When you stop to think about it, there is no particular reason why the future should lie ahead of us and the past behind. We use direction as a metaphor for time, which doesn’t actually have a spatial dimension at all. And so we are free to talk about time moving backward into the future, as they do in the tribal language of the Aymara Indians of South America. Or time moving from east to west, like the sun, as Aborigines in the Pormpuraaw community of Australia express it. Or even moving vertically, as Mandarin speakers in China might say. Scientists talk about the “arrow of time,” by which they mean entropy, a measure of disorder in physical systems. As an example, when Humpty Dumpty has his great fall, he goes to pieces rather than finding wholeness, all the king’s horses and all the king’s men notwithstanding. However, although entropy always moves in one direction only, from past to future, if you will, we’re still speaking metaphorically when we talk about the arrow of time.
Alone among the attributes of the natural world, time has no physical characteristics. It is undetectable by any of the five senses. Hence, the use of spatial metaphors in talking about it, since we can’t actually point to the thing itself. And although time can be measured with great scientific precision by various kinds of chronometers, it is not clear that these instruments are measuring anything other than their own movements. Time certainly exists as a measurement of change. But it is less clear what, if anything, is actually happening when we talk about the passage of time. I can remember things happening before now, and I may have a sense of time passing, but am I tangibly experiencing anything other than memory superimposed on my perception of what is happening right now? If my memory were erased, would time disappear as well?
Starting with Parmenides and his pupil Zeno in ancient Greece, philosophers have surmised that time is essentially an illusion. St. Augustine believed that time resided in the mind rather than in the external world. So did Emmanuel Kant, who argued that time was an a priori attribute of the mind that enables us to comprehend the world, rather than being a property of the world itself. Scientists took a different tack, at least initially. In his treatise on the laws of motion, Isaac Newton famously declared, “Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration.” Absolute time, along with three-dimensional space, was the framework within which Newton’s laws operated -- his “system of the world,” as he characterized it.
Then, of course, Einstein threw the whole enterprise into a cocked hat. According to his special theory of relativity, the speed of light was the only absolute, and there were no longer any present moments that extended throughout the universe. Time thereafter was strictly local, speeding up or slowing down according to the velocity of an observer relative to a given frame of reference. It quickly lost its status as an independent entity altogether and was folded into four-dimensional spacetime. Einstein’s teacher, Hermann Minkowski, who first proposed that idea, said, “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.” However, in this new block universe of spacetime, you can account for the temporal sequence of events but not for the flow of time. Indeed, physicist Julian Barbour argues there is no flow of time, only a succession of self-contained “nows” that exist in perpetuity. In the 2,500 years since Parmenides, we have come full circle, and the passage of time is once again deemed to be illusory.
Perhaps we should circle back and take a closer look at how time came to be viewed as something akin to a conjuring trick. Parmenides, a 5th-century BCE Greek philosopher and founder of the Eleatic school, was a monist who believed in the oneness of physical phenomena, which meant that all perception of time, motion and plurality were illusory. His pupil Zeno devised a series of paradoxical “thought experiments” meant to demonstrate the logical fallacy of any assertions to the contrary. The most famous of these was a hypothetical footrace between Achilles and a tortoise. Given a head start, the tortoise could never be overtaken, Zeno argued, because in the time it took Achilles to close the gap, the tortoise would advance some distance farther on. And once Achilles had closed the gap again, the tortoise would have advanced still farther. And so on, ad infinitum. Although mathematicians believe integral calculus -- which didn’t exist in Zeno’s day – “solves” the paradox, it only identifies a limit to the infinite regression of time and distance, it doesn’t actually close the gap. If time and space are infinitely divisible, as Zeno postulates, then motion is a logical impossibility; Achilles never overtakes the tortoise. Curiously, Einstein, who was also fond of thought experiments, wound up in pretty much the same place, without specifically addressing Zeno’s paradoxes. In his special theory of relativity, time and space are fused together in four-dimensional spacetime, and every event is like the still frame of a motion picture. Nothing truly moves.
Although Zeno’s paradox has taunted mathematicians and philosophers for more than two millennia, it has rarely been regarded as anything more than a curiosity. The fact remains that if there were an actual footrace between Achilles and a tortoise, no one in his right mind would bet on the tortoise. Motion might be a logical impossibility under the terms of Zeno’s paradox, but this has never slowed anyone up in the slightest. If motion is real, then something else has to give. A young theorist from New Zealand named Peter Lynds argues that motion is possible only if you eliminate instantaneous time and, along with it, anything constituting a flow of time. He argues, "With some thought it should become clear that no matter how small the time interval, or how slowly an object moves during that interval, it is still in motion and its position is constantly changing, so it can't have a determined relative position at any time, whether during a interval, however small, or at an instant. Indeed, if it did, it couldn't be in motion."
If temporal progression isn’t real, how do you account for our universal sense of time passing? Lynds, like Augustine and Kant before him, regards time as an attribute of mind rather than of the world. “It's something entirely subjective that we project onto the world around us,” he writes. A timeless universe turns out to be very much like the one Newton envisioned, minus the cosmic clock ticking away in the background. You can still measure the interval between events using a clock and still factor time into all your equations. But time is now nothing more than an abstract measurement of change rather than a constituent part of the universe. Clocks do not measure time; they are time.
Since time was apparently never part of the physical world to begin with, its removal changes nothing. However, the metaphysical implications are profound. Theologians – Augustine among them – have long argued that God exists outside of time. He is the “high and lofty One who inhabits eternity,” as the Prophet Isaiah once put it. This characterization was no doubt meant to place God well beyond the reach of time-bound mortals. Except now it turns out that we mortals also inhabit eternity, whether we realize it or not. Theologians who emphasize the otherness of God may need to rethink their position now that it appears we who are created in his image occupy the same timeless ground.
Lera Boroditsky, "How Languages Construct Time," in Time, Space and Number in the Brain (2011)
Peter Lynds, "Time and Classical and Quantum Mechanics: Indeterminacy vs. Discontinuity," Foundations of Physics Letters (2003)