Improbability of the Many-Worlds Interpretation?

[CynicusRex]

"Many-worlds implies that all possible alternate histories and futures are real, each representing an actual "world" (or "universe"). In layman's terms, the hypothesis states there is a very large—perhaps infinite—number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes." –Wikipedia

If the MWI would be true, wouldn't there be at least one reality where human civilization advanced much faster than we did and therefore: contacted all other universes; destroyed all the universes; colonized all other universes; etc.

Since, as far as we know, this has not happened, doesn't this make the MWI improbable?

 

[rootone]

As far as I know, most and maybe all MWI proposals have the different worlds causally disconnected.
So whatever goes on in world A can never affect events in world B.
No matter how advanced a civilization may be, it simply isn't possible for it do anything to entities in a hypothetical alternate timeline.

 

[stevendaryl]

As far as I know, most and maybe all MWI proposals have the different worlds causally disconnected.
So whatever goes on in world A can never affect events in world B.
No matter how advanced a civilization may be, it simply isn't possible for it do anything to entities in a hypothetical alternate timeline.

It's not quite comparable, but it reminds me of a comic about an alien race so technologically advance that they can travel from the comic book to the real world. Now, that would be scary.

 

[DrChinese]

If the MWI would be true, wouldn't there be at least one reality where human civilization advanced much faster than we did and therefore: contacted all other universes; destroyed all the universes; colonized all other universes; etc.

Since, as far as we know, this has not happened, doesn't this make the MWI improbable?

As I understand MWI, there is no expectation that all permutations of nature exist. For example, there is no guarantee that there is a world in which I became the US President in 2017. Accordingly, there would be no guarantee that the advanced civilization you imagine exists.

 

[Khashishi]

These "other universes" are not other places, but other possibilities. How do you colonize another possibility?

 

[CynicusRex]

These "other universes" are not other places, but other possibilities. How do you colonize another possibility?

If there is a reasonable possibility some other universe—a place—exists, it might be possible to colonize it. No?

As far as I know, most and maybe all MWI proposals have the different worlds causally disconnected.
So whatever goes on in world A can never affect events in world B.
No matter how advanced a civilization may be, it simply isn't possible for it do anything to entities in a hypothetical alternate timeline.

Has it been proved that traveling to another universe is impossible? I'm just a layman with a question. I'm not defending my theory or anything. It just seems to me, if traveling to another universe hasn't been disproved, it might be possible.

 

[entropy1]

Has it been proved that traveling to another universe is impossible? I'm just a layman with a question. I'm not defending my theory or anything. It just seems to me, if traveling to another universe hasn't been disproved, it might be possible.

I think one reason for it to be impossible could be conservation of energy: if one copy of you goes to A and another copy to B, then, if A travels to B, there would be two copies in one universe for instance.

 

[DrChinese]

Has it been proved that traveling to another universe is impossible? I'm just a layman with a question. I'm not defending my theory or anything. It just seems to me, if traveling to another universe hasn't been disproved, it might be possible.

Since no one has proved that there are any other universes to travel to yet, it might be difficult to prove it can't be done.

But regardless: you misunderstand scientific theory if you think that it's about imagining something and then doing it. As a general rule, the purpose is not to think up things you can disprove. Rather: Theory is a useful description of some pattern which applies to some scope.

 

[bhobba]

If the MWI would be true, wouldn't there be at least one reality where human civilization advanced much faster than we did and therefore: contacted all other universes; destroyed all the universes; colonized all other universes; etc. Since, as far as we know, this has not happened, doesn't this make the MWI improbable?

You are assuming such is possible. In the MW interpretation its not possible to contact the other worlds.

Thanks
Bill

 

[bhobba]

Has it been proved that traveling to another universe is impossible? I'm just a layman with a question. I'm not defending my theory or anything. It just seems to me, if traveling to another universe hasn't been disproved, it might be possible.

Of course not - but the theory pretty much says they are separate and remain so without delving into the details.

Basically MW is an interpretive thing. After decoherence you have a number of possible outcomes. MW interprets those outcomes as separate worlds - that they can't communicate is pretty much implied by defining what they are. Does it prove it - of course not - the definition doesn't say that - but it would seem at odds with its intent.

