Understanding the Double-Slit Experiment: Photon Direction and Predictions

[glederfein]

I have been studying the double-split experiment and read different descriptions of it, but there's something I don't quite understand about the actual setup of the experiment:

In the version of the experiment when photons are fired at the slits one-by-one:

1. Are the photons fired at random directions?
2. If I measured the direction in which each photon is fired, would I be able to predict through which slit the photon will pass and therby alter the result?

 

[naima]

If you use a laser to fire photons in the direction of the left slit you will get no interference

 

[dauto]

I have been studying the double-split experiment and read different descriptions of it, but there's something I don't quite understand about the actual setup of the experiment:

In the version of the experiment when photons are fired at the slits one-by-one:

1. Are the photons fired at random directions?
2. If I measured the direction in which each photon is fired, would I be able to predict through which slit the photon will pass and therby alter the result?

It's possible to collimate the beam so that you can be able to tell which way the photon went, but if you do that the interference pattern is lost. To create an interference pattern the beam must be wide enough (the position must be uncertain enough) that you simply cannot tell which way the photon went. The rule of thumb is: if you can tell which way the photon went (no matter how), than you get no interference pattern. That happens because the localization of the photon makes its momentum (and wavelength) so uncertain that interference cannot happen.