Is God a Mathematician_ - Mario Livio [102]
There was one major question that Newton left completely unanswered: How does gravity really work? How does the Earth, a quarter million miles away from the Moon, affect the Moon’s motion? Newton was aware of this deficiency in his theory, and he openly admitted it in the Principia:
Hitherto we have explained the phenomena of the heavens and of our sea by the power of gravity, but have not yet assigned the cause of this power. This is certain, that it must proceed from a cause that penetrates to the very centres of the Sun and planets…and propagates its virtue on all sides to immense distances, decreasing always as the inverse square of the distances…But hitherto I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypotheses.
The person who decided to meet the challenge posed by Newton’s omission was Albert Einstein (1879–1955). In 1907 in particular, Einstein had a very strong reason to be interested in gravity—his new theory of special relativity appeared to be in direct conflict with Newton’s law of gravitation.
Newton believed that gravity’s action was instantaneous. He assumed that it took no time at all for planets to feel the Sun’s gravitational force, or for an apple to feel the Earth’s attraction. On the other hand, the central pillar of Einstein’s special relativity was the statement that no object, energy, or information could travel faster than the speed of light. So how could gravity work instantaneously? As the following example will show, the consequences of this contradiction could be disastrous to concepts as fundamental as our perception of cause and effect.
Imagine that the Sun were to somehow suddenly disappear. Robbed of the force holding it to its orbit, the Earth would (according to Newton) immediately start moving along a straight line (apart from small deviations due to the gravity of the other planets). However, the Sun would actually disappear from view to the Earth’s inhabitants only about eight minutes later, since this is the time it takes light to traverse the distance from the Sun to the Earth. In other words, the change in the Earth’s motion would precede the Sun’s disappearance.
To remove this conflict, and at the same time to tackle Newton’s unanswered question, Einstein engaged almost obsessively in a search for a new theory of gravity. This was a formidable task. Any new theory had not only to preserve all the remarkable successes of Newton’s theory, but also to explain how gravity works, and to do so in a way that is compatible with special relativity. After a number of false starts and long wanderings down blind alleys, Einstein finally reached his goal in 1915. His theory of general relativity is still regarded by many as one of the most beautiful theories ever formulated.
At the heart of Einstein’s groundbreaking insight lay the idea that gravity is nothing but warps in the fabric of space and time. According to Einstein, just as golf balls are guided by the warps and curves across an undulating green, planets follow curved paths in the warped space representing the Sun’s gravity. In other words, in the absence of matter or other forms of energy, spacetime (the unified fabric of the three dimensions of space and one of time) would be flat. Matter and energy warp spacetime just as a heavy bowling ball causes a trampoline to sag. Planets follow the most direct paths in this curved geometry, which is a manifestation of gravity. By solving the “how it works” problem for gravity, Einstein also provided the framework for addressing