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Philosophy and physics may seem like polar opposites, but they regularly address quite similar questions. Recently, physicists have revisited a topic with modern philosophical origins dating over a century ago: the unreality of time. What if the passage of time were merely an illusion? Can a world without time make sense?



While a world without the familiar passage of time may seem far-fetched, big names in physics, such as string theory pioneer Ed Witten and theorist Brian Greene, have recently embraced such an idea. A timeless reality may help reconcile differences between quantum mechanics and relativity, but how can we make sense of such a world? If physics does indeed suggest that the flow of time is illusory, then philosophy may be able to shed light on such a strange notion.



British philosopher J.M.E McTaggart advanced this idea in 1908 in his paper titled, “The Unreality of Time.” Philosophers widely consider his paper to be one of the most influential, early examinations of this possibility. Looking through McTaggart’s philosophical lens, a reality without time becomes a little more intuitive and, in principle, possible.



A Tale of Two Times



McTaggart’s argument against the reality of time has a number of interpretations, but his argument starts with a distinction about ordering events in time. The “A” series and “B” series of time form an integral part of McTaggart’s argument, and I’ll unravel this distinction with an example historical event.



On July 20, 1969, Apollo 11 became the first manned spacecraft to land on the moon. For argument’s sake, consider this event to represent an event during the present. Several days in the past (July 16), then, Apollo 11 lifted off the ground. Additionally, several days in the future all of the mission astronauts will land back on earth, safe and sound. Classifying an event as “several days past,” or “several days future,” falls under the “A” series. For the moon landing, some events (e.g. Lincoln’s assassination) are in the distant past; some events are in the distant future (e.g. the inauguration of President Obama); and other events fall somewhere in between.



Under the “A” series, events flow from one classification (i.e. past, present and future) to another. On July 16th, the moon landing would have the property of being in the future. The instant the Apollo 11 landed on the moon, that event would be present. After this moment, its classification changes to the past.



The “B” series, however, doesn’t classify events on this scale ranging from the distant past to the distant future. Instead, the “B” series orders events based on their relationship to other events. Under this ordering, Lincoln’s assassination occurs before the moon landing, and Obama’s inauguration occurs after the moon landing. This relational ordering seems to capture a different way of looking at time.



Two Times, One Contradiction



With this distinction in place, McTaggart additionally argues that a fundamental series of time requires a change to take place. Under the “B” series, the way these events are ordered never change. Obama’s inauguration, for instance, will never change properties and occur before the moon landing and vice versa. These relational properties simply don’t change.



But the A series does embody the change that we might expect from the flow of time. Events first have the property of being in the future, then they become present events. Afterward, they drift into the past. Under the A series, time does have an objective flow, and true change does happen. In McTaggart’s mind (and perhaps the mind of many others), this change is a necessary aspect of time.



But herein lies the contradiction. If these events do change in this sense, they will have contradictory properties. McTaggart argues that an event can’t be in the past, in the present, and in the future. All of these properties are incompatible, so the A series leads to a contradiction. Consequently, time, which requires change, does not truly exist. Welcome to the timeless reality.



Wait a Minute…



Certainly, many philosophers and physicists still believe in the reality of time and have objected to McTaggart’s argument. There are a number of fascinating caveats and counterexamples that you can read about elsewhere. Nonetheless, McTaggart’s work has influenced a number of philosophers’ approach to time, and his work has inspired many philosophers to incorporate physics into their arguments.



For instance, when Albert Einstein introduced special relativity, he seriously disrupted our “folk” conception of the flow of time. In special relativity, there is no absolute simultaneity of events. In one reference frame, two events may appear to happen at the same time. An observer on a speeding rocket ship, however, may observe one event happening before the other. Neither observer is “right” in this situation: This is simply the weirdness that special relativity entails.



Consequently, many philosophers have used special relativity as evidence against a theory supporting the A series of time. If absolute simultaneity doesn’t exist, it doesn’t make sense to say that one event is “in the present.” There’s no absolute present that pervades the universe under special relativity.



But McTaggart’s entire argument may help us better understand strange physics at the intersection of quantum mechanics and general relativity. In an attempt to reconcile these two theories, some well-known physicists have developed theories of quantum gravity that imply the world lacks time in a fundamental way.



Brad Monton, a philosopher of physics at the University of Colorado Boulder, recently published a paper comparing McTaggart’s philosophy with prominent theories in physics, including quantum gravity. During an interview, I asked him how some of the “timeless” ideas in quantum gravity compared to McTaggart.



“They’re on par with the radicalness,” he said. “There’s a lot of radicalness.”



Monton cautioned, however, that quantum gravity does not imply the same lack of time that McTaggart may have had in mind. Physicist John Wheeler, as Monton notes, has postulated that time may not be a fundamental aspect of reality, but this only happens on extremely small distance scales.



Some of these ideas in quantum gravity may be radical, but several respected names in physics are seriously considering a reality without time at its core. If a quantum gravity theory emerges that requires a radical conception of time, McTaggart may help us prepare.



As Monton writes in his paper: “As long as McTaggart’s metaphysics is viable, then the answer to the physicists’ queries is “no” – they are free, from a philosophical perspective at least, to explore theories where time is unreal.”



Many quantum gravity theories remain speculative, but there’s a chance that timelessness may become a prominent feature in physics. If that’s the case, then hopefully philosophers of science will help us wrap our heads around the implications.