Two scientific papers published in the past week develop new lines of tabletop experiments that allow scientists for the first time to probe previously untestable questions about black holes, gravity and relativity.
One promises the first opportunity ever to observe an exotic species of radiation from black holes that was predicted by Stephen Hawking. The other pushes into the history books as the first experiment to observe the unreconciled worlds of both gravity and quantum mechanics together.
In a letter published in Thursday’s edition of Nature, theorist Ulf Leonhardt of the University of St. Andrews, Scotland, proposes modifying the experimental setup for studying stored light—a recently discovered process of stopping a light wave cold in its tracks—to mimic the event horizon of a black hole.
Doing so, Leonhardt says, could be done with minimal extra effort but potentially maximal new science.
“It is something like a naked event horizon, because there’s no black hole there,” he said.
Leonhardt’s proposed modifications involve generating an “optical molasses”—the gaseous or solid-state medium that actually stops light—whose light-stopping power begins to dilute at the edges.
The overall effect, in turn, is like the environment immediately surrounding a black hole, where an outside observer would see light slowing down more and more as it approached the point-of-no-return (the event horizon). Once light actually reaches a black hole’s horizon, it stops entirely—just like the light caught in optical molasses in stored light experiments.
“We would be mimicking the effect of gravity by using extreme states of matter,” said Edi Halyo of Stanford and the California Center for Physics and Astrophysics.
Leonhardt’s new twist would effectively create a pencil-sized event horizon simulator, which could then be used to test some of the phenomena long theorized to be found just outside a black hole’s grim gates.
On the top of the list would be experimental testing of a quantum mechanical mechanism first proposed by Hawking in 1974.
According to Heisenberg’s uncertainty principle, nature enforces its laws with a fudge factor large enough that a pair of particles such as two photons can appear out of nowhere, so long as they disappear just as quickly.
Odd as it may seem, these fluctuations in the so-called quantum vacuum (aka zero-point field) have been observed in such experiments as the “Casimir effect”—where the quantum vacuum actually pushes two metal plates together. The vacuum’s effects could be much more pervasive, too: In 1994, a team of American scientists argued that the quantum vacuum may be the ultimate wellspring of inertia.
Hawking realized that near a black hole, some of these virtual particles created by the quantum vacuum accidentally fall prey to the extreme gravity and disappear into the hole—leaving the partner to wander off like a kid who lost his dance partner at the prom. This stray particle (or photon) appears to the outside world like it came from the black hole—and is, in fact, the only form of radiation that a black hole emits.
Likewise, Leonhardt said, light produced by the quantum vacuum can also fall into the stored light field and cause its partner to wander off in a Hawking radiation-like process.
“Everyone believes Hawking’s prediction of radiation from a black hole,” said physicist Matt Viser of Washington University in St. Louis. “But we’ve never been able to test it.
“If we can find the analogue of Hawking radiation in this system, it would definitely be very exciting.”
On the other hand, last week’s issue of Nature presented a paper by a team of French physicists led by Valery V. Nesvizhevsky of Grenoble’s Institute Laue-Langevin announcing the first-ever test of quantum mechanics as it plays out under the influence of gravity.
Because gravity is such a weak force—some 39 orders of magnitude weaker than electromagnetism—it is only with the latest generation of extremely sensitive apparatus that such fundamental measurements can be pondered.
So far so good, says Thomas Bowles of Los Alamos. What’s important about Nesvizhevsky’s experiment is not just the result—the system acted as theory predicts—but the setup the team developed to get this result. This experimental apparatus could, for instance, readily be adapted to test the “equivalence principle” of general relativity.
“Because this technique is so incredibly sensitive, one can now start to probe issues that are at the basis of science,” he said.