Space by itself and time by itself are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.
– Hermann Minkowski
E=mc2
– Albert Einstein
This is more or less the centennial of Einstein’s theory of relativity. Einstein’s work turned the world of physics completely upside down. It overturned more than two hundred years of scientific thinking, as well as Newton’s apple cart. Isaac Newton had been one of the world’s most profound thinkers and had laid out a framework for understanding gravity that not only agreed with common sense, but it also worked. The thing is that it still does work. Engineers use it to send spacecraft into orbit and on distant journeys to the planets and beyond. Close enough for government work as they say. It is however incomplete in fully describing the gravity of the reality we actually live in.
Einstein realized this and proposed a theory where gravity is but a consequence or side effect of mass, space, and time.
The good professor Einstein actually had not one but… count em… two theories of relativity. The first one is the so-called ‘special’ theory. This was first published in 1905. What makes it so special you ask? It really isn’t special at all; it’s just that it applies to a special case of study where the frames of reference in question are moving in uniform motion relative to one another.
Arguably the most famous equation in the world is one of the features of special relativity and basically states the equivalence of mass and energy.
The other fun aspects of special relativity involve things like time dilation and length contraction and event occurrence with respect to observers who have different frames of reference to each other. Typically those frames of reference which are moving in uniform relative motion to each other. In common parlance: moving clocks will be seen to tick more slowly than clocks at rest, and an object in motion when measured in the direction of motion will be shorter than an identical object that is not moving. Special relativity is also responsible for the absolute speed limit of light, and for leading to the insights of the blending of space and time into the famous ‘space-time continuum’.
The General Theory on the other hand applies where the frames of reference are not moving in uniform relative to one another; most specifically where the acceleration due to gravity is present. Einstein published this in 1916. General relativity provides a geometric model of gravitation; stuff like the curvature of space-time in relation to the energy and momentum of matter. It goes a long way in explaining really big things like stars and black holes, and even the expansion of the universe. Gravity bends light and even influences the passage of time…how cool is that? It’s no wonder that on high gravity days at the climbing gym or crag that time seems to pass so slowly.
Relativity is a highly proven and successful theory. But there’s a catch however. There is one thing it doesn’t do, and that is to be able to reconcile itself with quantum mechanics, which of course is the realm of the very, very small. Quantum theory as we understand it today is also highly proven and successful. The search has been on for years without much success for the so called ‘Theory of Everything’ that will resolve Relativity and Quantum mechanics into one whole and consistent view of reality. There are lots of really smart people working on this. Currently this dilemma poses one of the ‘last unanswered’ problems of physics.