We have more reason to celebrate in the month of November besides Thanksgiving holidays. This year is the 100th birthday of Einstein’s theory of General Relativity. Since I will be in Las Vegas during the holiday season, there’s more reason and a very excellent excuse to throw a party without the traditional turkey dinner, the stuffing, gravy, and cranberry sauce—– we can instead celebrate the brilliant discovery of a theory which completely altered our way of viewing the universe and our understanding of space and time.
In 1905, Einstein formulated the special theory of relativity linking space and time and unifying mass and energy. It was a revolutionary theory but it didn’t include gravity. Einstein wanted to unify gravity with space-time and mass-energy to put together a complete picture of the universe. In November of 1915, Einstein accomplished this unification and laid out his General theory of Relativity which shocked humanity’s collective mind.
The theory tells us that space-time is not fixed but can be bent and warped and twisted by the presence of mass and matter. This warping of space-time is the manifestation of what we come to know as gravity. Needless to say, this was another major revolutionary invention and wasn’t accepted by many physicists at the time. According to the theory of general relativity, light would be bent when passing through the sun due to the mass curving space around it. In the year 1919, Sir Arthur Eddington led the naval expedition to the Portuguese island of Principe and observed the bending path of the light emitted by distant stars passing near the sun. The amount of bending was a perfect match to the general theory of relativity and Einstein became a celebrity over night and rocked the world of science. He appeared worldwide on the front page of major newspapers.
Einstein was confident about his new theory even before he became famous. Despite the success of Newtonian law of gravity, it couldn’t account for the orbit of Mercury and Einstein worked out the calculations with his general relativity and predicted the precession of the orbit of Mercury. This gave Einstein a great pleasure knowing that the general theory of relativity indeed works very well . The equations of general relativity also gave rise to solutions for an extreme curvature of space which we call black hole. These days black holes have been observed and confirmed although we still don’t have a complete understanding of how they come about and how they will end.
In this centennial of Einstein’s successful year, we want to take a look at the impact of general relativity on how we come to understand the universe and what benefits we have from our daily living. Einstein predicted that the passage of time depends on the strength of gravitational field. That means clocks will run slower in stronger gravity than in weaker gravitational surroundings, an effect we now call Gravitational time dilation. This has a real implication for the GPS system built into your phone. The fact that the clocks in satellites run more quickly than their Earth-bound clocks must be taken into consideration. If this time dilation effect is not accounted for, the difference in the clocks would lead the GPS system to inform you that you were in the wrong location. This effect is quite large for each day, the offset would be about 6 miles and within a very short period of time, the GPS system would be completely useless.
Despite all the successes of general relativity, there is still one more prediction that has not been confirmed directly. Since mass can warp space, and when the mass is moving, it can set up vibrations in space—- what we now call gravitational waves. According to general relativity, two orbiting neutron stars over a long period of time will show a decrease in their orbital period, about 75millionths of a second per year which is due to the emission of gravitational radiation. The evidence was considered very significant that two scientists were awarded the Nobel Prize back in 1993. Of course, it would be much better if we can gather direct evidence and be able to observe gravitational waves in the future. A series of experiments are underway using a range of technologies here on Earth. We hope to observe gravitational ripples as they pass over Earth. When this is achieved, it’ll mark another major victory of Einstein’s theory of general relativity which is already one of the most elegant intellectual inventions of all time.
In short, mass and matter tell space how to curve, and the curvature of space tell mass how to move. A great quote by John Wheeler. So lets begin celebrating Einstein’s 100th birthday and we have til the end of this week. Bring the party on!