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

General newbie questions regarding the speed of light

  1. Oct 10, 2007 #1
    I'm just begininning to undrestand the concepts laid out in the special theory of relativity, however I do have some issues which I'm having trouble coping with and perhaps someone on here can help me sort them out. I understand that some of these questions are probably not worded correctly, but please have patience as I'm just begginning to learn about the theory.

    So my first question and the main reason why I started this thread is : Why is it that no frame of reference can pass by another frame of reference at a relative speed faster then "c"? It's hard to swallow that no two frames of reference in our entire universe are moving in such a way and at such velocities that they never move at speeds higher then "c"?

    And my second question is:

    Is the belief among those who understand the theory of special relativity that time and space is ACTUALLY not absolute in reality? That what's occuring in the universe right now might be the same for us inertial observers on earth, but not necessarily to those travelling at different velocities to ours? I know it's an amateurish question, but I can't help asking since my inuitions about the absolutness of time/space are seriously limiting my ability to grasp the concepts of WHY time dilates or WHY lengths contact. Is it hopeless to question WHY it's happening, is there an answer that will satisfy intuition, or should I just be satisfied with knowing HOW, understanding the equations, predictions, consequences?

    Case in point: Right NOW I'm sitting on my laptop typing this out. Isn't this very MOMENT the exact same for EVERYONE in the universe regardless of speeds, distances, direction of movement, etc, etc???? Now at THIS VERY MOMENT that I'm typing this (or that you're reading this), is it true according to the theory that individuals in this very same universe are at a moment where THIS isn't happening right NOW (that I'm NOT typing right now, or that you're not reading this right now)? If I'm NOT typing this right now then what am I doing? Since its verifiable that i'm typing this right now, isn't it absolute fact that anywhere you are in the universe, moving at any speed i'm still typing this RIGHT at this moment?? Do some individuals in our universe exist in a state where I'm JUST booting up my computer right now, or already sleeping after posting this?? Does no one else see how crazy this **** is if it's true? Doesn't this just absolutley boggle everyones mind? How is it that I'm the only one that thinks this is absolutle bat **** crazy?

    If this is too mumble jumbo I understand, but I really needed to get this off my chest, and you guys are the only ones that seem to be capable of helping me or at least setting me straight. Thanks for your time.
  2. jcsd
  3. Oct 10, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Consider the SR velocity composition formula:

    v = (v1+v2) / (1 + v1*v2/c^2)

    If A is moving with a velocity of v1 relative to B, and C is moving with a velocity of v2 relative to B, the velocity composition formula tells you the velocity of A relative to C.

    No matter what v1 and v2 are, v is less than 'c'. Of course, this doesn't tell you why the velocity composition formula is true, but at least you'll hopefully see where your blind spots are.

    Yes. Different observers have different concepts of 'now'. This is known as the 'relativity of simultaneity'.

    You might want to note that relativity has a concept of cause and effect that is not related to 'now'. So pre-relataivity thinking might be that earlier events can affect later events, but later events cannot affect earlier events - cause must preceed effect, with a universal ordering of events.

    Post relativity thinking is that events cannot have effects outside their light cones.

    So there is still a concept of cause and effect in relativity, but it can't be used to create a universal ordering of events.
    Last edited: Oct 10, 2007
  4. Oct 10, 2007 #3
    I understand the reason as to why the answer can never exceed 'c' mathematically, I can't understand however, WHY 'c' must be used. What is the explanation given OTHER then "it works"? How do I have to alter my perception of the intuition of time that I have, in order to have one that's compatible with the theory?

    This is exactly what I'm talking about when I say I can't fight off my intuition about how wrong this feels. How does this happen? Do you just accept it as fact because the measurements work out, or do you actually understand why it behaves this way? Or perhaps you believe that there's no reason why it SHOULDN'T behave like this?
  5. Oct 11, 2007 #4
    It feels wrong to me, too. About c, I cannot say much, but I try to play down all relative concepts (such as simultaneity or relative speed) and stick to absolute concepts whenever possible.

    If maximum signal speed is finite, as it seems, we cannot really observe a distant event in real-time. So why should it matter, if we (by some assumptions) calculated it to be simultaneous to some other event? Maybe we have calculated incorrectly - we never know without direct observation. I'm also very doubtful of any speed measurements, since the finite signal speed is always somehow involved.

