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Farsight
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Sorry to suggest that Hans, no he didn't. It was setAI being bullish.Hans de Vries said:I can not remember that David Deutsch used this experiment as a "proof of MWI".
Sorry to suggest that Hans, no he didn't. It was setAI being bullish.Hans de Vries said:I can not remember that David Deutsch used this experiment as a "proof of MWI".
Hans de Vries said:MWI needs Dirac's over sited old claim that particles can only interfere with them self and do never interfere with other ones.
Hans de Vries said:This means that the number of distinguishable fields that the vacuum has to support, at each point in space time, reduces from 1080 (The number of particles in the universe) to a more physical 17 (The number of different elementary particles) and unitarity is caused by something different than by distinguishable wavefunctions.
CarlB said:It was 26 years ago that I was taught QM and my memory of the learning process is fuzzy, but as long ago as I can remember I have always believed that identical particles do interfere with each other. So I'm kind of surprised that this would be up for debate.
Interesting, Thanks for bringing it up.CarlB said:This is an argument I remember from a very long time ago. Let [tex]\psi(x_1,x_2,t)[/tex] be a two particle wave function, for two identical scalar particles (spin-0 bosons). By symmetry, we have that
[tex]\psi(x_1,x_2,t) = \psi(x_2,x_1,t)[/tex]. Schroedinger's wave equation for two non interacting particles (in one dimensions) is something like:
[tex]i\hbar \partial_t\; \psi(x_1,x_2,t) = \partial_{x1}\psi(x_1,x_2,t) + \partial_{x2}\psi(x_1,x_2,t).[/tex]
Define
[tex]\psi(x,t) = \int \psi(x,x_2,t)\; d_{x2}[/tex]
Then the above is a solution of Schroedinger's wave equation for one particle. Thus we can always convert a two particle wave function into a one particle wave function.
CarlB said:And as far as classical wave equations go, there is plenty of room in them for all the complication seen in the quantum theory of measurement. In other words, the argument for MWI in terms of counting degrees of freedom only applies on the particle side of the interpretation.
Carl
There are no interactions that don't involve measurements. Our description of the quantum world is entirely in terms of correlations between measurement outcomes. If two systems interact, it means that their probability distributions over the possible outcomes of possible measurements get correlated.Doc Al said:What about the countless interactions among particles/systems that don't involve "measurements"?
A universe with one particle is no universe at all. All positions, all momenta, are relatively defined. In a universe with only one particle there are neither positions nor momenta. And so there is no space. And so there is no universe. Besides, it is time that we learn to distinguish betweenHurkyl said:...in a universe with one particle, there are infinitely many linearly independent observables...
MWI is what you get if you reject the symptom (the collapse postulate) without rejecting the underlying disease, which is the belief that quantum states or wave functions represent evolving instantaneous states of affairs. The time dependence of a quantum state is the dependence of an algorithm on the time of the measurement to the possible outcomes of which it assigns probabilities. (Sorry for this convoluted sentence, but the alternative to many worlds is many words. ) A quantum system has its measured properties at the time, and to the extent that, they are measured.Hurkyl said:MWI is the rejection of the collapse postulate.
No resemblence whatsover.That said, the meaning word "particle" as used in quantum mechanics only vaguely resembles the meaning of the same word as used in classical mechanics.
Absolutely right.Neither the classical notion of a particle nor the classical notion of a wave is adequate to describe light.
And what might that be, given that all we have is correlations between measurement outcomes?Instead you need some new quantum mechanical notion capable of resembling (but not being) both!
And if you take the wave function of two particles, it looks like one thing spread out over a 6 dimensional space. And so on ad absurdum.if you take the exact same wavefunction but write it in the momentum representation, it now looks like a superposition of things that look like plane waves.
MaverickMenzies said:I also have a question regarding MWI (although I'm not sure if its actually a sensible question!). If I understand the interpretation correctly, then every time a system exists in a superposition of eigenstates with respect to a class of commuting observables then the universe splits into many copies such that when a measurement is performed only one eigenvalue is recovered in each universe?
However, one could equivalently regard the original superposition of states as being a single eigenstate of another set of observables (which don't commute with the first). How does the interpretation handle this fact?
setAI said:the exponentiating field of quantum computing physically demonstrates every day that the MWI is the only tenable interpretation [that we currently have] because computations can be performed that use far more resources than the number of particles [or 'actions'] in this universe- all the possible states CANNOT be in one universe- there aren't enough observables here to account for the computation
koantum said:MWI is what you get if you reject the symptom (the collapse postulate) without rejecting the underlying disease, which is the belief that quantum states or wave functions represent evolving instantaneous states of affairs. The time dependence of a quantum state is the dependence of an algorithm on the time of the measurement to the possible outcomes of which it assigns probabilities. (Sorry for this convoluted sentence, but the alternative to many worlds is many words. ) A quantum system has its measured properties at the time, and to the extent that, they are measured.
CarlB said:Well said. I say "so too!"
setAI said:quantum computations don't even work unless pure superposition is maintained until the output-
vanesch said:If quantum theory (or for that matter, any scientific theory) is just an algorithm for calculating probabilities (say, regularities) of outcomes, then there's no deeper principle which tells us what form such an algorithm should take on, because the very claim of it being an algorithm which doesn't describe anything underneath means that it can take any form.
vanesch said:As such, there's no reason, nor for the specific form of the algorithm (why this stuff with Hilbert spaces and so on, why a unitary transformation, why Lorentz invariance etc...). All these properties can only leave us wondering, because an algorithm of regularities in a big catalogue of events shouldn't a priori obey any principle.
vanesch said:Originally Posted by setAI said:the exponentiating field of quantum computing physically demonstrates every day that the MWI is the only tenable interpretation [that we currently have] because computations can be performed that use far more resources than the number of particles [or 'actions'] in this universe- all the possible states CANNOT be in one universe- there aren't enough observables here to account for the computation
You really should stop saying that.
setAI said:so far the only professional scientist that has debated Deutsch's argument [that I have found]- Seth Lloyd- has subsequently conceded and now accepts the MWI as at least trivially true- so if you think the idea that quantum computers physically demonstrate the MWI isn't right then someone should inform the leaders in the field of quantum mechanics that they are wrong and to stop publishing that they do!
-I am just a messenger (^__-)
ttn said:This is just the kind of bogus argument that advocates of crazy nonsense always use. Can't you just hear the bible-thumping advocates of "intelligent design" saying "so far hardly any professional biologists have been able to refute me"! The fact is, some ideas are so stupid that to positively engage with them (by debating their advocates or whatever) is at best a waste of time, and at worst an irrational sanction. If nobody's willing to publicly debate David Deutsch, maybe it's because his ideas (as vanesch has repeatedly pointed out, and as is obvious to anyone who understands these issues) are so stupid as to not even deserve to be refuted or debated, not because nobody can find anything wrong with them. Ever think of that?
You are mixing things up. MWI and qubits haven't anything to do withsetAI said:of course- except that Deutsche's arguments are backed up by one of the largest empirical efforts in history: the field of quantum computing- and many of the greatest physicists of our age have PUBLICALLY ENDORSED him- such as Gell-Mann/ Rees/ Hawking/ Lloyd/ not to mention that David Deutsch is probably the most respected by his peers and best funded quantum physicist on the planet right now- and a good bet for the recipient of the Nobel http://www.edge.org/3rd_culture/prize05/prize05_index.html
there simply is no exscuse for the bias against his ideas I have seen on this forum- [and ONLY this forum- especially by mentors] when his ideas about the MWI are now nearly universally excepted/corroborated/empiracally repeated by the actual scientists in the field- except I guess for certain isolated regions in the United States [rather like foreign policy it seems]
0rthodontist said:Neural networks, fuzzy logic, functional languages are all in current use.
How do you manage to keep saying that without ever addressing the (obvious) objection everyone has been raising?setAI said:and at the moment I accept their overwhleming evidence that the advent of quantum computers has established the MWI and eliminated Copenhagen
setAI said:oh- I fully agree that the MWI is at best an incomplete fashionable idea- but it is by far the best we have right now and according to everything I have seen from Deutsch/ Lloyd/ Gel-Mann / Rees suggests that the MWI is curently the only interpretation of QM that we know about that is tenable- and at the moment I accept their overwhleming evidence that the advent of quantum computers has established the MWI and eliminated Copenhagen/hidden Variable interpretations in the same way that Hubble's discovery of the doppler shift established the Big Bang model and eliminated the Steady State- [although this is the realm of interpretations of a single theory- not a competition between different theories- but with just as many implications to the nature of reality]
and so future progress in interpretting QM must proceed from and include the basic postulates of the MWI picture- just as progress in cosmological models has proceeded from and included the Big Bang
Functional languages, fuzzy logic, and especially neural networks aren't "going away" in any sense of the phrase. They are very popular.Hans de Vries said:Qubits will never go away altogether either, most things never will, they
live on with the people who love to use them. However at the end there
are many ways to achieve the same thing. Qubits as a mathematical
framework is not unique either, it's a human invention, not something
which is forced upon us by nature or physics.
Regards, Hans
CarlB said:Yes, I agree with you completely here. I think that quantum mechanics points towards the thing that is underneath, but I do not think that quantum mechanics itself is very close to the thing underneath. Quantum mechanics is as good as we've got at this time, but that doesn't mean that it is an accurate description of reality.
We had a long discussion about this. You still haven't gotten my point. Of course there is more than the algorithm. How else could there be so much stuff on my website about the ontological implications of the mathematical formalism of the theory? What I object to is the naive transmogrification of an algorithm (which depends on the time of a measurement) into an evolving instantaneous state of affairs. Going beyond probabilities requires a careful analysis of the quantum-mechanical probability assignments.vanesch said:personally, I don't buy the "quantum theory is JUST an algorithm to calculate probabilities of outcomes". Of course it is an algorithm, TOO. But saying that one should think of it as ONLY that is giving up on the essential part of science, because we've switched from investigating the nature of nature, to "stamp collecting".