> If you believe every outcome happens then one thing happening with higher probability than another loses meaning. The decision theory argument put forward by some many worlds proponents fails to solve this as it just provides a calculation you can do that produces the Born probabilities. It doesn't provide a convincing interpretation of them.
What further interpretation is needed? To the extent that we have a number we can measure, we have a calculation that can predict it. More interpretation would be nice, but fundamentally every interpretation of QM has this problem, it's not a unique problem with MWI.
> It assumes an infinite multiverse that is splitting into infinite variants in every infinitesimal instant, in possibly the grossest violation of conservation of momentum and energy (and therefore the corresponding symmetries) that could be imagined.
It assumes continuous unitary evolution that preserves momentum and energy in exactly the way you'd expect, exactly the way we already assume they work. Accepting that you can reasonably make calculations about infinitesimal changes is literally the foundation of physics, it's how Newton was able to make a theory of gravity (and people were just as unhappy about it then).
Of course it would falsify quantum mechanics, because MWI is nothing but the most austere version of QM. But people always speak of Copenhagen as if it was the standard interpretation of QM and everything else must be measurable different from it. In another world, MWI would have been the first consensus interpretation, and people postulating unfalsifiable wave collapses would be the weirdos.
Also, MWI doesn't assume infinite worlds, it predicts them. It's a consequence, not a postulate.
Labels are important and I think progress on Everett QM (or, maybe “unitary QM”?) has been held back by the label “Many Worlds Interpretation”.
“Worlds” is inaccurate since there is still only one world (the ordinary, uncontroversial, tensor product Hilbert space of textbook qm). It the stuff in the world that enters into superposition, among which stuff are the atoms making up “observers”.
Similarly “completion” would be a better word than “interpretation”.
The multiverse may be a consequence of Everett’s theory, but the reality shattering nature of that consequence warrants scrutiny of the theory.
Consider one of Xeno’s paradoxes. One may postulate that to go from A to B that first one passes through a midpoint C, and by naively applying induction (I.e. without knowing under what circumstances am infinite sum converges to a finite value), one concludes that a consequence is motion itself cannot exist.
However, the absurdity of this consequence should mot lead us to doubt the existence of motion, but quite the opposite, it hints to us that the either the postulate is false, or otherwise we are missing some essential knowledge that allows to determine the correct consequences of that postulate.
The Copenhagen interpretation is the most obvious thing to compare it to, but personally I'm more interested in de Broglie-Bohm (pilot wave), which might have eclipsed it in the 20th century if people had paid attention when Grete Hermann pointed out Jon von Neumann's mistake.
"The most austere version of QM" is marketing bullshit and not widely accepted (or particularly meaningful). Many worlds does postulate its many worlds rather than predict them as it provides no way their existence can ever be tested for.
Equating Schrodinger's equation with many worlds is both intellectual dishonesty and begging the question. Any interpretation of conventional quantum mechanics involves Schrodinger's equation. Penrose's ideas (referenced by Sean Carroll in quotes others have posted on this thread) involve violations of Schrodinger's equation because they are actual new theories that differ from standard quantum mechanics. In other words, Penrose is doing actual physics rather than blowing smoke up people's arses. This fact doesn't convey some falsifiability on many-worlds, which is just one of several interpretations of QM that all make the same predictions.
> Many worlds does postulate its many worlds rather than predict them as it provides no way their existence can ever be tested for.
Many Worlds just postulates that if you put a cat in a superposition of two quantum states (alive + dead), there is no sensation associated with that superposition. The cat is in a superposition of having the sensation of being alive, and having the sensation of being dead, but there is no observable quality of "being in a superposition of alive and dead". Both of the superimposed pure states of the cat feel decidedly one way or the other.
Hence, when a human is in a superposition, we would not know it. Both of our superimposed states are experiencing the feeling of being in a pure state of looking at an instrument measuring a photon with polarization ↑, or looking at an instrument measuring a photon with polarization ↓. Being in an (↑ + ↓) superposition doesn't feel like looking at an instrument display and seeing a blurry reading. We feel like we saw the instrument reading ↑, and we (as an element of the quantum system) cannot interact with the branch of the wavefunction that we are superimposed with in which we saw the instrument reading ↓.
This isn't a separate postulate, it's just straightforward quantum theory applied to quantum systems large enough to have opinions about their own state.
Pilot Wave and Objective Collapse are physical theories, Copenhagen is an administrative agreement, Many Worlds is pop-sci religion.
> Many worlds does postulate its many worlds rather than predict them as it provides no way their existence can ever be tested for.
Your conclusion does not follow from your premise. MWI does postulate different stuff from Copenhagen, but the existence of many worlds is not one of them. Just because I postulate Peano arithmetic doesn’t mean I also postulate 2+2=4. The latter is but a consequence of the former.
My understanding of MWI, is that it just "believes" the equations. Specifically, if the equations say the complex amplitude is not null, then this stuff is real. And that’s about it. It’s not our fault the equations describe non-null amplitudes, that if real are many worlds.
This interpretation business could apply to macro-scale physics as well: when you send a probe so far out there that it crosses the limit of the observable universe, does it cease to exist? What if there’s people inside, will they die? One would be hard pressed to argue from their discontinued existence: why would we disbelieve the equations of relativity, that work so well for everything we can observe?
Conversely, what makes you think that the equations that work so well on everything we can observe, would somehow stop working (or stop applying) on the stuff we cannot observe? I know you didn’t explicitly disbelieve the equations, but since we can’t at the same time believe in their general applicability and disbelieve MWI, it’s hard to interpret your refusal to acknowledge MIW as the most probably hypothesis as anything but scepticism about the applicability of those equations.
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To throw you a bone I personally do have a reason to be wary: physics is currently inconsistent, and when we get a credible theory of everything, it may very well use different equations that do not describe many worlds, make long range probes vanish, or both.
copenhagen is popular for historical reasons including our historical obsession with the mind-body duality. many physicists i talk to secretly admit to MWI/universal wavefunction making more sense but it is metaphysics and academia is a conservative institution, especially for metaphysics.
> all you need to do to falsify the many worlds-interpretation is to do an experiment where the wave function is not under the Schrödinger equation.
Which, Carroll conveniently fails to note, happens every time we make a measurement on a system that is not in an eigenstate of the observable being measured. For example, every time an H polarization measurement is made on a V+ polarized qubit, which happens all the time in quantum computing, the result is either H+ or H-, with 50% probability of each; but before the measurement the qubit was in state V+. The time evolution from V+ to H+ or H- is not time evolution under the Schrodinger equation.
MWI proponents explain this away by saying there is another branch of the wave function where the result is the other one (for example, H- if we observed the result H+), so that the whole time evolution is still under the Schrodinger equation. (Note that "the whole time evolution" here has to include the entire universe, not just the qubit being measured.) But MWI proponents also have to say that this other branch of the wave function is in principle unobservable. In other words, the MWI itself says it is not falsifiable.
In a general, philosophical sense it probably is non-falsifiable. But... where does it end?
What if we took the measurement 5ns, 500ms, 5 seconds, 5 hours later, because of, you know, things. Billions of universes springing up every second does not seem plausible to me and it does sound like an escape hatch: we don't know why it happened that way, so let's just say there is another unobservable universe where it happened the other way round.
Which is nice, I don't have to think about it anymore. But... it kind of reminds me of the flying arrow or a turtle you can never reach. If you are thinking in the same problem space you will never reach the correct conclusion. So, what we really need is the different mode of thought (I wish I would know to suggest something ;) ), not an escape hatch.
> Billions of universes springing up every second does not seem plausible to me
Why? Do you have any reason for this, or is it just a gut feeling?
If it’s just a gut feeling, I’ll note that this was exactly the reaction most of the intelligencia had to Capurnicus and Galileo. Turns out the universe is a caster place than we previously knew.
Occam's razor?
Occam's razor cuts both ways :-)
Some people when they hear that MWI implies that there are many worlds and that the wave function branches they think that implies that MWI is "adding" something so Occam's razor should work against it.
OTOH what MWI postulates that the Schrodinger equation is all there is and the collapse of the wave function is observed because the observer is entangled with the system it measured.
In order to make an intuitive sense of what's going on, MWI invokes the "worlds" and the "branching" but that's just a way for us to grasp how it would feel if ourselves are part of the entangled system. But as many analogies whose purpose is to tickle the intuition it doesn't work for everybody and may be confusing the discourse more than helping it.
But the thing is: the everett interpretation is simpler and it requires less additional rules and mechanisms to explain what we observe. So Occam's razor should favor it.
Simplicity in terms of weighing theory cost isn't about rules, but about explanatory posits. MWI posits exponential growth in discernible state about which sufficient information must be carried by whatever grounds the wavefunction. But you don't get this multiplicity of state/information for free. This is a theory cost that is as profligate as one can imagine. It is magical thinking to imagine that the universe gets this for free.
I don't know where the expression comes from, but "the only numbers that need no justification are zero and infinity". Think of it this way, if you found a coordinate system within the universe that goes from 0,0 to 100,100, you'd have to ask "but why exactly 100?" whereas it would be less surprising if the system had no apparent limit and just could go to infinity on any axis.
The multiplicity of MWI is like that: just a maximally extended coordinate system.
it’s not “billions of universes springing up”, it’s just simple wavefunction evolution according to the laws we already know.
“collapse” seems much more arbitrary, especially as we are able to construct larger and larger objects that are best described by wavefunction mechanics. it seems obvious we just become entangled with the experiment through decoherence. idk, it probably is non-falsifiable
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I mean, obviously. If you let a system interact with the external world that system on its own will not follow the schrodinger equation.
Carroll is suggesting we try to observe this behavior from a sufficiently large isolated system.
> If you let a system interact with the external world that system on its own will not follow the schrodinger equation.
Not only that--even the combined system + what it is interacting with won't.
> Carroll is suggesting we try to observe this behavior from a sufficiently large isolated system.
But you can't, even with the entire universe, if measurements have single results. In the MWI, they don't; any measurement has all possible results. But we don't observe that, we observe measurements to have single results, so the MWI has to jump through hoops to explain that discrepancy away--and in doing so, makes itself unfalsifiable.
sure it’s unfalsifiable in that there is always an interpretation of copenhagen that will predict the same thing as MWI, but if we are able to construct a very large object that doesn’t show “collapse”, it seems vibe-wise that the same process is what is happening to us when we make a measurement, just more chaotically (aka decoherence)
> But we don't observe that, we observe measurements to have single results
What do you think it would feel like to observe a measurement to have multiple results simultaneously? And do you think MWI predicts any branch of the wavefunction would exist in which a human would be experiencing that sensation? It doesn't; it predicts a superposition of (human observes result A + human observes result B), with ~zero probability of (human observes both results A + B). So I'm not sure which hoops you think MWI needs to jump through to match our subjective experience.
There are objective collapse theories that lead to different physical predictions. Copenhagen is not actually a well defined theory because it does not define measurement.
If we measured objective collapse it would falsify MWI.
> There are objective collapse theories that lead to different physical predictions.
Yes, that's true. So far all of the ones whose predictions can be tested have failed the tests. But if one were to succeed, yes, that would falsify the MWI.
Well. They're not discernible states if they are in another world. That's the very definition of "another world".
From an outside viewer that hasn't been entangled with you, no branching happens and they see the overall state evolve according to the Schrodinger equation.
Imagine that w create very small machines that could perform some observations and record them into some internal log.
Now imagine creating a particle in some state and have that machine measure that state and record it internally. Provided that the particle+machine system is isolated from the environment (and us) you'd probably agree that the particle and machine are entangled and that the state of the "log" is not in a defined state until we measure it.
Now, that information processing machine is not a human but I think it may be useful to describe how the external world would look like from it's point of view. For example, if that machine performed several experiments on that test particle or other particles it would note in its log data that would confirm that it obeys the Born rule.
In that sense, it's "useful" to imagine how things look like from the "point" of view of that machine and it's possible outcomes in the many worlds, although these worlds might not really matter to us since we can describe the system that includes that machine as having been in a mixed state all along until we humans actually observe its "log"
The likelihood that people start to object to this though experiment gets higher and higher s as soon as one starts to imply that we humans ourselves are exactly like that machine.
For me, the observation that our ability to accept or reject this interpretation is so much tied to our intuitions about our own _self_ is a strong hint that actually favours the theory. We humans have been known to be trapped by our point of view
>Well. They're not discernible states if they are in another world. That's the very definition of "another world".
Discernible in principle. Either the states are distinguishable from within the system (e.g. two branches that disagree on the state of the world), from an outside observer, or a God's eye view. If none of this is true then there is no other world.
>Now imagine creating a particle in some state and have that machine measure that state and record it internally. Provided that the particle+machine system is isolated from the environment (and us) you'd probably agree that the particle and machine are entangled and that the state of the "log" is not in a defined state until we measure it.
Lets further imagine a large collection of these machines connected in series. Each machine can perform one of two experiments at any given trial, and which experiment they run depends on the most recent outcome of the prior machine in the series (e.g. the direction of a particle's spin). There are an exponential number of scenarios as the number of machines in series grows. If we imagine this system entangled until we observe it, and we expect that the definite state is consistent in terms of which particle the machine measured and the corresponding prior outcome in the series, then the entangled system just has to carry discernible state in proportion to the exponential state space of the system. If not, then the system isn't in an indefinite state until observed. The concerns about exponential theory cost remain. Unless the state space is bounded by collapse or some other means, the theoretical cost to MWI or any theory based on Schrodinger without additional posits is supreme.
>In that sense, it's "useful" to imagine how things look like from the "point" of view of that machine and it's possible outcomes in the many worlds, although these worlds might not really matter to us since we can describe the system that includes that machine as having been in a mixed state all along until we humans actually observe its "log"
There's a tension here that needs to be released somehow. On the one hand, you have an external observer determining the system is in a superposition of states. On the other hand, you have the perspective from within the system of everything being definite. You can't just carry both forward simultaneously without addressing the disagreement on the state of reality. Either there are really an unbounded number of branches, the branches are bounded by some mechanism, or there are no branches and the evolution of reality happens asynchronously.
The sibling reply is correct, but I'll give a different answer connected to the analogy I gave that you are replying to. Occam's razor prefers simpler theories not smaller universes.
The idea that the universe consists of seemingly uncountable numbers of stars, each of which is another Sun and perhaps has planets of its own, is vastly more detail than the old heliocentric model of one Sun, a fixed number of planets, and stars being pinpricks in the tapestry surrounding the solar system. Nonetheless Occam's razor suggests that we accept the view that the stars are suns and we are just one planet among many billions upon billions, because although that universe requires vastly more detail to describe, it is governed by a simpler set of rules. Unifying heaven and earth under a single, simple set of physical laws, discovered by Newton, makes for a simpler scientific model, even if it introduces the possibility of the universe being vastly larger than we originally thought.
Similar situation with many-worlds. If MWI is right, then the "universe" if interpreted to include all reachable branches of the so-called multiverse, is vastly larger than we previously believed. But it is also simpler, in a strict Occam's razor sense, because it does away with the concept of collapse. Wave function collapse is an additional physical rule in the Copenhagen interpretation, whereas it's just an illusion and not an enumerated part of physical law in MWI.
Occam's razor is not just about simple rules. The original formulation is: "plurality should not be posited without necessity". Necessity here is empirical adequacy. A theory that posits an unbounded, exponential growth in states fails the simplicity test against an empirically adequate theory that posits bounded state.
The philosophy of science has advanced significantly since the 12th century, and scientists don’t generally use the original definition anymore.
I have never, ever heard it expressed in this statistical mechanics sense, and it doesn’t line up with most advances in physics (my field) which have been accompanied by increased number of states but fewer rules.
> What do you think it would feel like to observe a measurement to have multiple results simultaneously?
I have no idea.
> do you think MWI predicts any branch of the wavefunction would exist in which a human would be experiencing that sensation?
No. As you say, the MWI claims that in each branch of the wave function, the human experiences a single result of a measurement.
However, that is completely different from the way QM normally treats individual branches of the wave function in an entangled superposition, which is what we are talking about--"measurement" in the MWI is just an interaction that entangles a "measured system" with a "measuring device" (and eventually with "the brain of a human who looks at the measuring device to read off its result") and results in an entangled superposition of all of those subsystems. Normally, in individual branches of an entangled superposition of multiple subsystems, each subsystem has no well-defined state at all. Only the full system, including all branches of the superposition, has a well-defined state.
That would imply that in the case of a "measurement", the measuring device in each individual branch, and the brain of the human that looks at it, would have no well-defined state at all. It would not imply that in each branch, the measuring device and the brain of the human that looks at it has a well-defined state that corresponds to a particular measurement result being recorded and observed.
In terms of your "feel like", the way QM normally treats individual branches of the wave function in an entangled superposition, it should not "feel like" anything in any individual branch. The human should only "feel like" something, if at all, in the full wave function, containing all the branches. And of course nobody has any idea what it would "feel like" in such a state--but it clearly would not "feel like" having observed a particular result of a measurement.
> I'm not sure which hoops you think MWI needs to jump through to match our subjective experience.
It needs to explain why, somehow, when a "measuring device" or a human observer is involved, the way QM treats individual branches of entangled superpositions drastically changes. See above.