Sean Carroll writes:
Quantum Hyperion | Cosmic Variance: The orbit of Hyperion around Saturn is fairly predictable; happily, even for lumpy moons, the center of mass follows a smooth path. But the orientation of Hyperion, it turns out, is chaotic — the moon tumbles unpredictably as it orbits, as measured by Voyager 2 as well as Earth-based telescopes. Its orbit is highly elliptical, and resonates with the orbit of Titan, which exerts a torque on its axis. If you knew Hyperion’s orientation fairly precisely at some time, it would be completely unpredictable within a month or so (the Lyapunov exponent is about 40 days). More poetically, if you lived there, you wouldn’t be able to predict when the Sun would next rise.
So — is Hyperion oriented when nobody looks? Zurek and Paz calculate (not recently — this is fun, not breaking news) that if Hyperion were isolated from the rest of the universe [except for the gravitational pull on it by Titan and Saturn], it would evolve into a non-localized quantum state over a period of about 20 years. It’s an impressive example of quantum uncertainty on a macroscopic scale.
Except that Hyperion is not isolated from the rest of the universe. If nothing else, it’s constantly bombarded by photons from the Sun, as well as from the rest of the universe. And those photons have their own quantum states, and when they bounce off Hyperion the states become entangled. But there’s no way to keep track of the states of all those photons after they interact and go their merry way. So when you speak about “the quantum state of Hyperion,” you really mean the state we would get by averaging over all the possible states of the photons we didn’t keep track of. And that averaging process — considering the state of a certain quantum system when we haven’t kept track of the states of the many other systems with which it is entangled — leads to decoherence. Roughly speaking, the photons bouncing off of Hyperion act like a series of many little “observations of the wavefunction,” collapsing it into a state of definite orientation.
So, in the real world, not only does this particular moon (of Saturn) exist when we’re not looking, it’s also in a pretty well-defined orientation — even if, in a simple model that excludes the rest of the universe, its wave function would be all spread out after only 20 years of evolution. As Zurek and Paz conclude, “Decoherence caused by the environment … is not a subterfuge of a theorist, but a fact of life.” (As if one could sensibly distinguish between the two.)
But gravity works--presumably, at some level--by massive objects constantly bombarding each other with gravitons, so we are also averaging over all the possible states of gravitons that we are not keeping track of, aren't we? That should cause decoherence too, shouldn't it?
I am confused. Not as confused as I am about the powers of the Vice President of the United States as President of the Senate, but confused.
