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 Now online (2)belb Combo ...and 292 guests Last 5 registered Oplandisks nothingstar N_loop yipe foxtrotromeo Browse members... |
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you have to distinguish: In ordinary non-relativistic quantum mechanics everything works fine and everything is finite, atomic theory does not inherit any conceptional difficulties. It describes the nature of systems of point particles, like the hydrogen atom, composed of a core and an electron. The key role plays the wave function, a density function of probablity evolving in time. It's value is the probability of finding the whole particle at a certain point in space. The uncertainty princinple states that there is a limit for the product of the uncertanties of momentum (velocity) and spatial position. For very high energys, we need a relativistic treatment of point particles - a theory developed by P.A.M. Dirac in the 1920's. This theory also works together with electromagnetism, the theory of electromagnetic fields. In fact it follows that also these fields have to be quantized in order to describe nature accurately (feynmann and many others, 1940-ca. 1955). That's where we arrive at quantum electro dynamics. It inherits many conceptional problems and renormalization is needed and everything, but in fact there has never been found a more exact theory, it was tested for nine digets behind the comma and it simply is true - however it's not the whole story. |
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has anyone here ever seen that tv-series called "quantum leap"?? |
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there are more forces in nature , for example strong interaction, the force that determines the structure of the core of atoms. It was developed in the 60's and it is not fully done yet because the quarks, the interaction particles, are confined, i.e. they mustn't leave a baryon (like neutrons and protons and so on). From an experimental point of view thie theory is true too though calculations are very difficult and where only possible with recent computer technology. But it was a very successful theory after all (prediction of the top-quark, found in 1995) and many quantative predictions. Now we have one thing left: Gravity. It plays a very special role becaus eit's symmetry group is very different from the others. quantum electro dynamics has U(1), electroweak theory SU(2) (developed 1970-80 i think) and QCD (the last one) has SU(3). gravity has diff(4) and it can't be included in a general scheme. there are many approaches on how to include it some are more direct, some more indirect. I did a small work on a very indirect one (non-commutative field theory) and am now busy with getting into a more direct approach (2d quantum gravity). (click on the button in my profile and paste 'dumbo.html' with 'index.html') |
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The point, tho, is that we cannot say that is absolutely right, because the renormalization was invented to iron out a flaw in the theory, therefore isn't necessarily true - because it was invented for that specific purpose... this is akin to the philosophical view of quantum physics, in that "what we look for, we see": since it is incomplete, it can't be taken up as being an absolute true thing -- later on, someone else might come up with a theory that entirely bypasses renormalization and takes into account factors that forced the introduction of Renormalization. After all, isn't the history of Quantum mechanics built up on assumptions being held for years until someone comes along with a new model that takes more into account and bypasses certain contrived theoretical musings? And then were subsequently themselves debowelled when ANOTHER new theory came along to explain anomalies that were unsatisfactorily explained in the prior theory. I understand what I have just written is very confusing, but hell, it's hard to speak coherently about Quantum Mechanics :D |
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get a room you two! :P |
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I'm hoping that he doesn't come up with damnable evidence against what I just said - I'm WAY out of my depth here and mainly paraphrasing what I can remember of the book I'm reading :D :D |
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i second that we are not done yet although we can't find any experimental data that does not fit into the standard model - it's incomplete from a theoretical point of view cause it consists of two separated parts (quantum field theory and gravity) that can't be unified. |
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regarding schrödinger's cat, how many people here have realized that they are both alive and dead? |
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the box is a system with no interaction with the part of the world the observer is in, by definition. (you could replace the cat by another observer too, it's symmetric) hence the statement is alive or is dead has no meaning as long as no contact is established (wave functions are separated). A mathematical description of the cat necessarily involves the uncertainty of wether the cat is dead or alive, an outside description hence consits of superpositions of these states (something like 0.3*dead+0.7*alive, in principle). If you establish contact again you change the time evolution and entangle the state of the cat with the state of the universe hence the superposition breaks and it goes into one of the states (only approximately, because you have altered the hamiltonian (=energy-operator) by opening the box). thus you find the cat dead or alive. however, thermodynamical systems don't show this behaviour (difficult proof though) but it has been tested for particles and whole atoms. Even for Bose einstein condensates which consists of at at about 100.000 atoms, it's been found(they are no thermodynamical systems). that's not really a paradox, it's just that nature is way more complicated as one might expect when he takes a hammer, smashes his toe and feels the pain. |
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so, the cat, as an observer itself... what is its experience? |
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(interesting how you said the cat could be replaced with an observer as if the cat were dead either way) |
