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Trying to see light as a wave

  1. Jun 28, 2006 #1
    I understand the fact the light can be proved to be both a particle and a wave, but one thing I don't understand:
    Every other wave I have known, travels through a medium of some sort. A wave consists of a particle moving one way and then back to its original position. Then the particles next to it move the same way and back to their original position. So, the wave can be said to be moving, but the particles consisting of the wave after it passes, stay in place. A light "wave" baffles me.. If there is nothing to move one way and back, how can a wave progress?

    http://www.kettering.edu/~drussell/Demos/waves-intro/waves-intro.html [Broken] shows animations and explanations of what I mean as a wave.
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  3. Jun 28, 2006 #2


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    Here is a very rough description of the electromagnetic account of an EM wave. Maxwell's equations say, among other things, that a changing electric field causes a magnetic field, and a changing magnetic field causes an electric field. So imagine you start with an electric field in simple harmonic motion. Perhaps it's in a wire and driven by some broadcast signal. By its sinusoidal changing it generates a magnetic field, curled around the wire. And that magnetic field, per the equations,will inherit the sinusoidal variation, but 90 degrees out of phase. And so the changing magnetic field generates a further out electric field in sinusoidal variation, and that generates a next magnetic field, and.... so on. The waves are propagated through space as successive electric and magnetic layers, and don't need a medium to compress or whatever.

    The quantum picture is different, but not so awfully different.
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  4. Jun 28, 2006 #3


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    You're thinking of a mechanical wave, which do indeed require a medium in which to travel.

    However, light is composed of electric and magnetic fields. It is a beautiful feature of nature that:

    1) changing electric fields induce magnetic fields
    2) changing magnetic fields induce electric fields

    These are two of Maxwell's equations. Combining these two equations, you arrive at a "self-reinforcing" sort of wave, one which trades energy back and forth from the electric to the magnetic field as it travels. This wave does not require any physical medium in which to travel.

    - Warren
  5. Jul 1, 2006 #4
    what u r saying is about mechanical wave. Mechanical waves require material medium and occur by actual movement of material particles.
    Light is an electromagnetic wave it can travel in vacuum ie it doesnt require a material medium. NO PARTICLES ARE ACTUALLY MOVING.
  6. Jul 1, 2006 #5
    Light is neither: it behaves like a state vector in the quantum mechanical formalism, which shares properties of both waves and particles (but never at the same time) in the classical mechanical formalism.

    EDIT: And as CarlB will tell you, it can also be described by a density matrix.
  7. Jul 2, 2006 #6
    In fact your demonstation shows that all you need for a "wave" is synchronized changes of state at different points in space. There need be no interaction, mechanical or otherwise, between the state changing adjacent elements. After all, the objects on TV or computer screens don't actually move or interact -- only the right pixels turn on and off at the right times. That's all that is needed to create an illusion of motion (wave or any other) at a coarse grained/macroscopic level of observation.

    In fact all the fundamental field equations of physics, EM field and quantum matter fields (Dirac, Schrodinger), can be viewed as macroscopic limit of some underlying cellular automata (a la game of life, but with different rules). The waves in that case are simply coarse grained form of statistical properties of large number of cells, all sitting in place, changing their on/off states.

    You can read on this aspect of field equations in the papers by Garnet Ord (who is a mathematician, not physicst). Lots of related work with the similar idea was done by MIT's Digital Mechanics group. See http://kh.bu.edu/qcl/ [Broken], in particular papers by http://kh.bu.edu/qcl/pdf/smith__m19906d796b63.pdf [Broken] (more detail in his thesis), http://kh.bu.edu/qcl/pdf/smith__m19906d796b63.pdf [Broken] (http://kh.bu.edu/qcl/pdf/hrgovcih199262030511.pdf [Broken]), and http://kh.bu.edu/qcl/pdf/toffolit199064697e1d.pdf [Broken] (see also numerous other papers by him and his http://pm1.bu.edu/~tt/publ.html [Broken]). Note that MIT group shows how such cellular system can generate fully relativistic equations, including the simple cellular automata mechanism behind the Lorentz transformations. Gerard `t Hooft has in recent years also pursued a similar chain of ideas in a series of papers on "deterministic quantum mechanics".

    In other words, as the explicit counterexamples in those papers show, there is no basis in the prevailing folk wisdom that the advent of relativistic physics has somehow done away with the underlying medium (aether). The physicists of the early 20th century simply couldn't think up of the right medum that could model the relativistic physics and then jumped to conclusion that this absence of the proof is the proof of the absence of such medium. You can thus understand the EM & matter fields as a coarse grained view of such medium filling up all of the space-time (where the zero values of E and B are also states of that field) and the state changes of the immobile elements of that medium follow the relativistic wave equations. Note also that the space-time parameters appearing in the macroscopic equations are different parameters from the more elemental space-time parameters of the underlying immobile elements states & their changes. Hence, one can view the relativistic fields, which are our present model of physical reality, as a Matrix-like virtual reality simulated by an unimaginably poweful underlying distributed computer, implemented as a vast network of very elemental little computers.
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