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

The reality of Relative Velocities in Special Relativity

  1. Jan 21, 2015 #1
    Lets suppose we have a observer O and two objects A and B a speed is 0.5c and B speed is 0.3c than we can calculate other relative speed acoording to these informations.We know that (according this info B sees A going to 4/17c but we can pretty sure thats true because we cant ask. him How can O predict this speed or he know that theres special relativity and nothing is really what it seem.
     
  2. jcsd
  3. Jan 21, 2015 #2

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    Yes.

    I calculate 3/17 for the speed of A relative to B (assuming they are both moving in the same direction relative to O)

    [Edit: the above was incorrect.]

    Why can't we ask B? B is an observer just like O and A; there's no reason why he can't directly measure A's speed relative to him.

    The SR rules for velocity addition have not been extensively tested with macroscopic objects, true. But SR in general has been very extensively tested, and the results would make no sense if the SR rule for velocity addition were not true, since it forms a consistent part of the whole theoretical framework of SR.
     
    Last edited: Jan 22, 2015
  4. Jan 21, 2015 #3

    Nugatory

    User Avatar

    Staff: Mentor

    They have, however, been tested. Google for "Fizeau velocity addition".

    (PeterDonis knows this already, of course - this comment is for Quarlep).
     
  5. Jan 22, 2015 #4

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    You better get a new calculator. 0.5-0.3 is 0.2 not 0.15. The OP was correct.
     
  6. Jan 22, 2015 #5
    I want to ask another question.Whatever I wrote in Relative velocity equation I get small then c isnt it.I mean the equation requires it.Lets suppose I have two velocities smaller than c than I put it this equation these velocity Is there a chance to get bigger than c I think theres no probability of that but I want to be sure.
     
  7. Jan 22, 2015 #6

    Ibix

    User Avatar
    Science Advisor

    Correct. The velocity addition formula can be derived directly from the Lorentz transforms, which by definition don't allow anything moving below the speed of light to reach or exceed it.
     
  8. Jan 22, 2015 #7

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    Oops, you're right! I multiplied instead of subtracted. :oops:
     
  9. Jan 22, 2015 #8

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Blame it on the calculator!!! Those buttons are so close together.
     
  10. Jan 22, 2015 #9

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    Unfortunately I did this one in my head... :eek:
     
  11. Jan 22, 2015 #10

    phinds

    User Avatar
    Gold Member
    2016 Award

    Uh ... are the parts too close together? :D You could blame it on your hat.
     
  12. Jan 22, 2015 #11
    Quarlep
    You can check it yourself by substituting 1 for either of the values.
     
  13. Jan 22, 2015 #12

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    Not wearing one. I appreciate the effort to salvage my self-esteem, though. ;)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The reality of Relative Velocities in Special Relativity
Loading...