First off, quantum mechanics enthusiasts, chill ;) As we all know, Schrödinger's Cat is just the celebrity from the interesting "thought" experiment (hopefully only) imagined by the Austrian physicist Erwin Schrödinger, to discuss, if the concept of quantum superposition applied to the world of our scale, how reasonable would be the result?
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Second, quantum mechanics fans, chill - I'll be explaining all of the difficult concepts along the way, including "quantum superposition", so just sit back and enjoy! ;)
Quantum superposition principle just tells us the following:
=> Very small particles can be in valid quantum mechanical states that are sums of two or more valid quantum mechanical states.
Applied to the world of our scale, let's say that one has a girlfriend. They don't know whether their girlfriend is at home or at a bar, but they know that its Saturday night, so the girlfriend is more likely to have hit the bars than be at home. If we apply the quantum superposition principle above to this situation, then we can say that the girlfriend is both at home and at a bar simultaneously, UNTIL her partner calls her to find out where she is. When she receives the call and answers (we just hope), then her "superpositional" state "collapses" to one simple state: being at a bar.
Now, Schrödinger attempts to initiate a discussion exactly like the where-is-the-girlfriend story above, among his contemporaries, with his famous thought experiment "Schrödinger's Cat", to criticize an interpretation of quantum mechanics widely supported among the physicists - stay tuned.
To put it simply, this thought experiment is all about having the following stuff in a closed box:
- a cat
- a hammer
- a bottle of poison
- a controller that releases the hammer to break the bottle hides under his desk
- a small box with radioactive material that triggers the controller to release the hammer when one atom in the small box decays (that is, when its nucleus falls apart and emits particles)
These radioactive atoms in the small box decay with a certain probability at a given time, and whenever one of the atoms decays, the controller detects the radiation and releases the hammer.
The hammer breaks the bottle.
The poison spills out.
The cat dies.
So, at any time point:
If the hammer was already released, the cat is dead.
If the hammer was not released yet, the cat is alive.
As we've stated above, Erwin Schrödinger pointed out the above-mentioned situation to challenge an interpretation of quantum mechanics (for the curious among you, the Copenhagen interpretation):
According to this interpretation, through the principle of quantum superposition, when we have an atom in a closed box and don't "look at it" (i.e. measure it with a detector), then we cannot tell whether it has already fallen apart or not. It has both decayed, and not, because it is small enough to be governed by the counter-intuitive laws of quantum mechanics. Whenever it is measured by a detector, its superpositional state collapses into one of following the simple states: decayed OR didn't-decay.
BUT, in the experiment above, the cat's life depends on whether the atom has decayed or not, so does this superpositional state of the atom reflect to the world of our scale, rendering the cat both dead and alive?
This is, of course, very absurd and Schrödinger exactly tried to point that out. But, in an ironic way, now his thought experiment is mostly used as a metaphoric example to describe the principle of quantum superposition!
People who agreed with Schrödinger and were against the above interpretation of quantum mechanics, described the following interpretation of quantum mechanics (the De-Broglie-Bohm interpretation, for people who want to read more on that):
A quantum particle is never in two different states at once. We JUST don't know what it's going to do at a specific time point, but we know what it's more likely to do and what not. So, when we are not looking, the particle is in ONE of the two or more possible state, even though we can't tell which, but ONE! The reason we can't predict which state we will find the particle in upon measurement is that we don't know enough about the particle, so it's behaviors "seem" random to us.
According to this interpretation of quantum mechanics, quantum mechanics is "incomplete" and when we find out everything about these particles, we will be able to tell whether the cat is dead or alive without looking into the box.
ATTENTION: Of course, the cat, at the world of our scale is either dead or alive, even when we are not looking! What we are not so sure about are the states of the radioactive atoms in the box when we are not looking into the box. Did they decay, or not? When were are not looking, the quantum superposition principle says, for these small particles, quantum laws apply, so BOTH!
Interestingly, even though the De-Broglie-Bohm interpretation sounds a lot more intuitive, nowadays the Copenhagen interpretation (the one that sounds more magical and interesting) is more widely accepted.
BUT FOR A REASON! Because depending on which interpretation you choose, you have to accept or deny something that can be very difficult to accept or deny sometimes, depending on how you feel:
Superdeterminism: "The notion that results of all future events in the universe are set and decided, and we're just living through something like a "replay". Everything is determined to happen or behave in a certain way, and there are no choices. No free will. "
"Whoa, stop there!" you might say.
"We were talking about particles and cats, and now you're talking about whether free will exists or not?!"
Would you like me to explain the connection in between?
I think it's a better idea if I read your reactions first, and answer your questions, and THEN explain:
"What makes the choice between a "dead-and-alive cat" and a "dead-or-alive cat" the same as the choice between "free will" and "no free will?"
in my comments! :)
(Damn, even the question itself sounds paradoxical. )
Thank you very much for following through this long post, and please feel free to let me know whether you have any questions or whether I've been inaccurate in any part of my post above! :)