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Before opening the box, the observer does not know if the cat is alive or dead, however, a camera placed internally "knows" all the time, which is really happening.

Does this camera cancel the result of the experiment, even if it is not consulted?

EDITING:

I made this edition just to clarify my doubts.

The camera could be replaced by a piezoelectric attached to the wall of the box, that contains the experiment. Both, the (flat) crystal and the wall, record the cat's heartbeat. When the observer opens the box, both, its walls and the piezoelectric already know the history of the cat's life in advance, regardless of being consulted.

This invalidates the observer's opinion in the moment he opens the box?

Why the inner walls of the box, which are also "observers", do not invalidate the experiment as it was designed?

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    $\begingroup$ Is this helpful? $\endgroup$
    – Charlie
    Sep 16, 2020 at 23:33
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    $\begingroup$ Maybe and maybe not? $\endgroup$
    – Jon Custer
    Sep 16, 2020 at 23:49
  • $\begingroup$ The real problem is this: Before an experiment is performed, the universe is in an indeterminate state. The outcome of the experiment can collapse the universe into a state where the theory the experiment is intended to confirm is incorrect, and a more complex theory is needed. It even applies to thought experiment. This is why the Copenhagen Interpretation is no longer satisfactory. $\endgroup$
    – mmesser314
    Sep 16, 2020 at 23:52
  • $\begingroup$ The cat is both alive and dead until someone views the film in the camera. $\endgroup$
    – Adrian Howard
    Sep 17, 2020 at 0:05
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    $\begingroup$ please see my answer here to understand the difference between quantum mechanical observations and classical tools (which is the cat) physics.stackexchange.com/questions/266606/… $\endgroup$
    – anna v
    Sep 17, 2020 at 4:29

4 Answers 4

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Yes, it is possible to put a movie camera in the box with the cat. But when you open the box and look at the movie, you will see the cat die or not die, but not both. And that will agree with what you see when you then look at the cat: it will be either dead or alive. The movie film will be entangled with the cat. By opening the box and looking, you randomly select one state or the other, and that's all you see is the selected state. Some call this "collapse of the wavefunction", and some call it "branching of the world". For most practical purposes it's the same thing.

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    $\begingroup$ This experience says that, for the outside observer, the cat is both alive and dead, but the camera already "knows" what the real condition of the cat's life is, even before the box is opened. It seems to me that this situation invalidates the opinion of the external observer. I am wrong? $\endgroup$
    – João Bosco
    Sep 17, 2020 at 1:34
  • $\begingroup$ No, it doesn't mean that. For the outside observer, the camera and cat are essentially parts of one quantum object which includes both living and dead states As soon as that quantum object is measured/observed, the observer becomes entangled with the quantum object and ends up in a superposition of two independent states: one in which he sees the camera/cat dead, and one in which he sees the camera/cat alive. That's the Many Worlds View. In the Copenhagen view, the observer randomly sees one or the other state and the possibility of the state he doesn't see drops to zero. $\endgroup$
    – S. McGrew
    Sep 17, 2020 at 4:35
  • $\begingroup$ Really nice answer. $\endgroup$
    – Árpád Szendrei
    Sep 17, 2020 at 16:34
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Write $a$ and $d$ for the alive and dead states of the cat.

Write $A$ and $D$ for the "contains a picture of a live cat" and "contains a picture of a dead cat" states of the camera.

Write ${\cal A}$ and ${\cal D}$ for the "saw a live cat and a picture thereof" and "saw a dead cat and a picture thereof" states of your friend Jeeter.

The cat starts out in state $a+d$. Then the camera goes off. Then Jeeter looks in the box. Then you question Jeeter about what he saw.

Theory I: Before the camera goes off, the cat spontaneously jumps into either state $a$ or state $d$. Then the camera goes off, entering state $A$ or $D$ accordingly. Then Jeter peeks in the box and enters state ${\cal A}$ or ${\cal D}$ accordingly. Now the whole system is either in the state $aA{\cal A}$ or $dD{\cal D}$. You run into Jeter and question him. Depending on the system state, he responds either that the cat is alive or the cat is dead.

