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

Value of c and strange things that don't happen when at c

  1. Oct 31, 2009 #1
    Hi there,

    I became really serious studying the basic principles of physics few months ago. There is a plenty of good study material out there, so there was no need to ask anybody anything. Until now.

    The problem is with the speed of light c and why it has properties and value it has.
    Few questions which I would like to elaborate:

    1. Is there really no theory or framework from which I can derive the value of speed of light without knowing in advance what its value should be nor by measuring it directly? Basically I'm looking for an equation where c is the only unknown and when I add all values right into it I get the correct value of c (no tautologies please :)

    2. The basic principle of the Special relativity is that all observers perceive a light wave move at c regardless of their relative speeds to each other and to the light source. So it must be true that if you reverse it, relative to a light wave all (matter) objects move with one speed the c. For a light wave this would be as real as it gets. If matter moves at c, it's mass becomes infinite and time in the matter world stops for the observer (it essentially becomes a singularity. I quess that's why there is the fear that LHC will create miniscule black holes by accelerating some particles too close to the speed of light).
    If this reversal of perspective is true, why the path of light rays we see doesn't act as if any mass object it encounters was a singularity?
  2. jcsd
  3. Oct 31, 2009 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    I'm not sure what you mean by this. "c" is a physical property and can only be derived by physical measurement. Here, http://galileoandeinstein.physics.virginia.edu/lectures/spedlite.html, is a nice description of how Ole Römer did it in 1676. And, of course, that measurement has to be made in a particular system of units- meters and seconds, or miles and hours. You can't just "solve" a purely mathematical equation for it, if that's what you mean.

    As you say, mass (of matter with nonzero rest mass) becomes infinite (more correctly it is the momentum that becomes infinite) which is why that can't[/t] happen. And "time stops" for an observer moving at the speed of light which is why light cannot have a "frame of reference" and it is meaningless to talk about motion "relative to a light wave".
  4. Oct 31, 2009 #3
    Well, I was hoping for an explanation that will tell me why the value of c has the value it has and not some other value. Perhaps derived from some properties of spacetime that limit it to c.
    I understand there must be a finite value of c. Otherwise travelling backwards in time would be possible in some frames of reference. (Is this really true?) But as far as I know, it would be perfectly OK, if it was a little bit higher or lower.

    But if the current view is that it is an inherent property of our spacetime, then fine, I can live with that. I just wanted to know how far is the cause-consequence knowledge in in this particular case.

    OK, so even when I would give up observer's frame of reference of a light wave and switch to a reference frame of an observer with a nonzero rest mass and relative speed of 0.999c (which has a meaning now) all other mass objects I would encounter with this relativistic speed would have a pretty high momentum but their gravitational fields' effects (bending of spacetime) would not change compared to when I was in rest in relation to them?

    So gravitational field is not relative to speed because mass is not relative, only momentum is.
    Then why is weight relative to speed of an observer?
  5. Oct 31, 2009 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    I'm not sure what you're looking for either, but if you measure the electric permittivity of free space, and the magnetic permeability, you can calculate c.

    But the more modern view is that c is the natural "limiting speed" of the universe. And that meters and seconds are human units.

    You can ask why the value of the meter and the second have the values they do, but the answer will have more to do with physics than history.

    But this doesn't seem to be what you're after - hopefully the fact that the speed of light can be computed from Maxwell's equations, which depend on the permittivity and permeability of free space will be what you're after?
  6. Oct 31, 2009 #5

    Jonathan Scott

    User Avatar
    Gold Member

    Although historically it was an experimental result, since Special Relativity the value of c is now primarily a conversion constant between units used to measure space and units used to measure time, and of course it is now defined in that way too.
  7. Oct 31, 2009 #6
    For perspective, there is a LOT we don't know: not only why the speed of light is a certain value, we also do not know things like the why the mass of the electron, proton and neutron are what we measure, and we don't know what constitutes about 96% of the universe, dark matter and dark energy. In fact, we don't know if we live in the only universe or whether there are an infinite number of universes.

    So lot's of good stuff remains to be discovered!
  8. Oct 31, 2009 #7
    This is a derivation of a constant from another constants which is a tautology for me. But that is actually OK. This is what I wanted to know. It tells me that this is the most fundamental layer of reality which we can measure and that we have not discovered another deeper level from which mechanics the value of c is a natural consequence (in any units system).

