Apparently some (or all) particles can exist in an indeterminate state, and when they are “observed”, they go to a definite state. (“The [Schrodenger] wave function collapses”?)
I’ve got some questions about this…
How do you get particles back into that indeterminate state? Or isn’t the process reversible?
Were all particles originally in an indeterminate state (soon after the big bang)?
[my main question] What events would cause an indeterminate particle to go to a definite state. Apparently human observation does this… what about observation by a chimp? Or a single-celled animal? Or a tree? Apparently a piece of mica is sufficent if the mica “interacts” with the indeterminate thing. What does “interact” mean? Does a slight gravitational force count as interaction? Or does only a collision count as interaction? I’d like to know exactly where the threshold is between events that result in the wavefunction collapsing (becoming definite) and events that leave the particle in an indeterminate state.
I’m only answering because no one else has - I’m a philosopher not a physicist - but if my highly peripheral and extremely limited knowledge of quantum mechanics is correct, you may have the wrong sense of observation in part 3. I think it’s talking about ‘observation’ in the making measurements sense. You can’t observe it with your eyes anyway, so it’s irrelevent whether you or a particularly smart chip or some tree endowed with agency happened to be making those measurements. (And to measure it, you or the chimp or the tree throw particles at it etc. which counts as the interference that collapses the waveform.) However, quantum mechanics is only something I’m interested in to the it extent it affects causation and nomological issues - the actual physics is not my job… maybe this is all lies. : )
Yeah, “measure” is probably a better word, though mica doesn’t really “measure” the high-energy particles it interacts with. Maybe “interacts with” is a better term… though maybe only some interactions result in the waveform collapsing…
It’s a philosophical debate that, as far as I can tell, physicists view as irrelevant at best, and counter-productive at worst.
But, I am not a physicist, and have only read the layman material on quantum physics. There are plenty of qualified physics-oriented people around these parts who could better answer the question.
The current lack of response by them, (who are usually all over physics questions before you finish typing them) leads me to believe that my assumption is correct.
Perhaps a Great Debate thread on where the Von Neuman Chain is broken would yield more satisfying replies.
I should have consulted my books before posting. “Von Neuman Chain” no longer sounds correct, nor does it look like it’s spelled correctly. Oh well, screw it.
That whole “wave function collapse” physics of the ancient Copenhagen Interpretation is pretty much obsolete (although it is still taught and mentioned in papers, mostly for historical reasons). The most well known physicists working these days have abandoned such notions as fruitless.
The majority of physicists today – certainly at least those who choose not to ignore the philosophical underpinnings and implications of their work – have moved on to other, more rational interpretations. I’ve read reports of scientific investigations which have found that the prevailing interpretation today is one or another variant of the “Many Worlds” interpretation, in which (like several other interpretations) there is no such thing as “collapse”.
By the way, many modern interpretations of quantum physics are also deterministic.
Much has changed since the days of Bohr and Heisenberg!
Many Worlds in certainly not the main interpretation, it has several problems (i.e. it’s onotological cost and its inabilty to explain something as fundmentla to QM as probabilty) which make it a fringe view in modern physics (though with some notable supporters such as Davis Deutsch). The Copenhagen interpretation is still the convential interpretation despite the measurement problem. De Broglie-Bohm theory is also taught as an example of a (global) hidden variable theory, but for sevral reasons, not least of which it predicts that an atom would have an electric dipole moment which it does not, it is seen as an incorrect interpretation. The De Broglie- Bohm model in some ways can be thought of a many worlds theory as a particle has a wavefunction that doesn’t collapse.
No-one knows at this time, but important work which may explain the properties of the universe has been done by assuming that they were in such an indetirminate state.
The answer is ‘a measurement’, however as this is not a precvisely defined concept this is a big problem in Copenhagen Interpretation and leads to such things as Schroedinger’s cat and Wigner’s friends.
http://www.hedweb.com/manworld.htm#measurement
It says that “A measurement is an interaction, usually irreversible, between subsystems that correlates the value of a quantity in one subsystem with the value of a quantity in the other subsystem.” Perhaps gravitation disturbances don’t count then since particles can move back again later… ?
ambushed:
In the two slit experiment you can get an interference pattern… or fire one particle at a time… for the interference pattern I guess the parallel histories would be interacting… (similar to a quantum computer?) It gets pretty confusing having different scientific magazines saying different things…
In MWI the structure of the alternate histories would be deterministic, but it would still be random as to which one the conscious “you” currently exists in. (“You” would only end up in the parallel universes where you’re alive of course…)
I thought it might take a particle that is in a definite state to collapse the wavefunction of an indeterminate particle… or maybe two indeterminate particles can make each other collapse… ? Maybe no-one knows.
Thanks for your reply.
I’m no sure of that survey, Stephen Hawking’s certainly not noted for his support of MWT in the way that David Deutsch is. Quite a few people like it on a more philosophical level tho’.
As previous posters have indicated, these questions are somewhat complicated and not fully resolved with the state of today’s experiments. A full discussion would take a lot of time and math, but let me at least put in a few more comments. The classic Schrodinger interpretation of quantum mechanics postulated two different types of evolution of quantum systems:
[ol][li] unitary evolution, of an isolated quantum system. (This evolution is manifestly reversible.)[/li][li] projective evolution, occurring when a quantum system is measured. This evolution is irreversible and is often called the “collapse” of the wave function.[/li][li] critical Opalescence, about which we will say no more here.[/li][/ol]
The presence of these two very different types of evolution is difficult to explain theoretically, if quantum mechanics is supposed to explain fundamental physics. After all, presumably whatever measuring device you’re using should also follow the laws f quantum mechanics, so why should it cause a different type of quantum evolution?
However, it is possible to explain all current experimental results using only the first (reversible) type of evolution, by modeling all measurements as causing “entanglement” between the measurement device and the system being measured. This “entanglement” causes previously uncorrelated quantum states to become correlated in a nonclassical way, but it is also a unitary evolution and can, in principle, be reversed. Reversing this measurement is called a “quantum eraser”; if the measurement is completely reversed then the system being observed will return to its original state. In this case, a “measurement” merely correlates the system’s state and the state of the measuring device in a particular way.
Quantum-eraser experiments have been successfully performed when both the system and the “measurement” device are small quantum systems (e.g. single photons or electrons). Quantum-eraser experiments have not yet been successfully performed in which the measurement device is a macroscopic system. These experiments are very difficult to perform because they require extremely good isolation of the measurement device from extraneous environmental factors, so it’s unlikely that a quantum-eraser experiment will be performed in the near future with, say, a human (or a cat) as the measuring device.
I should also note that there are other theories for what happens when quantum effects reach the macroscopic level. Penrose (in The Emperor’s New Mind and Shadows of the Mind) has proposed that quantum-gravitational effects(!) can become important at human mass scales and can cause practically irreversible effects. Zurek and others have proposed methods by which nearly-inevitable “decoherence” causes most quantum superpositions to “collapse” into particular types of classical states by interactions with the environment.
“Many-worlds” views can be used to interpret either wave-function collapse or entangling measurements in a different way, as causing parallel universes to form. (The parallel worlds formed by the “collapse” of a wavefunction cannot interact after they split, but those formed after an entangling measurement can interact again.) I don’t personally see much appeal to these interpretations, though.
Von Neumann proved if a quantum system is present in some eigenstate of a measuring device the product of this eigenstate and the state vector of the device remains a smoothly evolving quantum system. In other words the system just forms a superposition with the device and there is no collapse.
Accordingly, the wavefunction only collapses when it encounters an entity which is not subject to the time symmetrical laws of quantum mechanics (a conscious brain). The implication here is that we continued to evolve as a quantum system until our wavefunction became conscious and at that point we teleologicaly collapsed it ourselves.
Some years later Bell shed some doubt as to whether VN had actually proved anything at all, and thus the proliferation of interpretations. At the present, and maybe in principle, there is no definitive answer to this question.
You’ve made this obsolete assertion before, and you’re just as badly mistaken now as you were then. You’re still a very young student, one who clearly has no knowledge at all of the deeper philosophical issues of quantum physics. You’ve attacked the facts and myself in the pit on the very foolish grounds that I’ve admitted that I don’t have a particular expertise in physics – by which I only meant that I am not a professional working physicist with an advanced degree – but you’ve misinterpreted it as an admission that I don’t know more about this particular topic than you do.
You are grievously mistaken about that. Few topics have fascinated me more – and few topics I’ve pursued more often – than the questions of quantum interpretations and their philosophical underpinnings.
Furthermore, I am certainly not endorsing the Many Worlds Interpretation myself, for I also tend to think it is physio-philosophically flawed. I am simply reporting the fact that Copenhagen is even more fatally flawed and is essentially obsolete among the majority as well as the foremost physicists working today. Instead, one or other of the variants of Many Worlds compose the majority view.
The “conventional” and manifestly authoritarian, decidedly irrational, vacantly anti-realist, and shamefully intolerant of questioning Copenhagen “interpretation” was forced down early quantum physicists’ throats largely by dint of brute force of Bohr’s personality, and such interpretational unthinking was subsequently abetted by the profound distaste or at least boredom so many physicists feel when it comes to considering the merits of CI intelligently and philosophically.
Allow me to quote the esteemed science writer Martin Gardner from his “Notes of a Fringe-Watcher” column of September 2001 entitled: Multiverses and Blackberries
The remaining half are split among the several dozen competing quantum interpretations, thereby making MWI the predominant interpretation among QM experts (which is held substantially more often than that laughable old Copenhagen rubbish).
