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

Extra dimensions

  1. May 9, 2005 #1
    Im trying to educate myself on relativity with things such as Einstein's Dreams and The Inflationary Universe before i jump into the actual theory. Right now i understand 3 dimenions: 1, 2, and 3 for x, y, and z. Ive heard the 4th dimension is time, but i am having a tough time understanding how this could be.

    In Einstein's Dream thus far ive read some on this 4th dimension where the author attempted to seperate the common notion of what time is between physics time.

    Could someone explain the 4th (time) dimension; are there others?
  2. jcsd
  3. May 9, 2005 #2
    Hey! I was having the same problem understanding the fourth dimension/time as well; in fact, I posted a thread (called Oddity of Time in the Special and General Theory forum) about it - my wording was a little bad when I asked the question, but I eventually got the answers I needed. Some people gave me websites to go to, so look at those because they are extremely helpful.

    Time is a difficult concept to understand, so I know where you're coming from!
  4. May 9, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, Einstein modeled the universe on 4 dimensions...3 of space and 1 of time. We can move freely in the 3 dimensions of space and one-way (forward) in the dimension of time (although at different rates). If you dig aroung the archives, you'll find some past discussions on the reality of Time.

    For Einstein's model, there are no other dimensions. String Theory/M-Theory proposes many more dimensions, but they have yet to be proven to exist.
  5. May 9, 2005 #4
    if string theory is proven, is einstein's theory void? Also, ive noticed that people tend to regard newtons laws as accurate until you look at space and super gravitational bodies, but if it breaks down in space shouldnt it break down on earth, which happens to be in space.

    how could you have relativity for space and newtonian mechanics for earth. :confused:
    Last edited: May 9, 2005
  6. May 9, 2005 #5
    I'll give it a shot. This is something I have read here that may help. To locate an object in space we need x,y and z coordinates. Now due to the relative motion of the universe these coordinates are changing all the "time". So to narrow it down we need a 4th coordinate and that is time.

    Say we want to get together, I give you an address which leads you to a location. Now if we never specify a time chances are we will not arrive at the same location at the same time. Time is related to movement and since everything moves time becomes a real coordinate.

    As far as other dimensions I think you need to look into string theory as it predicts 11 or so dimensions. I won't pretend to understand that though.
  7. May 9, 2005 #6


    User Avatar
    Gold Member

    Note for starters, that Newtonian mechanics and relativity are merely explanations for how our reality works. When we say 'Newtonian mechanics is breaking down', we merely mean it is inadequate to describe what we see.

    Newtonian mechanics explains perfectly 99% of the things we see here on Earth and in space. Almost everything that happens on Earth and in much of space falls within a non-relativistic range - speeds, masses, distance, etc.

    It is only when we deal with things that are ultra-heavy, ultra-fast or ultra-energetic that Newtonian predictions and relativistic predictions begin to drift apart in their predictions.

    For example, in Newtonian mechanics, time is an absolute, fixed and immutable measurement; speed does not alter the experience of time. If Newton had access to a particle accelerator, and witnessed fast-moving particles exhibiting increased mass and time dilation, he would be at a loss to explian it with his current description of the universe. His explanation of the universe begins to break down at these high speeds.

    His explanation of what would happen to a super-massive star that collapses would also not agree with relativity. A black hole does not make sense in Newtonian physics.

    Again though, 99% of the time Newtonian physics is adequate for explaining what we see.
    Last edited: May 9, 2005
  8. May 11, 2005 #7
    Does this not make newtonian mechanics false, given that this mere 1 percent is false; if one percent of a dna strand were false, it would still look like a humans but it wouldnt make a human.

    God_am: I like youre explanation of 4D very much, it makes perfect sense. This would be something called a worldline, where an event in space happes at multiple instances. Say a piece of rock moving along, it is at point A, B, and then C after that. So no position is absolute, but it has static coordinates in 4 dimensions at one instant only, and dynamic coordinates in 3 dimenions.

    Makes me wonder, what would happen if you started playing around with these 4D coordinates in mathematical terms, like taking the integral of a 4D object from point A to point J.
  9. May 11, 2005 #8
    Time is a dimension as x, y and z, this means when you perform transformations, as
    Lorentz trans in SR and gerenal trans in GR, time transforms together with space, and they are mixed together. It is not Einstein who point out SR is a theory in 4 dimension, Minkovski did that years later.

    Of course, you can write all equations in 3 dimension form and treat time separtely. But then it's difficult to check whether the theory satisfies the Lorentz inverance.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook