What is General Relativity?
General Relativity is the modern theory of gravity that replaced Newton's theory of gravity. General Relativity deals with accelerated motion such as is the case with gravity. Special Relativity deals with constant rate motion.
The PBS Nova Einstein Revealed episode presented Einstein's General Relativity equation for the equivalence of space-time and matter-energy. This equation says that matter and energy is a concentrated form of space and time!!!
Guv = 8(pi) Tuv
Space and Time is on the left.
Matter and Energy is on the right.
Guv is the Riemann tensor of curvature (space time).
Tuv is the energy tensor of matter (matter energy).
8(pi) is the concentrating factor.
But what does 8(pi) mean? Four 2(pi) 360 degree revolutions? One rotation for each of four dimensions? Three dimensions of space and one dimension of time?
Brian Greene's book, The Fabric Of The Cosmos (chapter 10 footnotes), shows how Einstein introduced a cosmological constant into this equation to counter gravity and thus provide for a static universe. Einstein abandoned this idea due to the redshift work of Edwin Hubble, which helped establish that the known universe is expanding. The cosmological constant idea is now back in vogue as a measure of dark energy driving the constantly increasing rate of expansion of the universe.
Kip Thorne, author of the book Black Holes and Time Warps, hosted the PBS Nova Time Travel episode (10-12-1999). This interesting show examines if relativity allows black holes or worm holes to be used as a possible means of time travel.
Einstein's best known equation, e=mc2, comes from Special Relativity and says mass is a concentrated form of energy, where c2 is the concentrating factor.