In 1915, Einstein publishes his general theory of relativity. It's a simple idea that creates a big bang in the world of physics. When, or perhaps more appropriately if, fully understood, the theory describes a world where space and time evolve dynamically: no longer absolute and eternal, but relative, no longer a fixed stage on which the great dramas of life play out, but non-Euclidean and non-Newtonian sets constantly changing and being changed by the actors and their actions inside them. Man, no longer a play thing to immutable and inscrutable fate, has cut the strings and is executing a mathematical and cosmic dance with the gods. Le point fixe qui bouge, in Lecoq terms.
Perhaps you are thinking that this is a bit dramatic or poetic for a description of modern physics, but then you would be forgetting that Einstein himself was a dreamer. In fact, he describes a vivid dream involving cows, a farmer and an electric fence as the inspiration for his special theory of relativity. "Science does not know its debt to imagination," writes Emmerson, and so it is that we owe a debt to the artists, to the writers and dreamers who travel and trade in the land of imagination, for nothing can be theorized or studied that hasn't first been imagined.
Beyond the big bang (tautology? is there anything before the big bang?), Einstein's theory creates the theoretical and mathematical framework which predicts and describes black holes, gravitational lensing and much more. In terms of gravity as geometry, people offer the image of a rubber sheet stretched out with bowling balls and other balls rolling around on it. If you visualize how each ball would create it's own dip and pull in the rubber sheet around it and what effect that might have on a nearby object, then all you have to do is add two more dimensions (one in space and one in time) to understand general relativity's description of the world.
Though it may be hard to understand now, one of the most revolutionary ideas inherent in general relativity was that the universe (space and time) might not be fixed and eternal. When you start plugging in the numbers out pops a picture of the universe which includes a big bang, i.e. a beginning, as well as expansion. And this beginning and expansion leads to an obvious question. If the universe starts at the big bang, 'how does it end?'
Remember that people have been burned at the stake, or worse, for asking similar questions, or worse, proposing answers to them. Take Giordano Bruno for example. He was burned just for suggesting that the earth rotated around the sun and not the other way around (thus displacing man from the center of the universe). Even at the late date of 1915, and to a man of logic and reason like Einstein, the idea that the universe could have a beginning and an end is quite inelegant, frightening even. You have to be a real sensitive type or extremely paranoid to worry about a problem that is, by all reasonable calculations, billions of years in the future!
But worry he does, until finally he is led to commit what he considered the biggest blunder of his career. In order make the equations of general relativity describe a static world where the universe isn't expanding or contracting at the drop of a photon, Einstein inserts into them what he calls the Cosmological Constant. As it turns out, the universe is expanding and this has repeatedly been confirmed by observation. Moreover, the Cosmological Constant is not even mathematically very, well, constant, as it still tends to result in descriptions of an expanding or contracting universe when you start adding it all up.
In a further twist, it turns out that an accurate mathematical model of the universe, i.e. general relativity, may need a cosmological constant, not for aesthetic reasons about whether we think it makes sense for the universe to be able to expand or not, but in order to take into account the effects of the vacuum, or ground-state energy, which act to push against the pull of all the matter. Although this isn't what Einstein had in mind, it does sort of mitigate the blunder. In fact, it might make it one of the more inspired, and dare we say, creative acts of theoretical legerdemain in the history of physics. It was certainly a leap of faith. But, wait, we're talking about science! Next thing you know you are going to be telling me that Darwin was an intelligent designer...
So, did general relativity change the world? One might say that it didn't change it at all, just our fundamental understanding of everything in it and how it all works. And that, as the poem goes, has made all the difference. To continue the Frosty metaphor, it's two roads diverging not just in a wood but because of the wood and by your traveling there (and let's not forget the effect that roads and the wood have on you either).
But the whole sha-big-bang does leave one wondering, how is it all going to end? Well theoretically speaking there are basically two possibilities: continuing expansion or contraction. Either there is not enough matter and energy, and therefore gravitational pull, to stop the expansion of the universe and it keeps on going until there are nothing but vast distances between every soon to be cold and extinct particle, or there is enough and the universe's expansion is eventually slowed down and stopped, at which point it begins to contract back in on itself into a final fiery and dense singularity (aka big bang bis or part deux).
Interestingly, a year after Einstein publishes his theory, our friend Robert Frost publishes the following poem:
Fire and Ice
Some say the world will end in fire,
Some say in ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
Did Frost subscribe to scientific journals? Did he somehow understand Einstein's general relativity (before anyone else) and what it implied about the beginning and end of the universe? Probably not, but actually that is what makes this poem even more prescient (as in preceding science as imagination always must), Frost has correctly described the two possible ends of the universe, fire and ice, and their anthropomorphic equivalents, desire and hate. So which is it?
Well in 1998, observations of supernovas have shown us that the universe is not just expanding, it's accelerating! More precisely the rate of its expansion is accelerating. While there is still room for new discoveries or understandings, like the effect of quantum gravity on these calculations or the whole dark matter thing, this acceleration pretty much tells us how it's all going to end, I'm talking about life, the universe and everything. In the words of that immortal poet Robert Van Winkle, it's going to be "ice, ice, baby."
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