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# What Is the Space-Time Continuum?

Everyday Einstein explores the 4 dimensions of the space-time continuum

By
Lee Falin, PhD
Episode #081

A few weeks ago, a listener wrote in with this question:

"Can you please give me a brief description of the space-time continuum?"

That's a big question, but I'll do my best. As you probably know, we live in space, which is a 3-dimensional thing. The fact that space is 3-dimensional means that you can move in three different ways. You could think of those as side-to-side, up and down, or forwards and backwards.

Scientists usually assign letters to those directions: x, y, and z. So if you move 4 steps to the right, you would move 4 steps along the x direction or the x "axis" as scientists call it. If you move 4 steps to the left, you would move 4 steps along the negative (or opposite) x axis..

Of course you can also move diagonally, but this is really just a combination of two or more of those three ways of moving. So if you took one step forward and to the right, you would be moving along the x axis and z axis at the same time.

## Stand in the Place Where you Live

Let's imagine that the middle of your living room is the centre of the universe. So we assign that spot the coordinates of x = 0, y = 0, and z = 0. This location is called the origin.

We'll also say that if you move north or south from that spot, you're moving along the x axis, if you move up or down, you're moving along the y axis, and if you move east or west you're moving along the z axis.

We could write the coordinates (or location) of your current position like this: (0, 0, 0).

If you move one meter to the right, we could say that your new position is x=1, y=0, z=0, or (1, 0, 0). Then if you jump into the air, we could say that your new position (while in the air) is x=1, y=1, z=0, or (1, 1, 0).

Now it's important to note that we arbitrarily said that x=1 means 1 meter of distance from the centre of the living room (or origin). We could have said that x=1 means 1 foot, or 1 inch, or even 1 mile. It doesn't matter as long as we're consistent with our measurements. The direction we assigned to x, y, and z also don't matter, as long as we keep them the same during our discussion. We could have just as easily said that z means left and right instead of x.

Space-time adds a 4th dimension to this idea.