Albert Einstein's theory of special relativity is an explanation of how a change in an object's speed affects measurements of its time, space, and mass.
Experiments that helped to establish a theory of electromagnetism showed waves in an electromagnetic field (which we see as light) zip through empty space at a speed of 299,792,458 metres per second (about 186,000 miles per second).
Never any faster. Never any slower.
This single speed limit also happens to be the same no matter how fast an observer is moving, which really doesn't make much sense at first. So in 1905, Einstein wrote a paper detailing a strange explanation.
How does speed affect time and space?
Isaac Newton's three laws of motion describe a relationship between a force, an object's mass, and its acceleration. These laws apply to everybody, regardless of how they happen to be travelling.
A clown entertaining passengers on a train can trust their juggling balls will go up and down at a slow, steady rate. On the other hand, as their train whips through a station, anybody peeking through the window can expect to see those juggling balls whip by at high speed. Nothing odd there.
If those balls were waves of light they could only move at a single speed, no matter who was watching. The person on the platform would also see them move along at the same pace as the juggler, in spite of the train's velocity.
For slow moving juggling balls, this is hard enough to imagine. Sped up to lightspeed, it requires some clever thinking to make sense of.
Einstein's solution was to view time and space as relative factors. According to the theory of special relativity, acceleration changes how time and distance compare between observers, especially as that acceleration becomes super fast.
A person on the platform would see the train pass with a super thin, super slow juggler on board, his thin juggling balls almost locked in space as he passes at high speed. The juggler wouldn't think they were slow or thin at all, but would instead see a wide person on the platform move at high speed.
Neither is 'wrong' in what they see. 'Right' just depends on the context of acceleration.
What does E = mc2 mean?
Einstein came up with a new equation to describe how mass and force relate as a way of supporting his new idea. He wasn't the only one pondering this relationship at the time – other physicists, such as Henri Poincaré, were also showing how energy and mass were closely linked.
For an object at rest, Einstein showed how its total mass equals its energy times the square of the speed of light. Or, put another way, E = m x c^2.
Since the speed of light is constant, mass and energy are two sides of the same coin. By adding energy, such as by pushing it, you also add mass. As the accelerating object gains more mass, it requires even more energy to accelerate, which in turn makes it even harder to push.
Special relativity might seem weird, but more than a century after Einstein wrote his proposal, it's become one of the most well supported theories in physics.
For anything that trudges along at a mundane – everyday pace whether it's juggling balls, rockets, bullets, or bull ants – the rules of special relativity don't really make much of a difference. But for any object at insanely fast 'relativistic' velocities (that means speeds that approach that of light) special relativity really packs a punch.