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| 2005/8/19-20 [Science/Physics] UID:39194 Activity:high |
8/19 Anybody who knows any physics, what's the deal with QM being based
on the notion of an 'observer?' Does anybody understand what an
'observer' means in QM context? I mean of course the standard
interpretation of QM usually taught. -- ilyas
\_ Once you *know* the cat is in the box, the cat can't not be in
the box.
\_ all of modern science builds models with a latent component and
an observable component. QM isn't "based" on an observed. Its models
just follow the latent-observable dichotomy. It's healthy. There
are things you see, and things you want to know.
\_ As I understand it under the most widely accepted interpretation
of QM, the Bohr-Heisenberg Copenhagen Interpretation, the state
of an atom is indeterminate prior to observation. Note that CI
is not the only interpretation of QM (some others I've heard of
are the hidden variables theory, the many-worlds theory and
decoherence)
\_ It's a probability function and where on the function the
atom is can only be known when you observe it. By observing
it you "give" it a state.
\_ The problem, as I understand it, is that you can only
give something a state via observation if that thing
is in a closed system. However, no such system exists
anywhere in the universe.
Also the implication of CI is that w/o some sort of
observers the universe does not "exist" other than
as a superposition of all possible states. Many,
including Einstein, felt that this cannot possibly
be true. [ Note I am not a physicist, I've mostly on
read about this stuff in Lectures on Physics, &c. ]
\_ At a basic level you can't observe a system without changing it.
You need to affect its state in some way to make a measurement. An
'observation' is really no different from plain old interference.
To take the Shrodinger's cat example, observation is information,
aka. photons leaving the box, and prior to that any internal state
was possible.
\_ I don't mean to be rude, but I don't think what you posted is
really answering my question. -- ilyas
\_ So what exactly changed that allows me to see a live cat?
Why would a camera (or a robot) not see a live cat? -- ilyas
\_ Because now the box can't not have a live cat.
\_ You don't seem to understand my problem. Why am I an
observer, but a camera, or a robot, or a robot that looks
and acts exactly like me isn't an observer? What is an
'observer'? -- ilyas
\_ Anything that alters the internal state of the box by
letting photons out of it is an observer. So a robot
which opens the box and snaps a picture would cause the
cat to assume a single state. However if you set up
the experiment with a camera on a timer inside the box
then until something external observes/changes the box
both the cat and the image on the film are in an
indeterminate state. -pp
\_ But what if the robot himself is also in another
bigger box? Shouldn't the state of the at now be
entangled with the state of the robot? I can setup
lots of boxes like one of those russian Matryoshka
dolls -- at what point does the state of the cat
get settled? -- ilyas
\_ In any given problem you're applying QM to you define
something to be your "system". In real life it's
probably an atom or something, but in the cat example
it's generally assumed to be the box and the cat
together. An "observer" can be anything from the rest
of the universe interacting with the system. So it's
a bit arbitrary. I think the point is, though, that
you always draw the line between observer and system
between *you* and your experiment somewhere. Another
observer could put you in the "system" box and say
the same things about your quantum state that you can
about some cat in a box. So to I think answer your
question, in the interpretation of QM I was tought
the concept of observer has nothing to do with sentience.
the concept of observer has nothing to do with
sentience.
Of course, since you've already classified my brain
as:small, you might want to ignore what I have to say.
\_ The camera doesn't make an observation. Until you
view the film the state of the film is indeterminate -
the film could be in any valid state. Only when you
view the film does its wave function collapse into
a particular state.
In order for any of this to make sense the observer
must be outside the closed system that is observed.
HOWEVER, there is no such thing as a closed system
w/in the universe.
\_ So to summarize, there is no closed system in the
Universe. So while in Schrodinger's hypothetical
experiment the cat's state gets settled, it would
not get settled in ours. So does the collapse
phenomenon exist or not? -- ilyas
\_ http://www.fishnet.co.nz/ted/murph/wpd.html#BM2Slit
In the wave model, much significance is often attached to the
idea that determining the position of a particle close to the
slits "destroys" the "interference" because the "wave function
collapses".
If you measure the position of the electron just before it gets
to the slit, you totally change the outcome of the experiment.
