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Why Does Lightspeed = C?

  1. Mar 14, 2009 #1
    Why does the speed of light equal C?
    (ie. 3 x 10^8 m/s)

    Why isn't it 5 x 10^8 m/s, or 7 x 10^5 m/s, or 4 x 10^25 m/s?

    What makes lightspeed the particular speed it is?

    What is the underlying basis for lightspeed being what it is?
  2. jcsd
  3. Mar 14, 2009 #2
    The permeability of free space mu0 is 4 pi x 10-7 henrys per meter

    The permittivity of free space epsilon0 is about 8.85 x 10-12 Farads per meter

    Therefore electromagnetic waves (light) travel at

    c = sqrt [1/(mu0 epsilon0)] = 2.9979 x 108 meters per sec.

    Review Maxwell's equations.
    Last edited: Mar 14, 2009
  4. Mar 14, 2009 #3
    Its funny considering that this question appears often. We can't really be so sure as to why but if you want to know how, the post by Bob S explains it well enough.
  5. Mar 14, 2009 #4
    Will we ever come up with a way to discern the underlying causal reason behind why lightspeed = C?

    We associate light with electromagnetism, as opposed to Strong Force, Weak Force, etc.
    Do we know that the gluons for Strong Force, and the W or Z particles also work at speed of light?
    Isn't it possible that the greater apparent strength of the Strong Force is due to its gluons acting at faster speeds, for example?
  6. Mar 14, 2009 #5
    Is 'c' really the speed of light in a strong gravitational field ? In there we have a big time dilation so light may propagate slow from far away observer.
  7. Mar 14, 2009 #6
    I asked my physics prof why the speed of light is always the same relative to the observer , and i said it must be pretty hard to explain y it is always constant and he said it is impossible to explain.
  8. Mar 14, 2009 #7


    Staff: Mentor

    We know the reason why the speed of light is 299 792 458 m/s. It is that way because starting in 1983 the BIPM defined http://www.bipm.org/en/si/si_brochure/chapter2/2-1/metre.html" [Broken] so that the speed of light is exactly 299 792 458 m/s.
    Last edited by a moderator: May 4, 2017
  9. Mar 15, 2009 #8


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    Is this in the Physics FAQ somewhere? It should be.

    I've never actually heard an explanation of why c is the speed it is before this.
  10. Mar 15, 2009 #9


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    There is no way to explain why the universe is the way it is. We opserve it to be that way, that is all we can possibly say. Physics cannot and does not answer "why"
  11. Mar 15, 2009 #10
    But is there perhaps some underlying layer of physical phenomena which might be exposed as the basis for why spacetime has its particular characteristics? (eg. speed of light = C, etc)

    How could we go about exposing any such underlying layer, if it exists?

    For instance, Michelson-Morley attempted to expose or disprove aether. They didn't however disprove the idea of foam. If foam exists, then wouldn't exposing its nature then reveal the reason for C and other spacetime characteristics?

    Just using that as an example, how would we go about trying to expose or disprove the existence of a dynamic quantum foam, just as Michelson-Morley did for static aether?
    What would be the most useful experiment from which to draw observations for this purpose?
  12. Mar 15, 2009 #11
    c is a dimensional quantity and hence its value has no operational meaning, see here:


    c is thus a mere conversion factor that arises because we use incompatible units for physical quantities that are fundamentally the same (e.g. space and time, energy and mass, etc.). Before we knew that the different quantities were physically the same, we already had defined units for them. Also we had assigned different incompatible dimensions to the quantities.

    Then, later, when we discovered the formulas that relate the supposedly incompatible quantities, the formulas come with dimensional conversion factors that undo our assignment of the dimensions.

    So, the speed of light question has to do with the way we would choose units for space and time before we found the fundamental laws of physics that relate them. Of course, we would choose such units that are practical in our daily lives. The second is a small unit of time which is so small that it is barely relevant. The same is true for the centimeter.

    Now, in natural c = 1 units, the second is vastly larger than a centimeter. So, the question then is why, given Lorentz invariance at the fundamental level, we are so insensitive to distances in the time direction, compared to distances in te spatial direction.
  13. Mar 15, 2009 #12
    I always thought the speed of light is something like the 'infinity' for speeds.

    Just like no matter how much you add or subtract from infinity you will still get infinity, no matter how much you are traveling with respect to something traveling at c, it will still appear to go at c.

    But I'm no physics expert (in fact I'm just an amateur).
  14. Mar 15, 2009 #13
    Yes, you can say that c is to relativity what an infinite speed would be in classical mechanics. In fact, the limit c to infinity is the so-called classical limit in which relativity becomes classical mechanics.

    This also explains why relativistic effects are of order (v/c)^2 and not of order v/c. Classical mechanics can handle a finite but non-invariant speed of light ok. E.g., there exists a classical formula for the Doppler shift in which c appears. The difference between classical mechanics and relativity is that c is invariant in relativity, which is an additional effect apart from the mere fact that the speed of light is finite.
  15. Mar 15, 2009 #14


    Staff: Mentor

    Expanding on what Count Iblis said, c is a dimensionful universal constant, and as such is always related to the way we define our system of units. It does not speak to the state of the universe as much as our human conventions. Basically, all it says that a meter is much smaller than a second.

    If you want to talk about fundamental constants that describe the universe then you want to talk about http://math.ucr.edu/home/baez/constants.html" [Broken]. Those are the ones that describe the universe rather than our unit systems. What you are probably more interested in is questions like "Why is the fine-structure constant equal to 1/137.03599?" And nobody knows the answer. It is hoped that a proper Theory of Everything will be able to derive the fundamental constants, or at least reduce their number, but that is still just a hope.
    Last edited by a moderator: May 4, 2017
  16. Mar 15, 2009 #15
    If c is derived from permiability and permittivity of free space, they are more fundamental than c. Is it right?

    And how do they calculate permiability and permittivity of free space?
  17. Mar 15, 2009 #16


    Staff: Mentor

    They don't calculate them, they measure them. Measuring c and calling it fundamental is really no different from measuring the permeability and permittivity of free space and calling them fundamental.
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