When you encounter quantum mechanics for the first time, one of the hardest things to wrap your mind around is the collapse of the wave function. To make the strangeness of this idea lets say that the state of the wave function describing its position is a superposition of several basis states. For the sake of argument say that each basis state describes the position of the particle at different locations. So we could imagine that prior to measurement if the wavefunction was a superposition of a large number of states you could say that in some sense the particle was distributed throughout the room. Then you make a measurement to find out where the particle is, and detect it somewhere, lets say by the professors desk. The act of measurement causes the wave function to "collapse" to that particular basis state. Then it begins evolving again if you leave it alone for a bit.
The collapse is easy enough to describe if you're talking mathematics. But conceptually it sounds pretty wacky, if not impossible. If a neutron passes through the room you are sitting in right now, surely it isn't in 1,000 places at once in the room? Then you look at it and BAM it just collapses down to one specific location? You can kind of understand this, I suppose, by thinking about the wave nature of a quantum system, so a wave can be widely distributed throughout space.
Well lately in quantum theory people have been talking about so-called "weak measurements" that allow you to measure the state of a quantum system without disturbing it too much. In this interesting article, they describe an idea proposed by Andrew Jordan at the University of Rochester, where using weak measurements, one can unmeasure a particle and return it to its original state. In other words the collapse of the wave function can be reversed.
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