The “space-time continuum” is one of the concepts that came out of relativity theory that most people have heard of, but do not fully understand, like the related formula E=mc². Science articles maintain an awareness of both, but science fiction and resulting memes do more to keep each in people’s consciousness. Alas, spacetime is considerably harder to grasp than Einstein’s famous equation, but that doesn’t mean it has to be treated as incomprehensible to anyone without a physics degree.

In Life, the Universe and Everything, Douglas Adams had great fun with a discussion on “eddies in the space-time continuum”. The humor arose as much from the idea that people had generally heard of the continuum without really knowing what it was, as the puns and absurdity. 

In one sense the idea is simple – instead of there being three dimensions of space, with time being something completely different, spacetime is a four-dimensional thing. Events are represented in spacetime by four coordinates: three based on where something occurs, relative to a defined point of origin, and the fourth being the time at which it happens. 

Although Einstein made the concept of spacetime essential through relativity theory, physicists had been toying with the idea of a unified spacetime in which time was treated as a fourth dimension for some years before, including coining the name. They are united by the speed of light (in a vacuum), which ensures that the effects of an event take time to be experienced in other places.

The problem most people have with this is that we experience time so completely differently from length, width, and height that the whole thing seems ridiculous. If, for example, we realize we have come too far in a particular direction, we can usually turn around and go back. We wish the same were true for time. Ursula Le Guin slyly proposed the idea of an anomaly in the space-time continuum from which time is running out, since there never seems to be enough of it, while noting the same is not observed for the other dimensions.

Physicists struggle to explain why time is so different from the other dimensions. Nevertheless, its status as a fourth dimension, albeit a special one, is provable. We also know time is connected to the other three dimensions to the extent that they often can’t be measured accurately without each other.

Under the conditions we experience in our everyday lives, treating space and time as separate isn’t a problem, which is why the idea of spacetime is so counterintuitive. However, if we were traveling at close to the speed of light relative to something important to us, the situation would be very different. 

A key feature of Special Relativity is that time slows down when traveling close to the speed of light as compared to the experience of a stationary observer. Likewise, at near lightspeed, space becomes contracted in the direction of motion. This means that if someone traveling very fast were to measure two events and compare their results with someone moving slower, they’d get different separations in both space and time. However, using the speed of light to convert between units of space and time, both observers measure the same spacetime distance (assuming they’re measuring accurately).

Moreover, this space-time continuum can be distorted, for example by powerful gravitational forces that affect time as much as they bend space. While controversial when Einstein proposed the idea, it’s now possible to verify the way large masses warp spacetime, for example by comparing clocks in orbit with those on the Earth. 

Measurements of the movements of massive objects, such as pulsars, around each other confirm the predictions of General Relativity regarding the workings of spacetime with greater and greater accuracy. These measurements are still being taken, partly because alternative theories to General Relativity exist. However, since these also accept the nature of time as a dimension, and its existence in a continuum with the dimensions of space, even if one of the alternatives is eventually proven superior, it wouldn’t invalidate spacetime.

Nevertheless, the existence of these competing theories is a reflection on the fact that, while we know the space-time continuum exists, there’s a lot we don’t understand about it. Physicists’ inability to reconcile General Relativity and quantum mechanics has been a sore point for decades. Many have suggested that quantized spacetime might be the solution, but so far no one has found a way to make that convincing.