Everything outside ourselves (ourselves too, but let's keep it simple) is represented in quantum theory by a wave function that gives the probability that an incident of sensation will lead to a certain specified result. Everything we know about the world consists of discrete incidents of sensation, flashes of nervous impulse triggered by a messenger carrying energy and momentum from the outside world. We experience these incidents as a continuum because our crude instruments of perception (eyes, ears, ...) smooth over the myriad individual detection events.
Each such event ,Äì I like to call them clicks; Bohr called them registrations ,Äì has a definite outcome. There is no uncertainty regarding the world we actually see. It is a network of sensory clicks. Each conscious being possesses a unique memory of a world of clicks. When we compare our very definite world with another's, we are not surprised that their pattern of clicks is not exactly the same as ours, but statistically speaking they are sufficiently close. In the world of large things, the deviations in the pattern of sensory events between two observers is small. We can speak to one another of what we experience, and expect to be understood.
In the world of the small, however, human perception lacks the necessary resolu-tion. Observers must objectify experience by capturing clicks in some macroscopic ex-ternal medium, such as a photographic emulsion, that any observer can then peruse. Bohr's word registration conveys the flavor. The objectifiable experience is one in which a microscopic event has been magnified and registered irreversibly in apparatus resembling a mousetrap.
All detectors have this property, including our own senses. A tiny transfer of energy by a message from the system we are observing provokes an irreversible avalanche of events in the detector that creates a sort of phenomenal lump ponderous enough to resist destruction by further gentle probing. What is real to you and me is these lumps, these registrations of microscopic events that comprise the macroscopic world. Any phenomenon large enough to measure serves as a detector of some prior interaction with the microscopic domain. In this way the frustrating elusiveness of objects too small to pin down is translated into a series of well-defined features of a universe that anyone can examine.
Reality, as I understand the word, is ultimately a social phenomenon. If other witnesses cannot see it for themselves ,Äì or reproduce the experience of seeing it, or imagine a way that it might have been reproduced or seen by others ,Äì then it is not physically real. I admit this criterion is vague and needs to be refined for personal phenomena like pain and other unshareable sensations (modern medical imaging seems capable of rendering even these in a shareable way).
Bohr would say more definitely that if the phenomenon is not registered it is not real. Only registered phenomena can be shared. Clearly "reality" is a term that makes most sense for macroscopic things. Perhaps we should call it "experiential reality" or even "existential reality." Quantum theory causes headaches because we cannot help using our macroscopic language ,Äì whose underlying world-view may even be hardwired into our brains ,Äì to speculate about the microscopic world. The macroscopic laws of motion, the laws of Newton and Einstein, track the movement of "big atoms" with well-defined positions in the course of time.
The macroscopic picture of the world is one of a tapestry of continuous big-atom world-lines in space-time. The microscopic picture, however ,Äì well, there is no microscopic picture. There is a discrete set of registrations, each of which is a marker that may together with other markers suggest a track approximating the world-line of a macroscopically observable object in space-time. But there are no world-lines in our experience corresponding to the microscopic elements themselves prior to registration. The tapestry breaks up into a pointillist scattering of isolated clicks.
It's a Muddle of Models!!! Experiential models, Reality models, subatomic/quantum models!
That's one of my favorite RAW phrases ;-)
Is this from your dads book?
Good stuff.
A lot of that depends on the Copenhagen interpretation, which may or may not be the correct way to interpret it.
But QM is still very interesting, even with other interpretations (Many Worlds, etc) used.
Well, this was from the chapter on the Standard Model, so it's to be expected.
And generally speaking there's more of a consensus agreement around the Copenhagen Interpretation than the Multiverse theory, so wha'cha gonna do?
pretty good stuff, LMNO!
so, basically what i get from this, is that from all the weird, funky shit that happens on the micro/quantum level, only the events that happen to manage to cause a macro-level ("registered") event, can be considered "real".
im afraid there's no barstool small enough to take this apart.
good stuff.
Copenhagen is one explanation. Many-worlds is another. There are a lot more than just those. But yes, Copenhagen is generally the easiest to "understand" even if it does lead to weird paradoxical stuff that other versions may not have.
It also seems to actually produce results when working the math, as opposed to simply creating fodder for bad sci-fi novels.
