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

Why is the speed of light the same for all observers?

  1. Dec 15, 2013 #1
    I never understand why the speed of light is the same for all observers irrespective of their motion relative to the source of light. Now suppose I am sitting at the back of a vehicle which is travelling at the speed of 0.999999999999c and light approaches me from behind the vehicle. i.e. I am going away from the source of light while I can still see the light. Now I attach an instrument to my car for measuring the speed of light. Won't it measure 0.000000000001c.

    Please help me understand why will the instrument still record 0.999999999999c and not 0.000000000001c according to the theory of relativity.
     
  2. jcsd
  3. Dec 15, 2013 #2

    jtbell

    User Avatar

    Staff: Mentor

  4. Dec 15, 2013 #3

    Saw

    User Avatar
    Gold Member

    I suppose you may now be asking yourself why velocities add in a different way in relativistic physics. if you want to answer yourself this, just look at the way that the relativistic formula for addition of velocities is derived.
     
  5. Dec 15, 2013 #4
    I know we use the lorentz transformation to add velocities but doesn't it seem a bit absurd?
     
  6. Dec 15, 2013 #5

    Saw

    User Avatar
    Gold Member

    Why? You may be thinking that the Galilean alternatives (c+v) and (c-v) are self-evident but they are aren't. They are also derived. Think of it: how do you derive them?

    (Will go to play tennis now...:)
     
  7. Dec 15, 2013 #6

    tom.stoer

    User Avatar
    Science Advisor

    In physics we cannot explain why nature is the way it is. We can only try to find a mathematical representation which describes how it works, and we can derive predictions.

    In this specific case we find experimentally that electromagnetic waves travel at the speed of light, and we find that this is true for all observers (all reference frames). Then we find theoretically that Maxwell's equations predict exactly this behavior. In addition we can use special relativity (including Lorentz transformations, relativistic addition of velocities, ...) to formulate a comprehensive theoretical framework.

    So we know how to describe nature. And we should believe in this description b/c it makes correct predictions.

    Unfortunately this is all we can say in physics.
     
  8. Dec 15, 2013 #7
    so true!
    Just to emphasize those points, everybody who studies special and then general relativity and especially quantum mechanics must change their way of thinking.

    It turns out you probably think space and time are fixed and immutable; turns out they are not, but 'conspire' together in such as way that enables all observations of lightspeed to be the same. It turns out the speed of light that is fixed and immutable...

    yes, it took an 'Einstein' to recognize that, so don't feel bad about it not seeming natural to you. If you are not familiar with the struggles of the greatest minds of the early 1920's, look up 'luminiferous ether'......


    A. Zee
     
  9. Dec 15, 2013 #8
    for clarity: nobody knows.

    Nobody even knows why light exists.
     
  10. Dec 15, 2013 #9

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Would you please describe this instrument that measures the speed of light. How does it work? Do you know of a place where you can buy one?

    Are those the only two choices? If I bought an instrument that measures the speed of light and it gave me one of those results, I'd want my money back. I could sell you an instrument that measures the speed of light. It would be a little box with a hole in one end. When you shine a light in it, a photo detector inside powers a little display that says "c".

    But if you actually wanted to measure the speed of light you would have to start a timer when the light reached you, then put a mirror in front of you some measured distance away and stop the timer when the reflection got back to you. Since the light traveled twice the distance to the mirror, you would calculate the speed of light to be that double distance divided by the reading on the timer. As long as you are not changing your speed while you're doing this, you will always get the same answer, c. Why? That's just the way the universe is.
     
  11. Dec 15, 2013 #10

    Bill_K

    User Avatar
    Science Advisor

    Then there are the people who measure the speed of neutrinos. They obviously don't use a mirror! :wink:
     
  12. Dec 15, 2013 #11

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Aren't they comparing the speed of neutrinos to the speed of light? We can also compare the speed of light coming from two different sources that are traveling at different speeds and determine that the speed of the light does not depend on the speed of
    the source. These are both examples of races to see which one wins or if it is a tie but they don't measure the absolute one-way speed, don't you agree?
     
  13. Dec 15, 2013 #12

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

  14. Dec 17, 2013 #13
    Yes, all experiments showed this and so we believe in this. But Special relativity did provide an answer for that. Length contraction and time dilation are nothing but an explanation for this phenomenon, is what I have known.

    Your instrument will calculate speed of light, by using distance of the source and time taken by light to reach it. Since both the values decrease while in motion, when you will calculate the speed, it will turn out to be 'c'.

    But even I'm still not able to use and calculate it mathematically. I am not sure who's length and time we would consider and how exactly the 'instrument' will find it. Maybe anyone would like to help in that?
     
  15. Dec 17, 2013 #14

    adjacent

    User Avatar
    Gold Member

  16. Dec 17, 2013 #15

    jtbell

    User Avatar

    Staff: Mentor

    If you post a specific example of a situation which confuses you, and your attempt in calculating it, with some description of what exactly is confusing you, we can help you with it. Please start a new thread for it rather than hijack this one.
     
  17. Dec 17, 2013 #16
    :rofl:
     
  18. Dec 17, 2013 #17

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    No, that is not right. There is no instrument that can measure the one-way speed of light that you are describing here. You cannot measure the time it takes for light to travel a distance. I already described how you measure the round-trip speed of light in post #9:

    Where did you get the idea that both the length and the time decrease while in motion? Please tell me what you are thinking.

    Even if you had such an instrument that you calibrated to work correctly in one inertial state (by synchronizing two clocks some distance apart), it would no longer work if you accelerated it to a new inertial state or pointed it in a different direction.
     
  19. Dec 18, 2013 #18
    I refered to the time dilation and length contraction. But not sure how it applied to that example. Equally confused.

    I don't think practically it is possible to do this, due to the high speed of light. Do you know experiments that are actually done? Do they have the same set up or some other techniques?
     
  20. Dec 18, 2013 #19

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Are you aware that Dilation means "expansion", not "contraction"?

    Sure, read this.
     
  21. Dec 20, 2013 #20
    Yes it does, but that also means there are fewer 'ticks' while moving a fixed distance at a slower speed.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook