There are four main reasons for saying time is not real: it is (a) subjective, (b) conventional, (c) inconsistent, and (d) emergent.
(a) Does time depend upon being represented by a mind? Without minds, nothing in the world would be surprising or beautiful or interesting. Can we add that nothing would be in time? Yes, said St. Augustine, who claimed time is nothing in reality but exists only in the mind’s apprehension of that reality.
(b) Philosophers generally agree that humans invented the concept of time, but some argue that time itself is invented as a useful convention, like when we decide that a coin-shaped metal object has monetary value. Money is culturally real but not objectively real because it would disappear if human culture were to disappear, even if the coin-shaped objects did not disappear.
Although it would be inconvenient to do so, our society could eliminate money and return to barter transactions. In the article, “Who Needs Time Anyway?”, Craig Calendar said:
Time is a way to describe the pace of motion or change, such as the speed of a light wave, how fast a heart beats, or how frequently a planet spins…but these processes could be related directly to one another without making reference to time. Earth: 108,000 beats per rotation. Light: 240,000 kilometers per beat. Thus, some physicists argue that time is a common currency, making the world easier to describe but having no independent existence.
(c) Bothered by the contradictions they claimed to find in our concept of time, Parmenides, Zeno, Plato, Spinoza, Hegel, and McTaggart said time is not real. McTaggart believed he had a convincing argument for why a single event is a future event, a present event and also a past event, and that since these are contrary properties, our concept of time is self-contradictory.
In the mid-twentieth century, Gödel argued for the unreality of time because the equations of general relativity allow for physically possible universes in which all events precede themselves. It shouldn't even be possible for time to be like this, Gödel believed, so whatever the theory of relativity is about, it is not about time.
(d) It also has been argued that time is not real because it is emergent. Leibniz argued it emerges from the order relations between pairs of events, and Minkowski argued it emerges from spacetime.
In 1994, Julian Barbour said, “I now believe that time does not exist at all, and that motion itself is pure illusion.” He argued that there does exist objectively an infinity of individual, instantaneous moments, but there is no objective happens-before ordering of them, no objective time order. There is just a vast, jumbled heap of moments. Each moment is an instantaneous configuration (relative to one observer's reference frame) of all the objects in space. If the universe is as he describes, then space (the relative spatial relationships within a configuration) is ontologically fundamental, but time is not, and neither is spacetime. In this way, time is removed from the foundations of physics and emerges as some measure of the differences among the existing spatial configurations.
The above arguments are not trivial, but I would like to respond to them.
(a) Regarding subjectivity, notice that our clock ticks in synchrony with other clocks even when no one is paying attention to the clocks. Second, notice the ability of the concept of time to help make such good sense of our evidence involving change, persistence, and succession of events. Consider succession. This is the order of events in time. If judgments of time order were subjective in the way judgments of being interesting vs. not-interesting are subjective, then it would be too miraculous that everyone can so easily agree on the temporal ordering of so many pairs of events.
(b) A good reason to believe time is not merely conventional is that our universe has so many periodic processes whose periods are constant multiples of each other over time. For example, the frequency of rotation of the Earth around its axis, relative to the "fixed" stars, is a constant multiple of the frequency of oscillation of a fixed-length pendulum, which in turn is a constant multiple of the frequency of a vibrating violin string. The existence of these sorts of relationships—which cannot be changed by convention—makes our system of physical laws much simpler than it otherwise would be, and it makes us more confident that there is something convention-free that we are referring to with the time-variable in those physical laws.
(c) Regarding the inconsistencies in our concept of time that Zeno, McTaggart, Gödel, and others claim to have revealed, I suggest we say that either there is no inconsistency, or else their complaint be handled by revising the relevant concepts. For example, Zeno's paradoxes were treated by requiring time to be a linear continuum, very much like a segment of the real number line. Yes, the mathematicians changed important characteristics of Zeno’s concept of time, but the change was very fruitful and not ad hoc and so cannot be accused of violating time’s very essence. Gödel's complaint can be treated by saying he should accept that time might possibly be circular; he needs to change his intuitions about what is essential to the concept.
(d) Suppose time does emerge from events, or spacetime, or even Barbour’s moments. Scientists once were very surprised to learn that water emerges from H2O molecules. But having learned that molecules are more fundamental than water, should we make the metaphysical leap to saying water is not real? Should we not say instead that now we more deeply understand what water is? If so, we can draw a similar conclusion for time.
So, let’s say that time is real, that it is objective rather than subjective, that it is not primarily conventional, that any inconsistency in its description is merely apparent or inessential, and that time is real regardless of whether it is emergent.
Department of Philosophy