qft123 Posted July 4, 2012 Author Posted July 4, 2012 Particle wave duality is a myth, which arose in the early times of quantum mechanics when only classical particles or classical waves were known and the myth was born to try to explain the experiments where the objects under study did not behave as classical particles or classical waves. The idea that photons sometimes behave as waves and sometimes behave as particles is a recurrent, but completely incorrect, claim. A well-known joke about this is In modern physics the photon is defined as a particle, always. Everything around us is made of particles as CERN remarks: http://public.web.ce...rdModel-en.html An electron or a photon are always particles and all particles behave as particles. Danger! Particle does not mean "little-hard-sphere-following-Newtonian-laws". Then how would you describe a "Particle" in quantum terms and not classical physics??
juanrga Posted July 4, 2012 Posted July 4, 2012 (edited) Then how would you describe a "Particle" in quantum terms and not classical physics?? Theoretical particle physicists like to relate elementary particles with irreducible representations of the quantum Poincaré group. In more experimental terms an elementary particle is unambiguously defined by properties such as mass, charge, and spin. The spin of an elementary particle is a purely quantum property that has not classical counterpart (i.e. the quantum spin would not be confused with the classical concept of spinning motion. Quantum spin is something different). If you took a look to the IUPAC link given above you can see that the electron is described as a particle with mass me, charge e, and spin 1/2. All known elementary particles are described by mass, charge, and spin (e.g. a photon is a massless particle with zero charge and spin 1) Composite particles are defined by giving their composition, e.g. a proton is a collection of two quarks-u plus a quark-d. Although for problems where the internal structure of a composite particle is not important, one only considers its total properties such as total mass, total charge... For instance, in molecular physics and chemistry the proton is very often considered as if was elementary, because the details about its structure are not relevant for those problems. Other properties of the particles such as energy, momentum, etc. are obtained by the usual quantum mechanical methods. One writes down the quantum operator for the property and obtains its eigenvalues for a given set of quantum eigenstates. This is how we obtain the energy of an electron in a hydrogen atom for instance. Edited July 4, 2012 by juanrga 1
studiot Posted July 4, 2012 Posted July 4, 2012 (edited) And using Juan's definitions the term particle and elementary particle correspond exactly to the classic definition of the term. La plus ca change la plus c'est la meme chose! +1 Edited July 4, 2012 by studiot
qft123 Posted July 5, 2012 Author Posted July 5, 2012 And using Juan's definitions the term particle and elementary particle correspond exactly to the classic definition of the term. La plus ca change la plus c'est la meme chose! +1 Studiot, You speak french?? Tres bon!
Klaynos Posted July 5, 2012 Posted July 5, 2012 Sorry, post 40 should have another post before it by mississippichem, reading: Answer it is both. How about the bicarbonate ion in solution in the presence of HCl? Answer both at the same time. [/Quote] Nailed it! +1 IMO the whole premise of the thread rests on a false dichotomy. The best answer is that quantum objects behave like quantum objects. Some of that entails some wavelike characteristics and some particle like characteristics. To me, electrons look like state vectors in a Hilbert space. The English language was not designed with QM in mind, but bras, kets, path integrals and the theory of Hermetian operators were. That's good enough for me. [/Quote] The mobile interface beat me somewhat. It is apparently REALLY easy to accidentally delete posts. Sorry everyone.
immortal Posted July 5, 2012 Posted July 5, 2012 Oh when did the accepted consensus did not mattered to anyone. One says that the wavefunction is real and physical and other one says that wave-particle duality is a myth. Please keep your crackpot science in a different forum. “Some time before the discovery of quantum mechanics people realized that the connexion between light waves and photons must be of a statistical character. What they did not clearly realize, however, was that the wave function gives information about the probability of one photon being in a particular place and not the probable number of photons in that place. The importance of the distinction can be made clear in the following way. Suppose we have a beam of light consisting of a large number of photons split up into two components of equal intensity. On the assumption that the intensity of a beam is connected with the probable number of photons in it, we should have half the total number of photons going into each component. If the two components are now made to interfere, we should require a photon in one component to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy. The new theory, which connects the wave function with probabilities of one photon, gets over the difficulty by making each photon go partly into each of the two components. Each photon then interferes only with itself. Interference between two different photons never occurs. The association of particles with waves discussed above is not restricted to the case of light, but is, according to modern theory, of universal applicability.” by P.A.M. Dirac. (The Principles of Quantum Mechanics, 4th Ed, Pg 9)
pmb Posted July 5, 2012 Posted July 5, 2012 Oh when did the accepted consensus did not mattered to anyone. One says that the wavefunction is real and physical and other one says that wave-particle duality is a myth. Please keep your crackpot science in a different forum. To whom are you referring?
