Spacetime is Curved and Mass is Energy:
Relativistic Physics
An Introduction
Around the turn of the century, physicists were optimistically proclaiming the end of physics (much as some are doing today); practically everything there was to know about the universe had already been discovered. They were not expecting a major reconsideration of their fundamental assumptions on the nature of light and time and space. In the Newtonian worldview, it was almost taken for granted that time and space were absolute; properties such as length, width, height, and time would be the same against an absolute reference point regardless of the location or velocity of the person making the measurements.
Experimental evidence, however, begged to differ.
In 1905 Einstein published a paper on the Special Theory of Relativity to expose the fundamental error of classical physics: absolute spacetime. He based his theory on two assumptions:
the laws of physics are the same for every non-accelerating frame of motion (the General Theory of Relativity ten years later dealt with, well, more general cases, wherein accelerated frames are also allowed, but the corresponding math is a lot more complicated);
the speed of light is constant, everywhere, always.
As a consequence of relativity, many of the properties we think immutable -- time, mass, space -- change as we approach the speed of light.
Mass at v =
M at rest *  |
Length at v=
L at rest / |
Time interval at v =
T at rest * |
 |
Thus, not only does the rocket ship above contract as it approaches light speed, its mass goes to infinity and time slows down.
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| v=0 |
v=.4c |
v=.6c |
v=.8c |
v=.95c |
v=.99c |
Mass is Energy
Applications of Relativity
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