Those who have read about quantum mechanics have heard about the Many Worlds Interpretation (MWI) proposed by Hugh Everett in 1957. It is an idea seems unbelievable when one first hears of it because it implies the existence of many, a huge number in fact, of unobservable worlds that exist in parallel to our own but of which we are unaware. One needs to get over the initial feeling of incredulity before one can judge it properly on its merits.
Everett proposed his solution to a conceptual problem in quantum mechanics in that as quantum states evolve undisturbed, they can consist of a superposition of distinct observable states. According to the standard Copenhagen interpretation of quantum mechanics, when we try and observe what specific state it is in, i.e., we make a measurement, the very act of measurement disturbs that superposed quantum state and forces it into one of the observable states. The specific one it ends up being in is unpredictable except in terms of the probability of it occurring. This is the basis of the famous Schrodinger’s Cat, where the cat is both alive and dead until we look at it, and then we always observe that it is either alive or dead.
The poorly understood process by which the act of measurement ‘collapses’ the full quantum superposed state into one observable state in the Copenhagen interpretation has been used to challenge that interpretation. Everett proposed an alternative interpretation, that an act of measurement does not collapse the wave function at all but that multiple universes are created, each corresponding to one particular observable outcome of the measurement. We observe the one that we are in, but there are other unobservable universes in which ‘we’ also exist but where the outcome of the measurement was different. In the Schrodinger cat example, the act of measurement results in two universes being created, one in which the cat is alive and one in which the cat is dead. In one universe, ‘we’ (being devotees of the Copenhagen interpretation) will interpret the wave function as having collapsed to give a dead cat, while in the other universe ‘we’ will say that the wave function collapsed to give a live cat. Since the ‘we’ in one universe cannot observe the ‘we’ in the other, this interpretation explains the observed result just as well as the one where the superposed state collapses to one state.
As one can imagine, the idea of multiple universes ever-multiplying seems somewhat extravagant and for that reason alone some people reject the idea. But Sean Carroll argues that a lot of people reject it for the wrong reason, that they hink that the splitting of the universes is an extra postulate of the Everett’s when in fact there are no extra postulates at all but the splitting is predicted to occur purely as a consequence of the same postulates of quantum mechanics that all physicists agree on. So there is no a priori reason to prefer the Copenhagen interpretation over the MWI.
I tend to agree with Carroll but I think we need to be a little circumspect about the use of the word ‘predict’ in this context. That word seems to imply that the outcome is a necessary consequence of our information about the initial state. There is a difference between a prediction of a theory and a prediction by an individual about a possible outcome.
For example, the search for Higgs boson was based on a prediction of the Standard Model of particle physics. Not finding it would be a problem for the Standard Model. But that is not what we mean with the MWI. This situation is more like when we toss a coin and one person calls heads. That is a prediction about the future based on the situation now and the prediction can turn out right or wrong but it does not challenge the underlying theory that there was an equal chance of it being heads or tails.
In this case, we have a situation in which everyone agrees that before the measurement, we had a superposed state and after the measurement we will have a single observable state. The question is how we get from the before state to the after state. Both pathways (Copenhagen and MWI) are possible and there is no reason as yet to prefer one or the other. Either one is compatible with the basic ideas of quantum mechanics. So I prefer to stick with the word ‘interpretation’ rather than ‘prediction’.
Of course in science the way we judge between different models is to devise a test that can distinguish the two. But despite the hard work of many scientists, no way has yet been found to distinguish the Copenhagen and MWI pathways. That does not mean it will not be found. After all, recall that for a long time people thought that one could not distinguish between the local reality model of quantum mechanics preferred by Einstein and the non-local ‘action at a distance’ of the Copenhagen model, as sharply illustrated by the Einstein-Rosen-Podolsky paradox published in 1935. But John Bell showed in 1964 that such a test could be devised and the difficult experiments started producing results the mid-1980s, supporting the Copenhagen model. So it took about 50 years to get some insight into which might be correct.
Solving difficult questions takes time and we should be patient and not reject ideas simply because they seem preposterous on the surface. As Arthur Eddington said “Not only is the universe stranger than we imagine, it is stranger than we can imagine.” (There are different versions of this quote with J. B. S. Haldane saying “The universe is not only queerer than we suppose, it is queerer than we can suppose.”)
