 It's very rare for much to withstand the test of time in science. Revision often leads to retraction, and that surefire theory that we thought was good today may actually not be so good tomorrow. But a hundred years ago, an Austrian physicist who had already changed the physics world put out a theory that made us rethink everything about the universe. And this is your space pod for November 20th, 2015. When Einstein introduced special relativity in 1905, he shocked the scientific world by putting the universe in a relativistic framework. But after that, he began to ponder, how could we put gravity into that framework? Well in 1907, he started an eight-year journey that would eventually lead to the theory of general relativity. Initially, it was treated as a scientific curiosity, some kind of brain buster that Einstein probably dropped on the world for fun. But as the numbers were crunched and the validity vetted, it was quickly hailed as the biggest advance in cosmology since Newtonian gravity. It explained many things, such as the shifting perihelion of the planet Mercury, solved large problems in physics that couldn't quite be explained with just Newtonian gravity and created a fine cosmological mesh with special relativity. General relativity sounds complicated. I mean, after all, it did take one of the greatest minds, the better part of a decade in order to figure it out. But in that complexity comes the beautiful simplicity of just how well it explains the physics. Spacetime is curved by matter, and that curve is experienced as gravitational force. That simple sentence sums up one of Einstein's greatest contributions to physics as we understand it. Satellite's trajectories, paths of light, and the passage of time that you and I experience is all relative to this curvature of spacetime. It's what allows things like gravitational lensing, black holes, and other curiosities of physics to occur. But as with everything in science, we must ask ourselves an important question. How do we know that general relativity actually exists? Luckily, there's been quite a few tests. English astronomer Arthur Eddington led an expedition in 1919 to observe a solar eclipse and compare the positions of stars near the sun with their predicted locations on an accurate star chart. If general relativity was at work, then those stars should appear slightly off. And what do you know? The imagery proved that the light from the stars was bent ever so slightly. In 2003, the Cassini spacecraft sent radio signals near the sun and the expected frequency was compared to what was actually received, and the signal showed that relativity was at play, showing up within 1,500th of the expected prediction. In 2004, a spacecraft called Gravity Pro Beat was launched to test relativity as it related to the Earth, specifically an effect called the geodetic effect. And this is a measurement of the effects of the curvature of spacetime caused by the Earth. And that effect was confirmed to within half a percent of what models predicted. Gravitational lensing is another way that we're able to predict general relativity. They won't happen without general relativity. And very amusingly, Einstein predicted that we would never have the optical systems to allow us to actually see gravitational lenses. Oh, if only Einstein could see what we can now. A new test is soon to start. Two Galileo navigation satellites for Europe were launched on August 22, 2014, and a fault in the Russian upper stage caused them to be placed into the wrong orbits. And over the past year, the European Space Agency has worked on raising the low points of their orbits so that the apogee and perigee only fluctuate by 8,500 kilometers. That difference allows the two satellites to experience a higher gravitational pull at one point in the orbit than at another point, meaning the atomic clocks on board will run a little slower closer to the Earth and a little faster away from the Earth. And we can compare these times to atomic clocks here on Earth. This orbital mishap has accidentally gifted us what will be the most precise test of general relativity ever. Thanks for watching The Space Pod. I'm Jared Head. What do you think about general relativity and Einstein's contributions to physics? Well, let me know down in the comments. I want to see what you've got to say. And also, don't forget to like and subscribe to us on social media. And a big thank you to all of our current patrons that we have on Patreon. If you really enjoy these space pods and are able to do so, we ask that you please seriously consider contributing to our Patreon campaign for them. The possibility of stopping these space pods has come up because we want our space pods to be self-sustaining, and currently they are not. So if we can get just a little bit more support, it should be able to make the continuance of space pods a reality. So until the next space pod, keep exploring.