    Local events and causality, instead, are absolute concepts. If two particles collide at certain point of spacetime, they do, and there is nothing relative in this. Two events A and B either are causally connected, or not related anyhow. Only these kind of concepts are real to me.
  6. Oct 11, 2007 #5
    Hi Newb good question, as im a beginner and asking myself the same sort of questions. Although im not sure you will get the answers you require to make things feel right in your head.
    I think what happens in the universe is so different to what we feel is 'normal' in our everyday lives that you may never be able to 'picture it' in your brain and make it feel normal.

    Like you I dont understand why c is the limit, and how things such as a 'limit' can even exist.
    Obviously there are situations like two galaxies moving away from each other at 0.8c means the distance is increasing at greater than c from a 3rd party perspective, but if you look from either of the galaxies themselves, the other one doesnt seem to be going away faster than c. (I think im right there?)

    What i've always thought in the past, using your example of the laptop and typing, people at a great distance may see you are actually still typing now, but is that just because the light is only just reaching them, or for some other reason. (ie, it 'feels' like you are typing now just because thats what they are 'seeing' but it actually happened a while back.)
    Or more recently im thinking I am wrong there and the event is 'actually' happening later.
    if only there was a frame of reference that covered the whole universe it would make things a lot easier.

    ill read that above link from Pervect and see if its any clearer.
    it does make my head spin but i love it.! and i keep on learning all the time.
    Last edited: Oct 11, 2007
  7. Oct 11, 2007 #6
    I think you're partly correct here, although it would be helpfull if someone who actually knew the answer could explain it, but I believe that your first example of people at great distances seeing the "past" is simply due to signal delay, and has nothing to do with the actual REALITY of the situation.

    If a year from right now someone on a planet 1 Light-year away, looked into a telescope they'd essentially see light hitting them from a year ago and they'd be able to see me sitting here on my laptop. However, with simple math (distance of earth from their location / speed of light) they'd be able to understand that what they're actually seeing occured a year ago.

    What relativity suggests, is that it has nothing to do with signal delay, and that AS A MATTER OF FACT, weird things are happening and things that are simultaneous to someone aren't to others moving at a different relative speed. Space and time IN REALITY, is not absolute, but the speed 'c', somehow is.... At least that's what I take from it.

    Maybe I just have to think about it more, but it's just so counter-intuitive that I have to go back and step by step break up my beliefs till I come up with something closer to the truth.... But one thing I can't/won't do is simply accept the calculations (which I believe are correct) and accept that my current beliefs are right (since they obviously are not).
  8. Oct 11, 2007 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    It doesn't bother me nearly as much as it does you, and I can incorporate the idea into my intuition, which is what I think you mean by "understand".

    One thing you might try is to attempt to not use any notion of "now", i.e. a way of synchronizing clocks, in any of your calculations. Regard only time intervals measured by a single clock as being "real". Restrict yourself to comparing two clocks only when they are at exactly the same location.

    Probably the biggest stumbling block here is the concept of velocity, which as it is usually defined requires two clocks and a way of synchronizing them. But one can regard "celerity" as a different concept which represents the generic idea of "speed" even though it's numerically different (see for example http://arxiv.org/abs/physics/0608040 for a defintion of celerity, and thanks to robphy for pointing this article out to me.) In nutshell, one uses only one clock to measure celerity, for instance if one want to measure the celerity of a plane, one could put a clock on the plane, measure out a course of known length in the lab frame, and divide the distance in the lab frame by the time on the plane clock it takes to traverse this distance. Celerity, unlike velocity, has no upper bound.

    To convert celerity to velocity, one assumes velocity is some function of celerity, which is independent of direction and location (i.e. velocity is an homogeneous and isotropic function of celerity). This is basically the usual assumption of homogeneity and isotropy one needs to do SR.

    With the exception of velocity, there's usually no reason one even has to define any notion of simultaneity. The other reason I'm aware of for defining a notion of "now" is to make sense of cause and effect, which I already talked about a bit.

    Other than the particular definition of velocity, and for use in "cause and effect" (which I've already talked about) I am not aware of any particular requirement to use the idea of "now". Perhaps you're used to using it, but try doing without it for a bit, and see what happens.
    Last edited: Oct 11, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook