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C constant

  1. Sep 12, 2006 #1
    I get the theory of special relativity, it is the logical conclusion drawn from the two facts that:
    a) the laws of physics are the same in all reference frames
    b) the speed of light is constant in all reference frames

    what I dont get is why einstein thought the speed of light was constant in all reference frames. What proof of that is there even now? I know maxwell came up with c which is like 1/(m0*e0) or something which kinda suggests it but why couldnt that just be the initial velocity of light if the emitter was at rest with the "global reference frame".

    Also, Im not convinced that theory is true. I mean, Ive heard of all these experiments "proving" general and special relativity, but Ive never seen any documentation of any or any real numbers. Ive searched to... Can someone please give me some good links before I start talking about a giant physics conspiracy?
     
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  3. Sep 12, 2006 #2

    russ_watters

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    Well first, there are already a couple of active threads asking this same question. Have a look at them.

    Second, an awful lot of people have put an awful lot of effort into this concept over the past 100 years. Don't be so quick to think there is a conspiracy: accept your own ignorance and make an effort to learn.

    Here's a start: google "gps relativity" and "Michelson Morley experiment"
     
  4. Sep 12, 2006 #3
    dude I was joking, I dont actually think relativity is wrong Im just curious how einstein came up with it. Ive been taught it 3 times and they always tell me the postulates but they dont say how he came up with them or why they are known to be true. And btw, my effort to learn was asking on a forum..
     
  5. Sep 12, 2006 #4
    and Ive seen the michelson-morley experiment, it proves nothing to me except that there cant be an aether stationary to the sun's reference frame.
     
  6. Sep 12, 2006 #5

    rbj

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    no, what it means is that the aether is moving around the Sun with the Earth. in the spring season when the Earth is moving the opposite direction around the Sun as it is moving now, the aether continues to follow us around the sun which is why it's so damn hard to detect a fringe shift in the Michaelson-Morley experiment. really, the aether is really :wink: there, it's just that we can't detect it because we are never moving through it. and that is because it moves along with the Earth.
     
  7. Sep 13, 2006 #6

    Aether

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    He didn't say that for "all reference frames", he said that for "all inertial reference frames".
    Inertial reference frames are defined as those reference frames (or coordinate systems) in which Newton's laws hold, and one of the requirements for that is that all one-way speeds are isotropic.
    They are called "postulates" to indicate that they are to be accepted as true for the sake of argument. They can't be proven by experiment.
    Lorentz ether theory (aka GGT) is empirically equivalent to SR.
     
    Last edited: Sep 13, 2006
  8. Sep 13, 2006 #7

    russ_watters

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    Fair enough, but there are an awful lot of crackpots out there and we're wary of that here.
     
  9. Sep 13, 2006 #8
    Sorry to butt in, but I'm one of the crackpots. I've been looking at this for a while now, without the baggage of a formal eduation in the subject.

    Apologies if I've got this wrong, but are we saying that Relativity doesn't apply in a Universe where acceleration is possible ?

    Can we really have an 'accepted' scientific theory that only works if nothing accelerates ?

    ( Deliberately argumetatively phrased, but a serious question anyway ? )
     
  10. Sep 13, 2006 #9

    ZapperZ

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    You got it wrong.

    What have been talked about is confined only to Special Relativity. It is General Relativity, not the topic of this discussion, that includes accelerating and gravitational fields. Physics and physicists are not that dumb to ignore such a huge hole in a theory, especially when there are plenty of empiricial evidence.

    Zz.
     
  11. Sep 13, 2006 #10
    Ok. Apologies. Just trying to get someone to explain to me how it works. No matter how many texts I see, it just doesn't sink in. Guess I'll have to keep trying.
     
  12. Sep 13, 2006 #11
    I agree with Michael that the "Michelson Morley experiment" does not prove a thing except that there is no aether to effect the speed of light. If I never see this posted as a proof of SR again I will be happy.

    Russ. Could you please give us a short little explanation of how the global positioning system proves SR. Thanks.
     
  13. Sep 13, 2006 #12

    ZapperZ

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    First of there, there's nothing in physics that is "proven" the same way you do for mathematics. Experimental observations are used to verify a theory or principle. The more observations of different types are discovered, the higher the degree of certainty, and the more convincing it becomes.

    Secondly, here are all the experimental results that are totally consistent with the postulates of SR. So knock yourself out.

    1. "Severe Limits on Variations of the Speed of Light with Frequency", B. Schaefer, PRL v.82, p.4964 (1999). Also see Physics News Update report at http://www.aip.org/enews/physnews/1999/split/pnu432-2.htm . This is the most accurate measurement to-date that c is independent of frequency/wavelength. If photons have any mass, or if c isn't a constant, this would manifest itself as a variation in speed at different frequencies. So far, none has been detected.

    2. http://www.aip.org/enews/physnews/2000/split/pnu484-1.htm . This is the most recent and accurate determination that the speed of light is independent of the speed of the source.

    3. http://www.aip.org/enews/physnews/2002/split/590-1.html . Again, this is the mostp recise test yet that the speed of light is independent of the direction of propagation.

    4. "Tests of Relativity Using a Cryogenic Optical Resonator", C. Braxmaier et al., PRL v.88, p.010401 (2002). Ether? What ether? This is the most precise determination to-date that the speed of light is independent of the velocity of the lab frame. The experiment used a version of the famous Morley-Michealson interferometer called the Kennedy-Thorndike test. You may read the Physics News Update report at http://www.aip.org/enews/physnews/2002/split/571-1.html .

