Equivalently we can say that neither QFT nor string theory gains anything from the empty hand-waviness of the copehagen interpretation, Everett's many-world's, etc.
Those also don't really play a role in the professional life of a physicist, nowadays you really do stare at computer screens a lot and try to find things in data, it's not like physicists sit around and discuss many-world theory as part of their day job.
These are just the popular things that make it out into public discourse
I have an unrelated question to the physics nerds. Is it even possible to determine whether the universe is deterministic or not from inside the universe?
Let's imagine a simplified example. I give you two sets of numbers (let's pretend they are atom coordinates). One set is purely random, another one is generated with a very simple formula (example: f(n) = SHA256(N + salt)). I will give you as many of the numbers as you want. Can you determine which set is which (if you don't know the formula ahead of time)?
It can also be explained by branching. Run a simulation of a universe, and every time a random bit is required, fork into two separate processes. One where the "random" bit is 1 and another where the bit is 0. From the inside, it would seem indistinguishable from true randomness. But the system as a whole is purely deterministic.
How would forking occur? Forking requires copying data. Does the universe get cloned infinitely many copies all the time? That's quite an extraordinary claim.
Note that you're assuming that a nanosecond is a tiny amount of time. It's tiny relative to what we're used to, but as far as what "implements the universe" goes, a nanosecond could be enough time for a huge amount of occurrences to happen within. We don't know.
This answer seems to be assuming the universe must be implemented on a classical computer and computed in a reasonable amount of time (that's not slowing down exponentially over time). That's not necessarily true.
I feel that assuming that the nature of how the universe is computed matches up with our first intuition feels a bit like the assumption in geocentrism that the universe matches up with our intuition of Earth being the center and the most significant body.
Even if you accept the basic premise of the universe being a simulation, there are still many ways it could work with MWI. The simulator's universe needn't have classical physics; the simulation could be running on quantum computers or something more advanced. Or the simulator's universe could be classical with a classical computer doing the simulation at exponentially-decreasing speeds as it has to simulate the increasing number of branches. It wouldn't make any difference to us how long the simulation takes to compute us, as long as the simulating machine doesn't break down and succumb to entropy. The simulator's universe could be something like Conway's game of life, and the simulation is running on a turing machine pattern which will never decay or break down. (I might be borrowing more than a few ideas from Permutation City here.)
When I was younger and had most interest in non-useful things (in a boring sense) I came to "the theory" that each time observation or logical induction is performed, the universe splits into many variants ahead of time and these variants that produce "oops" in terms of contradiction simply disappear. Each time you see something weird but real, it is oops that survived, because you didn't see something contrary yet (and now you cannot, because only one thing has to be remaining).
That made me sad, because we could come to hyperdrives and FTL journeys to alien worlds, but some people in 20th century made few observations and conclusions that now prevented fun forever. Magic things were easy before the technology, now they are physics-hard. Our universe is spoiled in a very wrong way.
Amazingly, we have an experiment that shows that this is not the case, if you make some pretty reasonable assumptions. It's based on Bell's theorem. Your statement is very intuitive, and the fact that it may be wrong is very surprising.
Not everyone agrees on what is "reasonable". I have no problem giving up locality, but "true" randomness (i.e., information generated without an algorithm) seems like a philosophical cop out.
Note that something can also be globally deterministic but indeterministic from the perspective of a subsystem. In this sense, the universe would appear random as far as we're concerned, but not random to whoever is simulating the universe (this is often called superdeterminism, but I think that's a silly word—the universe is either deterministic or it isn't).
The probability of an event depends on your knowledge. Whether something is deterministic is a question of probability. Hence whether something is deterministic can depend on your knowledge. If the universe is deterministic to its simulator but random to us, I would call it random. Besides this difference in terminology, I think we agree.
Do you agree that the probability of an event depends on your knowledge? For example, what is the probability that the bottom card of a shuffled deck is an ace? Peek at the top card, it's an ace of spades, now what is the probability?
Would you agree that something is determined iff its probability is zero or one? I'm guessing the disagreement is here - what definition would you use?
You are right, according to quantum mechanics. If you have a pair of electrons in a product state and measure the spin of one of them, you instantly know what the "other particle's" spin is going to be (if you knew what the initial state was). When you "peek at a card", you "make a measurement" and alter the system.
For anything that exists outside of a light-cone around the first electron, a measurement of the second electron will be truly random from the measurer's perspective.
One concerns a lack of knowledge, like in your cards example. In that case, the probability is an expression of your state of understanding of the deck, and is not a property of the deck itself. The physical details of the deck and the situation it is part of could be entirely deterministic and we could still talk about this kind of probability. In this case your knowledge is independent of whether it is probabalistic or not.
The other is whether the universe contains fundamental randomness, such that you could say it is literally "probabalistic". And in this case whether that is true or not is independent of our knowledge of the probabilities.
There's a third kind -- indexical randomness, or not knowing who you will be -- that appears in many-worlds. Many worlds is in some sense completely deterministic. Yet with indexical uncertainty you still cannot possibly ever know which way you will see a photon go in a half-silvered mirror. Is that fundamental randomness or not?
I would say, not. There's no actual randomness from an objective point of view, it's just a lacking in your understanding of things.
I think that map/territory confusions are the source of so many problems and should be rigourously avoided. I think it would be a map/territory confusion to consider it a kind of actual randomness.
> I think it would be a map/territory confusion to consider it a kind of actual randomness.
Even though it makes perfect sense to apply probability theory to it? We found a perfect coin, one that we know that you can't predict in advance even in principle, and you want to avoid considering it a kind of randomness? Why does the mechanism by which the universe implements randomness -- in this case via determinism -- matter?
The many worlds case I was responding to is different to the coin case you are taking about. In the MW case we know enough to see that the situation is not at all random. In your example you're postulating a coin that may well actually be random.
I don't understand what you mean here. I think "the many worlds case I was responding to" in your comment is justinpombrio's "which way you will see a photon go in a half-silvered mirror", but that's functionally identical to the "indexical coin flip" referred to by the LessWrong article, where the coin flip causes a bifurcation, landing heads in universe A and tails in universe B. From within the multiverse it is impossible to know ahead of time which universe you will observe, same as the photon example. It's "random" in the sense of being absolutely unpredictable (otherwise MWI would not be experimentally equivalent to alternative interpretations of quantum physics).
So, does indexical randomness fit your definition of "fundamental" randomness? And what did you mean when you said "a coin that may well actually be random" if it wasn't the indexical kind of coin?
> does indexical randomness fit your definition of "fundamental" randomness?
No, it does not. MW is determinsitic. The fact that you don't know which universe you will observe is a limit on your knowledge. It does not make the things actually random.
There are other interpretations of QM where there is literal randomness, independent of an observers knowledge.
Or to give a slightly better example, suppose you're playing russian roulette with a six-shot revolver. You empty the revolver of bullets, then put one bullet in, spin the barrel, point it at your head and fire.
What is the chance that you're shot? Is it 1/6? What about if you open it up and look in the barrel to see if there's a bullet there?
Nothing about the location of the bullet has changed.
Are you suggesting this example somehow shows that changing knowledge/probabilities determine whether what's going on there is deterministic or not, as is the issue under discussion?
Going to piggyback with a question unrelated to yours but tangental to the topic:
What about the universes in the multiverse where 'multiverse theory' is wrong, and there is no multiverse? Are they islands? Are they like multiverse 'diodes' which may allow only one-way travel into that universe, but from which return to the larger multiverse is impossible?
This sounds a lot like the question of "does the set of all sets contain itself?" which is naturally a part of Russell's paradox. I can't offer more, but I don't think the concepts are dissimilar.
Just to clarify something: the standard many-worlds interpretation (such as discussed in the original article and elsewhere in this thread) doesn't contain anything about other alternate branches having different physical laws or being fundamentally different, and in MWI the only way that branches interact with each other is that probabilities are affected when multiple branches have their physical states match up completely (after possibly taking different paths) -- there's no way for intelligent beings to interact with meaningfully different branches for example. The many-worlds interpretation isn't a baseless what-if-there-are-alternate-universes thought experiment that anything could happen in, but instead is arguably the straight-forward interpretation of what we know about quantum mechanics.
Real physics nerd, non-popsci answer: everything that is obersed rather than conjectured about the universe implies that it has no "outside" and your question is a bit like asking what is North of the North Pole. It just isn't a meaningful concept, like "before the big bang" or "what did the universe expand into."
While that is a nice thought it's in no way supported by any evidence or observation, or really any hope of evidence for observation. MWI or some other multi universal theory could have merit, but in their current formulation they're not even falsifiable.
> everything that is obsersed rather than conjectured about the universe implies that it has no "outside"
"outside" is not very rigorous. For example, we seem to be observing objects vanishing behind the comoving horizon. It seems unlikely that the individual cosmic horizons centred on each of our microscopic components is destroying these objects, and it seems unlikely that they will ever reenter the comoving horizon. The metric expansion induces other interesting horizons, too, and each has an "inside" and an "outside". But how many of these horizon-crossings are directly detectable by the objects crossing them? (Reflectively, we are each exiting the horizons of distant observers at slightly different scale factors because we aren't occupying the same point in spacetime).
"More of same" for some distance outside of e.g. the cosmological event horizon is wholly reasonable. We can even put lower bounds on "some distance" depending on how we look at the homogeneity and flatness problems. They're big. IIRC Guth's original cosmic inflation work predicted that the Hubble volume is no more than 10^-26 of the total causually connected volume at the start of inflation. We can also put bounds on any sort of gradient on various apparent constants such as c and G, and the region in which they are virtually certian to have the same values we have experimentally here is also big.
However we currently can't do much better than that. The bits and pieces that were close to us in the hot dense phase of the universe are mostly inaccessible to us now, but there is every probability that they have identical local physics to us. There may be bits and pieces that were insufficiently close to us in that phase that have wildly different physics; and we do not really know anything about the still denser phase of the universe. It is perfectly reasonable to consider observables generated by detailed guesses that e.g. avoid an actual singularity like Carroll & Chen (who propose one or more other universes evolving towards de Sitter space from some arbitrary shared values surface) for instance. But even there, "outside" gets tricky.
Moreover, "inside" vs "outside" is not really the best way to approach the underlying question, "what is the nature of the metric expansion of space?" where the real answer should identify the cosmological frame and its preferred coordinate system in which almost all matter remains at essentially the same spatial coordinates from the big bang to the infinite future, with the scale factor relating to radar distances (and radar beam wavelength changes) between objects at distant spatial coordinates. Then we can admit that there are various ways to interpret the radar observables, with the "easiest" one being a purely local evolution of the vacuum at each point along the radar's path, which in the preferred cosmological frame can be seen as dark energy, but which with other systems of coordinates can be seen as anything from "the local creation of more space" to a Doppler effect to (somewhat less usefully) a change in the vacuum's refractive index.
> what is North of the North pole
Well, a change of coordinates on the Earth gets rid of that particular coordinate singularity, doesn't it? We can get rid of all sorts of oddities by swapping the coordinates we're using, which is probably the greatest strength of general covariance.
What we can't do by changing coordinates (even to accelerated systems of coordinates) is eliminate the Earth's oblateness.
Likewise, we can change from FLRW coordinates to other coordinates on the cosmological frame and get rid of (or introduce) all sorts of oddities. We can't, however, get rid of angle-brightness-redshift relations.
It is a bit silly to introduce all sorts of additional action-at-a-distance forces to explain the Earth's oblateness in an ...
My understanding of QM is only surface-level, so there may be subtleties I'm missing, but this presentation's arguments against the many-worlds interpretation seem pretty weak.
Page 28 grudgingly alludes to the idea that MWI could be simpler than other theories ("Objection based on surprisingly common misconception that standard QM defined solely by Schrödinger's equation ... It is only within a many-worlds framework that this view could begin to make sense"), but later introduces MWI as a contender beginning with a nonsense news headline and then characterizes it as intuitively bizarre for proposing a large multiverse, as if the size for its proposed multiverse was an obvious point against it. It seems to me there's an implied misuse of Occam's razor: "Entities are not to be multiplied without necessity" is most properly applied to the number of rules in a theory, not the amount of matter a theory predicts. (If you apply it based on the amount of matter a theory predicts, then Occam's razor surely should rule out theories that dim clusters of light in the sky correspond to trillions of galaxies like our own, and should prefer theories that predict that they're illusions, reflections, or something otherwise insignificant that fits the observations.)
