Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed of light

  1. Jan 7, 2006 #1
    As there are no stupid questions ...

    Do we have an idea why speed of light is constant ?
    Is it because a photon has no mass because it is impossible to apply acceleration to something that has no mass. Isn't time in itself absolute but observation of time relative because we have to take speed of light as measurement rod ?
    So time is what allows us to observe it ...
  2. jcsd
  3. Jan 7, 2006 #2
    I don't know what you mean by time being absolute. If you think of time as being simply another dimension then observers who are moving relative to one another have time axes which are rotated with respect to eachother, so in that since it is not absolute.

    I don't think there's any explination as to why the speed of light is what it is other than "that's as quickly as information propagates in the universe." Remember that light isn't the only thing which travels at that speed, other phenomena such as gravitational waves do as well.
  4. Jan 7, 2006 #3


    User Avatar

    The speed of light in a vacuum is the constant c. It is the fastest velocity possible, because it is the speed of a particle with a mass zero.
    Is it because a photon has no mass because it is impossible to apply acceleration to something that has no mass.

    It depends on the frame of reference. Like if you're standing on the Earth, and hold a ball in your hand. Its not moving. Actually it is. The Earth is spinning around its axis at about 450 m/s, the Earth is spinning around the sun at about 107,000 km/s, the Sun is spinning around the galactic core at 200 km/s, the galaxy is flying towards the great attractor at 600-1000 km/s.
  5. Jan 7, 2006 #4
    hmm odd, ive heard about this before but didnt know the speed at which we were moving, and thats pretty damn fast.

    i wonder how soon until we arrive. if im thinking correctly itd be like 10,000 years. not likely very good as for earth.

    as for the original thing.
    The speed of light evidence says constant
    the barry setterfield report goes into large detail. and such. basically they dont really have any data to say that the speed of light has changed or is changing.

    though i have a similar thread around which im playing advocate and seeing if the speed of light was infact slower when matter was much closer together and has been increasing with time. as matter has been putting some distance between themselves.
  6. Jan 7, 2006 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Fortunately for the Earth, you're not thinking correctly.
  7. Jan 7, 2006 #6
    What is the mechanism? In present theory such as special relativity it is the space continuum or as Einstein prefered to call it, physical space, that regulates the speed of light. Sound waves travel at a set speed through air and likewise photons are regulated by physical space. But relativity did not agree with physical space being a medium like air or even the aether.

    So is physical space an absolute spatial entity? Yes and no. According to Einstein's view later in his life, it is absolute if you include time. How does physical space regulate the speed of photons? Relativity gives no explanation. We really still don't know why actually?

    Is the speed of light always constant? Yes and no. Relativity just says that the speed of light will always be measured to be the same by observers. I call these the 1st and 2nd party observers. But we can calculate the speed of light be otherwise. For example photons shooting away in opposite directions will have a speed of 2x light speed relative to each other based on a 3rd party stationary observer at the point of emission. Same thing if a rocket shoots away at the speed of light and sends a light beam in the opposite direction. A 3rd party observer can calculate that the light is traveling at nearly 2x the standard speed of light relative to the rocket.

    But the speed of light is always the same relative to the absolute physical space in a vacuum and in a zero gravitational field. That is if we could measure it in absolute speed terms. But as Einstein implied, that's meaningless, as it is not possible to know whether physical space is moving relative to the observer, whether clocks have slowed down, and length measuring devices have shrunk as they move through physical space.

