# Single Photon Travel

1. Oct 12, 2004

### primal schemer

Hi All,

Just wondering about something. First of all, if I have a light source that emits a single photon at a time, and a machine that can detect photons, am I right in assuming that the photon can only be detected by the detecting machine in one area. For example if the light source is in the middle of the room, pointing left, then the detection machine must be on the left hand side of the room, "looking at" the light source in order to detect the photon??

If this is correct, am I also right in saying that the photon can take any path from the light source to the detector, and that while the probabity is huge that it will go in a straight line to the detector, it may follow any path???

Ok, 2 questions. Firstly, Why can it go in any path?? (I think that this is an unanswered question in physics, but I thought Id ask it anyway)

And secondly, I read somewhere (cant recall where) that all paths cancel each other out to ensure that light almost always travels in the straight line. How does this happen???

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Now, another scenario for the same setup. This time in between the light source and the detector, there is a barrier with only a tiny gap directly in between the source and the detector (that is you can draw a straight line between the source and the detector through the gap). Light now doesn`t travel in a straight line (you would have to move the detector down to detect it). Is this because all other possible paths cannot cancel eachother out (due to the barrier)?? or is there some other reason.

Help appreciated,

Primal schemer

2. Oct 13, 2004

### Mk

Hmm, very interesting questions, for the second I think you are mixing up with electricity; electricity takes all paths there. This may be due to the whole electromagnetic force, photon and electron thing. Hope I could help.

3. Oct 13, 2004

All paths are indeed possible, but as you say, the 'alternative routes' cancel out, making light appear to go in straight lines.

If you remove some of the possible routes with a thin slit or a diffraction grating, the light can be seen to go in different directions.

(Feynman's QED is the best book you can get if you want to understand this further.)

4. Oct 14, 2004

### Mk

Well, if you had a vacuum chamber the width of one photon, and however long you want, and the chamber was made of the photon detectors, then what would come up?