6
$\begingroup$

My understanding is that the Many-Worlds interpretation essentially says that the entire universe is one big wavefunction, and what we interpret as "measurement" or "collapse" is just when our conscious mind splits into a particular branch. I've also heard that this is just an interpretation, and makes no testable predictions to differentiate it from, say, the Copenhagen interpretation.

But if the universe is one big wavefunction, shouldn't some of the branches be able to interfere with each other via quantum interference? Would this be a prediction that contradicts the predictions of other interpretations?

For example, under Many-Worlds, could we, in principle, put a human in some kind of capsule and send them through some version of a double-slit experiment? My understanding is that Copenhagen would predict that capsule would always go through one slit or the other because the human inside is observing it, but under Many-Worlds we'd see interference.

Obviously, there's a lot of practical obstructions to doing this, but I'm wondering if there's something I'm missing with the principle itself.

Improve this question
$\endgroup$
5
  • $\begingroup$ We have a lot of questions here concerning MWI & consciousness, but most of them are closed. But see physics.stackexchange.com/q/297011/123208 physics.stackexchange.com/q/734154/123208 physics.stackexchange.com/q/709903/123208 $\endgroup$ Commented Feb 5 at 19:37
  • $\begingroup$ Not in this world, but perhaps there is another world where I feel differently. $\endgroup$ Commented Feb 5 at 19:39
  • $\begingroup$ To see interference with humans passing a double-slit, you need enough humans to compute a statistic. One experiment is not enough. And all experiments must represent the very same quantum state for any conclusion to be valid. So, no, it does not work, even in principle. $\endgroup$ Commented Feb 5 at 21:16
  • $\begingroup$ @StéphaneRollandin Why is it not possible, in principle, to repeat a macroscopic double-slit experiment enough times to compute a statistic? I understand why it is insurmountably difficult, but is it physically impossible? $\endgroup$ Commented Feb 5 at 21:54
  • $\begingroup$ See for example this answer of mine: physics.stackexchange.com/a/707744/109928 $\endgroup$ Commented Feb 5 at 23:04

3 Answers 3

Reset to default
11
$\begingroup$

It is a feature of all interpretations of QM that they are indistinguishable from one another. “Many-worlds” is no different from “Copenhagen” except the words they use to describe what’s happening; they make the exact same predictions.

The few circumstances where many-worlds claims to be different from more-standard theories, e.g. quantum suicide and immortality rely on assumptions like “conscious observers are actually a quantum mechanical thing”, which stems out of taking the word “observer” too literally when taking QM/SR/GR classes. If many-worlds actually offered a prediction different from Copenhagen’s, and if we could conduct such an experiment, it would already have been done.

On that note:

For example, under Many-Worlds, could we, in principle, put a human in some kind of capsule and send them through a double-slit experiment? My understanding is that Copenhagen would predict that capsule would always go through one slit or the other because the human inside is observing it, but under Many-Worlds we'd see interference.

“Observer” does not equal “human”. An observation is an event whence the quantum-mechanical “information” about a particular property of a particle is released, not when a human looks at it or knows about it or something. This is a common mistake for newcomers to QM.

Improve this answer
$\endgroup$
7
  • $\begingroup$ So what I've gotten wrong is that, if we send a human through a double-slit experiment, then that human is not an observer? In other words, the Copenhagen interpretation would also predict interference? $\endgroup$ Commented Feb 5 at 19:08
  • $\begingroup$ I mean, sending a human through a double slit would constitute slicing them to little bits… but the human doesn’t automatically count as an “observer”. If you continuously make observations of a particle as it goes through double slit, it actually does not interfere even according to Copenhagen (or so I think); this is related to the quantum Zeno effect. $\endgroup$ Commented Feb 5 at 19:10
  • $\begingroup$ That was my point - if the human riding along counts as an observer, then there can be no quantum effects because the wave function never gets a chance to "spread out". But in MWI, "observers" are quantum too, so if there was no interaction with the rest of the universe during the experiment, the wavefunctions that make up the human's body could undergo interference. Predicting what that interference would look like, and preventing said interaction with the universe, is infeasible from a practical standpoint, but based on these answers it seems it's not prevented by any physical law. $\endgroup$ Commented Feb 7 at 0:02
  • $\begingroup$ And as others have pointed out, what I've just described is essentially a reformulation of Wigner's Friend. $\endgroup$ Commented Feb 7 at 0:03
  • $\begingroup$ But a human riding along does not count as an observer. A human can't go through the double slit - a human isn't a particle. Human's atoms are constantly "observing" each other when they interact chemically. The important thing to understand: MWI isn't different than Copenhagen. They make the exact same set of predictions because they are the exact same model, except for those requiring human consciousnesses to act as observers (see also quantum suicide/immortality). $\endgroup$ Commented Feb 7 at 0:16
9
$\begingroup$

