Einstein said there would be days like this.
This fall 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 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, Colo., 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.” At the University of California, Santa Barbara, researchers are planning an experiment in which a small mirror will be in two places at once.
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 fall 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 about whether, in effect, physicists were justified in trying to change quantum theory, the most successful theory in the history of science. Dr. Leggett said yes; Dr. 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 Z. Albert, a professor of philosophy and physics at Columbia. Regarding the 1935 paper, Dr. Albert said, “We know something about Einstein’s genius we didn’t know before.”
The Silly Theory
From the day 100 years ago that 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.
If light was a particle, how did it know which way to go when it was issued from an atom?
“The more success the quantum theory has, the sillier it seems,” Einstein once wrote to friend.
The full extent of its silliness came in the 1920’s 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, usually referred to as E.P.R., 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.
Imagine that a pair of electrons are shot out from the disintegration of some other particle, like fragments from an explosion. By law certain properties of these two fragments should be correlated. If one goes left, the other goes right; if one spins clockwise, the other spins counterclockwise.
That means, Einstein said, that by measuring the velocity of, say, the left hand electron, we would know the velocity of the right hand electron without ever touching it.
Conversely, by measuring the position of the left electron, we would know the position of the right hand one.
Since neither of these operations would have involved touching or disturbing the right hand electron in any way, Einstein, Podolsky and Rosen argued that the right hand electron must have had those properties of both velocity and position all along. That left only two possibilities, they concluded. Either quantum mechanics was “incomplete,” or measuring the left hand particle somehow disturbed the right hand one.
But the latter alternative violated common sense. Such an influence, or disturbance, would have to travel faster than the speed of light. “My physical instincts bristle at that suggestion,” Einstein later wrote.
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.”
Enjoy the Magic
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 S. Bell, a particle physicist at the European Center for Nuclear Research near Geneva, who had his own doubts about quantum theory, took up the 1935 E.P.R. 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. Apparently a particle in one place could be affected by what you do somewhere else.
“That’s really weird,” Dr. Albert said, calling it “a profoundly deep violation of an intuition that we’ve been walking with since caveman days.”
Physicists and philosophers are still fighting about what this means. Many of those who care to think about these issues (and many prefer not to), concluded that Einstein’s presumption of locality – the idea that physically separated objects are really separate – is wrong.
Dr. Albert said, “The experiments show locality is false, end of story.” But for others, it is the notion of realism, that things exist independent of being perceived, that must be scuttled. In fact, physicists don’t even seem to agree on the definitions of things like “locality” and “realism.”
“I would say we have to be careful saying what’s real,” Dr. 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.”
Indeed the effects of spooky action, or “entanglement,” as Schrödinger called it, only show up in retrospect when the two participants in a Bell-type experiment compare notes. Beforehand, neither has seen any violation of business as usual; each sees the results of his measurements of, say, whether a spinning particle is pointing up or down, as random.
In short, as Brian Greene, the Columbia theorist wrote in “The Fabric of the Cosmos,” Einstein’s special relativity, which sets the speed of light as the cosmic speed limit, “survives by the skin of its teeth.”
In an essay in 1985, Dr. Mermin said that “if there is spooky action at a distance, then, like other spooks, it is absolutely useless except for its effect, benign or otherwise, on our state of mind.”
He added, “The E.P.R. experiment is as close to magic as any physical phenomenon I know of, and magic should be enjoyed.” In a recent interview, he said he still stood by the latter part of that statement. But while spooky action remained useless for sending a direct message, it had turned out to have potential uses, he admitted, in cryptography and quantum computing.
Nine Ways of Killing a Cat
Another debate, closely related to the issues of entanglement and reality, concerns what happens at the magic moment when a particle is measured or observed.
Before a measurement is made, so the traditional story goes, the electron exists in a superposition of all possible answers, which can combine, adding and interfering with one another.
Then, upon measurement, the wave function “collapses” to one particular value. Schrödinger himself thought this was so absurd that he dreamed up a counterexample. What is true for electrons, he said, should be true as well for cats.
In his famous thought experiment, a cat is locked in a box where the decay of a radioactive particle will cause the release of poison that will kill it. If the particle has a 50-50 chance of decaying, then according to quantum mechanics the cat is both alive and dead before we look in the box, something the cat itself, not to mention cat lovers, might take issue with.
But cats are always dead or alive, as Dr. Leggett of Illinois said in his Berkeley talk. “The problem with quantum mechanics,” he said in an interview, “is how it explains definite outcomes to experiments.”
If quantum mechanics is only about information and a way of predicting the results of measurements, these questions don’t matter, most quantum physicists say.
“But,” Dr. Leggett said, “if you take the view that the formalism is reflecting something out there in real world, it matters immensely.” As a result, theorists have come up with a menu of alternative interpretations and explanations. According to one popular notion, known as decoherence, quantum waves are very fragile and collapse from bumping into the environment. Another theory, by the late David Bohm, restores determinism by postulating a “pilot wave” that acts behind the scenes to guide particles.
In yet another theory, called “many worlds,” the universe continually branches so that every possibility is realized: the Red Sox win and lose and it rains; Schrödinger’s cat lives, dies, has kittens and scratches her master when he tries to put her into the box.
Recently, as Dr. Leggett pointed out, some physicists have tinkered with Schrödinger’s equation, the source of much of the misery, itself.
A modification proposed by the Italian physicists Giancarlo Ghirardi and Tullio Weber, both of the University of Trieste, and Alberto Rimini of the University of Pavia, makes the wave function unstable so that it will collapse in a time depending on how big a system it represents.
In his standoff with Dr. Ramsay of Harvard last fall, Dr. Leggett suggested that his colleagues should consider the merits of the latter theory. “Why should we think of an electron as being in two states at once but not a cat, when the theory is ostensibly the same in both cases?” Dr. Leggett asked.
Dr. Ramsay said that Dr. Leggett had missed the point. How the wave function mutates is not what you calculate. “What you calculate is the prediction of a measurement,” he said.
“If it’s a cat, I can guarantee you will get that it’s alive or dead,” Dr. Ramsay said.
David Gross, a recent Nobel winner and director of the Kavli Institute for Theoretical Physics in Santa Barbara, leapt into the free-for-all, saying that 80 years had not been enough time for the new concepts to sink in. “We’re just too young. We should wait until 2200 when quantum mechanics is taught in kindergarten.”
The Joy of Randomness
One of the most extreme points of view belongs to Dr. Zeilinger of Vienna, a bearded, avuncular physicist whose laboratory regularly hosts every sort of quantum weirdness.
In an essay recently in Nature, Dr. 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,” Dr. Zeilinger wrote.
Dr. Zeilinger suggested that reality and information are, in a deep sense, indistinguishable, a concept that Dr. Wheeler, the Princeton physicist, called “it from bit.”
In information, the basic unit is the bit, but one bit, he says, is not enough to specify both the spin and the trajectory of a particle. So one quality remains unknown, irreducibly random.
As a result of the finiteness of information, he explained, the universe is fundamentally unpredictable.
“I suggest that this randomness of the individual event is the strongest indication we have of a reality ‘out there’ existing independently of us,” Dr. Zeilinger wrote in Nature.
He added, “Maybe Einstein would have liked this idea after all.” New York Times