What do you think of this?
No Big Bang? Quantum equation predicts universe has no beginning
More information: Ahmed Farag Ali and Saurya Das. “Cosmology from quantum potential.” Physics Letters B. Volume 741, 4 February 2015, Pages 276–279. DOI: 10.1016/j.physletb.2014.12.057. Also at: arXiv:1404.3093[gr-qc].
Saurya Das and Rajat K. Bhaduri, “Dark matter and dark energy from Bose-Einstein condensate”, preprint: arXiv:1411.0753[gr-qc].
I’ve always been more partial to Cramer’s Transactional Interpretation of Quantum Mechanics. There was a recent article about it here: The Quantum Mechanics of Fate.
A couple of common misconceptions about MWI should be mentioned;
1) Occam’s Razor disfavours it. “There are too many universes!”. This objection seems silly to me. It’s like saying quantum electrodynamics is unattractive because there are too many electrons and photons.
2) It violates conservation laws. “Stuff being created out of nothing!”. This is just a misunderstanding of what branching entails. Every branch comes with its own probability, and a smaller “share” of the starting “stuff”. So energy is conserved both along a branch, and across branches.
I’m confused… Does that mean that when I drink water that has been infused with good vibrations through quantum resonance, I have one chance out of two to turn this panacea into poison?
I favor David Z Albert’s Many Minds Theory over the Many Worlds Theory.
It is more plausible, and also funnier.
Occam’s Razor even favors MWI over interpretations with a collapse postulate. It’s similar to the situation of a photon that exits our Hubble volume. Once it escapes, we don’t expect to ever see any evidence that indicates whether it continued or winked out of existence. But we expect that it continues, because it’s the extra winking-out mechanism, rather than extra stuff, that carries a probability penalty that would have to be balanced by evidence. Likewise for superpositions that decohere.
Coming from the computer/information science side of things, I’ve heard some researchers suggest that the successful implementation of quantum computers would be considered significant, if circumstantial, evidence in favor of the MWI. The reasoning being that physics places well-understood constraints on the amount of bits that can be computed with a given number of particles and if QM allows us to do more, we must be, in some sense, using the computational resources of other universes. I wonder if Mano has a position on this approach.
As a sort of positivist, at a certain level I don’t even recognize the meaningfulness of the many-world interpretation and the Copenhagen interpretation. I don’t use the word “hypothesis” because it makes no actual predictions in the normal sense of the word “predict”. Let me know when they get an actual, honest to goodness, falsifiable prediction. Until then, I’ll be in the “shut up and calculate” camp.
@nrdo
As a professional computer scientist (read: professional programmer with an undergrad degree), and as someone with an amateur understanding of the physics of quantum computers, that sounds wrong. It sounds like it’s based on bad premises. My guess is that your source has a metaphysical belief that if quantum computers must be directly analogous to classical computers w.r.t. information theory and entropy, and that if quantum computers work, then they must have access to extra bits in order to match the information requirements of classical computers. I don’t see why that should be. Perhaps quantum computation is just fundamentally different than classical computation. Further, as classical computation is just a statistical result of quantum processes (just like all of classical physics), I really see no particular problem with this position.
PS: Mostly unrelated, but: It is important to remember that a Turing machine can compute anything a quantum computer can compute. The existence of quantum computers do not change our fundamental definition of “computation”, and the Church-Turing thesis remains unchanged.
http://en.wikipedia.org/wiki/Church%E2%80%93Turing_thesis
It’s just that quantum computers can compute the solutions to certain problems much faster than classical computers, and perhaps arguably with much less physical moving parts. To put it another way, your cited argument seems to be an objection to the second part “with much less physical moving parts”.
nrdo,
I wish I had something intelligent to say in reply to your question but I just don’t understand enough about quantum computers to do so.
I was always fascinated by MWI and while I certainly do not pretend to understand quantum theory beside the layman interpretation we all understand, I have always wondered if these “splits” are constant or if eventually they can merge back together.
Here is an example with the cat once again (the poor kitty). Let’s say that the cat wee put in the box has cancer and he is suffering. While he is in the box we do not know if he is dead or alive. If we open the box and we find it dead, we are in the dead cat universe. But let’s say we are instead in the live cat universe, but we are forced to euthanize the kitty immediately after opening the box.
Even if we have had a split at the moment the box was opened, the end result is a dead cat. Actually, a dead cat that has had no chance to affect either universe in a meaningful way. In the dead cat universe, we take the cat and dispose of the body. In the live cat universe, we take the cat, euthanize him and dispose of the body.
Are we still ending up with 2 universes? Or is it possible that universes, like rivers, can eventually join back up to form a single reality regardless to what happened to the cat?
In other words, if each quantum event split into different universes, it would create, since the beginning of time, an infinite number of realities. But if instead those realities could merge back together if the split did not affect the continuum of both universes how would that change the hypothesis? Would this be even something worth considering or would it be inconsequential in the grander scheme?
Sean Carroll has written about the Many Worlds Interpretation on his blog a number of times. One of his posts last year helped me understand the many worlds interpretation quite clearly. In that post he argues:
cconti,
Interesting idea but the problem would be that though the cats might be both dead in the two universes, many other things would have evolved differently in the two universes and thus getting them to merge seamlessly would be impossible. There may be more profound reasons why it can’t happen but this practical one was the first that came to my mind.
cconti @11:
The “splits” involve quantum decoherence, in which interference between the different components of a wavefunction is lost via interaction with the environment, and the observable states emerge. Decoherence is pretty much a one-way street.
The “shut up and calculate” idea in physics sounds like a hypothetical “shut up and observe” approach to exploring an unfamiliar building.
The lobby of the high rise you’re in has a bank of elevators. This is centrally located in the building. The floors available to those elevators are labeled 30 at the highest, descending to Street Level and finally Lower Level.
However there is a wing of this building with five additional, narrower levels. They aren’t in a position where the lobby elevators could serve them. Also, you have heard several people saying they are going to the “Basement” located beneath the Lower Level. This is also not accessible by the lobby elevators.
So as a thinking being, (and let’s say, unable to explore further) you can’t help but come up with models of how all this is put together.
Options: There is an elevator or elevators which serve 31-35 as well as the basement (B).
31-35 are served by separate elevators.
31-35 and B are only accessible by the stairs.
B is a delusion in the minds of the people who said they are going there.
31- 35 are purely ornamental and not accessible at all.
There is a range of plausibility based on what we know about the architecture typical of the city and the time period when and where it was built, safety and accessibility regulations and the likelihood of meeting several psychotic people with an identical delusion all working in the same place.
The arguments about the implications of modern physics seem to me like an infinitely more complex version of this principle, with the added problem of far fewer people available who have relevant knowledge and experience.
Amazing discoveries have been/are being made and naturally those able to even think about these things on a high enough level try to correlate them into a coherent “picture” that takes all the facts into account and does not entail contradictions or violations of already established natural laws.
The many worlds interpretation is one of the logical possibilities. I don’t understand how it’s inappropriate for scientists to try to map out reality as best they can.
@Ed
Again, I’m a sort of positivist. I contrast that with a realist. A realist is someone like you (no offense) who thinks it’s an important question to answer. I don’t even see a meaningful question. If the answer to the question literally has absolutely no value in making predictions of future observations, then I don’t see the point in talking about it. It’s not an interesting question.
Of course, my position here applies only to material questions about our shared material (or super-material) reality. It applies to all pure scientific questions. It does not apply to questions of math, logic, beauty, morality.
My position does apply to the question of the manyworlds interpretation. The manyworlds interpretation is using the same language as a scientific theory, but it is not a scientific theory. It is masquerading as a scientific theory. It purports to be a stronger statement than mere quantum theory, but it also offers no way to falsify it, even in principle. For that reason, it is not science. It is not right. It is not even wrong.
http://rationalwiki.org/wiki/Not_even_wrong
IMO.
PS:
Let me continue, sorry. I should explain it like this. Perhaps we’re all brains in vats, like The Matrix movie. It’s consistent with all of the facts, although someone might object because of Occam’s razor. The theory is uninteresting to the extent that it offers no differing predictions of conventional realism. For that reason, as a sort of positivist, I don’t care if the “truth” is that we’re brains in vats or in some “real” world. It doesn’t affect my life one bit. I will still be hungry tomorrow, and I will still need to acquire food -- whether “virtual” or “real” -- in order to sate my very real hunger.
It is in this sense that I really don’t care about the manyworlds hypothesis. It’s not science because it’s unfalsifiable, and it’s not interesting because it offers no predictive power precisely because it’s unfalsifiable.
I want to make a stronger statement, similar to that of logical positivists: Namely, I even question what you mean by the manyworlds hypothesis. I only understand reality in terms of observations of real people (or observations of other hypothetical observers). Questions of ultimate reality are completely useless because science can never answer them, ever. Any purported answer may just be an intermediary with a deeper, inaccessible “real” reality underneath it. IMHO, the only sensible approach is to be a pragmatist, and respond to the manyworlds interpretation as “What an interesting idea. Does it have any implications? ‘No’ ? Ok then.”
OK, I see what you mean (I think). It’s not falsifiable in other words? I can see where that’s a good standard to keep from going off track into pointless speculation. And too much induction can lead to unwarranted conclusions. Not everything hypothetically consistent with the facts is true.
But I can’t help being fascinated with the idea of someone coming up with a big picture of what things are actually like even if it doesn’t have practical value.
Or, to be more accurate and nuanced, a model of what things are probably like with the qualifications that the model must be constantly altered and updated to accommodate new information and that there are probably limits to human comprehension.
Could there be a discovery that would be evidence for multiple universes? But even then it would be hard to say whether or not they were universes that diverged from ours according to the many worlds idea, or whether there were simply other universes with separate origins.
One must be wary of using falsifiability as a criterion for whether something is scientific or not because it has proven to be untenable. I have written about this extensively elsewhere on my blog (search on the word falsifiability) but that is pretty much the consensus view among philosophers of science.
A theory does need to be testable, however. By that I mean that it should make predictions that can, at least in principle if not in the foreseeable future, be tested. The difference between testability and falsifiability is that with the latter, a failure of the test leads immediately to falsification. With the former, failure of a test leads to a weakening of the case for the theory while passing the test strengthens it. It is the accumulation of positive or negative evidence that eventually leads to a consensus judgment (not proof) among scientists as to whether a theory is true or false.
One could criticize MWI for not having been able to generate any tests as yet. But one could make the same argument against the Copenhagen interpretation of wave function collapse. But that does not mean that one can never do so. As I said with the EPR question, sometimes it takes a while to devise a test to distinguish between two positions and that is why I hesitate to criticize scientists for working on it.
@Ed
Yes
@Mano
Sorry -- I was using “falsification” in a looser sense. I usually conflate “testable” with “falsifiable”. Sorry about the confusion. I think we’re agreeing on the major point.
However, one new point of contention: I don’t think there’s a particularly useful reason to make a difference between “testable” and “falsifiable”. Is that how the terms are commonly used in the literature? I did not know. Almost no theory is overturned with just a single test, which seems to be your definition of “falsifiable”. Rather, every theory is evaluated separately, and the amount of evidence to overturn a theory is different for every theory. In other words, all proper reasoning is Bayesian, and this is just a simple consequence of Bayesian reasoning. A consequence is that there is no bright line distinguishing your definition of “falsifiable” from “testable”.
Sometimes I like the term falsifiable because it emphasizes the way so many belief systems are walled away from any evidence or logic that could threaten them.
Any possible economic event is “evidence” for their dogma in the minds of both hard core Marxists and free market people. If you pray and survive, your god rewarded your devotion. If you pray and die, the reward comes in the next life.
Questioning can even be the CAUSE of failure. It offends the gods or produces energies that screw up the magic spell. It leads to subversion of the revolution or proper market functioning. If nothing could change a person’s mind on an issue. they are saying that on that issue (not necessarily others) their thought process is frozen in place and pretty much useless.
EnlightenmentLiberal,
I think we agree that testability and falsifiability can be treated almost synonymously if we are using both in terms of the level of evidentiary support.
My concern is that the term falsifiability is sometimes used as a purely semantic measure of whether a theory is scientific or not, irrespective of evidence. For example, take the theory that the planet Jupiter is made of cheese. Is this a scientific theory? I would argue that it is not because there is absolutely no reason to think it is true and there is no evidence to support it. It does have the potential to be tested but it is only if the tests turn out to yield positive evidence would we take it seriously.
But it is undoubtedly a falsifiable statement and if that is the measure of whether a theory is scientific, then we would have to grant it that status irrespective of any evidence.
To address quantum computing: my understanding is that there is no “computational power of other universes” necessary in any way.
Think of it this way: You have a compass. You want to know what direction North is. You are able to test all directions simultaneously as the magnet in the compass straightens to the exact direction. You don’t need to do something like a binary search to continuously compute and refine your directions. The laws of physics (in this case, magnetism) force the correct solution.
That’s analogous to quantum computing. You can test a huge array of data by abusing the nature of physics to snap to the correct solution. In this case it’s the superposition of electrons.
To follow up: Quantum computers can do certain operations in a single step that classical computers cannot (the best example is the computation in Shor’s Algorithm). Classical computers can fake these operations but it doesn’t really make sense as it doesn’t speed anything up.
Shor’s Algorithm works by simultaneously checking all possible solutions and finding a correct solution to a period function.
How many branches?
If an event has an exact 50:50 split, it is “obvious” that 2 universes are created (whatever that means).
If an event has a 50.1:49.9 split, it is also obvious that 501 + 499 = 1000 universes are created (499 is prime).
If an event has a pi-3:4-pi split, given that pi is irrational (and transcendental), how many universes are created?
And if the answer is “infinite”, why isn’t it also infinite in the 50:50 case? Or in the 100:0 case?
@ doublereed
I’m aware that you can understand quantum computing in terms of the laws of physics “guiding” the solution.
However, there are definitely some well-known researchers (including David Deutsch, who published the first mathematical model of a Quantum Computer that Shor used in developing his algorithm) who argue that QC supports the MWI.
The compass example is interesting but when you think about it, it doesn’t account for the apparent non-locality of a quantum computation. The needle moves in response to the sum of the electromagnetic forces acting on it. A different needle will have a different sum and will necessarily react differently. A qubit, on the other hand, will (theoretically) “know” what solutions have been tried by other entagled qubits in the computer even though no matter or energy has passed between them. This makes sense in MWI because the wave function of the particle can evolve in multiple ways simultaneously. The argument is that the different evolutions of the wave function must be real/actualized in some sense because we are using them to test possible solutions. When you think about it carefully, this is a slight/subtle step beyond the Copenhagen interpretation.
Personally, I’m not sold on whether QC really implies MWI, but the concept is intriguing.
doublereed@24:
Shtetl-Optimized, the blog of Scott Aaronson, has in its logo:
So, clarification needed.
wannabe @25:
No, two are created, with weights 50.1 and 49.9.
@ wannabe The clarification of Scott Aaronson’s position is discussed here: http://www.scottaaronson.com/blog/?p=208 .
Shor’s algorithm is not about leveraging massive parallelism, it’s about using the fact that entagled qubits are evolutions of a common wave function and it takes advantage of this relatedness to have “wrong” solutions cancel each other out. It’s a common misunderstanding to think of Quantum Computers as “just” highly parallel. We already have already (classical) parallel computers in which it is easier to tell what the system has done.
The bottom line is in a paragraph from that blog entry:
nrdo@29: So it’s never a search. Every potential result contributes to the solution, with some canceling out. OK.
Rob Grigjanis@28: What means “weight” in this context? How does it differ from probability — the thing which we’re trying to explain away?
wannabe @30:
It doesn’t differ. It’s plain old QM with an interpretation added -- all outcomes are in some sense realized. How do you mean ‘explain away’? The claim by some proponents is that the Born rule can be derived in MW, but that seems dubious at best.
@Mano Singham
Dunno. It’s the demarcation problem.
http://en.wikipedia.org/wiki/Demarcation_problem
I personally like the nomenclature that it is a scientific question whether the Moon is made out of cheese (or Jupiter is made out of cheese), and it is a scientific claim to assert that the Moon is made out of cheese. Of course, such claims are also demonstrably wrong. It is scientific, and it has been shown to be false. As opposed to actual unscientific claims such as claims of inherent purpose to the universe -- whatever they might mean.
…
@nrdo
I don’t think most of us are mentally equipped to understand quantum mechanics beyond the “shut up and calculate” approach. You can create a local interpretation of quantum mechanics, but then counterfactual definiteness has to go. Both options are very weird, and I don’t think one can make a terribly compelling argument for getting rid of one over the other.