In principle, the answer to the question is yes. What you are describing is a version of the so called Wigner's friend experiment, which was used to argue a similar point. See in particular this paragraph from Wikipedia:

Several authors, including Everett, John Archibald Wheeler and David Deutsch, call many-worlds a theory or metatheory, rather than just an interpretation.Everett argued that it was the "only completely coherent approach to explaining both the contents of quantum mechanics and the appearance of the world." Deutsch dismissed the idea that many-worlds is an "interpretation", saying that to call it an interpretation "is like talking about dinosaurs as an 'interpretation' of fossil records."

Note however that observing interference between macroscopic systems is extremely unpractical. For example, one will have first to calculate the full wave function of the macroscopic system with all of its degrees of freedom. For comparison, representing on a computer the full wave function of a single iron atom with its 26 electrons, will require something like $10^{78}$ bits (3 spatial degrees of freedom per electron), which is only slightly less then the total number of protons in the universe.

Another caveat is that the possibility of interference between macroscopic systems might not be unique to MWI. There are probably as many different versions of each interpretation of QM as the number of physicists practicing the theory!

Improve this answer
$\endgroup$
13
  • $\begingroup$ This depends on how you represent it. If you use a spatial grid with Planck length units, perhaps. A Dirac MCHF program that does a fairly good job will run on your phone. $\endgroup$ Commented Feb 5 at 19:50
  • 1
    $\begingroup$ @my2cts The calculation assumes 10 grid point per d.o.f., so it's an atomic scale, not Planck scale. And it is still far from a great resolution ... $\endgroup$ Commented Feb 5 at 20:22
  • $\begingroup$ Can you give the details of this calculation? $\endgroup$ Commented Feb 5 at 23:17
  • $\begingroup$ Are you familiar with quantum chemistry? $\endgroup$ Commented Feb 5 at 23:18
  • $\begingroup$ @my2cts Just place a rectangular grid on 78 dimensional space with 10 grid points along each dimensions ... nothing fancy here $\endgroup$ Commented Feb 6 at 7:45
7
$\begingroup$

The many worlds interpretation (MWI) is what you get if you take the equations of motion of quantum theory seriously as a description of how the world actually works and work out their implications as you would for any other scientific theory.

In classical physics the equations of motion for the $x$ position of a particle are written in terms of a function $x(t)$ such that if you measure $x$ at time $t$, you get the answer $x(t)$.

In quantum physics the equations of motion are written in terms of matrices called observables. The eigenvalues of those observables are the possible results of measuring the relevant quantity. Quantum physics predicts the probability of each of those possible outcomes.

In many experiments, the result of an experiment depends on what happens to all of the possible values of the relevant observable: this is called quantum interference. For an example see Section 2 of this paper

https://arxiv.org/abs/math/9911150

When you walk through a doorway you don't have to take account of all of the possible routes through the doorway. A commonly stated explanation for this fact is that somehow all of the possible values of the observables describing your trajectory vanish except the one you see: this is called collapse. Collapse is incompatible with the equations of motion of quantum theory. Some physicists have tried to modify the equations of motion of quantum theory to include collapse:

https://arxiv.org/abs/2310.14969

It is more common in textbooks to simply state that collapse happens and to give no explanation of how it happens. It's difficult to test a theory like this because it is extremely vague.

Quantum theory without collapse models measurements as interactions that produce records of some property of the measured system. A record is a piece of information that can be copied and copying information out of a quantum system suppresses interference. This is called decoherence:

https://arxiv.org/abs/1911.06282

Any object you can see in everyday life is undergoing such interactions on scales of space and time smaller than those over which they change significantly, e.g. - light reflecting off an object conveys information about its position to other systems, as does the pressure an object exerts on whatever it is resting on and so on. As a result of decoherence such objects tend to obey the laws of classical physics to a good approximation.

Decoherence doesn't eliminate the other values of the monitored observable, it just prevents interference between them. As a result quantum theory describes a reality in which all of the systems around you exist in multiple versions that form layers each of which acts approximately like the world as described by classical physics:

https://arxiv.org/abs/1111.2189

https://arxiv.org/abs/quant-ph/0104033

Hence the name of the MWI.

You couldn't interfere multiple versions of a human being because we're extremely large complicated sacks of chemicals that have to constantly exchange material with the outside world to stay alive, e.g. - breathing, heat exchange etc.

In Section 8 of his 1985 paper "Quantum theory as a universal physical theory" David Deutsch gives an account of an experiment where an AI implemented on a quantum computer could do an interference experiment on himself and know that he was in an unsharp state during that experiment:

https://boulderschool.yale.edu/sites/default/files/files/Deutsch.pdf

Deutsch interprets this as a test of the many worlds interpretation (MWI) versus the Copenhagen interpretation (CI) since the CI wouldn't allow an observer to undergo interference and the MWI would.

There are a lot of Wigner's friend type experiments on the quantum theory of observers such as the Frauchiger and Renner paper:

https://arxiv.org/abs/1604.07422

and there are other more recent papers along similar lines:

https://arxiv.org/abs/2209.06236

https://arxiv.org/abs/2407.06279

More generally the MWI is testable in the same way as other scientific theories:

https://arxiv.org/abs/1508.02048

I should also note that the idea of collapse isn't a good fit for the description of many kinds of quantum measurements, such as repeated, continuous and unsharp measurements:

https://arxiv.org/abs/1604.05973

Collapse theories (and pilot wave theories) also don't reproduce the predictions of relativistic quantum theories:

https://arxiv.org/abs/2205.00568

In practice we don't have to do exotic experiments involving interfering AIs or anything like that. The other interpretations are underdeveloped modifications of quantum theory and should be understood in that light.

Improve this answer
$\endgroup$
9
  • 1
    $\begingroup$ I find it hilarious that people say MWI doesn't have any testable predictions... when the Copenhagen interpretation violates so many ways we understand the universe to behave. The fact that MWI doesn't require, for example, communication faster than the speed of light in a static spacetime, is evidence against Copenhagen and for MWI. We just somehow normalized "quantum" as "weird" and Copenhagen is just so weird, right? It's acausal, non-local, CPT violating, non-deterministic, non-linear, discontinuous... great fit for a "weird quantum theory"! :D $\endgroup$ Commented Feb 6 at 7:52
  • 2
    $\begingroup$ @Luaan Advocates of the CI have two options. (1) Make their theory so vague it becomes untestable and useless. (2) Come up with a specific variant that has the kinds of problems you mention. Both options are bad. $\endgroup$ Commented Feb 6 at 8:10
  • $\begingroup$ Possibly a pedantic note: posts on arXiv that haven't yet gone through peer review aren't strictly speaking papers, but preprints. Also your note about Deutsch's paper isn't correct: it's not an account of an experiment (implying something that was actually performed), but a description of a thought experiment. Quantum computation isn't yet at the point where even a simple AI could be run, so such an experiment can't yet be performed. $\endgroup$ Commented Feb 6 at 17:23
  • 1
    $\begingroup$ @Luaan indeed, Copenhagen should have been dead on arrival considering that it post-dates special relativity by some two decades. And you can read works like Maudlin's book that expound on the nonsense that trying to interpret things with collapse leads to, without drawing the obvious conclusion that collapse is nonsense. (Maudlin also gets the cat wrong by writing down equations for it that obviously violate unitarity.) $\endgroup$ Commented Feb 7 at 1:36
  • 1
    $\begingroup$ Am I wrong, or do portions of this answer directly contradict the currently most-upvoted answer by @controlgroup? $\endgroup$ Commented Feb 7 at 17:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.