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

Time is not the 4th dimension

  1. Dec 3, 2004 #1
    Time is not the 4th dimension. Well... depends on perspective. Why do people say time is the 4th dimension? I know you can say it's a dimension and there are already 3. But why not say mass is a dimension? Each point in space has a certain mass. What's the difference? Einstein said it and it became convention? That better not be the reason :mad: :rolleyes:.
    I think it would be best when describing time with space and need to call it a dimension to say something like, "If you use time as a dimension then..."
  2. jcsd
  3. Dec 3, 2004 #2


    User Avatar
    Science Advisor
    Gold Member

    I've said thsi before to say time is the foruth diemsnion is is some ways menaingless as your not saying what it is a dimensions of.

    Of course the reason people say this is that we do phsyics in spacetime which has four dimensions and we may view time as one of them.
  4. Dec 3, 2004 #3
    if time isnt a dimension, then what do you put in place of it when you describe an event?
  5. Dec 4, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It is a simple matter of mathematics. It requires 4 numbers to specify an event in Space/time. 3 spatial coordinates and the time at which the event occurred. It is not necessary to specify the mass. Notice that I am specifying a SPECIFIC type of problem, one dealing with SPACE/TIME events. In a different problem, one NOT dealing with space/time the meaning and number of dimensions may be entirely different.

    If you were dealing with problem where mass was changing with position, you may well need a set of equations were mass is the forth dimension, but that would be a different problem, if fact if you were track your mass and position through out your life you would need 5 dimensions, 3 for your position,1 for the time and 1 for your mass.

    Once again the number of dimensions of a system is the minimum number of points required to completely specify the state of that system.

    To say that time is not a dimension of space/time is simply ludicrous.
  6. Dec 4, 2004 #5
    Calling space 3 dimensional implies that the three quantities used to describe a point are similar. Thus 1 metre North is thought to be the same as 1 metre East - if you have a metre rule pointing North then you can turn it to point East. Equivalently, different observers may have different meanings for 'forwards' and 'sideways'. What special relativity says is that time is similar to space - although different observes will agree on the spacetime distance between events, they may not agree on how this splits up between time and space.
  7. Dec 4, 2004 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    Time and space mix together. What is a pure space-like separation for one observer is a mixture of space and time separation to another moving relative to the first.

    It's this mixing together of space and time by the equations of relativity that that makes space-time a unified concept, usually called the space-time continuum.
  8. Dec 4, 2004 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you use time as a fourth dimension you will find the theory of relativity, which makes predictions that have been verified by experiments. If you use density as a fourth dimension you get nothing interesting. That's why we say that time is the fourth dimension. What that really means is that time is the fourth dimension of spacetime, which is a useful concept. You could say that density is the fourth dimension of "spacedensity", but that seems kind of pointless doesn't it?

    (It doesn't really make sense to speak of the mass at a certain point. That's why I replaced mass with density).
  9. Dec 4, 2004 #8


    User Avatar
    Science Advisor
    Gold Member

    Without time, spatial dimensions are meaningless. As Integral said, you cannot quantify distance [or force for that matter] without time.
    Last edited: Dec 4, 2004
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Time is not the 4th dimension
  1. The 4th Dimension. (Replies: 5)

  2. The 4th dimension? (Replies: 13)