Does quantum mechanics say everything is random?

[SeventhSigma]

Does quantum mechanics say "Everything's random at the quantum scale, but on a macroscopic level, it's pretty much deterministic. It's still random, but deterministic in the sense that the variance attributable to quantum events is vanishingly small"?

 

[bcrowell]

"Everything's random" is much too broad to be meaningful. There is zero probability of a violation of conservation of energy, even at the atomic scale. There is zero probability that the charge of an electron will fluctuate.

It's also difficult to define randomness in a meaningful way. The evolution of a wavefunction according to the Schrödinger equation is completely deterministic. There is no randomness in the many-worlds interpretation.

IMO there is no meaningful answer to your question unless you refine it by defining more clearly what you mean.

 

[SeventhSigma]

Okay, what I mean is that we say, on a quantum level, that some things are unpredictable. We can predict things according to distribution, but some things we can't label as fully deterministic even if the wavefunction is.

Therefore, we've always said "it's possible" to materialize on the other side of the moon, but we say it's super improbable because the position function for anyone particle becomes crazy unlikely when we're talking about a distance that large (untold standard deviations away), and also unlikely when you consider that ALL particles of your body would have to do this at once, etc.

In other words, on macroscopic levels, we're not *fully* deterministic -- but even in systems where we can predict outcomes with virtually 100% accuracy, the quantum unpredictability will have little impact on the unexplained variance.

 

[BruceW]

In standard interpretation, when a measurement is made, the outcome is random, with probability given by the state vector of the system.
Defining when measurement happens is tricky. A good rule is to say that if the resulting state vector can practically be returned to the original state vector, then measurement has not yet occurred.
When a classical object gets entangled with the quantum state, the state vector now describes a many-particle state which is practically impossible to write down. Therefore measurement happens when a classical object gets entangled with the original state.
So the 'randomness' gets brought in when the quantum system interacts with a classical object. The classical object could be a Geiger counter, or a cat, or the air around us.
So the quantum state evolves deterministically due to the Schrödinger equation, until a classical object makes a measurement, which causes state reduction (which is non-deterministic).
There is another interpretation called many-worlds, where state reduction never happens, so everything is deterministic in many-worlds interpretation.