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If it is true that particles are in different potential locations until an observer comes along and collapses the wave function, then how can a blind man throw a ball and hit a wall if the particles are spread out?

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    $\begingroup$ It appears that you are mistakingly assuming that it takes a person seeing something in order for a wavefunction to collapse. Furthermore, the wave function not collapsing does NOT imply that 'a ball goes through a wall'. You should look around on this website for questions on this topic; You'll find some clarifications. $\endgroup$ – Danu Mar 20 '14 at 13:37
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    $\begingroup$ Blind or not, no one can throw a ball through a wall via tunneling. $\endgroup$ – Kyle Kanos Mar 20 '14 at 13:37
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    $\begingroup$ I would not downgrade this question. This is a very good opportunity to define an observer in QM. $\endgroup$ – PhilMacKay Mar 20 '14 at 16:26
  • $\begingroup$ I think if the question said "someone with their eyes closed" instead of "blind man" nobody would downvote it. I didn't downvote it, but I think it's unnecessarily phrased in a way that could be offensive to some. $\endgroup$ – DavePhD Mar 20 '14 at 16:41
  • $\begingroup$ All right! Let's edit the question to make it better then! $\endgroup$ – PhilMacKay Mar 20 '14 at 17:57
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I think the root of your question is that there is a misunderstanding of the concept of "observer" in quantum mechanics.

If the ball is observed on this side of the wall, the probablity that it will later be observed on the other side of the wll is so small that it is essentially zero (because the ball is a macroscopic object.)

Now, what does it mean to state "the ball is observed"? An observation does not have to be made with eyes. By touching the ball, the blind person is essentially observing it in his hand. He could also observe the ball indirectly, say if it's a ball rolling in the snow, then he could feel the path the ball created in the snow.

In addition, the observer doesn't have to be human, consious, or evn living. Any macroscopic object can observe the ball. The act of observing is essentially the process of interracting so many times (think each atom in the hand interract with an atom in the ball) with another object that the observer's state is essentially entangled with the object. In the case of macroscopic object, just the contact with ambiant lighting is enough to entangle the object with its surrounding.

I will leave the definition of entanglement to another question!

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