Double slit experiment observation on and off

[wes hunter]

Might I be so bold as to ask a question about the "double slit experiment". Was wondering...have scientist ever considered...turning the observation on and off...? Is it even possible...? Experiment says...when observed...particles react one way...and when not observed...they react a different way. Have they ever tried turning the camera 'on and off'...throughout...?

 

[bhobba]

Experiment says...when observed...particles react one way...and when not observed...they react a different way.

That's not quite what it says.

QM is a theory about observations. When not observed the theory is silent. Switching it on and off will tell us nothing new because what's going on when not observed it doesn't say.

Thanks
Bill.

 

[Nugatory]

The word "observation" may be misleading you here. What matters is whether there is any interaction of any kind (for example, the particle collides with a random air molecule floating around somewhere between source and screen, or light shines on it, or there is a detector in one ofthe slits, or a polarizing filter behind one of the slits, or... ) whose result requires that the particle had to go through a particular slit. If there is, then the particle will behave as if only that one path was available to it. If there is not, then the particle will behave as if both paths are available to it.

The experiment in which sometimes both paths are possible and other times only one is possible has been done many times, and always matches the quantum mechanical prediction: An interference pattern is formed by those particles that could have gone through either slit and is not formed by those that could not. It's worth mentioning that you can't see this with a single particle, as each particle just makes a dot on the screen either way. To see the interference you have to run the experiment with a large number of particles and watch the individual dots build up the pattern.

If you google for "Kim delayed choice quantum eraser" you'll find some good descriptions of one of the more interesting experiments along these lines.

And if you are wondering why we keep on hearing about "observations"... It's historical. Physicists were using that word early in the 20th century when these concepts were first being thrashed out. By the time is became clear that "observation" was the wrong word it was too late - that usage had caught on in the popular press.

 

[dlgoff]

... the particle will behave as if only that one path was available ... the particle will behave as if both paths are available ...

That's a good choice of word(s) IMO. I don't think I've ever heard it said that way before. But maybe I just haven't read enough.

 

[Nugatory]

That's a good choice of word(s) IMO. I don't think I've ever heard it said that way before.

I picked it up from @DrChinese who explains the double-slit experiment in terms of the "availability" of which-path information.

 

[DrChinese]

I picked it up from @DrChinese who explains the double-slit experiment in terms of the "availability" of which-path information.

Ha! I probably learned the idea from you first...

 

[DrChinese]

Might I be so bold as to ask a question about the "double slit experiment". Was wondering...have scientist ever considered...turning the observation on and off...? Is it even possible...?

Not surprisingly, this has been done (assuming I understand your question correctly). You place perpendicular polarizers in front of each of the 2 slits. The polarizer acts to tell you which slit a photon goes through - but only because of the polarizers' relative positioning (as perpendicular). So there is NO interference pattern. If you rotate a polarizer so they are now parallel: there IS an interference pattern.

In both cases, there are polarizers in the path. If observation was as simple as having a polarizer present, neither case would yield an interference pattern. But that's not what happens. So it is more complex than that, as Nugatory, bhobba and dlgoff allude.

You can therefore alter the setup so you have 100% knowledge of which slit the photon went through, 0% knowledge, 50%, or any value between 0 and 1. This is done by rotating the polarizers to a suitable relative positioning. You can turn it on and off as you like without otherwise disturbing the setup.

 

[naima]

A question about path information. If you know that the particle is near the slit, this information is complete, if it is localized on the screen it is very small or null. Is there a function of the position giving the value of this information?