Isolated Systems Are a Myth, and Time Symmetry Is the Exception

Source: Theories of Everything | Published: 2026-09-21T14:38:42Z

In his new book *On Time*, physicist Jim Al-Khalili argues that no truly isolated systems exist in the universe — the arrow of time doesn't emerge from time symmetry, but rather time symmetry itself is the rare approximation.

Consciousness Was Never the Answer

It's worth retracing how the measurement problem got distorted. The founders of quantum mechanics originally said: a quantum system must undergo an irreversible interaction with a "classical system" for measurement to be complete. That requirement was then twisted, step by step — first into "there must be a measuring instrument," then "a person must operate it," and finally "there must be a conscious observer." Questions like "Why can't the cat complete the measurement itself?" only led people further astray.

We now know the correct answer is decoherence: a quantum system becomes entangled with its surrounding environment, and when that environment is large and complex enough, the entanglement evolves into decoherence. Al-Khalili draws an analogy from thermodynamics — look at a single gas molecule, and there's no temperature, no direction of time; scale up to vast numbers of molecules and temperature, pressure, and the thermodynamic arrow of time all emerge. Quantum measurement follows the same logic.

But decoherence only solves half the problem. It kills off the off-diagonal elements of the superposition — the "simultaneously dead and alive cat" — but both options, the live cat and the dead cat, are still there. When you open the box, you see only one outcome. Where did the other go? This is the final step of the measurement problem, and the point where different interpretations genuinely diverge.

Every Interpretation Has Its Lump Under the Carpet

When Al-Khalili was doing his PhD, the philosophical foundations of quantum mechanics were off-limits. "Want to understand what it means? Go study philosophy. We're doing physics — look at everything our equations let us build." He and a few fellow doctoral students quietly formed a reading group, which they called the Carlsberg Group — named after the Copenhagen brewery that funded Bohr's institute, a wry nod in that direction.

His considered position is realism: there is a reality that exists independently of observation, and physics exists to figure out what it's like. Within that framework, his current preference is Bohmian mechanics — electrons are always particles, carried by a "pilot wave," with definite positions we simply cannot know in advance. But he's candid about it:

"I want it to be correct. That's different from believing it's correct. We don't know yet."

He uses a carpet analogy: whichever interpretation you choose, it's like laying wall-to-wall carpet and finding that a lump keeps appearing — you can shove it behind the sofa or next to the TV, but you can never get rid of it. The price of Bohmian mechanics is abandoning locality: particles are connected by instantaneous physical links that exceed the speed of light, directly violating special relativity. His friend Antony Valentini, a major advocate for Bohmian mechanics, goes further — if the theory is correct, Einstein wasn't just wrong about quantum mechanics, he was wrong about relativity too, and particles across the universe are genuinely communicating faster than light, capable in principle of synchronizing clocks across the cosmos. That's a price that keeps Al-Khalili at arm's length, even though he very much wants Bohmian mechanics to be right.

What Is the Electron Doing When No One's Looking?

This is a question he was asked directly in the interview. The answer depends entirely on which interpretation you hold.

Bohmian mechanics gives the cleanest answer: the electron is a particle, always in motion, passing through conductors, orbiting nuclei, colliding with other electrons — we just don't know its precise location. Copenhagen's answer: the question is meaningless. The wave function tells you everything that can be known, and if it doesn't tell you the electron's exact position, that's because the electron has no exact position before measurement — not "it has one but we don't know it," but rather "there simply isn't one." The many-worlds interpretation says: the electron exists in every possible state, distributed across different branching realities. Others argue that what's truly real isn't the particle but the quantum field of the electron — the question "what is the electron doing?" should be replaced with "what is the field doing?"

Al-Khalili explicitly sides with Einstein. Bohr's epistemological position was that "physics is the study of what we can say about the world"; Einstein's ontological position was that "physics is the study of what the world is actually like." He believes that generations of quantum physicists have betrayed the mission of physics by dismissing ontological questions as meaningless.

"Maybe we don't know. But that doesn't mean nothing is happening when no one's looking."

The Block Universe, Properly Understood

The block universe is another concept that's routinely misread. Al-Khalili clarifies a common misconception: the block universe isn't simply a "frozen spacetime." It is the four-dimensional spacetime structure that emerges from special relativity — three spatial dimensions plus one temporal dimension. Because we can't visualize four dimensions, the usual approach is to drop one spatial dimension and represent the whole thing as a three-dimensional loaf of bread: each slice is all of space at a given moment, stacked along the time axis.

There's a subtlety, though: how you slice the loaf depends on the observer's reference frame. An observer at rest relative to you and one in motion relative to you will cut the same block of spacetime at different angles — two events you consider simultaneous, they might see as sequential. So the block universe has no single "God's-eye" standard slicing; its time dimension only becomes definite once an observer is chosen. The real point of the image is this: past, present, and future exist with equal reality within spacetime. "Now" is no more special than last week or a million years from now — just as your location is no more special than any other point in the universe.

The Flow of Time Is Psychology; the Arrow of Time Is Physics

This is the central argument of On Time, and the part of the interview where he presses hardest.

The flow of time and the arrow of time are two different things. Most discussions conflate them.

The flow of time is a psychological perception: we experience a continuously changing present, a sensation that time passes like a river. But this doesn't appear in any physical equation. The t in dynamical equations is coordinate time — you can evolve it forward or backward and the equations don't care. "Time itself does not 'flow' in any meaningful sense" — this belongs to what he calls "manifest time," a matter for psychology, not physics.

The arrow of time is something else entirely, and it is objectively real: the direction of entropy increase, the direction of causation, the direction of cosmic expansion. These arrows existed before life appeared on Earth, independent of any observer.

