You may have noticed that over the last few weeks, journalists (and everyone else) were all weepy-eyed, marking the milestones of the last year and decade. ‘Twas the season, after all. Well, we’ve entered a new year, and the name of the new game is to figure out all of the interesting anniversaries. I therefore propose that 2010 be known as the Semisesquicentennial year of the Philosophy of Quantum Mechanics.
Some while ago, I made a list of some of the more interesting, conversation, and controversial papers. Topping the list (chronologically), is the original Schrodinger’s Cat paper, which was written in November, 1935. That’s 75 years ago this year, if my subtraction works.
Schrodinger wondered how it was that the wavefunction description of particles the microscopic world could be squared with the “collapse of the wavefunction” when microscopic systems were “observed.” As he put it:
One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.
The standard (Copenhagen) interpretation of quantum mechanics is that, prior to opening the box, the cat is neither dead nor alive, but in a superposition state of the two. This is patently absurd, which was precisely Schrodinger’s point. Does a scientist need to open the box? Is a grad student sufficient? Shouldn’t a cat’s consciousness be enough to collapse the wavefunction? Does it even require consciousness at all?
75 years later, Quantum Mechanics still doesn’t have a particularly good answer to how, precisely, the wavefunction collapses.
We’ve done significantly better on another question raised the same year. In March of 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen (EPR), published a paper entitled, “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”. You’re probably familiar with Einstein’s lament, “God doesn’t play dice.” The EPR paradox gives fullest expression to his objections to the random elements of quantum mechanics. Einstein believed that everything that appears random about quantum mechanics is actually based on “hidden variables” in the system. He imagined two particles created in such a way that their spin exactly cancels.
This image will make even more sense if you’ve read chapter 3 of our book.
EPR complained that if the spin of particle “A” really were random then by measuring it’s spin, particle “B” would have to instantly fall into the opposite spin state, even though we never touched it. How did it know? If the two were separated by a great distance then the signal would (presumably) have to travel between the two at faster than the speed of light.
“Wouldn’t it be be simpler,” EPR proposed, “to simply assume that the spins of the two particles are pre-programmed?”
Einstein’s hidden variables was an effective, albeit untestable, argument for almost 30 years, until in 1964, when John Bell derived his famous inequality. It’s a bit too mathematical to reproduce here (but we do a version based on Mermin’s reality engine in our book), but basically Bell showed that by measuring the correlation between many randomized systems (with random orientations), one could, in principle, distinguish between the randomized version of quantum mechanics, and Einstein’s hidden variables.
Unlike Schrodinger’s cat, Einstein’s variables can be definitively answered. In 1982, Alain Aspect and his collaborators experimentally put the EPR paradox to the test.
A: No Hidden Variables!
This has some bizarre implications. It means that by measuring the spin of an entangled electron, the corresponding positron is forced to be in the opposite spin state faster than the speed of light! Crazy, you say? Einstein even referred to this as “spooky action at a distance.”
Spooky, perhaps, but no action. I defy you to use this as a communication device.
My point in all of this is that while we can a) build all sorts of technology based on quantum mechanics, and b) have all sorts of philosophical conversations about what it all means, most of the real groundwork was laid 75 years ago.
Not a bad time to reflect on what we’ve learned since then.
p.s. Happy New Year!