Thanks
Bill

 

[Michael Price]

As I understand MWI, there is no expectation that all permutations of nature exist. For example, there is no guarantee that there is a world in which I became the US President in 2017. Accordingly, there would be no guarantee that the advanced civilization you imagine exists.

No, many worlds guarantees a very large number of worlds where you are US president. Even in 2017, and even if you're not a US native. It's possible, therefore it exists, according to MWI.

 

[PeterDonis]

many worlds guarantees a very large number of worlds where you are US president.

Only if such a world is physically possible by unitary evolution from the initial conditions. Discussions of the MWI usually blithely assume that anything that seems "possible" to us based on our ordinary everyday intuitions must be physically possible by unitary evolution from the initial conditions, but I think that assumption is actually way too broad. See my posts and the discussion in this previous thread (and also the other previous thread I linked to in post #4 of that one):

https://www.physicsforums.com/threads/is-anything-possible-in-the-many-worlds-interpretation.895347/

 

[akvadrako]

It seems like it's physically possible for Dr. Chinese to become president. To start he could acquire a quantum random number source and use it to bet on the stock market, becoming the richest person on Earth. What could conceivably prevent that from happening?

 

[Michael Price]

It seems like it's physically possible for Dr. Chinese to become president. To start he could acquire a quantum random number source and used it to bet on the stock market, becoming the richest person on Earth. What could conceivably prevent that from happening?

Yes. And by possible here, we include stuff that violates the 2nd law, as DeWitt stressed, which pretty much means anything we can classically imagine happens. Dr Chinese becoming President is small potatoes by comparison. Much weirder stuff will occur on other timelines.

 

[Mentz114]

Yes. And by possible here, we include stuff that violates the 2nd law, as DeWitt stressed, which pretty much means anything we can classically imagine happens. Dr Chinese becoming President is small potatoes by comparison. Much weirder stuff will occur on other timelines.

My universe is as weird as it gets. There are people who actually believe that MWI is true !

 

[Vanadium 50]

Only if such a world is physically possible by unitary evolution from the initial conditions.

That is a key point. None of us (with the exception of Mr. Price) know what is physically possible by unitary evolution from the initial conditions in our universe, which are not known nearly well enough to calculate it.

 

[PeroK]

That is a key point. None of us (with the exception of Mr. Price) know what is physically possible by unitary evolution from the initial conditions in our universe, which are not known nearly well enough to calculate it.

It's even possible in some worlds that a deposit account might outperform technology stocks. Or, perhaps that's going too far!

 

[Michael Price]

That is a key point. None of us (with the exception of Mr. Price) know what is physically possible by unitary evolution from the initial conditions in our universe, which are not known nearly well enough to calculate it.

I know that Schrödinger's cat will be both dead and alive by unitary evolution, because both outcomes are possible (by design).

The boundary conditions of the universe are only relevant if I wish to know what is going on in Andromeda or beyond.

 

[Vanadium 50]

I know that Schrödinger's cat will be both dead and alive by unitary evolution, because both outcomes are possible (by design).

But that doesn't mean you know that there are universes where Dr. Chinese is President.

 

[Michael Price]

But that doesn't mean you know that there are universes where Dr. Chinese is President.

Most classic (in both senses) historical what-ifs are the same as Schrodinger's cat. As I said, since we, following Dewitt, are allowed violate to the 2nd law in MWI, Dr Chinese being President is no big deal. Mutterings about unitary evolution or the boundary conditions of the universe misses the point when it comes to our local part of the cosmos, at the classical level

 

[DrChinese]

It seems like it's physically possible for Dr. Chinese to become president. To start he could acquire a quantum random number source and use it to bet on the stock market, becoming the richest person on Earth. What could conceivably prevent that from happening?

I'm liking where this discussion is going. Not too sure about the president thing though.

 

[PeterDonis]

It seems like it's physically possible for Dr. Chinese to become president.

That's not the same as saying it's physically possible for him to be president, right now, with nothing else changing. But the latter is what @Michael Price appears to be claiming in post #11.

