A determinist holds that everything is determined by the physical world around them and free will is an illusion.

A multiverse suggests there are an infinite number of alternative universes in existence, of which we only experience one.

But if the world does not have free will, and everything is determined, then a multiverse will never exist because the existence of our universe determines it cannot exist.

Is this true? A lot of determinists also believe in a multiverse, but how?

  • Well blow me down. I hadn't thought of this. I think you might be right. The argument holds up for me although I'm still pondering. Nice one! – PeterJ Jul 6 '18 at 11:23
  • I don't understand why you take determinism to only be possible in our "universe"; you can easily state that there are other deterministic systems in the other "universes" (if we're inclined to take your definition of multiverse and not the Many-Worlds theory as the answers below suggested), exactly like we (might) have in our "universe". You also imply a causal link between the "universes", which isn't necessary. – Yechiam Weiss Jul 7 '18 at 9:58
  • I don't see how being a determinist contradicts the possibility of existence of our universes. There could be even a universe identical to ours (for any internal observer). No contradiction. What it can contradict is many-worlds QM interpretation. But even then it is possible for determinism to be the case. In the end, determinism means the world is computable. – rus9384 Jul 7 '18 at 22:46
  • If determinism means the world is computable, 100% predictable, there are no variations. If there are variations, or areas that are not predictable, that allows for choice or free will, of the freely acting agent. If there is total predictability why does there need to be alternate realities, because if they are mirrors of this world, they must be identical. This is not the popular concept of multiple universes, this is my point. – PeterJens Jul 8 '18 at 7:49
  • @PeterJens "if there are mirrors of this world, they must be identical" - a) why do you consider the "other worlds" as "mirrors"? b) why do you think they must be identical? In fact, if they are identical, they are most probably the exact same world we live in (Leibniz would use this idea to pursue his "best possible world" thesis). If there are actually other worlds, they must be non-identical, most probably with different logic system and physical laws. A nice article that follows your overall stance. – Yechiam Weiss Jul 8 '18 at 16:06

You might be misunderstanding the Everett ("many-worlds") interpretation of quantum mechanics. On the Everett interpretation, there's one universe, and its change in time is described deterministically by the Schrödinger equation. But observers inhabit different "worlds," meaning that they will give different macroscopic/classical reports of events. One observer will report that the cat in the box is dead, another will report that the cat in the box is alive. Both observers inhabit the same, single universe; but their macroscopic/classical experiences of that single universe are different. The change of worlds in time is only described probabilistically by the Schrödinger equation.

Or, you might be thinking of classical universes, described by Newtonian physics. Newtonian physics isn't deterministic. But bracket that point. A system is deterministic if its future state is necessitated by its past state and the laws that govern its change over time. So you can have many deterministic classical universes with different future states so long as either their past states or the laws that govern them are different.

  • Under the Everett interpretation, there can still be uncertainty about which observer I am. Perhaps this kind of indexical uncertainty would be perceived to not violate determinism. On the other hand, for example Hellie's vertiginous question suggests that there needs to be something that determines which perspective is mine... – present Aug 6 '18 at 2:46

Free will is not compatible with physicalist materialism, except in a compatabalist sense of being a useful illusion. Whether there is true randomness in the world cannot account for free will, or not.

The multiverse is not a fixed eternal visitable place, and neither are parallel universes. The past exists as it is embodied in now, in conserved quantities and so on (including information conservation, in the Many Worlds view). Alternative probabilities exist in probability space, which has real measurable impacts on our reality, over certain specific -compactified- scales. https://www.universetoday.com/48619/a-universe-of-10-dimensions/

Many Worlds recovers determinism in quantum mechanics, but in a higher dimensional space that ours is a subset of.

the existence of our universe determines it cannot exist.

What does that mean? It doesn't make sense to me.

  • If in our world everything is determined by its existence in the physical world, there are not alternatives possible, or else determinism is not true. Once there are alternatives, no matter the source or trigger point of the alternatives, things are not determined. – PeterJens Jul 6 '18 at 10:10
  • @PeterJens What do you mean, exactly, by "our world"? – Chelonian Jul 6 '18 at 13:47
  • "our world" - the world we experience and know as opposed to other worlds or parts of the universe we inhabit but do not know. – PeterJens Jul 6 '18 at 13:55
  • Physicalist materialism is compatible with either a deterministic or quantum world. A quantum world can be considered determinstic across a Many Worlds interpretation. Quantum randomness cannot give you free will, causality deriving from mind states, except in a compatible sense of being a kind of supervening heuristic explanatory layer. Randomness is irrelevant to this, if you are a physicalist materialist. – CriglCragl Jul 6 '18 at 15:01

Free will is not the opposite of causality but of coercion. Free will means that I have agency, based on my own internal mental states as well as the situation around me. It does not mean I can violate the laws of physics. It means I'm awake, aware, and don't have a gun pointed at me. (Or some other physical threat, etc.) An entity is said to have free will when it has internal mental states that can change its behavior.

Non-determinism would not save the notion of free will that people say is impossible. If I'm walking down the street and then, for no apparent reason, my arm flings out and smacks somebody, that's not free will. The un-caused isn't free will. The randomly produced without cause is not free will.

Multiverse theory is a (fairly abstruse) variation of quantum mechanics. It was suggested as a way to interpret certain things in QM that people had difficulty adding to their list of things they thought they understood.

It better not be required to understand anything about human behavior, since the idea only arose a little more than 60 years ago. Whatever did people do about late-night arguments over too much alcohol before that?


And, so far, nobody has managed to formulate an experimental test of multiverse theory. That is, nobody has managed to show that the existence, or non-existence, of these other universes, can be shown to make any detectable difference.

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