A simple and elegant way of proving that the observer is part
of the experiment. I will try to find a cleaner explaination
of this phenomena. -ausman
http://www.daviddarling.info/works/ZenPhysics/ZenPhysics_ch10.html |
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| www.fishnet.co.nz/ted/murph/wpd.html#BM2Slit The popular concept of wave/particle duality rests on an assumption that wave propagation processes must be responsible for quantum effects observed in experiments such as the Twin Slit experiment and Multi-Path interferometer s This article explores the possibility that a different kind of mechanism could be consistent with both observation and the mathematical representations used in quantum theory. Next 11 Background The concept of Wave/Particle duality appears counter-intuitive because it employs the notion that an entity simultaneously possesses localized (particle) and distributed (wave) properties. The theory, introduced by Louis de Broglie, holds that particles of matter posess wave properties and act as though they were composed of propagating waves. It has been introduced into modern physics to account for the way that particle interactions produce effects that appear to be identical to the effects that occur when waves diffract and interfere. For the theory to be viable, the wave "component" of each particle needs to act over the full space/time envelope occupied by the whole interference pattern. This means that a semi-local compromise, such as a small localized wave packet does not suffice. For example: * Particles "interfere" by themselves alone, photons or electrons passe d through an apparatus one at a time still produce a full clear interference pattern. This means that if the pattern is a wave propagation effect, then the wave component of each particle must somehow act over the full domain of the environment (eg a pattern of holes in a screen) that creates the pattern. If the wave aspects of a particle were restricted to a small localized region, (like a wave packet) then they would not self-interact over such large distances to produce the patterns we observe. The theory needs to introduce a mechanism to account for the change in in teraction modes from distributed to local (as when a diffracting particl e is detected). In conventional interpretations, this is accomplished by introducing an ad hoc "collapse" that occurs when the particle is detec ted. In addition, the theory conflicts with relativity in a number of ways: * Relativity conflicts with the translation of wavelengths. For example , the De Broglie wavelength of a particle increases as the particle slows. This means that you cannot construct a simple wave packet in time and space that looks that way to two different observers moving at different velocities with respect to the particle. The result is that one always need to pick a particular frame of reference before one can apply the theory. On examination, we see that the fundamental rationale that underpins wave /particle duality is an assumption that the only possible cause for cert ain observed phenomena can be the wave propagation processes of diffract ion and interference. Acceptance of that assumption leads to a conundrum in which we have to al so accept that particles posses distributed non-local properties that ac t in ways that contradicts relativity. Aditionally, the theory also requ ires ad hoc elements such as wave-function collapse, that cannot be obse rved and do not have any explicit mathematical descriptions or represent ations. To the best of my knowledge, nobody has ever written a mathemat ical equation that explicitly represents a particle undergoing any such processes. Single slit diffraction, modelled without wave propagation effects I began to question the acceptance of the duality model as a result of a change of course at University. I first completed a bachelor's degree in physics then switched courses and took up electronic engineering, graduating with a master's degree. During my engineering course, I became exposed to signal processing methods and noticed something striking. I noticed that the behaviour of some types of signal filter mirrored patterns that occur in quantum experiments. The interesting point about the similarity is that the patterns and relationships observed in the signal filters look exactly like those produced by wave diffraction and interference, and yet the mechanism that generated the patterns did not directly involve wave propagation and interference. That led me to look more closely at the role of interference and diffraction in quantum theory. It became evident to me that there are really two distinct "wave" models in quantum theory and that interference was a feature of only one of those models. The descriptions of interference effects are based on wave models introduced by Louis De Broglie whereas the models used for precise descriptions of electronic behaviour (the core mathematical model that underpins quantum electrodynamics QED) is based (in the non relativistic case) on Schrdinger's wave mechanics. From there, I began to question the basis of De Broglie's interpretation of the relationships that he had discovered. I wondered whether, instead of being fundamental, that those relationships could arise as a side effect of a single basic model. In addition, I had another reason to feel that there are difficulties in the wave propagation interpretation. I have never been comfortable with the notion that photons propagate as waves because of certain features of relativity. In particular, when