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SMQ
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Quantum Physics
« on: Sep 23rd, 2008, 2:01pm » |
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on Sep 23rd, 2008, 11:55am, Sir Col wrote:| ...quantum mechanics seems such twaddle to me. Whenever I've had serious conversations with physicists and asked them to explain it to me they seem to agree. Of course they thought they were articulating the validity of the theories perfectly well, but the more they spoke, the more they proved my point. Most attempts to persuade me of its validity usually end with the speaker calling me an ignorant fool with a brain too small to understand. I admit I am a simple man, but if it really were a simple theory and explains our reality so well then why hasn't it superseded the Newtonian deterministic system of reality? Why do we continue to waste the time of young people on an ineffective system that fails to describe our universe? |
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So let's set aside the details of the theory for a moment and just look at the results:
- Thermal (black body) radiation: Hot things glow, and, somewhat surprisingly, the color and intensity of the glow depend mostly just on temperature and not on specifically what is glowing. Around the start of the 20th century several physicists developed various equations to explain the experimental results of measuring the light and heat put out by hot glowing objects. It turns out that quantum principles provide an equation which has since matched all experimental results to the limits of measurement, while classical physics failed spectacularly at explaining the same results.
- Superconductivity: Classical mechanics predicts (correctly) that electrical resistance should diminish at very low temperatures, but it was surprisingly found to vanish entirely in some substances, with a sharp transition from normally conducting to superconducting states. A quantum-mechanical theory not only explained the phenomenon, but correctly predicted the existence of type-II superconductors.
- Superfluidity: In a parallel to superconductivity, at very low temperatures, Helium, rather than solidifying, becomes superfluid, i.e. it loses all viscosity and flows without resistance. This (readily-observable, macroscale) phenomenon is not explained by classical mechanics, while quantum mechanics goes further and correctly predicts the observed differences between superfluid Helium-3 and Helium-4 (due to the odd versus even number of elementary particles in their atomic nuclei).
- Bose-Einstein condensation: Bose and Einstein predicted that at extremely cold temperatures the quantum nature of mater should have macroscopically-observable effects. It was almost eighty years until technology was available to reach the required temperatures (fractions of a degree above absolute zero), but Bose-Einstein condensates have been studied under laboratory conditions for over ten years now, and Bose and Einstein's 80-year-old predictions have held up extremely well.
- The scanning tunneling microscope: The most accurate microscope ever invented -- able to "see" individual atoms -- is an application of quantum tunneling: a phenomenon in which an electron is occasionally observed to have passed through an gap or insulator which classical physics predicts it should not have been able to pass through.
Over the last century a very large number of experiments have been devised to test and refine various aspects of quantum theory, with all results being consistent with a fundamentally quantized understanding of nature at extremely small scales. Furthermore, the many and various technologies developed from quantum theory bear out its usefulness as a descriptive model of reality. Whatever your opinion of the "realness" of the underlying theory, it seems to work.
--SMQ
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Grimbal
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Re: Quantum Physics
« Reply #1 on: Sep 23rd, 2008, 3:21pm » |
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"If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet." -Bohr
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Sir Col
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Re: Quantum Physics
« Reply #2 on: Sep 23rd, 2008, 3:31pm » |
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Wow! Thank you for your introduction, SMQ. I would genuinely like to understand this better...
(1) When you talk about formulae from quantum theory correctly predicting behaviour, what makes them specifically quantum mechanical formulae as opposed to formulae that just predict?
(2) Does modern Physics assert that classical models are wrong in the sense that they are not universally valid but work perfectly in restricted cases? Or does it claim that classical models at best provide good approximations for a restricted case, but never work perfectly and only expose their flaws in the extreme cases?
(3) How is quantum matter affected by forces: magnetic, electric, gravitational, and so on?
(4) With reference to coincidental occurrence of quantum matter, is this an actual proposed reality or an admittance that at this point no better explanation exists? If the former, then how does Physics deal with the self-evident law of non-contradiction?
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SMQ
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Re: Quantum Physics
« Reply #3 on: Sep 24th, 2008, 8:52am » |
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You're welcome! Bearing in mind that, all appearances to the contrary aside, I'm not actually a quantum physicist (I know, shocking, what?)  , I'll try to answer your questions as best as I can. For further investigation, Wikipedia's non-technical introduction to quantum mechanics appears to be a good place to start. It gives a broad overview with emphasis on the experimental results that lead to the development of the theory and plenty of links to information on the details.
