What if the measurement device in the double slit experiment were a light year away?

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.

Brokkr ,

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.

krayj , (edited )

There is a great article on space.com that covers this exact scenario.

space.com/667-quantum-astronomy-cosmic-scale-doub…

First, though, your premise is a bit off. Zooming in still wouldn’t change the speed of light or change how fast the photons take to get from point A to your zoom lens. Zooming doesn’t give you a time or distance shortcut - all zooming does is decrease the angle of view of whatever you are pointied at. The only thing that matters in the double slit experiment is whether you observe them enroute or if you observe the screen after impact. If the screen were between you and the photon sources and you zoomed in, the photons would still hit the screen first and the photons you observe through the lens would come after.

The TL/DR of that article I cited earlier is that we still know the field would collapse. The more interesting question (and the one they pose in the article that remains unanswered) is: how fast does the collapse propagate back to the source? Is the propagation delay of the collapse instant/infinite (like what would be described by entanglement) or is the speed of the collapse still subjected to the speed of light (which is the same for the propagation delay of gravitational waves)?

jesterraiin ,

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?

That level of precision implies technological advancement, that would result with far better equipment to perform the test and measure the results, providing they would be still needed.

I know it sounds like an attempt to brush the question off, but it’s Spherical Cow and/or Newton’s Flaming Laser Sword territory. 😉

AbouBenAdhem ,

A measurement device is necessarily local: if it’s “zooming in” from a light year away, it’s using transmitted particles to observe—and those particles are traveling (and entangled) with the particles you’re trying to observe.

echo64 ,

When you say zoom in, what you are actually asking is what if the wall was a light year away, and you’re building the delayed choice version of the experiment, details here …m.wikipedia.org/…/Delayed-choice_quantum_eraser

But basically, the universe knows, and you can’t worm your way around it.if you detect which slit the photons flow through, then you lose the interference pattern.

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