Theory II: The cat is still in state $a+d$ when the camera goes off, collapsing it into either state $a$ or state $d$. The camera enters state $A$ or $D$ accordingly. Then Jeter peeks in the box and enters state ${\cal A}$ or ${\cal D}$ accordingly. Now the whole system is either in the state $aA{\cal A}$ or $dD{\cal D}$. You run into Jeter and question him. Depending on the system state, he responds either that the cat is alive or the cat is dead.

Theory III: The cat is still in state $a+d$ when the camera goes off. There is no collapse, so the cat-plus-camera is now in state $aA+dD$. Jeter peeks in the box, collapsing the state to either $aA$ or $dD$, and he enters state ${\cal A}$ or ${\cal D}$ accordingly. Now the whole system is either in the state $aA{\cal A}$ or $dD{\cal D}$. You run into Jeter and question him. Depending on the system state, he responds either that the cat is alive or the cat is dead.

Theory IV: The cat is still in state $a+d$ when the camera goes off. There is no collapse, so the cat-plus-camera is now in state $aA+dD$. Jeter peeks in the box, but does not cause a collapse, so the cat-plus-camera-plus-Jeter is now in state $aA{\cal A}+dD{\cal D}$. You run into Jeter and question him, causing the system to collapse to either $aA{\cal A}$ or $dD{\cal D}$, whereupon Jeter tells you either that the cat is alive or the cat is dead accordingly.

All of these theories (and many many others with many many more steps along the way) have exactly the same testable implications, so there are no reasons other than aesthetic ones for preferring one theory to another. Different people's aesthetic senses might lead them to prefer different theories, but no observable phenomenon can prove any of them wrong.

The details of all this were worked out by John von Neumann and appear in his book on the Mathematical Foundations of Quantum Mechanics.

Edited to add: Theory V (probably the most satisfying of the bunch): Write A and D for the "believes the cat is alive'' and ``believes the cat is dead'' states of you. The cat-plus-camera-plus Jeter system has reached state $aA{\cal A} + dD{\cal D}$ state as in Theory IV. You run into Jeter and question him, after which the entire system is in the state $aA{\cal A}{\bf A} + dD{\cal D}{\bf D}$, in which state everything remains.

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To preserve the quantum superposition, the cat must be completely isolated from its environment, which means it cannot interact with the photons necessary for the camera to function. In other words, the camera cannot observe the cat without a flash, which is tantamount to making an observation.

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  • $\begingroup$ Not until the picture is viewed. $\endgroup$
    – Adrian Howard
    Sep 17, 2020 at 0:08
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    $\begingroup$ Are you saying the thought experiment doesn't work if the box contains a light bulb and a battery (but not a camera), as well as the usual randomizing device? Assume the randomizing device is not affected by photons. $\endgroup$
    – alephzero
    Sep 17, 2020 at 3:00
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Yes, the cat can be filmed. You will se a movie where the cat dies (if the radioactive atom decays), or not. Watching the film, or simply filming it, is not the reason for the "collapse" of the cat state. The cat itself is a perfectly legit "observer", so it is never in a dead/alive superposition (there is no procedure to prepare the "cat+Geiger counter+radioactive atom" in such an entangled state in the first place). The Geiger counter that should activate the trap is itself an observer. Filming the cat adds nothing to the "experiment" (also the presence of the cat adds nothing, the Geiger counter is sufficient.. the cat is there just for poetic reasons).

It is sometimes said that "the cat is both part-dead and part-alive until an observation is made". This is a misconception (Schrödinger himself was very clear on this point). The decay (or non-decay) of the atom will trigger (or not trigger) the poison mechanism, and that alone is where the transition from quantum behavior to classical behavior occurs. In short: if you appreciate the fact that this experiment is just a nice story to provoke the deep question "where is the transition from the quantum to the classical behaviour?", you will immediately see that many of its parts (the box, the poison, the cat itself, the Wigner friend, or the camera) are just elements to make the story more "colorful". In fact, Schrödinger’s purpose in putting forth his thought experiment was simply to illustrate how easy it is to arrive at an absurd prediction (an half-dead-alive cat) if you misinterpret quantum mechanics: to measure an observable you do not need an "observer" (every irreversible interaction will do the job).

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    $\begingroup$ I also think that the cat is expendable. $\endgroup$
    – João Bosco
    Mar 1, 2021 at 2:56

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