View Full Version : How long does a particle's position remain defined after it is observed?

2012-Nov-17, 03:15 AM

For example, in the photon double slit experiment, how long does a particle keep it's definite position after either being observed going through the slit or interacting with the wall? It seems that a particle can retain it's definite position at least long enough to get from the slit (when detected) to the wall where it leaves it's pattern. Or does the particle now have a defined position permanently?
Hopefully that makes sense!


2012-Nov-17, 03:39 AM
It depends on your setup.

If you do the classic double slit and measure particles passing through one slit, the interference pattern disappears. The smearing effect of knowing the position seems to be permanent.

But wait! There's more. What if our screen we measure the pattern on, itself, has two slits in it? Then, there will be no interference pattern on the first screen, but what passes through the new slit creates one further down.

In essence, it's permanent, if you're able to include all possible sources of measurement and diffraction. It technically happens before the particle even covers the path due to the melding of different amplitude functions. Like saying it's a wave and interferes with itself as a wave, until you measure it as a particle, then you destroy a whole bunch of possibilities which existed before.

A good example is Feynman's lectures on reflection:
He touches it at 47:00, but better to start at 46:00.

2012-Nov-17, 07:28 AM
You can think of it a bit like this:

A wavefunction describes the time evolution of a system from its initial state. This initial state may have a lot of uncertainty. So between the source and the measurement apparatus the wavefunction has a lot of fuzziness, a lot of possible states it could be in. It is describing a photon which was emitted somewhere in the source with an energy in the range allowed by the source. When you measure the photon's position at the apparatus you fix its position. From that point the wavefunction describes a photon that was emitted somewhere in the source, has a range of energies as allowed by the source and was at point X and time T. The number of states consistent with this is smaller than were available to the original wavefunction. The more measurements you make the more constrained or defined the initial state is so the fewer states are available to evolve into.

So in the case of the double slit the interference pattern vanishes because you no longer have two whole sets of possible states able to interfere. Because you have constrained one set then they cannot interfere unless you arrange to 'lose' that information. This is the basis of the quantum eraser experiments (http://en.wikipedia.org/wiki/Quantum_eraser_experiment).