Time Is Not an Illusion: Irreversibility Is More Fundamental Than Symmetric Equations
Jim Al-Khalili argues that time is real and directed, and that irreversibility is more fundamental than time-symmetric equations; he also proposes replacing thermodynamic entropy with quantum entanglement entropy as the basis for the past hypothesis.
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The argument · tap a timestamp to hear it
The direction of time is built in, not emergent
Jim stakes out a hard position: time is real, tangible, part of four-dimensional spacetime, not just something a clock measures. He goes further, saying the directionality of time is not emergent from directionless fundamental laws but is built into the universe. Sean reminds the audience that this is in fact a big controversy — many physicists think time is merely a tool or that its direction is a product of coarse-graining. Jim's intuition is that if we look from inside the universe, irreversibility is the more fundamental thing.
— JANo system is isolated, so irreversibility is the norm
The traditional narrative is: fundamental equations like Newton's and Schrödinger's are invariant under time reversal, so the arrow of time must emerge from entropy increase or coarse-graining. Jim turns this around, arguing that time-symmetric equations are just an idealization, because no system is truly isolated — even quantum systems are constantly entangling with their environment. Time-symmetric equations apply only to isolated systems and are a special case of the master equation; in reality everything exchanges energy and information with its environment, and irreversibility is the norm. He admits this is a minority position, but thinks it is the foundation closer to the real world.
— JAThe special moment can only be the Big Bang, not last week
If you treat the entire universe as an isolated system, and it also obeys time-symmetric laws, then by the second law yesterday's entropy should be higher than today's, not lower. The only way out is to push that special moment back to the very beginning: the Big Bang. That is the past hypothesis. Jim stresses that this is not an extra assumption but part of the universe's initial conditions, on a par with fine-tuning of constants. He agrees with Sean: if you must pick a special moment, the Big Bang is far more reasonable than last week.
— JAThe laws of physics contain no flow of time, yet we feel it
Even if low-entropy initial conditions explain the arrow of time, Sean presses: where do psychological experiences like ‘now’ and the sense of passing come from? Jim honestly admits that the laws of physics contain no flow of time, yet we feel time flowing. He says this is the gap between physical time and manifest time, and he is not even sure how to state it clearly. That candour is more valuable than pretending to have solved it, and it shows the problem of time is far from closed.
— JAA DNA mutation may come from one proton's quantum tunnelling
Jim, from the perspective of a nuclear physicist, gives the most concrete case in quantum biology: the two strands of the DNA double helix are joined by hydrogen bonds, and a hydrogen atom is essentially a proton. Löwdin proposed in the early 1960s that a proton might tunnel through the energy barrier to the opposite strand, causing a mutation during replication. Jim's team has done increasingly refined calculations to pin down the ratio of tunnelling to thermal excitation. He points out that quantum biology is not the old line that ‘atoms themselves are quantum’, but rather asks whether life has evolved tricks that exploit long-lived coherence and entanglement.
— JAA more fundamental version of the past hypothesis is entanglement entropy
The thermodynamic past hypothesis requires specifying a low-entropy microstate. Jim and the philosopher Eddy Chen tried a more quantum version: assume the universe began in a pure state with extremely low entanglement entropy, with subsystems barely entangled with one another. As it evolves, subsystems gradually entangle with each other, and the entanglement entropy of the whole universe rises monotonically. Conceptually this is more fundamental than thermodynamic entropy, but Jim freely admits there are still many holes — such as how finely the subsystems should be divided. This direction is still unfinished.
— JAAfter the universe's heat death, time still passes
Many people say time may disappear at extreme scales. Jim counters: even if the universe reaches thermal equilibrium in tens of trillions of years, with no change left to measure time by, time still passes. He admits this is not time in the relativistic sense that holds for every reference frame, but it brings time almost back to Newton's absolute clock — a kind of event that really happens. For him, even if time emerges from something more fundamental, that does not mean it is not real.
— JAIn their own words · checked verbatim
For me, time is a real thing. It's tangible. It's part of four-dimensional spacetime, the fabric of our existence, of reality itself.
JA4:07
And my view, maybe this is something we will get into later on, is that it's the other way around, that time's direction, directionality, irreversibility is more fundamental.
JA5:23
I never say it's a hydrogen atom, it's a proton. I don't care about electrons. That's chemistry.
JA45:51
It decoheres in femtoseconds. What's the chance?
JA54:32
And what we have tried to argue is that the universe started off with very low entanglement entropy, that all the parts of the universe, the subsystems of the universe, were in pure states.
JA1:03:12
I have no issue with the idea that time may be emergent from something more fundamental, quantum entanglement or something like that, in the same way that maybe space is something that is emergent from something more fundamental. But I don't think that means it's not real.
JA1:09:53
That itch has to be scratched. And if the Greeks were scratching it and we're still scratching it, what's wrong with that?
JA1:13:40
Figures
| Time of the universe's initial low entropy | about 14 billion years ago | 31:03 |
| Ambient temperature inside living organisms | about 300 kelvin | 54:32 |
| Timescale of quantum decoherence | on the order of femtoseconds | 54:32 |
| Jim's teaching career | 30 years | 1:06:20 |
| Age at which Jim took early retirement | 63 | 1:06:20 |
| Timescale for the universe to reach thermal equilibrium | several trillion years | 1:11:06 |
| Jim's grandchildren | 10-month-old twins | 1:06:20 |
Glossary
- past hypothesis
- The assumption that the universe was in a special extremely low-entropy state at the Big Bang, invoked to reconcile time-symmetric laws with entropy increase.
- entanglement entropy
- Entropy arising from quantum entanglement between subsystems, measuring the quantum correlations hidden in an overall pure state.
- master equation
- An equation describing the evolution of open quantum systems; the Schrödinger equation is only a special case for isolated systems.
- decoherence
- Entanglement of a quantum system with its environment causing interference terms to vanish, which shows up as the emergence of classicality.
- non-Markovian
- When the environment's evolution depends on the system's history, so information can flow back rather than being lost monotonically.
- many-worlds interpretation
- The view that measurement does not collapse the wave function and that all outcomes are branching parallel universes.
How to listen
Physics learners who care about the nature of time, researchers in quantum foundations, and tech investors who want to know how much evidence quantum biology actually has.
The first 12 minutes of publishing anecdotes and interdisciplinary talk can be skipped.