Fundamental question on waves

  1. Caution:This is probably an extremely stupid question but I couldn't find any answers on the web.

    So, do waves really exist in the physical world the way we plot them on a graph? or is it just our way of representing an entity? and if they truly physically exist, how do we know for sure that they exist
    in that form? I can understand sound waves,water waves etc. in terms of vibrations of particles, but i want to know if the waves we plot on a graph exist in the real world(do they actually look like that physically)? I hope I have made my question clear. Thanks
     
  2. jcsd
  3. Pythagorean

    Pythagorean 4,582
    Gold Member

    The graph just tells us how a particular quantity varies with time and/or space. In the case of acoustic waves it is the pressure, in the case of electromagnetic wave it is the magnitude of the electric (or magnetic) field. In some cases, a particle's vertical position is represented; for example, when modeling a vibrating string, you describe each section of the string in terms of it's vertical position and the tension from neighboring section of the string.

    "Wave" is the name for the dynamics that occur in such systems, regarding a particular relationship between the quantity's (whether pressure, field magnitude, or position) rate of change in space with its rate of change in time. This is the wave equation:

    [​IMG]

    In QM, the concept becomes a little more abstract, being related to the probability of measuring the particle in a particular state.
     
  4. DrClaude

    DrClaude 2,003
    Science Advisor

  5. Yes, I understand. But does the graph of a given wave represent how the wave moves actually in space or is the graph simply a method of representation like for eg: How SHM is represented on a unit circle (by projection on the diameter as the particle moves)?
     
  6. Yes! My question is if the same holds true for waves we cannot see with our naked eye, like matter waves or radio waves or light waves? Do they actually exist in the form of waves (like your picture showed in the case of a string) and how they are shown in a graph, or is representing them on a graph as waves simply for convenience, and not the actual way in which they move in space.
     
  7. Pythagorean

    Pythagorean 4,582
    Gold Member

    Not necessarily. For instance, here's what pressure waves would actually look like if you could see the density of air. The top image shows our plot, the bottom image tries to demonstrate what's actually happening in terms of particle motions.

    [​IMG]
     
  8. DrClaude

    DrClaude 2,003
    Science Advisor

    Similarily, for radio or light waves (or any other part of the electromagnetic spectrum), the wave doesn't extend in space. It is the electric and magnetic fields that oscillate as the wave propagates.
     
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