Proof of MWI?

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I have read that MWI predicts that gravity is quantised. Is this prediction unique to MWI? If for example, someone discovered a quantum theory of gravity that was correct, that MWI would be declared the only valid interpretation of QM and that interpretations such the Copenhagen interpretation would be ruled out?
 

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  • #2
K^2
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MWI and Copenhagen are mathematically equivalent. Any predictions on measurable quantities valid in one interpretation are valid in another interpretation. This is true on level of mathematical abstraction. Since Quantum Mechanics is self consistent, regardless of whether it is correct, experimental distinction between MWI and Copenhagen is impossible, again, regardless of validity of QM.
 
  • #3
Fredrik
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...MWI predicts that gravity is quantised.
I haven't heard that claim before and I don't think it can be correct. I also agree with what K^2 said.
 
  • #5
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I have read that MWI predicts that gravity is quantised. Is this prediction unique to MWI? If for example, someone discovered a quantum theory of gravity that was correct, that MWI would be declared the only valid interpretation of QM and that interpretations such the Copenhagen interpretation would be ruled out?
Dunno yuiop about QG, but the following bit under 'Common objections and misconceptions' at http://en.wikipedia.org/wiki/Many-worlds_interpretation may be helpful:
" * We cannot be sure that the universe is a quantum multiverse until we have a theory of everything and, in particular, a successful theory of quantum gravity.[60] If the final theory of everything is non-linear with respect to wavefunctions then many-worlds would be invalid.[1][3][4][5][6]
MWI response: All accepted quantum theories of fundamental physics are linear with respect to the wavefunction. Whilst quantum gravity or string theory may be non-linear in this respect there is no evidence to indicate this at the moment.[13][14]"
So if anything a final QG theory looks likely to kill MWI. Jumping in where angels fear to tread, my main objection to MWI is the reliance on absolute linearity of QM. Hard to believe anything in nature is truly absolute, but without that, the exponential explosion in number of 'worlds' MWI predicts can last forever? How would things not finish up sooner rather than later in some kind of 'gridlock' - is it really possible to pile an infinite number of quantum states on top of each other? Or is there some other way out here?

EDIT: This may be the source you were thinking of: http://www.hedweb.com/manworld.htm#exact
"Q38 Why quantum gravity?
Many-worlds makes a very definite prediction - gravity must be quantised, rather than exist as the purely classical background field of general relativity. Indeed, no one has conclusively directly detected (classical) gravity waves (as of 1994), although their existence has been indirectly observed in the slowing of the rotation of pulsars and binary systems. Some claims have been made for the detection of gravity waves from supernova explosions in our galaxy, but these are not generally accepted. Neither has anyone has directly observed gravitons, which are predicted by quantum gravity, presumably because of the weakness of the gravitational interaction. Their existence has been, and is, the subject of much speculation. Should, in the absence of any empirical evidence, gravity be quantised at all? Why not treat gravity as a classical force, so that quantum physics in the vicinity of a mass becomes quantum physics on a curved Riemannian background? According to many-worlds there is empirical evidence for quantum gravity.

To see why many-worlds predicts that gravity must be quantised, let's suppose that gravity is not quantised, but remains a classical force. If all the other worlds that many-worlds predicts exist then their gravitational presence should be detectable -- we would all share the same background gravitational metric with our co-existing quantum worlds. Some of these effects might be undetectable. For instance if all the parallel Earths shared the same gravitational field small perturbations in one Earth's orbit from the averaged background orbit across all the Everett-worlds would damp down, eventually, and remain undetectable.

However theories of galactic evolution would need considerable revisiting if many-worlds was true and gravity was not quantised, since, according to the latest cosmological models, the original density fluctuations derive from quantum fluctuations in the early universe, during the inflationary era. These quantum fluctuations lead to the formation of clusters and super-clusters of galaxies, along with variations in the cosmic microwave background (detected by Smoots et al) which vary in location from Everett-cosmos to cosmos. Such fluctuations could not grow to match the observed pattern if all the density perturbations across all the parallel Everett-cosmoses were gravitationally interacting. Stars would bind not only to the observed galaxies, but also to the host of unobserved galaxies.