Quote from: LMNO on August 27, 2007, 01:59:21 PMIt also seems to actually produce results when working the math, as opposed to simply creating fodder for bad sci-fi novels.
math+results
OR
bad sci-fi fodder
???
difficult choices, ITT.
Quote from: LMNO on August 24, 2007, 02:41:45 PM
Well, this was from the chapter on the Standard Model, so it's to be expected.
And generally speaking there's more of a consensus agreement around the Copenhagen Interpretation than the Multiverse theory, so wha'cha gonna do?
Break into another Universe and show those Copehagen types they know NOTHING!
Cain,
pimping Incunabula.org since 2005
Quote from: LMNO on August 24, 2007, 02:41:45 PM
And generally speaking there's more of a consensus agreement around the Copenhagen Interpretation than the Multiverse theory, so wha'cha gonna do?
The copenhagen interpretation itself is also still 'under construction'. A big puzzle in this interpretation is what causes the so called collapse of the wave function, meaning: what changes a quantum statistical probability to a physical thingie. In the original interpretation there was this thing called an 'observer' that causes the collapse, however nobody could interpret what properties this observer should have. (To solve this so called measurement-problem people even considered religion, or psychic stuff).
However nowadayas there is a pretty good and quite generally accepted solution to this problem called decoherence. (unfortunately, decoherence is hardly covered in standard QM courses/books and thus not well known). The main idea of decoherence is that apart from the system (the thing you measure) and the observer (the thing that measures), there is always a background. The background consists basically of those thing you don't/can't measure, but that are there; in other words: the system feels the background, but the observer cannot see it. now it turns out that if the background is 'large' enough, the quantum nature of the system effectively dissappears. so e.g. if we look at a single electron in vacuum (almost no background), we see all the quantum stuff. If we look at the electric field (which is caused by electrons) of a radio (so a large background) we don't see the quantum stuff. In my opinion decoherence gives by far the best description of the transition betweeb quantum and 'classical' behavior.
Bo
QuoteQuantum theory does not tell all we would like to know about things. It does not attempt to describe "things" at all, only their potential impact on our senses (or on any other registration device). Physicists like to theorize about simple systems that are conveniently isolated (more or less) from their surroundings, such as a single electron moving about an atomic nucleus.
But real things can be large and complicated. Schr??dinger envisioned a wave function for a cat to emphasize the absurd inadequacy of the quantum viewpoint. The 'wave function' does not have to look at all like a wave. Its key feature is a list of probabilities for registrations corresponding to a set of well-defined events ,Äì one probability for each event. For the cat the events are determinations that the animal is dead or alive.
Schr??dinger imagined a cat confined to a box containing a flask of poisonous vapor linked to an apparatus that would smash the flask when a detector clicked in response to the decay of a radioactive nucleus. Radioactive decays occur at random with a characteristic average time ,Äì the "half-life." After the lid is closed, you wait one half-life. At that time, quantum theory implies a wave function that gives the cat a 50% chance of being observed alive when you open the box. Well, is the cat dead or not? The wave function does not judge.
To Schr??dinger, that is a ridiculous state of affairs. Obviously the wave function could not be telling everything about the cat. Quantum theory appears to be saying that until the box is opened the cat is in a smeared-out state, a superposition of possibilities, in this case half dead and half alive. Your act of opening the box appears to resolve the situation. Does your act decide the cat's fate? Must you bear responsibility?
No. The wave function does not pretend to describe the cat. The information it contains is about measurement probabilities, not entirely about what causes them. The cat's fate is sealed as soon as a radioactive emission effects an irreversible consequence in the world ,Äì certainly by the time the first detector clicks. We simply do not know what has happened until we open the box. If we want to reassure ourselves that our action did not kill the cat, then we can perform an autopsy to determine the instant of demise.
Bohr grasped intuitively the mouse-trapping that converts the possibilities inher-ent in a microscopic system into macroscopic reality. Much of the century passed, however, before physicists developed a satisfactory theoretical account of this process. It is complicated by the fact that irreversibility entails the disturbance of many pieces, as in the dissipation of energy in friction, or the dispersal of a drop of ink in water. The quantum version of the process is called decoherence, but the image of a mousetrap will serve our purpose.17
17. Decoherence
"The mechanisms of decoherence are different from (though related to) those responsible for the approach of thermal equilibrium. In fact, decoherence precedes dissipation in being effective on a much faster timescale, while it requires initial conditions which are essentially the same as those responsible for the thermodynamic arrow of time." E. Joos, in the Introduction to "Decoherence and the Appearance of a Classical World in Quantum Theory" 2nd Ed.,by D. Giuliani, E. Joos, C. Kiefer, J. Kupsch, I.-O. Stamatescu, and H.D. Zeh, Springer, Berlin 2003. This is a technical monograph, but parts should be comprehensible to non-specialists.