    In the 14th Feb. issue of Phys. Rev. Lett., there is not one, but TWO new experimental results that put a severe limit on any possible violation of the Lorentz transformation (which is built-in in Special Relativity). These two experiments present the most accurate result so far that c is velocity and earth-orientation independent. You may read the summary of one of this result at the AIP Physics News Update:

    http://www.aip.org/enews/physnews/2003/split/623-2.html

    or better yet, read the actual papers in PRL. These two experimental evidence will be numbered 5 and 6 (in addition to the first 4 that were isted earlier).

    5. "Tests of Lorentz Invariance using a Microwave Resonator", P. Wolf et al., PRL v.90, p.060403 (2003).

    6. "New Limit on Signals of Lorentz Violation in Electrodynamics", J.A. Lipa et al., v.90, p.060403 (2003).

    7. This time, the evidence comes from the most accurate measurement to date of the uniformity of c using a modern version of the infamous Morley-Michealson experiment.[1] Using cryogenic optical resonators, they measured for the possible anisotropy in the speed of light for over a year (as the earth moves through space in its orbit around the sun and thus, changing its orientiation). The showed with unprecedented accuracy that the upper limit for any possible variation in c would have to be lower than 2.5 x 10^-15, which is 3 times more accurate than previous measurements.

    Muller et al., PRL v.91, p.020401 (2003).

    8. we have another experimental evidence for the constancy of the speed of light - this time coming from very low frequency radio waves in the frequency range of 5 to 50 Hz.[1] Again, this measurement places the upper limit on the photon rest mass (if any) at less than 4 x 10^-52 kg (yikes!).

    M. Fullekrug, PRL v.93, p.043901 (2004).

    Do you want more?

    Zz.
     
  14. Sep 13, 2006 #13

    ZapperZ

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    Then maybe you should write a paper to debunk this, because complaining about it on here means nothing.

    And why, since you agreed with Micheal, that MM experiment "... it proves nothing to me except that there cant be an aether stationary to the sun's reference frame.." have anything to do with the sun in the first place is beyond me. Pick whatever "stational ether" in whatever inertial reference frame that you want. Now predict how such thing can be detected.

    http://www.physicscentral.com/writers/2000/will.html

    Zz.
     
    Last edited: Sep 13, 2006
  15. Sep 13, 2006 #14
    1. ... bases this estimate on the observed arrival of gamma rays from distant explosive events in the cosmos, such as gamma-ray bursters. If the speed of light (c) were slightly different for the different frequency ranges, then some light waves would show up before the others, but this is not the case. ...


    This seems to be the entire proof that the speed of light is constant ? If it were different for different frequencies of light, a distant event would first appear red / blue rather than white ?

    Is this logically correct ?
     
    Last edited: Sep 13, 2006
  16. Sep 13, 2006 #15

    I'm not so sure ... looking at the results of the experiment it seemed to prove that the speed of light isn't constant ??

    I think that's the point, it disproved a constant speed of light so they had to come up with something else so that everything they'd said over the preceding years wasn't completely wrong ??
     
  17. Sep 13, 2006 #16

    ZapperZ

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    Look at the dispersion law!

    Pass a "white" light through a prism (it is, after all, a "medium"). Why do you think all the different frequency separates?

    Furthermore, different frequencies carry different amount of energy. Unless you are also suggesting another crackpottery of light having a rest mass and that this mass varies with its energy content, this means that all of its energy is in its "motion", meaning different energy would have different velocity.

    Zz.
     
  18. Sep 13, 2006 #17
    No. Don't get the question. Isn't that just because of the different frequencies ? Apoligies for my ignorance, but how is it relevant ?

    I'd agree that different frequencies carry different amounts of energy, but again, how does this help ?
     
  19. Sep 13, 2006 #18

    ZapperZ

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    Different frequencies have different index of refraction in a medium. The index of refraction is the ratio of the speed of light in vacuum to the speed of light in that medium. Now do you get it?

    If all the energy is only in motion, this is the kinetic energy, is it not? You don't see the connection with velocity here? Er... what level of physics do you have? This is not meant as a degoratory question. I have to know at least SOME level of physics to which I can assume you know to base my answers on. You do see the problem here, don't you? I feel like I have to backpeddle several steps to explain an answer to an answer.

    Zz.
     
  20. Sep 13, 2006 #19

    Your reply is a bit 'derogatory', but reasonably accurate and I'm not that easily insulted.

    I can see where you're going with the refraction ratio and pretty much where you're going with the kinetic energy thing, but what has this got to do with the original question :


    1. ... bases this estimate on the observed arrival of gamma rays from distant explosive events in the cosmos, such as gamma-ray bursters. If the speed of light (c) were slightly different for the different frequency ranges, then some light waves would show up before the others, but this is not the case. ...


    How does this prove that the speed of light is constant ?
     
  21. Sep 13, 2006 #20

    ZapperZ

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    Er... "different frequency ranges" .... meaning different energies, meaning different velocities IF you put all the energy into motion. Isn't that what I've been trying to point out?

    I have no idea how to make this any clearer.

    You still did not elaborate on your background, so I will continue to assume that you do know at least some undergraduate physics. I will not backpeddle anymore.

    Zz.
     
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