How do we know they behave differently when they’re observed in general? I know they can tell they’re being observed by interacting with the detector but how do we know that their behavior changes?
In: Chemistry
Simply put, we observe (i.e. measure) them. We must measure the electrons at some end point; however, if we try to observe (measure) them earlier as well, it can significantly change the later measurement in a dramatic way. So in both cases we observe the electrons at the end but by adding an additional observation we can change the outcome. This means observing (measuring) them changed their behavior.
For example, an electron that has 2 paths it can take to the final detector (which can’t tell which path it took) makes an interference pattern that can only be explained by information from both paths. This leading us to believe it must have interacted with (or gone through) both paths. However, when researchers add a way to check which path each electron actually took, the interference pattern is gone and they get a pattern that looks like one would expect from particles that only look one path. In either case, we have to measure them in the end but the act of measuring them earlier changes the very nature of that final measurement.
And in fact, that information doesn’t even need to be collected. Just the fact that the information is out there somewhere, where someone could possibly (no matter how unlikely) use it to figure out which path the electron took, destroys the interference pattern. Interestingly, if you (or something else) somehow physically destroy this path information so it’s irretrievable, you get the interference pattern again. This is called the quantum eraser.
By the way, when physics say “observe” in this context, it’s not the general meaning like from an intelligent being, but rather it’s a measurement that reveals a particular trait of the particle. Such traits (called observables) include location, speed, and direction to name a few. This is to say, particles don’t have some supernatural sense of being watched by people.
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