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

Homework Help: Slightly confused with Spacetime Diagrams

  1. Jan 26, 2008 #1

    So I am supposed to draw and find the displacement vector between to points in spacetime, coordinates being (3,2,0,0) and (-2,1,0,0). I somehow feel slightly stupid for asking this, but I was reading the textbook and looking through my lecture notes. What I basically did that draw a standard 2D coordinate system wit ct and x as axis, add the two points draw the vector from the (-2,1,0,0) to (3,2,0,0) and find the distance using the line element formula in Minikowiski space:

    [tex] \Delta S^2 = -(\Delta ct)^2 + \Delta x^2 + \Delta y^2 + \Delta z^2 [/tex]

    Does that sound about right? I might be reading too much into this as the -2 for the ct component is throwing me off.


  2. jcsd
  3. Jan 27, 2008 #2


    User Avatar

    The sign of a componoent doesn't change anything.
  4. Jan 27, 2008 #3

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Are you worried about having negative time? t is just a coordinate, so having a negative value for t is no more problematic than having a negative value for the x coordinate, i.e., negative x doesn't mean negative space.

    If today is chosen as the origin for the t coordinate than tomorrow has t = +1 day and yesterday has t = -1 day = -24 hours.
  5. Jan 27, 2008 #4
    Thanks, I was just confused cause one half of the textbooks just show positive t and x and the other half show a full coordinate axis.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook