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Nature of light

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  1. May 4, 2015 #1
    why light shows both particle and wave nature?
     
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  3. May 4, 2015 #2

    Drakkith

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    We don't know. That simply appears to be the way nature works. Quantum Electrodynamics is the theory that fully describes our understanding of light at the quantum scale, but it is very difficult to explain if you know nothing about the basics of quantum physics. In any case, it only explains how light works, not why it is the way it is.
     
  4. May 4, 2015 #3

    bhobba

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    See our FAQ:
    https://www.physicsforums.com/threads/is-light-a-wave-or-a-particle.511178/

    The wave particle duality is actually a myth:
    http://arxiv.org/pdf/quant-ph/0609163.pdf

    Unfortunately the way physics is taught they sometimes start out with things you later learn is wrong - but we all have to start somewhere. Feynman commented on it and lamented it is like that - but as he knew well, teaching is about what students need to eventually understand - which sometimes entails being a bit loose with the truth at the start.

    You have however asked a question the correct answer to which means we cant be speak in 'half' truths.

    Thanks
    Bill
     
  5. May 6, 2015 #4
    I get the feeling myth is maybe a bit of a strong word no? Maybe misnomer is better? I know he didn't consider the FAQ but his question didn't ask about wave-particle "duality" strictly, he asked about why light had both wavelike and particle like qualities. Although we don't know why this is really the case we know from basic Electromagnetic theory that light is a transverse wave that carries energy, linear momentum and mass akin to the properties of traveling particles. The connection was made by Einstein who showed that the flow of energy associated with the translation of relativistic particles at the rate of the speed of light is precisely the flow of energy associated with these light waves.
    EDIT: Apologies for the FAQ comment - I thought you were suggesting OP question didn't quite comply with rules, i.e didn't do enough background reading or something..
     
    Last edited: May 6, 2015
  6. May 6, 2015 #5

    bhobba

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    Its a myth.

    Its perpetrated by the semi-historical approach most beginner level QM texts use. Here is a MUCH better way of looking at QM:
    http://www.scottaaronson.com/democritus/lec9.html

    Advanced texts like Ballentine don't even mention it because it was done away with when Dirac came up with his transformation theory:
    http://www.lajpe.org/may08/09_Carlos_Madrid.pdf

    I know this and other myths, like virtual particles are real, are very difficult to shake because its widely used in popularisations and beginner texts. This forum is likely the first place the truth is told for many posters. It leads to long threads where people wedded to the myth quotes this and that - but they really go nowhere because it doesn't change the facts.

    Thanks
    Bill
     
    Last edited: May 6, 2015
  7. May 6, 2015 #6

    bhobba

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    That's not what he did.

    He showed the photoelectric effect could be explained with light considered as particles.

    Interestingly it is now known that model isn't required - but that really requires a whole new thread. Best to start another along those lines if you want to pursue it.

    Thanks
    Bill
     
  8. May 6, 2015 #7

    zonde

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    Are you sure it's (much) better? It doesn't seem so:
    "Come read about why quantum mechanics, far from being a mysterious, arbitrary structure foisted on us by experiment, is something that mathematicians could easily have discovered without leaving their armchairs. (They didn’t? Minor detail…)"
     
  9. May 7, 2015 #8
    Could you please elaborate a tiny bit more on this ?

    [["More generally, if you want to flip over an N-dimensional object by a continuous motion, then you need to go into the (N+1)st dimension.

    Exercise 6 for the Non-Lazy: Prove that any norm-preserving linear transformation in N dimensions can be implemented by a continuous motion in N+1 dimensions.

    But what if you want every linear transformation to have a square root in the same number of dimensions? Well, in that case, you have to allow complex numbers. So that's one reason God might have made the choice She did."]]

    It seems to me that the last paragraph says that we need complex numbers in order to have continuous motion in our 3d space , otherwise we'd need 4d space for that. Right ?

    However, when first reading the first paragraph, I was thinking: well of course, that (3+1) must the time dimension... if that makes any kind of sense. So I'm a bit puzzled by this.
     
  10. May 7, 2015 #9
    ..One variable is involved -- Observer, act or some form of interaction which breaks (in a sense) duality. Well, we can say ways about QM. As a layman. It boils down to broad approaches; QM has both particle/wave property, waves is a property/illusion of particle (QM), particle is a property/illusion of wave(classical) and so on. The most common is the copenhagen interpretation (majority of the QM textbooks). Others include, MWI and many of it's variations(BST) -- the most literal, simplest yet paradoxically rich and head on/direct approach to QM; Consistent histories -- It has both classical and schrodinger eqn; Epistemic interpretations; statictical interpretations; modal interpretations; transactional interpretation and my personal favorite RELATIONAL interpretation.

