What Physics Actually Knows About Time

There's a particular kind of intellectual vertigo that hits when a physicist explains something you thought you understood, and you realize you never understood it at all. Theoretical physicist Brian Greene produces that sensation reliably, and in a recent WIRED video, he does it with time itself—arguably the one thing every human being alive has in common.

The video runs through nearly four centuries of physics in about eighteen minutes, from Newton's cosmic metronome to string theory's intimation of a timeless substrate underlying reality. It's well-executed science communication. But what makes it worth sitting with isn't the tour—it's the honest admission at the end of it: we still don't actually know what time is.

Newton's Convenient Assumption

Greene frames Newton's treatment of time almost affectionately, as "the easy way out." Newton hypothesized a grand cosmic clock ticking at a uniform rate for every observer, everywhere, always. It was a scaffolding assumption—useful, internally consistent, and wrong enough to matter.

What's underappreciated about Newton's framework is how well it worked. His equations predicted planetary positions with astonishing precision, and they still underpin most of classical engineering. The assumption that time is absolute wasn't sloppy—it was exactly as precise as it needed to be for the speeds and gravitational fields humans encountered in the seventeenth century. The scaffolding held until we started asking questions it wasn't built to answer.

Einstein asked one of those questions at age sixteen: what would a beam of light look like if you could ride alongside it? The mathematics said the light couldn't appear stationary. But intuition said it should. That tension, unresolved for years, eventually produced special relativity—and with it, the demolition of Newtonian absolute time.

The Constant That Breaks Everything

The core of special relativity is a single, seemingly modest claim: the speed of light is the same for every observer, regardless of their motion. Greene spells out why this is genuinely strange.

If someone throws a baseball toward you, its apparent speed depends on whether you're moving toward or away from the thrower. That's just how velocities add. But swap the baseball for a laser beam and that logic stops working. You can sprint directly toward the source or flee it at any speed achievable by conventional means, and the light still approaches you at exactly 299,792,458 meters per second.

Speed is distance divided by time. If speed stays constant when both the distance and time components are changing—which they must be, for different observers—then space and time themselves have to be doing something strange to compensate. Greene puts it cleanly: "space and time themselves cannot be constant, but rather they must change in exactly the right way so that their ratio, which is what speed is, is itself constant."

This isn't an abstraction. The 1971 Hafele-Keating experiment put atomic clocks on commercial jets, flew them around the world, and compared them to clocks left on the ground. The airborne clocks showed less elapsed time—by exactly the amount Einstein's equations predicted. The time dilation is small at jet speeds, but it's real and measurable, and it matters: modern GPS systems have to correct for both special and general relativistic effects or they'd accumulate errors of kilometers per day.

General relativity adds gravity to the picture. In Einstein's 1915 framework, gravity doesn't just pull on objects—it pulls on time itself. The stronger the gravitational field, the slower time runs. Near the event horizon of a black hole, this effect becomes extreme enough that an external observer would see a clock freeze entirely. There is no universal "now" in this framework—whether two events are simultaneous depends entirely on the observer's position and state of motion.

The Arrow Problem

Here's where things get philosophically thorny, and where Greene's argument gets most interesting.

Relativity explains how time dilates. It doesn't explain why time runs in one direction. Nothing in Einstein's equations—or Newton's, or Maxwell's equations for electromagnetism—distinguishes between forward and backward in time. Run any of these equations in reverse and they remain valid. The mathematics of fundamental physics is, as Greene puts it, completely "agnostic when it comes to the direction of time."

And yet nothing in our experience is agnostic about it. Eggs shatter; they don't reassemble. Coffee and milk mix; they don't spontaneously unmix. You remember yesterday, not tomorrow.

The standard answer to this asymmetry is entropy—the tendency of systems to move from ordered states to disordered ones, described by the second law of thermodynamics. A new egg is a low-entropy, highly ordered arrangement of molecules. Scattered yolk on a kitchen floor is high-entropy disorder. The second law says systems move from the former to the latter, and that progression defines what we call "forward."

But Greene identifies a serious problem with this explanation, and it's one that the physics of temporal direction literature has been wrestling with for decades. If the fundamental laws don't distinguish past from future, and entropy is derived from those laws, then entropy shouldn't distinguish past from future either. The second law, applied symmetrically, predicts that entropy should increase in both temporal directions from any given moment—not just forward.

He illustrates this with a half-melted ice cube. Intuition says it got there by melting from a solid cube. But the laws of physics, applied backward, say it should have gotten there from a puddle that spontaneously coalesced. Past and future are, in the fundamental equations, on equal footing.

The Big Bang as the Actual Answer

The resolution Greene offers—and it's the best one physics currently has—is that the arrow of time isn't written into the laws themselves. It's written into the initial conditions of the universe.

The hypothesis is that the Big Bang began in an extraordinarily low-entropy, highly ordered state. Since then, the universe has been running downhill toward disorder. That asymmetry between early and late isn't demanded by the laws—it's a fact about our particular universe's starting point. The reason eggs can exist at all, Greene argues, is that conditions at the Big Bang were ordered enough to produce orderly objects in the first place.

This is a coherent position. It also immediately generates a question Greene doesn't flinch from: why was the Big Bang so ordered?

"Answer, I have no idea. Nobody on planet Earth has any idea, either. We have ideas that we have thrown around, but there is no consensus on what may have imposed this high degree of order on the Big Bang itself."

That's a senior physicist talking about the foundation of one of the most important explanatory frameworks in physics. The honesty is notable and worth emphasizing. The arrow-of-time question isn't solved—it's been relocated. We've traded one mystery for a slightly deeper one.

Time as an Emergent Property

The final and most speculative thread Greene picks up concerns whether time is fundamental at all. Most physical theories treat space and time as the stage on which everything happens—they're assumed, not derived. But certain approaches to quantum gravity, including aspects of string theory, have produced mathematical structures where time doesn't appear in the foundational equations. It would emerge, under certain conditions, from something more basic.

This isn't a fringe idea—it shows up in serious technical literature, including work on the Wheeler-DeWitt equation, which describes the quantum state of the universe and is famously devoid of any time variable. It's also deeply unresolved. As philosopher Simon Saunders has argued, time's fundamental status remains physics' most poorly understood question—and our models may be structurally limited in their ability to capture it.

Greene frames his own intuition carefully: he suspects a future reformulation of fundamental physics will describe a timeless substrate from which time emerges contextually, much as temperature emerges from the collective motion of particles rather than being a property of any individual molecule. Temperature isn't fundamental—it's a pattern that appears at scale. His intuition is that time may be the same kind of thing.

What that would mean for our lived experience of time—for memory, anticipation, the felt sense of duration—is a question physics alone probably can't answer. The experience of time isn't just a scientific problem; it's a phenomenological one, and the gap between "time is an emergent statistical pattern" and "I am waiting for something and it feels long" remains as wide as it's ever been.

The striking thing about Greene's survey isn't how much we know—it's the shape of what we don't know. We have precise, experimentally verified equations for how time behaves under different conditions. We have a plausible (if incomplete) account of why it seems to run in one direction. We have serious theoretical work suggesting it might not be fundamental at all.

What we don't have is an answer to the question a precocious child might ask: what is time, actually?

Physics has made that question harder to answer, not easier. That's usually what progress looks like.

— Amelia Nwofor, Science Desk Editor