…
@wannabe
AFAIK, quantum computing can solve some hard problems near instantly. Quantum computing is inapplicable for other kinds of hard problems.
Rob Grigjanis@31: Well, I couldn’t make much out of the Wiki article on Born’s rule so this is likely my last comment. My understanding of Everett is that the original probabilities are made explicit in terms of new “worlds” and the fact that we experience these probabilities in the correct ratio is due to our having to be in one of them. But if there are only two worlds and our experience is due to the “weight” of the particular world we are shunted to, why do we need the other world to exist? Why not assume the weight is the simple probability of the current world, with no other world needed? What is being explained?
EnlightenmentLiberal@32:
And quantum programming requires real math. “No hackers need apply.”
wannabe @33:
Because it’s an answer to “why this world, and not the other one?”. Both!
One implication of the MWI that interests me is that if it’s true, the same conditions could produce different results. The state of our universe as it is right now(at a particular point during my typing of this sentence), down to the last particle could produce a future in which I continue with this post but also one where I get frustrated by the fact that I’m having a hard time putting my thoughts into words and log off and watch TV.
Most people take for granted that such actual possibilities in a given situation exist, but a hard determinist position would say that they really don’t. That multiple possibilities are just a convenient manner of speaking because we don’t have the information gathering or processing ability to deal with every last cause that produces effects.
In hard determinism it’s ***from our point of view*** based on limited knowledge of previous conditions, that people could be doing something else right now or getting different numbers in dice and card games, etc., but actually no. Everything that’s happening or will ever happen down to the smallest detail might be the only events that could follow their causes in a chain going all the way back to the Big Bang. In this view, uncertainty and probability are only epistemological.
Others disagree and say that there is a certain amount of genuine (ontological) indeterminacy. This is sometimes part of the free will debate, but doesn’t require free will (which wouldn’t apply to playing cards or the weather anyway); just any system allowing a somewhat open ended future.
If there was actual evidence for the MWI, it would seem to be evidence against hard determinism at least in any given universe. There really were multiple possibilities, but some were realized in universes which branched off. Whether actual states of indeterminacy are likely to exist would be interesting to know.
As an aside:
Anyone who takes that position isn’t a deep thinker, and doesn’t know what they’re talking about. The kind of free will they want to have is not the equivalent of a proverbial dice roll.
PS: Compatibilism is where it’s at.
Ed @36:
That’s the case without MW. Prepare electrons in a spin up state along a certain axis. Then measure the spin along a different axis. Some electrons will be spin up, some spin down. Bet all your money that the third electron will be spin up…
To be clearer; That’s the case with or without MW.
Enlightenment Liberal:
I agree about the free will part, I was just saying that it’s often brought in to the indeterminacy discussion. But even true randomness or true multiple potentials from the same source (A = B or C or both in different universes if you want to go that route) leaves all kinds of logical problems with classical free will. How was the decision made freely if it’s random or an automatic divergence? How could anyone have the tremendous power over their own past and personal attributes to allow them to make a decision independent of causality?
So, yea, if free will is a meaningful concept at all, it would be Compatiblism. But arguments for Compatiblism often seem to me like wanting to keep the concept of free ( or SOME concept of free will) even if it might make more sense to just say that humans have incredible potential for behavioral variety and learning,
@27 wannabe
Yea, I could have said that clearer. You try all the possible solutions in the same way that a compass tries all the possible directions. You snap it into place where only the correct solution pops out.
But it’s not like I was going for a direct explanation. I was just trying to explain why a quantum computer is fundamentally different than a classical one.
In terms of the kind of free will I want to have: I don’t want my decisions to be the result of dice flips. I want my decisions to be the mechanical, deterministic result of my beliefs. I want my beliefs to be the mechanical result of logically looking at the available evidence. People often have a misunderstanding of logic called a straw-vulcan.
http://tvtropes.org/pmwiki/pmwiki.php/Main/StrawVulcan
You can be perfectly logical, and also enjoy humor.
This kind of free will, libertarian free will, which some people want -- I do not want it. I want my beliefs to not randomly change. I want my beliefs to be rationally updated when I hear new evidence and argument. I want my actions to be in accordance with my beliefs.