 

[PeterDonis]

The boundary conditions of the universe are only relevant if I wish to know what is going on in Andromeda or beyond.

That's not correct. A significant part of the boundary conditions of the universe are in our past light cone, right now, so they are certainly relevant for trying to predict what is possible here and now by unitary evolution.

 

[PeterDonis]

by possible here, we include stuff that violates the 2nd law, as DeWitt stressed, which pretty much means anything we can classically imagine happens

If things that can violate the 2nd law can happen in some MWI worlds, then the MWI undermines itself, because the possibility of violations of the 2nd law means we can't trust the evidence we have of the past. But if we can't trust the evidence we have of the past, we can't trust the experimental data that led us to adopt QM, and led some people to postulate the MWI, in the first place.

 

[Michael Price]

That's not correct. A significant part of the boundary conditions of the universe are in our past light cone, right now, so they are certainly relevant for trying to predict what is possible here and now by unitary evolution.

I meant, to determine what happens in the lab we only need the boundary conditions defined on the lab.

 

[PeterDonis]

to determine what happens in the lab we only need the boundary conditions defined on the lab.

And if the MWI is true, those boundary conditions are already going to include a huge number of terms in a vast superposition, based on what happened in the entire past light cone up to that point and all of the possible quantum branch points that occurred. So there doesn't need to be any "world splitting" in the lab itself to have multiple worlds according to the MWI; multiple worlds are already included in the lab's boundary conditions. For example, if you are going to claim that there is a world in which @DrChinese is the US president, that doesn't mean we had to do something in the lab to switch timelines to that one.

But the question is, which multiple worlds are already included in the lab's boundary conditions? Is there a world included in those boundary conditions in which @DrChinese is the US president? Supposing that there is some world in which @DrChinese is the US president, how many other things are different in that world as compared to this one? Is there even a lab in the same place at the same time? And if there isn't, it makes no sense to restrict attention to "the boundary conditions defined on the lab" but also talk about the possibility of @DrChinese being president, because those two things are mutually exclusive: if we are in a world in which a lab at the same place at the same time exists, then @DrChinese is not US president in that world, because none of the worlds in which @DrChinese is president have a lab at that time and place.

And in fact, even that doesn't fully unpack the issue, because to even have a world in which @DrChinese is the US president, it has to be that there is a US with a president, and a person who is properly identified as @DrChinese, and that person has to have gone through all the process of getting to the point where he is elected US president. And you can't tell whether such a world is even possible by just looking at the boundary conditions of your lab. And we can go further: there has to be a planet Earth that had humans evolve on it, with a history the same as our Earth; which means there has to be a solar system the same as ours, of the same age, in the same galaxy, etc., etc.

The point I'm making is that all this blithe talk about "multiple worlds" fails to pay attention to specifically how such multiple worlds get created, if the MWI is true. They don't get created by magic. They don't get created just because we humans can imagine them. They get created by having genuine quantum mechanical uncertainty, "Schrodinger's Cat" type uncertainty where single quantum branch points have macroscopic consequences, involved in particular key events in particular timelines. And the only point in the entire universe's development where we are pretty sure that actually happened was at the end of inflation, when the quantum fluctuations in the inflaton field got transferred to the Standard Model fields and the particular pattern of small-scale inhomgeneities was formed that were to eventually evolve into our present galaxies and stars and planets. Everything after that is classical evolution and need only have led to a single macroscopic world. And if other universes exist in MWI "worlds" that correspond to other possible patterns of small-scale inhomogeneities created at the end of inflation, they won't even have a solar system like ours, or a planet Earth like ours, or humans like us, or a US with a president--let alone have all that stuff be the same but still have just enough difference to have @DrChinese as the US president in 2019.

 

[akvadrako]

They get created by having genuine quantum mechanical uncertainty, "Schrodinger's Cat" type uncertainty where single quantum branch points have macroscopic consequences, involved in particular key events in particular timelines. And the only point in the entire universe's development where we are pretty sure that actually happened was at the end of inflation, when the quantum fluctuations in the inflaton field got transferred to the Standard Model fields and the particular pattern of small-scale inhomgeneities was formed that were to eventually evolve into our present galaxies and stars and planets. Everything after that is classical evolution and need only have led to a single macroscopic world.

It's not clear to me that Michael Price is saying Dr. Chinese is president right now. For me that doesn't make sense, since he clearly isn't president based on the information contained in this lab. I'm not even sure you can compare times between different worlds as they are casually disconnected from the point of branching.

But I also think you can't just consider classical evolution since inflation. When people perform quantum measurements now, it forms new branches. This is most clearly true when people use quantum outcomes to make macroscopic decisions. It's not clear if you are suggesting an alternative view, where quantum outcomes are overwhelmed by classical evolution and the branch we are living in is actually deterministic.

I also think quantum branching and decoherence is ubiquitous, though it's more controversial. This is based on the only analysis I've seen on the topic, which I linked in the other linked thread, looking at how the average coin flip is an amplification of quantum uncertainty.

 

[PeterDonis]

When people perform quantum measurements now, it forms new branches.

Yes, but how often does that happen? And how often does the outcome of such an experiment affect who gets elected President?

This is most clearly true when people use quantum outcomes to make macroscopic decisions.

And how often does that happen?

It's not clear if you are suggesting an alternative view, where quantum outcomes are overwhelmed by classical evolution

No, I'm not. I'm just saying that "Schrodinger's Cat" type situations, where the outcome of a single quantum measurement leads to macroscopically different states of a macroscopic object like a cat, are rare.

I also think quantum branching and decoherence is ubiquitous, though it's more controversial

Yes, more controversial indeed.

 

[bhobba]

Yes, but how often does that happen?

At a deeper level what exactly, defined entirely in terms of QM because everything is QM, is a quantum measurement? A lot of progress has been made in solving that, but issues still remain, It may even be incomplete giving Einstein the last laugh. People often talk about issues with QM, but the above is not often mentioned.

Thanks
Bill

 

[akvadrako]

Yes, but how often does that happen? And how often does the outcome of such an experiment affect who gets elected President?

Thanks! Your position is clear and comes down to a technical point which hasn’t been deeply explored beyond that paper I mentioned. Indeed MW supporters like Sean Carroll casually assume branching happens all the time without really showing the details.

But I have done stuff like use an online service tied to a quantum random number generator to create a 1GB program and run it. So some other version of me should be capable of doing anything that a compressed and focused AI can possibly explain.

Maybe it doesn’t happen all the time, but even the occasional event is enough to make a big difference.

 

[DarMM]

So some other version of me should be capable of doing anything that a compressed and focused AI can possibly explain

Can you explain this a bit more?

 

[akvadrako]

Can you explain this a bit more?

Well if I manage to create a branch for every possible program of size X, in this example 1GB, some of those programs should contain AI that can do any arbitrary thing, like make Dr Chinese president or explain QM completely satisfactorily. My point is about how essentially MW does allow everything to happen, even *if* it needs to be invoked explicitly.

 

[Buzz Bloom]

This thread discussion has reminded me of a variation of the MWI a coworker explained to me about 15 years or so ago. I am wondering it anyone at PF knows of any publication that discusses this variation. I tried to find one searching the Internet, but I had no luck.

The MWI variation is that the multiple universes are not real, but instead are contingent. This mean that each of the various possible combinations of all of the possible future measurements that could possibly happen, according the the wave function of everything, define one possible contingent universe. When a measurement occurs, the single real universe becomes constrained to have it's wave function of everything reconfigured to have new initial conditions consistant with the new measurement. The collection of all the contingent universes are also reduced to just those which are consistant with the new measurement.

 

[DarMM]

Well if I manage to create a branch for every possible program of size X, in this example 1GB, some of those programs should contain AI that can do any arbitrary thing, like make Dr Chinese president or explain QM completely satisfactorily. My point is about how essentially MW does allow everything to happen, even *if* it needs to be invoked explicitly.

By AI here do you mean strong AI that genuinely figures out how to make @DrChinese president and tells you how to go about it? How do we know such a thing exists in the 1GB program space? Even assuming the human brain works like a "program" in any way, we know it exceeds a petabyte in storage?

One odd one is that technically there is some subset of the state space of the gunpowder where due to entropy decrease the gun doesn't fire for an incredibly rare entropic decrease. So if you get somebody to fire a machine gun straight at you there's a world where every bullet failed to fire.

This is beginning to sound like a Greg Egan novel, pretty cool thread. I think it breaks the record for @DrChinese mentions in one thread.

 

[akvadrako]

By AI here do you mean strong AI that genuinely figures out how to make @DrChinese president and tells you how to go about it? How do we know such a thing exists in the 1GB program space? Even assuming the human brain works like a "program" in any way, we know it exceeds a petabyte in storage?

I don't mean a fully general AI, just the most perfect program to accomplish one task. It doesn't even really need to figure it out - it just knows and explains the steps to take. 1GB is arbitrary but it's about the size of the human genome. Surely it's enough for all the content needed: the perfect ads, bots, private bitcoin keys to bribe and passwords / encryption keys to hack voting machines.

Do physicists not really think about this stuff? How information rules everything?

 

[PeterDonis]

One odd one is that technically there is some subset of the state space of the gunpowder where due to entropy decrease the gun doesn't fire for an incredibly rare entropic decrease. So if you get somebody to fire a machine gun straight at you there's a world where every bullet failed to fire.

You're assuming that there is quantum uncertainty involved in gunpowder firing. That's an example of the kind of assumption I am objecting to.

 

[DarMM]

Do physicists not really think about this stuff? How information rules everything?

"Everything" and "rules" meaning what exactly?

I also wouldn't connect a clarification request from me about what exactly you were referring to to be related in any way to what physicists think about or don't think about. I just wasn't sure precisely what you meant.

 

[DarMM]

You're assuming that there is quantum uncertainty involved in gunpowder firing. That's an example of the kind of assumption I am objecting to.

Doesn't tunneling permit exploration of the chemical state space even if it's very small? My (very rough) intuition is that quantum effects in chemistry are suppressed when you reach arsenate molecules.

 

[PeterDonis]

Doesn't tunneling permit exploration of the chemical state space even if it's very small?

The state space of what? A single gunpowder molecule? (Well, I know it isn't made up of molecules of a single chemical species, but you get the idea.) You can't fire a bullet with a single gunpowder molecule. You need something like an Avogadro's number of them. Yes, quite possibly some miniscule fraction of that Avodagro's number of molecules will be in a "can't fire" state due to quantum tunnelling, but so what? Enough will be in that state to fire the bullet. Or if the bullet doesn't fire it will be for classical reasons, like you left the ammunition unused too long and exceeded its shelf life, or the firing pin didn't work. It won't be because quantum uncertainty put the Avogadro's number of gunpowder molecules into a superposition of "can fire" and "can't fire", which is what would need to happen for quantum uncertainty to cause a splitting of worlds according to the MWI when the firing was attempted.

 

[DarMM]

Yes, quite possibly some miniscule fraction of that Avodagro's number of molecules will be in a "can't fire" state due to quantum tunnelling, but so what?

Because then isn't there a chance they'll all be in the "can't fire" state?

 

[PeterDonis]

Because then isn't there a chance they'll all be in the "can't fire" state?

No. Or at least, if you're going to allow for that possibility, then you are allowing for possibilities that invalidate our belief in past data, and that version of the MWI undermines itself, as I argued in post #24. The chance that enough of the molecules will happen to all have tunneled into the "can't fire" state, and none of them will have tunneled back into the "can fire" state, at the exact instant that you try to fire the bullet, is comparable to the chance that @DrChinese actually was president of the US 5 minutes ago, but then a quantum fluctuation happened that changed everything on Earth, including all of our memories, to the state we all perceive and remember now. And if you allow for possibilities like that, all bets are off and there's no point in doing science at all.

 

[DarMM]

What prevents it though if you permit any arbitrary tunnling outcome to occur? What principle would stop me from allowing it in the space of outcomes?

 

[PeterDonis]

What prevents it though if you permit any arbitrary tunnling outcome to occur? What principle would stop me from allowing it in the space of outcomes?

If you take the MWI entirely literally, nothing does. And I'm saying that doing that makes the MWI undermine itself, since it tells us we can't trust our memories and records of past data, because we could be in one of the worlds where a quantum fluctuation changed them. And what forces us to adopt QM in the first place is our memories and records of past data. If we can't trust those, what reason do we have to even believe QM, let alone the particular MWI interpretation of it?

 

[DarMM]

I'll need to chew on that, very interesting.

What do you make of the fact that in "single world" QM there is still a small chance that such an event could occur. Is it basically that such an event essentially has never occurred so we can ignore it, but that in MWI you know that there are definitely "science nullifying" worlds.

I suppose there would also be worlds where the Born rule was never validated due to unfortunate fluctuations causing it to be rejected.

 

[PeterDonis]

What do you make of the fact that in "single world" QM there is still a small chance that such an event could occur.

That depends on whether you think sufficiently small probabilities--something like 1 expected event per $10^{70}$ years, say, where our universe has only existed for $10^{10}$ years--are actually meaningful. I personally think probabilities that small are just a sign that the theory is incomplete, just as the presence of singularities in certain GR solutions is a sign that GR is incomplete.

 

[PeterDonis]

I suppose there would also be worlds where the Born rule was never validated due to unfortunate fluctuations causing it to be rejected.

According to the MWI, yes, there should be worlds (lots of them) in which the Born rule does not hold. I have seen papers claiming to show that the set of such worlds is a "set of measure zero", or something similar, but I'm personally not convinced by such arguments.

 

[DarMM]

That depends on whether you think sufficiently small probabilities--something like 1 expected event per $10^{70}$ years, say, where our universe has only existed for $10^{10}$ years--are actually meaningful. I personally think probabilities that small are just a sign that the theory is incomplete, just as the presence of singularities in certain GR solutions is a sign that GR is incomplete.

You might be interested that Roland Omnès has similar views to be seen on pages 82-84 and 235-236 of his "Understanding Quantum Mechanics". Ultimately these are related to earlier views due to Émile Borel in:
"Le jeur, la chance et les théories scientifiques modernes" (1941)

 

[Klystron]

Do physicists not really think about this stuff? How information rules everything?

Information theory expands the latter question to:
"How information describes everything?"
Otherwise, are we not limited to rule-based descriptions?

Broader question:
Do physicists (scientists) think everything can be known?

Self-referential form:
Does Information inform everything knowable?

 

[Hans de Vries]

And if the MWI is true, those boundary conditions are already going to include a huge number of terms in a vast superposition, based on what happened in the entire past light cone up to that point and all of the possible quantum branch points that occurred.

I should try not to get involved with quantum philosophical discussions, but okay, having done this sort of "hypothetical MWI" calculations in the past:

- There are ~$10^{80}$ elementary particles in the universe.
- Interactions / State transitions take place at the femto second level in stars.
- The Universe is circa $4*10^{32}$ femto seconds old.
- Each event may actually span much more than 2 branches, say $N$ branches.

So very, very roughly there may be $N^{10^{112}}$ different universes right now.
The total number of particles (and thus energy) of the multiverse increases by a factor of $N^{10^{95}}$ per second.

Each of the $10^{80}*N^{10^{112}}$ particles in the multiverse somehow knows exactly to which of the $N^{10^{112}}$ universes it belongs and only interacts with the particles labeled to be in the same universe. Each particle would need some universe label to distinguish between other universes. The number of bits you would need for one such label for one particle would be far more then the total information content of an entire universe.

The whole universe must split near instantaneously over the entire size of the universe according to some absolute reference frame to which time is synchronized, otherwise the order of splitting and the universe labeling would mess up completely. The average time between two splits is $10^{-95}$ seconds during which the split has to propagate circa $5*10^{10}$ light years.

So the minimum speed at which the splitting propagates must be something in the order of $10^{114}$ times faster as the speed of light.Yeah really, MWI makes the task of understanding the physics of nature so much easier...

 

[PeroK]

I should try not to get involved with quantum philosophical discussions, but okay...

Perhaps you should read this first:

http://www.preposterousuniverse.com...ion-of-quantum-mechanics-is-probably-correct/
It might save you from tilting at straw men.