Time Is Real, Space Is Emergent: Lee Smolin on Finishing Einstein's Revolution

Lex Fridman opens by asking what Einstein's unfinished revolution is, the title of Smolin's latest book. Smolin says it is the pair of revolutions that produced relativity theory, special and especially general, and quantum theory. He notes that Einstein was the first, in 1905, to realize that a radically different theory would be needed to resolve the wave-particle duality of the photon, which makes him one of the founders of quantum mechanics.

Einstein later distanced himself from the theory as it was developed in the 1920s. First he did not believe it at all, and then he became convinced it was consistent but incomplete. Smolin shares that view: quantum mechanics needs additional degrees of freedom, such as particles and forces, to give a complete description of each phenomenon, which is what realism demands.

When asked what bothers him most, Smolin points to the measurement problem. The basic formulation of quantum mechanics gives two ways for a situation to evolve in time, one when nobody observes it and one when a measurement happens, and the two contradict each other. A second reason the revolution is unfinished is that general relativity describes big things and quantum theory describes little things, and we don't understand how they relate.

He says there are two routes. If quantum mechanics as understood now is correct, one brings gravity into the quantum domain through a quantized theory of gravity. If, as Smolin believes, quantum mechanics must be completed, then finding that completion should also incorporate spacetime and gravity.

Fridman asks what spacetime is. Smolin calls it an intellectual construction we make of events in the universe. He believes the events and the causal relationships between them are real, while the smooth four-dimensional geometry that satisfies Einstein's equations is a good approximation at some scales and not something fundamental.

He defines fundamental as part of the description as far down as you go, and says time is fundamental while space and spacetime are not. Time, in his view, is the continual creation of events from existing events. He also believes the past is a story of events that took place while the future does not yet exist, so with no events there would be no time at all.

Smolin admits that an event is a primitive concept, with no definition in terms of other things. In general relativity it is where two particles' paths intersect or something changes. In the theories he is developing, it is an elementary happening with no tie to energy or mass. He credits his collaborators Marina Cortês and, earlier, the philosopher and law professor Roberto Mangabeira Unger, whose thinking influenced his own.

In this picture, events have an order in which they cause each other, but they do not live in space. Space is an emergent property that arises at a sufficient level of complexity and cannot be derived directly from the fundamental level. He says this emergence has been demonstrated in detail in some models, both computationally and analytically.

Fridman says it seems crazy for locality not to be fundamental, since we can affect nearby things and not distant ones. Smolin separates several definitions. In quantum field theory there is a precise one: field operators at spacelike-separated events commute, and this property is well tested.

The other definition comes from Einstein and was made precise by John Bell. Take two entangled particles and measure one; the assumption is that what is real about the distant particle is not affected by the choice of what to measure at the first one. This is called Bell locality and leads to an inequality that correlations must satisfy.

Experiments that create pairs of photons or other particles violate the inequality by many sigma, so this definition of locality is false. Smolin says it has been tested over a number of kilometers, though he doesn't recall whether it was 25 or 107. For him, making causality fundamental and breaking away from locality is a crucial step toward completing the revolution.

Asked how to finish the revolution, Smolin says he can only describe what he is trying and what he has abandoned, working as one ant in an ant colony. In both quantum foundations and quantum gravity there are more than a dozen approaches. Each works partly and then meets an obstacle no one can get around, and none of them completely solves the problems he cares about.

He calls the situation either terrifying for students or full of opportunity for the right student. He thinks the field has evolved into communities around particular approaches. Borrowing a metaphor from Eric Weinstein, he describes them as sitting on hilltops in the landscape of theories and throwing rocks at each other.

His proposal is that people come down into the valleys, party and talk, and say "yes, and" instead of "no": your idea goes this far, and perhaps combined with mine it can go further.

Fridman brings up Sean Carroll, a proponent of the many-worlds interpretation. Smolin praises Carroll as an articulate ambassador of science who takes philosophy seriously and does his homework, although they often end up on opposite sides. Smolin says Carroll told him something in June 2016 that changed his approach to a problem, which he wants to tell Carroll before saying publicly. Fridman urges the two to have a conversation.

On many worlds itself, Smolin says it doesn't appeal to him and doesn't answer the questions he wants answered. His own search for principles made the first principle that there is just one world that happens once. He nonetheless thinks hard about the interpretation as part of the physics community.

His main technical concern is probability. Many worlds keeps only the smooth Schrödinger evolution and drops the measurement rule, yet probabilities belong to the part that was thrown away, and experimentalists use them to check the theory. Everett's attempt to count branches as frequentist probabilities gets subtle when there are infinitely many possibilities, and Smolin understands that it failed.

Later work brought in decoherence, which Everett didn't know about. The Oxford philosophers of physics Simon Saunders, David Wallace and Harvey Brown, along with David Deutsch, use decision theory to ask how one would bet if the Everettian multiverse were true. When he checked with Saunders and Wallace while finishing his book, they weren't sure any of the attempts was yet correct. Of Carroll's approach using self-locating observers, Smolin says it didn't make sense to him, though he admits he didn't study it hard.