Time is relatively well-described in current physical theories 鈥� it鈥檚 just that those descriptions are perplexingly at odds both with each other and with our perception of what time should be.
Time is not absolute
贰颈苍蝉迟别颈苍鈥檚 general theory of relativity established time as a physical thing: it is part of space-time, the gravitational field produced by massive objects. The presence of mass warps space-time, with the result that time passes more slowly close to a massive body such as Earth.
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This effect, although tiny in our own neighbourhood, has been confirmed in experiments. Clocks, for instance, run faster on mountain peaks than they do at sea level, and more slowly at our feet than they do by our heads. While we may think of time as a constant, metronomic beat against which the events of the universe play out, that is an illusion borne out of our own imprecise perception of time. Relativity says there is no single beat to which the cosmos moves.
Time has no direction
The irony of relativity鈥檚 abolition of absolute time is that this sort of time keeps our other basic physical theories ticking 鈥� from Newton鈥檚 laws of dynamics to the equations governing the evolution of the quantum world.
But these equations come with their own wrinkle: they are all fully reversible in time, running backwards just as well as forwards. Relativity gives no direction to time, either: time just 鈥渋s鈥�. That is at odds with our own perception, in which time determines our direction of travel, propelling us headlong into the future, whether we want it to or not.
The only exceptions to the rule of physical non-reversibility are the equations governing the flow of heat. For this reason, many physicists have sought the source of the sort of flowing time we perceive in thermodynamics 鈥� with so far unsatisfactory results.
Time has no 鈥渘ow鈥�
贰颈苍蝉迟别颈苍鈥檚 relativity also says that the passage of time is affected by motion, with moving objects seeing less time passing. So not only does how much time elapses vary from place to place, but different observers looking at the same place but moving at different speeds will see different amounts of time passing.
So even 鈥渘ow鈥� is relative, and you can鈥檛 even draw one objectively agreed line between all the points in the universe currently experiencing it. From its own perspective, each event has its own past, formed of those areas from which signals travelling at light speed, the cosmic speed limit, have had time to travel and so influence it. The event also has a future, formed of those areas to which light signals can propagate and feel its influence.
But other observers will see those pasts and futures differently. And outside each of those carefully delimited pasts and futures are vast swathes of the cosmos that are neither past nor future, but also not 鈥渘ow鈥�. Our grammar of time, again born out of local experience, fails to describe what those areas might be.
Time is not (yet) quantum
Most physicists believe 贰颈苍蝉迟别颈苍鈥檚 relativity is unlikely to be the final word on time. Time emerges out of the gravitational field, and all other fields and forces we know of are described by quantum physics. Gravity should be too. In this view, only by unifying our understanding of gravity and quantum phenomena can we hope to fully understand time. Alternatively, understanding more about the nature of time may point us towards the form of that one true theory.
This article appeared in print under the headline 鈥淔inding the flow鈥�
