# The Zeilinger experiment being FTL?

1. Nov 14, 2015

### SlowThinker

Reading the recent Insight series on Block Universe, Blockworld and its Foundational implications Part 5, the second paragraph talks about an experiment where a photon detected earlier is affected by the position of a detector far away:

First, let me round the focal length of the lens to 1 meter.
The way I understood the article, I could install 2 detectors D1 (at 1m and at 2m), and by turning on one or the other, I'd be affecting the pattern at D2. But if I position myself midway between those detectors, I only need time 0.5/c to activate a detector, while D2 is 3.5 meters away.
So I need time 0.5/c to send a message 3.5/c into the past?
The message being, which of the 2 patterns are being detected.

The article does not really go into details about the "Single counts Coincidences" and "Conditional counts in D2", perhaps the explanation is hiding there?

2. Nov 14, 2015

### zonde

Yes, that's right. If you count all the clicks in D2 you never get interference pattern. You can see interference pattern when you consider only the clicks that have matching click in D1 (placed at f).

3. Nov 14, 2015

### Heinera

Yes, that's where the explanation is. The interference pattern is conditional, i.e., you will only see it if you count only the photons for which the second photon was registered. This information about the second photon can only be transmitted at the speed of light or less (that is why you have this line connecting the detectors in the figure). If you include all photons unconditionally, you will not see an interference pattern.

(Edit: zonde beat me to it.)

Last edited: Nov 14, 2015
4. Nov 14, 2015

### SlowThinker

Now I see that the two density profiles have a very different scale on the x-axis.
At the first sight it looks like these can be distinguished very well, since there is no way to add more photons to the lower image to make it look like the upper image. But with the different scales, it's possible. So that's the answer, thanks.

5. Nov 14, 2015

### Staff: Mentor

Sure it is - you fill in exactly the missing gaps if you don't check for coincidence.