The first time a physicist told me they wanted to get rid of space-time, on a cold January morning earlier this year, I stopped typing my interview notes and clutched my żěè¶ĚĘÓƵ mug of tea. Space-time is the very fabric of physical reality – the four-dimensional framework that holds everything in the universe. But now the expert on my computer screen was telling me: “The idea of space-time somehow has to go. The notion of space-time can’t really be a totally fundamental one, and has to be replaced with something more abstract, more primitive, deeper.”
By March, when another physicist told me that space-time “may well be a fiction”, I had wrapped my mind around the idea a bit more. As it turns out, physicists really are trying to leave space-time behind – and they have good reasons for doing so.
The idea of space-time, as opposed to simply space and time, is only about 120 years old. Before that, the two were thought to be separate; you could argue about them separately and the physics equations describing them were separate as well. The giants of modern physics – Albert Einstein, Hermann Minkowski and Hendrik Lorentz – changed this in the early 1900s.
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The unification of space and time is a prominent feature of Einstein’s theory of special relativity, which explains what happens close to the cosmic speed limit – the speed of light. “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality,” in 1908.
Seven years later, Einstein completed his theory of gravity, better known as the theory of general relativity. Yet again, space-time was one of its key ingredients. In fact, general relativity originated the idea that space-time is like a fabric: you can imagine our universe being akin to a stretchy four-dimensional sheet that warps and dimples under the weight of planets, stars and other celestial bodies. If you get too close to a planet, you end up orbiting around it because you are slipping down the curve it has made in space-time. We typically ascribe this motion to the force of gravity.
Space-time was such a useful concept that, from then on, it started showing up everywhere. Special and general relativity are very effective ways of describing our world, so why would we want to get rid of one of their key ingredients, space-time? As is often the case, we can blame quantum theory. For decades, physicists have been unable to combine it with general relativity.
The tricky question here is how gravity can be made quantum. And since gravity is really a consequence of a space-time that can warp, dimple and curve, this question becomes more dramatic – how do we make all of space quantum?
That plan runs into problems at very small scales. Quantum theory identifies a scale beyond which our standard definition of space doesn’t make sense any more. It is called the Planck length, and it is about 100 billion billion times smaller than a proton. We can’t observe anything so small directly, but the mathematical analysis is clear – combining general relativity and quantum theory at such short distances leads to nonsensical results.
Black holes, which relentlessly consume all matter, present a problem too. Some scientists explain their behaviour by suggesting they are simply made from space-time itself. At the same time, black holes are known to have a temperature and another property known as entropy, which physicists can use to calculate how many microscopic states fit inside of each one. But microscopic states of what? If we were discussing a cloud of gas instead of a black hole, counting microscopic states would mean enumerating all the possible configurations of atoms within that gas. If black holes are made from space-time, their entropy can be translated into the number of microscopic states that are possible for whatever bits of stuff make them up.
Because of this, some of the most popular theories of quantum gravity directly engage with the idea that space-time is made from something – rather than being the fundamental medium for physical reality. In string theory, the route to space-time is complex and requires cosmic strands of energy. In loop quantum gravity, the key ingredients of reality are chunks of space-time that get woven together into the esoterically named “spin foam”.
In some theories, quantum entanglement – the inextricable link that can only exist between quantum objects – provides, as , “warp and weft that give rise to the geometry of the world”. Some researchers believe that the universe should be described as a network that space-time emerges from.
And then there is the “cosmohedron,” a gorgeously abstract object that exists in some purely mathematical space and yet seems to be able to reconstruct some simplified models of the universe. Specifically, it can reconstruct the models’ quantum wavefunction, which captures every physical detail of an object. As a result, if researchers derived a cosmohedron for our universe, they would never have to refer to space-time in order to calculate something about it – they would just construct this complex jewel-like shape instead.
In fact, it is difficult to find a physicist who will publicly stand up for the space-time that we have known and loved for the past century. In 2024, , at the University of Edinburgh in the UK. When pressed on the specifics of what black holes may mean for space-time, however, he wavered on his position. “I think if I were forced to place a bet, I might bet that space-time is emergent. But I would not bet very strongly,” he said.
If reality is made from fundamental building blocks that then combine to give rise to space-time, would we ever notice? Should we worry about the prospect of living in a universe whose secrets are inscribed in a mathematical jewel and not much else?
That depends on how much of an emotional stake you have in your current conception of reality. If you are ardently on team space-time, the next few decades of physics research – be it observations of black holes or tabletop experiments aiming to detect quantum properties of gravity – may force you to rethink what hides underneath the world that your senses, and your mind, conspire to show you.
At the same time, none of the theories in question want to deny that space-time can still be useful. They just posit that it isn’t the most fundamental ingredient of the cosmos. You could still use it when explaining why a ball that you throw in the air eventually falls, or to understand why you have to move to go from one room into another. As long as you don’t find yourself magically shrinking to the size of the Planck length or visiting any black holes, you really shouldn’t worry.
But historical precedent also opens the door for slightly more excitement. While most people in the 1900s didn’t know that they should update their idea of reality to include a unified space-time, Einstein’s theories did find practical uses for this concept. For instance, the Global Positioning System that enables many satnavs would never work were it not for special relativity. Could purging space-time from our best theories of the cosmos have consequences for some future technology? This is a question that is nearly impossible to answer right now – but I now know I should keep asking physicists about it.
