Physics & Math / Subatomic Particles

In this test, a beam of light is projected through two parallel slits cut in an opaque barrier and then onto a white screen. When light hits the screen, it does not produce just two overlapping regions of brightness. Instead, something strange appears: a series of alternating light and dark stripes, called an interference pattern. The 19th-century explanation for this was that light is a wave and that light waves overlap after passing through the slits. The light waves seem to behave much like water waves on the surface of a pond: Where two crests meet, the wave gets higher, creating a bright stripe; where a crest meets a trough, the two cancel out, and the wave vanishes, yielding a dark zone.

With the development of quantum theory in the early 20th century, the explanation became far weirder. Physicists realized that light is not a wave exactly but rather a wavelike particle called a photon. That discovery suggested a new experiment. In principle, it would be possible to send light through the slits one photon at a time and collect them on photographic film. Common sense says there should be no interference pattern in this case: There is only one photon in the apparatus at any given moment, so there is nothing for the light to interfere with.

Then in 1909 a young British physicist named Geoffrey Ingram Taylor actually ran the experiment and witnessed the bizarre result. As the photons accumulate on the film, the same old interference pattern of alternating bright and dark stripes gradually appears, defying common sense. In this case, there is only one thing each photon can interact with—itself. The only way this pattern could form is if each photon passes through both slits at once and then interferes with its alternate self. It is as if a moviegoer exited a theater and found that his location on the sidewalk was determined by another version of himself that had left through a different exit and shoved him on the way out.

Since then, other researchers have repeated the experiment with electrons, atoms, even with relatively bulky molecules containing as many as 70 carbon atoms. The results never vary. Individual atoms and molecules go through both slits at once. Yet for some reason the laws of physics take away that ability for large objects like paper clips, people, and planets. “Something has got to go wrong with quantum mechanics somewhere,” Penrose says. “I regard this as a major problem that is going to require another revolution. But rather few people seem to agree with this viewpoint.”

When pressed, quantum theorists usually fall back on what is known as the Copenhagen interpretation. The idea was promoted in the 1920s by Danish physicist Niels Bohr and his protégé German physicist Werner Heisenberg. In their view, we do not see quantum effects in the everyday world because the act of observation changes everything, fixing the many possibilities allowed by quantum mechanics as one. As a result, when we look, we only see one version of events, with every object firmly anchored to one position at a time.

The flaw in the Copenhagen interpretation is that it has no basis in theory—it is more like a story that scientists tell to make sense of facts that otherwise would seem nonsensical. It also suggests that the universe does not become fully real until someone observes it. Einstein found this idea abhorrent. “I like to think that the moon is there even if I am not looking at it,” he fumed in response to Bohr. Nevertheless, the Copenhagen interpretation was voted the preferred explanation of quantum weirdness by physicists at a conference in 1997.

The runner-up explanation is an even stranger view of reality. Called the many worlds interpretation, it was proposed in 1957 by Princeton University doctoral candidate Hugh Everett III. Its adherents take the laws of quantum theory at face value: Every possible quantum outcome really exists—but in worlds parallel to our own. In one universe, Penrose is talking with me in Oxford; in another, he is watching a monster-truck rally. From this perspective, people and particles behave much the same way. We just do not see them in many places at the same time because each potential location is tucked away in a different universe (see “Quantum Schmantum,” Discover, September 2001, posted on our Web site, www.discover.com).

Penrose cannot believe anyone finds either the Copenhagen interpretation or the many worlds picture satisfactory. “If you take the equations of quantum mechanics up to the level where you can actually see things going on, you’re driven to an absurd viewpoint. People are led into views of the world which are pretty fantastical. And rather than say, ‘This is a bit wild, let’s try to do something a bit more commonsense-ish,’ they come up with theories that are completely wild.”

After struggling for years to come up with a better explanation, he finally has a solution.

Other Penrose Questions #2
What is gravity?
For nearly 40 years, Penrose has worked on twistor theory, a radically original description of gravity, space, and time. Rather than treating space-time as an empty arena in which physical events unfold, Penrose postulates that objects called twistors build the fabric of space-time from the ground up.

Other Penrose Questions #3
Can a pattern have no pattern?
Using only a notebook and a pencil, Penrose devised a way to seamlessly cover a flat service in a nonrepeating pattern with just two different shapes, now called Penrose tiles. This feat had been considered impossible. Researchers have since learned that certain chemicals naturally organize themselves into these patterns, some of which are now used to make nonstick coating for pots and pans.