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Q & A: Does looking change the outcome?

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Most recent answer: 03/21/2016
Hi, the observer effect is haunting me and I really wish we could all play with some sort of reasonably sized and priced observer effect machine. To help me better assimilate this concept can we talk about it as a thought experiment like Schrodinger cat around a hypothetical burglar alarm founded on this principle. Let�s call it �The Someone-Looked-Omatic Burglar Alarm�.It might be setup like this:The particle version of the double slit device is setup with a slit detector so that whenever a particle is detected going through one of the slits a light bulb inside a large vault flashes. The particles are shot at the rate of 2 per second. Thus about 1 time per second the light inside the vault should flash (with an observer) but it will be random. Outside the vault is a detector that can distinguish between the resulting pattern of fringes / no fringes. When the vault door is closed the state of the light (observed slit) is unknown (right?) so the apparatus should then result in a fringe pattern. If the door is opened by someone and the state of the detector light becomes "known" then the fringe pattern should disappear. Is this experimental setup possible?
- Jacques Williamson (age 48)
Phoenix, AZ

I'm not sure I totally understand your experimental design, but the short answer is that it doesn't matter whether someone actually "looks" at the which-slit information or not. The situation is the same whether you open the box or leave it closed—if the which-slit information is attached to each photon, interference won't happen.

Here's the rule: if there's no information we could use to tell the photons apart, we say they are "indistinguishable," and will interfere. If there is some information we could use to tell them apart, they are "distinguishable," and will not interfere. Exactly why this is true is a matter of interpretation in quantum mechanics, but many experiments have confirmed it.

Interestingly, even if you do mark which slit each photon went though, you can still erase that information afterwards and see interference again. For example, you could put a horizontal polarizer over one slit and and a vertical polarizer over the other, so that the polarization of the photons would mark which slit they went through. This destroys the interference because the photons are no longer indistinguishable. But if you put a polarizer at 45 degrees ("diagonal," halfway between horizontal and vertical) after the slits, you can erase the which-slit information. Horizontal and vertical photons both have a 50% chance of being transmitted through the diagonal polarizer, and they both emerge with diagonal polarization. The photons are now indistinguishable, and will interfere and show fringes. This kind of experiment is called a "quantum eraser" and has been done in the lab in lots of different ways. It doesn't have anything to do with whether a human observer looks at the outcome or not (as far as we know, that doesn't ever matter) but it's still a cool property of interference.

Rebecca H.

(published on 03/21/2016)

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