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As you will all know, when one tries to detect which slit an electron has gone through with close up observation, it changes from behaving like a wave and producing an interference pattern to behaving like a particle and producing two lines on the screen.

Is there a full explanation as to why the electron changes its behaviour when observed?

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can't you just answer rather than downvoting? –  Olly Price Apr 2 '12 at 22:52
    
Related: physics.stackexchange.com/q/16711/2451 –  Qmechanic Apr 2 '12 at 23:12
    
probably it can be asnwer by the feynman's infinite possibilities proposition - whatever can happen happens. –  Vineet Menon Apr 5 '12 at 12:00
    
BTW, this behavior is not specific to electrons. We have some issues observing neutrinos, let alone building double slits for them, but I'd bet even they would behave this way. –  MSalters Apr 5 '12 at 12:15

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up vote 6 down vote accepted

The (never ending!) confusion about the double slit experiment exists because people don't understand what an electron is. It isn't a particle, and it isn't a wave - it's an excitation in a quantum field. The electron can interact with it's environment in ways that make it look like a particle, and it can also interact in ways that look like a wave, and whenever you interact with an electron you change it's state. So if you interact with an electron in a "particle like" way you change it so that future interactions are also going to return "particle like" results.

In the double slit experiment it doesn't make sense to ask which slit the electron went through, because it isn't a particle and didn't go through one slit. Arguably it doesn't even make sense to say it went through both slits, because that statement is still influenced by "particle like" thinking, but possibly this is getting excessivle philosophical.

Anyhow, you specifically asked "Is there a full explanation as to why the electron changes its behaviour when observed?" and the answer is "yes". In this context "observing" means "interacting with", and when you interact with an electron you change it's state, and therefore you change the way it can interact with the slits. If you interact with the electron in such a way as to localise it, i.e. pinning it down to one position then this change will change it's subsequent interation with the slits and prevent the interference pattern forming.

In principle you can choose a specific interaction, e.g. having it pass through a gas and ionise the gas molecules, and you can describe it's interactions with a gas molecule then the slits in a mathematically rigorous way. Well, you might be able to - sadly this level of detail is beyond my skills :-)

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That answer is unclear, and even sounds incorrect.

The fact is that in quantum mechanics we only know the mathematics, and the results of experiments. Quantum mechanics is one of the most solid theories we have, and it has made plenty of predictions we have later proved to be correct, over and over. We don't understand the underlying "logic" behind those weird effects, however. There are multiple "interpretations" that are offered to explain that quantum behaviour.

In the Copenhagen interpretation, for example, there is indeed a change of behaviour when you observe a quantum particle: it goes from wavelike behaviour to particle-like behaviour -- the so-called "wave function collapse". And contrary to what a lot of people struggling to understand quantum mechanics think, this wave function collapse is NOT caused by physical interaction with the measurement instrument. It is the mere fact of recording the state that causes this collapse. The measurement does NOT "bump" into the particle, or something like that, causing it to change behaviour by purely classical means. In any case, the Copenhagen interpretation is arguably scientifically untenable -- or at least incomplete, as it doesn't explain why the wavefunction collapse really happens. And it even involves a "fudge factor" whereby you ignore what the math is suggesting to you (see the video I recommend at the end of this answer, to fully understand what I mean here)

Another interpretation is the many worlds interpretation. It suggests that all possible outcomes of a wave function actually happen in a multitude of superpositioned "universes". When you observe the path taken by the electron, for example, you (and everything else that is already entangled with you -- possibly the entire universe?) enter an entangled state with that particle, and as a result the wavefunction applies to you AND the particle. In other words, you are simultaneously in all possible states of observing the particle in every single one of all of it's possible paths. That's why each you that is part of any one of those specific entangled states, is aware of only one outcome out of all of the possibilities: hence the particle-like behaviour you now observe.

There are a few other interpretations. But mathematically speaking what happens is that the observer enters an entangled state with the observed particle. As a result the whole "complex" of both the observer and the observed is in the superposition (or wave-like) state. This sounds a lot like the many-worlds interpretation, although it doesn't necessarily suggest that the latter is what the physical universe behaves like...

I recommend the YouTube video: "The Quantum Conspiracy: What Popularizers of QM Don't Want You to Know" (a Google Tech Talk). It explains all those things much better than I can.

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It is called Quantum Decoherence which explains why macroscopic world doesn't show quantum behaviors (like we can't exist at more than one place). All quantum behaviors exist due to the fact of isolation. We can observe consequences of those behaviors, but observation of real behavior isn't possible because it violates isolation. A single photon is good to intrude privacy.

In double-slit experiment, interference pattern is explained best with wave model of electrons (all denizens of quantum world are both particles and waves). For particle model, we need to introduce superposition of particles which means they can exist at more than one places. We can see consequence of superposition as interference patterns. But, when we try to observe this superposition in real, the superposition is lost because of Quantum Decoherence.

Why?
We don't know yet. Its like something is trying to keep quantum world secret.

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protected by Qmechanic Dec 29 '12 at 12:06

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