Thursday, September 28, 2006

Quantum Weirdness

The quantum is that embarrassing little piece of thread that always hangs from the sweater of space-time. Pull it and the whole thing unravels. ~Fred Alan Wolfe

“Quantum weirdness” is almost a redundancy. Everything to do with quantum theory is as weird as anything in Lewis Carroll's Wonderland. Einstein hated it. Richard Feynman reveled in it. And we reap the benefits of its implementations.

Of course, it's also the reason I'm not a physicist. Firstly, the math is brutal; if I never see another eigenfunction, I'll die a happy man. Second, trying to wrap your brain around the incongruities of the quantum world is not easy. I think I could have managed the second if I could have survived the first.

Consider, if you will, wave-particle duality. Every physics student has done the double slit experiment in one form or another. Basically, you shoot electrons at a card with two thin slits in it. The electrons pass through the slits and impinge on a target (like a cathode ray tube, or televison screen, as it's more colloquially known). The pattern that results is an interference pattern, alternating dark and light bars, as though waves passed through the slits. You get this pattern even if you shoot the electrons one at a time. So even though you're shooting little discreet particles at the slits, the pattern of their impact is that of waves.

But it doesn't stop there. Suppose you're a clever little scientist, and you decide you're going to find out exactly what's going on. So, you devious devil, you put detectors at each slit so you can actually detect each electron as it passes through the slit. The devices do not impede or alter the motion of the electron in any way. Thus prepared, you began shooting your electrons at the double slit card. Sure enough you can detect each electron clearly passing through one hole or the other, acting like particles and not the least bit wave-like. But, when you look at the target, expecting the interference pattern, you get a most unpleasant surprise.

Instead of the wave interference pattern, you have two clumps of strikes, exactly what you'd expect if discrete particles passed through discrete slits.

Neils Bohr explained this effect in what has become known as the “Copenhagen Interpretation.” He introduced the concept of complementarity, that is, particles act like both waves and particles, including things like electrons which are really neither. Moreover, in attempting to measure in the quantum world, the observer interacts with the system to such an extent that the system isn't independent any more. In other words, by choosing to measure the particle nature of, say, an electron, we wipe out the wave nature of that electron. We are said to “collapse the wave function.”

This is the Uncertainty Principle in action. Erwin Shrodinger, whose wave function equation was doing the collapsing, didn't like what was implied. The Copenhagen Interpretation was saying that, until you observed the electron it existed in a “superposition” of states, both wave and particle. Einstein was similarly distressed because this led to the concept of “spooky action at a distance.”

Imagine two electrons that are “linked” such that one has spin in one direction while the other must have the opposite spin. This is called “quantum entanglement” If you separate the two by some means, both electrons still have the possibility of either spin state. But which state is not determinable without observation, so essentially both electrons have both spin states. When you look at one and check its spin, the wave function is collapsed, and the other particle assumes the opposite spin. This happens instanteously, even if the particles have been separated by a light year, which means the state of the second particle has been determined by information sent at faster than the speed of light.

This is not some metaphysical construct. There is direct evidence of quantum entanglement and “spooky action”. See the Brian Greene reference below for an excellent discussion of the topic.

At any rate, all this stuff bothered Schrodinger enormously, so he posed his own conundrum. Supposed you had a box containing a radioactive source, a geiger counter, a vial of poisonous gas, and a cat. You rig the apparatus such that, if the geiger counter detects that the source has undergone radioactive decay, the vial is broken which releases the poison and kills the cat.

Now you can determine a time period over which there is a fifty-fifty chance for radioactive decay to occur. So you turn on the counter for this period of time. If decay occurs, the cat dies. If it doesn't, the cat lives to annoy you another day. According to the Copenhagen Interpretation, since the outcome is dependent on a probabilistic quantum state change, until you actually open the box to see what happened, the cat exists in both states, or neither, depending on your point of view. It's in a superposition of states that is all states until you make the observation by opening the box and collapse the wave function. (For a much more complete explanation, check out the John Gribbin book listed below).

You could well argue that all of this is playing with math and metaphysics, but a group of scientists is determined to see if superposition actually can be detected, without harming any cats in the process.

A University of Maryland team
headed by Keith Schwab has created a “nanoscale resonator”, essentially an incredibly tiny pendulum on a chip. On the same chip, these people have crammed a “single electron transistor” which changes its current based on changes in position of the resonator. With this device, they have demonstrated the Uncertainty Principle's “back effect.” Essentially, when you measure the position of the resonator, you alter its momentum, which causes it to heat up and act “noisier” than it would if the measurement hadn't been done.

Just another day at the lab, you say, because this effect has been observed often. But, using a technique known as electron tunneling, they can cause the transistor to absorb energy which causes the resonator to cool down from 500 millikelvin to 300 millikelvin. To put this in perspective, zero degrees Kelvin is absolute zero, where nothing moves. We're talking chilled here.

It is just possible that at this ridiculously low temperature, the resonator could enter a state of superposition, which the scientists hope to actually observe. In other words, they would see the resonator both “dead” and “alive”.

What they're looking for is the point at which classical physics gives way to quantum physics, a boundary that has thus far eluded experimental discovery. Think about this for a moment. The idea of superposition essentially means that all possible states exist simultaneously; until now, the act of observation has always caused the wave function to collapse into one state. So Mr. Schwab is looking into a realm that has only been dreamt of by theoreticians and science fiction authors.

Not to mention being experienced by Schrodinger's cat.

In Search of Schrodinger's Cat, John Gribbin, Bantam Books, 1984
The Fabric of the Cosmos, Brian Greene, Alfred A. Knopf, 2004

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