    Anyways, at the end of the day. We can't really tell and to what extent which one is truer since most of the things i mentioned above has it's own flavors and some change in formalism -- mathematical theory. The only bottlenecks we have are our threshold in our experimental endeavors and consistency evaluations. If it's appears in superposition and known to decohere then it tells something about some interpretation. I accept the picture as 'threshold' and see how far we can go about it but i wouldn't go too far to say that it is the absolute way. Illusions/blackswan/phenomenon are apparent in nature but it doesn't matter. What matters most is what 'contributes' to that phenomenon to appear/show that way. It is a standard macroworld idealization that is lost in QM (Not all but most)..

    https://www.sciencenews.org/blog/context/tom’s-top-10-interpretations-quantum-mechanics
     
  11. May 7, 2015 #10

    bhobba

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    Well, having waded through tons of beginner books its way way more transparent - but each to his/her own I suppose.

    Thanks
    Bill
     
  12. May 7, 2015 #11

    bhobba

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    Sure.

    Here is a more detailed argument along the same lines.

    Suppose we have a system in 2 states represented by the vectors [0,1] and [1,0]. These states are called pure. These can be randomly presented for observation and you get the vector [p1, p2] where p1 and p2 give the probabilities of observing the pure state. Such states are called mixed. Now consider the matrix A that say after 1 second transforms one pure state to another with rows [0, 1] and [1, 0]. But what happens when A is applied for half a second. Well that would be a matrix U^2 = A. You can work this out and low and behold U is complex. Apply it to a pure state and you get a complex vector. This is something new. Its not a mixed state - but you are forced to it if you want continuous transformations between pure states.

    QM is basically the theory that makes sense out of pure states that are complex numbers. There is really only one reasonable way to do it - by the Born rule (you make the assumption of non contextuality - ie the probability is not basis dependant, plus a few other things no need to go into here) - as shown by Gleason's theorem.

    The following gives a more rigorous development of this idea:
    http://arxiv.org/pdf/quant-ph/0101012.pdf

    The key idea, and what distinguishes QM from standard probability theory, is you must have continuous transformations between pure states. But in modelling physical systems it pretty much what is required ie if a system changes to a another state after one second, it must go through another state after half a second.

    Thanks
    Bil
     
    Last edited: May 7, 2015
  13. May 7, 2015 #12

    bhobba

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    Remember what I said before:

    They have been discussed in many threads on this forum. Going over the same thing again will serve no purpose. If it is still unclear please go over the link I gave before about myths in QM:
    http://arxiv.org/pdf/quant-ph/0609163.pdf

    Thanks
    Bill
     
  14. May 7, 2015 #13
    I agree with bhobba the idea of a particle does not make sense from a modern quantum mechanical perspective, so why then do we observe particles in experiments? The best answer I have seen to date is that when we make an observation, we are actually entangling our own wave function with that of the photon, electron, whatever. This means what the particle we observe is actually just a reflection of our own state within the entangled wave function. This of course leads to interpretations of quantum mechanics like the Many Worlds Interpretation by Everet, which although not widely accepted has a lot of useful insight.
    There are however other interpretations, such as the Many Interaction Worlds Approach, proposed recently, in which particles do exist in an infinity of semi-classical interacting "parallel" worlds (worth the read, very interesting perspective). The point is that we don't actually know why we observe particles but theories describes waves, and while there are many explanations, there has not been proposed one satisfactory one on which the scientific community agrees.

    Hope this helps guys
    Brage
     
  15. May 7, 2015 #14

    bhobba

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    We do - but its part of that rather difficult area known as Quantum Field Theory:
    http://www.mat.univie.ac.at/~neum/physfaq/topics/position.html

    Its true, however, ordinary QM doesn't explain why some things have position as an observable - it just assumes it does.

    Thanks
    Bill
     
  16. May 7, 2015 #15
    Please read my whole statement and yes Bob i did read the article before but thanks anyways. I gave a broad statement. What i meant was It is difficult to determine what is a true interpretation, competing theory, and what is merely a change in formalism. An interpretation is technically an interpretation of the mathematical theory.

    I don't take positions (realist, orthodox and agnostic in Griffiths QM) and always fascinated on the different solutions and interpretation of QM. In relational view they don't give much attention to real or not or even facts for that matter. It is a purely dynamic description formulation.
     
  17. May 7, 2015 #16

    bhobba

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    I did - and you clearly mentioned the wave particle duality 'QM has both particle/wave property'.

    Thanks
    Bill
     
  18. May 7, 2015 #17
    Ah. No. I only say duality in a broad way. In essence, standard QM is wave and particle is a projection or special case as was sited in the paper. I can also say the other way around.
     
  19. May 7, 2015 #18
    I just meant that Maxwell Calculated the energy and linear momentum of his E-M waves. Then Einstein came up with relativity and showed that relativistic particles traveling at c had the same energy and linear momentum.
     
  20. May 7, 2015 #19

    atyy

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    "Because" matter also shows particle and wave properties :) For that matter "spacetime" also shows particle and wave properties. So it is natural in quantum mechanics for things to have particle and wave properties.
     
  21. May 7, 2015 #20

    zonde

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    It is quite meaningful to talk about light as particles in QM. The reason is that energy of light comes in discrete packets. That is characteristic property of particles. Waves do not show such property.
     
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