Page 39 and 40 both quote paragraphs from books that seem more convincing to me than the presentation's seemingly hand-waving refutations. Page 40's refutation appears to do nothing but insist upon PWT's "epiphenomenal 'pointer'", naming our branch as real and explaining away the many deep interactions of other branches as only being "mathematical significant" rather than having "ontological significance". ... Maybe this is a bad time to mention my preference for the Mathematical universe hypothesis ("which posits that all computable mathematical structures (in Gödel's sense) exist"), which seems to make the idea of something having mathematical significance but not ontological significance meaningless.
Occam's razor is so commonly misused and abused. It seems to be more often used to support whatever pet theory someone has rather than as an impartial tool.
The "nonsense news headline" and followup discussion are less critiquing MWI than poking fun at David Deutsch. This is because Deutsch is a prominent critic of PWT.
The arguments against MWI from Occam's Razor are indeed weak. The stronger critiques are the difficulty of deriving the Born rule, or indeed quantum randomness at all, from the deterministic MWI. This is presumably what the authors hint at in their "doubtful it [pure Hamiltonian evolution] makes sense even there (in MWI)".
Why do you say that quantum randomness is a problem in MWI? I thought it was one of the neat things of MWI that it doesn't require randomness. There are no random events: instead you have a branch for each outcome. The only randomness is which branch you find yourself in as an individual.
It's a problem because it closes off further investigation - same as the anthropic principle. It's an answer for everything, but doesn't tell us anything about where to look next.
So if it's right, it needs to rigourously exclude everything else.
On the other hand objective randomness is a black box. You can describe how it behaves at the surface level (so functionally it is usable), but by definition you can't have a finite model of it, so what is inside? No answer by definition - something infinite.
So why X happens instead of Y?
1. Let's propose an unexplainable, unmodellable infinite entropy source under everything
The first is somehow psychologically less satisfying answer for me, although - of course - I accept that calculation works either way. The second at least not an unimaginable infinite, and in the end leaves me with less "why?".
MWI doesn't mean anything and everything can happen because there's infinite universes. The equations of quantum mechanics describe the ways that worlds can branch apart and the proportions of those branches.
Sorry if I'm underestimating your familiarity with MWI here; if I am, szemet has a much better response. I'm just a little concerned that some people in the thread don't realize MWI refers to something much more specific and grounded than what-if-multiple-universes, and I wanted to dispel that notion.
I hear a lot that MWI implies that everything that can happen does happen. What I find frustrating is that the obvious next interesting question then is 'well what can happen?', and people don't seem to talk too much about that.
If you want a more layman's explanation of what sort of things would cause a branch and how the branches would differ: one simple example is that when an excited atom emits a photon, it doesn't emit it in a random direction, instead there's a superposition of it emitting the photon across all paths away from itself. In many of these paths, the photon will interact with particles differently than happens with other paths, and the paths will decohere into different branches of the world with separate consequences from the photon hitting in a different place in each branch. In one branch, the photon could hit a particle in the air and affect its temperature and velocity by the tiniest amount. In another branch, the photon could instead hit a receptor in a person's eye and immediately trigger a reaction that would not have otherwise occurred.
Small differences between the branches could build up into bigger differences over time, especially when you consider the sheer number of concurrent interactions that are creating overlapping superpositions which decohere in many different ways. It could be that there's enough branches that most things that could have reasonably happened by chance do happen in some branch.
(If you're wondering how we could know in theory that the world may branch like this, it's because branches where particles take different paths but then later have all of their positions line up together interfere with each other. See the two-slit experiment.)
In short the problem is that there's no explanation for why you only experience a single branch, and no explanation of why the probabilities associated with those branches are given by the Born rule.
