He added, "The EPR experiment is as close to magic as any physical phenomenon I know of, and magic should be enjoyed." Recently, 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. Schroedinger 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 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," 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; Schroedinger's cat lives, dies, has kittens and scratches her master when he tries to put her into the box.
Recently, as Leggett pointed out, some physicists have tinkered with Schroedinger'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 Ramsay of Harvard last fall, 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?" Leggett asked.
Ramsay said 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," Ramsay said.
David Gross, a recent Nobel Prize 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 Zeilinger of Vienna, a bearded, avuncular physicist whose laboratory regularly hosts every sort of quantum weirdness.
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.
Zeilinger suggested that reality and information are, in a deep sense, indistinguishable, a concept that 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," Zeilinger wrote in Nature.
He added, "Maybe Einstein would have liked this idea after all."
*April the cat stalks off to tinker with the superstring theory, obviously yet another physics theory invented by some cat or other, possibly Schrodinger's: ze vorld consists of things like little bits of string...* Hehe! (But really, I scratch my head, as baffled as Bohr and Einstein, by the quantum weirdness of it all ;-))