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 there a difference between past and future?

  1. Apr 19, 2012 #1
    Please teach me this:
    We know that there is a approximate symmetry of time T in physics(together with CP symmetry).Then why there is an asymmetry between past and future(we can not reverse the present world to past world)?
    Thank you very much in advance.
  2. jcsd
  3. Apr 19, 2012 #2


    User Avatar
    2017 Award

    Staff: Mentor

    arrow of time

    Two important concepts:
    - space is expanding: "The past is a state with a 'smaller' universe"
    - entropy is increasing: "The past is a state with less entropy"

    In other words: If there is some reason to have a small, expanding low-entropy state, you'll get a universe with a well-defined arrow of time like the universe we are in.
    If you begin with a high-entropy state, you won't get life as we know it.
  4. Apr 19, 2012 #3


    User Avatar
    Science Advisor

    The origin of asymmetry can be traced back to the initial conditions. Entropy increases with time, which means that initial entropy was very low. The fact that entropy increases with time is not really a physical law (even though it is sometimes referred to as the second "law" of thermodynamics), but merely a property of a particular solution which happens to describe the state of our universe.
  5. Apr 19, 2012 #4
    OK,I see,thank you very much.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook