space and time are not separate things --2/11/2026

11 Feb 2026

space and time are not separate things --2/11/2026

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Today's selection-- from The Biggest Ideas in the Universe by Sean Carroll. The dawn of the theory of relativity:

“In Newtonian physics, you would have been welcome to consider space and time together as a single four-dimensional spacetime in which events are located. But nothing would have forced you to do that; space and time had their independent identities, and nobody ever got them mixed up. It was with the theory of relativity, put together in the early twentieth century, that talking about spacetime became almost unavoidable. In relativity, it's no longer true that space and time have separate, objective meanings. What really exists is spacetime, and our slicing it up into space and time is merely a useful human convention.

“One of the major reasons why relativity has a reputation for being difficult to understand is that all of our intuitions train us to think of space and time as separate things. We experience objects as having extent in ‘space,’ and that seems like a pretty objective fact. Ultimately it suffices for us because we generally travel through space at velocities far less than the speed of light, so pre-relativistic physics works pretty well.

“But this mismatch between intuition and theory does make the leap to a spacetime perspective somewhat intimidating. To make things worse, presentations of relativity often take a bottom-up approach, starting with our everyday conceptions of space and time and seeing how they become altered in the new context of relativity. We are told stories of stretching rods and distorted clocks, all of which are technically accurate but can obscure the beauty and simplicity of the underlying concepts.

Albert Einstein, physicist, 1879-1955, Graphic: Heikenwaelder Hugo,1999

“We're going to be a little different. Our route into special relativity might be thought of as top-down, taking the idea of a unified spacetime seriously from the get-go and seeing what chat implies, rather than starting from more familiar-seeming notions and seeing how they become modified in this new context. We'll have to stretch our brains a bit, but the result will be a much deeper understanding of the relativistic perspective on our universe.

“The development of relativity is usually attributed to Albert Einstein, but he provided the capstone for a theoretical edifice that had been building since James Clerk Maxwell unified electricity and magnetism into a single theory of electromagnetism in the 1860s. Maxwell's theory explained what light is-an oscillating wave in the electromagnetic fields-and seemed to give a special place to the speed at which light travels. The idea of a field existing all by itself wasn't completely intuitive to scientists at the time, and it was natural to investigate the question of what was actually ‘waving’ in a light wave.

“Various physicists investigated the possibility that light propagated through a medium they dubbed the ‘luminiferous aether.’ But nobody could find evidence for any such aether, so they were forced to invent increasingly complicated reasons why this substance should be undetectable. Einstein's contribution in 1905 was to point out that the aether had become completely unnecessary, and we could better understand the laws of physics without it. All we had to do was to accept a completely new conception of space and time. (Okay, that's a lot, but it turned out to be totally worth it.)

‘Einstein's theory came to be known as the special theory of relativity, or simply special relativity. His foundational paper was titled ‘On the Electrodynamics of Moving Bodies,’ in which he argued for new ways of thinking about length and duration. He explained the special role of the speed of light by positing that there is an absolute speed limit in the universe—a speed at which light just happens to travel when moving through empty space—and that everyone would measure that speed to be the same, no matter how they were moving themselves. To make that work out, he had to alter our conventional notions of time and space accordingly.

“But he didn't go quite so far as to advocate joining space and time into a single unified spacetime. That step was left co his former university professor, Hermann Minkowski, in 1907. The arena of special relativity is today known as Minkowski spacetime.

“Once you have the idea of thinking of spacetime as a unified four-dimensional continuum, you can start asking questions about its shape. Is spacetime flat or curved, static or dynamic, finite or infinite? Minkowski spacetime is flat, static, and infinite. Einstein worked for a decade to understand how the force of gravity could be incorporated into his theory. His eventual breakthrough was to realize that spacetime could be dynamic and curved, and the effects of that curvature are what you and I experience as ‘gravity.’ The fruits of this inspiration are what we now call general relativity.

“So special relativity is the theory of a fixed, flat spacetime, without gravity; general relativity is the theory of dynamical spacetime, in which curvature gives rise to gravity. Both count as ‘classical’ theories even though they replace some of the principles of Newtonian mechanics. To physicists, ‘classical’ doesn't mean ‘non-relativistic’; it means ‘non-quantum.’ All of the principles of classical physics, including our ability to chink of things in terms of Hamiltonian dynamics or the principle of least action, are fully intact in the relativistic context, even if we have a somewhat more sophisticated notion of space and time.”