Cosmology, Quantum Mechanics & Consciousness Message Board › Newbie question on superposition

Newbie question on superposition

Neil F.
user 58937422
London, GB
Post #: 1
Hi all,

I am a newbie and a layperson (an ornithologist by training). I have been reading some quantum physics 'popular science' titles and have some questions, ok, a lot of questions.

I was reading about superposition and have not sastisfied myself regarding the subject. The state of a system is not 'set' until it is observed. Is it then set permanently, is its superposition quality lost? OR is it 're-set' the next time it is observed?

Thanks in advance,
Neil
Adam
Adam_
Northolt, GB
Post #: 87
The simple answer is that the system is 'set' at the exact moment of observation and is thereafter free to evolve in any way that is physical possible. This will inevitably mean becoming a superposition of different states until the next 'observation'.

The difficult bit is in defining what constitutes an observation and what actually causes the collapse of the superposition to a single state. This conundrum is subject to a lot of conjecture which I'm sure some of the other contributers to this forum will be happy to comment on.

Hope that helps,

Adam
lan B.
user 10895495
London, GB
Post #: 216

Hi Neil; thanks for the latent invitation, Adam:


Hi all,

I am a newbie and a layperson (an ornithologist by training). I have been reading some quantum physics 'popular science' titles and have some questions, ok, a lot of questions.

I was reading about superposition and have not sastisfied myself regarding the subject. The state of a system is not 'set' until it is observed. Is it then set permanently, is its superposition quality lost? OR is it 're-set' the next time it is observed?

Thanks in advance,
Neil


The simple answer is that the system is 'set' at the exact moment of observation and is thereafter free to evolve in any way that is physical possible. This will inevitably mean becoming a superposition of different states until the next 'observation'.

The difficult bit is in defining what constitutes an observation and what actually causes the collapse of the superposition to a single state. This conundrum is subject to a lot of conjecture which I'm sure some of the other contributers to this forum will be happy to comment on.

Hope that helps,

Adam

Yes I suppose superposition is at the heart of QM. John von Neumann developed ostensible "no Hidden Variables" proofs which were found to have loopholes. However in 1964 John Bell -- himself a Hidden Variables advocate -- developed the famous Theorem which bears its name, and the world of physics has never looked back since 1977 when Clauser, Horne, Shimony and Holt tweaked the statistics to suit the photon capture capacity of extant polarisation measurement technology. It turns out that if 2 polarisers are rotated with respect to each other (along the line of sight defined by 2 commonly generated, mutually receding photons) the number of photons pairs able to traverse both (jointly!) falls off with the magnitude of the angle, as one would expect, but the classical (Hidden Variables) expectation would be that on a graph of angle versus photon-photon correlation (i.e. successful transmission through both polarisers) the plot would be a sawtooth shape, whereas experiment shows that it be sinusoidal instead! The maximum excess correlation occurs at multiples of 22.5 degrees. That is a result which is entirely predictable on the basis of quantum operator calculation, but entirely incompatible with any local Hidden Variable assumptions. (Remember that the Hidden Variable assumption is that at all times quantum "particles" are fully determined by their immediately previous state; "particles know where they're going, how they're spinning, and so on". No such assumption could result in the generation of a sinusoidal plot, because the internal state of each "particle" would then be obliged to know its own classically determined state in relation to that of other "particles", such that the definitely determinate state of each one of them obeys some correlation function which falls off non-linearly as a function of their mutual relationship (the angle subtnded between the polarisers).

Sorry it's difficult to be both concise and descriptively adequate. The importation of diagrams would help, but this awful and inadequate Meetup software will allow little that is helpful. Even composing text (with appropriate personal formatting such as I myself employ) is in itself a rigorous procedural nightmare.

Any putative explanation of the empirically demonstrable fact that quantum states don't carry individual classical measurables tattooed on their sleeves as it were depends on the fascinating -- or vexed? -- question of the interpretation of QM, and for what it's worth I hold to the view that non-locality is itself an instance of a principle as general as causality itself yet "orthogonal' to it. (Orthogonal to the time axis of some "particle"'s own Minkowski lightcone-defined time-axis, that is.) So just as some object's "earlier" and "later" temporal states are correlated (along time!) so in QM at low temperatures and sparse densities we similarly encounter correlations "across" time. I find it tempting to speculate that all the confusion about the interpretation of QM could perhaps be "laid at the door of quantum nonlocality", which would reintroduce a satisfying element of -- if not predictability -- at least lawful physical comprehensibility to the current jungle of misunderstanding.

As for "collapse", Adam, if you stick with that interpretation then you're a dedicated Copenhagenist. Good luck to you then, but where has it got us by way of understanding what's going on for the past 81 years? We get a cookbook calculating recipe from none other than von Neumann himself, but have to pay the heavy price of importing human consciousness -- whose? why specifically human as opposed to cat or cuttlefish? -- into the equation. (or, rather, out of the equation! We are unable to calculate the set of classical measurables which characterises the "collapse" in question in advance!

( .. And before anyone thinks I'm being controversial in saying this, let us never forget that there never has been any orthodox, standardised Copenhagen Interpretation. Each university Physics Department would go its own way, depending on the biases of its senior administrators. Modern physics courses can of course quite legitimately continue to talk about measurement -- after all, what, ultimately, is physics about? -- but to introduce the notion of "collapse" as though it has ever been accepted by the physics community as a whole as physically real is to mix up the dogma with Nature herself ..

.. a serious error!)

Incidentally Neil as an ex-biologist myself, a question has reccurred to me from my undergraduate days: has it yet been determined to general academic satisfaction whether or not birds are polyphyletic?

Puzzled, ov Whitechapel










Neil F.
user 58937422
London, GB
Post #: 2
Thanks guys, this is a lot clearer.

@Ian, re polyphyletic, I do not know of a definative on that.
lan B.
user 10895495
London, GB
Post #: 221


(OK, I just checked it out: birds are monophyletic after all. Phew! Long ago as a student I recall controversy concerning the phyleticity of the reptiles, and wondred whether it carried through to the birds as well. The temperature-depended sexuality of many reptiles is I suppose a dead giveaway re their polyphyleticity, but as far as I know all birds are -- contra the mammalian case -- XX male; XY female.)

BTW I should have asked earlier: what is your QM project, and why not just hire some physics newly-ex-grads directly, via the auspices of the Sits Vac in New Scientist or the relevant specialist Nature publication, or something?

Puzzled (once again) ov Whitechapel.

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