    So now, only the problem of why gravity is not relative to speed is opened. Can please anybody explain that?
    Last edited: Oct 31, 2009
  9. Oct 31, 2009 #8


    Staff: Mentor

    The value of c is due to the choice of units. It can have any value depending on the units you choose to use.
  10. Oct 31, 2009 #9
    Light is EM radiation. It propagates at C, and it is fundamental property of our universe. You can ask why we perceive time at the rate we do, and that would be much more interesting question.

    Now basic carrier of EM radiation is photon. It has no mass, and its energy depends solely on its frequency. You can even have photon with no energy, assuming that it has zero frequency, and it would be still propagating at the speed of light, although we could never detect it because it has no energy.
    Your question comes to accelerated frames. I suggest that you search for "tween paradox" in older threads.
  11. Oct 31, 2009 #10
    I'm quite familiar with the twin paradox, thank you. I know that observers in non-stationary frames of reference observe time dilation, length contraction and increase of mass. What I would like to know is if that relativistic increase in mass (momentum) has any effect on intensity of gravitation field of the object in relative motion? In other words, if gravitation is relative to speed or not? And why.
  12. Oct 31, 2009 #11
    Yes, if you accelerate object with mass, you are adding to its energy-mass content, and thus increasing its gravitational potential.
  13. Oct 31, 2009 #12


    Staff: Mentor

  14. Oct 31, 2009 #13
    OK, if this is true and gravity is indeed relative to observer and varies with the observer's relative speed (this comes from what you've said) then there must be a HUGE difference in a trajectory of high energy cosmic rays around Earth in comparison to particles traveling with conventional relative speeds. I can imagine that for such particles really heavy objects like neutron stars would literally become black holes when they would fly with relativistic speeds close enough.
    It would also open a possibility to create a black hole for an observer just by increasing his relative speed relative to the object. Even when before the acceleration the object wasn't one. Sorry, if it sound crazy but this is what it implies.

    Maybe according to transformation equations these effects won't occur. Is there a way I can find out how big this effect will be for various relative speeds and masses?
  15. Oct 31, 2009 #14
  16. Oct 31, 2009 #15
    OP is struggling with notion that from photons frame mass is traveling at C. You are about to confuse him even more.
    To OP:
    Consider astronaut piching baseball in a empty space. They separate at 30 m/s. Astronaut has 1000 times more mass then ball. Can we calculate when their frames separate, how much is each frame accelerated, or we can just tell that they are separating at 30 m/s?
  17. Oct 31, 2009 #16
    You assume you can look at the situation from some 3rd absolute frame of view from which you can look at both frames at once. But you can't. That's what SR is about. I'm not sure if this has something to do with my question though or maybe I just didn't understand your point.
  18. Oct 31, 2009 #17
    No, you can look from their initial frame, and calculate that astronaut has been accelerated 1000 times less then a ball.
    Now consider light bulb emmiting a photon. Photon has no mass, but it has momentum. It ventures to space and slightly recoils light bulb in a opposite direction.
    Using the same principle as above you can see that photon is the one traveling at C, not the object with mass.
  19. Oct 31, 2009 #18
    I know what you are trying to say. But this is only from perspective from the initial frame of reference. If you were an observer standing in the initial frame you would observe what you describe. If you were in the frame of reference of the bulb you would see the photon running away by speed c and you (the observer in the initial frame) would recoil slightly in the same direction as the escaping photon. From the photon's perspective (if there was any possible) the bulb and you would suddenly accelerate away with c. There is no way you can tell that any of those frames is truer than other. This is the beauty of the Special Relativity. There is no other lesson in that.

    We still do not know what would happen with gravity of the bulb and observer in relation to the photon (or better a 0.999c accelerated particle).
    Last edited: Oct 31, 2009
  20. Oct 31, 2009 #19


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you are interested in learning the basics of physics you would not start with relativity and the speed of light. If you want to learn the basics, start with classical mechanics and electromagnetism.

    With learning electromagnetism you would find that Clerk Maxwell derived the speed of light from electromagnetic theory in 1867. As mentioned earlier in this thread the value depends upon the permittivity and permeability of free space.

    [tex] c = \frac 1 {\sqrt {\mu_0 \epsilon_0}} [/tex]

    This is not a result that is included in "basic" physics, you need sophicated math, (partial differential equations} and a good understanding of electromagnetism. You will not see this derivation until upper division undergrad work in most universities.
  21. Oct 31, 2009 #20
    Thanks Integral. I will certainly look into it. However. for now I find this problem of derivation of the value of c a closed case.

    Maybe I should open a new thread on the question of the relativity of gravity alone so that those 2 topics don't mix anymore. Is that question that's more interesting for me right now.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Value of c and strange things that don't happen when at c
  1. Beyond c what happens? (Replies: 4)