Space Time:The Fabric of the Universe
 
 
 
 
 
 
 
 

    "Space and time are now dynamic quantities: when a body moves, or a force acts, it affects the curvature of space and time - and in turn the structure of space-time affects the way in which bodies move and forces act."

                                                                                       Stephen Hawking-A Brief History of Time

        In the classical Greek and Roman cosmologies space and time were considered to be distinct and seperate.  This belief remained for over two thousand years as the majority viewpoint, however that all changed in 1915 with Einstein's Theory of General Relativity.  This theory contains a few main points, the first being time slows as one approaches the speed of light.  The next proposes that gravity is a force that is caused by the fabric of space time.  To give the reader a visual image you should imagine that if their were no masses in the entire universe than the space time fabric would be flat like one giant sheet of thin rubber.  Should a mass occur, which tends to happen in this universe, then the mass will dip down into the space time fabric, causing a warp.  The amount that the space time will bend depends on the mass of the object that is on the fabric.  This means that as objects become greater in mass than they will have a higher gravitational pull.  This is why the moon orbits the Earth and the Earth orbits the Sun, and the Sun orbits some heavy gass cloud in the center of the galaxy, and so on.  Because the heavier balls will creater larger ripples and eventuall a dip in the space time fabric, they will have a force in which to keep balls in its orbit.  The following diagram should help to explain this furthur.

                                                                          This diagram illustrates how gravity functions on the level of our
                                                                    solar system.  The large mass in the center is the sun which has created
                                                                    a large warping of the space time fabric.  Suppose that a mass of smaller
                                                                    size rolls along this fabric than it will be sucked into the space time warp
                                                                    much in the same way that a marble would fall into a gap.  There is an
                                                                    animation which diagrams this further here.  Choose the Nova insignia on
                                                                    the site map and go under "search" and once there try space time.  Choose
                                                                    the Einstein site and you should see this animation.
 

    Einstein's revolutionary theory explained gravity in a way that Newton could not.  Where Newton suggested that gravity was a force that just "was", Einstein theorized that gravity could be explained by the curvature of space time.  Under the theory of relativity gravity is not a force like the other forces in the universe; it is a consequence of the fact that space time is not flat but warped by the mass and energy inside of it.  One cosequence of this is that objects travel in straight lines, and are not held by some mysterious force, gravity, rather they simply follow the curves in space time.  This means that objects move in straight lines along four-dimensional space time but move in elliptical circles in three dimensional space.  The following anaology appears in Stephen Hawking's book, A Brief History of Time.
                                   This is rather like watching an airplane flying over hilly ground.  Although it follows
                                   a straight line in three dimensional space, its shadow follows a curved path on the
                                   two-dimensional space.
    Another prediction of space time is that of the Mercury's orbit.  Because Mercury is the planet closest to the sun, which means that it feels the effects of gravity the most prominantly, and its orbit is highly elliptical, the theory of general relativity predicts that the orbit will rotate around the sun.  The following diagram should illustrate this point more clearly.
 
 

This diagram is a bit exagerrated since the elongated axis of mercury should only rotate about
one degree in ten thousand years.  This effect had been noticed before 1915, when coincidentally
Einstein's theory was introduced and there was a time when one could observe this warping of Mercury's
orbit.  These observations were some of the first confirmations of the Theory of Relativity based on experimentation.
 
 
 
 

        Another aspect of the universe which is affected by the warping of space time is that of light.  Although light appears to travel in straight lines, it actually is bent, curved, and altered by the fabric of space time.  This means a star that is on the opposite side of the sun from us on earth, but is just visible over the curve of the sun, will not be where we think that it is.  Although it will look like it is straight out in front of us, it is actually around the corner of the sun because the space time warp morphs the light.  We see only the result of the light that is being bent around the sun.  This theory usually can not be tested since the sun is shining us right in the eyes, creating a glare that will not allow us to view stars.  However, during a total solar ecclipse, it is possible to test out this theory.  Since we are constanstly orbiting the sun, we are able to notice the changes between where are star appears to be during the solar ecclipse, and where it actually is later on in orbit.  When the results of these two tests are compared, Einstein's theory have been confirmed.

        Now its time for a recap of the material which I just described.  Keep in mind that what this is a brief summary, not a thorough examination of space-time.  For an in depth insight into just how space time works I will have a set of links at the end of the page.

                                     1.  Space time is the fabric of the universe.  This means that it holds the mass of the universe on its surface.
                                                2.  Larger masses create larger dimples and larger depressions in the space time fabric.  This accounts for gravity.
                                                                    This shows how planets can orbit suns.
                                                3.  There is empirical evidence for the theory of relativity which generated the space time idea.
                                                                    The orbit of Mercury
                                                                    The bending of light by the gravity of the sun during a total solar ecclipse.
         All of these ideas stem for Albert Einsteins's general theory of relativity.  In a sense this theory and the implications which it presents form a new modern cosmology, that is it changes the way that we view ourselves in respect to the universe.  In the next section I will discuss how this cosmology compares to the ancient cosmologies that we have studied, and what implications that our cosmology has on ourselves as a culture.  You can find that essay here.