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I saw this video of the double slit experiment by Dr. Quantum on youtube. Later in the video he says, the behavior of the electrons changes to produce double bars effect as if it knows that it is being watched or observed.

What does that mean? How is that even possible? An atom knows if it is being watched? Seriously? Probably, likely are the chances that I dint understand the video?

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That video has prompted questions here before. The first half of it is pretty standard explanation of quantum mechanics for laypeople, but at some point in veers off into new age woo and silly quantum mysticism. The basic answer is that QM describes the way the universe works very accurately. It is futile to assign wacky philosophical explanations to it. The universe will do what the universe will do, and QM is simply a description of it's behavior. –  Colin K Nov 8 '11 at 22:11
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Don't let Dr Quantum touch you there... He's not a real doctor. –  Mikhail Nov 9 '11 at 3:51
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remember what Dr. Feynman said about QM...If you think you understand QM, then you didn't understand it! –  Vineet Menon Nov 9 '11 at 4:47
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Before I attempt to answer your question it is necessary to cover some basic background, you must also forgive the length but you raise some very interesting question:

There are two things that govern the evolution of a Quantum Mechanical (QM) system (For All Practical Purposes (FAPP) the election and the double-slit/Youngs apparatus you mention I will take to be a purely QM system), the time evolution of the system (governed by the Schrödinger equation) which we will denote as $\mathbf{U}$ and the State Vector Reduction or Collapse of the Wave Function $\mathbf{R}$. The Schrödinger equation describes the unitary/time evolution of the wave function or quantum state of a particle which here we will denote as $\mathbf{U}$. This evolution is well defined and provides information on the evolution of the quantum state of a system. The quantum state itself, expresses the entire weighted sum of all the possible alternatives (complex number weighting factors) that are open to the system. Due to the nature of the complex probabilities, it is possible for a QM system, like your electron traveling through the Youngs apparatus, to be in a complex superposition of multiple states (or to put it another way, be in a mixture of possible states/outcomes that the given system will allow).

For your system lets assume for simplicity that there are two states $|T\rangle$ the state associated with the electron going through the [T]op ‘slit’, and $|B\rangle$ the state associated with the electron passing through the bottom ‘slit’ (for simplicity we will ignore the phase factors associated with the QM states. See here for more information about the phase factor associated with Quantum States). So, just before the electron strikes the wall it is in a superposition of states $\alpha|T\rangle + \beta|B\rangle$, where $\alpha$ and $\beta$ are complex number probabilities that represent the likely hood of the particle being in the respective states. Now, in order to determine which path/’slit’ the electron actually took (either $|T\rangle$ or $|B\rangle$) we have to make some kind of ‘observation’/measurement (as was pointed out above). This measurement is what causes process $\mathbf{R}$ to occur and subsequently the collapse of the wave function which force the superposition of states $\alpha|T\rangle + \beta|B\rangle$ to become either state $|T\rangle$ OR $|B\rangle$. It is this QM state reduction or wave function collapse caused by process $\mathbf{R}$ that invokes all the mystery and the very strange nature of QM. There are numerous paradoxes (EPR-Paradox, Schrödinger’s cat etc. see here for an overview and some background) that stem from this measurement procedure/problem. At this point I can now address your questions: “What does that mean? How is that even possible? An atom knows if it is being watched? Seriously? Probably, likely are the chances that I didn’t understand the video?”

So it is the process $\mathbf{R}$ that causes this issue so you are right to ask what does it mean when someone says “it knows that it is being observed”. To answer the above I will ask one of my own questions: “Is $\mathbf{R}$ a real process?”. I ask this because there are two ways of viewing $\mathbf{R}$. Some physicists view the collapse of the wave function and the quantum superpositions of complex probabilities (the use of state vectors) as real physical properties, others do not (even Dirac, Einstein and Schrodinger himself, did not take the probabilistic view of QM as serious view of what was actually happening in reality, rather they took it as a mathematical formalism that allowed these physical processes to be predicted). If you are to deem the state vector as a real entity then you must accept the consequential blur between what happens at the quantum level and what happens at the macroscopic/large scale level. This leads to the Feynman’s multiple history view of QM where all of the possible outcomes of a QM system occur and this itself leads to the “Many-World” interpretations of QM. I for one (along with the like of Penrose, Einstein etc.) believe the current picture of QM is not complete and that there is some physical process causing the collapse of the wave function.

The wave function collapse is what causes the electron to choose a QM state, and the act of observation/measurement does seem to cause this collapse. However, this give rise to the question “Is it the act of human observation/consciousness that causes this collapse?”. It is impossible to argue this is the case. To go into more depth I will have to bring in the idea of quantum entanglements, which is essentially what was described above as a superposition of two QM states. These entanglements are what “collapse” when observations/measurements are made and are what constitute $\mathbf{R}$. So the real question is what causes dis-entanglement of two superposed states. There are some very interesting theories that postulate that the state vector reduction is gravitationally reduced and not the act of any observation. These ideas also have a bearing on the question of human consciousness! These details and in depth discussion on this subject can be found in the very accessible book: “Shadows of the Mind” by Roger Penrose.

I hope this was of some help.

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The video shows that the interference pattern goes away when one tries to measure which slit the electron went through. The point is that in order to measure which slit the electron went through, one must disturb the electron (shoot some light at it, for example). And amazingly, this interaction is enough to destroy the interference pattern. In some sense, though there is still some mystery about this. One says that the measurement (which implies an interaction) collapses the wave function (which describes the electron motion). The double slit experiment is a good place to start to get into the strange world of quantum mechanics!

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simple and clear....I guess the narrator wanted to convey the simple fact about uncertainty principle! –  Vineet Menon Nov 9 '11 at 4:48
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