...in the universe, that is. Susan Foreman in "The Unearthly Child" informs us there are five - counting space and time.
In the 1960s that was advanced physics. Today? Today we're not sure. String theory supports dozens of dimensions, but most of them are too small for us to see. And today we think space and time are basically aspects of the same thing.
But now, just to confuse the matter, physicist are thinking there may only be two.
What? Are we saying we live in Flatland now? This all, like the idea of almost infinite dimensions, goes back to string theory. String theory suggests that the entire universe is an optical illusion - a hologram created by light diffraction. Another piece of evidence - the fact that it seems you can't have anything smaller than the Planck length - is that the pixel of the universe? And the Holometer at Fermilab is supposed to find out.
I don't buy it, but then neither would the people of Flatland buy three dimensions. So... Also, the Holometer may answer other basic questions about space and time.
And if the universe really is a hologram, what does that mean for us? (Why am I suddenly flashing back to Reynolds' Terminal World? Oh, right, pixel sizes).