immortal Posted July 5, 2012 Posted July 5, 2012 To whom are you referring? I really don't care who the hell they are, this forum is anonymous, I don't have to care how old he or she is or what degree he or she has. I only care about the content of one's post and the arguments one make. Seen in this context this forum is a great place for learning.
studiot Posted July 5, 2012 Posted July 5, 2012 (edited) Immortal, I didn't understand the thrust of your post either. I think pmb was simply trying to ask which of the two options you posted you thought was the correct one. I don't think his reply was personalised. As regards to your extract from Dirac, isn't that view superceded by the modern idea of virtual particles? Edited July 5, 2012 by studiot
qft123 Posted July 5, 2012 Author Posted July 5, 2012 I really don't care who the hell they are, this forum is anonymous, I don't have to care how old he or she is or what degree he or she has. I only care about the content of one's post and the arguments one make. Seen in this context this forum is a great place for learning. This reply was certainly not required.. Are you refferong to Juan?
qft123 Posted July 5, 2012 Author Posted July 5, 2012 I really don't care who the hell they are, this forum is anonymous, I don't have to care how old he or she is or what degree he or she has. I only care about the content of one's post and the arguments one make. Seen in this context this forum is a great place for learning. This reply was certainly not required.. Are you reffering to Juan?
immortal Posted July 5, 2012 Posted July 5, 2012 Immortal, I didn't understand the thrust of your post either. I think pmb was simply trying to ask which of the two options you posted you thought was the correct one. I don't think his reply was personalised. As regards to your extract from Dirac, isn't that view superceded by the modern idea of virtual particles? Particle wave duality is a myth, which arose in the early times of quantum mechanics when only classical particles or classical waves were known and the myth was born to try to explain the experiments where the objects under study did not behave as classical particles or classical waves. The idea that photons sometimes behave as waves and sometimes behave as particles is a recurrent, but completely incorrect, claim. A well-known joke about this is In modern physics the photon is defined as a particle, always. Everything around us is made of particles as CERN remarks: http://public.web.ce...rdModel-en.html An electron or a photon are always particles and all particles behave as particles. Danger! Particle does not mean "little-hard-sphere-following-Newtonian-laws". Are particles and waves somewhat related? Yes. An electromagnetic wave is a large collection of photons. Therefore a wave has momentum and energy. Note, however that the description of a collection of photons as a electromagnetic wave is only an approximation. Quantum optics is more general and requires a photon treatment. Here... I found you a link. One of the most direct experiments we have confirming the quantum behaviour http://news.bbc.co.uk/1/hi/8570836.stm I can tell you without a shadow of a doubt, that not only is the wave function real, but it is also physical. I didn't quoted individually because there were too many posters making speculative claims which contradicts the accepted consensus. This reply was certainly not required.. Are you reffering to Juan? No, I was talking in general.
StringJunky Posted July 5, 2012 Posted July 5, 2012 Particle wave duality is a myth I can tell you without a shadow of a doubt, that not only is the wave function real, but it is also physical. An overwhelming sense of personal certainty is antithetic to good scientific practice and can blind a person to further advancement. 2
studiot Posted July 5, 2012 Posted July 5, 2012 (edited) An overwhelming sense of personal certainty is antithetic to good scientific practice and can blind a person to further advancement. Yes indeed, quite so. +1 However there is more to it than that. I have been promoting the idea that it doesn't have to be one or the other but there is more to this than contained in the original post for We do know that photons behave like a wave and a particle both, that's what particle wave duality tells us.. Light and electricity possess four apparantly contradictory characteristics that 1)Can be explained either by waves or by particles 2)Can be explained only by waves 3)Can be explained only by particles 4)In addition they do not posses some characteristics attributable to particles or waves. All these four characteristics are present at all times. Bohr recognised this when he introduced 'complimentarity' which roughly explained means that the phenomenon cannot be explained by a series of either/or questions to which there are only yes/no answers. He called this type of reasoning classical logic and introduced non classical logic to account for activity. The actual activity observed depends upon the external conditions, not the quantum entities. These conditions 'extract' the characteristics of interest. So how does that play out with the original post? LIGHT We do know that photons behave like a wave and a particle both, that's what particle wave duality tells us.. 1)So which source of light behave likes a particle and which one like a wave, How do we know that?? 2)If a particle has larger wavelength it behaves like a wave and the one which has smaller wavelength behaves like a photon? 