In physics, relativity deals with the way things are in different reference frames. A restricted principle of relativity was expressed by Galileo Galilei early in the 17th century AD. He said in effect that experiments conducted in the enclosed cabins of two ships, one at rest and the other in uniform motion, could not determine in which ship the experiments took place. One of the implications of the principle was that if you knew the velocity of an object in one reference frame you could calculate its velocity in another by adding the relative velocity of the two frames to the known velocity of the object.

Albert Einstein was well aware of Galilean Relativity and its implications. Experiments testing that relativity principle had been carried out with astonishingly precise results for about 300 years. But Albert knew something that Galileo did not. He knew that the speed of light was the same in all reference frames in uniform motion relative to one another and regardless of the motion of the source of the light. While the debate went on as to which principle was in error, Galilean relativity or the constant speed of light, Einstein in his twenties asked himself what were the consequences if both were true. He spent the rest of his life explaining those consequences.

We will certainly not follow in Einstein's footsteps here. The best I can do is set forth as statements of fact some of those consequences. Einstein's relativity principle is commonly divided into two parts. The first published in 1905 is called special relativity and is the relativity of uniform motion, motion in which all reference frames and objects. move in straight lines unless interfered with. The general principle of relativity (1916) is the relativity of non-uniform motions allowing for accelerated or rotated reference frames and objects as well as reference frames and objects subject to gravitation.

From special relativity we get the following:

1. No object, or even actionable information, can travel faster than light. To do so violates the law of causality, which holds that no effect may happen before its cause in any reference frame.

2. Space and time are joined in a single entity called spacetime.

3. The time and distance between events will vary among reference frames but the spacetime interval. interval between events, given by the square root of the difference in squares of time and distance, will be the same in every reference frame.

4. Simultaneity is reference frame dependent. Two observers may not agree that events are simultaneous and both be correct.

5. If two events happen at the same place in a reference frame, the interval between them is purely time-like and may be measured directly by a clock at rest in that reference frame. That interval is called the proper time between the events and is the longest elapsed time possible between those events.

6. In a given reference frame, objects in motion age more slowly than do objects at rest. The faster the motion the greater this effect. Objects that move a light speed age not at all.

7. Mass and energy may be exchanged, the exchange rate being the square of the speed of light (C). E=MC².

From general relativity we get:

1. Gravity and acceleration are indistinguishable in their effect on an object. If gravity and acceleration are properly balanced, the net effect on objects may be that neither is felt, as for instance in the case of a satellite in orbit.

2. Gravity is explainable as curvature in spacetime induced by the presence in spacetime of mass and energy. The more and the closer the mass+energy sum, the sharper the curvature of spacetime and the stronger the effects of gravity.

3. Light is affected by the curvature of spacetime, seemingly deflected by gravity.

4. Objects in more sharply curved local spacetime age more slowly than objects in less sharply curved local spacetime.