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Relativity, speed of light and stuff

  1. Aug 26, 2008 #1
    Hi guys!

    this is my first post here, and I would like to ask a few questions concerning relativity, speed of light and concerning stuff.
    I know that a lot has already been talked around here about this areas, but I have not found one thread that puts this issue in the way I would like to put it.

    First, I would like to make just a few points about my background:
    -I am not a physic, nor I have some background in physics, apart from school teaching, obviously
    -I've read some books, threads, posts and articles about physics, but that is pretty much my background!
    -So, please, if you mind to answer this thread be critic. If I'll say something very stupid, please state so! just let me know. I really love to learn and discuss this topics.





    So, my issues:

    My very first doubt urged when I read about einstein. I am a strong fan of Newton, but I have to admit I do not "like" the way Einstein aproaches all concepts in his theories.

    My point is just a simple one: I do not understand why do physics consider light so much special! Why is its speed the limit!

    I have read a lot about this, and how the very nature of light (not a wave, but a wave-particle) is so important to define speed of light as the limit. or even as it is calculated from the relations within universe laws.

    My point is just this one - if we are sublight beings (we work tops at light speed minus chemicals relations in our neurons), if all our creations are sublight (a computer cant work at light speed) ... ... ...

    how can we measure light speed if we work, tops, at sublight? how can we say that there is nothing that can travel faster than speed of light if we cant "see" at over speed of light?


    this is - if we had no eyes, and only ears, wouldnt we think that sound speed would be the greatest speed ever? wouldnt Einstein put "c" as the speed of sound?

    of course we never experienced for sure faster than light experiences. but is that possible? we have no machines working at that speed! Is there a flaw in my reasoning?

    and i even cant understand that argument people usually say, that if one travels that speed of light it would almost like we do something before it occurs - how is that???

    yes, we would do something and then travel faster than the speed of light, but that's not time travel. It is just like making a sound and then travelling at mach 3, only multiplied many times, but not even close to "change" time line

    This is not so obvious for me, and I have never read anything that answered this question clearly.


    I have also read something stating that it actually makes sense once we are the ones to "analyse" all this phenomena, and it makes sense to the actual observer (us, humans) but for me physics is the study of nature rules, and not nature rules as seen by humans. At least, that's the way I see it!


    Hope to discuss this further with you!


    Cheers,
    ricmat
     
  2. jcsd
  3. Aug 26, 2008 #2

    berkeman

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    Staff: Mentor

  4. Aug 26, 2008 #3

    JesseM

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    The "specialness" of the speed of light basically has to do with the way the laws of physics look in different coordinate systems. If you know something about Newtonian physics you may be familiar with the idea of inertial coordinate systems, where different inertial (non-accelerating) observers can construct coordinate systems to assign position and time coordinates to different events, and each observer's own rest frame is the one where his own position coordinate isn't changing with time. In Newtonian coordinate systems, if you have two observers in motion relative to one another, then naturally they will get different values for the speed of some object in their own coordinate systems. This means that in Newtonian coordinate systems, if Maxwell's equations of electromagnetism work in one observer's coordinate system they can't work in the coordinate system of a different observer moving relative to the first, since Maxwell's equations predict that the speed of light is c, and if they were c in the coordinate system of the first observer and the second observer was moving at speed v relative to the first, then the second observer would measure the light to be moving at v+c or v-c. So Einstein's work was in part an attempt to construct a different set of coordinate systems than the Newtonian ones, such that Maxwell's laws could work correctly in every observer's coordinate system. The set of coordinate systems he found are related to one another by a set of equations called the "Lorentz transformation", which is different from the "Galilei transformation" which relates the coordinates of different inertial observers in Newtonian physics. Einstein also made the postulate that if we use such coordinates, all the fundamental laws of physics (not just Maxwell's laws) will turn out to obey the same equations in all these coordinate systems, a property known as "Lorentz-invariance". This postulate has shown to be correct as more fundamental laws continued to be discovered (asking why all the laws of physics obey this symmetry is not really something that physicists can answer, their job is only to discover what the laws of nature are like, not why the laws have the particular form they do and not some other). You could indeed construct a different set of coordinate systems where the speed of sound waves (or some other object) was the same in each coordinate system, the difference is that the known laws of physics would not show the same sort of symmetry with respect to these coordinate systems.
     
  5. Aug 26, 2008 #4

    rbj

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    it's not just visible light.
    nor is it just electromagnetic interaction (of which light is).

    all fundamental interactions (caused by a generating agent) have their effect (on the responding agent) delayed by a time (as observed by a third party equal distant from the two agents) that is proportional to the distance between the cause and effect agent. whether the two of us are holding charges that are attracted (by the E&M action) to each other or the two of us are much bigger and holding planets that are attracted (by gravitation) to each other, if i wave my charge (or planet) around, your charge (or planet) will be disturbed accordingly. the time between my agitation and the disturbance you detect will be longer (as observed by the third party) if the distance between you and me is longer. that implies a speed of interaction. that speed is the same whether it's E&M (light), gravity, or the nuclear forces. and there aren't any other interactions that we know about. it's a property of space and time (and the relationship betwixt the two), not specifically of light, nuclear forces, or gravitational attraction.
     