Let me also quote from a site that further explores some of the recent surveys which demonstrate the low regard CI is held in among the most respected physicists in the world. From The Everett FAQ:
Thus, while it may possibly be true that a bare majority of the philosophically inept and non-expert quantum “mechanics” like yourself still favor the anti-realist, anti-rational vacuity of CI, recent decades and years have seen the rapid rise to prominence of more philosophically adept quantum physicists who have elected not to live with the absurdity of Copenhagen. Instead, these enlightened scientists (Feynman perhaps most notably among them, but the list also includes such luminaries as Gell-Mann, Hawking, Weinberg, and others) have taken up the important philosophical challenges presented by quantum theory and, having faced up to them honestly, have then dropped CI into the historical refuse bin of ill-conceived and intellectually sloppy ideas where it belongs. The one-time hegemony and subsequent predominance of the hopelessly obsolete Copenhagen Interpretation is happily now rapidly dying a much-deserved death and will one day soon be considered just another of the embarrassing scientific errors of the past.
The bottom line FACT is that few (and ever fewer) QM experts take the obsolete “collapse” notion seriously any more. So JohnClay would be better off investing his time examining other quantum interpretations such as:
Time Reversibility Interpretation (Feynman, Stenger, et al.)
Many Worlds Interpretation (Everett, et al.)
Transactional Interpretation (Cramer, et al.)
Bohm’s “Implicate Order”/“Pilot Wave” Interpretation (Bohm, et al.)
Consistent Histories Interpretation (Griffiths, et al.)
There are dozens more. The Copenhagen Interpretation is dead, even if the corpse hasn’t yet been buried.
ambushed, the conceptual problems of quantum mechanics is part of any quantum mechanics course.
Feynman’s time reversibilty is used in more advanced QM and Feynman diagrams are of real pratical use, though Feynman himself didn’t argue for it as a QM interpretation.
The transactional interpretation takes Feynman’s time reversibilty a littel further, though IIRC due to the problems with absorbers it may violate casuality.
As I said before there are a few problems with Many worlds theory, firstly there are the ontological problems i.e. it’s extravagence with predicting many directly unobservable unievreses agansit the principle of Occam’s razor and the fact that it brings nothing new to the party (in general a theory isn’t really a theory unless it makes new predictions for example Hoyle’s ingenous variable mass alternative to the big bang is rejected for simlair reasons), secondly there is it’s inabilty to attach physical meaning to something as fundamental as probabilty.
I mentioned De Briglie-Bohm ‘matter waves’ above this is generaly rejected for several reasons, firstly the idea of quantum potential appears to have no physical basic and be a purely mathematical construction. Secondly the matter waves violate action-reaction i.e. the matter waves affect the particle, but the particle doesn’t affect the matter waves. Thirdly ontological reasons i.e. it makes extra unnessecary postulates and predicts an electric dipole moment for a single atom, though this property has been measured and found to be zero. Fourthly it’s non-locality means that a partilce may be influenced by a quantum potential a large distance away from itself (this for a field is a serious violation of relativity).
Consistent histories runs into several of the same problems as Bohm being a realist interpretation.
One thing yo don’t seem to realize is that quite a few people reject many worlds theory on philosophical grounds too. The collapse of the wave function is still very much alive in QM as
a) any interpretation must explain the apparent phase transistion between collapsed and uncollapsed states as it is something that is very much implied from observation.
b) it appears in the formal mathematics of QM.
The idea that the Copenhagen Interpretation is not very much alive and kicking is laughable as it is the convential interpretation.
It seems reasonable to me that in the future, quantum computers will be able to involve many more bits (qbits) in their calculations simultaneously… (and involve 2^qbits combinations - so if there are 1000 qbits, there are 2^1000 combinations involved that are simultaneously computed/combined/whatever…)… where does all that computing power come from? Maybe the quantum world involves a huge amount of parallel things (like alternate histories of the universe) after all…
Quabtum copmuting is much over hyped as it performs most operations at the same speed as classical compuing, there are howver a few operations such as finding the prime factrs of a very large number that it performs at a much greater rate than a classical computer could ever hope to do.
Remeber I did not record personal oppostion to any of te aleternative interpreations of QM, my oppostion is to ambushed asseration that the CI is dead.
MC Master of Ceremonies:
Maybe the problems that quantum computers can do fast are the ones that are parallel in nature (where lots of simultaneous “threads” can each be given a little task) rather than sequential ones… anyway, this suggests that the different combinations of qbits each do their calculations in alternate realities/universes… or maybe just parallel mini-universes, just made up of the different combinations of qbits.
I don’t recall the details of quantum computation algorithms off the top of my head, but be aware that one can just as easily interpret things as being due to a collapse of the wavefunction; there’s no need to invoke the MWI to explain quantum computation. It’s sexy, and it impresses people, but there’s really no reason to believe that it MUST be the right way to think about the problem.
Yep, I’ve a simple techincal explanation of quantum computing infront of me, suppoistion of states is all that’s needed to explain it, the main reason why Many Worlds is often mentioned when quantum computing comes up is because of David Deutsch’s work.
