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WILL time end? It is a disturbing prospect, more chilling even than the end of our universe, because in most of the ordinary scenarios of cosmic doom there remains the comforting possibility that a new universe might rise from the ashes of the old.
But if time itself can end, then we surely have no get-out clause. There will be no time for anything new to get started. That will be that.
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In recent years, cosmologists have been trying to face this final curtain. Perhaps it is not such a grisly proposition for them, as they are already used to the many ends of time that come out of Einstein’s general theory of relativity. This gravitational theory implies that when a star collapses into a black hole, its matter is crushed to a single point of infinite density called a singularity, a full stop where all quantities become infinite and time ends.
You could go and see what it is like by heading for the nearest black hole and jumping in, something we might all get to enjoy if the universe ends in a “big crunch” and everything collapses down to one cosmic singularity.
General relativity is probably not an exact theory, however, and physicists suspect that its perfectly point-like singularities will be blurred out slightly by the effects of quantum mechanics. If that is the case, when space-time goes through the mangle of a black hole or big crunch there may be enough room for it to survive and bounce back on the other side.
Having escaped a violent end, time might simply coast to a halt. One of the distinctive things about time is that it only points one way, heading from past to future. This directionality stems from the fact that the known universe used to be super-dense (see “Time’s arrow”). When everything was neatly tucked away in one spot, it was in a highly ordered state, but as space expanded everything was free to become more disorderly.
After countless trillions of years, when all the stars have burned out and even the black holes have evaporated, all matter in our universe may become evenly spread out. Everything will be as disordered as it can possibly be. Then there will be no direction to time, and not much happening either.
On a subatomic scale, however, particles will still be colliding with one another and occasionally these random collisions will lead to something more interesting. Rare statistical flukes could produce an ordered object – a glass of beer, say, or a puzzled lemur – which will briefly be subject to the effects of time once more. So in this picture time is very ill, but it is not quite dead.
Time’s true demise may be decreed by the multiverse. Many models of the cosmos involve a form of expansion called eternal inflation, in which new universes are constantly being created, each with different properties. Cosmologists want to get a handle on the range of possibilities – how many of these universes have stars, how many have matter, what proportion is hospitable to life – but they have hit a snag. In an infinitely multiplying universe there is an infinite number of versions of everything, and it becomes impossible to calculate probabilities.
To get around this, some cosmologists pretend that most of the multiverse doesn’t exist. By imposing an arbitrary cut-off in space and time, they can calculate probabilities for our local patch of the multiverse. It seems to work. For example, they use this to predict a value for the cosmological constant – the repulsive force that boosts the expansion of the universe – that’s in the same ballpark as that measured by astronomers.
While that sounds like good news for cosmologists and their calculations, it is bad news for time. , a theorist at the University of California, Berkeley, has pointed out that these probabilities are only consistent if the real multiverse is finite in time. If the cut-off multiverse reflects reality, Bousso calculates that time probably only has a few billion years left. “It’s a crazy sounding proposition, but in physics one has to be careful ruling things out just because they seem crazy,” says Bousso.
Time has the right to ask for a second opinion, in the hope of a better prognosis. Alan Guth of the Massachusetts Institute of Technology and Vitaly Vanchurin of Stanford University in California say they can calculate probabilities in the multiverse without having to decapitate time. They still do not rule out the possibility, however.
If such an End of Ends is possible, what will it look like? at the University of Maryland in College Park has done an experiment to find out. As a stand-in for the universe, he used a material made from stripes of plastic deposited onto a film of gold, which bends light in a peculiar way. In this metamaterial, one axis acts like time drawing light rays inevitably onwards – a motion mathematically identical to light moving in space-time.
Smolyaninov joined some of this metamaterial to a piece of ordinary material where light can move freely in any direction, meaning there is no axis of time. He found that at the boundary between the two realms, where “time runs out”, light piles up to create a powerful electric field. Theory predicts that without energy losses in the material, the electric field would increase to infinity.
“Our physical vacuum probably behaves like a metamaterial, so our experiments probably make some sense,” says Smolyaninov. If the analogy holds, then in real space all energy fields would be boosted to huge values, raising the temperature and filling the last split second of existence with an inferno of particle creation. If you think everything is going to hell, maybe you are right.
A more serene vision of the end comes from , a philosopher who has collaborated with cosmologists in building a peculiar picture of reality that he calls Platonia. In Platonia, all possible configurations of matter exist. There is no passage of time, merely a set of unconnected instants, or “nows”. We experience the illusion of time because many of these nows are arranged as if they had evolved through time. Barbour thinks that the possibilities in Platonia should be infinite, and so the comforting illusion of time should be infinite too. If time doesn’t exist, it won’t end.
“Prediction is difficult,” said physicist Niels Bohr, “especially about the future.” So perhaps it’s not surprising that there is no last word on the end of time.
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