Here's the latest entry in our "ask the physicist" series. How do you find out if Schrodinger's Cat is alive or dead, for real?

Yesterday, we talked about why the speed of light is the ultimate speed limit, and before that, I talked about the expansion of the universe.

Today's question comes from Bill-H, who should feel free to email me to get his free copy of "A User's Guide to the Universe." He asks (among other things):

Regarding collapse of a wave function - a quote in a Grant Callin book has the aliens saying to us "So you have developed an entire physics based on the premise that a sentient observer is required to collapse a wave function? How delightfully egotistical! How amusing!"

Questions:

1. What are the characteristics of an "observer" needed to collapse a wave function? Is a cockroach a qualified observer for the purpose? A paramecium? A virus?

In my first two entries, I've gone for questions that physics actually has an answer to. If you're still paying attention at this point, you're about to be rewarded with a frank admission that we (that is, Physicists) have no real idea how wave functions collapse.

Here's a ten-second quantum mechanics course. At a fundamental level, everything in the universe behaves like a little probability wave. Particles are literally in many places at once, each with some probability. Take an electron and fire it at a screen with two slits cut through it, and astonishingly, the electron will go through both slits simultaneously. But suppose you set up a pair of cameras to monitor *which* slit the electron goes through and suddenly – poof – the "wave function collapses" and it really goes through only one of the two. Somehow observing the system directly affects it.

I'm going to anticipate a couple of smarty-pants objections right off the bat:

**1.** You might have this idea that an electron is like a microscopic Tasmanian Devil that's just jittering around so quickly that we can't say for sure where it is. That's not how it is. As far as we can tell, there's absolutely no way to know which slit an electron went through unless we're directly observing it.

**2.** Someone is going to point out that when we "observe" one slit or the other, it really means that we're firing little energetic photons at it. It's not surprising that that will affect the system. Well have I got news for you! In 1978, John Wheeler proposed his "delayed choice" experiment in which a detector was set up to observe the slits *after* a particle had already passed through it, and this experiment was actually performed a few years ago. It turns out that you can retroactively force the wave function to collapse. That is messed up.

You don't experience quantum weirdness in your everyday lives, so it takes some work to concoct an example where it shows up. The most absurd case, and certainly the most famous, is Schrödinger's cat. The idea is that some sadist traps a cat in a box with a vial of poison and a (randomly decaying) radioactive isotope. If the isotope decays, the cat dies. If it doesn't, the cat lives. But I can assure you that it will be most resentful for the rest of its days. The only way to collapse the wave function (and retroactively force the isotope to have decayed or to have not decayed) is to open the box and find out.

This is pretty embarrassing. What's so special about the observer? Do you need a Ph.D., or will an undergraduate degree do? Isn't a cat a good enough observer? Mine observe me all the time.

The weirdest thing about all of this is that you're *made of* quantum particles. The entire universe should actually have one giant, ridiculously complex wave function, and you, me, the cat and the electron are all part of it. It's kind of like the force.

You're probably thinking that we don't collapse the wave function at all when we open the box. We're just finding out whether we have to have a cat funeral or not. And you're not the only one. Schrödinger wasn't seriously proposing the cat experiment. It was really his not-so-subtle way of mocking the "Copenhagen interpretation" of quantum mechanics, the one you normally hear about.

If the Copenhagen interpretation bothers you, there are other options. David Bohm's "Causal Interpretation" is based on the idea that regardless of whether you observe it the electron really does go through one slit or another; the cat is dead or alive, and opening the box just informs you, not the universe. There is no collapse of the wave function; there are just hidden variables that we can't measure.

Another fan favorite is Hugh Everett's "Many Worlds Interpretation." The basic idea is that we live in one universe of (infinitely) many. Every quantum possibility branches off into a new parallel universe. In the double-slit experiment, the electron goes through one slit or another, and the interference comes from the interaction with electrons in other universes. This being io9, I'm pretty sure you're all familiar with the concept.

We can be so cavalier about interpretations because all of them produce identical predictions in our universe. Quantum mechanics has allowed us to built very effective circuitry and the like without ever needing to worry about the philosophical implications. The physicist David Mermin probably summed it up best: "Shut up and calculate."

Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (Wiley: 2010). He is an associate professor of Physics at Drexel University. He is in the process of answering questions from readers on io9.

## DISCUSSION