The Quantum Mechanic

New tests of Einstein's 'spooky' reality

By Dennis Overbye The New York Times

THURSDAY, DECEMBER 29, 2005

Einstein said there would be days like this. This autumn scientists announced that they had put a half dozen beryllium atoms into a "cat state." No, they were not sprawled along a sunny windowsill; to a physicist, a "cat state" is the condition of being in two diametrically opposed conditions at once, like black and white, up and down, or dead and alive.

These atoms were each spinning clockwise and counterclockwise at the same time. Moreover, like miniature Rockettes, they were all doing whatever it was they were doing together, in perfect synchrony. Should one of them realize, like the cartoon character who runs off a cliff and doesn't fall until he looks down, that it is in a metaphysically untenable situation and decide to spin only one way, the rest would instantly fall in line, whether they were across a test tube or across the galaxy.

The idea that measuring the properties of one particle could instantaneously change the properties of another one (or a whole bunch) far away is strange to say the least - almost as strange as the notion of particles spinning in two directions at once. The team that pulled off the beryllium feat, led by Dietrich Leibfried at the National Institute of Standards and Technology, in Boulder, Colorado, hailed it as another step toward computers that would use quantum magic to perform calculations.

But it also served as another demonstration of how weird the world really is according to the rules known as quantum mechanics.

The joke is on Albert Einstein, who, back in 1935, dreamed up this trick of synchronized atoms - "spooky action at a distance," as he called it - as an example of the absurdity of quantum mechanics. "No reasonable definition of reality could be expected to permit this," he, Boris Podolsky and Nathan Rosen wrote in a paper in 1935.

Today, that paper, written when Einstein was a relatively ancient 56 years old, is the most cited of Einstein's papers. But far from demolishing quantum theory, that paper wound up as the cornerstone for the new field of quantum information.

Nary a week goes by that does not bring news of another feat of quantum trickery once only dreamed of in thought experiments: particles (or at least all their properties) being teleported across the room in a microscopic version of "Star Trek" beaming; electrical "cat" currents that circle a loop in opposite directions at the same time; more and more particles farther and farther apart bound together in Einstein's spooky embrace now known as "entanglement."

Niels Bohr, the Danish philosopher king of quantum theory, dismissed any attempts to lift the quantum veil as meaningless, saying that science was about the results of experiments, not ultimate reality.

But now that quantum weirdness is not confined to thought experiments, physicists have begun arguing again about what this weirdness means, whether the theory needs changing, and whether in fact there is any problem.

This autumn, two Nobel laureates, Anthony Leggett of the University of Illinois and Norman Ramsay of Harvard, argued in front of several hundred scientists at a conference in Berkeley, California, about whether, in effect, physicists were justified in trying to change quantum theory, the most successful theory in the history of science. Leggett said yes; Ramsay said no.

It has been, as Max Tegmark, a cosmologist at the Massachusetts Institute of Technology, noted, "a 75-year war." It is typical in reporting on this subject to bounce from one expert to another, each one shaking his or her head about how the other one just doesn't get it.

"It's a kind of funny situation," N. David Mermin of Cornell, who has called Einstein's spooky action "the closest thing we have to magic," said, referring to the recent results. "These are extremely difficult experiments that confirm elementary features of quantum mechanics." It would be more spectacular news, he said, if they had come out wrong.

Anton Zeilinger of the University of Vienna said that he thought "the world is not as real as we think."

"My personal opinion is that the world is even weirder than what quantum physics tells us," he added.

The discussion is bringing renewed attention to Einstein's role as a founder and critic of quantum theory, an "underground history" that has largely been overlooked amid the celebrations of relativity in the past Einstein year, according to David Albert, a professor of philosophy and physics at Columbia.

Regarding the 1935 paper, Albert said, "We know something about Einstein's genius we didn't know before."

From the day 100 years ago when he breathed life into quantum theory by deducing that light behaved like a particle as well as like a wave, Einstein never stopped warning that it was dangerous to the age-old dream of an orderly universe.

"The more success the quantum theory has, the sillier it seems," he once wrote to friend.

The full extent of its silliness came in the 1920s, when quantum theory became quantum mechanics.

In this new view of the world, as encapsulated in a famous equation by the Austrian Erwin Schrödinger, objects are represented by waves that extend throughout space, containing all the possible outcomes of an observation - here, there, up or down, dead or alive. The amplitude of this wave is a measure of the probability that the object will actually be found to be in one state or another, a suggestion that led Einstein to grumble famously that God doesn't throw dice.

Worst of all from Einstein's point of view was the uncertainty principle, enunciated by Werner Heisenberg in 1927. Certain types of knowledge, of a particle's position and velocity, for example, are incompatible: The more precisely you measure one property, the blurrier and more uncertain the other becomes.

In the 1935 paper, Einstein and his colleagues Podolsky and Rosen (usually referred to as EPR) argued that the uncertainty principle could not be the final word about nature. There must be a deeper theory that looked behind the quantum veil.

Bohr responded with a six-page essay in Physical Review that contained but one simple equation, Heisenberg's uncertainty relation. In essence, he said, it all depends on what you mean by "reality."

Most physicists agreed with Bohr, and they went off to use quantum mechanics to build atomic bombs and reinvent the world. The consensus was that Einstein was a stubborn old man who "didn't get" quantum physics.

All this began to change in 1964 when John Bell, a particle physicist at the European Center for Nuclear Research near Geneva, took up the 1935 EPR argument. Somewhat to his dismay, Bell, who died in 1990, wound up proving that no deeper theory could reproduce the predictions of quantum mechanics. Bell went on to outline a simple set of experiments that could settle the argument and decide who was right, Einstein or Bohr.

When the experiments were finally performed in 1982, by Alain Aspect and his colleagues at the University of Orsay in France, they agreed with quantum mechanics and not reality as Einstein had always presumed it should be.

Physicists and philosophers are still fighting about such results. Many who care to think about these issues (and many prefer not to) have concluded that Einstein's presumption of locality - the idea that physically separated objects are really separate - is wrong.

"I would say we have to be careful saying what's real," Mermin said. "Properties cannot be said to be there until they are revealed by an actual experiment."

What everybody does seem to agree on is that the use of this effect is limited. You can't use it to send a message, for example. Leonard Susskind, a Stanford theoretical physicist who called these entanglement experiments "beautiful and surprising," said the term "spooky action at a distance" was misleading because it implied the instantaneous sending of signals. "No competent physicist thinks that entanglement allows this kind of nonlocality."

But in an essay recently in Nature, Zeilinger sought to find meaning in the very randomness that plagued Einstein.

"The discovery that individual events are irreducibly random is probably one of the most significant findings of the 20th century," Zeilinger wrote. "I suggest that this randomness of the individual event is the strongest indication we have of a reality 'out there' existing independently of us."

He added, "Maybe Einstein would have liked this idea after all."