But here lies a problem that has dragged on for more than a century: the fundamental dynamical equations of physics are time-symmetric — replace t with -t and the equations still hold. Physicists have long asked: how does an irreversible arrow of time emerge from fundamentally time-symmetric laws?

No System Is Truly Isolated — Symmetry Is the Special Case

Al-Khalili thinks this question is being asked backward.

His argument rests on a simple but overlooked observation: no system is truly isolated from its environment. Imagine a box of gas molecules pushed into a corner — the act of pushing has already caused that box to interact with the outside. The piston moves in, someone does work, and the system is no longer isolated. Time-symmetric equations are an idealization applicable only to perfectly isolated systems — and there are no truly isolated systems in the universe. The only thing that could claim true isolation is the entire universe itself, because it has no exterior.

For everything inside the universe, the arrow of time is present from the very beginning — it doesn't emerge from time symmetry. Time symmetry is the special case, the approximation, the idealization.

This is also why he's skeptical of the "past hypothesis" — the idea of positing a special low-entropy moment at the Big Bang. It exploits the fact that there is no time before the Big Bang to sidestep the time-symmetry problem, breaking time-translation invariance and swapping one hard problem for another. He accepts it as a temporary patch, but not as a final answer.

His Disagreement With Carlo Rovelli

Rovelli wrote a blurb for On Time, but the two hold opposite positions on the nature of time. Rovelli believes time is not fundamental — everything reduces to relations, and time itself is not real. Loop quantum gravity treats time as an emergent quantity rather than a basic one.

Al-Khalili's rebuttal is direct: you can't say time is an illusion and simultaneously say the directionality of time is real. To have an arrow, you must first have time for that arrow to exist within. The reality of the arrow of time logically implies the reality of time itself.

He also acknowledges that his disagreement with Rovelli is structurally similar to Rovelli's disagreement with Greene over string theory, or Greene's disagreement with Sean Carroll over many-worlds: smart people holding different positions in the absence of decisive evidence. "If there were physical evidence, whoever was falsified would have to concede. Right now, there isn't any."

Whether time is fundamentally real or emergent — he stays open to the latter. Just as temperature emerges from molecular vibration, and consciousness emerges from the coordinated activity of tens of billions of neurons, time might emerge from some more fundamental quantum domain. But he insists that emergence doesn't mean unreality. What emerges can be entirely real.

Quantum Gravity Needs Three Legs

His view on quantum gravity is somewhat contrarian. Most people frame it as unifying quantum field theory with general relativity, with string theory and loop quantum gravity as the two main paths.

Al-Khalili thinks a third leg is missing: thermodynamics. Physics rests on three great pillars, and time enters each framework in a completely different way — in quantum mechanics it's a coordinate, in relativity it's a dimension of spacetime, in thermodynamics it's an arrow. Unifying only the first two leaves time without a coherent unified status. He believes any genuine theory of quantum gravity must incorporate all three.

He doesn't oppose string theory, but he's not enthused — decades of work, enormous mathematical power, legions of brilliant people, yet no experimental results to show for it. He has personal respect for Rovelli and Smolin, the founders of loop quantum gravity, but their path on the question of time diverges from his. The candidate theory he finds most appealing is causal set theory — a relatively niche approach that places time and causation at the center: causes precede effects, time is real, and it doesn't emerge from something deeper.

"Approximate" Is More Honest Than "Wrong"

The interview touches on a remark by Roger Penrose: Einstein called quantum mechanics "incomplete," but Penrose thinks that's too generous — if a theory requires major revision, it's simply "wrong," and "incomplete" is just a polite word for the same thing.

Al-Khalili doesn't fully agree with Penrose. He thinks "approximate" is more accurate than either "incomplete" or "wrong." Newtonian mechanics isn't wrong, and it isn't incomplete — it's approximate. It works within its domain of validity, and beyond that domain it needs to be replaced by something better. "Einstein probably should have said quantum mechanics is approximate, not incomplete."

Relativity may be equally approximate. Which of the two — quantum mechanics or relativity — will need to yield more ground, or whether both require fundamental revision, is precisely what the competing quantum gravity candidates are fighting over. He won't place a bet, and he doesn't think anyone should right now.

You Have to Earn Academic Credit Before You Can Spend It on Outreach

The latter part of the interview turns to science communication, and some of what he says is worth heeding for early-career researchers.

During his PhD, he spent much of his spare time reading biographies of quantum mechanics' founders — Bohr, Heisenberg, Feynman, Dirac. Those books made quantum mechanics feel magical again. Not mysterious in a mystical sense, but thrilling in the way of "why is this thing so strange, and so beautiful?" He stayed on with his original supervisor for his doctorate rather than moving elsewhere.

On science communication, his view is clear: you have to earn academic credibility before you spend it on public outreach. When people ask how to break into science communication, he says the first step is figuring out which path you actually want — "being a science communicator" and "being a scientist who communicates" are two different careers. If you choose the latter, you have to do the science first. PhD, postdoc, faculty position — outreach can happen in parallel, but it must stay subordinate until your academic footing is solid. "I now describe myself as a 'gentleman scientist' — research is optional, but writing and broadcasting are my main work. That's a stage I only reached late in my career."

In writing this, his fifteenth book, he went to press without having fully worked out his own position in advance. It was during late-night revision sessions, struggling to find a new analogy, working out how to explain the relationship between isolated systems and the arrow of time, that his thinking slowly crystallized. The book is published and the answers are in it — but by the time of the interview, new questions were still being raised, and he realized there were things he still hadn't thought through.