Congratulations! 100 years ago today, on Thursday, November 25th, 1915, Albert Einstein presented his General Theory of Relativity. After having been working on it for eight years he finished one of the most beautiful and robust scientific theories. In these 100 years, the theory has passed from not being accepted at first, to not having found any deviation from the observations and experimental tests. In 1905 Einstein presented his special theory of relativity, that unified space and time, matter and energy. This first theory showed us how these values depended on the observer and changed our view of space and time as non being rigid, but deformable. At that time, many scientists were about to find it: Poincare, Lorentz, Fitzerald. Indeed, these scientists had already found the equations that Einstein presented in his theory. But they didn't understand their meaning. They though those mathematical expressions weren't but artifacts to get to the right numbers to agree with experiments. They believed they didn't have any physical significance, though. Einstein, in a very elegant manner, presented those same equations from simple principles and gave them physical meaning. Conceptually, he had to work on the theory for ten years to understand it, and only three weeks to develop the mathematics to describe the phenomena. The General Theory of Relativity went the other way round. He visualized the physical theory with one thought experiment, but he needed eight years to develop the mathematical description. The Special theory was on the air in science at the time Einstein came through it. If it hadn't been him, another would have presented and understood it in a few years. The General theory was much more complex to envision, and Einstein had the great vision of a bright mind that could have taken decades to others at that time. It was not a straightforward work, Einstein went back and forth and had to learn mathematical concepts he didn't know. Due to the geometric aspect of the theory, it soon attracted the attention of many mathematicians. The first test to the new theory was the total eclipse of the Sun in 1919 (Einstein had preciously calculated the orbit of Mercury, where Newton's theory had a little discrepancy, Einstein's theory hadn't). At that time, the equipment was not very precise and the pictures taken by Eddington in that eclipse wouldn't have been published today [1]. But he was right. Since then, the theory of relativity has been tested in many different ways, showing always to be a correct theory [2, 3, 4, 5, 6, 7, 8, 9, 10]. But we have not yet managed to reconcile General Relativity with Quantum Mechanics. The fact that the first rules physics when huge masses are in play, and the latter rules physics of elementary particles makes it difficult to observe or design experiments where both aspects are into play. Masses of elementary particles don't have enough mass to create a detectable gravitational field. There are many theories that try to unify both of them. The most relevant are string theory and quantum loop gravity. Both approaches introduce new perspectives to our view of the universe and matter, but their predictions are far from being tested, which makes it almost impossible to verify whether one of them is correct. Physics final goal is to describe Nature and we need to test theories. If one theory doesn't describe Nature, it doesn't matter how beautiful it is, we can not consider it as a real physical theory. Also, the dark matter and dark energy conundrums we are dealing with in the latest years, have pushed some physicists to think that, maybe, General Relativity has to be reconsidered and it needs some modifications to account for the Universe as a whole. In any case, up to date, none of this alternative gravity theories has better success than relativity to describe the universe with or without the dark sector. But, besides the unification with quantum mechanics, and the understanding of dark matter and dark energy, the last 100 years have given us great success of General Relativity. We understand much better the universe, stars, galaxy formation that we did one century ago. And we also have General Relativity in our pockets. Due to the difference in speed and in gravitational field between us and the GPS satellites, our clocks don't run at the same pace as those in the satellites. There is a difference of 43 nanoseconds in one whole day. This difference is predicted by relativity and it might seem small, but if we don't consider it in the GPS protocol, our devices would be wring by 11 kilometers after 24 hours, making this positioning system useless. So, we should be very happy and thankful to Einstein and his General Relativity for letting us understand better our Universe, and for helping us drive our way to where we are going to have our turkey this Thanksgiving Day.
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## Andrés AragonesesPhysicist, working in quantum optics and nonlinear dynamics in optical systems. Loves to communicate science. ## Archives
October 2017
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