## 100 Years of General Relativity

Posted by on Wednesday, November 25, 2015

Exactly one century ago today, Albert Einstein submitted a paper for publication in which he generalized his already successful special theory of relativity. The single paper contained the bulk of the theory of general relativity in a clean finished form, and would forever change not only all of physics, but also our view of nature itself.

It must be said as well that there is some controversy on the authorship, as the brilliant mathematician David Hilbert had submitted a nearly identical theory five days earlier, but it didn't get published until the following spring. Einstein and Hilbert had been discussing this theory for many years, and each had lectured at the other's institution and so they clearly knew what each was working on. And unfortunately, Hilbert's published paper had been revised and the original paper was lost, so we will never know exactly what he had proposed. It is generally believed now that Hilbert's theory had an error in it, but that is not certain. However it must be stressed that both of these men were true academics and they never fought each other other authorship. Hilbert and Einstein both loved the theory they had created, and were more interested in the scientific progress than the credit.

This story really begins in 1905, Einstein's now famous miracle year, in which Einstein showed that requiring the laws of physics to be independent of the observer would necessarily make time and space two aspects of a single four-dimensional spacetime. (In truth, Einstein didn't quite make this jump to a four-dimensional spacetime, but his paper lead Minkowski to provide this more intuitive version of special relativity.) This new theory changed the way we viewed space and time, made mass and energy equivalent, and eliminated the long held belief in a universal time that would be the same for everyone.

For the next ten years Einstein struggled to expand his new theory to include gravity. He realized that in many ways, gravity in one frame of reference is equivalent to acceleration of an observer in another frame of reference without gravity. Through a long and convoluted series of ideas, Einstein came to realize that spacetime itself must be dynamic. It must curve around mass and energy. It would shrink and expand. Space would change over time. And what we once believed to be the force of gravity was in fact the effects of moving through a curved spacetime.

And that lead other people over the decades to discover other mysteries of general relativity. Schwarzchild found the first solution of Einstein's equations, which described the curvature of spacetime around a spherical mass, and was shocked to find that there would be a density of matter that stopped even light from escaping - leading to the creation of the theory of black holes. Then Friedmann, Robertson and Walker discovered that on large scales spacetime should be expanding, a prediction proven by Hubble. More recent discoveries have included wormholes that connect distant regions of space, configurations of matter that seem to allow travel through time, and the possibility of exotic matter engines that allow travel faster than light. And with all the solutions already discovered, they amount to a tiny fraction of the possible solutions. The best is yet to come.

However it is not just interesting theoretical oddities that make general relativity such an interesting theory. The effects are actually large enough that they must be included in many everyday objects. Every GPS unit in every car and every smartphone requires general relativity to provide accurate measurements. Every space probe that has traveled to distant worlds has required general relativity to avoid going off course. And there is no doubt that in the coming years, the uses of this interesting theory will continue to grow and become an even bigger part of our lives.

And all because one century ago one man dared to dream of what it would be like to ride on a beam of light.

It must be said as well that there is some controversy on the authorship, as the brilliant mathematician David Hilbert had submitted a nearly identical theory five days earlier, but it didn't get published until the following spring. Einstein and Hilbert had been discussing this theory for many years, and each had lectured at the other's institution and so they clearly knew what each was working on. And unfortunately, Hilbert's published paper had been revised and the original paper was lost, so we will never know exactly what he had proposed. It is generally believed now that Hilbert's theory had an error in it, but that is not certain. However it must be stressed that both of these men were true academics and they never fought each other other authorship. Hilbert and Einstein both loved the theory they had created, and were more interested in the scientific progress than the credit.

This story really begins in 1905, Einstein's now famous miracle year, in which Einstein showed that requiring the laws of physics to be independent of the observer would necessarily make time and space two aspects of a single four-dimensional spacetime. (In truth, Einstein didn't quite make this jump to a four-dimensional spacetime, but his paper lead Minkowski to provide this more intuitive version of special relativity.) This new theory changed the way we viewed space and time, made mass and energy equivalent, and eliminated the long held belief in a universal time that would be the same for everyone.

For the next ten years Einstein struggled to expand his new theory to include gravity. He realized that in many ways, gravity in one frame of reference is equivalent to acceleration of an observer in another frame of reference without gravity. Through a long and convoluted series of ideas, Einstein came to realize that spacetime itself must be dynamic. It must curve around mass and energy. It would shrink and expand. Space would change over time. And what we once believed to be the force of gravity was in fact the effects of moving through a curved spacetime.

And that lead other people over the decades to discover other mysteries of general relativity. Schwarzchild found the first solution of Einstein's equations, which described the curvature of spacetime around a spherical mass, and was shocked to find that there would be a density of matter that stopped even light from escaping - leading to the creation of the theory of black holes. Then Friedmann, Robertson and Walker discovered that on large scales spacetime should be expanding, a prediction proven by Hubble. More recent discoveries have included wormholes that connect distant regions of space, configurations of matter that seem to allow travel through time, and the possibility of exotic matter engines that allow travel faster than light. And with all the solutions already discovered, they amount to a tiny fraction of the possible solutions. The best is yet to come.

However it is not just interesting theoretical oddities that make general relativity such an interesting theory. The effects are actually large enough that they must be included in many everyday objects. Every GPS unit in every car and every smartphone requires general relativity to provide accurate measurements. Every space probe that has traveled to distant worlds has required general relativity to avoid going off course. And there is no doubt that in the coming years, the uses of this interesting theory will continue to grow and become an even bigger part of our lives.

And all because one century ago one man dared to dream of what it would be like to ride on a beam of light.