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 using quantum entanglement entropy instead of thermodynamic entropy as the basis for the past hypothesis.
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The argument · tap a timestamp to hear it
Time's direction is built in, not emergent
Jim takes a hard stance: time is real and tangible, part of four-dimensional spacetime, not just something measured by clocks. He goes further, saying that time's directionality is not emergent from directionless fundamental laws but is built into the universe. Sean reminds the audience that this is highly controversial—many physicists think time is just a tool or that direction is a product of coarse-graining. Jim's intuition is that, viewed from inside the universe, irreversibility is the more fundamental thing.
— JANo system is isolated, so irreversibility is the norm
The traditional narrative is that fundamental equations like Newton's and Schrödinger's are invariant under time reversal, and the arrow of time must emerge from entropy increase or coarse-graining. Jim turns this around: time-symmetric equations are idealizations because no system is truly isolated—even quantum systems are constantly entangled 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 surroundings, and irreversibility is the norm. He admits this is a minority view but believes it is closer to the real world.
— JAThe special moment must be at the Big Bang, not last week
If the whole universe is treated as an isolated system that 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 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, alongside 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.
— JAPhysical laws 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 passage come from? Jim honestly admits that physical laws contain no flow of time, yet we feel time flowing. He says this is a gap between physical time and manifest time, and he is not even sure how to articulate it clearly. This candor is more valuable than pretending to have solved it, and it shows the problem of time is far from closed.
— JADNA mutations may come from a proton's quantum tunneling
From a nuclear physicist's perspective, Jim presents the most concrete case in quantum biology: DNA's double helix is held together by hydrogen bonds, and the hydrogen atom is essentially a proton. Löwdin proposed in the early 1960s that a proton might quantum-tunnel through the energy barrier to the opposite strand, causing a mutation during replication. Jim's team has done increasingly refined calculations to determine the ratio of tunneling to thermal excitation. He points out that quantum biology is not the old saw that 'atoms are quantum' but asks whether life has evolved to exploit long-lived coherence and entanglement.
— JAA more fundamental version of the past hypothesis uses entanglement entropy
The thermodynamic past hypothesis requires specifying a low-entropy microstate. Jim and philosopher Eddy Chen have tried a more quantum version: assume the universe began in a pure state with extremely low entanglement entropy, with subsystems barely entangled. As it evolves, subsystems become increasingly entangled, and the universe's total entanglement entropy rises monotonically. Conceptually this is more fundamental than thermodynamic entropy, but Jim admits there are many gaps—for instance, how finely to partition subsystems. This direction is still unfinished.
— JAAfter the heat death of the universe, time still passes
Many say time might disappear at extreme scales. Jim counters: even tens of trillions of years from now, when the universe reaches thermal equilibrium and no change can be used to measure time, time still passes. He admits this is not time in the relativistic sense that holds in every frame, but it makes time almost like Newton's absolute clock—a real occurrence. 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 |
| Temperature inside living organisms | about 300 kelvin | 54:32 |
| Quantum decoherence timescale | femtosecond scale | 54:32 |
| Jim's teaching career | 30 years | 1:06:20 |
| Jim's early retirement age | 63 years old | 1:06:20 |
| Timescale for the universe to reach thermal equilibrium | tens of trillions of years | 1:11:06 |
| Jim's grandchildren | 10-month-old twins | 1:06:20 |
Glossary
- past hypothesis
- To reconcile time-symmetric laws with entropy increase, the hypothesis that the universe began in an extremely low-entropy special state at the Big Bang.
- entanglement entropy
- Entropy arising from quantum entanglement between subsystems, measuring the hidden quantum correlations in an overall pure state.
- master equation
- An equation describing the evolution of an open quantum system; the Schrödinger equation is just a special case for isolated systems.
- decoherence
- The disappearance of interference terms as a quantum system becomes entangled with its environment, manifesting classical behavior.
- non-Markovian
- Environment evolution depends on the system's history, allowing information to flow back rather than being monotonically lost.
- many-worlds interpretation
- The view that measurement does not cause wavefunction collapse; all outcomes branch into parallel universes.
How to listen
Physics learners concerned with the nature of time, quantum foundations researchers, and tech investors who want to know how much evidence quantum biology actually has.
The first 12 minutes of publication anecdotes and interdisciplinary talk can be skipped.