Thursday, March 11, 2010

Wigner's friend's cat

Of all the science concepts you've heard a dozen times and still don't know, probably the worst offender is Schroedinger's Cat. How many times has someone on a TV show explained it to someone else (and thus, to you)? Odds are, every time that happened, they told it wrong, and gave it the wrong point. To explain why, let's go back a step, and visit with Thomas Young.

The double slit experiment is simple. You have an impermeable membrane with two slits cut into it. On one side, a source of light; on the other, a screen.

If light is made of particles, then you expect to see two bright dots. Don't believe me? Imagine the same experiment but with a man throwing baseballs at a wall with two holes. While one or two baseballs might bounce in an odd way now and then, the vast majority will land in two spots. But if light is made of waves, you'll see an interference pattern as depicted above, and just like if this were done with water waves in a tank, because the waves spread as they pass through the slits and then interfere with one another.

Thomas Young used this experiment to "prove" that light was a wave, in 1803, settling a debate that went back to the time of Isaac Newton versus Christain Huygens (in Huygens' favor). And light remained a wave until a guy named Einstein had something to say about the photoelectric effect, but that's another story. What's really important here is something that comes along much later.

Take this same experiment, and make it so the light source is so very, very weak, that only one photon's worth of energy is being released at a time. So there's only one "chunk" of light in the experiment at any given moment. You still get the interference pattern. That single piece of light can interfere with itself. But that's not where it gets really weird.

Now fit the slits with a passive sensor that can tell you if the light went through one side or the other. This sensor does not deflect, absorb, or otherwise impact the light directly in any way other than by passively observing it, measuring it. And yet, the moment you add it, the experimental outcome changes. The interference pattern vanishes and you get two bright spots. The mere act of knowing where the photon went forces the photon to have only gone one place; as long as you don't know, it doesn't choose, but goes both ways, and thus interferes with itself. But wait, it gets weirder.

Put the light source very far away, so it takes several seconds for it to get to the apparatus. Release the photon, and then while it's in transit, flip a switch which decides whether the sensor in the slit is activated or not. The photon will change its behavior retroactively based on whether the sensor is there at the moment it arrives at the slit.

In a way that science is only beginning to glimpse, information is a fundamental building block of physics. Changing information is just as impactful on the world as changing atoms.

Where's the cat I promised? Well, Schroedinger, quite understandably, found this property of photons to be inexcusable. Physicists put up with it because it was subatomic particles doing this, and they could handwave away the effects on the macroscopic world -- the world of billiard balls and people and trains -- because all those infinitesimal immeasurabilities and uncertainties cancelled out statistically in the aggregate of the trillians of atoms in a single object. So you could still measure the path of a bullet and figure out where it would hit. In a way it's analogous to how Einsteinian relativity proves that Newton was wrong about kinematics, but you can still calculate the orbits of the planets, because at speeds much lower than the speed of light, the difference was too small to measure anyway.

So Schroedinger set out to show the science community that it wasn't like relativity: the problem was not something that magically went away in real world examples. He invented a "reductio ad absurdem" argument to demonstrate it. Place a cat in a box, with a single particle of some radioactive substance set to a detector. If the particle decays, the detector releases poison and the cat dies. If it doesn't, the cat lives. Since radioactive decay is, according to quantum physics, random and subject to superpositions of states (just like how the photon could be in either slit, so was therefore in both slits until we measured which one it was in), the particle was both decayed and not decayed, and thus, the cat alive and dead, the two states "interfering" with one another the same way the light waves interfered and caused a pattern on the screen.

His intent was to show, through the absurd consequence of the thought experiment, how there was a problem that needed to be answered. And many physicists generally take it that way. But some scientists, and virtually every screenwriter in the world who hears about it, takes it as being a real consequence of quantum physics, suggesting that there's some mysticism lurking just beneath the surface. So easy is it for people to miss the point that even one of Schroedinger's contemporaries, Wigner, made the same mistake. He posited an experiment in which his friend conducted the Schroedinger's cat experiment, looked inside the box, but hadn't yet told him the results. So, Wigner concluded, his friend was also in a superposition of states, and isn't that absurd? Therefore, he concluded, Schroedinger's idea makes no sense. Schroedinger must have groaned and said to himself, "duh, that was my whole point!" (Only in German.)

In fact, as we understand it, the detector that was attached to the poison vial would be sufficient to trigger the collapse of states, so the cat isn't in any superposition. Just like the slit sensor collapses the photon even before a person reads the output screen attached to it. A lot of people have put a bunch of mumbo-jumbo about a special role for consciousness into quantum physics (and there are other places where such ideas are worth consideration), but superposition of states doesn't depend on a conscious observer, just an act of measurement.

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