The Quantum Zeno Effect
Here’s something pretty cool…
Put a particle into an excited state. Then in a time very short compared to the lifetime of the excited state (i.e. so the probability of having decayed back to the ground state is very, very low) check to see if the particle is still in the excited state. This will collapse the wavefunction and (almost always, because you picked the timing that way) guarantee the particle is in the excited state. Rinse, lather, repeat, and you can keep the particle in the excited state indefinitely.
But what’s really cool is that you don’t have to do an active test (i.e. see if the particle is in the excited state) — a passive test works just as well. For example, you can send in a photon that can only be absorbed if the particle is in the ground state.
So you can basically keep the particle in the excited state by doing almost literally nothing — after all, in theory you could use photons that you were planning to use for something else. And since almost none of them will ever be absorbed, you can still use the photons for whatever else you were doing, but with the side-effect of keeping the particle excited.
Pretty neat.
February 26th, 2006 at 21:57
Huh?
If the chance of decay in any time slice is epsilon, but you check at 1/epsilon different time slices, during one of those tests you’ll suddenly find that the particle dropped out of the excited state, no?
Unless there’s something more clever going on here than you lay out (is it proven that a drop from state X to state Y takes more time than the sample interval allows?), this seems like nonsense.
(which doesn’t mean it’s not true - not only do all false QM things sound like nonsense, but so do 80% of the true things)
February 26th, 2006 at 22:40
The neat thing isn’t so much that the particle is kept from decaying for a probabalistically long time, it’s that you can do that without actually doing anything (in that the unabsorbed photon, which can, by virtue of being unabsorbed, go off to do other useful things). You’d naively think you’d have to be pumping energy into the system to keep the particle in the excited state for that length of time, but you don’t.