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If there was no mistake and it actually is the case that a universal wave function that's in a very ordered state at the beginning of time is predicted to simulate a continuously branching timeline after that time, the theory that there is a universal wave function that's in a very ordered state at the beginning of time is called the many worlds interpretation.

What I don't understand is how it's even possible that the universal wave function simulates a continuously branching timeline. I don't know much about quantum mechanics so can you give an answer I'm more likely to be able to understand?

Let's take for example an led light bulb with a direct current running through it. I suspect that if you run square root of half the voltage through it, it emits the same spectrum at half the brightness. According to the Copenhagen interpretation, if you look at it when it's half the brightness, you have one future that observes it as having half the brightness, not two futures one of which observes it at full brightness and one of which observes it as being off.

On the other hand, I believe that under the theory of a universal wave function that's very ordered at the beginning of time, when the timeline splits for the first time ever, the universal wave function simulates multiple universal wave functions each of which starts at a time after that time and each of those simulates multiple universal wave functions each of which starts at an even later time so the universal wave function simulates a universal wave function where you brain is not in superposition and then later goes into superposition after you observe the energy level of an atom, and that's why people sometimes say your brain wasn't in superposition before the observation when it actually was and only went in superposition after the observation.

If you brain is following a universal wave function that is not in superposition, how is it possible that an intermediate state of the atom gives you two noninteracting futures instead of a single future which observes the intermediate state like happens in the case of an led light bulb at half the brightness according to the Copenhagen interpretation. Could there be a mistake? Did people actually figure out using only the math itself that a universal wave function with a very ordered state at the beginning of time simulates a continuous branching timeline, or were they so sure of certain assumptions that they just assumed the truth of a certain mathematical statement and didn't do the math to check whether that statement was actually true? One possible mistake somebody could make is that a smooth object has a microscopically thin layer of hexane on it so we observe water to have a different contact angle with it than the math shows that the theory predicts it will have so they assume the theory predicts it and don't do the math to check.

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    $\begingroup$ Hopefully a many-worlds proponent will answer your question, but note that the many-worlds interpretation is not a theory, only a way to make sense (for some people) of the quantum formalism. It does not predict anything that bare (uninterpreted) QM does not predict in itself, and as far as I know the notion of universal wavefunction is mostly hand-waving and does not refer to a well-defined theoretical construct. See for example arxiv.org/abs/1512.06845 $\endgroup$ – Stéphane Rollandin Sep 15 '18 at 20:46
  • $\begingroup$ @StéphaneRollandin: It does not predict anything that bare (uninterpreted) QM does not predict in itself Indeed, many people, e.g., Sean Carroll, prefer to define MWI simply as standard quantum mechanics. as far as I know the notion of universal wavefunction is mostly hand-waving and does not refer to a well-defined theoretical construct. The paper you reference does not seem to have anything to do with your assertion. $\endgroup$ – Ben Crowell Sep 16 '18 at 18:30
  • $\begingroup$ @BenCrowell. This paper is just an example among many and I am not particularly fond of it. That being said, here is the most relevant quote: 'the negative answer would render the universal wave function an ideal mathematical entity devoid of any empirical meaning.' (of course the negative answer is provided by the paper). This is enough for me to call it an ill-defined theoretical construct. I can provide other papers, from other perspectives, if you are interested. $\endgroup$ – Stéphane Rollandin Sep 16 '18 at 18:41
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The part of your description I would want to change is this:

when the timeline splits for the first time ever, the universal wave function simulates multiple universal wave functions each of which starts at a time after that time and each of those simulates multiple universal wave functions

Change this description to this:

The universal wave function evolves in time according to Schrödinger's equation which is a deterministic equation so just results in a single universal wave function at a later time.

Now it is generally accepted within mainstream physics that some parts of this wave function correspond to macroscopic measuring devices.

If a macroscopic measuring device interacts with a microscopic quantum system say with two states UP and DOWN, again it is accepted with mainstream physics that solving the Schrödinger equation for this interaction results in:

  1. an entanglement between the UP and DOWN states and two macroscopically distinct states of the measuring device (say two pointer positions),

  2. a process called decoherence which effectively destroys quantum interference between these two parts of the wave function.

Therefore within the one universal wave function we can identify two independent classical branches of the macroscopic measuring device.

At this point you have an interpretational choice, both consistent with experimental physics:

Choice A (Standard Interpretation): only one branch is the true branch.

Choice B (Everettian Interpretation): both branches are real.

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In standard QM, we have two main postulates:

  1. The evolution of the wave-function via Schrodinger equation and

  2. The collapse postulate that determines what we measure.

In the many worlds interpretation the collapse postulate is dropped. Hence, it is not equivalent to standard QM. It's a different theory with a different interpretation. It's a realist theory in that it takes the wave-function to be ontologically real (despite the fact it lives in an infinite-dimensional space) and this is interpreted to mean that there is a branching universe. How exactly this occurs is a problematic within the theory.

The main critique against this interpretation is that though logically coherent, it is not physically coherent, since when we look around us we only see one universe.

In a sense, it suffers from the same problem as the classical physics, but in the opposite sense. Classical physics expected its description of the world to suffice at the atomic level. It turns out that this expectation was false. Likewise, many world advocates expect quantum mechanics, which suffices to describe the atomic world to suffice for the world we see around us - even though it's straight-forward purely on empirical grounds to show that this expectation is wrong - we do not see many worlds - and nor, logically speaking, can we ever see many worlds.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – ACuriousMind Sep 20 '18 at 16:08

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