WE ARE born. We die. We call the span that separates these events time. Its passage is perhaps the most fundamental feature of our human experience, yet we are incapable of saying exactly what it is. Worse – the laws of physics don’t help. That time exists is undeniable, but the way we experience it makes no sense.
“There’s an old joke about time – it’s nature’s way of keeping everything from happening at once,” says physics Nobel laureate Steven Weinberg of the University of Texas, Austin. To us mortals, time is the passage of the sun and seasons, the progressive wrinkling of our skin as we age – irreversible markers of a present that is moving forwards, and a future that is ineluctably becoming the past. Unlike space, time has a natural order. If A influences B, then B is always later in time. This is the central feature of time as we perceive it: as a flowing entity that orders our lives.
Advertisement
There’s only one problem with this, says David Deutsch of the University of Oxford: it’s nonsensical. We see ourselves as living in a present that marches down an imaginary timeline at a set pace. The imagery implies the existence of some sort of universal ticking time setting the beat against which all else is measured. “But what is that other time?” says Deutsch. We’ve only succeeded in creating a new problem.
Metaphysics special: Philosophy’s biggest questions unravelled
What is reality made of? How do we know we exist, or if God exists? Join us as we test nine fundamental questions against the best scientific knowledge
Read more
In Einstein’s relativity, our best large-scale theory of the universe, there isn’t even a single objective metronome tick. Time becomes folded up with space into a malleable, four-dimensional space-time, and its passage depends on how fast you are moving, or the strength of the surrounding gravitational field. Yet of the four dimensions of space and time, time remains somehow special. “If I tell you what’s happening in a region of space like the solar system, then we can predict what will happen in that same space at a different time, but not what is happening at the same time in a different space,” says Deutsch.
Quantum mechanics, the other pillar of modern physics, reinforces this view of time as something apart, while producing a picture completely at odds with relativity. Here, there is an objective “god’s eye” time that allows you to see all events encapsulated in time, including the future, from outside it. But while all quantum-mechanical observables – things that can be calculated about reality – depend on it, time is not itself an observable, so cannot be calculated. It can’t even be reliably measured: the principle of quantum uncertainty makes it impossible to distinguish the order of two events that are very close in time. “It gets harder to prevent effect preceding cause,” says Weinberg.
https://www.youtube.com/watch?v=umfjGNlxWcw
So between quantum theory, relativity and our own aberrant perception of a flowing time, we’re left in quite a pickle. “All three conceptions are inherently problematic,” says Deutsch, “quite apart from conflicting with each other.”
One hope is that a brighter, shinier theory will unite quantum theory and relativity, and also illuminate time’s true nature, but such a theory is still a long time off. In the meantime, Deutsch thinks our best clue comes from calculations done by physicists Don Page and William Wootters more than three decades ago. They showed that pairs of quantum “entangled” particles, whose properties influence each other even at a distance, naturally evolve to provide anyone inside a universe with an illusion of time passing – while to anyone looking from outside the universe nothing would be happening at all.
support for this idea of time as an illusory emergent phenomenon. If true, it could begin to explain some of our difficulties. Just as we are bound by geometry to view Earth as flat even though it is curved, so we are not able to experience “real” time. “Our perception is on a very coarse-grained scale,” says Deutsch. “We don’t see the multiplicity of stuff joining up and being entangled.”
That’s still speculative, and far from a general physical theory of time. But perhaps we shouldn’t lose too much sleep over it. No amount of understanding time will change the truth about ours – that it is limited. Time to make the most of it.
This article appeared in print under the headline “Is time an illusion?”