Although there are many versions of MWI, the general claim is that a single axiom (a unitarily evolving wavefunction) is sufficient to explain our observations of the universe. Without getting into what it means to "explain" something, it seems fair to demand that a sufficiently intelligent agent with no prior knowledge of our physics should be able to predict what the theory says about their future observations. However no one would be able to make any experimental predictions based on the above axiom. There's nothing there to suggest that if the agent becomes entangled with a quantum system then they would only experience a single branch of the entanglement. Even if you add that in, there's nothing to suggest which branch the agent will experience (this is roughly the preferred basis problem). Even if you add that in, there's nothing to suggest with what probability the agent will experience that branch.
You could always add in the above as additional axioms. You could postulate some physical content to the Born-rule, or to something weaker from which to deduce the Born rule. But this would break the illusion that MWI requires fewer axioms. Instead, a great deal of energy has been expended into deducing the Born rule purely from that single axiom. There have been many very clever attempts at this, but none convincing enough to settle the matter.
>there's no explanation for why you only experience a single branch
Isn't that obvious given that the particles of your brain can't interact with the particles of your brain in physically-different branches?
>it seems fair to demand that a sufficiently intelligent agent with no prior knowledge of our physics should be able to predict what the theory says about their future observations. ... Even if you add that in, there's nothing to suggest which branch the agent will experience
Don't you get that same problem with other interpretations that instead assume interactions can have a truly random result on some probability distribution?
Not immediately obvious to me. An agent could calculate the entangled state of themselves (including brain particles), the system and the measuring apparatus. This overall entangled state would in principle be pure, although the agent's own reduced state would be in a superposition which could be written in one of an infinite number of ways. What the agent should expect to experience in this situation is not at all clear, to me.
> Don't you get that same problem with other interpretations...
Most other interpretations have an explicit postulate relating physical probabilities to parts of the mathematical formalism. The agent can use this to make predictions about future observations.
Pilot waves are not a theory. No experiment can exclude them, unless it falsifies the entire quantum theory of mechanics. One reason to dislike pilot waves is that people, speaking as scientists, take them too seriously. This is not to say that scientists shouldn't talk about them, just that we shouldn't pretend we're doing science when we do.
In particular, don't pretend that anyone can be scientifically right or wrong about the human interface to quantum mechanics. Some interfaces cause fewer errors than others do, and the rest is Vi and Emacs.
Speaking subjectively, there are two reasons that I prefer Everett's relative state interpretation. (It's the serious version of the approach that these slides send up as many worlds.)
Relative states make it blatantly obvious why I can't put a contract on my great-great-grandfather. In other approaches, you have to think about this.
In the pilot wave "theory", there are superpositions, a.k.a. "pilot waves", and a bunch of other things. In the Everett worldview, by contrast, there are superpositions, full stop.
Everett does leave some grey areas, roughly speaking the quantum version of, "Is 7 a random number?" I doubt that any interpretation of quantum mechanics can clarify that, and Everett at least acknowledged the problem and made it fairly explicit.
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[ 3.2 ms ] story [ 60.7 ms ] threadProblem is, neither QFT nor modern deeper theories such as strings gain anything from this kind of metaphysical image.
These are just the popular things that make it out into public discourse
Let's imagine a simplified example. I give you two sets of numbers (let's pretend they are atom coordinates). One set is purely random, another one is generated with a very simple formula (example: f(n) = SHA256(N + salt)). I will give you as many of the numbers as you want. Can you determine which set is which (if you don't know the formula ahead of time)?
If the universe gets cloned one time every second that would still be a huge claim.
A million times every nanosecond is even a bigger claim.
Now imagine how many states the universe can be in, and how many times a second can be meaningfully divided.
If you can solve the "one clone every second" then I would be satisfied.
I feel that assuming that the nature of how the universe is computed matches up with our first intuition feels a bit like the assumption in geocentrism that the universe matches up with our intuition of Earth being the center and the most significant body.
Even if you accept the basic premise of the universe being a simulation, there are still many ways it could work with MWI. The simulator's universe needn't have classical physics; the simulation could be running on quantum computers or something more advanced. Or the simulator's universe could be classical with a classical computer doing the simulation at exponentially-decreasing speeds as it has to simulate the increasing number of branches. It wouldn't make any difference to us how long the simulation takes to compute us, as long as the simulating machine doesn't break down and succumb to entropy. The simulator's universe could be something like Conway's game of life, and the simulation is running on a turing machine pattern which will never decay or break down. (I might be borrowing more than a few ideas from Permutation City here.)
