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

A Brief History of Time

  1. May 29, 2012 #1
    In the opening chapter of "A Brief History of Time" Hawking writes, "We now know it is impossible to have an infinite static model of the universe in which gravity is always attractive". Can someone please explain this to me? I'm don't quite understand it.
  2. jcsd
  3. May 29, 2012 #2


    User Avatar
    Science Advisor
    Gold Member

    A static universe would only be a temporary condition. Such a solution is far too fragile to endure. This is why Einstein added the cosmological constant to his field equations. He, like most others of his time, believed the universe was static and the only way to get a universe that did not eventually implode was to insert some kind of repulsive force to neutralize gravity.
  4. May 29, 2012 #3
    Picture a ball thrown up in the air. It will either keep traveling up or be pulled back to Earth. It can't just hover in mid-air.
  5. May 29, 2012 #4

    The lumps would get bigger through gravity until the whole thing was one big lump.
  6. May 30, 2012 #5
    In other words it's an unstable situation like trying to balance a pencil on its point. There will always be small perturbations of some type. If it increases gravitational attraction in a static condition somewhere, there would be runaway collapse and vice versa if gravitational temporarily decreased anwhere....runaway expansion.
  7. Jul 5, 2012 #6
    I have a few questions after reading this book. Rather than creating a new thread, I decided to piggyback on this thread instead. Please enlighten me with your constructive feedback.

    1) In an earlier chapter of this book, he presented an argument that no life form (presumably on earth) can exist in 2-dimension. This idea was illustrated with an animal with a digestive system run'ning through its body, thus as the food travels through the body of tihs life form, it will "cut" the body into two halves, and thus the body cannot be kept together and the subject dies.

    However, I think this argument is flawed in the sense that it assumes the life form must have a digestive system linking the mouth to its exit simultaneously.

    A very simple 3D example is pushing a pea through a lump of bluetac. We reshape the lump of bluetac and gradually creates an exit and pushes the pea out.

    Moving to a 2D world example: Put a drop of oil on one edge a bowl of water, then move
    the oil patch along the diameter of the bowl to the other edge of the bowl. The oil patch is 'food', and the surface of the water (which is 2D) is the 'body' of the 2D life form. This life form reshapes and encompasses the 'food' and absorb the 'food' as it travels from one part of the body to another part. Note that the mechanism of absorbing the nutrition does not necessary mean organs like stomach or intestines.

    Why this was not considered as an alternative way for a life form in the book?

    2) The book talked about time travel and mechanism to achieve it.
    Time travel is certain a huge challenge for entity like us, who lives in a 3-dimensional world with the concept of time governing us (e.g. "life"). Time is not in a form which we can appreciate like an object in the 3-dimensional world.

    But what if there are entities that exist and are not bound by the dimension of 'time' as we do? Then time travel for them is as easy as taking a stroll down the road for us.

    One simple example is movie. The film captures a series of scenes over a period of time. For entities like us, we live 'within' this sequence and thus travelling through time is limited to only moving forward in the sequence. We cannot move backwards or jump to another point in time.

    Imagine another entity, can experience all these scenes simultaneously and optionally enters each scene? If so, then time travel is easy.

    Please enlighten.

  8. Jul 5, 2012 #7


    User Avatar
    Gold Member

    Although I think it very likely that there are solid reasons why a 2D "creature" could not actually live, I like your thought process and I think you have demolished the "splits it in two" argument.

    As to your second point, I think this is speculative to the point of being nonsense, but perhaps others here will disagree w/ me.
  9. Jul 5, 2012 #8
    Thanks Phinds.

    I agree my 2nd point is highly speculative.
    What I am not understanding is the theory of time.
    Like gas, by controlling the conditions, it can turn into liquid and solid.
    Does / what if such conditions exist with "time"?
    Thus my imagination goes to what if "time" is already all there and it is
    just a matter which sequence we are experiencing.
    A far fetched thought is that, maybe prophets were such kind of entity
    that can experience "time" out of sequence hence give predictions.
    Last edited: Jul 5, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook