If gravity intervenes, as Penrose expects, it forces the mirror either to remain at rest or to move—but not both—and the outcome is totally different. Now the photon cannot follow both paths because gravity anchors the mirror to a single state. Consequently, each photon will follow one path only, so it cannot interfere with itself; half the time that path will lead it to the detector. Thus if an X-ray triggers the detector, the quantum duplicate of the mirror must have disappeared, and Penrose’s view of reality must be correct.
The expense and technical difficulties of aiming X-ray lasers at targets thousands of miles away in outer space had seemed insurmountable, but Dirk Bouwmeester, a former postdoc under Penrose who is now a professor of physics at the University of California at Santa Barbara, saw a way to make it feasible. Along with colleagues William Marshall and Christoph Simon, he devised a way to bring Penrose’s experiment literally down to Earth—to a tabletop in Bouwmeester’s lab.
The revised experiment relies on a relatively simple visible-light source rather than an X-ray laser. Still, everything about Bouwmeester’s setup will push the boundaries of laboratory physics. To give the mirror the same kick a more energetic X-ray photon would produce, the light photons will have to reflect back and forth between two mirrors a million times. Until now, the largest objects ever studied in a state of quantum superposition were soccer-ball-shaped carbon molecules called buckyballs. Bouwmeester is trying to detect the same effect on a mirror that is a billion times bigger. “If we were able to observe that, it would be spectacular, a test of quantum mechanics in a completely new regime,” he says.
The team at Santa Barbara is running the experiment right now, but with a significantly smaller mirror than needed to test Penrose’s theory. If the current tests succeed, Bouwmeester will gradually increase the size of the mirror up to the necessary tenth-of-a-human-hair diameter. He and his colleagues are also working out ways to shield the experiment from the vibrations, stray photons, or temperature changes that would ruin the results. “It is not something that will happen overnight,” he says. “We need to isolate the quantum world from our world and see what happens. If everything works well, I expect some results four years from now.”
Penrose, who turns 74 in August, is hopeful that he will see the day when his ideas are vindicated. Not many physicists share this optimism. Tony Leggett, a Nobel laureate at the University of Illinois at Urbana-Champaign, suspects the experiment will fail to show that gravity has any effect on quantum systems. “I take the quantum paradox as seriously as Penrose does,” Leggett says. “I’m personally convinced that somewhere between the level of the atom and human consciousness, something has to come in which changes the structure of quantum mechanics.” The problem is that quantum theory has never yet failed to predict the outcome of any experiment. Without evidence of some such flaw in the theory, physicists are left groping in the dark for ways to improve it. “I think the odds of them being right are less than 5 percent,” he says.
David Deutsch, a theoretical physicist at Oxford University’s Centre for Quantum Computation, is a leading proponent of the many worlds theory. He turns the tables on Penrose, arguing that his quest is based more on aesthetics than science: “If something is wrong with a theory, or there is some experimental anomaly, those are motivations for changing a theory. When your motivation comes from a metaphysical reluctance for reality to be a certain way, then historically that kind of motivation has never produced the right answers.”
Penrose responds that he is not changing quantum mechanics; he is merely putting it to a new, more rigorous test. “You can say we haven’t seen any violation of quantum mechanics, but that’s absolutely what you’d expect, because no experiment has ever been performed that comes remotely close to the level you’d need to see any violations. So unless you try to get to this level I’m aiming for, it’s not at all surprising that we haven’t been able to see any deviations,” he says.
If Bouwmeester’s experiment succeeds, it will show that the fantasy of being in two places at the same time really is impossible. As a kind of compensation, it will also show that the number of places science can go is far greater than we have come to believe. Most physicists today trying to unite Einstein’s theory of gravity with quantum mechanics focus on microscopic realms beyond the reach of any conceivable experiment. Perhaps the solution that eluded Einstein is much closer at hand, in the strange territory where quantum mechanics just barely emerges into the human world.
The one Penrose rises from his one chair, preparing to pick up Max, his 4-year-old son, from school. He has no doubt that Max’s generation will learn physics lessons different from the confusing, incomplete story that Penrose got from Dirac all those years ago.
“Is quantum mechanics the last word?” Penrose asks. “There is no reason to believe that.”
Other Penrose Questions #5
Can a computer be intelligent?
Penrose believes that the human brain performs feats that are beyond computational processes. He cites a famous proof by the ligician Kurt Gödel on the limitations of all mathematical systems as an idea that no computer could ever devise.
The Experiment: Can a mirror be in two places at the same time?
The Orthodox Quantum Mechanics View
A light source shoots particles of light, or photons, at a beam splitter. According to standard theory, when a photon arrives at the beam splitter, it splits into two states. One is reflected toward mirror 1; the other goes through the splitter to the tiny mirror, moving the mirror on the way out and restoring it to its initial position on the way back after reflecting off mirror 2. It is impossible to know which path the photon takes, so the tiny mirror exists in two states (moved and unmoved) at the same time. Both states of the photon perfectly retrace their paths and interfere with each other, so no photons ever hit the detector.
The Penrose View
If Penrose is right, gravity forces the tiny mirror into a single state.
As a result, the photon now can follow one path only. It either goes through the
beam splitter toward the tiny mirror (dashed lines), or it reflects off the beam
splitter toward mirror 1 (solid lines). In either case, the photon returns to
the beam splitter and is directed to the detector half the time. If this happens
in the real experiment, then we will know there is something wrong with
conventional quantum mechanics.
