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Post by Progenitor A on Sept 14, 2010 8:03:54 GMT 1
The photon particle-wave duality of which we are aware.
We all accept that , don't we?
Well consider this.
A photon can only occupy a quantum energy level we are told - when an electron in some atoms falls to a lower energy level (by one quantum level) a photon is emitted.
And we can regard this photon as a wave or particle
But if we regard it as a wave, then waves have particular characteristics. One of these characteristics is that waves occupy an area - there is no such thing as a point-wave.
So the energy (1 quantum level of it) of the photon is spread over an AREA
In other word the quantum of energy is diffused so that at any smaller area than the whole area covered by the wave, the energy content is less than one quantum level.
But we are told we cannot sub-divide quantum levels of energy
Therefore the photon cannot be a wave, can it?
Here's another problem
A characteristic of waves is that they propagate, that is move. We are familiar with this from radio and light waves move and 'spread out' as they travel. The generatinmg source (a radio transmitter) is using energy to keep up this propagation. If the energy source is turned off we lose the radio wave.
Now the electron is a source of waves - in some cases it is a wave are told. It is radiating energy, all the time, ever since it was created in a big bang. Even if the radiated energy is tiny, over billions of year the total radiated energy becomes quite a lot.
Where do electrons (and photons and other electrically-charged particles - any charged particle in motion generates an em field) get the energy to radiate for so long?
Did they start off at the 'big bang' all with the same massive energy, and what we see now is a faint echo of the massive energy they once had?
Odd isn't it
I suppose I will be told to F Off from the usual sources once more!
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Post by olmy on Sept 14, 2010 8:43:59 GMT 1
Yes. Quantum mechanics is odd and trying to apply 'common sense' or intuition to it is doomed to failure. I suppose I will be told to F Off from the usual sources once more! Well, it depends what the point of your post is. If you are simply trying to say "isn't this silly (according to my infallible common sense), so those daft scientists must be wrong" then something along those lines would be appropriate. This is one of the best tested theories ever - it underpins the technology you are using to make the post. If you actually want to understand, then we can get somewhere, although I suggest a good popular book would be a better start than here. For example "Quantum" by Jim Al-Khalili is a starting point. The problem with your post and giving a detailed answer is that it attempts to cover a great deal based on 'common sense' and misconceptions. For example, energy is quantized in certain situations like electrons in atoms and hence photons emitted from atoms have discrete energies (that's why we get spectral lines). It is not the case that photons "occupy a quantum energy level" per se. I suggest you start with a single question (preferably after having read a good introduction). |
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Post by Progenitor A on Sept 14, 2010 9:09:35 GMT 1
If you actually want to understand, .......... I tend to agree with far superior brains to mine that it is not possible to 'understand' QM at this stage of investiagtion. It is possible to construct mathematical models that describe what is happening and also happily allow verifiable predictions about the behaviour of sub-atomic particles. Although models can help in understanding, in themselves they are NOT understanding. At college I had a simple and quite reliable test of the more complicated subject: if the lecturer could not describe them in understandable English then they did not really understand what they were talking about Such is how I perceive physicists today; they cannot express their knowledge in understandable language and this indicates that they do not really understand their subject - an idea that Feynman submits to Take this simple question that I asked of a self-confessed physicist elsewhere: How can a unified particle spin cw and acw simultaneously? The answer? Abuse and an invocation to accept the 'magic' of QM! Unless people who make such statements can explain what they mean, then they do not know of what they speak. The best that can be said is this: 'We have a mathematical model that fits with the observations and that mathematical model tells us that a particle can spin in all directions simultaneously. We cannot offer more than that' That is how it is |
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Post by olmy on Sept 14, 2010 9:42:03 GMT 1
At college I had a simple and quite reliable test of the more complicated subject: if the lecturer could not describe them in understandable English then they did not really understand what they were talking about You say it was 'quite reliable'. How did you test the reliability? In my experience, the skill of being able to explain stuff to non-specialists is a quite separate skill to a technical speciality itself. Hence, you can have an outstanding contributor to a field, appointed to a university, who cannot teach to save their lives (well, you used to have this situation, anyway). Quantum Mechanics is hard to understand at all and impossible to put in terms of an 'intuitive picture'. There is really no reason at all why we should expect the universe, at its basic level, to operate in accordance with our intuition. Our intuition evolved in a very restricted corner of the universe. To expect everything to be explainable in intuitive terms is just blind arrogance. Having said all that, some people are better at explaining the non-intuitive concepts 'in simple terms' than others. Some people are more receptive learners than others, too. |
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Post by speakertoanimals on Sept 14, 2010 12:40:40 GMT 1
A photon can only occupy a quantum energy level we are told No, an ELECTRON occupies a quantum energy level, and emits a photon when it moves to a different energy level. First, a particle is not a particle nor a wave, but a quantum wavefunction which can have properties of both depending on the circumstances. The fact that the energy of a photon can be spread out if the photon wavefunction is is no more mysterious than the fact that the electron charge is spread out if the wavefunction for the electron is. Having energy density spread out doesn't mean that you can split part of it off, and get the energy of half a photon. Ity follows from how quantum theory works, that measurement collpases the wavefunction, so that when we measure a photon, we go from photon spread out, to photon (a whole photon) detected as being where we have detected it. The problem here is you are trying to criticize quantum theory based on classical properties of waves, but that is not what quantum theory is, and it is not how the wave function (the wavey bit of quantum theory) is related to physical measurements, such as the position of a photon. More silliness. First, electron waves which is what an electron is, aren't the same as the waves that electrons can radiate (em waves). Nor is an electron radiating energy all the time! This is just nonsense. Second, by referring the radiating em waves, you have forgotten a second sort of em field -- the stationary field around a bar magnet, for example. So, the energy transmission aspect of electron waves is just the simple fact that a travelling electron, moving from right to left, transmits energy from right to left -- but it is not EMITTING energy, just a moving wave carrying energy, just as moving em waves carry energy. Whereas an electron in an atom is more like a stationary wave, holding energy in one place rather than transmitting it. Really, even using just simple classical bad analogies, your arguments are very easy to refute, and if you knew a little more classical physics, you would see that. |
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Post by speakertoanimals on Sept 14, 2010 12:47:01 GMT 1
If you can't explain it in plain english you don't understand it is familiar tosh, that a certain poster on the
BBC boards used to repeat ad nauseam.
Quantum theory (or practically any part of physics, chemistry, biology, or maths) involves specialist concepts that involve specialist vocabulary. There just aren't everyday words to handle the concepts involved. Second, there are precise concepts in science that best need a different (and designed to be precise) language to properly express them, and that language is maths.
There are MANY concepts in maths and science that just don't have any common or garden analogs, even ones that come anyway close, hence no surprise if these concepts can't be adequately explained using just plain english. And anyone who thinks they can knows so little beyond the everyday, and has such a profound lack of imagination, that they aren't worth trying to teach in the first place.
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Post by Progenitor A on Sept 14, 2010 13:09:45 GMT 1
A photon can only occupy a quantum energy level we are told No, an ELECTRON occupies a quantum energy level, and emits a photon when it moves to a different energy level. And what is the energy level of the emitted photon? |
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Post by speakertoanimals on Sept 14, 2010 13:30:15 GMT 1
The DIFFERENCE in the energy levels of the electron.
The photon has a definite energy corresponding to how it is produced, but photons in general don't occupy distinct energy levels, because unless we consider just photons in a box (which we do when we are computing the black body spectrum), photons on a vacuum can have ANY energy.
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Post by Progenitor A on Sept 14, 2010 13:55:16 GMT 1
The DIFFERENCE in the energy levels of the electron. The photon has a definite energy corresponding to how it is produced,.......... What is the energy level of an emitted photon when an electron drops to a lower energy level? If you mean the difference between the two energy levels of the electron, is that not a quantum energy level? Aren't you being purposefully pedantic? Do you simply delight in contradiction I wonder? |
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Post by alanseago on Sept 14, 2010 13:58:16 GMT 1
That brings back another memory from school: "When an electron suddenly drops to a lower orbit, light is emitted". I presume the value can be calculated.
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Post by Progenitor A on Sept 14, 2010 14:12:56 GMT 1
That brings back another memory from school: "When an electron suddenly drops to a lower orbit, light is emitted". I presume the value can be calculated. 10.2 eV |
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Post by speakertoanimals on Sept 14, 2010 15:51:20 GMT 1
What is the energy level of an emitted photon when an electron drops to a lower energy level? If you mean the difference between the two energy levels of the electron, is that not a quantum energy level? Aren't you being purposefully pedantic? Do you simply delight in contradiction I wonder? No, just showing that you don't understand the basic terminology! So, yes a free photon emitted by an atom has an energy that corresponds to an energy level (for a free photon), but the critical bit is that the energy levels for a free photon aren't quantized (i.e. occupy only a discrete set of possible energy levels), whereas the energy levels for an atom are quantized (i.e. a set of discrete energy levels corresponding to bound states of the electron). It's where the quantum in quantum theory comes from. The point about the photon energy being spread out is just back to the basics of quantum theory and what the wavefunction means. So, we cannot split particles, we have one photon, even if spread out. So what that means is that although the probability of finding the photon is spread out, when we measure where the photon is, we just get ONE definite answer, with ALL the energy, even though the wavefunction for that photon is spread out. Same for electrons -- wavefunction spread out, charge density spread out, but when we measure the position, we find it in ONE place (since we are measuring position). It's the collapse of the wavefunction from a spread-out wave, to a very localized (subject to measurement accuracy) wavepacket. The point being that the measurement process involves an interaction with something else (the measuring apparatus). We can see this very directly when we do one photon double slit experiments. Each photon passes through both slits, yet gives a single blip on the screen at the other end. We can only see the wave nature of the interference pattern when we repeat the experiment many times, then we can see the spread-out probability distribution on the screen from which the measured position of any individual photon is randomly drawn by the god that doesn't throw dice......... |
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Post by speakertoanimals on Sept 14, 2010 16:05:26 GMT 1
That brings back another memory from school: "When an electron suddenly drops to a lower orbit, light is emitted". I presume the value can be calculated. That is kind of the whole point behind quantum theory. The fact that for a given element, the light that emitted only belongs to a set of discrete frequencies (spectral lines), and those are different for different elements. It was realised that one way to try and explain the discrete lines was if the electron could only occupy distinct energy levels, different for each atom. The bohr model could do this for hydrogen, by making an ad hoc assumption, that the electron waves had to fit in a whole number of (half) wavelengths, in effect. Which gave roughly the right answer. The rest of quantum theory was then based on how to write down the real problem in quantum theory for simple atoms like hydrogen (central charge, one electron), computing the energy levels, and checking them against experiment. When it could be seen that some lines were actually double, or some collection of very closely spaced lines, this led to including other terms, such as the way the magnetic dipole of an electron interacts with the magnetic field generated by a charge orbiting. Atoms with many electrons are complicated, because each electron sees not just the central electric charge of the nucleus, but all the other electrons as well. So, can only be solved approximately without using computers. Then we also get other effects, such as what happens to the energy levels when you put the atom in a magnetic or electric field, and all the results confirm the detailed predictions from quantum theory. |
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