The Most Controversial Thought Experiment In Physics
Channel: Veritasium · Duration: 1:42 · source
[00:00] Put a cat in a box with a radioactive atom.
[00:03] Add a radiation detector that triggers the release
[00:06] of poisonous cyanide gas.
[00:08] Now, although it was only meant as a thought experiment,
[00:11] Schrödinger helpfully notes this device must be secured
[00:15] against direct interference by the cat.
[00:17] Anyway, the whole point of the experiment
[00:20] is to magnify the state of the atom
[00:21] up to the state of something macroscopic and tangible.
[00:24] He could have picked anything.
[00:26] It didn't have to be alive,
[00:27] but Schrödinger selected a cat.
[00:30] If the atom decays, the detector detects radiation,
[00:33] releases the poison, and the cat dies.
[00:36] If the atom doesn't decay,
[00:38] the detector doesn't detect radiation,
[00:39] poison is not released, and the cat remains alive.
[00:43] Since the state of the cat and detector apparatus
[00:46] are directly tied to the state of the atom,
[00:48] we say they are entangled.
[00:51] Where things get weird is that according to quantum mechanics,
[00:54] the state of the atom does not have to be
[00:56] either decayed or not decayed.
[00:58] Generally, it's in a superposition of both,
[01:01] decayed and not decayed at the same time,
[01:03] assuming no measurements have been made.
[01:05] This superposition state of the atom
[01:08] gets entangled with the detector and then the cat.
[01:12] So after some time, the wave function
[01:14] of everything inside the box is in a superposition of
[01:18] the atom has not decayed, poison not released,
[01:20] cat alive state, and the atom has decayed,
[01:24] poison released, cat dead state.
[01:26] So according to quantum mechanics,
[01:28] the cat really is both alive and dead at the same time.
[01:33] Only when we open the box and make a measurement
[01:36] does the wave function collapse,
[01:37] and the cat actually becomes either dead or alive.