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What if the measurement device in the double slit experiment were a light year away? (self.askscience)

submitted 1 year ago by AB7ORH7D to c/askscience

8 comments fedilink hide all child comments

In the famous double slit experiment, setting up a measurement device that watches which slit an electron passes through changes the eventual outcome on the screen, causing the wave function to collapse.

If the measurement device were a light year away and were precise enough to "zoom in" and see which slit the electrons went through, what would happen on the final screen?

Surely, if the measurement device were off, then the electrons would behave like waves and not particles. An interference pattern would appear on the final screen as there is no observer. On the other hand, if someone a light year away turned on the measurement device, this far-away person (and the measurement device) wouldn't know which slit the electrons went through until a year later. And, the electrons going through the slits wouldn't "know" they are being observed because no information (ie the measurement device turning on) can travel faster than the speed of light.

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[–] Brokkr 15 points 1 year ago

The "observation" doesn't occur when a person sees the result, but rather when the electron or photon interacts with the device (in this case the wall). The wall is making the observation. In this situation "observation" doesn't have the traditional meaning, but rather refers to an interaction event.

So the same average result will happen no matter where the device is, the only thing that changes is its proximity to you.