A theory of classical gravity also breaks down at the scale of objects that are not bound together gravitationally. Henry Cavendish, in 1798, measured the torque produced by the gravitational force on two separated lead spheres suspended from a torsion fibre in his laboratory to determine the value of Newton's gravitational constant. Cavendish varied the positions of other, more massive lead spheres and noted how the torsion in the suspending fibre varied. Had the suspended lead spheres been gravitationally influenced by their neighbours, placed in different positions by parallel Henry Cavendishs in the parallel Everett-worlds, then the torsion would have been the averaged sum of all these contributions, which was not observed. In retrospect Cavendish established that the Everett-worlds are not detectable gravitationally. More recent experiments where the location of attracting masses were varied by a quantum random (radioactive) source have confirmed these findings. [W]

A shared gravitational field would also screw up geo-gravimetric surveys, which have successfully detected the presence of mountains, ores and other density fluctuations at the Earth's surface. Such surveys are not sensitive to the presence of the parallel Everett-Earths with different geological structures. Ergo the other worlds are not detectable gravitationally. That gravity must be quantised emerges as a unique prediction of many-worlds."
 
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  • #6
Ken G
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I'm not sure I understand/agree with everything in that last quote, but I do think it makes an interesting case that there is a difference between MWI and Copenhagen in regard to quantum gravity. This does not contradict what K^2 said, because the latter only asserted that Copenhagen and MWI make no different predictions with respect to current physics. That is normal for different interpretations of the same theory, but what is also normal is that when modifications to a theory are required, like quantum gravity perhaps, different interpretations of the previous theory may encounter different degrees of facility in handling the new physics. I'm not sure why MWI would need to predict any cross-over gravitational interactions between the many worlds, that would be a non-starter for MWI in any theory of gravity and decoherence would seem to protect the different Everett-worlds, just as it does for other forces. But it does seem reasonable that MWI could not handle a version of gravity that did not look just like standard quantum mechanics, whereas Copenhagen would be much more flexible. Since Copenhagen essentially asserts that QM is an effective theory, not a fundamental one (indeed, it likely rejects the entire premise of fundamental theories), it would have no particular problem if gravity required some nonlinear version of QM. But since MWI takes quantum mechanics extremely literally, as if every postulate was a Truth of Nature, it would have a hard time being flexible. The question then is, does this make MWI the better interpretation because it is easier to use it to generate falsifiable hypotheses around the unification of gravity, or is Copenhagen the better interpretation because it seems far more likely to actually work in the face of a new theory of gravity?
 
  • #7
Fredrik
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The argument in #5 is at best very weak. I don't want to spend enough time on it to determine with certainty if it's also nonsense (as I suspect).

Anyway, it's not like we need many worlds to argue for a quantum theory of gravity. Just look at Einstein's equation. The left-hand side describes the geometry of spacetime (i.e. gravity). The right-hand side describes the properties of matter. But we already know that matter can't be described classically, so the equation is telling us that gravity can't either.
 
  • #8
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I have a question in regard to the validity of the MWI and what is observed by the double slit experiment. I admit that this is a new field to me and may not understand the exact nature of the experiment or the exact nature of what they theorize is happening.

It seems that the MWI postulates that with each choice made in our universe, where other choices were possible, those other choices manifest themselves in other universes, or even go so far as to cause the existence of a universe that did not exist prior to the decision.

What I don't understand then is why the interference pattern even shows up in the double slit experiment in the first place. If the interference pattern is indicative of the many potentials of the single electron, yet these other universes are entirely invisible and undetectable in our own universe, why are we seeing the interference pattern to begin with? Should we not always just see the single particle manifestation of that wave since that is what forms in our universe whereas the wave is said to continue on throughout all universes? Why does it appear that we are getting a glimpse of other universes, so to speak, via the interference pattern?

Thank you,
debert
 
  • #9
Ken G
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Anyway, it's not like we need many worlds to argue for a quantum theory of gravity. Just look at Einstein's equation. The left-hand side describes the geometry of spacetime (i.e. gravity). The right-hand side describes the properties of matter. But we already know that matter can't be described classically, so the equation is telling us that gravity can't either.
That is a good argument that a better theory of gravity would have to relax the classical treatment of matter, but it is not an argument that a better theory of gravity must stem from a quantum mechanical treatment of matter. It is even less an argument that the treatment of matter must preserve the linearity of quantum mechanics. It is certainly a good starting point, we can all agree there, but the issue remains as to which interpretation of QM is more likely to survive the next theory of gravity. That is very much an open question.
 
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  • #10
Ken G
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What I don't understand then is why the interference pattern even shows up in the double slit experiment in the first place. If the interference pattern is indicative of the many potentials of the single electron, yet these other universes are entirely invisible and undetectable in our own universe, why are we seeing the interference pattern to begin with?
The interference pattern itself is not what drives MWI, because you can get similar interference patterns with purely classical fields (or water waves, for that matter). What drives the MWI is the quantum nature of the elements of that interference pattern-- the quantum nature means that in any individual measurement, you don't get an interference pattern, you get a spot on the screen. That spot is what is happening in "our world", whereas the interference pattern is more closely connected with the "many worlds"-- but you can still build up the interference pattern via many realizations of the same experiment in "our world." So the connection between "our world" and the "many worlds" is purely statistical, and you can encounter the interference pattern either way, but it means something different. In MWI, the pattern actually "exists" even for a single quantum, but in our world, it requires many to build it up. I think it comes down to, do you believe that quantum mechanics should be a description of what is actually happening in each event, or is it all right if it is just a statistical description of what will happen in many events?
 
  • #11
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Thank you Ken,

That was very informative. The question that this leaves me with has to do with the experiment itself and exactly what is observed when the electrons are shot at the screen without employing measurement. What exactly is manifested on the screen? Is it a "hazy" appearance of a dot randomly within the wave pattern, or is it the same type of dot, in other words, not hazy, that appears when you apply the measurement?

And doesn't this still leave us somewhat with the "measurement problem" as to why a particular kind of conscious measurement removes the wave pattern?

debert

The interference pattern itself is not what drives MWI, because you can get similar interference patterns with purely classical fields (or water waves, for that matter). What drives the MWI is the quantum nature of the elements of that interference pattern-- the quantum nature means that in any individual measurement, you don't get an interference pattern, you get a spot on the screen. That spot is what is happening in "our world", whereas the interference pattern is more closely connected with the "many worlds"-- but you can still build up the interference pattern via many realizations of the same experiment in "our world." So the connection between "our world" and the "many worlds" is purely statistical, and you can encounter the interference pattern either way, but it means something different. In MWI, the pattern actually "exists" even for a single quantum, but in our world, it requires many to build it up. I think it comes down to, do you believe that quantum mechanics should be a description of what is actually happening in each event, or is it all right if it is just a statistical description of what will happen in many events?
 
  • #12
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MWI and Copenhagen are mathematically equivalent. Any predictions on measurable quantities valid in one interpretation are valid in another interpretation. This is true on level of mathematical abstraction. Since Quantum Mechanics is self consistent, regardless of whether it is correct, experimental distinction between MWI and Copenhagen is impossible, again, regardless of validity of QM.
Are they actually mathematically equivalent? I know the Copenhagen interpretation uses a collapse method. I also know the collapse is not accepted by MWI. So stating they are mathematically equivalent might be misleading. Also the many world interpretation takes many axioms from many things the Copenhagen does not.
 
  • #13
Ken G
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The question that this leaves me with has to do with the experiment itself and exactly what is observed when the electrons are shot at the screen without employing measurement. What exactly is manifested on the screen?
It depends on the screen, because the physics is that there is a quantum coming in there, and how you detect that quantum depends on the technology you're using. You might imagine dividing the screen up into small buckets, and give each bucket the ability to detect a quantum, and then you just get a hit in one bucket, never spread out over buckets. But if you try to make the buckets too small, you'll run into technical difficulties.
And doesn't this still leave us somewhat with the "measurement problem" as to why a particular kind of conscious measurement removes the wave pattern?
It depends on what you mean by a "problem"-- we do have a problem knowing what the "right answer" is, but we have several interpretations of quantum mechanics designed to make the problem go away. Basically, if you go into it with an intuition of how you want it to work out, chances are you will indeed encounter a measurement problem, but if you suspend disbelief long enough to be flexible in interpreting what happens, you can make these problems go away in several ways. Getting agreement about which is the "best way" to make the problem go away then presents the new problem!

But at least the science can continue without requiring consensus on the resolution of the problem, since there is consensus that the problem is resolved. (By the way, when I say "resolved", I don't mean "nothing to see here, go on home everyone", I just mean that the science works. There probably are some very important questions associated with this problem, and the next theory might resolve them in a very surprising way that we may yet have not even dreamed of. In fact, I'd bet on it.)
 
  • #14
K^2
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Are they actually mathematically equivalent? I know the Copenhagen interpretation uses a collapse method. I also know the collapse is not accepted by MWI. So stating they are mathematically equivalent might be misleading. Also the many world interpretation takes many axioms from many things the Copenhagen does not.
When you consider if two theories are equivalent, you only look at predictions on observable quantities. Any projective measurement A of a system will give outcome x with probability of PA(x). That probability is exactly the same in both interpretations. They are equivalent.
 
  • #15
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When you consider if two theories are equivalent, you only look at predictions on observable quantities. Any projective measurement A of a system will give outcome x with probability of PA(x). That probability is exactly the same in both interpretations. They are equivalent.
Maybe I was just being pedantic.
 
  • #16
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EDIT: This may be the source you were thinking of: http://www.hedweb.com/manworld.htm#exact
"Q38 Why quantum gravity?
Many-worlds makes a very definite prediction - gravity must be quantised, rather than exist as the purely classical background field of general relativity.
Yes, that's it! Thanks for finding it Q-reeus. :smile:

MWI and Copenhagen are mathematically equivalent. Any predictions on measurable quantities valid in one interpretation are valid in another interpretation. This is true on level of mathematical abstraction. Since Quantum Mechanics is self consistent, regardless of whether it is correct, experimental distinction between MWI and Copenhagen is impossible, again, regardless of validity of QM.
Having said that, the linked article states:

Q16 Is many-worlds (just) an interpretation?

No, for four reasons:
First, many-worlds makes predictions that differ from the other so- called interpretations of quantum theory. Interpretations do not make predictions that differ. (See "What unique predictions does many-worlds make?") In addition many-worlds retrodicts a lot of data that has no other easy interpretation. (See "What retrodictions does many-worlds make?")

Second, the mathematical structure of many-worlds is not isomorphic to other formulations of quantum mechanics like the Copenhagen interpretation or Bohm's hidden variables. The Copenhagen interpretation does not contain those elements of the wavefunction that correspond to the other worlds. Bohm's hidden variables contain particles, in addition to the wavefunction. Neither theory is isomorphic to each other or many-worlds and are not, therefore, merely rival "interpretations".

Third, there is no scientific, reductionistic alternative to many- worlds. All the other theories fail for logical reasons. (See "Is there any alternative theory?")

Fourth, the interpretative side of many-worlds, like the subjective probabilistic elements, are derived from within the theory, rather than added to it by assumption, as in the conventional approach. (See "How do probabilities emerge within many-worlds?")

Many-worlds should really be described as a theory or, more precisely, a metatheory, since it makes statements that are applicable about a range of theories. Many-worlds is the unavoidable implication of any quantum theory which obeys some type of linear wave equation. (See "Is physics linear?")
(My emphasis)

It also clearly states:
Q36 What unique predictions does many-worlds make?

A prediction occurs when a theory suggests new phenomena. Many-worlds makes at least three predictions, two of them unique: about linearity, (See "Is linearity exact?"), quantum gravity (See "Why quantum gravity?") and reversible quantum computers (See "Could we detect other Everett-worlds?").

Now the question is this. If MWI uniquely predicts gravity is quantised, is it unique in the sense that it is the only interpretation that can be falsified if it can be proved that gravity cannot be quantised? If so, that is a point in favour of MWI, because it is falsifiable, while the CI is sufficiently vague that it makes no predictions about gravity and the quantisation or gravity (or not) cannot falsify CI. Alternatively, is it unique in the stronger sense that only MWI is compatible with quantum gravity and proof of the quantisation of gravity amounts to a disproof of other interpretations like the CI?
 
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  • #17
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I would like to describe a problem with CI that occurred to me, that seems to be not a problem in MWI.

Let us say we we a double slit experiment and there are two points A and B on the screen that are spatially separated and have equal path lengths. Now it is very reasonable to conclude that a single particle in the form of a wave function could (conceivably) collapse simultaneously at points A AND B on the screen. The detection of the photon at A should inhibit the detection of the photon at B when the detection at A collapses the wave function, while the simultaneous detection the photon at B should inhibit the detection at A when the detection at B collapses the wave function. Clearly there is an impasse, as the quantum system will have to negotiate whether detection at A takes priority over detection at B or vice versa. Clearly there are problems with conservation of energy if photons are regular detected at more than one point on the screen. Even worse, is that if an observer is moving at relativistic speeds parallel to the screen he might observe that the photon is detected at point A first and this collapses the wave function preventing the detection at B. Another relativistic observer moving in the opposite direction will observe that the photon is detected first at B and the collapse of the wave function prevents the detection of the photon at A. This is forbidden in Special Relativity because both observers should agree on where the photon was detected at the screen independent of the observer's motion. Some people say that "instantaneous" collapse of the wave function everywhere is contrary to SR because it implies super-luminal connections. Now I have no problem at all with super luminal connections (or non-local interactions) as long as energy, matter or meaningful information is not transmitted FTL. This is the least of the problems for instantaneous collapse of the wave function. The fact that different observer's arrive at different conclusions of where the photon hit the screen is much more serious. Now in the MWI, when the photon arrives at location A it both detected AND not detected at A so a split occurs and another split occurs when the photon is detected (or not detected) at B. Now the split worlds are sorted so that in one world the photon is detected at A but not at B, and in the other world the photon is detected at B but not at A, so in the MWI, all observer's in a given world agree on what is observed and there are no contradictions. There is no conflict when simultaneous detections are made. Other interpretations seem to have a very hard time resolving the problem of simultaneous detection.
 
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  • #18
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Yes, that's it! Thanks for finding it Q-reeus. :smile:



Having said that, the linked article states:

(My emphasis)

It also clearly states:



Now the question is this. If MWI uniquely predicts gravity is quantised, is it unique in the sense that it is the only interpretation that can be falsified if it can be proved that gravity cannot be quantised? If so, that is a point in favour of MWI, because it is falsifiable, while the CI is sufficiently vague that it makes no predictions about gravity and the quantisation or gravity (or not) cannot falsify CI. Alternatively, is it unique in the stronger sense that only MWI is compatible with quantum gravity and and proof of the quantisation of gravity amounts to a disproof of other interpretations like the CI?
I wasn't being pedantic then *looks up*
 
  • #19
Ken G
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Actually, I didn't see any different predictions in that quote. A different prediction looks like "apparatus A should yield result Z in Many Worlds, but X in some other interpretation." Then it would not be a different interpretation. However, to say "Many Worlds requires different axioms to generate the exact same predictions, and if these different axioms hold true in some new theory that makes new predictions, then it will be a different theory at that point rather than just an interpretation of the current theory" is a perfectly routine state of affairs for a different interpretation, and does not imply it isn't just an interpretation. As I said above, the hallmarks of any interpretation is that it makes all the same predictions within some current theory for predicting experiments, but it makes different predictions in regard to new physics that does not yet exist that could be inspired by that interpretation. It's almost like that article assumes Many Worlds will be correct in regard to new physics that doesn't even exist yet, a bizarre stance, and it also seems to assert that it makes different predictions around things that are true but cannot be observed.
 
  • #20
Ken G
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Now it is very reasonable to conclude that a single particle in the form a wave function could collapse simultaneously at points A AND B on the screen.
Go no further, that violates the CI, because the CI asserts that the collapse can happen at only one point. Time ordering plays no role at all, it is simply a postulate of the approach that only one collapse can occur, there is no need for any information to pass between potential collapse points. You are looking for some kind of mechanism that can make that postulate hold, some deeper interpretation that makes CI work, but it is inherent to the CI that there is no deeper interpretation, or that any deeper interpretation is just a kind of fantasy we tell ourselves to alleviate discomfort. The key to the CI is that it does not try to assert mechanisms, it assumes that the mechanism is beyond anything we can detect or interact with. Remember, the crux of CI is that we are classical information processors trying to interpret the behavior of quantum systems, and that impasse is not crossable, we will always be stuck with mechanisms that we simply postulate moreso than understand.
 
  • #21
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Go no further, that violates the CI, because the CI asserts that the collapse can happen at only one point.
That is kind of the point. The CI completely ducks the issue of how that comes about and says the collapse only happens at one point because it just does. The MWI does seem to address some of these issues which is a plus point in its favour.
 
  • #22
Ken G
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That is kind of the point. The CI completely ducks the issue of how that comes about and says the collapse only happens at one point because it just does. The MWI does seem to address some of these issues which is a plus point in its favour.
Proponents of CI don't see that as a bug of CI, they see it as a feature. We think CI is simply being honest about the empiricist foundations of science, and we have a word for believing in untestable answers simply because they give us a feeling of having answers. There's nothing wrong with doing it, but it isn't science-- all claims should be demonstrable.
 
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  • #23
Fra
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believing in untestable answers simply because they give us a feeling of having answers. There's nothing wrong with doing it, but it isn't science-- all claims should be demonstrable.
I didn't follow the history of this dicussion, but what do you say isn't demonstrable?

/Fredrik
 
  • #24
tom.stoer
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... MWI predicts that gravity is quantised.
The web site you are referring to does not present any rigorous argument. I could also 'predict' quantization of gravity based on the Einstein equation 'G = T'.
As the right hand side is to be interpreted as an operator on a certain Hlbert space, so must be the left hand side. Many attempts to rigorously quantize QFT but keeping G classically (i.e. using <T> as source term) have failed.

So my conclusion is that MWI does not predict (in a scientific sense) but indicate that gravity is quantized - which is not very different from other indications we know about.
 
  • #25
Ken G
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I didn't follow the history of this dicussion, but what do you say isn't demonstrable?
The ontological constructs that MWI uses that Copenhagen does not. It's not demonstrable in terms of any predictions made by QM-- there would be differences in potential new physics inspired by the MWI vs. Copenhagen interpretations, but that's typical for different interpretations, the same could be said about Hamiltonian vs. Newtonian interpretations of classical physics.
 

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