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to clarify some confusion - the whole thing only works if the system are small enough cause it has been shown that thermodynamical systems show decoherence on their own. But replacing the cat with a bunch of atoms or a 'simple enough' quantum system restores the the paradox. (the problem here is that QM is a one-particle description and a many - particle theory like quantum field theory shows some different effects) imagine i take out the dead cat and go into the box myself. The original experiment has a mechanism that kills me if a certain atom decays. That brings in an element of uncertainty, let's say in 30% of the cases it decays within some time period and in 70% it doesnt. If we 'close' the box - prevent all interaction - you can try to give a mathematical description for what is happening inside the box. Now it is crucial that i am a quantum system and not a thermodynamical system: Your description of me is a wave function that consists of two just as i said above. The amplitude of the 'death' states is increasing with time because it is more likely that the atom has decayed at a later time. That's your description. I for myself in the box do also evolve seperated from the rest of the universe and - if i'm a thermodynamical (i.e. large) system show decoherence and go into a certain state (1*dead+0*alive, or the other way round). But if i was a pure quantum system (that effectivly means a thermodynamical system with temperature zero) i could make a description of the outer world as a mixture of the states of it, just like the outside obsevrver does. If the Box is opened we both change the rules of time evolution and the intermediate states of both our descriptions are projected (i.e. collapse) on the states of our new, common, description. It's important that that only works for small systems with a low number of degrees of freedom. Introducing thermodynamics fucks the thing, it's a difficult proof though, from the 1980's i think |
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I prefer to ignore this stuff for now. It almost seems like a scientist(s) took the piss and decided to make up some lunacy to see if it would catch on. |
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I can see your point. |
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There's a funny play on the Schroedinger's cat experiment in the excellent anti war novel "Johnny got his Gun" by Dalton Trumbo. In it a man in a full body cast dies, but for days the nurses continue to change his urine and blood bags without noticing. One particularly caring nurse does something (I forget what) and discovers he is dead. The main character is angry at her becasue she "killed" him by discovering his death. |
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Isn't the Schrodinger's Cat theory a bit outdated now, with the new String Theory? |
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schroedinger's cat is not a theory, it's an experiment from the beginning of the theory (almost 100 years ago now) to illustrate some basic features on quantum systems, nothing more. you can't really say a theory is outdated, it describes the atomic structure correctly and provides a deep understanding of nature. quantum field theories (like qed, qcd and string theory) are more fundamental though, but they are (in principle) nothing more but technically complicated quantum theories of fields. also string theory is a quantum field theory with some special features but - it has not been verified by the experiment yet, partly resulting from the lack of predictive power: calculations are so difficult that hardly anything follows directly. but it is very interesting from a theoretical point of view, the mathematics are just beautiful. |
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schrodinger's cat is a theory - how can it be an experiment? It has never been done, so it remains theoretical. |
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no, it's just an thought experiment to move some weird features of quantum mechanics (which is the actual theory) from atomic level to the level of daily experiment and it hasn't been done yet because it doesn't work on this scale. But the experiment can be done and is done every day in any physicisty labratory (in fact the superposition principle is even needed to explain such a simple object as the hydrogen atom) |
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My older brother is accepting a degree in quantum physics from WVU on May 18. |
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we are just now learning about this in my physics class. it's only mildly interesting at the mo. our teacher gave a couple lectures on lightning. i wonder if i should take a theoretical physics class in college/uni. |
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you should!! i have been studying theoretical physics for four years now and start the phd in a year (hopefully) |
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impressive! best of luck on your Ph. D =) |
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marlowe, do you still have the Commander Keens? i just read the other thread, could you maybe share them with me? |
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it will take me another three years, damn |
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yes, I believe so :D let me check that thread out :) |
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........... and evaluating the hamiltonian of the particle over the manifold defined by the system of homomorphic solutions to the wave equation of the system under consideration, as solved for the fundamental frequency (the Eigen vector solutions), we find that the momentum of the particle is given by the simple expression of 6.305*10^(-J0(npi)/n!)*exp(-2h/(pi*mu))*exp(-1,n) where J0(x) is the zeroth order bessel function and n is the desired quantum state. The solution set for the first five natural modes are given as {0.23456,0.083746, 0.00387645, 0.00048596, 0.000078645} Normalizing the set we get the solution {1,2,3,4,5}. Disclaimer: I don't know any math or quantum physics. *runs away laughing, and later repents being a pseudo geek* |
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