Physicist Jim Al-Khalili on Whether Time Is Real

Run Newton's equations forward or backward in time and they work identically. The same holds for the Schrödinger equation in quantum mechanics, and for the equations governing how the universe expands. Time appears in all of them as a parameter, a lowercase t you assign a value to, not as something that flows. And yet you are reading this sentence after the previous one, not before it. That asymmetry, between what the equations say and what you experience, is the problem Jim Al-Khalili spent a conversation at the Royal Observatory Greenwich trying to untangle for New Scientist.

Al-Khalili, a physicist and author of On Time, draws a distinction that most popular coverage of this topic collapses: manifest time versus physical time. Manifest time is the psychological experience of duration, the way a dentist's waiting room seems to stretch a half-hour into an afternoon while a good party compresses it. Physical time is what Einstein and Newton wrote equations about. These are related questions, but they are not the same question, and conflating them produces a lot of avoidable confusion.

His position on the flow of time is precise: "I believe that the flow of time is an illusion. But that doesn't mean that time itself is not real." The distinction matters. In physics, nothing in the governing equations encodes a flowing present moment. Time is a coordinate. You can solve a system forward to t = 10 or backward to t = 2, and the mathematics does not care. The sensation that now is special, that the present moment is the only one that exists while the past recedes and the future approaches, has no foothold in those equations.

Eternity gets a hearing here. Under the block universe picture that follows from Einstein's special relativity, and which was formalized by Hermann Minkowski, all points in time exist with equal ontological status. The past is not gone; the future is not yet to arrive. They simply occupy different coordinates in a four-dimensional spacetime structure. Al-Khalili describes this as the picture "most physicists" find most coherent with relativity, while acknowledging that it sits uncomfortably against the felt reality of passing time. The presentist view, where only the present moment exists, and the growing-block view, where the past is real but the future is not, are harder to square with what relativity actually says. The block universe framework keeps emerging as the default position precisely because relativity makes no room for a privileged now.

Three pillars that do not fit together

The deeper problem Al-Khalili identifies is structural. Physics rests on three frameworks that each treat time differently, and nobody has successfully combined all three.

General relativity treats time as a dimension of spacetime, curved by mass, capable of being stretched. Quantum mechanics treats time as a label, an external parameter that tells you when to evaluate the state of a system. Thermodynamics gives time a direction: entropy, roughly the measure of how disordered a system is, increases over time, and this gives you an arrow pointing from past to future. As Al-Khalili notes, most efforts to unify physics focus on combining quantum mechanics with general relativity, producing candidates like string theory and loop quantum gravity. What those efforts typically neglect is thermodynamics, and Al-Khalili argues that neglect is precisely why a complete theory of time remains out of reach. "We need to bring these three together," he says. The directionality problem that results from ignoring thermodynamics is not a minor footnote; it may be the central obstacle.

The entropy problem, and why the standard resolution is not quite satisfying

The thermodynamic arrow of time says that entropy increases toward the future. Ludwig Boltzmann did much of the foundational work on this in the 19th century, and James Clerk Maxwell, among others, noticed the resulting tension immediately: if all the dynamical equations of physics are time-symmetric, where does the one-way direction come from?

The conventional answer invokes what physicists call the past hypothesis: the universe began in an extremely low-entropy state at the Big Bang, and entropy has been increasing ever since. Because there is no past before the Big Bang, you do not need to explain why entropy was also lower further back; there is no further back. Al-Khalili acknowledges the logic but calls it "clever," which in this context is not entirely a compliment. It resolves the regress by stipulating a special initial moment rather than deriving irreversibility from deeper principles.

Al-Khalili's own preferred move is more radical. He argues that time-symmetric equations are not the fundamental starting point; they are idealizations that apply only to isolated systems, and isolated systems do not actually exist. Every real physical system interacts with its environment. When you account for those interactions, irreversibility is not something you need to explain on top of the equations; it is already there, built into the fact that nothing is ever truly isolated. Quantum decoherence, the process by which quantum systems continuously entangle with their surroundings and lose their distinctly quantum character, provides a physically grounded mechanism for this irreversibility, one that does not depend on the act of measurement or on an observer opening a box. "For me, quantum decoherence is a more basic irreversibility than thermodynamic entropy," he says.

This is a substantive claim, not a rhetorical reframing. If irreversibility is primary and time-symmetry is derived, the standard framing of the arrow-of-time problem may be asking the wrong question.

Whether time could be emergent

One possibility that physicists and philosophers have raised is that time is not fundamental at all but emerges from something deeper, the way temperature emerges from molecular motion or wetness emerges from collections of water molecules. Al-Khalili takes this seriously as a possibility, pointing to ongoing research into the emergence of both space and time from quantum entanglement. But his own instinct runs the other way: "I think that time is there, absorbed into reality at the basic level." He is skeptical that emergence fully resolves the problem rather than relocating it, and notes that strong emergence, of the kind some philosophers invoke for consciousness, has no agreed-upon physical mechanism.

The question connects directly to what various quantum gravity programs actually say about time. Al-Khalili finds causal set theory, which treats spacetime as fundamentally discrete and takes time's reality seriously, a more intellectually honest approach than frameworks in which time is an afterthought. String theory, he suggests, treats time too loosely for his comfort.

Time travel, briefly

Al-Khalili's answer on time travel is a qualified yes, and the qualifications are important. An observer moving at high velocity or orbiting a sufficiently massive object ages more slowly than a stationary observer. The film Interstellar used this correctly, he notes, crediting Kip Thorne's involvement in ensuring the physics was sound. Backward time travel is different in kind. It is not ruled out by current physics, but every proposed mechanism produces logical paradoxes, and Al-Khalili thinks the most promising escape routes require accepting parallel realities or branching universes. He calls it possible in a technical sense while doubting it in a practical one.

What remains open

The conversation at Greenwich does not arrive at a resolution, because none exists. Al-Khalili says outright that he does not expect the problem to be solved in his lifetime, while insisting that this is not a reason to stop taking it seriously. Physics has a long history of declaring problems intractable before solving them. The question of what time actually is, as distinct from how to calculate with it, has resisted centuries of intelligent attention.

What Al-Khalili leaves us with, via the New Scientist interview, is a cleaner map of where the genuine difficulties lie: the manifest/physical distinction, the three-pillar unification problem, the unresolved status of irreversibility, and the question of whether time is fundamental or emergent. None of those are the same problem. Progress probably requires treating them separately before any synthesis becomes possible.

The unsettling implication is that the version of time you experience, the flowing, directional, present-anchored sense of duration that organizes every moment of your life, may correspond to nothing in the fundamental equations. And the version the equations describe may be so unlike lived experience that connecting the two requires conceptual tools physicists do not yet have.

Priya Sharma covers science and health for BuzzRAG.