I can't understand the relation between Schrödinger's Cat and this amazing experiment of the double slit.

It seems like in the double slit we know if we observe the particle the wave function collapse and the electron acts like a normal particle, so doesn't that solve the problem in Schrödinger's Cat?

When we open the box the atom that should help release the poison acts like a particle,the poison is in the bottle so the cat stays alive, don't know really?!


There is a close connection in that both require the collapse of a superposition, and I think understanding one does help in understanding the other.

In the double slit experiment the position of the electron is not measured until it hits the screen, or photographic plate or whatever you're using to view the interference pattern. This means that before the electron hits the screen it does not have a well defined position. It's very important to understand this point. It is not the case that the electron has a position but we don't know what it is - the electron is delocalised over the region between the electron source and the screen and it does not have a more precise position in the usual sense of the word.

When the electron interacts with the screen screen we see it as a point on the screen, so suddenly it has a well defined position. What happened?

Well if the electron was in a superposition of all possible positions on the screen, that means immediately after impact the screen must be in a superposition of all its possible interactions with the electron. In this state the position of the interaction between the electron and the screen isn't well defined. And when you interact with the screen by looking at it, you must go into a superposition of states of seeing the interaction at all possible places on the screen. And when you tell me the result I must go into a superposition of all possible things you could have told me. And so on: the superposition ripples out into the whole universe.

But this isn't what happens. Why not?

At this point opinions differ and there are a whole host of theories about what happens. The commonest are probably the Copenhagen interpretation and the Many Worlds interpretation, but several other views exist as well. I've provided links for these, but be warned that the explanations are all heavy on the maths. However they all agree that while it's easy to keep a simple object, like an electron, in a superposition it gets increasingly hard as the object gets more complicated (in physics speak it has more degrees of freedom).

So you don't see the screen as a superposition because it's far too complicated to exist in a state of superposition for longer than the tiniest moment. Instead you see the interaction of the electron and screen as a well defined spot. Similarly the cat/box/poison is far too complicated to survive as a superposition for any significant time. Instead the cat is always either alive or dead.

  • $\begingroup$ when you say that they both require the collapse of a superposition,you mean that they both agree with the Copenhagen interpretation? because as far as I know schrodinger use the cat experiment to prove the silliness of that interpretation. $\endgroup$ – farahastro Aug 19 '14 at 15:57
  • $\begingroup$ +1 I really like this answer, honestly I'm utterly surprised as to how it hasn't received any upvotes yet. I have two general questions after having read your answer, if I may: (i) So in all correctness, without having to adhere to an interpretation of QM, one can say that when an atomic system (e.g. here the electron and its position) is in a superposition state (of positions here), then its position is in principle not well-defined, as in it has nothing to do with our experimental capabilities, rather that is the nature of atomic systems, right? $\endgroup$ – user929304 Jul 1 '16 at 12:07
  • $\begingroup$ (ii) On the one hand, we know that the screen is a macroscopic system and the electron a Q-mechanical one, thus when these two are to interact, the Q-system decoheres and can be described by the classical laws of motion, but doesn't this imply that on the screen we should not see fringes? since the system decohered. On the other hand, I convince myself as follows: as a result of the decoherence due to the interaction with the screen, the electron falls into a well defined positional state and thus can be described by a point on the screen. Is the latter the correct assessment? $\endgroup$ – user929304 Jul 1 '16 at 12:07
  • $\begingroup$ @user929304: yes, your last sentence is basically correct. Note that decoherence alone isn't enough - we need decoherence + many worlds to give us the observed classical result. Explaining exactly why gets very mathematical very fast :-) $\endgroup$ – John Rennie Jul 2 '16 at 6:39

1) The central maximum of the diffraction pattern is between the slits. 2) Shoot single electrons at the double slit. The same diffraction pattern accumulates. 3) Any attempt to identify through which slit a given electron passes collapses the double slit diffraction pattern into two single slit patterns. 4) All of it works just fine using C60 buckminsterfullerene as the diffracted entity, MW = 720.6 amu, or a big molecule like phthalocyanine derivatives C48H26F24N8O8 MW = 1298.7 amnu.


Tell us how classical physics allows a 1300 amu molecule to pass through both slits simultaneously. Compare width separation to molecule size. Now, the giggle,

5) Quantum eraser and quantum double-eraser experiments. How does it know to be classical or QM at the slit when the decision is made after the slit?

  • $\begingroup$ I did not downvote but the confusion arises by using the word "particle" which attributes classical particle behavior, as in your "how does it know..." for a quantum mechanical entity. The very backbone of a quantum mechanical entity is that its mathematical model describes a probability distribution for its appearance at a measurement. A probability controlled by the boundary conditions of the quantum mechanical problem . The knowledge is in the probability function. There have been experiments where the slit was known for individual particles' appearance, and the interference still exists $\endgroup$ – anna v Feb 2 '14 at 5:16
  • $\begingroup$ en.wikipedia.org/wiki/… . It is the change in the boundary conditions of the QM problem that detectors at the slit destroy the pattern. One can have condition as shown in a link in the link where the pattern survives the detection. The attempt changes the wavefunction if it is not fine enough. Nothing but the probability maps two slits simultaneously. The particle goes through one . $\endgroup$ – anna v Feb 2 '14 at 5:21

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