Lorne Booth
Sep 5, 2018 · 2 min read

You might regret asking: how do we know time is curved? While curved, it is not a sphere so Groundhog Day won’t happen. It is probably more correct to think of time as being stretched farther apart as one looks farther away and as being pinched at the point where you are. It’s a bit like being at the North Pole and observing all the parallel latitudes lines of Earth emanating from the equator and converging on you. We know the latitude effect is due to planetary surface curvature.

How do we know for time? The cop out answer is that Einstein postulated that space and time together curve in some mathematical sense, which led to rather unique predictions, and those predictions have passed every test that has been tried. Actually, Newton’s theory was one of curved time only, although he didn’t use these words. Newton showed us that an object thrown horizontally straight from a tower follows a curved path over time, until it hits the ground. This is Newton’s curved time in action. Einstein fused space and time together and discovered that they both curve. Space only curves noticeably in presence of extreme masses. Time is always curving noticeably (if only in the math).

The real answer is the word curvature is an analogy. It’s a visual way of extending ideas about three dimensional space to the four dimensions of spacetime. All we really know is if you move one parsec out and move one century into the future, and then move one parsec back in the opposite direction and one century into the past, you don’t end up where you started, all this done in a coordinate system, of course, since you can’t really go backwards in time. It’s just like moving north 100 km from the equator (or from any other point), then east 100 km, then south 100 km, and then west 100 km. You won’t end up at your starting point on a curved surface like the Earth. We intuitively know this is because of planetary surface curvature with Earth movements. We suppose it is spacetime curvature in the case we can’t picture so well.

If I had to speculate on a more physical explanation, I’d say it is related to the bubble of time that started with the big bang and continues to expand. We usually measure short durations between events on bubble surfaces that close to each other in distance. Things seem normal. When we measure long durations between events on bubble surfaces that are far from each other in distance, we get some strange effects related to how we measure things. Distance and duration lose some of their normal meaning. But I’m just speculating here.

    Lorne Booth

    Written by