When I was younger and had most interest in non-useful things (in a boring sense) I came to "the theory" that each time observation or logical induction is performed, the universe splits into many variants ahead of time and these variants that produce "oops" in terms of contradiction simply disappear. Each time you see something weird but real, it is oops that survived, because you didn't see something contrary yet (and now you cannot, because only one thing has to be remaining).
That made me sad, because we could come to hyperdrives and FTL journeys to alien worlds, but some people in 20th century made few observations and conclusions that now prevented fun forever. Magic things were easy before the technology, now they are physics-hard. Our universe is spoiled in a very wrong way.
https://en.m.wikipedia.org/wiki/Bell%27s_theorem#Bell_inequa...
EDIT: tone.
Not everyone agrees on what is "reasonable". I have no problem giving up locality, but "true" randomness (i.e., information generated without an algorithm) seems like a philosophical cop out.
Note that something can also be globally deterministic but indeterministic from the perspective of a subsystem. In this sense, the universe would appear random as far as we're concerned, but not random to whoever is simulating the universe (this is often called superdeterminism, but I think that's a silly word—the universe is either deterministic or it isn't).
Would you agree that something is determined iff its probability is zero or one? I'm guessing the disagreement is here - what definition would you use?
For anything that exists outside of a light-cone around the first electron, a measurement of the second electron will be truly random from the measurer's perspective.
One concerns a lack of knowledge, like in your cards example. In that case, the probability is an expression of your state of understanding of the deck, and is not a property of the deck itself. The physical details of the deck and the situation it is part of could be entirely deterministic and we could still talk about this kind of probability. In this case your knowledge is independent of whether it is probabalistic or not.
The other is whether the universe contains fundamental randomness, such that you could say it is literally "probabalistic". And in this case whether that is true or not is independent of our knowledge of the probabilities.
http://lesswrong.com/lw/jlb/logical_and_indexical_uncertaint...
I would say, not. There's no actual randomness from an objective point of view, it's just a lacking in your understanding of things.
I think that map/territory confusions are the source of so many problems and should be rigourously avoided. I think it would be a map/territory confusion to consider it a kind of actual randomness.
Even though it makes perfect sense to apply probability theory to it? We found a perfect coin, one that we know that you can't predict in advance even in principle, and you want to avoid considering it a kind of randomness? Why does the mechanism by which the universe implements randomness -- in this case via determinism -- matter?
So, does indexical randomness fit your definition of "fundamental" randomness? And what did you mean when you said "a coin that may well actually be random" if it wasn't the indexical kind of coin?
No, it does not. MW is determinsitic. The fact that you don't know which universe you will observe is a limit on your knowledge. It does not make the things actually random.
There are other interpretations of QM where there is literal randomness, independent of an observers knowledge.
What is the chance that you're shot? Is it 1/6? What about if you open it up and look in the barrel to see if there's a bullet there?
Are you suggesting this example somehow shows that changing knowledge/probabilities determine whether what's going on there is deterministic or not, as is the issue under discussion?
What about the universes in the multiverse where 'multiverse theory' is wrong, and there is no multiverse? Are they islands? Are they like multiverse 'diodes' which may allow only one-way travel into that universe, but from which return to the larger multiverse is impossible?
"outside" is not very rigorous. For example, we seem to be observing objects vanishing behind the comoving horizon. It seems unlikely that the individual cosmic horizons centred on each of our microscopic components is destroying these objects, and it seems unlikely that they will ever reenter the comoving horizon. The metric expansion induces other interesting horizons, too, and each has an "inside" and an "outside". But how many of these horizon-crossings are directly detectable by the objects crossing them? (Reflectively, we are each exiting the horizons of distant observers at slightly different scale factors because we aren't occupying the same point in spacetime).
"More of same" for some distance outside of e.g. the cosmological event horizon is wholly reasonable. We can even put lower bounds on "some distance" depending on how we look at the homogeneity and flatness problems. They're big. IIRC Guth's original cosmic inflation work predicted that the Hubble volume is no more than 10^-26 of the total causually connected volume at the start of inflation. We can also put bounds on any sort of gradient on various apparent constants such as c and G, and the region in which they are virtually certian to have the same values we have experimentally here is also big.
