# Which slit/way at time of observation or passing slits?

1. Dec 11, 2011

### San K

in a single particle, double slit experiment

is there any way/experiment/argument that can tell if

the which-slit decision/finalization is made at the time A or time B below:

A) when the photon is observed (by a telescope etc), say a few microsecond after it has passed through the slits
or
B) when the photon encounters the slits

2. Dec 11, 2011

### Joncon

I'm not sure how experiments could confirm this one way or another but I believe the which slit "decision" is made when the particle is observed. In fact, the particle never travels through just one slit, it's just detected as if it did.

http://www.mathblog.ellerman.org/2011/11/a-common-qm-fallacy/ [Broken]

Last edited by a moderator: May 5, 2017
3. Dec 11, 2011

### San K

well said and good link, thanks Joncon.

I wonder if a photon goes through both slits does its energy/size/frequecy/amplitude halve? (In the two new wavefronts that emerge from the two slits)

or is it that just the probability gets "distributed"?

for example in mach zender when we block one path, we still record the photon with same energy, if it went through both paths then a layman would expect "half of the original photon energy" to be detected.

Last edited by a moderator: May 5, 2017
4. Dec 11, 2011

### Joncon

How could we ever know? Does a photon actually HAVE an energy/size/frequency/amplitude before it's detected?

It would seem to be an incredibly inefficient way for the universe to operate, if it had to split the photon up into an infinite number of parts (corresponding to the infinite number of paths it could take) and then join them all back together when it's detected!

5. Dec 13, 2011

### San K

It seems the photon carries "memories/states/factors" for both interference and non-interference at all times prior to detection/collapse.

6. Dec 13, 2011

### StevieTNZ

The context of the experimental set-up determines whether the particle travels via both or one path. (Or so I believe)

7. Dec 14, 2011

### Ken G

Here's another way to think about it-- rather than imagining that the photon has to wait to know what questions we are asking before it can decide what to be, just recognize that physics is our way of, in essence, communicating with nature. So it is not the photon that needs to know what question we are asking, before it can answer-- it is we who need to know, when we receive an answer, what question we were asking that some detection is the answer to. To me, the way quantum behavior depends on the apparatus is a clear indicator of the role that our analysis approach is playing in the outcomes of quantum mechanics.