But you're right about the single largest measured system. Doesn't mean that a tennis ball doesn't have a wave function. It does. It's just incredibly complex, and we can probably never know it or write it down. Kinda like trying to write down the position and momentum of every water droplet in a cloud.
No, it doesn't necessarily mean that very large objects such as the universe, or even medium large macroscopic objects such as a Tesla Semi Truck or a cat have a single quantum wave function.
What you described is an extrapolation of the laws of quantum mechanics to very large systems, which is called the "Copenhagen view" of quantum mechanics, which actually only a minority of quantum physicists support (!):
The "Copenhagen view" has a number of deep problems:
- Who are the "observers" that "measure" and cause the collapse of a wave function?
- Who were the "observers" one microsecond after the Big Bang? The universe couldn't have expanded without the wave function collapsing.
- Who are the "observers" inside a black hole?
- Is Schrödinger's cat dead or alive before we open the box? If "both", did you kill it by opening the box?
So there are other, scientifically rigorous explanations for the "observer problem":
- that there's a dampening of quantum fields with larger objects,
- that there's a "many worlds" multiverse,
- or super-determinism,
- or a simulation universe,
- or "objective collapse" where larger quantum systems automatically collapse their wave functions,
- etc.
We simply don't know (yet) whether large, complex wave functions exist, we don't know which of the variants above actually exist - but we
do know that the Copenhagen view is not universally accepted!
BTW., some of these are falsifiable, with viable experiments proposed that would test whether very large quantum system have wave or particle behavior.
(Anyway, this comment is OT and not OT at once, until a moderator measures it: the "TMC uncertainty principle".)
