Before you tag my question as a duplicate I want to make sure you understand it.

Earlier I saw a video about Schrödinger's cat that says once we open the box to see if the cat were alive or dead the universe splits into 2 separate almost identical universes or something, one in which the cat dies and one in which the cat lives. But then I started questioning if the 50/50 chances of the cat dying are really 50/50 because I read that total randomness is impossible. So that means that the 50/50 isn't actually random, but it's already defined if the cat is going to live or die. Does that mean the universe never split?

So my question is: Can someone please explain to me if the 50/50 in Schrödinger's experiment was really random and how did he make something truly random?

PS : I'm not a very smart guy so please take it easy if I'm just speaking nonsense.

  • I made an edit which you may roll back or continue editing. One thing you might add to your question through an edit is a link to the video you saw or its name. This would provide context which might help someone focus when providing an answer. Welcome! – Frank Hubeny Aug 30 '18 at 14:53
  • 50/50 is not about true randomness. A very bad pseudo RNG returning (t - 1) mod 2 will be 50/50. t is current number, t - 1 is previous number. A true RNG would be the one which returns t-th bit of transcendental number. – rus9384 Aug 30 '18 at 16:40
  • @rus9384 The trillionth digit of pi is as deterministic an event as one could imagine. It's the same every time and you could easily write a computer program to print it. – user4894 Aug 30 '18 at 17:10
  • @user4894, the thing is that t is never the same. Each time you use the device, t increases by 1. – rus9384 Aug 30 '18 at 17:21
  • @rus9384 The sequence of digits is exactly the same every time. You have a funny definition of random if it's exactly the same every time. Even the statistical randomness of the digits of pi -- a much weaker concept of randomness -- hasn't been proved. – user4894 Aug 30 '18 at 17:37

Schrödinger's cat is a thought experiment which strains the capabilities of the interpretations of quantum mechanics as they have to deal with macroscopic objects that are described as being in a superposition of states. If you run the math on this experiment, you get.... the math on this experiment. You get the solution to an equation in the form of a wavefunction.

It is up to the interpretations of QM to assign meaning to those equations, and it is these interpretations which get strained.

The Copenhagen interpretation declares that when you open the box, you "collapse" the waveform to a concrete state where the cat is either alive or dead. It states that whether the the cat lives or dies is randomly selected.

Now if we really poke hard at what these interpretations say, you really have to use a slightly weaker wording. We say that the state of the cat when we open the box is indistinguishable from a random result. That phrasing opens doors. It is entirely possible that there is actually some supreme being which has a book which covers the entirety of time, past and future, and in that book it states that the cat lives. However, without this omnipotent knowledge, this is the best we can do.

The Many Worlds Interpretation is the one which discusses the idea of the universe splitting. Unfortunately, that was not a very convenient word choice. It got popular, because it had a certain visual appeal. The interpretation more specifically states that the observed values (such as the liveliness of the cat) are dependent on the subject, not just the object. In MWI, observations are not objective truths. The mathematics for this superposition permit treating the universe as if it were two parts, one with an observer that sees a living cat, and one that sees a dead one.

Pilot wave theory does not have randomness nor universe splitting. It has a universe-wide wave function and particles that interact with it. In pilot wave theory, whether the cat is dead or alive is deterministic, but it depends on the state of every object in the universe (even those outside of the light cone of the experiment). As such, it is unknowable whether the cat is alive or dead, until you open the box, even though it has a definite value.

Each of these interpretations has to "give up" something that we hold dear in the classical world in order to reconcile the real experiments done in the field of QM. Copenhagen gives up determinism, letting naked random variables go scurrying around. MWI gives up objective realism. Beauty is truly in the eye of the beholder in MWI, because the state of the observed can only be stated with respect to a given subject. Pilot wave gives up locality completely, letting every single "decision" made at the quantum level depend on the state of the entire universe.

As for whether there exists a truly random thing, consider that a random variable requires there to be meaningful mapping into a probability space. In a lot of cases, with exactly 1 world, where events happen exactly 1 time, this definition is fraught with difficulty. A less strict reading would be to talk of "nondeterministic" behavior rather than random behavior. All random behavior is nondeterministic, but one can consider non deterministic things (such as minds) which are not random. The philosophical discussion of determinism vs non-determinism is well documented, both in other questions and in the literature at large.


The idea behind Schoedinger's cat is that we don't know whether the cat is alive or dead until we check. We can set the probabilities to be as close to 50-50 as we like, depending on how well we make our apparatus. However, something can be random without being 50-50, as long as they're not even theoretically predictable.

We're dealing with something called the collapse of the wave function. Anything can be described with a wave function. (The one for me as I sit here is far more complex than we could possibly deal with, but it exists.) The wave function includes all possibilities with certain probabilities. Then we observe the system (in this case, open the box) and see what we have, and we find it's one of the possible results and only one. What happens? There's no obvious reason why it couldn't continue to exist with multiple possibilities, but it doesn't. This is called the collapse of the wave function.

Some people have thought that the Universe splits into however many new Universes it has to, so that there's a Universe with a live cat in the box and one with a dead cat. That's one interpretation (called the "many worlds" interpretation) but there are others, and trying to make these interpretations make testable predictions is at best extremely difficult.

  • randomness doesn't usually mean not known – user34654 Aug 30 '18 at 21:56
  • @user3293056, true randomness means unpredictable. If it is known, it is predictable. – rus9384 Aug 31 '18 at 0:05
  • oh i thought 'random' meant undetermined, not unpredictable – user34654 Aug 31 '18 at 9:31
  • i mean the word means different things to different people, 50/50 chance, unknown result, undetermined result, or that everything is possible – user34654 Aug 31 '18 at 10:31
  • not really a criticism of your answer, just concerned in case misunderstanding – user34654 Aug 31 '18 at 11:33

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