The joint is jumping over at Peter Woit’s Blog, even though his is even newer than mine. (Blog statistics are just what academia needs: another quantitatively precise and wholly meaningless measure of worth.) I suspect it’s because Peter tends to say provocative and controversial things that people readily disagree with (about emotional topics like, say, string theory), whereas I am so sweetly reasonable that everyone cannot help but agree with everything I say.

Peter did ask a question to cosmologists that I didn’t get to, so I thought I should take a swing: “What is ‘string cosmology’?” If the response were to make any sense, I should explain something about string theory, which means explaining something about quantum gravity, which means explaining something about ‘quantum’ even without gravity. I don’t know how far we’ll get, but explaining quantum mechanics is a worthy goal in its own right.

Quantum mechanics (QM) is one of the top two most profound ideas in the history of physics. The other member of the top two is classical mechanics, the system developed by Galileo and Newton and their friends, which was eventually superseded by quantum mechanics. (The ordering of the top two is tricky, and there’s no consensus on number three.) Nevertheless, QM is consistently misrepresented (or even misunderstood) by professional physicists, and its basic ideas aren’t nearly as clear to people on the street as they should be.

Classical mechanics is simple. For any physical system (balls on a billiard table, planets moving around the sun, the whole universe) you tell me the “state” that the system is in at some time, and I can use the laws of physics to predict what the state will be at any other time. Specifying the state typically means specifying the positions and velocities of all the components. This kind of system is at the heart of the “clockwork universe” that came out of the Enlightenment.

Quantum mechanics came about in the early 20th century. Surprisingly, the description of classical mechanics in the previous paragraph also applies perfectly well to quantum mechanics: you tell me the state, I’ll use the laws of physics to evolve it forward in time (or backward, for that matter). The crucial difference lies in a feature so profound that it’s hard to conceptualize: in quantum mechanics, what you can see (the observable properties of the system) is related to, but not the same as, what there really is.

So, for a single particle, classical mechanics tells us that it has a position and a velocity. The lesson of quantum mechanics is sometimes garbled into the idea that “we can’t be perfectly certain where the particle is or how fast it is moving.” The truth is more profound: there is *no such thing* as “where the particle is,” or “how fast it is moving.” Instead, there is something called the **wavefunction** that describes the state of the system. The wavefunction answers the question, “when we observe the system, what is the probability we will observe it to have a given position or velocity?” In classical mechanics we can observe anything we want about the state, but in quantum mechanics we can’t, we can only predict probabilities for what might happen when we make an observation.

What actually happens when we make an observation is the source of great philosophical angst. The old “Copenhagen interpretation” held that the wave function changed instantaneously and non-locally, into a state that was concentrated around the result of our observation. The newer (but still pretty venerable) “many-worlds interpretation” says that we the observers are also described by wave functions, and the measurement process mixes up our wave function with that of the thing we’re looking at in such a way that we only ever experience unique outcomes for observations, even though everything is evolving smoothly. As crazy as it sounds, many working physicists buy into the many-worlds theory (and, like approval for gay marriage, there is a significant demographic slant, in which younger people are more open).

Quantum mechanics is not so much a theory as it is a framework in which we can propose all sorts of specific theories. The most empirically successful are quantum field theories, in which the elements of our physical reality are fields defined on spacetime (quantities that take on values at every point, like an electric field). In quantum field theories, the actual field values are one of these unobservable things; what we can actually see is discrete excitations of the fields that we call “particles.” Quantum field theory successfully describes every experiment ever performed and every phenomenon ever observed, with one glaring exception: gravity. For a force that is so important, it’s truly embarrassing that we can’t fit it into our favorite framework. That’s why so many physicists think that the search for a consistent quantum theory of gravity is so interesting and vital.

P.S. (When reading Peter’s most recent post, please keep in mind the date posted.)