By Marcel Gleiser, NPR, April 15, 2015
One hundred years ago, a 36-year-old Albert Einstein presented the complete formulation of the General Theory of Relativity to the Prussian Academy of Sciences. Across the world, events and conferences will be celebrating what is considered, without hyperbole, the most beautiful of physical theories, marrying mathematics with physical concepts in deeply meaningful and elegant ways. Some consider it the highest intellectual achievement in history.
Whatever your take, it is undeniable that Einstein's theory is a magnificent example of the power of the human imagination as it attempts to decipher nature's most hidden secrets: Welcome to the universe of curved spaces, black holes, the Big Bang, wormholes and even multiple universes.
First, why "general"? It distinguishes it from Einstein's "special" theory of relativity, proposed in 1905, which focused on motions with constant speeds. The special theory revolutionized our views of space, time and matter. Einstein showed that our perception of reality is myopic, distorted due to our sluggishness; could we perceive motions with speeds close to the speed of light (186,000 miles per second), we would see objects shrinking in the direction of their motion, clocks slowing down, and masses increasing with speed. These weird effects are all around us, but imperceptible for the speeds we are used to. (Consider this: If you blink your eye, light goes about 7 1/2 times around the Earth.) They are routinely observed in high-energy physics, where subatomic particles collide with speeds close to the speed of light. Even a GPS corrects for relativistic effects: The satellites that provide us the service move fast enough in their orbits to justify the small correction for improved accuracy. (They also use general relativistic effects due to Earth's curving of space around it.)
Einstein knew that the special theory was not the final word. After all, objects don't move at constant speeds in straight lines, but accelerate and turn corners. His general theory was designed to include all motions. To his surprise, and in a moment of spectacular intuition, Einstein realized that any theory that includes accelerated motion must also be a theory of gravity. Why? Because accelerated motion can mimic gravity and vice-versa. We know this from riding fast elevators. As you start going up you feel heavier. But how could that be? You didn't gain weight by stepping into the elevator! The accelerated motion upwards acts just as gravity, as if Earth had suddenly become more massive. Conversely, as the elevator goes down fast you feel lighter. If it simply fell down, you would float in mid-air, without any feeling of weight.
This weightlessness, by the way, is what astronauts feel when they are in orbit. It's not absence of gravity that makes them float; they are effectively falling down as they circle the Earth. The circular orbit is a product of Earth's gravity pulling them downwards and the tangential motion with constant speed that keeps them moving around.
Einstein struggled to get his theory right; the math was really hard. At the time, the Theory of Gravity was due to Isaac Newton, who figured that gravity could be described as a force that fell with the square of the distance. Newton assumed gravity was instantaneous and that it acted across space like a ghost. Although mysterious, the theory worked really well, for must purposes. Einstein proposed something completely different. Instead of an action at a distance, gravity was due to the curvature of space around an object: The more massive the object the more curved the space around it. It's like when you sit on a mattress: Close to you, the mattress gets deformed, and it will get more deformed the heavier you are. If you now picture space as the mattress (you need to add an extra dimension, as the surface of the mattress is two-dimensional and we live in three dimensions) you see what Einstein imagined.
Space became elastic, malleable; it responded to the distribution of matter in it. Conversely, an object travelling in a curved space would have its trajectory deviate from a straight line. As the physicist John Wheeler quipped, "Mass tells space how to bend and space tells matter how to move.”
Being a physicist, Einstein knew that his wild idea would only be taken seriously if he could provide real-world examples where the new effects could be verified. Testability, the core of the physical sciences! He proposed three tests of his new theory. One was a correction to the orbit of Mercury, which precessed mysteriously around the sun like a wobbling top. Newtonian theory couldn't get the numbers right, but Einstein, using the idea that Mercury felt the curvature of space around the sun, got it right. Another effect was the bending of starlight as it passed near the sun. In 1919, astronomers set off to the western coast of Africa and to the city of Sobral in Brazil to measure the deviation of the light from stars during a solar eclipse. (This way they could compare the two situations, with the sun nearby, and far from, the path of the starlight). The results were convincing enough to validate Einstein's theory. Newton's theory was an approximation valid for weak gravity. In the new theory of gravity, the shape of space and the flow of time respond to the distribution of mass in the universe.
Within weeks of the announcement of the eclipse measurements, Einstein became a superstar. On Nov. 7, 1919, Londoners woke up to dramatic headlines from the Times: "Revolution in Science. New Theory of the Universe. Newtonian Ideas Overthrown." Likewise, The New York Times reported three days later: "Lights All Askew in the Heavens; Men of Science More or Less Agog over Results of Eclipse Observations. Einstein Theory Triumphs. Stars Not Where They Seemed or Were Calculated to Be, but Nobody Need Worry." The "worry" here presumably refers to the fear that the stars would fall on our heads. Indeed, there is no need to worry about that. Time magazine would elect Einstein "Person of the Century."
Perhaps the greatest lesson from Einstein and his theories of relativity is that reality is not what it seems. What we perceive as real is a distortion, a cognitive fabrication due to how our brains take in the world around and within us. Science expands our vision of reality, revealing what often appears to be strange — and unforeseen connections and possibilities. As we continue our struggle to understand nature and its mysteries, it is good to remember Einstein's immortal words: "What I see in Nature is a magnificent structure that we can comprehend only very imperfectly, and that must fill a thinking person with a feeling of humility."
Marcelo Gleiser is a theoretical physicist and cosmologist — and professor of natural philosophy, physics and astronomy at Dartmouth College. He is the co-founder of 13.7, a prolific author of papers and essays, and active promoter of science to the general public. His latest book is The Island of Knowledge: The Limits of Science and the Search for Meaning. You can keep up with Marcelo on Facebook and Twitter: @mgleiser.