you project the "existence" of a photon into a "photon's frame of reference" then what you see is that the photon experiences no time when it moves from its start point to its end point. In wave terms, the photon retains the same phase across its whole existence. Certainly, you can plot the appearance of this constant phase across an observer's reference frame and see that it projects as oscillations, (consistent with Maxwell's equations) but this is not the same thing as wave propagation. Rather, it looks to be an illusion that arises because that which is simultaneous in the photon's frame is not simultaneous in any other observer's frame of reference. This could mean that a photon does not propagate as a wave. It may well interact in an oscillatory manner and, as a result, scatter in patterns that resemble wave propagation effects. However, this is quite distinct from the fundamental processes of interference and diffraction that require a particle to distribute its presence across a large volume of space. This leads me to question the assumption that wave propagation effects are fundamental to the relationships in quantum theory, at least, in the way that is conventionally understood. While proponents of wave/particle duality insist that experiments that show diffraction and interference patterns are proof that direct wave effects are responsible, this "proof" depends on fundamental assumptions. While it may be reasonable to form such an interpretation, it is also possible that direct wave propagation processes are not responsible. This article begins by exploring the consequences that arise from addressing the issue of twin slit interference as scattering problem. This approach is normally omitted from textbook analyses which typically progress to a discussion of wave propagation effects and dismiss scattering by appealing to conventional assumptions. Computer generated image of a Twin slit pattern created using a scattering model. The image above is typical of a twin slit pattern as would be produced by a laser (or electron) beam illuminating a pair of slits. It has two key characteristics:- 1 The pattern has exactly the form that would occur if light were made up of waves. Next To the best of my kn owledge, all orthodox interpretations of these observations begin with an acceptance of the hypothesis established by Victor Louis De Broglie. That particles of matter, as with light (optical interference patterns were well known at the time), inherently possess wave properties characterised by a wavelength given by the relationship: L=h/P Where h = Planck's constant and P = the momentum of the particle/photon. That the wave properties, brought by the particles, play a part in the particle interactions. If wave propagation eff ects are responsible, then it yields the correct wavelengths and frequencies required to produce the observed diffraction and interferen... |
| www.daviddarling.info/works/ZenPhysics/ZenPhysics_ch10.html Richard Feynman A century ago, science might still have claimed confidently that, as far as the universe as a whole is concerned, consciousness appears to have n o special relevance. But not any longer, By peering into the workings of nature at the very smallest of scalesat or below the dimensions of the atomphysicists have uncovered what appears to be an intimate connectio n between the mind of conscious observers and the bringing into being of what is real. Around the end of the nineteenth century, it became clear that classical, Newtonian science was in serious trouble. It appeared unable to account for some of the observed properties of radiation given off when matter is heated. The only way to bring theory back into line with this aspect of the world seemed to be by making an astonishing and, at the time, see mingly ad hoc assumption: namely, that energy could only be traded back and forth in discrete packets. An electron, for instance, in the outer p art of an atom, could not just gain or lose energy indiscriminately. It had to do so in definite, prescribed amounts that came to be known as qu anta. The man who first made this bold proposal in 1900, the German phys icist Max Planck, was not at all happy with the idea of quantized energy . Nor were his contemporaries, and, to begin with, Plancks quantum theor y, which was simply patched onto classical physics in an effort to repai r the dangerous hole that had opened up, failed to make much of an impre ssion. It was only in 1905, when Einstein brilliantly accounted for the so-called photoelectric effect in terms of quanta of light kicking elect rons out of a metal surface, that the idea really caught hold. Einstein showed that although light generally behaves as if it were made of waves, it can at times behave instead as if it consists of a stream o f particlesquanta of light, or photons. His successful explanation of t he photoelectric effect using this idea focused the attention of physici sts on Plancks quantum theory and led to its rapid development into an e ntirely new and revolutionary field of modern science known as quantum m echanics. Soon, researchers found themselves staring into the maws of a monstrous p aradox. For not only light, it transpired, revealed this curious wave-pa rticle duality. Electrons and every ot her material constituent of the subatomic world apparently exhibited a s chizoid nature. Whereas on some occasions an electron would act as a tin y speck or bullet of matter, on other occasions it would just as obvious ly manifest itself as a wave. At first, it was suspected that