Farnsworth: "There is it. The edge of the Universe!"

Fry: "Far out. So there's an infinite number of parallel Universes?"

Farnsworth: "No, just the two."

Fry: "Oh, well, I'm sure that's enough."

Bender: "I'm sick of parallel Bender lording his cowboy hat over me!" -Futurama

Our existence here in this Universe is something that we know is rare, special, beautiful, and full of wonder.

Some things happen with amazing regularity and predictability: the occurrence of days-and-nights, the tides, the seasons, the motion of the heavenly bodies, and so much more. The physical laws that govern the Universe are very, very well understood, and that understanding has helped us construct a rather comprehensive view of exactly what our observable Universe consists of, where it came from, and what it looks like.

*not*an entirely predictive system! Sure, laws like gravity are predictive and deterministic: in other words, if we knew the positions and momenta of all the particles,

*and*had infinite computational power, we could figure out the properties of any particle an arbitrary amount of time into the future. (Or the past, for that matter.)

But then quantum physics came along.

And it turns out that knowing the positions and momenta of particles -- even of every particle in the Universe -- *isn't enough* to determine the properties of that particle in the future. Give me an atom of Uranium, and sure, you *know* it will decay. But you can't predict when!

You can predict the *probability* that any particular Uranium nucleus will decay after a given amount of time, and you can -- if you get a large enough sample of Uranium -- predict some properties of the larger ensemble that the individual particles make up. But there is no way, regardless of what you do, to predict what any one particular particle will do. And the same quantum weirdness, or indeterminism, turns up in other system, such as firing a single photon at a screen with multiple openings in it.

Sure, if you fire enough photons, you can be confident in the pattern that will emerge, statistically. That's what quantum mechanics allows you to predict with great accuracy.

But if you are asking about the properties of one *particular* particle -- where it winds up, what path it took, etc. -- there is **no way to know**. This is one of the most mind-boggling, puzzling aspects of the quantum reality of our Universe.

And at the same time, remember, our Universe, our physical, observable Universe, is full of a huge amount of this stuff!

When you add everything up that we know of: photons, neutrinos, protons-and-neutrons (or quarks and gluons, if you want to go more fundamental), electrons, antimatter, and everything else, we know that there are **at least some 10 ^{90} particles** in the observable Universe. The Universe has been around -- since the era of the Big Bang -- for some 13.8 billion years, or some 4 × 10

^{17}seconds, or (if you prefer units of Planck time) about 8 × 10

^{60}units of Planck time.

Now think about all that time, and think about *one* particle. Any one you want, but just one.

How many times did that one particle experience a quantum interaction with another? How many times did its position or momentum change? How many times did one particular quantum possibility happen for that particle, and hence, not the other possibilities?

The answer, for each of these 10^{90} particles, is **a lot**. Each time a nuclear reaction takes place inside a star -- something that happens maybe 10^{20} times each second in our Sun alone -- a huge number of particles experience a quantum interaction. And if *just one* of these interactions had a different outcome, our Universe would be in a different quantum state than the one it's actually in.

If just *one* randomly directional process -- like matter-antimatter annihilation -- had occurred in a *slightly* different direction, like it was off by 0.000000001°, our Universe would be different. If a single radioactive atom decayed just an attosecond later than it actually did, our Universe would be different.

And with all the particles interacting in all the ways they have over the Universe's history, you can make some calculations to try and determine *how many* of these quantum "decisions" have been made, and what the odds are that our Universe would exist with every quantum phenomenon shaking out exactly the way it has.

Well, the number of possibilities is somewhere around -- are you ready for a big number? -- 10^{1090!}, which should be read as ten-to-the-((ten-to-the-ninety)-factorial). Which, unless you're a professional mathematician who specialized in number theory, is probably the biggest number you've ever seen or conceived of. (For comparison, I'm going to show you *only* 1000!, or 10^{3}!, below.)

"So what," you might scoff! "A number can be as big as it wants, but if the Universe is *infinite*, then there are an infinite number of realizations that are just like this, and every quantum possibility can happen *somewhere*!"

Easy there. Those are some big assumptions. First off, there's an assumption underlying the idea that parallel Universes could be real, something that's glossed over by many-worlds interpretation enthusiasts.