As I understand it, there are two fundamental concepts in quantum mechanics from which the rest follows:
1) Certain attributes of matter and energy are fundamentally discrete rather than continuous. For example, electromagnetic radiation at a certain frequency can only have certain discrete energies, and at small scales there is therefore a minimum energy which can exist or be transferred at a certain frequency. This means that atoms can only absorb or emit light at certain frequencies -- those whose minimum energy corresponds to transitions in electron "orbits" with the same change in energy -- a fundamental result in explaining why "neon" tubes emit different colors with different gasses in them, or why a semiconductor diode allows current to pass in one direction but not the other. Other quantized (discrete) attributes include the momentum of electrons in atoms and the vibrational energy of atoms in molecules. In all cases some parameter -- the frequency of the light, the number of electrons in the atom, the arrangement of atoms in the molecule, etc. -- constrains some form of energy -- radiation, momentum, vibration, etc. -- to take on only a discrete set of values rather than a continuous one, and furthermore these discrete values are all positive integer multiples of some minimum value: the "quantum" of that energy under that parameter.
2) Certain pairs of attributes of matter and energy cannot be mutually known beyond some precision. For example, it is impossible to know both the position and momentum of an electron beyond some degree of accuracy: any possible measurement of one disturbs the other to an unpredictable extent, and the more accurate the measurement the greater the degree of the unpredictable disturbance. The achievable precision is so small as to be of no consequence in everyday experience, but when you start trying to define the orbits of electrons around an atomic nucleus you run into trouble: because the momentum of a given electron is discrete (and so known to a relatively high precision to the level of uncertainty in the controlling parameter), it's exact position is fundamentally unpredictable. You can measure where it was, but in so doing you knock it into an unpredictable new orbit (and so lose track of its exact momentum).
These two principles lead to an underlying mathematical formulation in terms of infinite matrices and probability density functions. The discrete values correspond to the rows and columns of the matrices -- one row or column for each allowed value -- and the fundamental uncertainty of certain attributes leads to a description of their likely values rather than their exact values. These probability density functions turn out to be wave-like, such that a stationary particle is represented by a standing wave, and the elements of the infinite matrices are equivalent to the coefficients of the Fourier transform of the probabilities. This is a surprisingly elegant result!
What's more, it turns out to be an extraordinarily powerful model, leading to explanations of previously inexplicable phenomena, numerous predictions verified by experiment, and new avenues of pursuit for technological innovation. It also leads to such non-intuitive results as wave-particle duality, where elementary particles are seen as having both a "wave nature" and a "particle nature" which manifest themselves differently in different experiments, observer effects (a la Schrodinger's cat) where parts of the universe don't take on definite form until "someone" notices them, and "spooky action at a distance" where measuring something can change the possible outcomes of a separate experiment some (potentially large) distance away.
To attempt to answer your questions, then:
(1) The formulae are specifically quantum mechanical because they arise from the above model of elementary particles as probability density functions manipulated through infinite discrete matrices. It is the very treatment of certain attributes as discrete and certain pairs of attributes as mutually imprecise that gives rise to the model which generates the formulae. Just as relativity theory drove the development of new mathematics to describe its results, quantum theory has done the same, and the predictive formulae are expressed in the language of that mathematics.
(2) Modern physics claims that classical physics is an excellent approximation of quantum physics on human scales, where the number of particles involved is large. This is analogous the claim that Newtonian mechanics is an excellent approximation of relativistic mechanics when the speeds involved are small.
(3) The mathematical treatment of forces at the quantum level (quantum field theory) goes way over my head, but it basically boils down to macroscale forces being due to interactions of elementary particles on a quantum scale. Certain particles -- photons, W+, W-, Z0, gluons, possibly gravitons -- are force carriers, while other particles -- electrons, quarks, neutrinos -- interact with one another by way of those carriers to produce the forces we observe on a large scale.
(4) It's an actual proposed reality, but... Einstein's famous principle of mass-energy equivalence, E = mc2, still applies at quantum scales, and different quantum particles fall at different places on the mass-energy continuum. Quarks (the components of protons and neutrons) are very "masslike" and so have a definite aversion to occupying the same point in space at the same time, while photons (the carriers of electromagnetic energy) are very "energylike" and so don't mind sharing their position with other photons. The other particles fall in between. Quantum physics says that matter prefers not to occupy the same position in space and time only because there are powerful forces acting between certain classes particles to keep them separated, and not because of any more fundamental principle.
--SMQ
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Sir Col
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Re: Quantum Physics
« Reply #4 on: Sep 24th, 2008, 2:43pm » |
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Scary as it sounds, it seems to be making more sense to me this time round; although I still don't like it! But as Grimbal quoted, those two "experiences" seem to be inextricably linked.
Thanks for taking the time to expound these ideas so well.
I'll have to find some time to carefully read that Wikipedia article.
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