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Is Special theory of relativity only about speed of light?

  1. Sep 29, 2011 #1
    I am a beginner in this field. I have read Special theory of relativity by Einstein, but the one written for beginners, not the technical. I have also read couple of undergraduate books which had STR. From my understanding, STR is a theory about transformation from one frame to another, both inertial. My question is, does all the notion of time and space being relative arise solely due to the fact that different observers traveling at different speeds perceiving events differently due to light reaching them at different times?
    Is light reaching observers differently the underlying idea of STR? I am confused. Or is it got to do with the fact that Space time curves at different relative speeds?..
  2. jcsd
  3. Sep 29, 2011 #2
    It's more than that; just as Newton's mechanics is more than that.
    No, it's not like that - but welcome to Physicsforums :smile:

    The underlying idea of SR is that the classical theory of optics and electromagnetics did not correspond to the relativity principle, while measurements strongly suggested that everything does obey the relativity principle. The main point of SR is the principle of relativity for all physics. The speed of light simply played an important role in the derivation of the right equations. And those equations predict that times and distances will be measured differently in standard inertial reference systems that are moving relative to each other.

    In the time leading up to SR it had been found that accelerated particles did not follow Newton's theory, and that their max. speed probably was equal to the speed of light. This concept of a limit speed is more essential to SR than the speed of light.
    Last edited: Sep 29, 2011
  4. Sep 29, 2011 #3


    Staff: Mentor

    Hi nikeadidas, welcome to PF!

    That is a very footwear-oriented handle :smile:
    Unfortunately, with the approach that many people take it is easy to arrive at that misconception. In fact, the three main effects of SR (time dilation, length contraction, relativity of simultaneity) have nothing to do with perception at all. In SR although we use the term "observer" a lot, an observer is considered to be intelligent and knows about the finite speed of light. So when an observer receives a signal he can work out when it was actually sent. In other words, the three main relativistic effects are what remain after an intelligent observer corrects their "perception" for the finite speed of light.

    The wierdness of SR arises not because light's finite speed causes delays but rather because light's speed is frame invariant. In other words, the wierd thing is that if you measure something to be moving at c I will also measure it to be moving at c even if we are moving at v wrt each other.
  5. Sep 29, 2011 #4


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    Just to clarify a bit about what DaleSpam said about light moving at c according to everyone. He means as measured relative to themselves.

    For example: Assume that I am running past you at 0.5c carrying a flashlight. As I pass you, I turn on my flashlight. At the same time you turn on your own flashlight.

    Here's what happens according to me: The light from both flashlights run neck and neck at c relative to me. (after 1 sec the front of both beams will be ~300,000 km in front of me.)

    Here's what happens according to you: The light from both flashlights run neck and neck at c relative to you. (after 1 sec the front of both beams will be ~300,000km away from you.) This means that according to you, after 1 sec the light will be only ~150,000 km in front of me.

    In order for both of us to come to such different conclusions of what happens with the same light, we must measure time and distance differently (which in turn leads to the three effects that DaleSpam mentioned), and that is the crux of what SR is about.
  6. Sep 29, 2011 #5
    Special relativity results from the very special and unique way in which the universe is structured. So, the speed of light is not the underlying reason for relativity, rather it is a notable consequence. However, physicists found that the speed of light was the same for all observers (and this has nothing to do with travel times between points), regardless of their inertial frame of reference (from Maxwell's theory and actual measurements). And once they understood this result, they knew something strange was going on. Einstein figured out what was going on -- again, motivated by the knowledge about the speed of light -- and presented this as the theory of special relativity.
  7. Sep 30, 2011 #6
    I'm utterly empty when it comes to the mathematics but I am working hard on understanding the concepts. The follow-on from stating that light's speed is constant (and which must be the case if space-time contract(s) to zero) regardless of the observer's POV is that Light itself does not move - space-time is only an issue at sub-light speeds. I know that this may or may not explain black hole behavior and how light is diverted by gravity and maybe we're getting to the stage where single-state results aren't possible. Many theories now have gone beyond quantum levels of multi-state scenarios so maybe light both moves and stays still at the same time?

    So what I'm trying to get my head around is that "speed"/"velocity" only applies when something traveling less than the speed of light is witnessing/experiencing light.

    If there is no "universe" as such through which light would experience moving, therefore, it seems like light is independent of the universe as a whole. Outside of an observer's point of view, does it not follow that all light must be everywhere at all times all at once - it is a single entity, like it would experience the universe as a singularity.

    I would normally think myself daft for asking some of these questions but then with the multi-verse theories starting to bubble through theoretical physics (Hawking's latest Black hole explanations), a lot that sounds like almost supernatural in nature is being pondered.

    In essence, has light been fully addressed as a problem in cosmology/physics past basic cause and effect empiricism? Is there still a lot we don't know in this regard? Are we approaching a point where multi-state solutions are melding with multi-verse/multi-dimensional solutions?
  8. Sep 30, 2011 #7
    Reading all of the above posts has made me revisit SR once again. I think what is confusing are the illustrations which are normally given, e.g the clock involving 2 mirrors. If you see, there the concept of time being relative comes due to the fact that the stationary observer observes the movement of light between 2 mirrors at different points in space. It gives an idea that the observer is simply perceiving time differently due to the fact that light waves reach him a bit late than what they would have had he been in the same frame of reference. To a novice like me, it gives an idea that this concept is applicable to only such hypothetical clocks, but what about clocks such as our wristwatch?
    However, in Feynman's book it has been explained beautifully. The very basic purpose/principle of relativity is that observer should not become aware of his inertial frame's movement while doing any experiment, whether concerning mechanics or electromagnetics. Thus, the clock in his hand has to run differently so as to account for Maxwell's laws in relative motion. It has to run according to that hypothetical clock, otherwise a difference between the two clocks will make the observer aware of his frame's movement through space.
    Why it behaves so is not known (I presume)...I guess its just a weird way the nature works!.
    Please guide me if I am wrong. I must confess I have'nt read how Lorentz transformations are derived. I work in a field which is not related to physics or Maths, so my mathematics is really constrained :)
    Last edited: Sep 30, 2011
  9. Sep 30, 2011 #8
    The point there is just to illustrate time dilation; and the same must be the case with any clock, else you could in principle measure absolute motion and the PoR would be broken.
    It would have been good if you had read Feynman's book first; the rest serves to enhance your understanding about how things work. :smile:
  10. Sep 30, 2011 #9


    Staff: Mentor

    Yes, that is why I don't like that style of presentation. We often see people with that same misconception and almost universally they have had exposure to the same type of teaching style. It seems to me that it is a confusion that could be avoided if teachers and introductory references would not focus so much on "thought experiments".
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