New York and Waterloo, Ontario, born 1955
Lee Smolin
Time is real, and it is the most fundamental thing there is. The present moment is not an illusion, the future is not yet there, and even the laws of physics are not eternal: they change, and they have a history.
The view
Smolin is an American theoretical physicist based at the Perimeter Institute in Waterloo, Ontario, and one of the founders of loop quantum gravity. His views on time are set out in Time Reborn (2013) and, with the philosopher Roberto Mangabeira Unger, in The Singular Universe and the Reality of Time (2014).
He started out close to Barbour. The two worked together, and Smolin has acknowledged his debt to him. He came to the opposite conclusion: that the timeless picture of physics is an error, and that a theory of the whole universe must take time to be real.
The argument
The Newtonian paradigm cannot be extended to the whole universe. Physics usually studies a small part of the world: we fix the laws, set up initial conditions, and watch what happens. That works because we can repeat experiments and stand outside the system. With the universe as a whole, we cannot. There is one universe, and no outside. To ask why its laws are these and not others, Smolin argues, we need an explanation that unfolds in time.
Timeless laws leave the laws unexplained. If the laws simply are, eternally, then there is no reason why they are what they are. If they evolved, there might be.
Cosmological natural selection. In The Life of the Cosmos (1997), Smolin proposed that black holes give birth to new universes, each with slightly altered constants of nature. Universes whose constants favour making black holes leave more descendants. Over many generations, a typical universe should be one tuned to make black holes, and that, he argues, is a testable prediction.
Laws are not given once for all; they too have a history. after Smolin, Time Reborn (2013)
The present is real. Smolin holds that what is real is what is real now, a moment in a succession of moments, and that the block universe mistakes a mathematical representation for reality. He has also suggested a "principle of precedence": nature may follow habits, repeating what it has done before, rather than obeying timeless laws.
Where he meets Deutsch
On the surface, almost nowhere, which is why Smolin is on this site. But they share a temperament. Both are realists: physics is about what really exists, not a tool for predictions. Both value explanation over mere fitting of data, and both want theories that can be tested.
Both also hold that the future is genuinely open, though they mean different things. For Smolin it is objectively not yet there. For Deutsch it is open relative to each observer, because the multiverse really contains several outcomes. And both reject the deterministic block of classical spacetime as the whole story.
Where they part
Does time flow? For Smolin, yes: the present is real and moments succeed one another. For Deutsch, the idea of a flowing present is not just false but incoherent.
Are the laws eternal? Smolin's laws evolve. Deutsch's multiverse is held together by deterministic laws that every universe obeys. He says outright that there are no universes in which the laws of quantum physics fail to hold.
One universe or many? Smolin rejects the many-worlds reading of quantum theory and looks for a realist theory with a single world. Deutsch regards the multiverse as the only explanation of quantum phenomena on offer.
What is real? For Smolin, only the present. For Deutsch, the belief that the moment we are aware of is more real than the others is a kind of solipsism.
The disagreement could hardly be sharper. If Smolin is right, Deutsch's chapter has the fundamentals backwards. If Deutsch is right, Smolin is trying to explain a feature of our experience by building it into the foundations.
| Smolin | Deutsch | |
|---|---|---|
| Does time flow? | Yes. Time is real and fundamental. | No. The idea is incoherent. |
| What exists? | The present moment. | Every moment, in every universe. |
| Are the laws fixed? | No. Laws evolve and have a history. | Yes. Every universe obeys quantum theory. |
| One world or many? | One universe, with a real history. | A multiverse. |
Further reading
- Lee Smolin, Time Reborn (London, 2013).
- Roberto Mangabeira Unger and Lee Smolin, The Singular Universe and the Reality of Time (Cambridge, 2014).
- Lee Smolin, The Life of the Cosmos (London, 1997), on cosmological natural selection.
- Lee Smolin, Einstein's Unfinished Revolution (London, 2019), on his case against many worlds.