However we currently can't do much better than that. The bits and pieces that were close to us in the hot dense phase of the universe are mostly inaccessible to us now, but there is every probability that they have identical local physics to us. There may be bits and pieces that were insufficiently close to us in that phase that have wildly different physics; and we do not really know anything about the still denser phase of the universe. It is perfectly reasonable to consider observables generated by detailed guesses that e.g. avoid an actual singularity like Carroll & Chen (who propose one or more other universes evolving towards de Sitter space from some arbitrary shared values surface) for instance. But even there, "outside" gets tricky.
Moreover, "inside" vs "outside" is not really the best way to approach the underlying question, "what is the nature of the metric expansion of space?" where the real answer should identify the cosmological frame and its preferred coordinate system in which almost all matter remains at essentially the same spatial coordinates from the big bang to the infinite future, with the scale factor relating to radar distances (and radar beam wavelength changes) between objects at distant spatial coordinates. Then we can admit that there are various ways to interpret the radar observables, with the "easiest" one being a purely local evolution of the vacuum at each point along the radar's path, which in the preferred cosmological frame can be seen as dark energy, but which with other systems of coordinates can be seen as anything from "the local creation of more space" to a Doppler effect to (somewhat less usefully) a change in the vacuum's refractive index.
> what is North of the North pole
Well, a change of coordinates on the Earth gets rid of that particular coordinate singularity, doesn't it? We can get rid of all sorts of oddities by swapping the coordinates we're using, which is probably the greatest strength of general covariance.
What we can't do by changing coordinates (even to accelerated systems of coordinates) is eliminate the Earth's oblateness.
Likewise, we can change from FLRW coordinates to other coordinates on the cosmological frame and get rid of (or introduce) all sorts of oddities. We can't, however, get rid of angle-brightness-redshift relations.
It is a bit silly to introduce all sorts of additional action-at-a-distance forces to explain the Earth's oblateness in an ...
Page 28 grudgingly alludes to the idea that MWI could be simpler than other theories ("Objection based on surprisingly common misconception that standard QM defined solely by Schrödinger's equation ... It is only within a many-worlds framework that this view could begin to make sense"), but later introduces MWI as a contender beginning with a nonsense news headline and then characterizes it as intuitively bizarre for proposing a large multiverse, as if the size for its proposed multiverse was an obvious point against it. It seems to me there's an implied misuse of Occam's razor: "Entities are not to be multiplied without necessity" is most properly applied to the number of rules in a theory, not the amount of matter a theory predicts. (If you apply it based on the amount of matter a theory predicts, then Occam's razor surely should rule out theories that dim clusters of light in the sky correspond to trillions of galaxies like our own, and should prefer theories that predict that they're illusions, reflections, or something otherwise insignificant that fits the observations.)
Page 39 and 40 both quote paragraphs from books that seem more convincing to me than the presentation's seemingly hand-waving refutations. Page 40's refutation appears to do nothing but insist upon PWT's "epiphenomenal 'pointer'", naming our branch as real and explaining away the many deep interactions of other branches as only being "mathematical significant" rather than having "ontological significance". ... Maybe this is a bad time to mention my preference for the Mathematical universe hypothesis ("which posits that all computable mathematical structures (in Gödel's sense) exist"), which seems to make the idea of something having mathematical significance but not ontological significance meaningless.
The arguments against MWI from Occam's Razor are indeed weak. The stronger critiques are the difficulty of deriving the Born rule, or indeed quantum randomness at all, from the deterministic MWI. This is presumably what the authors hint at in their "doubtful it [pure Hamiltonian evolution] makes sense even there (in MWI)".
So if it's right, it needs to rigourously exclude everything else.
So why X happens instead of Y?
1. Let's propose an unexplainable, unmodellable infinite entropy source under everything
vs.
2. X happens and so do Y, and one is observed at a certain probability due to anthropic principle - like this: https://www.youtube.com/watch?v=9R5OWh7luL4
The first is somehow psychologically less satisfying answer for me, although - of course - I accept that calculation works either way. The second at least not an unimaginable infinite, and in the end leaves me with less "why?".