    That's the way I understand it, what do you think?
    Last edited: Jan 7, 2006
  8. Jan 8, 2006 #7
    I am not that familiar with physics, my domain being biology.
    We are told the universe to be infinite but also constantly expanding in an accellerated way. So wil the universe tomorrow be more infinite than today ?
  9. Jan 8, 2006 #8
    I like to think of it like this:
    Depending on the number of dimensions that you take into account, not only massless particles move at speed [itex]c[/itex] but any particle. 4-velocities have magnitude [itex]c[/itex] for any particle, whether massless or mass-carrying. So this [itex]c[/itex] is a kind of universal speed for everything, which in a way simplifies things: it's simpler to consider a constant property than to consider a property that depends on circumstances.
    The seemingly random value of [itex]c[/itex] (i.e. 299792458 m/s) is a mere result of our unlucky choice of units for space and time. In physics, these units are usually chosen more conveniently such that [itex]c=1[/itex]. That results in a universal speed [itex]1[/itex] for all objects in space-time. This represents a parameter of such fundamental simplicity that there is hardly a way to make it even more simpler. One bit more simple would probably be no velocity at all but I guess this would effectively result in the non-existence of our universe.
    Last edited: Jan 8, 2006
  10. Jan 8, 2006 #9


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    A 4-velocity is defined using the tangent-vector to the particle's worldline.

    By computational convenience and convention, a massive particle has a 4-velocity normalized to c (or, in natural units, 1) and, therefore, has [in any inertial frame] a spatial velocity less than c.

    However, a massless particle has a tangent-vector with zero-norm (sometimes referred to as a "null" vector) and, therefore, has [in any inertial frame] a spatial velocity equal to c. So, a massless particle does NOT have a 4-velocity of [itex]c[/itex]... moreover, it can't be normalized since it has zero-norm.
  11. Jan 8, 2006 #10
    Thank you for your correction. For particles with spatial speed c, the 4-velocity is indeed not c (I suppose that, strictly spoken, it cannot even be defined properly despite the conventional use of the null vector in the lightcone).
  12. Jan 8, 2006 #11


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Strictly speaking, the 4-velocity is defined for massive particles, which have timelike 4-vectors and can be normalized. Indeed, the 4-velocity is useful in decomposing tensors into temporal and spatial components in that massive-particle's reference frame. For the photon (a massless particle), one emphasizes the 4-momentum of the photon, which is also tangent to the photon's worldline and is also a null-vector. Of course, there is no analogous reference frame for a photon.
  13. Jan 8, 2006 #12


    User Avatar
    Gold Member

    The universe is not infinitely large, no.

    Usually how it is described is "finite yet unbounded"

    The surface of a sphere is finite yet unbounded.
  14. Jan 9, 2006 #13
  15. Jan 9, 2006 #14


    User Avatar
    Gold Member

    Ok, well that post was totally out of left field...
    Now back to our regularly scheduled programming.
  16. Jan 29, 2006 #15


    User Avatar

    i think i have a feel for why the speed of E&M propagation should be the same for all inertial reference frames. it really just comes from Maxwell's Eqs. and the knowledge (verified by the "negative" outcome of the Michaelson-Morley experiment) that there is no ether medium that E&M is propagated in.

    consider the propagation of sound, for instance. if the wind is steady and blowing across your face at some velocity [itex]v[/itex] from left to right and you measure the speed of some sound coming from your left, you will measure it to be [itex]2v[/itex] faster than if it came from your right. that is because you are moving relative to the "ether" (air) medium that carries the sound wave. but there is no such medium for light or any other E&M wave.

    so then, how do we tell the difference between a moving vacuum and a stationary vacuum? if we can't, if there really is no difference between a moving vacuum and a stationary vacuum, that such a concept is really meaningless, then whether the light that you are measuring originated from a flashlight mounted on a rocket moving past you at [itex] c/2 [/itex] or from a stationary (relative to you) flashlight, how does that change the fact that a changing E field is causing a changing B field which is causing a changing E field which is causing a changing B field which is causing a changing E field, etc.? that propagation of an E field and B field disturbance, which has velocity [tex] 1/ \sqrt{ \epsilon_0 \mu_0 } [/tex]? how is it different for you or for the observer that is traveling along with the flashlight at [itex] c/2 [/itex]? whether you are holding the flashlight or moving past it at high velocity, Maxwell's Eqs. say the same thing regarding the nature of E&M in the vacuum and you will both measure the speed of that propagation to be [tex] 1/ \sqrt{ \epsilon_0 \mu_0 } [/tex].
  17. Jan 29, 2006 #16
    The speed of light is constant by definition. Einstein defined it as axiomatic constant. This constancy is not the aftermath of calculations, or of physical evaluation.

