FIFTH DIMENSION

We directly experience 3 spatial and 1 temporal dimensions (3+1). But here’s an interesting observation made by Kaluza and Klein almost a century ago: If you write the equations of general relativity in (4+1) dimensions, but imagine one spatial dimension is “curled up” (closed and periodic), it looks like relativity in (3+1) dimensions plus electromagnetism plus an extra scalar field (“dilaton”). So, in this theory, the 5th dimension is electromagnetism!

Kaluza-Klein theory is not believed to be correct, but it was a conceptual predecessor of the curled-up spatial dimensions assumed in string theory.