The term “spacetime” and the concept of time being the “fourth dimension” appear to contradict each other. Is time actually a 4th dimension, or are all dimensions captured in time?

Question

The term “spacetime” and the concept of time being the “fourth dimension” appear to contradict each other. Is time actually a 4th dimension, or are all dimensions captured in time?

In: 22

Answer 1

Dimensions are just a mathematical construct.

You can make up any dimensional space you want: First dimension could be even numbers and second dimension is odd numbers. That’s a valid 2D space.

The trick is to find a dimensional space that is useful and consistent with some situation.

For example, sailors on the planet earth might use 2D longitude and latitude because it makes math and navigation easier. Is Earth 2D? No, you can fly or dig a hole but that doesn’t lessen the usefulness of longitude and latitude for boats that sail on the surface of earth.

4D (Space + Time) is just a useful space definition for describing relativity, much like longitude and latitude is useful space definition for describing a location on the ocean.

Now, space time certainly seems “special” because it fits with relativity so well and relativity has been proven to describe “reality” by real world experiments. (So far.)

Mathematicians can create infinitely many 4D spaces. Einstein’s space time is just a particular 4D space that works well with the theory of relativity. Before that, 3D space and time were considered separate. Edit: And that space definition worked well with Newtonian physics.

Now some other physicists might come along and come up with a 10 dimensional space that handles relativity + quantum physics in a single unified theory. But that hasn’t happened yet.