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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
December 22, 2013
Episode #081

Page 1 of 2

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.

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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.

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