By the Fall of 1915, Albert Einstein was a bit grumpy.
And why not? Cheered on, to his disgust, by most of his Berlin colleagues, Germany had started a ruinous world war. He had split up with his wife, and she had decamped to Switzerland with his sons.
He was living alone. A friend, Janos Plesch, once said, “He sleeps until he is awakened; he stays awake until he is told to go to bed; he will go hungry until he is given something to eat; and then he eats until he is stopped.”
Worse, he had discovered a fatal flaw in his new theory of gravity, propounded with great fanfare only a couple of years before. And now he no longer had the field to himself. The German mathematician David Hilbert was breathing down his neck.
So Einstein went back to the blackboard. And on Nov. 25, 1915, he set down the equation that rules the universe. As compact and mysterious as a Viking rune, it describes space-time as a kind of sagging mattress where matter and energy, like a heavy sleeper, distort the geometry of the cosmos to produce the effect we call gravity, obliging light beams as well as marbles and falling apples to follow curved paths through space.
This is the general theory of relativity. It’s a standard trope in science writing to say that some theory or experiment transformed our understanding of space and time. General relativity really did.
Since the dawn of the scientific revolution and the days of Isaac Newton, the discoverer of gravity, scientists and philosophers had thought of space-time as a kind of stage on which we actors, matter and energy, strode and strutted.
With general relativity, the stage itself sprang into action. Space-time could curve, fold, wrap itself up around a dead star and disappear into a black hole. It could jiggle like Santa Claus’s belly, radiating waves of gravitational compression. It could even rip or tear. It could stretch and grow, or it could collapse into a speck of infinite density at the end or beginning of time.
Nowadays, some of the knowlegdes or applications would be ignored without the Theory of Relativity:
- GPS. GPS systems owe a debt of gratitude to Einstein and his thought experiments, without which they would not exist at all.
The highly precise clocks have oscillators that function not on springs or pendulums, but atoms.
The level of precision required for satellites is down to this atom clock, whose ticks must be known to an accuracy of 20 to 30 nanoseconds.
Because the satellites are constantly moving relative to the Earth, effects predicted by Einstein's theory must be taken into account.
In particular, the pull of gravity is stronger on Earth than in the satellite's orbit, meaning time is passing marginally faster in the latter than it is in the former.
The precision of atomic clocks makes the desired accuracy achievable and GPS technology corrects this discrepancy to make the location accurate.
- The Big Bang theory. Scientists realised that if you went back far enough in time, the universe would get increasingly smaller, or shrink, until the moment when it appeared.
This became known as the Big Bang and suggests that since that moment, the universe has expanded.
- Black holes. They are among the most mysterious objects in our universe - concentrated wells of gravity from which nothing, not even light can escape.
But without Einstein's general relativity equations, we could still be ignorant of the existence of black holes, as it was instrumental in their discovery.