Where did time come from?
Researchers are unravelling space-time to find out if one of its components is an illusion
“It’s that riding the wave of the increasing entropy of the universe that gives us the perception of the flow of time”
Prof Sean Carroll
Time is a constant part of our everyday lives – even as you read this sentence, its first words have disappeared into the past while the next paragraph looms up from the future. We’re so used to the experience of flowing time that we very rarely stop to think about what it really is or why it works in the way that it does.
While most cosmologists agree that time is an innate feature of the universe with its direction defined only by other laws of physics, a controversial new theory suggests that its flow could be driven by the fact that our universe is expanding – an idea which means that, at least in theory, time could one day be thrown into reverse.
Fortunately, even though time is hard to visualise, scientists have a well-established way of treating it as a fourth dimension: a direction in which phenomena can change their location, similar to the familiar three dimensions that locate objects in space.
“Time is not that hard to understand,” says Professor Sean Carroll, research professor at the California Institute of Technology and author of From Eternity to Here: The Quest for the Ultimate Theory of Time, optimistically. “I don’t think it’s a mystery, and I don’t think it’s been a mystery for a very long time. It was a bit simpler back when we had Isaac Newton and time and space were both absolute. Then we would have considered the universe to be made of space and everything in it, and the universe keeps happening over and over again – time is just the label we put on those different versions that happen one after another with things in different positions, a bit like the pages of a book.”
As he explains, our modern view of time – superseding Newton’s – was shaped by the breakthroughs of Albert Einstein a little over a century ago: “In Einstein’s view there’s actually ‘space-time’ [a structure with four dimensions that determines how objects are located in the universe], and how an observer slices that space-time into time and space is a little arbitrary – different people can look at it in different ways, and no one is right and no one is wrong. But still, in any one point of view there’s a sequence of moments. It’s a little more complicated, but really it’s not that hard – it’s certainly no more profound to ask what time is, than-to-ask-than, what space is.
“I guess I’m smuggling in an eternalist point of view here, which treats every moment of time as on an equal footing, as opposed to a ‘presentist’ point of view that says only the world right now is real. In a post-Einstein world that’s really not how many physicists think,” he continues.
But if every moment in time is equivalent, why can’t we move back and forth in time at will – why does time only appear to flow in one direction, and does it really ‘flow’ at all?
“An important subtlety is the difference between time as a label on different points versus the ‘arrow of time’ – the fact that the past and future are different for us in a way that is not true for space,” explains Carroll. “All directions in space are the same (at least outside the influence of gravity), but in time the two directions are very different, and there’s just one direction that we go in.”
Ask most physicists about the arrow of time and they’ll almost certainly explain it in terms of the second law of thermodynamics (the science that relates heat and energy, discovered and expanded upon during the 19th century). The second law is a simple statement that says the amount of disorder or ‘entropy’ in a closed system increases with time, unless energy is supplied to create more order.
The system can be anything, and its entropy can be thought of as the ‘useless’ energy within it. A classic example is a pair of boxes containing hot and cold gases with a connecting door between them. At first this particular arrangement’s entropy is low because most of its energy is in useful form – locked up in hot, fast-moving gas molecules in one box that could be used, for example, to pump an engine piston.
Open the door between the boxes, however, and over time the gas molecules of different temperature will inevitably mix together. Hotter gas molecules will collide with colder and slowermoving ones, transferring energy until eventually both boxes are filled with gas of an intermediate temperature. A once-orderly system with low entropy has now become a disorderly (high-entropy) system of jumbled molecules with less capacity to do useful work.
But what exactly does all this have to do with the direction of time? Well, according to current cosmology, the universe too can be thought of as a closed system – there’s only so much energy to go around and no way of supplying energy from outside to reverse the rise of entropy. The universe therefore started out in a highly ordered, low-entropy state (the Big Bang era where all matter had uniformly high temperatures), and entropy
has been increasing ever since. Today, much of its matter is still concentrated in low-entropy systems such as stars, but in the far future, as succeeding generations of stars burn out, the cosmos will succumb to ‘heat death’ in which matter and energy become more and more evenly and thinly spread.
“In my perspective, the arrow of time just comes from the fact that the entropy of the universe was smaller in earlier times and grows larger at later times. There’s nothing propelling the universe forward in time, it’s just you have all these different moments distinguished by the rule that earlier means lower entropy, later means higher entropy,’ says Carroll. “That explains why you can remember the past and not the future, why you can make choices about the future but not the past and so forth. There are good reasons why a person, considered as a series of people at moments in time with increasing entropy, feels that time flows in that direction.
“It’s more of a psychological effect than anything else – we carry around in our minds a moving image of what we were a second ago, what we will be a second from now, and we’re constantly updating on the basis of what we learn, how our surroundings change and so on. It’s that riding the wave of the increasing entropy of the universe that gives us the perception of the flow of time.
If, however, the idea is that there’s some active element of reality that pushes the universe forward in time to bring about change, then that’s not really part of physics as we understand it.”
Nevertheless, that possibility of a ‘driving force’ behind the flow of time is where a controversial new idea put forward by physicist Richard Muller of the University of California, Berkeley could come into play. Muller’s own 2016 book, Now:
The Physics of Time, suggests that time is a real phenomenon, and that more time is created as space itself grows.
While Carroll and many other cosmologists are doubtful about the need for a driving force behind the arrow of time, Muller does at least put forward a plausible way of testing his proposal. The idea comes from gravitational waves – the minute distortions of space-time that ripple out across the universe from certain cataclysmic events
involving large asymmetric masses.
Although such waves were predicted by Einstein in the early 20th century, they have only been detected at the super-sensitive Laser Interferometer Gravitational-Wave Observatory (LIGO) in the past couple of years. Muller and his Caltech collaborator Shaun Maguire argue that because the black hole collisions that generate gravitational waves create ‘new’ space, they should also create a small amount of new time. What’s more, the quantities of time involved (around a millisecond) are large enough to be measurable using existing LIGO instruments, so Muller’s intriguing hypothesis could either be proved wrong or pass its first observational test in the relatively near future.
While Muller’s model of time is a radical departure from those supported by the majority of cosmologists, the way it makes the flow of time ’real’, rather than something merely defined by the increase of entropy, gives it an obvious intuitive appeal. Technically, the difference between the roles of time and those of other dimensions is its lack of ‘symmetry’. Physicists define symmetry as the property of a system that remains unaltered by a ‘transformation’ or movement in terms of one or more dimension. A system can be transformed
General relativity allows flexibility in spacetime, perhaps including the possibility of ‘wormhole’ tunnels between one region of space-time and another
In 1908, Hermann Minkowski showed how space and time could be modelled as a fourdimensional space-time ‘manifold’