Sorry if I'm underestimating your familiarity with MWI here; if I am, szemet has a much better response. I'm just a little concerned that some people in the thread don't realize MWI refers to something much more specific and grounded than what-if-multiple-universes, and I wanted to dispel that notion.
If you want a more layman's explanation of what sort of things would cause a branch and how the branches would differ: one simple example is that when an excited atom emits a photon, it doesn't emit it in a random direction, instead there's a superposition of it emitting the photon across all paths away from itself. In many of these paths, the photon will interact with particles differently than happens with other paths, and the paths will decohere into different branches of the world with separate consequences from the photon hitting in a different place in each branch. In one branch, the photon could hit a particle in the air and affect its temperature and velocity by the tiniest amount. In another branch, the photon could instead hit a receptor in a person's eye and immediately trigger a reaction that would not have otherwise occurred.
Small differences between the branches could build up into bigger differences over time, especially when you consider the sheer number of concurrent interactions that are creating overlapping superpositions which decohere in many different ways. It could be that there's enough branches that most things that could have reasonably happened by chance do happen in some branch.
(If you're wondering how we could know in theory that the world may branch like this, it's because branches where particles take different paths but then later have all of their positions line up together interfere with each other. See the two-slit experiment.)
Although there are many versions of MWI, the general claim is that a single axiom (a unitarily evolving wavefunction) is sufficient to explain our observations of the universe. Without getting into what it means to "explain" something, it seems fair to demand that a sufficiently intelligent agent with no prior knowledge of our physics should be able to predict what the theory says about their future observations. However no one would be able to make any experimental predictions based on the above axiom. There's nothing there to suggest that if the agent becomes entangled with a quantum system then they would only experience a single branch of the entanglement. Even if you add that in, there's nothing to suggest which branch the agent will experience (this is roughly the preferred basis problem). Even if you add that in, there's nothing to suggest with what probability the agent will experience that branch.
You could always add in the above as additional axioms. You could postulate some physical content to the Born-rule, or to something weaker from which to deduce the Born rule. But this would break the illusion that MWI requires fewer axioms. Instead, a great deal of energy has been expended into deducing the Born rule purely from that single axiom. There have been many very clever attempts at this, but none convincing enough to settle the matter.
Isn't that obvious given that the particles of your brain can't interact with the particles of your brain in physically-different branches?
>it seems fair to demand that a sufficiently intelligent agent with no prior knowledge of our physics should be able to predict what the theory says about their future observations. ... Even if you add that in, there's nothing to suggest which branch the agent will experience
Don't you get that same problem with other interpretations that instead assume interactions can have a truly random result on some probability distribution?
Not immediately obvious to me. An agent could calculate the entangled state of themselves (including brain particles), the system and the measuring apparatus. This overall entangled state would in principle be pure, although the agent's own reduced state would be in a superposition which could be written in one of an infinite number of ways. What the agent should expect to experience in this situation is not at all clear, to me.
> Don't you get that same problem with other interpretations...
Most other interpretations have an explicit postulate relating physical probabilities to parts of the mathematical formalism. The agent can use this to make predictions about future observations.
Other theories don't rule out nonlocality anyway. And I'd rather have nonlocality than their non realism!!
In particular, don't pretend that anyone can be scientifically right or wrong about the human interface to quantum mechanics. Some interfaces cause fewer errors than others do, and the rest is Vi and Emacs.
Speaking subjectively, there are two reasons that I prefer Everett's relative state interpretation. (It's the serious version of the approach that these slides send up as many worlds.)
Relative states make it blatantly obvious why I can't put a contract on my great-great-grandfather. In other approaches, you have to think about this.
In the pilot wave "theory", there are superpositions, a.k.a. "pilot waves", and a bunch of other things. In the Everett worldview, by contrast, there are superpositions, full stop.
Everett does leave some grey areas, roughly speaking the quantum version of, "Is 7 a random number?" I doubt that any interpretation of quantum mechanics can clarify that, and Everett at least acknowledged the problem and made it fairly explicit.
Using weak measurements, average photon trajectories predicted by Bohmian mechanics observed.