms, the units that make up air, other gases and familiar objects. That means a vacuum is the next best thing to a space truly empty of anything at all—something physicists say can’t exist in nature as we know it, thanks to a phenomenon called the uncertainty principle. This holds that nothing can be in a state that is pinned down with perfect precision.
The uncertainty principle ensures that the vacuum teems with various subatomic particles that flit in and out of existence. They appear for an instant and disappear again, the energy fueling their existence “borrowed” from the void. Since their life is so fleeting, they are called virtual particles.
In the new work, Christopher Wilson and colleagues at Chalmers University of Technology in Gothenburg, Sweden said they coaxed photons into leaving their “virtual” state and becoming real photons—measurable light. The physicist Gerald Moore predicted in 1970 that this should happen if virtual photons bounce off a mirror moving at nearly the speed of light, in a phenomenon called the dynamical Casimir effect.
“Since it’s not possible to get a mirror to move fast enough, we’ve developed another method for achieving the same effect,” said Per Delsing, a physicist at Chalmers. “Instead of varying the physical distance to a mirror, we’ve varied the electrical distance to an electrical short circuit that acts as a mirror for microwaves.” The “mirror” consists of a device called a SQUID or superconducting quantum interference device, which is extremely sensitive to magnetic fields. By changing the direction of a magnetic field several billions of times a second the scientists said they made the “mirror” vibrate at one-fourth the speed of light.
“The result was that photons appeared in pairs from the vacuum, which we were able to measure in the form of microwave radiation,” said Delsing. “We were also able to establish that the radiation had precisely the same properties that quantum theory said it should have when photons appear in pairs in this way.” Quantum theory is the science of extremely small particles.
During the experiment, Delsing said, the “mirror” transferred some of its energy of motion to virtual photons so they could materialize. Göran Johansson, another physicist at Chalmers, said other particles might also be extracted from a vacuum in principle, but photons are easier. That’s because the equivalence of energy and mass, discovered by Einstein, implies that photons—being weightless—can be stimulated “out of their virtual state” with relatively little energy. Obtaining chunkier particles, such as electrons or protons, which make up atoms, “would require a lot more,” he added.
The scientists said the photons that appear in pairs in the experiment may be useful in the research field of quantum information, which includes the development of superfast “quantum” computers. But the main value of the work, they said, is that it aids our understanding of basic physical concepts, such as vacuum fluctuations. Some scientists believe these may have a connection with “dark energy” which drives the accelerating expansion of the universe, a discovery itself recognized this year with a Nobel Prize in physics.
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