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Why does light have invarient speed?

  1. Feb 29, 2008 #1
    What is the physics answer to the question of why light has an invariant speed
    to anyone and everyone, other than this is what light is? There must be a
    reason why light behaves this way (or perhaps not necessarily this way
    always). I'd think something must have happened external to the light to give
    it this peculiar property. Put it in another way, what's wrong with the
    classical physics where velocity would follow the law of vector arithmetics,
    when applied to light?
  2. jcsd
  3. Feb 29, 2008 #2


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    Why is a difficult question for much of science. The speed of light is constant is one of the requirements of special relativity, which, as I am sure you well know, has been experimentally verified for around a hundred years. Ordinary classical physics for the addition of velocity doesn't work for anything approaching the speed of light, not just light.
  4. Feb 29, 2008 #3


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    The parameter c was important to classical physics before it was known to be the speed of light. It is the ratio between the natural unit of charge for electric and for magnetic forces. It appears as a necessary constant in Maxwell's equations. "Ordinary classical physics" would be inconsistent if c were not constant. All of classical electromagnetism would be wrong if c were not an invariant constant. It also just happens to follow from Maxwell's equations that c is the speed at which light travels. The electrodynamics of moving objects requires the constant c (or epsilonnaughtmunaught) in order to include electric and magnetic forces in the same equation. Understanding all of this was why Einstein titled his relativity paper "The Electrodynamics of Moving Bodies" and not "The Speed of Light".
  5. Mar 5, 2008 #4
    c = 1/sqr(Uo*Ep)... where Uo is the permeability and Ep is the permittivity for free space.

    This relationship holds true because the speed of light (and of all electromagnetic phenomena) is determined at the aether level. It remains constant in all frames because it is not dependent on a coordinate system like matter with mass is. Since ratios like permeability and permittivity are determined at the aether level and the aether is immaterial and not bound by spacetime laws, 'c' can be frame independent.

    The speed of light is frame independent, but it is dependent on the physical properties of free space, and free space is immaterial, with no landmarks, therefore, not subject to the laws of Relativity, like objects in spacetime are. That's why the speed of light is a constant unaffected by the speed of the observer or the observed.

    The speed of light sets the scales. For fields to continue to work regardless of spacetime conditions there must be time and space distortions between the observer and the observed when dealing with relativistic speeds. This is where the principles of relativity and equivalency come from. Because a field's speed must not change regardless of relative motion, and because energy is finite, for reality to work, all parameters must be adjusted around the speed of light. This is why we get time dilation and length contraction.

    Since the speed of light, hence, the propagation speed of fields, must remain constant for all the other fundamental constants to continue to be proportionally the same, process (mass) has to increase in order to keep up... to a point, once we go over the speed limit and fields can't keep up, matter disintegrates. When we reach the speed of light, wavelength and frequency drop to zero, waves become flat, devoid of any information, and we are back to being immaterial empty space.

    Time and length contractions are real. They need to be in order for the Equivalency Principle and the laws of Thermodynamics to hold. Spacetime, or material space, is a product, not a fundamental or primary component of reality, and that is precisely what is claimed by Relativity. In spacetime, space-like separation is relative. If spacetime were primary then spatial extension wouldn't be variable or dynamic, but it is, it shrinks and expands, just like clocks run slower or faster, depending on energy usage vs. energy available.

    Since the speed of light is constant and closely related to the Compton wavelength and the Schwarzschild radius, the universe is the same everywhere, independently from existing spacetime conditions. This means that matter will always have the same properties and behave the same way everywhere, regardless of the existing spacetime conditions, that a carbon atom will look and behave like a carbon atom anywhere in the universe.
    "...quantum field theory says that associated with any mass m there is a length called its Compton wavelength, lc, such that determining the position of a particle of mass m to within one Compton wavelength requires enough energy to create another particle of that mass. Particle creation is a quintessentially quantum-field-theoretic phenomenon. Thus, we may say that the Compton wavelength sets the distance scale at which quantum field theory becomes crucial for understanding the behaviour of a particle of a given mass. On the other hand, general relativity says that associated to any mass m there is a length called the Schwarzschild radius, ls, such that compressing an object of mass m to a size smaller than this results in the formation of a black hole. The Schwarzschild radius is roughly the distance scale at which general relativity becomes crucial for understanding the behaviour of a given mass. Now, ignoring some numerical factors, we have:

    lc = hbar/mc


    ls = Gm/c^2

    These two lengths become equal when m is the Planck mass. And when this happens, they both equal the Planck length!"

    ------ John C. Baez (from "Higher-dimensional algebra and Planck scale physics", as it appeared on the book "Physics Meets Philosophy at the Planck Scale" by Craig Callender and Nick Hugget)
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