2012-01-04, 13:34
#1177
Citat:
Ursprungligen postat av manne1973
Han visar bara att det för en partikel i dubbelbrunnen finns två "grundtillstånd" med nästan samma energi. Detta ger ingen information om vad som händer om man tillför energi till systemet.
Här är ytterligare ett inlägg från Mr Cox där han försöker förtydlga sitt inlägg om att alla elektroner känner av alla andra elektroner i universum:
Dear all,
Let me add a bit more by way of clarification, because I think it's interesting. I've already posted a detailed analysis of the behaviour of a two proton - two electron system, and shown how the exclusion principle leads to a covalent bond in a Hydrogen molecule. Let me paste a couple of pages from my book The Quantum Universe - to save you having to buy it - and annotate it in a couple of places.
In the book, we do the double well as I posted previously.
This is how we describe the situation:
"It seems that we must conclude that the pair of identical electrons in two distant hydrogen atoms cannot have the same energy but we have also said that we expect the electrons to be in the lowest energy level corresponding to an idealised, perfectly isolated hydrogen atom. Both those things cannot be true and a little thought indicates that the way out of the problem is for there to be not one but two energy levels for each level in an idealised, isolated hydrogen atom. That way we can accommodate the two electrons without violating the Exclusion Principle. The difference in the two energies must be very small indeed for atoms that are far apart, so that we can pretend the atoms are oblivious to each other. But really, they are not oblivious because of the tendril-like reaches of the Pauli principle: if one of the two electrons is in one energy state then the other must be in the second, different energy state and this intimate link between the two atoms persists regardless of how far apart they are.
This logic extends to more than two atoms – if there are 24 hydrogen atoms scattered far apart across the Universe, then for every energy state in a single-atom universe there are now 24 energy states, all taking on almost but not quite the same values. When an electron in one of the atoms settles into a particular state it does so in full “knowledge” of the states of each of the other 23 electrons, regardless of their distance away. And so, every electron in the Universe knows about the state of every other electron. We need not stop there – protons and neutrons are fermions too, and so every proton knows about every other proton and every neutron knows about every other neutron. There is an intimacy between the particles that make up our Universe that extends across the entire Universe. It is ephemeral in the sense that for particles that are far apart the different energies are so close to each other as to make no discernable difference to our daily lives.
This is one of the weirdest-sounding conclusions we’ve been led to so far in the book. Saying that every atom in the Universe is connected to every other atom might seem like an orifice through which all sorts of holistic drivel can seep. But there is nothing here that we haven’t met before. Think about the square well potential we thought about in Chapter 6. The width of the well determines the allowed spectrum of energy levels, and as the size of the well is changed, the energy level spectrum changes. The same is true here in that the shape of the well inside which our electrons are sitting, and therefore the energy levels they are allowed to occupy, is determined by the positions of the protons. If there are two protons, the energy spectrum is determined by the position of both of them. And if there are 1080 protons forming a universe, then the position of every one of them affects the shape of the well within which 1080 electrons are sitting. There is only ever one set of energy levels and when anything changes (e.g. an electron changes from one energy level to another) then everything else must instantaneously adjust itself such that no two fermions are ever in the same energy level.
The idea that the electrons “know” about each other instantaneously sounds like it has the potential to violate Einstein’s Theory of Relativity. Perhaps we can build some sort of signalling apparatus that exploits this instantaneous communication to transmit information at faster-than-light speeds. This apparently paradoxical feature of quantum theory was first appreciated in 1935, by Einstein in collaboration with Boris Podolsky and Nathan Rosen; Einstein called it “spooky action at a distance” and did not like it. It took some time before people realized that, despite its spookiness, it is impossible to exploit these long-range correlations to transfer information faster than the speed of light and that means the law of cause and effect can rest safe.
This decadent multiplicity of energy levels is not just an esoteric device to evade the constraints of the Exclusion Principle. In fact, it is anything but esoteric because this is the physics behind chemical bonding. It is also the key idea in explaining why some materials conduct electricity whilst others do not and, without it, we would not understand how a transistor works."
We then go on to 3 wells, and then to 10^23 or so - which is the situation in small lump of silicon - and show that this multiplication of very closely-spaced energy levels, (correction added - the occupation of which is governed by) the Pauli principle, is the origin of the conduction and valance bands - i.e. the key to understanding how transistors work (which we also describe).
I'll admit that we just state that causality is preserved without proof in the book. The notion of causality in quantum field theory is actually a tricky one - there is a large literature on it if you do a search on Spires. But the description of the Universe as a single potential well, with an associated energy level spectrum, is surely valid unless one introduces new physics, which is not mandated by experiment - and I remind you that this rather counter-intuative picture is necessary at a macroscopic level (admittedly transistor-sized and not universe-sized) in order to understand the conduction and valence bands in semiconductors.
The more "presentational" question posed by some on the forum - namely that one shouldn't say that everything is connected to everything else for fear of misinterpretation - is interesting. In my view, the interpretation of quantum theory presented above is not only valid, but correct in the absence of new physics - and therefore everything IS connected to everything else. I was very careful to point out in the lecture that this does not allow any woo woo shite into the pantheon of the possible, as I think I phrased it.
My general position is that when communicating with the public we shouldn't spend our time triangulating off nutters. I'm having to deal with this in spades in my current series, Wonders of Life, where it is tempting to try to give creationists no ammunition at all by avoiding areas of doubt when describing the origin of life and the evolution of complex life on Earth. My strategy is to ignore such concerns, because these people shouldn't occupy any of our time! If we tried to take account of every nob head on the planet, we wouldn't have time to make the programs or write the books.
Brian