3)Photons have momentum, p=hv/c, do wave have some momentum? Electrons Electrons are present around the nucleus of an atom(we all know that) 4) Are they present there as particles, standing frequency, clouds or on orbits(which is the least i would prefer) ?? (1)&(2) No because it depends upon the external conditions as above. (3) Already answered by Swansont (4) Again depends upon conditions which is why I asked Juan about lecher lines and he avoided the issue. Consider a suitable source of electricity (at UHF) and pass it down a beam tetrode tube. You will get a beam of particles. That is what the tube does. The beam particle density does not vary appreciably and you may get a space charge effect. In effect the electrons rattle down the track like vehicles on a motorway. There are local clusters and but no wavelike structures appear.This is pure particulate behaviour. Now pass those same electrons from the tetrode down a lecher line. You will observe a periodic structure of voltage potential, sufficient to power a lightbulb at some points and zero at other. No particulate solution can lead to this behaviour. It is pure wave. Edited July 5, 2012 by studiot
pmb Posted July 5, 2012 Posted July 5, 2012 (edited) Let's get something straight. The wave-particle duality is no a myth. In fact its a subject that you can find defined and described in all textbooks on quantum mechanics. It has a very precise meaning which is defined here http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality. I have a graduate level quantum mechanics text called Principles of Quantum mechanics - 2nd Ed. by R. Shanmkar. From page 113 We found that entities such as the electron are particles in the classical sense in that when detected they seem to carry all their energy, momentum, charge, etc. in localized form: and at the same time that are not particlelike in that assuming they move along definite trajectories leads to conflict with experiment. It appears that each particle has associated with it a wave function [math]\Psi(x, t)[/math], such that [math]|\Psi(x, t)|^2[/math] gives the probability of finding it at a point c at time t. This is called wave-particle duality. That is the precise meaning of wave-particle duality. Edited July 5, 2012 by swansont fix math tag
studiot Posted July 5, 2012 Posted July 5, 2012 in that assuming they move along definite trajectories leads to conflict with experiment Just as I described in my lecher lines, however they can move along definite trajectories note the key word assuming.
pmb Posted July 5, 2012 Posted July 5, 2012 Just as I described in my lecher lines, however they can move along definite trajectories note the key word assuming. I think you may have misinterpreted that quote. Electrons cannot follow definite trajectories. That's what it was saying, i.e. its not a particle in the sense that it can move along definite trajectories.
studiot Posted July 5, 2012 Posted July 5, 2012 (edited) I think you may have misinterpreted that quote Read my description again, in post#64. They do in a beam tetrode or cathode ray tube. I even have an (old) textbook of electron ballistics pertinent to that era. However you may pass the same stream of electrons so that they form waves as in lecher lines. Consider the tetrode as one black box and the lines as another in series the electrons can pass through one black box as a particle and the next exhibiting wave properties. The boxes can also occur in the opposite sequence. Edited July 5, 2012 by studiot
juanrga Posted July 5, 2012 Posted July 5, 2012 And using Juan's definitions the term particle and elementary particle correspond exactly to the classic definition of the term. (i) It is not "Juan's definitions" but the standard physicists and chemists definition. And (ii) the quantum definition of particle is not a classical definition. I even stated how one of the properties of a quantum particle (spin) has not classical counterpart.
pmb Posted July 5, 2012 Posted July 5, 2012 Read my description again, in post#64. Consider it done. They do in a beam tetrode or cathode ray tube. A beam of electrons is not a trajedctory. If it was possible for an electron to follow a classical trajectory precisely then the curve would have a width of zero. The beam width of an eletron beam is very very very far from being a partilce trajectory. If someone is talking about such a trajectorey thn they are ignoring quantumn effects. That's done quite often in classical mechanics but never in quantum mechanics.
studiot Posted July 5, 2012 Posted July 5, 2012 (edited) Compare 1) Consider a suitable source of electricity (at UHF) and pass it down a beam tetrode tube. You will get a beam of particles. That is what the tube does. The beam particle density does not vary appreciably and you may get a space charge effect. In effect the electrons rattle down the track like vehicles on a motorway. There are local clusters and but no wavelike structures appear.This is pure particulate behaviour. 2) Now pass those same electrons from the tetrode down a lecher line. You will observe a periodic structure of voltage potential, sufficient to power a lightbulb at some points and zero at other. No particulate solution can lead to this behaviour. It is pure wave (1) is pure particulate. Yes in the real world the electron emitter is of finite dimensions and you get statistical variations so the beam is of finite dimesions. But there are no wavelike effects. (2) is pure wave. Tthere are no beamlike effects Do you have any idea how difficult it is to agree (and disagree) with both yourself and Juanraga at the same time. I am become the one true quantum particle Edited July 5, 2012 by studiot
juanrga Posted July 5, 2012 Posted July 5, 2012 (edited) It is ironic that a poster uses an old quote from Dirac about wavefunctions for posting attacks to posters who do not buy the duality myth. This poster calls "crackpot science" to what others call modern quantum physics. It is ironic because any modern textbook on quantum physics emphasizes how wrong Dirac was regarding wavefunctions! Yes, textbooks explain how the solutions to the Dirac equation cannot be wavefunctions, in despite that Dirac initially interpreted them as wavefunctions. Dirac was wrong about such wavefunctions, because he missed some advanced aspects of quantum physics. Today we know that the Dirac equation is not a valid wave equation for an electron. I am also perplexed why some posters insist on confounding a wavefunction (i.e. a function without physical meaning) with a wave (i.e. a physical system with physical properties). Since that the wikipedia has been cited. Let us take a look to the talk page: Sources, speaking on the "duality", either obsolete or are popular, educational, or philosophical literature. Serious contemporary theoretical sources don't mention about duality, they use more effective approaches, almost all based on PI. There is a good analogy with the notion of so-called "relativistic mass", which served its in the interpretation of relativistic effects in terms of Newtonian physics, but in the modern 4-dimensional formulation only creates a confusion Raoul NK (talk) 08:55, 20 September 2010 (UTC) A theoretical physicist writes about the wave-particle duality myth: But why then the wave-particle duality is so often mentioned? One reason is philosophical; the word "duality" sounds very "deep" and "mysterious" from a philosophical point of view, and some physicists obviously like it, despite the fact that a dual picture is not supported by the usual technical formulation of QM. Another reason is historical; in early days of QM, it was an experimental fact that electrons and photons sometimes behave as [classical] particles and sometimes as [classical] waves, so a dual interpretation was perhaps natural at that time when quantum theory was not yet well understood. From above, one may conclude that the notion of "wave-particle duality" should be completely removed from a modern talk on QM. In fact, as everyone can check the IUPAC definition of electron do not mention duality and the CERN glossary about modern physics do not mention duality. Edited July 5, 2012 by juanrga
pmb Posted July 5, 2012 Posted July 5, 2012 (edited) It is ironic that a poster gives an old quote from Dirac about wavefunctions in her/his personal attacks to posters who do not buy the duality myth. Nobody used a personal attack against anybody else in this thread. We disagree with you. I scanned and PMd the section of the wave-particle duality from my graduate text on quantum mechanics to the people in this thread. They already know what its all about. Historical Note: It was Author Eddington who coined the term wavicle The following is, in essence, what the wave-particle duality is all about. Simply put: All experiments on diffraction and interference effecs with articles show that a definite wavelength is associated with partilces of a a given momentum. That's the very definition of wave-particle duality. This is just straight textbook quantum mechanics. Has C. Ohanian describes it as follows in his text Modern Physics - 2nd Ed. on page 147 Are electrons classical particles or waves? They are niether. They are wavicles, or quantunm-mechanical particles--they have some properties of a classical partilce and some properties of a classical wave. We say that electrons exhibit duality: they sometimes behave like particles, and sometimes behave like classical waves. The electrons behave in one way or another depending on the experimental arrangement. This is the definition of what the term wave-particle duality means. Anyh atempt to claim otherwise is just plain wrong. Edited July 5, 2012 by pmb
studiot Posted July 5, 2012 Posted July 5, 2012 (edited) pmbThe electrons behave in one way or another depending on the experimental arrangement. Which is what I keep saying. Edited July 5, 2012 by studiot
pmb Posted July 5, 2012 Posted July 5, 2012 Which is what I keep saying. I appologize for being unclear. I'm very cautious whn I say things like that because, at least in my eyes, its quite tricky. For example: What exactly is an experimental set up? If I have 10,000 doub;e slit experiments set up each of which only one particle goes through. In each case the electron is localized when it hits the screen, where the position is then recorded. In each experiment there is no wave property other than the wave being used to prdict where the electron will be. The ensemble of experimental set ups shows that there is intereference as an enemble but there's no way that it can be said that electrons are interfering with each other. Do you see why I'm cautious?
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