    The constancy of speed of light results to a constant ratio of space/time. Therefore time is not absolute in itself. Time is related to space by a constant ratio that is expressed by the axiomatic constancy of speed of light.

    In this context, it is not "time what allows to observer the speed of light", but it is the other way around: it is the constancy of the speed of light that allows to observer "time" and "space" as two physical terms that can only be observed in an interdependent way. When we make scientific observation and calulations of time, we are actually observing/calculating space too. And when we make scientific observation and calulations of space, we are actually observing/calculating time too. We can not observe/calculate "time" or "space" indivintually and independently one from the other.

    So, when we say that "the speed of light is constant", we are actually saying that the ratio of the physical term "space" by the physical term "time", (space/time), is a universall constant, which is expressed/observed/calculated by a scientific methodology as the ratio that is expressed by the physical term "the speed of light".

    The constancy of speed of light is offen taken as a result of arbitrariness of a decision taken by Einstein. This is not so. You can read the history, of the decision making for the constancy of speed of light, in this web site of the American Institute of Physics: http://www.aip.org/history/einstein/essay-einstein-relativity.htm

  18. Jan 29, 2006 #17
    Sorry, but I disagree with this statement. I can define anything I want as being constant, that doesn't mean that when I go out and measure it I will find that it is constant in reality. The speed of light, and the fact that it's constant in all reference frames, is a consequence of the fundamental geometry of spacetime; the fact that Einstein's theory of Special Relativity actually works when we test it in the lab is evidence of the fact that his hypothesis that one could define the speed of light as being constant was correct.
  19. Jan 29, 2006 #18


    User Avatar

    i'm sure that will convince the proponents of VSL:

    even though the IOP article you mention says there is no proof that Einstein was aware of the Michaelson-Morley experiment, i find it a stretch to think that he was not aware of it. but i think that Einstein had already believed in the constancy of c and looked at the negative result of the M-M as a confirmation of what he had already believed. like, for Einstein, the negative result is exactly what he had expected. the "negative" outcome was, to Albert, a positive outcome because he would have viewed it as an experiment to confirm that the speed of light was the same for all observers, including observers moving though space (on this spaceship: Earth) at pretty high speed in opposite directions.

    i don't believe it was ever axiomatic. only the last competitor standing after all the other theories were disproved, either by experiment or by thought experiment.
    Last edited: Jan 29, 2006
  20. Jan 29, 2006 #19


    User Avatar

    not that i think this has anything to do with it, but it's a sematic issue to say that photons have no mass. i would say they have a mass of

    [tex] m = \frac{E}{c^2} = \frac{h \nu}{c^2}[/tex]

    but they have no rest mass (or "invariable mass") since the rest mass is

    [tex] m_0 = m \sqrt{1 - \frac{v^2}{c^2}} [/tex]

    and, by definition, [itex] v = c [/itex] for light.

    but photons behave the same way in the presence of graviation as any other particle (with mass) and even generate their own (very weak) gravitational field.

    i dunno why, but in the last couple decades, it has become really fashionable semantically to equate the term "mass" to the "invariant mass" or "rest mass" of a thing. not all of us do that.
  21. Jan 29, 2006 #20


    User Avatar
    Staff Emeritus
    Science Advisor

    There are many reasons why, the most basic reason is that invariant mass is not coordinate dependent.

    The trend of modern physics has been to express physics in language that does not depenend on the coordinate system used.

    Here's an example. Suppose I have an object that has a mass of 1 kg.

    I then perform a passive change of coordinates by moving towards the object at some velocity v.

    The invariant mass of the object, being a property only of the object itself, does not change. The "relativistic mass" does change.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Speed of light
  1. The speed of light (Replies: 4)

  2. The speed of light (Replies: 7)

  3. Light speed (Replies: 5)

  4. Light speed (Replies: 8)

  5. Speed of light (Replies: 18)