What Time Is It?

The three can only walk in to a bar, together, if that bar is located in a neighborhood of a black hole.

In the Nickelodeon series, “Bubble Guppies,” characters often ask the question: “What time is it?” In “From Eternity to Here, ” Sean Carroll, a professor at Caltech, explains what time is.

We use both space and time to help us make sense of the workings of the world around us. When someone invites us for afternoon tea, we ask them the location and the hour of the get-together.

Where: Lady Mendl’s Tea Salon, 56 Irving Place, New York.
When: Friday, 3:00 p.m.

In cosmology, anything that occupies a certain location in space, and a moment in time is called an “event.” The universe is the sum total of all events, of every point in space, at every moment of time, past, present, and future. And that entire collection is treated as a single entity, known as “space-time.” The universe, therefore, is a four-dimensional entity. And to pick out an event in it, we need four coordinates.

Three of those coordinates are supplied by space. To indicate the location of an object on Earth, we require its latitude (distance from the equator), longitude (distance from the prime meridian), and attitude (height above ground.) For an object in space, such as a star, its position is indicated by a pair of angles and its distance from Earth.

Time supplies the fourth coordinate.

Space and time are, in a way, similar.

Just as space is the medium through which we travel, so is time. Even if we sat on an armchair, and didn’t go anywhere, we’d still be moving—moving through time, that is. We’d still be aging. Time flows past us, from the past, through the present, toward the future. It’s the agent of change.

Time labels each instance of the world, and provides a sequence that puts the different instances in order (now, then, before, after.) Just as space is a measure of how far apart objects are, time is a measure of how far apart events are. For example, the duration or “distance” between 1:00 a.m. and 3:00 a.m. is two hours. Time is what clocks measure.

Although there’s a parallel between time and space, the two are radically different.

Directions in space are created equal. If one were adrift in space, far away from Earth or any other planet for that matter, every direction (north, south, east, west, up, down) would be indistinguishable. Every direction in space is good as every other. There’s no preferred direction. It’s Earth that orients us in space.

On the other hand, the two directions in time—the “past” and the “future”—are created different. The arrow of time moves only one way—forward—because the Big Bang has set a preferred direction in time.

Which is to say, that in regard to time, we live in the temporal vicinity of a pivotal occurrence: the birth of the universe. In regard to space, we live in the spatial vicinity of a pivotal object: Earth.

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