Quote from: LMNO on August 28, 2007, 03:01:15 PM
Quote
17. Decoherence
"The mechanisms of decoherence are different from (though related to) those responsible for the approach of thermal equilibrium. In fact, decoherence precedes dissipation in being effective on a much faster timescale, while it requires initial conditions which are essentially the same as those responsible for the thermodynamic arrow of time." E. Joos, in the Introduction to "Decoherence and the Appearance of a Classical World in Quantum Theory" 2nd Ed.,by D. Giuliani, E. Joos, C. Kiefer, J. Kupsch, I.-O. Stamatescu, and H.D. Zeh, Springer, Berlin 2003. This is a technical monograph, but parts should be comprehensible to non-specialists.
Well it's still only a footnote :) (but that's a footnote more then in my QM books...)
Bo
When I read between the spaces that separate your post from mine, I get the following feeling:
Nothing changes a statistacal probability to a physical thingie. A probability doesn't measure anything. It's a range of educated guesses. The wave-like quantum function is a probability wave, and the "particle" is nothing like a billiard ball shooting through space.
In essence, it's a really tasty-looking menu, but you shouldn't nibble on it.
The statistaical nature of QM indeed never gets lost, until you do the very measurement. QM can never tell you what whould happen to a single electron. It can only tell you what could happen.
What decoherence does for you is changing the quantum probability (which includes entanglement, so cats being alive and dead at the same time), to a 'normal' statistical state (cats being either alive or dead).
Bo
just to clarify for Bo (if necessary): with "tasty menu", LMNO means the "the map is not the territory" phrase, which we have reformulated here to
1 "the meal is not the menu",
2 "don't eat the menu",
3 and finally "it's a really tasty-looking menu, but you shouldn't nibble on it."
just saying
please continue your QM talks
More stuff from that book:
So much for experience. What about theory? Raw experience is chaotic even in the macroscopic world. Theory helps us organize this chaos by proposing models whose components simulate the things we actually see. If the theories work, that is, if they appear to simulate accurately the pattern of observable events, then we are inclined to attribute a kind of reality to their components.
We might call these models theoretical reality, which sounds like an oxymoron. This is the kind of reality, however, most relevant to human affairs. It is this image of reality that persists in conscious memory and motivates future actions. Short of inadvertent knee-jerks, or a more or less conscious surrender to impulse, we rarely act upon raw unfiltered experience, but rather upon images based on past experiences that we carry in our minds.
These are the images that give meaning to experience. They are the visions that suggest possible consequences of actions, and they have an extraordinary psychological power. Only these remembered images are continuous and permanent. The actual experience of physical reality is fragmentary and fleeting.
So we attribute persistence or trajectories or world-lines to all visible objects despite the hopelessness of observing them directly at every instant. We think of the world-lines as real because the theory that contains them ,Äì the possibly hard-wired model in our brains ,Äì works spectacularly well for big things. One of quantum theory's striking features is that it does not have any component resembling trajectories or world-lines for microscopic objects. That is, the quantum world view does not fill in with theoretical entities all the parts of nature that are missing from our direct experience.
Also:
The early literature on quantum theory, which is still widely read by scientists as well as by philosophers and historians, is an archaeological midden in which the precursors of our current understanding are mixed with misleading and irrelevant debris. Much confusion has been created by authors who have tried to reconcile this old language with the modern unambiguous understanding we have achieved, especially about the issue of waves versus particles.
So let me state clearly: One of the fundamental concepts of quantum theory is a wave, or more precisely, a field, but it is a probability field that does not itself carry energy or momentum, and can be considered "real" only insofar as the theory that contains it is successful.
On the other hand, at the fundamental level, the theory does not contain "particles" even as theoretical entities. What it does contain is registrations, and these do not appear in the mathematical formulas, but in the interpretive statements that link the theory with experience (Born,Äôs interpretation).