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Need some clarifications about light speed

  1. Jun 11, 2004 #1
    the other thread got the lockdown before i got a chance to gather all i could from it <sad>

    so i guess i have to start a new one.

    when people say "The speed of light is constant in a vaccum at C or 299,060 kps" (or whatever the freaking number is) what exactly do they mean?

    from the last post we've determined that all things are relative and all realities are valid, so when they did this test for light speed what was it relative to? seems kind of unsafe to say that it's constant everywhere there's vaccuum. maybe it's just in our solar system or our galaxy that light behaves this way.

    or even more so, light speed had to have been measured relative to some "stationary" position which isn't allowed because who's to say that position is actually stationary?

    so it comes down to that light speed is constant in a vaccuum RELATIVE to SOMETHING that is stationary.

    and BECAUSE it is constant then stationary frame(s) MUST exist. these two things are mutually co-dependant.

    to make it simpler. the speed of light can only be constant relative to something stationary. it CANNOT be constant relative to something that can move or does move.

    so here comes a kicker. if we devise a space instrument that consists of 4 strobes on poles, some receptors in the center, and some thrusters, oh ya and some computing power ;D. set this thing adrift in space with the right program to measure time from strobes to receptors, and use the thrusters to compensate for the deiscrepancies from all 4 strobes, then we can come up with an object that sit ABSOLUTELY STILL in space.

    relative to the whole freaking universe :D (if our assumptions are true)
  2. jcsd
  3. Jun 11, 2004 #2
    In #1, ram2048 said, "....the speed of light can only be constant relative to something stationary. it CANNOT be constant relative to something that can move or does move."

    Yes it can, and it is. The speed of light in a vacuum is constant relative to whoever measures it. That constant speed is c. It doesn't matter whether you're moving or what you're moving relative to (including the source of the light!). It doesn't matter. You'll always get c when you measure the speed of light.

    That seems counterintuitive. But we aren't born with intuition; it comes purely from our experiences in the world. So intuition can fail if we try to apply it to situations with which we have no experience. Most people have no experience with speeds anywhere close to that of light. So we have no valid intuition about what things are like at that kind of speed.

    A good analogy is how counterintuitive it must have seemed to people when they were first told the world was round. All their experiences were with travel over very short distances. And over short distances the world seemed flat. Why should that change for long distances? And if they kept going far enough around, wouldn't they eventually be upside down and maybe even fall off? Surely this round theory had to be wrong; it was so counterintuitive!

    Lesson? Intuition is useful for getting us through our day to day lives. But when you set out to understand basic truths about the physical universe, it's best to leave your intuition behind, and base your conclusions on what has been objectively measured. Only a very few, very brilliant physicists seem able to use intuition without running amock!
  4. Jun 11, 2004 #3


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    There have been many threads covering this.

    The constancy of c was first derived by Clerk Maxwell in about 1867 when he formulated the fundamental relationships of Electromagnetism (Now called Maxwell's Equations) in the form of the wave equation. In this equation the speed of the wave appears in a characteristic manner. The quantity which represented the speed of a electromagnetic wave (which had not been experimental observed at that time) was the quantity

    [tex]{\sqrt \frac 1 {\epsilon_0 \mu_0}} [/tex]
    Where the Permittivity of free space is
    [tex] \epsilon_0 = 8.85 x 10^{-12} \frac F m [/tex]

    and the Permeability of free space is
    [tex] \mu_0 = 1.26 x 10^{-6} \frac H m [/tex]

    If you work out the basic units and do the math you will find c. Maxwell observed this agreement with the experimental value of the speed of light with some amazement. To the best of my knowledge this was the first real evidence that light was electromagnetic in nature. The implications of this result are startling, simply because the speed of light is NOT DEPENDENT on the motion of the observer but on fundamental and well known constants of the universe. When this result was published it initiated a half century of turmoil in Physics community which finally ended in Einstein's publication of Special Relativity.
    Last edited: Jun 11, 2004
  5. Jun 11, 2004 #4

    Well, I know we've had this argument before, but it wasn't Maxwell's discovery that caused the turmoil. Maxwell believed there was an ether, and he believed that the speed in his wave equation was wrt the ether. That was perfectly consistent with all other wave motion. It wasn't until the MM experiment failed to detect the ether, that all the turmoil started. A wave without a medium? Impossible!

    In fact during the time between Maxwell's discovery and the MM experiment, many physicists believed that their work was pretty much complete, that there was no more physics to discover. You can check it out.
  6. Jun 11, 2004 #5


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    One would wonder then why they bothered with the MM experiment? The reason it was done was to COUNTER Maxwells result, thus it is known as the experiment that failed.
  7. Jun 11, 2004 #6


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    Well, even if there were an ether, determining the 'ether wind' would be an interesting physical experiment, much like Cavendish weighing the earth.
  8. Jun 11, 2004 #7
    i think you must have it wrong...

    if this were the case then shooting a photon at something 10 light seconds away, then running after it as 1/2 the speed if light, the photon would be moving at 1.5 c in order to maintain speed of C relative to me.

    it can't be constant relative to something moving. i think you must be measuring it wrong :|
  9. Jun 11, 2004 #8

    I agree. How on God's green earth can something alter it's relationship dependent on frame of reference and yet maintain a different relationship with another frame at the same time?
    Something is seriously wrong here.
  10. Jun 11, 2004 #9


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    Well let's have a look at what special relativity has to say then:

    We know that length contraction and time dialtion occur governed by the equations:

    x' = γ(x - vt)

    t' = γ(t - vx/c2)

    therefore we can say:

    Δx'/Δt' = (Δx - vΔt)/(Δt - vΔx/c2)

    So let's put in your figures which are v = c/2 and Δx/Δt = c (as the photons speed is constant we can say vphoton = Δx/Δt), using the substituion &Delta;x = c&Delta;t.

