IN OUR EXPERIENCE, NOTHING EVER REALLY ENDS. WHEN WE DIE, our bodies decay and the material in them returns to the earth and the air, allowing for the creation of new life. We live on in what comes after. But will that always be the case? Might there come a point sometime in the future when there is no “after”? Depressingly, modern physics suggests the answer is yes. Time itself could end. All activity would cease, and there would be no renewal or recovery. The end of time would be the end of endings.
This grisly prospect was an unanticipated prediction of Einstein's general theory of relativity, which provides our modern understanding of gravity. Before that theory, most physicists and philosophers thought time was a universal drumbeat, a steady rhythm that the cosmos marches to, never varying, wavering or stopping. Albert Einstein showed that the universe is more like a big polyrhythmic jam session. Time can slow down, or stretch out, or let it rip. When we feel the force of gravity, we are feeling time's rhythmic improvisation; falling objects are drawn to places where time passes more slowly. Time not only affects what matter does but also responds to what matter is doing, like drummers and dancers firing one another up into a rhythmic frenzy. When things get out of hand, though, time can go up in smoke like an overexcited drummer who spontaneously combusts.
The moments when that happens are known as singularities. The term actually refers to any boundary of time, be it beginning or end. The best known is the big bang, the instant 13.7 billion years ago when our universe—and, with it, time—burst into existence and began expanding. If the universe ever stops expanding and starts contracting again, it will go into something like the big bang in reverse—the big crunch—and bring time crashing to a halt.
Time needn't perish everywhere. Relativity says it expires inside black holes while carrying on in the universe at large. Black holes have a well-deserved reputation for destructiveness, but they are even worse than you might think. If you fell into one, you would not only be torn to shreds, but your remains would eventually hit a singularity at the center of the hole, and your timeline would end. No new life would emerge from your ashes; your molecules would not get recycled. Like a character reaching the last page of a novel, you would not suffer mere death but existential apocalypse.
It took physicists decades to accept that relativity theory would predict something so unsettling as death without rebirth. To this day, they aren't quite sure what to make of it. Singularities are arguably the leading reason that physicists seek to create a unified theory of physics, which would merge Einstein's brainchild with quantum mechanics to create a quantum theory of gravity. They do so partly in the hope they might explain singularities away. Still, you need to be careful what you wish for. Time's end is hard to imagine, but time's not ending may be equally paradoxical.
Well before Einstein came along, philosophers through the ages had debated whether time could be mortal. Immanuel Kant considered the issue to be an “antinomy”—something you could argue both ways, leaving you not knowing what to think.
My father-in-law found himself on one horn of this dilemma when he showed up at an airport one evening only to find that his flight had long since departed. The people at the check-in counter chided him, saying he should have known that the scheduled departure time of “12 A.M.” meant the first thing in the morning. Yet my father-in-law's confusion was understandable. Officially there is no such time as “12 A.M.” Midnight is both the end of one day and start of the next. In 24-hour time notation, it is both 2400 and 0000.
Aristotle appealed to a similar principle when he argued that time can have neither beginning nor end. Every moment is both the end of an era and the start of something new; every event is both the outcome of something and the cause of something else. So how could time possibly end? What would prevent the last event in history from leading to another? Indeed, how would you even define the end of time when the very concept of “end” presupposes time? “It is not logically possible for time to have an end,” asserts University of Oxford philosopher Richard Swinburne. But if time cannot end, then the universe must be infinitely long-lived, and all the riddles posed by the notion of infinity come rushing in. Philosophers have thought it absurd that infinity could be anything but a mathematical idealization.
The triumph of the big bang theory and the discovery of black holes seemed to settle the question. The universe is shot through with singularities and could suffer a distressing variety of temporal cataclysms; even if it evades the big crunch, it might get done in by the big rip, the big freeze or the big brake [see box on opposite page]. But then ask what singularities (big or otherwise) actually are, and the answer is no longer so clear. “The physics of singularities is up for grabs,” says Lawrence Sklar of the University of Michigan at Ann Arbor, a leading philosopher of physics.
The very theory that begat these monsters suggests they cannot really exist. At the big bang singularity, for example, relativity theory says that the precursors of every single galaxy we see were squashed into a single mathematical point—not just a tiny pinprick but a true point of zero size. Likewise, in a black hole, every single particle of a hapless astronaut gets compacted into an infinitesimal point. In both cases, calculating the density means dividing by zero volume, yielding infinity. Other types of singularities do not involve infinite density but an infinite something else.