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We’ve all heard it before: There’s no time like the present. Broadly speaking, of course, it means to “seize the opportunity right now,” or maybe in my case, to avoid procrastinating. From a psychological perspective, this makes a lot of sense. As humans we experience time “passing,” and there is a special quality to the present moment. Hypnosis and dreams aside, there is no way to directly experience either the past or the future in the same way we experience the present. But is the aphorism true? Does modern physics actually tell us that there’s no time like the present?

Our best current physical theory of space and time is general relativity. Prior to Einstein’s revolution over a century ago, physics considered time to be an “external parameter”—an independent, fundamental feature of reality not influenced by any other factor in the universe. Whether or not the passage of time is real or illusory (this is an age-old philosophical debate that predates Einstein and is indeed not settled by his theory), we now know that time intervals are not external or universally determined. Time is an internal component of a physical system, a dimension intertwined with three spatial dimensions. Taken together, this is“spacetime,” and is influenced by varying factors, including speed (relative to other observers or systems) and gravitational forces. Because the theory of relativity posits the constancy of the speed of light for all observers (even if they are moving relative to each other), spacetime itself must dilate and the concept of a time interval becomes elastic.

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As a result, there is no universal notion of the present that applies equally to all observers. What looks present to me could just as easily be in someone else’s future, and in a third person’s past. Simultaneity is relative. Suppose your spouse is on a business trip on the other side of the globe. Just as you’re thinking about how much you miss her, she hits the “send” button on an email to you saying how she misses you. Romantic, right? But while these two events occur simultaneously from your perspective, for an alien in a spaceship whizzing by the Earth the time order is different. If the spaceship is heading in the direction of your spouse, the alien will observe the email being sent before your wistful thought. If the spaceship is heading away from your spouse, her email will be sent after. So, as a consequence of the theory of relativity, there is nothing special or unique about the present moment. In fact there are plenty of times like your present—other observers will call your present their future or their past.

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Shape dynamics leaves us with a countless number of present moments, or Nows.

Our story does not end here. Although general relativity has been extremely well confirmed by experiment it is not a complete theory of the universe. It doesn’t incorporate the lessons learned from the other great pillar of 20th-century physics—quantum mechanics. Indeed the two theories, in their current forms, are incompatible. When studying black holes or the beginning of the universe, for example, physicists need to describe very massive objects (which general relativity is good at doing) packed into very small spaces (which is the realm of quantum mechanics). Such scenarios require a rethinking of physics and will hopefully one day lead to the discovery of a quantum theory of gravity. One of the difficulties behind such a search lies precisely in the different ways time is considered in the two theories. Canonical quantum mechanics maintains an earlier view of time as an external parameter unaffected by the universe itself. But we know time can’t be thought of in this way because general relativity tells us that time is influenced by gravity, which is not explained by quantum mechanics. Thus a new theory of quantum gravity will likely upend our concept of time once again, much like Einstein did a century ago. A 21st-century revolution in physics may be just around the corner.

While different proposals for quantum gravity have different things to say about the nature of time, perhaps one of the most interesting perspectives is a theory called shape dynamics. Shape dynamics does away with time altogether, and tries to explain the effects of gravity through the evolution of spatial shapes each of which tell a causal story of how the universe came to be. The theory leaves us with a countless number of present moments, or Nows. Every one of these moments is like a photo in an album which contains the historical record (in separate snapshots) of how we got to where we are today.

The proponents of shape dynamics call these timeless, instantaneous Nows “time capsules,” and argue that our brains stitch these together to give the illusion of time passing. Think about how we perceive the motion of a jumping cat: Our eyes pick up instantaneous “snapshots” of the cat at various points in the air, and our brain stitches them together to create the illusion of motion. Shape dynamics proposes that an analogous process is responsible for the appearance of time itself.

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As for our aphorism, shape dynamics agrees with relativity: It’s incorrect to say there’s no time like the present. If shape dynamics is correct then the universe consists of many, many presents, and there is no such thing as the continuous passage of time anyway.

But shape dynamics is far from the only approach to quantum gravity. What is the fate of time in string theory, for example? It turns out that here too it is incorrect to say that there is no time like the present. String theory postulates that the point-like fundamental particles of physics are best modeled as one-dimensional extended objects, which can take the form of either loops or segments of “strings.” The string theory framework requires one to adopt extra dimensions of space (there are at least 10 spatial dimensions, depending on the flavor of the theory) which are curled up on themselves so that they are not observed in the three-dimensional world we are used to. An analogy concerning a garden hose might be helpful here: Viewed from a distance (say, your bedroom window), a hose lying in the backyard appears to consist of only one dimension—its length. But if you were to walk up to it, you’d realize that the hose possesses a second dimension—its width.

Just as extra spatial dimensions emerge from string theory, it has been posited that time emerges from a timeless element in an appropriate version of string theory. In this, sense time is not a fundamental concept in string theory, and we can’t say there is no time like the present because there is no time at all.

Now, if you’ll excuse me, I need to get a bunch of stuff done. And there’s no time like now-ish, give or take.

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Mark Shumelda is a Ph.D. candidate in the philosophy of physics at the University of Toronto, and has taught extensively in both science and humanities in Canada’s North.

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