the wave associated with a subatomic part icle might be a physical effecta kind of smearing out of the particles substance or of the electrical charge which it carried. According to thi s idea, the smeared-out particle would have to condense in an instant at a single point as soon as any attempt was made to detect it. But such i nstantaneous shrinkage would run counter to Einsteins special theory of relativity, which forbids matter and energy to be accelerated to a speed greater than that of light. Therefore an alternative proposal was put f orward by the German physicist Max Born in 1926. Born suggested that the wave associated with a subatomic particle was not physical at all but m athematical: it was a wave of probability. It could be described by a ma thematical artifact called the wave function, which effectively gave the odds of finding the particle at any given point in space and time shoul d an attempt be made to look for it. Einstein railed against such a blat ant probabilistic motion at the heart of nature and issued his now famou s proclamation I shall never believe that God plays dice with the world. But most of his contemporaries disagreed with him, quantum uncertainty won the day, and mainstream science began to acquaint itself with the bi zarre idea that, at its most basic level, the material universe is not c oncrete and well determined but, on the contrary, is curiously abstract and conditional. It was no longer meaningful to think of an electron, for instance, as alw ays being definitely somewhere and somewhen in between the times when it was being observed. Unless an attempt was made to detect it, the sum to tal of what was and could be known about the whereabouts of a particle w as contained in its wave functiona purely statistical description. It could not be claimed, in the new quantum picture of the world, that pa rticles even truly exist outside of observations of them. They have no i ndependent, enduring reality in the familiar classical sense of being li ke tiny beads of matter with a definite (if not necessarily known) locat ion in space and time. The distinguished American physicist John Wheeler has expressed the central quantum mystery in these terms: Nothing is more important about the quantum principle than this, that it destroys the concept of the world as sitting out there, with the obser ver safely separated from it by a 20-centimeter slab of plate glass. Ev en to observe so minuscule an object as an electron, he must shatter th e glass. To describe what has happened, one ha s to cross out that old world observer and put in its place the new wor d participator. In some strange sense the universe is a participatory u niverse. Somehow, through the act of observation, subatomic particles are briefly summoned out of a kind of mathematical never-never land of potentiality and possibility into the solid world of tangible things and events. In q uantum parlance, an observation results in the collapse of the wave func tionan instantaneous telescoping-down of the probability spread to a lo calized point, a real particle. But what counts as a valid observation i n this respect? Who or what qualifies as an effective quantum observera measuring instrument such as Geiger counter, a human being, a committee of people? But the most widely accepted viewpoint, firs t advocated by the Danish physicist Niels Bohr and referred to as the Co penhagen interpretation, is that the sudden change in character or colla pse of the wave function is brought about, ultimately, by conscious obse rvershipthe registering of an event, such as the reading of an instrumen t, in the mind. And it appears the more so when one reme mbers that all of the material universe is comprised of subatomic partic les. Not one of these particles, according to modern physics, can be act ualized, or made properly real, without an observation that collapses th e wave function. Almost unbelievably, our most fundamental branch of sci ence implies that what had previously been assumed to be a concrete, obj ective world cannot even be said to exist outside of the subjective act of observation. Furthermore, if the Copenhagen interpretation is correct , then it is the mindthe mirror in which the object is reflected and bec omes the subjectthat serves as the essential link between mathematical p ossibility and physical actuality. The intervention of mind in the affairs of the subatomic world was specta cularly demonstrated a few years ago. In 1977, B Misra and George Sudar shan at the University of Texas showed theoretically that the decay of a n unstable particlesay, a radioactive nucleusis supressed by the act of observation. Like any quantum system, an unstable particle is described as fully as it can be by its wave function. Initially, this is concentra ted around the undecayed state. But as time goes on, the wave function s preads out into the decayed state so that the probability of decay gradu ally increases. Misra and Sudarshan showed that every time an observatio n is made it causes the wave function to snap back, or collapse, to the undecayed state. The more frequent the observations, the less likely the decay. And if the observations come so close together that they are vir tually continuous then, as in the case of the proverbial watched pot tha t never boils, the decay simply doesnt happen. This astonishing predicti on was verified by measurements carried out by David Winehead and collea gues at the National Institute of Standards and Technology, Boulder, Col orado, in 1990, using a sample of beryl... |