Einstein’s picture of spacetime, a four-dimensional fabric uniting space and time, breaks down at the smallest scales, where quantum mechanics takes over. For decades, physicists have searched for an account of how spacetime “emerges” from this deeper quantum reality. But philosopher Sam Baron argues that the whole idea of spacetime “emerging” from something else makes no sense. All our accounts of how things “emerge” from something more fundamental presuppose spacetime, so the idea that spacetime itself emerges is circular. His radical conclusion is that we must abandon the project of reconciling spacetime and quantum mechanics, and accept that spacetime, at least as Einstein described it, does not exist.

I don’t think spacetime exists. I know how that sounds. Pretty silly, right? We are very used to thinking of spacetime as the fabric of the universe. We are also pretty used to thinking of it as the arena in which we live our day-to-day lives. The loss of spacetime would thus seem to be devastating. Wouldn’t it effectively mean the annihilation of the universe? Or, at least, wouldn’t it be hard to plan for anything, since without spacetime there’d be no future and no sense in which there’s anything to come?  

Hear me out.

A quest in physics

The early twentieth century was transformational for physics. We developed the general theory of relativity and quantum theory. Despite the incredible success of these two theories, physicists are hell-bent on replacing them with a single theory of quantum gravity.

Why? Well, put simply, these two theories don’t provide a complete picture of the world. General relativity provides an account of how gravity works in terms of the curvature of spacetime. The incredible success of the theory leads us to believe that the world is as the theory describes and thus that spacetime is real. But while the theory is extremely successful at large scales, it simply doesn’t work when we try to apply it to extremely small scales where quantum effects dominate. Since quantum theory has little to say about gravity, this means we don’t have a story of gravitation that reaches all the way into the quantum domain.

There are many approaches to developing a quantum theory of gravity, and not much agreement on what the “right” approach might be. In a number of such theories, however, it seems that spacetime as we know it disappears at the very fine levels of detail where quantum gravitation is expected to apply. In some sense, this is not all that surprising. General relativity, as a theory, must give way. When theories give way like this, we often discover entirely new physics lurking below and revise the way we think about the world.

Emergence

Matters are not quite so simple, however. General relativity is a very good theory: it is extremely well confirmed. So, we can’t just throw it away. There are a couple of options for what to do.

One option is to take general relativity to be a completely true theory of a particular sub-domain of the universe, namely the domain in which quantum effects don’t dominate. Because general relativity seems to require the existence of spacetime to be true, we could then draw the conclusion that spacetime, as described by general relativity, exists but only within that domain. Since general relativity does not apply universally, we could then say that there are domains in which spacetime doesn’t exist.

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We need spacetime to explain how spacetime emerges, and that looks circular in a bad way.

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