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

Trying to Understand

  1. Mar 20, 2006 #1
    Regarding Universe Expansion:
    Does the fabric of SpaceTime play a role in regulating the expansion of the universe? And if so does anyone have any recommendation of intermediate readings that could help me to understand this role?

    Regarding String Theory:
    I know I haven't read eonough on this subject but I've got a conceptual block... A string to me is a complex structure. Yet this particular structure is suppose to be a fundamental thing. To me a point is a fundamental structure. I'm just having trouble conceptualizing I suppose. Any suggested readings here would be greatly appreciated as well.
  2. jcsd
  3. Mar 20, 2006 #2


    User Avatar
    Gold Member

    A point is a zero-dimensional object. A string is a one-dimensional object; it is one (and only one) degree of freedom.

    The whole point of a string being a string rather than a point is this:

    It is the zero-dimensional point particles that are causing all the infinities when reconciling GR and QM. A zero-D particle means that the "energy divided by time" transfer between particles ends up as an infinite (or undefined) number. If the fundamental object has at least one dimension, then transfer of energy (say, in a collision) is not infinite.
    Last edited: Mar 20, 2006
  4. Mar 20, 2006 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to Physics Forums, Prairie!
    To say the least, the expansion of the universe is the expansion of spacetime (direct role). But also, the expansion of the universe is regulated by gravity (which slows down expansion) and "dark energy" (which speeds up expansion). With Einstein's Relativity, 'gravity' can be thought of as the behavior of spacetime. Dark energy is still a mystery.

    I'm not sure I buy string theory, but first you should get rid of the image of a string that you may find around your house and try to imagine a tiny (no...even smaller) 1-D segment...perhaps like a little energy loop instead of an energy point. But isn't a string made up of points you ask? Perhaps mathematically, but how do you get any number of zero-dimensional things to add up to anything at all? Just a thought to help visualize it.
  5. Mar 20, 2006 #4
    Thank you both for the excellent explanations. My main interest actually concerns the relationship between Spacetime and matter. How the two are so intimately dependant, yet they do funky, and violent, things to each other.

    String theory is a wonderful thought exercise for me at this point. Pardon the pun. And I wish all the string theorists the best of success. But talk about going for the jugular... wow.

    Thanks again you guys.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook