What is “space time” and how does it “bend”?

Question

I’ve heard the term “space time” thrown around, and that gravity “bends space-time” but how does that work?

In: 16

Answer 1

spacetime is a unified model of space and time. In day to day life, we think of there being 3 dimensions in space, and time as something unrelated. However, in certain circumstances, it becomes clear that the 3 spatial dimensions and the 1 time dimension are closely enough related that it makes sense to think about not just space and time separately, but as one big four-dimensional “spacetime.”

Objects trace out paths through spacetime. If you see an object standing still, you can think of it as tracing out a path that’s always at the same place in space, but moving straight ahead through time. A moving object, on the other hand, has a path that’s tilted a bit. It stretches through time, but varies where it is in space depending on where it is in time, since the object’s moving.

Gravity doesn’t bend spacetime, gravity is the *result* of bent spacetime.

Objects will tend to move in straight lines through spacetime unless something else acts upon them. For instance, if two objects bounce off eachother, the paths they take would look like lines that got bent at the place they touched. But “Straight lines” behave strangely on curved surfaces. For instance, if you and a friend stood on the equator and walked in straight lines due north, at first it would look like you were walking perfectly side by side. But if you kept going, the curvature of the earth would mean that your paths would eventually cross at the north pole. If you wanted to keep apart, one of you would need to stop walking in a straight line and turn away. Gravity works in a similar way. Mass bends spacetime, and it bends it in such a way that a straight line through spacetime will tend to aim closer to that mass. Even though the orbit of the earth around the sun seems curved, it’s actually the straightest path the earth can take through spacetime, and when you stand on the ground, the weight you feel on your feet is actually the feeling of the ground forcing you to “turn” through spacetime instead of letting you continue in a straight path to the center of the earth.