The universe as imagined by Lucretius does not conform precisely to our current view of it, yet its affinities to ours are more striking than their differences, and there are continuities of meaning. They are of the same lineage.
The universe, as it is now supposed to be, is a vast system of matter, constantly expanding, consisting of innumerable worlds, like our solar system, star clusters, and galaxies of various sizes and shapes. This ever-expanding universe, which may be only one of many, began in an instant, about 13.7 billion years ago. What was before, no one knows — there are only the usual suspects — nor does anyone know when or how or if it will end, although it does not seem to be everlasting. A lifespan of 20 billion years is mortal for all that.
Lucretius supposed that atoms and space have no beginning and no end. Space is infinite in all directions, much as Isaac Newton conceived it, which should come as no surprise, for it is likely that he first encountered the idea of a world system by reading Lucretius’ poem.
Infinite space was filled with an infinite aggregate of atoms, each of them moving rectilinearly at the same speed and in the same direction, which, for convenience, was called “down.” They moved in parallel courses and so would never touch unless one or more of them shifted ever so slightly laterally. Now, Lucretius, following earlier Greek atomists, believed that the only active force in the world was impact, and that it was by impact that the dance of atoms begins, succeeding to regular circular motions, vortices, which cause groups of atoms to join together and enter into more or less stable and regular configurations, and so form worlds, which, like our planetary systems, star clusters, and galaxies, are beyond counting.
These worlds, like our star systems, are jerry built. They are not made to last, and although their duration may seem long when compared to the lifespan of animals like us, they will eventually perish, giving their material substance to new configurations, whence nature will once more work its generative magic.
The “swerve” is this infinitesimally small lateral motion that is supposed to make possible the generation of worlds; it is nature’s way of creation, unplanned, unpredictable, yet fateful. As Lucretius describes it, the swerve doesn’t happen once, but repeatedly, “at uncertain times and uncertain places.”
“If this did not occur, then all the atoms would fall like raindrops down through the void,” Lucretius theorized. “There would be no collisions, no impacts of atoms upon atom, so that nature would never have created anything.” (Book II, lines 216–93).
Notice how visual, how tangible Lucretius’ account of things is, and also how correct his reasoning! Although he did not have the advantage of instruments to try and experiment nature, he cultivated another indispensible element of scientific imagination — a keen observation of things and a rational capability to draw from experience plausible models to explain processes that could not be directly observed, and to fashion persuasive arguments to prove his theory.
Like Democritus, who started it all, he was not after myths or likely tales, but real explanations of things that were founded on experience and reason, on “the appearance and rational system of nature.”
Why did Lucretius believe that nothing exists forever, except atoms and void? Because as we look all around us, we observe things begin, increase, and then decay and perish, casting off or leaving behind the stuff of new beginnings, and new cycles of generation and decline. Without the swerve, there would be none of this.
But the swerve has another use. If the universe were a machine, whose parts and powers and operations were so connected that everything happened by necessity, then there would be no place for spontaneity. And if there were no spontaneity, there would be no freedom. And if no freedom, there would be neither creativity nor responsibility. And if not these, then art and morality would have never been invented. Nature would be an absolute despot.
Yet, according to Lucretius, nature has made provision for all of this by the simple device of the swerve. Free will, which Lucretius attributes to all animals, including us humans, is a power in us to act spontaneously, to begin an action and, by striving, to bring it to completion. He illustrates by describing the action of proud and well-trained racehorses: “The starting gates fly open, the horses are keen to go, but they can’t break out as fast as their minds would wish. For all the mass of matter must be stirred through the whole body, roused through every limb, before it can follow the prompting of the mind. So you see that heart or desire begins the motion, then mind and will join in and drive it on until it all reaches the body and limbs.” (Book II, lines 264–70).
How does the swerve account for all of this? It is a sign that nature is not fully determined. If we view nature from the bottom up, we see not clockwork, but striving, struggling, trial and error, success and failure. Out of all of this there occurs the emergence of feeling, instinct, desire and intelligence; of value, of pleasant and unpleasant, noble and vile, good and bad.
The late A.R. Ammons, a celebrated American poet, described poetic creation in a similar way. “It comes from anxiety … either the mind or the body is already highly charged and in need of some kind of expression, some way to crystallize and relieve the pressure … and an idea, an insight, an association occurs to you, then energy is released through the expression, a poem is written” and after this follows “a certain resolution and calmness.” 
This is the sequence: anxiety, thought, spontaneity, expression, order and contentment. John Locke supposed that all human action originates in a state of anxiety: the more acute the feeling, the more spontaneous the act, and all action, artistic or moral — even political action, noble or vile — is fulfilled in an expression that is represented in words and informs the mind with thoughts.
And so it has been with nature from the beginning. Nature is the greatest artist of all, and we her offspring and her imitators.