  6. Aug 27, 2008 #5

    Dale

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    Hi ricmat, welcome to PF

    I have to admit that I didn't understand special relativity for several years using exactly this same line of reasoning. The key difference between sound and light is that sound requires a medium in which to propagate and light does not.

    The speed of sound is only constant wrt its medium, and can be measured faster or slower depending on the speed of the medium wrt the measuring apparatus. We can detect the motion of the medium with many other techniques (e.g. finger in the wind) and accurately determine what the speed of sound should be in our reference frame.

    By contrast, since there is no medium for light, the speed of light is never measured faster or slower than c regardless of the speed of the measuring apparatus. We cannot in any way detect the motion of empty space (this is essentially what M&M were trying to do), and so the speed of light is always c in any reference frame.

    I hope this helps, because I understand your reasoning since I used it too for many years.
     
  7. Aug 27, 2008 #6

    atyy

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    Although we are sublight beings, we can measure the speed of light by making it go over a very great distance, so that the time interval measured is very long.

    But this is not so relevant. The constancy of the speed of light actually comes from the Newtonian idea that if you are in a car, the cars going in the same direction will seem to move more slowly, and cars going in the opposite direction will seem to approach more quickly than if you were standing by the road.

    In the Michelson-Morley experiment, the car is the earth going round the sun. At different times of the year, the earth is going in different directions, and so if light is moving with respect to some external medium, it should change its speed according to the time of the year. The change should be around 15 m/s, which perhaps you will more readily believe sublight beings can detect?
     
  8. Aug 27, 2008 #7
    Thank you guys for your quick answers.

    a lot of food for thought to digest, and little time to do so! so sorry for not being able to address all points, but i'll do soon.


    just a quick answer to DaleSpam:



    so great i find a person who has already thought the same way. I just have one question, as I seem to find a flaw in your reasoning: light does not travel the same way in each mean! as we can find in the link above mentioned:

    "Light traveling through a medium such as air (for example, this laser) travels slower than light through a vacuum."


    my question is - yes, sound do not travels in vacuum, but they are different kind of waves! one is mechanic the other one is electromagnetic, so that maybe the reason!
    I would just like to recall that before einstein we thought that there was no vacuum, and light traveled through ether... i have a phd friend in cosmology, who told me very recently that this very same thesis as come up more and more in present days again... it seem that there is no consensus among today scientist that there is no "something" like ether as opposite to vacuum....
     
  9. Aug 27, 2008 #8
    thats perfectly true, but if you became length contracted, time dilated, and experienced loss of simultaneity as you approached mach one then you would not be able to detect any change in the speed of sound.
     
  10. Aug 27, 2008 #9
    Ahh, but the speed of light in a medium is irrelevant. There's no law saying that the speed of light in glass is a universal constant - in fact it's perfectly okay to travel faster than the speed of light passing through a medium, and this can produce Cherenkov radiation.
    Special relativity is based upon the speed of light in a vaccuum only. There is something special about this speed BECAUSE then light is travelling only through a vaccuum... get me?
     
  11. Aug 27, 2008 #10

    Dale

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    The constant, c, refers to the speed of light exclusively in vacuum, which is the same for all inertial observers. I agree with FeynmanMH42's comments above.
     
  12. Aug 27, 2008 #11

    Dale

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    Yes, and if such things happened then the invariant speed would be mach one, light would travel at the speed of sound, and tornados would be relativistic phenomena.
     
  13. Aug 27, 2008 #12
    of course. but nobody would be claiming that because its velocity is the same for all observers that sound doesnt have a medium.
     
  14. Aug 27, 2008 #13

    Dale

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    Of course not. But think about it. Why not?
     
  15. Aug 27, 2008 #14
    I imagine that you mean that we cant stick our hand out the window and feel the breeze of the aether going by. but particles are waves too. If we are also waves in the aether then we should hardly expect to be able to do so.
     
  16. Aug 27, 2008 #15

    Dale

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    Precisely, there are many physical experiments that we can do to measure the velocity of air. There are no experiments that we can do to measure the velocity of the vacuum.
     
    Last edited: Aug 27, 2008
  17. Aug 27, 2008 #16

    rbj

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    i might claim that, given the premise (which we don't believe).
     
  18. Aug 28, 2008 #17

    Doc Al

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    The speed of sound is not the same for all observers.
     
  19. Aug 28, 2008 #18
    you seem to have missed the 'would' in my post.

    read post 8
     
  20. Aug 28, 2008 #19

    Doc Al

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    Oops. My bad.
     
  21. Aug 29, 2008 #20
    An engineer, a mathematician, and a physicist were travelling through Scotland when they saw a black sheep through the window of the train.
    "Aha," says the engineer, "I see that Scottish sheep are black."
    "no," says the mathematician, "You mean that some Scottish sheep are black."
    unable to agree, they asked the physicist to decide which was the correct statement.
    The physicist thought for a moment then said "there is one sheep in Scotland and it is black on one side".
     
    Last edited: Aug 29, 2008
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