Starts With A Bang

Ask Ethan: Why Must Time Be A Dimension? (Synopsis)

A time-lapse photo like this composition reminds us that photographs are normally snapshots of locations at particular moments, with each moment distinct and unique from the last. Image credit: flickr user Anthony Pucci.

“It is old age, rather than death, that is to be contrasted with life. Old age is life’s parody, whereas death transforms life into a destiny: in a way it preserves it by giving it the absolute dimension. Death does away with time.” -Simone de Beauvoir

If you want to describe your location here on Earth, only two values are usually sufficient: one for your latitude and one for your longitude. This is because the surface of the Earth is two dimensional, and you’re generally not either underground or airborne; if you are, a third value, representing your altitude, is also necessary.

On the surface of a world like the Earth, two coordinates, like latitude and longitude, are sufficient to define a location. Image credit: Wikimedia Commons user Hellerick.

But location typically assumes that we’re talking about a particular moment — right now — and that isn’t necessarily the case. Instead, if you want to know exactly where and when an object or event happens to be, you need a fourth value as well: its time. Those four values uniquely define an object in our 3+1-dimensional fabric of spacetime, but there’s a fundamental difference between the three “spatial” ones and the one “time” one.

Mark and Scott Kelly at the Johnson Space Center, Houston Texas; one spent a year in space (and aged slightly less) while the other remained on the ground. Image credit: NASA.

Time absolutely must be treated as a dimension, but there’s more to the story than the obvious reasons. Come get the nuance on this week’s Ask Ethan!