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

Time dilation confusions

  1. Feb 28, 2015 #1
    Hello! I have some confusions about this topic, I'm a physics newbie so my question may have no sense, please have mercy ;). It's supossed that relativity only considers observation as valid "what we see", but observation it's independant of what we know (we can't tell how distant we are from different objects without previous information, we need another relative system for that). Does this mean that relative observation is bidimensional? If so, it would be impossible to calculate absolute parameters whithout changing the observation point? If so the speed of light would be actually slower for some observers right? This thought has got me really intrigued.
  2. jcsd
  3. Feb 28, 2015 #2


    User Avatar
    Gold Member

    What specifically are you referring to ? "It's supposed that relativity..." doesn't make that clear, is this a statement you found somewhere? A link would be helpful.
  4. Feb 28, 2015 #3
    That in a relative system our observed parameters are different vs another relative system. For example, for observation we can only tell that the sun is a point moving on a plane, it's impossible for us to tell how far the sun is without absolute information.
  5. Feb 28, 2015 #4


    User Avatar
    Gold Member

    I don't understand what you mean by "bidimensional" observation.
    What are you observing?
    Absolute quantities in Special Relativity are quantities that don't depend on a choice of reference frame. Let's make it classical at first. Are the components of a vector absolute in classical mechanics? They are not. If you make a rotation for example (go to a rotated frame), the components will in general get mixed up. What is an absolute quantity under this rotation then? It's the vector's magnitude. Both the initial and the rotated frame, will measure the same length for the vector.
    In Special relativity the same thing also applied- you are mainly interested in "absolute quantities", except for if you are able to keep track of what is going on. The "absolute parameters" (at least as I interpret your word choice) don't depend on observers.
    The speed of light (the group velocity of the wave) is the same for all observers (that's a postulate).
  6. Feb 28, 2015 #5
    Thanks for the answer, I'm sorry If I don't understand the correct words well because of my lack of knowledge, I'm triying to use words that can express my question (also my english is not very good :P). Yes with bidimensional observation I mean what an object is observing, I use that because most of examples that Ive seen like the clock and the beam of light use relative observation. I get the point of the vector, when we rotate the absolute system stays the same as the relative, however we observe a complete diferent one.
  7. Feb 28, 2015 #6
    I apologgies for my rookieness, its just that I'm relavitely new to physics and I'm very exited about learning. I realize now that as long as the relative system is not moving at some speed, the parameters (vectors) dont change, and observation means absolute measure within the relative system. Thanks again guys for your previous answers.
  8. Mar 2, 2015 #7
    I'm not sure what your confusion exactly is about; but as your starting point sounds wrong, perhaps that's the cause of your confusion!

    When people loosely speak of "observed" in relativity, it should often not be taken literally.

    What people usually mean with "observed" is what they infer from measurements with instruments. And those instruments (or their calculations) have been adjusted to the assumption that light and radio signals propagate at the same speed in all directions relatively to their chosen reference system. If they choose a different reference system then they will consequently adjust their instruments or their calculations; and next they will "observe" other values although nothing changed. For more detail, you can search this forum for "relativity of simultaneity".
  9. Mar 2, 2015 #8
    Yes, that was exactly my confussion, so observation stands for instant measure of something thats happening within our frame. I will indeed serch for the "relativity of simultaneity" concept. Thanks for the answer!
  10. Mar 2, 2015 #9
    You're welcome :smile:

    However, when looking at those discussions I realize that they may be unclear or unhelpful as starting point (sorry!). It may be better to read first one of the basic explanations, for example here:
    http://www.bartleby.com/173/9.html (also the next page is useful, as it illustrates that also length measurements depend on our choice of reference system).
  11. Mar 3, 2015 #10


    User Avatar
    Science Advisor

    I think the crucial part of your original post was "If so the speed of light would be actually slower for some observers right?". The answer to that is NO! Indeed, all of the calculations of length contraction and time dilation are intended to make sure that the speed of light is the same for all observers.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook