Is there true randomness in the universe?
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“Randomness” is used to mean multiple different things. The answer somewhat depends on which meaning you’re using. Whenever asking “does true X exist?”, it’s useful to be very precise in what we mean by X.
All of the following are qualified, of course, by “as far as we know”.
The answer is generally “yes” but the degree of confidence depends on which meaning you’re talking about.
First meaning: “externally unpredictable” – that there exists a series of measurements one could take, and never be able to perfectly predict the next measurement in the series. Yes, quantum behavior exhibits true unpredictability. This actually means that true unpredictability is extremely *common* – we are surrounded by and made up of quantum particles. Of course, “on average” the unpredictability evens out. So here, the answer is just “yes, there is true randomness.”
Second meaning: “does not follow an internal predetermined, fixed pattern”. This is the idea that the pattern can’t be externally found, but still exists in some “internal state”. This is also known as determinism – and in quantum mechanics, as “hidden variable” theories. The “strongest” form of determinism, “superdeterminism”, can never be ruled out – this is the idea that literally everything in the universe is perfectly determined ahead of time, that there’s no such thing as choice, and that any apparent “choice” is all just part of the predetermination. This is, however, not a very useful model; just as “what if I’m a brain in a tank and all my knowledge of physics is fake” is not a very useful model. In terms of useful models, deterministic theories currently have some evidence against them and no significant evidence for them, so the answer is *probably* (but not *certainly*) “yes, there is true randomness.”
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