Three decades ago, talk of other universes was not seen by most physicists to be part of science. Most research in theoretical physics and cosmology concerned observable features in our universe and most papers and seminars referred to experimental results. However, since then there has been a gradual shift, during which it first became acceptable to work on theories that described not only our universe, but other possible universes, universes with less or more dimensions, or universes with different kinds of particles and forces. In the last few years, we have moved further away from theories of our one universe, as these other worlds went from being logically possible to hypothetically actual. It is now common to hear about the multiverse — a quantum cosmology that takes for granted that the visible universe that we see around us is just one of a vast or infinite number of universes.

The multiverse assumption often comes hand in hand with a metaphysical assumption regarding the nature of time. It has been argued by many experts in quantum cosmology that time is not a fundamental concept, but an approximate and emergent one. If this is correct, then we experience time in a timeless universe for reasons similar to why we, who live in a quantum universe, experience one that obeys classical physics: we are composed of very large numbers of fundamental particles and emergent statistical regular ities determine much of what we experience.

Furthermore, the combination of the multiverse assumption and the timeless assumption effectively gives us a static meta-universe. Even if our own universe evolves in time, at a deeper level it is part of a timeless, eternal, ensemble of universes.

There are good reasons for these conclusions, and like many others in the field of quantum cosmology I have explored them. However, in the last few years I have come to believe that these conclusions are profoundly mistaken. In collaboration with the Brazilian philosopher Roberto Mangabeira Unger, we have been trying to understand the source of the problems and develop an alternative notion of time and law on the cosmological scale. Our reasons for doing so are based partly on concerns about whether these theories are testable by doable observations, partly on the current results of attempts to realize the timeless approach and partly on philosophical considerations.

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