    Δx'/Δt' = (c&Delta;t - c&Delta;t/2)/(&Delta;t - c2&Delta;t/2c2) = (c&Delta;t/2)/(&Delta;t/2) = c

    Therfore the velocity of the phton in the primed frame is also c due to the effects of time dialtion and length contraction (of course should've expected this as time dialtion and lenght contraction are derived from the constancy of c in all inertial refrence frames).
  11. Jun 11, 2004 #10
    ram said, "it can't be constant relative to something moving. i think you must be measuring it wrong :|"

    After all the nonsense you've posted in the last few days, my intuition told me not to respond to your post. This time I should have followed my intuition.

    Jerk somebody else's chain; I've got plenty of time, but not enough for you. You'll never understand any physics.
  12. Jun 11, 2004 #11
    o.. k...

    time dialation has nothing to do with the rate of covering distance.

    velocity means it will cover X distance in X time. there is no variable for perception.

    the photon is GOING to travel 299,792,458 meters in one second and 2,997,924,580 meters in 10 seconds.

    this is a what's commonly refered to as being constant

    now if you say this is RELATIVE to ANYTHING, moving or non, it means that going 20 miles an hour or 299,792,457 meters per second, that photon will still be ACTUALLY moving 299,792,458 MORE than THAT speed.

    which makes it NOT constant.
  13. Jun 11, 2004 #12
    Integral said, "One would wonder then why they bothered with the MM experiment? The reason it was done was to COUNTER Maxwells result, thus it is known as the experiment that failed."

    Sorry, but you've got this totally wrong. There's a very famous story about Michelson getting his idea for measuring the speed of the earth through the ether after seeing a letter written by Maxwell suggesting such a measurement. Google maxwell and michelson; I'm sure you'll find it.
  14. Jun 11, 2004 #13


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    velocity does NOT mean "it will cover X distance in X time", but by making the simplifying assumptions of constant velocities along a single axis we are looking at the case where it does mean an object will cover "X distance in X time".

    The problem is you are using the assumptions of Galliean relativity which we now know to be the limiting case only.

    If you disagree with the results, go through the equations and their initial assumptions and pick the steps that you disagree with.
  15. Jun 11, 2004 #14


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    Even in Galliean relativty (which is what your argument is based on) you can see that the measured velcoties are dependent on the motion of the observers.
  16. Jun 11, 2004 #15


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    Gee! I wonder why MAXWELL would have ever sugested such an experiment. Seems to me you cannot see the forest for the trees.

    Please show me where an ether enters into

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

    Sugested reading
    Last edited: Jun 11, 2004
  17. Jun 11, 2004 #16
    i know of no other method of describing velocity other than direction + distance over time. Since the direction is irrelevant because we have stipulated movement in one axis it can be ignored.

    saying "the speed of light will not cover 299,792,458 meters in one second for a moving observer" is the same thing as saying it's NOT constant relative to the moving observer.

    which is what i've been trying to verify. the equations that Integral brought up base the speed of light upon "Permittivity" and "Permeability", two scales for quantifying space (i guess) but has anyone actually verified it using REAL TOOLS?

    We base SO much upon this constant light speed yet it seems to lack quite a bit of substansive proof
  18. Jun 11, 2004 #17
    doing some research i'm seeing that michelson's experiments in 1879 seem to find light speed at 186,350 miles per second with a likely error of around 30 miles per second

    so what happens when we take his experiment into space and give it an inertial reference speed (from earth) of 1/2 c?

    pointing the main vector of the light beam at the direction we're travelling, the splitter will shoot the beam off in two directions at right angles to one another.

    relative to the earth the light beam reflected "to the right" must travel with a "forward" motion at 1/2 C to keep up with the reference frame of the experiment.

    but relative to the earth, the light is actually travelling at an angle with a total velocity greater than the acceptable constant value of the speed of light
  19. Jun 11, 2004 #18
  20. Jun 11, 2004 #19
    That's easy. It's the velocity of a wave. And Maxwell believed it was the velocity of the wave with respect to the ether. Just like the velocity of all other waves is the speed at which they propagate through their mediums. The only issue anyone had with this was that the only other fundamental field that was known at the time (gravity) seemed to be independent of velocity. EM fields evidently weren't. But if there was really an ether, that made perfect sense. Then MM showed there was no ether, and they had to find something else for c to be relative to. Einstein postulated that c was the speed of light relative to whoever observed the light.
  21. Jun 11, 2004 #20
    IF lightspeed is in any way constant relative to something, it's going to conform to emitter theory (my opinion)

    basically wherever light is dropped into the universe or created from exists as a TRUE fixed position, universally so. Light's speed is constant and relative to that absolute position.

    what the universe does has an effect on how we measure it from there, be it bending space with gravity, or slowing the propagation medium with permeable mass (air water etc)

    think of it as a photon being a rotating wheel with a certain velocity equal to the rotation speed and size of the tire. this wheel has infinite traction with the universe when in contact with it. when introduced into the universe it immediately takes off at full speed depending on the medium it's in.

    well that's my take on it according to everything i've read about it so far. we'll see how that develops :D
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