In 1905, Albert Einstein published the theory of special relativity, which explains how to interpret motion between different inertial frames of reference — that is, places that are moving at constant speeds relative to each other.
Einstein explained that when two objects are moving at a constant speed as the relative motion between the two objects, instead of appealing to the Ether as an absolute frame of reference that defined what was going on.
Special relativity includes only the special case where the motion is uniform. The motion it explains is only if you're traveling in a straight line at a constant speed. As soon as you accelerate or curve — or do anything that changes the nature of the motion in any way — special relativity ceases to apply. That's where Einstein's general theory of relativity comes in, because it can explain the general case of any sort of motion.
Einstein's theory was based on two key principles:
The principle of relativity: The laws of physics don't change, even for objects moving in inertial (constant speed) frames of reference.
The principle of the speed of light: The speed of light is the same for all observers, regardless of their motion relative to the light source.
The genius of Einstein's discoveries is that he looked at the experiments and assumed the findings were true. This was the exact opposite of what other physicists seemed to be doing. Instead of assuming the theory was correct and that the experiments failed, he assumed that the experiments were correct and the theory had failed.
In the latter part of the 19th century, physicists were searching for the mysterious thing called Ether — the medium they believed existed for light waves to wave through. The belief in ether had caused a mess of things, in Einstein's view, by introducing a medium that caused certain laws of physics to work differently depending on how the observer moved relative to the Ether. Einstein just removed the Ether entirely and assumed that the laws of physics, including the speed of light, worked the same regardless of how you were moving — exactly as experiments and mathematics showed them to be.
Unifying space and time
Einstein's theory of special relativity created a fundamental link between space and time. The universe can be viewed as having three space dimensions — up/down, left/right, forward/backward — and one time dimension. This 4-dimensional space is referred to as the space-time continuum.
If you move fast enough through space, the observations that you make about space and time differ somewhat from the observations of other people, who are moving at different speeds.
The strange behavior of space and time is only evident when you're traveling close to the speed of light, so no one had ever observed it before. Experiments carried out since Einstein's discovery have confirmed that it's true — time and space are perceived differently, in precisely the way Einstein described, for objects moving near the speed of light.
Unifying mass and energy
The most famous work of Einstein's life also dates from 1905, when he applied the ideas of his relativity paper to come up with the equation E=mc2 that represents the relationship between mass (m) and energy (E).
Einstein found that as an object approached the speed of light, (c), the mass of the object increased. The object goes faster, but it also gets heavier. If it were actually able to move at c, the object's mass and energy would both be infinite. A heavier object is harder to speed up, so it's impossible to ever actually get the particle up to a speed of c.
Until Einstein, the concepts of mass and energy were viewed as completely separate. He proved that the principles of conservation of mass and conservation of energy are part of the same larger, unified principle, conservation of mass-energy. Matter can be turned into energy and energy can be turned into matter because a fundamental connection exists between the two types of substance.
E = MC2
Albert Einstein developed a theory about the relationship of mass and energy. The formula, E=mc, is probably the most famous outcome from Einstein's special theory of relativity. The formula says (E)= Energy (measured in joules), (m)= mass (measured in kilograms), (c)= the speed of light (186,000 miles per second, or 3 x 108 ms-1). In essence, it means mass is just one form of energy. Since the speed of light squared is an enormous number, a small amount of mass can be converted to a phenomenal amount of energy. Or, if there's a lot of energy available, some energy can be converted to mass and a new particle can be created. Nuclear reactors, for instance, work because nuclear reactions convert small amounts of mass into large amounts of energy.
Interesting Facts
Matter determines how space curves. Curved space determines how matter moves.
Due to the natural curvature of space, the shortest path between any two objects is never a straight line, but a curved line called a geodesic.
The speed of light in a vacuum is exactly 299,792,458 metres per second. Light reaches all objects from all directions at the same speed, regardless of their motion.
No physical object can travel at or faster than the speed of light. The speed of light is generally considered to be a physical speed barrier.
Einstein's special theory of relativity applies to the special case where acceleration = zero. His general theory includes the effects of objects moving with acceleration.