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In the double-slit experiment, if you shoot particles through the slits one by one and observe which slit they travel through, is there still an interference pattern on the screen behind the slits? If not, then how is our observation of the particle different than any other object in the universe being affected by it? Doesn't our observation of an event have essentially the same effect as any other object in the universe being affected by the event?

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No, if you observe which slit they traveled through then there is NOT an interference pattern. The act of observing, or more accurately, the need for the location of the electron to be resolved causes it to take on a definite position and then continue on from that position as a particle. If it is not observed or interacted with in some way that would make it's position relevant to subsequent events, then it's position is not resolved and it continues on as a probability wave, which results in an interference pattern.

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The interference pattern indeed appears also if you send the quantum particles one after the other through the double slit. This essentially is the proof that the interference pattern is not an effect of several particles interacting with each other.

But yes, it is not specifically the observation by a human (or other conscious being) which kills the interference, but merely the availability of the information in principle. That is, if your particle interacts with something else, and the interaction is of a way that the other object differs substantially whether your particle went through the left or right slit, then your interference pattern disappears. You can even make the interference patternm just less pronounced by having the difference in the external object sufficiently small; that is, there is not really a binary "interference pattern or no interference pattern" but you can gradually tune between full interference pattern and no interference pattern.

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Strangely enough, when you do a quantum mechanics experiment, you get a result that says something about what you already know. If you place a detector at one (or both) of the slits, you watch individual particles go through the slits, and you get a particle result from your detector (e.g., no interference pattern). If you don't know which slit the particles are going through, you get a wave result (an interference pattern), which essentially tells you that you have no information regarding which slit the particles are going through. This type of experimental result has bothered physicists since the very early days of quantum mechanics, but it implies that for a quantum mechanical experiment, the experimenter is part of the experiment whether he wants to be or not.

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  • $\begingroup$ Hi David, it is site convention not to add the name of the poster at the end of any post. $\endgroup$
    – Gaurav
    Commented Jul 27, 2015 at 6:19
  • $\begingroup$ Gaurav, I've never visited this site before, so I was unaware. Thanks for the information. $\endgroup$
    – David White
    Commented Jul 27, 2015 at 15:32

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