If somebody said that he hears voices in his head, or that he has a twin that no one can see, or believes in an invisible world, then you would infer that he is schizophrenic, or suffers some similar mental illness.
If somebody believes in Heaven, Hell, angels, and devils, and prays to an invisible god, you would infer that he is religious, but not crazy.
But what do you make of somebody with an advanced scientific degree who believes:
* You have identical twins in distant universes.
* Our world is just a simulation, running on a computer in a more advanced civilization.
* The universe constantly splits into parallel universes, where every possibility is played out for real.
* All events have been pre-determined, from the first second of the Big Bang.
* We have no free will, and all actions are controlled as with a robot or puppet.
All these things seem like symptoms of mental illness to me.
Another tipoff is having crazy ideas about randomness. Insane people sometimes think that nothing is random, and every coincidence is a manifestation of some bizarre conspiracy of causes. Or they think that randomness is one of the fundamental forces in the universe, disrupting everything. Either way, the insane man is troubled by imaginary demons that are sabotaging his life.
So how am I supposed to think about respected physics professors who believe in this crazy stuff? I do not have a good answer for this.
I have reviewed some debates on free will, from respected professors. I think that they are all insane. I wonder how they can even function in their daily lives with such peculiar beliefs.
As an example, see Sean M. Carroll's most recent podcast. He is pretty good at explaining physics, but he also believes in a lot of wacky stuff. He believes in the many-worlds multiverse. He claims to believe in free will, but he also believes in a deterministic multiverse. At 2:10:40, he gets a question about free will and determinism. He concedes that our branch of the multiverse may not be deterministic, but he adamantly argues:
There is a question about whether or not the laws of physics are deterministic or indeterministic, but that has zero to do with the question of free will. The strong sense of free will, the libertarian sense of free will, has to do with whether or not you personally can violate the laws of physics, just by thinking about it. And I don't think that's true, but whether or not it's true, has nothing to do with whether the laws are deterministic or indeterministic.
This is just nutty on multiple levels.
What does he mean by "laws of physics"? He includes many-worlds multiverse, even tho it makes no predictions and has no relation to reality. It is just a fantasy game, where he pretends that all possibilities are happening in some parallel universe. He does not include strong or libertarian free will, as he thinks that means willing a law violation.
If a law of physics is violated, then it is not really a law of physics. Defining free will as a violation does not solve anything.
Part of his problem is that he is always talking about what is "fundamental" or not. Some laws of physics are, and some are not. Some are emergent. Many of these opinions are based on what he thinks will be in some hypothetical Final Theory, from which all else will be derived.
My personal opinion is that consciousness and free will are real, and consistent with the laws of physics. We may or may not get better theoretical understandings of them in the future, but personal experience today convinces me of the nature of consciousness and free will. It is not contrary to any known law of physics.
But Carroll says free will has zero to do with determinism. He doesn't believe that he has any ability to make any choices, as they are all determined. There could be parallel universe branching to confuse matters, but that does not affect his view that all his choices are just illusions.
Elsewhere in the podcast, he explains his belief in eternalism. The present time is just another illusion, and all times are equally valid. Our minds just remember the past instead of the future, because entropy is increasing in the brain.
I think that Carroll would be considered insane, except that he is also able to explain coherently a lot of textbook physics.
On the subject of what is fundament, the "Ask a Physicist" blog tries to explain Carroll's many-worlds nonsense, without much success, and notes:
Very, very frustratingly, without declaring a measurement scheme in advance, you can’t even talk about quantum systems being in any particular set of states. For example, a circularly polarized photon can be described as some combination of vertical and horizontal states, so there’s your two worlds, or it can described as a combination of the two diagonal states, so there’s your… also two worlds. This photon is free to be in multiple states in multiple ways or even be in a definite state, depending on how you’d like to interact with it. For the world to properly “split” a distinction must be made about how the photon is to be measured, but that isn’t something intrinsic to either the photon or the universe.
It is this sort of argument that makes me doubt whether it is useful to even talk about something being fundamental. Which type of polarization is the fundamental one? It is impossible to say, as each type appears non-fundamental when viewed in the context of the other. You are apt to think one is fundamental if it happened to be explained first in whatever textbook you used.
Much of Physics is this way. Things seem fundamental when explained one way, but there is often an equivalent theory where something else is fundamental. For example, some people think particles are fundamental and fields are derived, while others think fields are fundamental and particles are derived.