3 ms·

This doesn't seem very convincing considering lasers expand too.

• teekert 11y ago
  But not backwards. The photons have a direction. Perhaps we need physicist to comment :)
• tinco 11y ago
  Isn't this just because not all of the laser is perfectly parallel? The household laser is created by exciting atoms to emit photons in a cylinder with mirror caps. One of the caps is 'perfect', the other is 99% reflective. The photons go in random directions, but will bounce around within the cylinder until they get absorbed (and re-emitted in a random direction). Photons that are (near) parallel to the cylinder have a much higher chance of going through the imperfect mirror. Thus what shoots out the end is a (near) parallel stream of photons.