You see, in quantum mechanics, we define a particle's properties by a wavefunction, and that function changes over time. Now, in some interpretations, that wavefunction *isn't* a real thing, with definite properties, that determines anything about that particle. Measurables are the real thing, and the wavefunction is just a calculational tool. But in other interpretations (like many-worlds), the wavefunction *is really a real thing*, and so every time a "quantum decision" can be made, **every** possibility happens somewhere, and what we experience as our Universe is simply a path being chosen.

Mathematically, these different interpretations yield the same measurable results. But if we want this latter interpretation -- the many-worlds one (with a huge number of parallel Universes and all) -- to be true, we need **at least 10 ^{1090!} Universes**-worth of space, time, and matter for it to happen in.

And while there are some good arguments that we do, in fact, live in a multiverse, the leap to having *that much* Universe to work with is staggering. Let me explain.

You see, the Universe, in its very early history, underwent a period of cosmic inflation, where the Universe expanded exponentially. For a period of at least ~10^{-30something} seconds, this was what happened to set up the Big Bang.

There are some good arguments that inflation has been happening for a very long time (detailed here), which means that there *could be* 10^{1090!} regions of spacetime identical (more-or-less) to our own.

But there's a huge leap between "at least 10^{-30something} seconds" and the "at least 10^{1090!} seconds" (or years, or Planck units, or whatever; the units are unimportant at this level) that having ** real parallel Universes** requires.

Now, this isn't to say it *can't* or doesn't happen, but it is a tremendous leap, and one that requires an inordinate extrapolation to make. We're still trying to figure out what came before inflation, how long it lasted, and whether there was a singularity or not to initiate it. Let's keep in mind how mind-bogglingly much one must assume if we want infinite parallel Universes to be real, and remember as we move forward in time through the Universe: some infinities are bigger than others. And that's what I have to say about the physics of parallel Universes!

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Before we had direct observational evidence of the atom, many scientists scoffed at the idea that they existed, even if the calculations/theories of the times indicated that they did exist. They believed that science could not explain the reality of the situation and should only be used as a calculation tool. This is instrumentalism. This view should have been laughed at by other scientists of the time, but it wasn't, it was a mainstream view.

If our best explanations seem to indicate that X could be true about the world, then it should be taken seriously, and not just brushed off as a calculation tool. To not do, is a failure of imagination and inhibits further progress. The multiverse denial seems so anthropocentric.

Ethan: See position space and momentum space on wiki and note the reference to the Fourier transform. Then take a look at weak measurement work by Aephraim Steinberg and Jeff Lundeen et als. Note Jeff's semi-technical explanation where he says this:

"So what does this mean? We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory".Think of the photon as a waveform in space, analagous to a seismic wave deep in the ground. It goes through both slits and interferes with itself. However when you detect it at one slit you perform a wavefunction-wavefunction interaction that operates akin to an optical Fourier transform. The photon is transformed into a dot at that slit so it goes through that slit only, and there is no interference. When you detect it on the screen you perform another wavefunction-wavefunction interaction that again operates akin to an optical Fourier transform. Hence you get a dot on the screen. No magic, no mystery, and no multiple universe is required.

Yeah, it seems like a perfect example of an argument from incredulity to me. Besides, I don't know of any

a priorireason to assume that all 10^10^90! or so universes would have had to exist from the beginning, rather than being spawned at each possible interaction.FWIW, I am kind of a fan of many-worlds, but I do acknowledge that there's essentially no evidence for it, and it may not even be possible to collect any evidence in principle, so I wouldn't be upset if some other interpretation turned out to be correct, or at least many-worlds were disproved. In some regards, I'd actually be rather relieved.

I am not reading this as multi-universe denial, as much as "Before we go off whole hog, let's get some questions answered, because this is a big one!"

Mike,

That is correct. You see, the recognition that the Universe went through a period of cosmic inflation coupled with our understanding of quantum field theory leads us to the conclusion that even though

ourlittle corner of the Universe has stopped inflating, inflation has been happening in most places for the last 13.8 billion years, and who-knows-how-long before that.http://scienceblogs.com/startswithabang/2011/10/28/why-we-think-theres-…

But you think there's a big difference between 10^-33 seconds and 13.8 billion years? Try the difference between either of those numbers and 10^((10^90)!), and you'll be thinking in exclamation points, too!

The problem is with referring to each one as a "real parallel universe". That's just a visualization, at least relative to Everett's original idea. There's one universe, which exists in a superposition of multiple states. I've seen other articles about the idea talk about violating conservation of energy when "creating" these other universes and I just facepalm, because that's not what this interpretation actually says. (Thank you for not going there.) There aren't a whole lot of universes, there are a whole lot of terms in the wave function that describes the universe. And in exchange, you get locality and determinism. This way of describing it is surely less interesting to sci-fi authors (though one could still imagine a technobabble explanation for "traveling between universes" - it could even use the word "phase" in a semi-accurate way!) but if you can grok the math, it makes a lot more sense. (And note that the little math needed to use bra-ket notation is all you'd really need.) Looking at it like that, I actually find it vastly simpler than any other interpretation.

Multiple universes is fun and very syfy to consider. However, such thoughts still describe the issue in three dimensional terms. What are the other dimensions that the mathematics suggest are there? Even the terms "what" and "there" rely on 3D ideas.

If we can conceptualize other dimensions, and consider the properties they offer, and the effect of the 4 we normally deal with, we might begin to understand quantum issues as intersections, wave functions as descriptions of reality and not just tools.

No analytical support? Einstein et al conceived of much of what we know long before they had the tools to confirm it. Creative thinking, consistency of argument and a recognition that current ideas are inadequate.

When Sean Carroll describes the Many Worlds interpretation, he describes it more like BenHead does. There aren't many parallel universes, there's just a single universe described by a single wave function which evolves according the the Schrodinger equation and that's it. "Many worlds" just comes from the fact that we are part of that wave function, too, and the various outcomes for us exist in a superposition just like it does for everything else, and what "I" am is just one of those possibilities which is why, subjectively, it looks like wave functions collapse.

He's made the "brief intro to QM" chapter of one of his books available online and it goes into the Many Worlds interpretation. Here's a linky:

http://preposterousuniverse.com/eternitytohere/quantum/

After reading that the Many Worlds interpretation made a lot more sense to me, and now I think it has a lot to recommend it.

It does still have the unnerving property there are 10^whatever other "me"s that exist just as much as I do. But not in the same sense as "parallel universes".

Jim:

"What" and "there" are well-defined valid terms for any arbitrary dimensionality. A "dimension" is an axis of measurement, and "there" is a position; in N-dimensional space "there" is defined by N values. Additional dimensions may be nigh-impossible to visualize, but it's trivial to conceptualize and there's lots of math already dealing with it and theories that incorporate that math. I don't think any of them have solved the fundamental interpretation issues with QM.

Not to say that a specific idea which includes extra dimensions couldn't solve the issue, but I don't think it's as easy as postulating a greater than 3+1 dimensional space.

BTW, seeing that "Turbo C++ IDE" window was a real blast from the past. :)

Just for a little clarification, the many-worlds-interpretation doesn't

requirethat parallel universes are real. The parallel Universes are -- in a conservative MWI view -- a visualization of what the wavefunction creates.But there are people who are putting together the huge amount of Universe-space created via eternal inflation and the MWI to argue that parallel Universes are real, and that for every quantum realization that occurs in our Universe, there exists a real parallel Universe where everything that ever occurred in our Universe occurred over there, except for that one difference.

That is what I've been talking about here; commenters BenHead and CB are correct that the MWI does not necessitate these real, infinite parallel Universes.

What eternal inflation? We can envisage an early universe where energy density was very high, much as it is down near a black hole where gravitational time dilation is high. So if the universe expands even at some sedate pace, any observers within that universe would assert that the expansion was extremely rapid. Like inflation. And we have no evidence that this occurred in only one region. So all this multiverse stuff is a speculation riding a hypothesis on top of a conjecture. It isn't science, it's pseudoscience. It's woo. But people sure do love their woo.

Well the other quantum possibilities encoded in the wave function would be as "real" as I am (and I think I'm pretty real).

But I think I understand your clarification about combining it with eternal inflation -- the idea is that they're really *parallel*, as in taking place in a separate inflation bubble that happened to exactly match the quantum state of ours up to some point of divergence, and with infinitely many such bubbles every quantum event will be the point of divergence between ours and some other bubble.

Is that correct?

Seems to me that by assuming that once a quantum event occurs differently in the two universes that they now diverge entirely after taking on a specific state, this is more Copenhagen + eternal inflation. If it was Many Worlds, each inflation bubble that started with the exact same quantum state would evolve the same way, each containing all the possibilities. Which makes me feel like I'm not understanding.

In any case, thanks Ethan.

Darn. When I said "we have no evidence that this occurred in only one region" I should have said "we have no evidence that this occurred in more than one region".

I think you're underestimating the necessary universes by a few orders of magnitude, Ethan. Each of those parallel universes would have their own evolution, and unless we somehow assume ours is special, then each of them would need that huge number of parallel universes itself, right? And so on...

When I read your columns, Ethan, I always regret that I did not get my degree in astrophysics. So fascinating!

it could possible be true due to quantum mechanics and string theory which states there a 7 dimensions instead of 4 as we thought of in the past. This is due to Einstein General relativity and quantum mechanics so there could be other dimensions that we can explore and see the laws of physics even govern them as they do in our 4 dimensional universe.

I honestly don't see the issue with the number being so big. The question is a

processone: do we have scientific evidence or theory which predicts there is a process for universe-splitting or universe-formation at quantum events. If the answer is "yes," then whatever number of universes you end up with is just the natural outcome. If the answer is "no," then you don't have any reason to expect any of them.Its sort of like compound interest or evolution. Non-experts are are constantly surprised/amazed at the amount of total change those processes can produce in a significant time too. Again though, it's just a question of whether you have a reasonable mechanism. If you do, then you ought to accept any counter-intuitive result you might get out of cranking that mechanism.

So, what happens to the law of conservation of mass-energy? Every particle change creates, from nothing, an entire new universe having all of the dark and barionic matter of the first. Just doesn't pass the test of common sense.

Does quantum computing offer any ability to exclude or favor any of the interpretations on offer? Some authors I've read have argued that if we're successful in building quantum computers this proves the reality of many-worlds (This was in In Search of the Multiverse, but I think David Deutsch has said similar things).

Um, the wave function evolves in a Hilbert space of undetermined dimensions. _Not_ our three- (or four-) dimensional physical space (or spacetime.) So yeah, there's extra dimensions to move around in. But they're more like the temperature-dimension or the momentum-dimension.

Ah, I see this has already been covered. My apologies. That'll teach me to skim ;-)

CB and marcel - thanks for the feedback

I first began to think of other dimensions when I followed the Herculean effort at CERN to drill deeper and deeper. Instead of trying to breach the vault door directly, mightn't one simply walk around the edge of the facade and unlock the vault from the inside?

My favorite idea for another dimension relates to human perception. Some call it a sixth sense, others intuition. I have no idea what causes this, but haven't you known people who seem to have insights that others lack? Like they can see into the future, or have an understanding of a person or situation that is remarkable. Are they more "in tune" with another dimension and able to better adapt to our 3+1?

Many worlds?

How many papers have the phrase "many worlds" in their title

- 17 papers published in 2013 on the Philosophy of Science preprint online data base

- 1 paper published in 2013 on the arXiv (e.g. physics) preprint online data base

As I suspected. The "many worlds" idea seems not to make much difference to physics and astronomy either experimental or theoretical. There are very few even thought experiment differences between the many worlds interpretation and other quantum mechanics interpretations. But the many worlds idea is very important philosophically.

"The reason for adopting the MWI is that it avoids the collapse of the quantum wave. (Other non-collapse theories are not better than MWI for various reasons, e.g., nonlocality of Bohmian mechanics; and the disadvantage of all of them is that they have some additional structure.)... The MWI is a deterministic theory for a physical Universe and it explains why a world appears to be indeterministic for human observers... However, THE ADVANTAGE OF THE MWI IS THAT IT ALLOWS US TO VIEW QUANTUM MECHANICS AS A COMPLETE AND CONSISTENT PHYSICAL THEORY which agrees with all experimental results obtained to date." date.http://plato.stanford.edu/entries/qm-manyworlds/

But is quantum mechanics "a complete and consistent theory" yet?

A few typical quotes suggest NOT if you include gravity.

"general relativity and quantum field theory are outright incompatible... there is no way to a peaceful coexistence; a shoot out is inevitable."

"The 21st century has thus inherited a fundamental crisis in physics, viz., the incompatibility of general relativity and quantum physics!"

"I would like to suggest that it is possible that quantum mechanics fails for large distances and large objects. Now, mind you, I do not say that quantum mechanics does fail at large distances, I only say that it is not inconsistent with what we do know. If this failure of quantum mechanics is connected with gravity, we might speculatively expect this to happen for masses such that GM2/~c = 1, of M near 10^−5 grams, which corresponds to some 1018 particles ” - Feynman (1957)

Also INTERPRETATION OF MANY WORLD INTERPRETATION VARIES DEPENDING UPON WHICH PHYSICIST YOU READ.

The importance of the MWI, for me, is that it is a current example that illustrates how important philosophical assumptions are to current physics. Physicists seldom will admit that philosophy plays any role in their theories. (e.g. "As a theory relevant to the origin of the universe, the Big Bang has significant bearing on religion and philosophy. As a result, it has become one of the liveliest areas in the discourse between science and religion. Some believe the Big Bang implies a creation, while others argue that Big Bang cosmology makes the notion of a creator superfluous." wikipedia

Why is there a problem with the wavefunction collapsing? Maybe that's what is happening (in as much as the model of the thing is the thing itself).

"But is quantum mechanics “a complete and consistent theory” yet?"

Again, it depends on what you're going to want for making the assessment of being complete and consistent.

Its successes, especially in areas where they were consequential results of the model but not a prediction or pre-requisite for it shows it's to that extent complete.

And consistent with what?

F=ma can be proved from "first principles" from the wave equation of QM (if you assume the Newtonian result is the "expectation value" result.

So it's consistent with classical mechanics.

Then again, you can point to places where it's inconsistent with other theories and where it doesn't apply (even if only because QMing a cat is a silly thing to find out if it purrs: ask a vet).

So the answer is yes and no.

One reason why science isn't interesting to the Jenny Housecoats of the world but religion "seems" "better": it's damn certain in its answers, and doesn't publicly waver.

Maybe it puts a limit on the

typesof universes out there - i.e., they must have net zero energy - but it certainly doesn't limit the amount. Ten-to-the-((ten-to-the-ninety)-factorial) times [universe which has zero net energy] is still zero net energy.* You could also have a bunch of net-positive and net-negative universes that balance out to meta-net-zero.But that is a curious and possibly informative question. If we hypothesize that all of those ten-to-the-((ten-to-the-ninety)-factorial) universes must, like ours, have a net zero energy, does that allow us to derive some understanding of what those universes must be like? Or does having the same net energy as our universe mathematically follow from QM (same number of particles and strength of gravity, just with the bits arranged differently), and thus not really tell us anything?

*Like ours. AIUI, according to how cosmologists and physicists count, our universe has a net energy balance of 0 because the 'negative' energy of gravity matches all the other energy and mass in the universe. In fact, this observation is the reason why Hawking came out a few years back and said that the universe could come from nothing.

, so mass and energy can be conserved no matter how many universes there are.

I could never understand the logic or rational for Many Worlds interpretation.

Multiverse from inflation does allow for similar universes. But that is not the same as Many Worlds. Yes.. if there are enough unique bubble universes, there is a chance that in one of them everything happened in just the right way to make another me with everything being the same except i won a lottery. But that's irrelevant to anything physics says about this universe.

Like I said in the beginning, I could never understand MW. Would like for someone to explain what it really means for a "function" to be physically real. I just don't understand what that means. No more than I understand "truth" to be physically real.

Where is that function? Is it big? Small? How much does it weight? Does it like pizza?

But it gets worse, because it's not that there are many universes. No.. you have our universe.. that goes on it's merry way, then I choose to wear red socks, and whoala.. a whole universe appears "somewhere" that has to replicate my universe exactly, just with i.e. green socks. Eghm... seriously? Appear from what, where, how? By what energy and cause? etc...

So ok.. forget that one. Let's look at it in a different way. They say we have a physically real function... So there it is, this "wave function" that is somewhere..., and in it is everything that has happened, happens, and will happen. It hasn't been made. It's without cause. It "knows" everything. And everything that happens "under" it has no more say in it than anything else. It does as it wills. In a way this sounds like a notion of deity. Except now you know it's useless to pray to it 'cause math says there is no "prayer" value in the function...

I took it to extreme, but no more than I feel MW camp has taken "randomness" to extreme.

"Would like for someone to explain what it really means for a “function” to be physically real. I just don’t understand what that means. No more than I understand “truth” to be physically real."

I've used this one many times, but again.

Evanescent waves.

Simplifying the maths for an E-M wave you can, instead of using sin/cos for the amplitudes of the electric and magnetic fields, you can keep a single *complex value* amplitude and have it rotate and the real part gives you the real (observed) electric field strength. The change of this gives you the magnetic field strength.

HOWEVER, using a complex number on the 19C maths used to describe total internal reflection there's an imaginary part that extends beyond the medium reflected in the original direction of travel. This decays exponentially and is entirely imaginary (as in it's a value multiplied by the square root of -1, there is no real part of the complex number).

HOWEVER, again according to that maths, if you put another refracting medium in this "evanescent" field, you will get the creation of a new EM field with a strength equal to the size of the imaginary vector of the evanescent wave at that distance from the totally reflected surface.

This was only a mathematical trick to make it easier to do the figures.

But when placing a refracting medium close to another one which had a wave being totally reflected internally to that medium, you got light coming out of the first medium, unlit by anything else.

A function that appears to be totally real: EM fields of a photon being a single-valued complex number rotating through i-r number space.

I think it's possible that the probability wave is more real than the classical particle since this helps describe the two-slit experiment in the case of limited numbers of discrete quanta entering the experiment and being individually measured.

It is also to some extend realised inthe lamb shift, where the probability of the electron acts as if it were an equivalent fraction of an electron with an equivalent fraction of an electronic charge and interferes with itself and the remainder of the atom. I.e. the electron is "smeared out" throughout the QM probability distribution of the electron in its shell.

This also explains why this electron isn't giving off massive amounts of synchrotron radiation: it's not moving at all: the wavefunction is stationary.

In a two-slit experiment, the probability field means that the probability of an event resulting in detection reflects the diffraction pattern in a wave-like experiment and the scattered beam in a particle-like experiment. Both types change the probability wave. Neither make the "electron" "know" that it's in a different experiment in-flight.

Maybe one way to change this would be to have a wavelike/particlelike experiment set up that can be changed as to what it's meant to detect quicker than the transit time from emission to detection locations for the particle/wave, then swap between them and see if the pattern changes.

Maybe it wouldn't help at all. I've not yet managed to work out what the consequence that would happen under any model of what "reality" "really is" and therefore make this experiment capable of eliminating a view or not.

This, however, doesn't require a MW view. Just that the wavefuntion is more "real" than the quanta that is detected.

Although "just" is a rather small word compared to what it means.

Rather like "In order to fly, you just have to throw yourself at the ground and miss".

Sinisia @27:

Isn't it the same sort of mechanism though? In the first case, you've got a set of QM principles that allows universes to form. The probability of allowed events never happing is infinitely small, so we can reasonably expect them to happen.

In the latter case, we've got a set of QM rules that appear to allow wavefunctions to collapse in many different ways. Since its allowed....[repeat above logic]

Hawking and Mlodinow's Grand Design hints at this (but doesn't cover it...gripe gripe gripe). In the book, they point out first that QM allows our present to have many possible futures. Then, secondly, they point out that QM allows our present to have many possible pasts. Okay. That leads very naturally to a discussion I wish they had included (but didn't): are there therefore many presents? Seems odd to think that the QM multiverse has an infinite number of futures, a lesser but still infinite number of pasts, but necks down to a single universe at each instant of the present. If Hawking is right about past and future, why should we think such a neck exists?

Sinisa:

Yeah it's always troublesome trying to think of math as "real" rather than as a description of reality. Even if unintuitive implications of that math end up being borne out by reality, like evanescent waves, you could just take it to mean the math was an even better description of reality than you thought (but not necessarily perfect). So let's just set that aside.

What Many Worlds is saying is that the wave function isn't just a description of the state of a quantum system prior to its "collapse" into a single classical state, but rather a description of reality at all times. What appears to be "collapse" is really just the measurement apparatus and the system being measured becoming entangled so only certain subsets of their states are consistent. An electron exists as a superposition of all possible locations. When the apparatus measures an electron "there", then only states of the electron where it is "there" are consistent. Yet because the measurement apparatus is itself a quantum system, it also exists as a superposition of every possible ("electron measured there", "electron is there") state.

What Many Worlds is asking you to believe is "real" is superposition and entanglement. It's asking you to believe that quantum systems subject to these phenomenon aren't just tiny collections of particles we study in a lab, but everything. Including the apparatus, and also -- now here's the tough part -- including you.

You are not a classical observer who makes quantum systems start behaving classically when you look at them. You are a quantum system that exists as a superposition of all valid states. However because you are heavily entangled with everything around you, those valid states for you correlate heavily with specific subsets of valid states for the rest of the universe. So each of the possible "you"s sees only that subset of consistent valid states for the universe, which is why the world mostly looks classical and why wave functions appear to "collapse".

The key point for me to make here is that there are no spontaneously generated universes -- the "many worlds" are really just subsets of the valid states of the universe as described by the wave function that all exist in a superposition. No Conservation of Energy problem. No new information is created, or for that matter destroyed. Unlike in Copenhagen, in Many Worlds quantum mechanics is (in principle) completely reversible.

A universal wave function that defines all possibilities in the universe shouldn't on its own strike you as odd. It is in this sense precisely the same as the set of classical laws of physics plus the state of everything in the universe, from which one could calculate all future (and past) states.

"" Multiverse from inflation does allow for similar universes. But that is not the same as Many Worlds."

Isn’t it the same sort of mechanism though?"

No, not really.

Multiverses do not start out (or get chosen from) states that are in one universe progressing.

The MWI in its broadest and most colloquial sense is saying that the possibility produces another world where the alternative outcome was the one chosen.

The "solution" this gives is that which universe you perceive as real is the one of those many possible outcomes that came out true.

I.e. you're about to roll a dice. Six universes come into being from that point where one universe rolls a 1, another has a 2, and so on.

When you, the observer, notice that the answer that came up was 5, the universes stay, but your continuing consciousness has been placed into the universe that rolled a 5.

Those other universes, in the most common use of MWI, continue to exist, since the result of the "random" dice roll was never random, only which one you happened to follow on to was random.

Multiverses were abandoned because no causality from this one can ever reach it, even to the extent of "The value of the constant G is 6.7x10^-11". Or anything else.

Unless some physical laws are required for a universe to "survive" (three space dimensions, for example, may be an absolute requisite for a universe that can survive more than a plank time).

It never linked, never was, never will be, to a universe that had you, or any particle, force, wave or propagation that you will find in this universe, past, present or future.

@CB

am not sure I understand MW the way I do. It's not about believing that we are all QM systems or that we as observers are QM entangled as well. I understand that and agree.

If we move it from electron cloud to two slit.. the way I understand MW (and as ethan writes as well)... there IS physicaly REAL universe parallel to this where the particle passes through the other slit. and so on for every interaction... or the cat. just because the math says there's a 50/50 odds that's dead or alive, the moment you open the box there isn't a new universe getting created where the cat dies... yet it seems to me that MW is arguing precisely that

p.s. sorry first sentence should be: "... the way YOU do."

p.p.s.

" in Many Worlds quantum mechanics is (in principle) completely reversible."

well, how does this reconcile with uncertainty or entropy rise?

Quantumly, (cromulent new word!) entropy is just the difficulty of getting the same state back. Colliding two billiard balls back to their original location goes from having one force (the initial cueing) that was done any old how, to having to have two forces putting the two balls back in their original places and, rather than be any old way, they have to be precisely reversed.

If in the meantime, one or other (or both) had hit another ball, the reversal would require more forces in the right direction to cause the reversal.

And, stochastically, there are many more "wrong" ways to get the balls moving back (but not to precisely the same location) than there are "right" ones, leaving the scene as it found it.

MWI has many more worlds where you don't go back to where you started but if you could navigate precisely, you may be able to find one.

I don't really know it helps, though.

think I'll end here because it's hard to keep track :) anyways the math is same, regardless of interpretation. and as long as we get same results, i guess it's ok to believe anything :)

Sinisa: "well, how does [MWI implying QM is reversible] reconcile with uncertainty or entropy rise?"

Same way as all the other laws of physics which are completely reversible. :) In principle if you could reverse the motion of every particle in a (classic ideal) gas resulting from mixing a hot and cold gas you could get it to return to its original state of two separate resevoirs In practice the odds of this happening spontaneously are infinitesimal, and therefore (at a macroscopic statistical level) entropy always rises.

"there IS physicaly REAL universe parallel to this where the particle passes through the other slit. "

Absent path information, the particle passes through both slits. Do you believe that is physically real? MWI asks you to believe that it is.

All that happens then is when you detect the path, then the detector becomes correlated (entangled) with the particle and each of the two possibilities of detection are not compatible with the opposite possibility of path.

Both paths still occur, though. Just now the superposition includes the correlated state of the detector. And then you, as you say either "Ah-ha! It went through slit A!" or "Ah-ha! It went through slit B!" Those two possibilities exist in a superposition, as do their further evolutions. But the "you" that saw it go through slit A cannot see anything about the "you" that saw it go through slit B because those states are no longer compatible.

So instead of viewing it as every interaction forking a new universe, you can look at it as every interaction causing a subset of the universe (which is all states allowed in the wave function) becoming invisible to you.

I'll leave this link here again as it explains it much better than I can:

http://preposterousuniverse.com/eternitytohere/quantum/

I'll finish by saying that while MWI has grown on me (not having an point of arbitrary irreversibility being part of why), I don't really "believe" it. Frankly I don't "believe" any interpretation of QM, and use Copenhagen as my mental model because it's simpler to conceptualize. I and my macroscopic experimental apparatus are classical observers and when quantum superpositions are observed they collapse and all the possibilities that I didn't observe cease to exist. Simple!

Consider that measurement will realise the answer of a quantum superposition. That doesn't have to be you looking at it either, it can just be something that can change if the superposition were in one state but not in any of the others.

Yes, the _last_ thing I want to do is imply that "observation" implies "observer" implies "human sentience" -- the pun-based "logic" that results in all kinds of woo. Observation is measurement is anything that's state will correlate with the state of the thing being measured, whether that's intentional or not (one of the ways this is made practically clear is by the difficulty designers of quantum computers have *not* measuring the state).

@CB #43: I think you (i.e., all of us physicists) have to be a bit careful when you say "anything." Generically, if you have something which couples to (correlates) "with the state of the thing being measured," what you end up with is an entangled quantum system.

In order to reduce or avoid that entanglement, and end up with a "classical measurement," what you need is a coupling which not only "correlates with the state of the thing being measured", but also acts as a projection operator, picking out one of the eigenstates of the system (by "picking", I mean a la the Born rule).

This turns out to be much harder, philosophically. In practical terms, what we have observed so far (pun entirely intended :-) ) is that you need something which couples to the environment, or to anything with a large number of stochastic degrees of freedom. That something acts as a projection operator, or to diagonalize the density matrix, or to "collapse the wavefunction," or whatever particular terminology you like this week.

There have been some really cool papers over the past decade or so, which have quantified this. For example, coupling an atom in a superposition to a microwave cavity, and measuring, for example, the time it takes the atom to end up in one of the two eigenstates as a function of how many photons are in the cavity. It's not instantaneous, and the time varies inversely with a power of the cavity population.

It's quite impossible to imagine about another universe 'cause we are bought up to believe that our earth is the only one that has air water and oxygen but I do believe in parallel universe because of all of my friends amazing facts and information and a lots of images

Amazing or what? Just amazing. God's Nature is amazing.

re #46: I thank you. It's some of my best work. I'm working on Nature v 2.0, using what I learned in creating this one.

Shaping up to be great!