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| How does the electron make a photon in an antenna? |
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| HP-ILnerd:
--- Quote from: raspberrypi on February 11, 2017, 10:43:23 pm ---OK but there must be different mechanism to radiate sub Infrared (longer) photons. When a wire generates a radiowaves no electrons are jumping from a higher orbital to a lower one as is the case to make IR light UV and Xrays (balmer series etc). The longer the jump the higer energy/shorter wave length photon. IR seems to emitted by phonons in matter as way to get rid of thermal energy. But what happens at lower energies? --- End quote --- Ah! I think I see the problem. The wavelength of a photon is not its physical length, but a property of how it propagates through space. Like all particles in the Standard Model, the photon is dimensionless. Consider the following experiment: Say you have a hole that you can block in a tiny fraction of a second. Through this, you try to shoot a photon where the period of it's wavelength is twice the time it takes to close the hole. Can you close the hole to "slice" the photon in half to catch half a photon? No. The photon made it through in its entirety or it did not. The E=hv equation has to be used in integral amounts, i.e., 1(hv) or 2(hv)...n(hv) but never 1/2(hv) or any other fraction. I think it's less confusing if you consider fields rather than particles or waves (notions which have their uses). Certain phenomena seem less magical then. Fun Sean Carroll lecture on particles and fields: |
| Rick Law:
--- Quote from: raspberrypi on February 11, 2017, 10:43:23 pm --- --- Quote from: Rick Law on February 11, 2017, 04:04:51 am ---Photon and EM wave are the same thing. To say EM wave make photons is rather like saying H2O makes water. Photon is a packet of EM wave energy. EM wave traveling is a bunch of photons traveling. When energy is released in an atom, such as when an electron falls from a higher energy state to a lower energy state, it will emit a photon - that is the same as saying it will emit EM wave. That released EM wave (photon) contains the energy it released. Mass and energy are the same thing. Photon (mass-less) carries momentum. Photon and EM wave relationship is a different concept from wave-particle duality. Wave-particle duality is the concept that all particles exhibits wave properties and the reverse is also true. You can pick any subatomic particle, be it photon, electron, alpha particle, or for that matter, any particle. When you treat it as a particle, you can measure it's particle properties. When you treat it as a wave, you can measure its wave properties. You will find alpha particles doing crazy things like being at two places at the same time when you are treating it as a wave. work are being done. We don't know all there is to know yet. --- End quote --- OK but there must be different mechanism to radiate sub Infrared (longer) photons. When a wire generates a radiowaves no electrons are jumping from a higher orbital to a lower one as is the case to make IR light UV and Xrays (balmer series etc). The longer the jump the higer energy/shorter wave length photon. IR seems to emitted by phonons in matter as way to get rid of thermal energy. But what happens at lower energies? --- End quote --- Exact same thing - you got a photon with less energy. That it was less energy doesn't matter, it just turns into a lesser photon. Eventually, the energy can get so low that uncertainty principal comes into play. Then you don't know what energy that photon has, or where that photon is - or if that photon even exist. Now you are into philosophy more than you are in Physics. Even ignoring inflation and the expansion of the universe, how do you measure a photon with frequency< (1 / (13.7 billion years)), or in other words a photon with wave length>13.7 billion light years? That photon if created at the birth of the universe has not completed one cycle of oscillation yet. You have to wait another 0.1 billion years for the first oscillation cycle to complete. We think that uncertainty principal appears valid. So below a certain point, it can just vanish perhaps to reappear at a later time in another form. What happen to that photon for the moment it vanished or does it exist? That I don't think you can find the answer in Physics yet. |
| T3sl4co1l:
--- Quote from: Rick Law on February 12, 2017, 10:17:57 pm ---Even ignoring inflation and the expansion of the universe, how do you measure a photon with frequency< (1 / (13.7 billion years)), or in other words a photon with wave length>13.7 billion light years? That photon if created at the birth of the universe has not completed one cycle of oscillation yet. You have to wait another 0.1 billion years for the first oscillation cycle to complete. We think that uncertainty principal appears valid. So below a certain point, it can just vanish perhaps to reappear at a later time in another form. What happen to that photon for the moment it vanished or does it exist? That I don't think you can find the answer in Physics yet. --- End quote --- Misapplying Fourier analysis doesn't invalidate what the universe does. ;) Consider, for example, the case of the Sun-Earth system, as a quantum "atom": - What is the quantum number of this system? - If the quantum number were to decrease by one, what is the wavelength of "photon" emitted? (It's a gravitational rather than electromagnetic system, so it would actually be a graviton, as such. For the problem, a boson is a boson, so that's fine.) - Based on what you know about the Earth-Sun system, what is interesting about this wavelength? (Borrowed from Griffiths' Quantum Mechanics. My favorite problem in the book.) Tim |
| Rick Law:
--- Quote from: T3sl4co1l on February 12, 2017, 10:25:28 pm --- --- Quote from: Rick Law on February 12, 2017, 10:17:57 pm ---Even ignoring inflation and the expansion of the universe, how do you measure a photon with frequency< (1 / (13.7 billion years)), or in other words a photon with wave length>13.7 billion light years? That photon if created at the birth of the universe has not completed one cycle of oscillation yet. You have to wait another 0.1 billion years for the first oscillation cycle to complete. We think that uncertainty principal appears valid. So below a certain point, it can just vanish perhaps to reappear at a later time in another form. What happen to that photon for the moment it vanished or does it exist? That I don't think you can find the answer in Physics yet. --- End quote --- Misapplying Fourier analysis doesn't invalidate what the universe does. ;) Consider, for example, the case of the Sun-Earth system, as a quantum "atom": - What is the quantum number of this system? - If the quantum number were to decrease by one, what is the wavelength of "photon" emitted? (It's a gravitational rather than electromagnetic system, so it would actually be a graviton, as such. For the problem, a boson is a boson, so that's fine.) - Based on what you know about the Earth-Sun system, what is interesting about this wavelength? (Borrowed from Griffiths' Quantum Mechanics. My favorite problem in the book.) Tim --- End quote --- You are thinking along a line that I am not in sync with. I am unsure how Fourier Analysis comes to play in your line of thinking. Along the line of energy being a function of wave-length (or frequency, same thing just inverse), Fourier Analysis doesn't come into play. So I am having problem following your line of thought here. I chose very long wave length is to illustrate the point of the philosophical difficulty in scaling mathematics to real-life. A frequency of 1/13.7 BillionYears is just a number for mathematics, you can plug that into any equation. But the universe is just 13.6 billion years old. So can you have an oscillation cycle that lasts 13.7 billion years? So whether such photon can exist or not is philosophical. (Again, forgoing the complexity of inflation and universe expansion.) I am also unsure of your example Earth-Sun system analogy in reference to the very low energy discussion. Do you scale Planck's constant with it? The issue is not the absolute size of the quantum, rather, the issue is how close is it to Planck's constant. The closer to Planck's constant, the bigger the uncertainty. At the size of the solar system, uncertainty due to the uncertainty principle is not even in the scale of rounding errors. So even if you are talking about a single particle of graviton, you are talking a huge amount of energy far exceed the scale of uncertainly. There would be no chance of it hiding within the grey area covered by the uncertainty principle. That said, much much much bigger "borrowing from uncertainty" came into play before - namely the big bang. |
| T3sl4co1l:
--- Quote from: Rick Law on February 13, 2017, 12:11:24 am ---You are thinking along a line that I am not in sync with. I am unsure how Fourier Analysis comes to play in your line of thinking. Along the line of energy being a function of wave-length (or frequency, same thing just inverse), Fourier Analysis doesn't come into play. So I am having problem following your line of thought here. --- End quote --- Simply highlighting a fallacy of analysis -- at the simplest level, wave-particle duality is identical to time-frequency duality, and the Heisenberg uncertainty principle is simply the relationship between time-domain bounds and frequency-domain bounds. That is to say -- the universe is more than happy to allow conditions which, within the scope of a Fourier analysis, should count as a photon (say) with a frequency lower than the age of the universe -- that you consider it as such, is merely your fault of applying an analysis that can only resolve things in terms of frequency, and not in other, more suitable terms. :) (What those terms are, is an exercise for the student, naturally...) Now, I'm not sure under what conditions you could ever observe such a thing -- :-DD -- but the takeaway point is, use what analysis is most suitable; Fourier analysis falls apart at "DC", where "DC" is merely however long you wish to look at a signal. Remember also that Fourier analysis (of our simplest, most favorite functions) is symmetrical: the transform must exist for all time, including all negative time and all positive time. Neither condition of which can be properly met in a finite-time universe! Fortunately, Fourier transforms fail softly, so we can dirty up our graphs by bounding them within realistic windows. We remind ourselves of the extents and limitations of our experiments, and perhaps we choose to exclude that pesky DC term from our subsequent analysis because it's an artifact of the transformation, or measurement. But remembering, also, that we should contemplate its origin, in case it's really there (the universe has a net charge..?!). :) In QM, Fourier isn't quite right, because QM isn't pure signal analysis. But the duality phenomenon is common to all wave systems, and so we should naturally expect to see similar concerns arise in all wave systems. Basically, for QM, you might find it's better to use a time-domain analysis than a frequency-domain analysis, in such a case. The frequency-domain (or momentum, or..) results arise from eigenvalues of the solved equation; the eigenfunctions give their spacial distribution. This works nicely when the problem is static (like the energy levels of a particle in a box, or the hydrogen atom), for which you expect a frequency analysis to work nicely (because it's not otherwise changing over time!). The choice of analysis, is a convenience to the solver -- consider solving for the time-domain waveforms of an RLC circuit (analytically, not with SPICE ;) ), versus with Fourier analysis. Once you've trudged through all the awful integrals and found your series of exponential functions, you still can't do much with it because if you want to change the input signal, you have to integrate the damn thing again (output signal = convolution of input signal with impulse response). AC steady state analysis is doing the whole thing in the Fourier domain, though they don't often tell you that that's what you're doing (hey, it's only the second course in the average EE curriculum). If nothing's changing over time, of course the two approaches converge; we don't even bother writing the integrals nor the reactances, and the whole thing reduces to DC resistor networks: EE101. ;) Conversely, Fourier analysis won't help you much with a switching supply circuit -- it's bad enough if the duty cycle is varying over time, but if the frequency is varying as well, you're pretty much screwed. :P Combined with the nonlinear parameters in a real semiconductor circuit, you're better off using energy arguments and events in time. --- Quote ---I am also unsure of your example Earth-Sun system analogy in reference to the very low energy discussion. Do you scale Planck's constant with it? --- End quote --- Nope, as stock. Basically take the already-solved hydrogen atom equations, and plop in the correct potential (gravitational vs. Coulomb) and masses. --- Quote ---The issue is not the absolute size of the quantum, rather, the issue is how close is it to Planck's constant. The closer to Planck's constant, the bigger the uncertainty. At the size of the solar system, uncertainty due to the uncertainty principle is not even in the scale of rounding errors. So even if you are talking about a single particle of graviton, you are talking a huge amount of energy far exceed the scale of uncertainly. There would be no chance of it hiding within the grey area covered by the uncertainty principle. That said, much much much bigger "borrowing from uncertainty" came into play before - namely the big bang. --- End quote --- The nice thing about this example is, it illustrates the problem of limited analysis over a much more human time scale than the Big Bang. Since, as you say, the math doesn't care, you can simply plug in any number -- why not ask the same questions of an atom 2 A.U. across, or 100pm across? :) For the Earth-Sun system, some pertinent questions are: - It is well studied that the Earth's orbit affects the orbits of its neighbors. If it radiates so little energy, how can this be? (Even given the Earth has been in its orbit for 4.5 billion years.) Surely, so little energy cannot distort space-time enough to do that! - If radiation is given off at the rate it seems to be given off at (and, well, why wouldn't it?), then how is it that we can seemingly measure its effect on a far shorter time scale? (The answer to this, on the truly quantum scale, is a field of active development, actually -- the mechanism is analogous to using parametric sensors to measure the presence of a signal, altering the signal slightly in the process but not absorbing it whole.) If you don't know/remember the QM to work it out by hand (it's a good study to work through, I encourage you to give it a try if you can!), the answer is here. SPOILER: http://www.physicspages.com/2013/01/15/earth-sun-system-as-a-quantum-atom/ Tim |
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