system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: evidence for other dimensions (beyond 3)
gpt
At the simplest level, a dimension is a number you need to describe something fully. For instance, in our 3d space, you need 3 numbers to describe where something is...up/down, left/right, forwards/backwards (or any one of an infinity of equivalent systems).

But actually, it's immediately clear that 3 numbers isn't enough to describe something. You need another one to describe *when* something is. Therefore, in this sense, time is a dimension. General relativity, our excellent theory of how space and time works, describes time in exactly this way - it gets more or less the same treatment as a spatial dimension, though it's mathematically a slightly different type of thing so it ends up not behaving quite the same...such as only going one way for us.

Now, you're probably actually thinking about extra dimensions in the popular science way, where people throw terms like '11 dimensions' around like confetti. These dimensions are theoretical, and arise from making a set of assumptions and seeing that these assumptions necessitate those dimensions. The central idea to this area of science (and all other areas, though the other ones often have existing practical successes) is trying to make a unique prediction from these assumptions. If that prediction could be verified, the assumptions would have strong evidence for being correct, and so would the dimensions. We *do not* currently have good scientific evidence for these theories, and the existence of more than 4 dimensions is not well accepted scientific theory at the moment.

To elaborate a little further, the extra dimensions discussed here aren't quite like the dimensions you think of as up, down, left and right. A good (and common) example is of an ant on a telephone wire. From some way away, this system seems 'one dimensional' to you because you can describe where the ant is with a single number - how far along the wire it is. However if you look closely, as the ant does, it's actually a two dimensional system; the ant has both a distance along the wire and a distance *around* the wire. This 'around' is still a dimension because it's a number that you need to describe where the ant is, but you couldn't see that dimension from far away, because it's so small as to seem irrelevant to you. The idea in string theory (or whatever) is that there may be analogous cases on quantum scales which may govern how particles behave without being obvious to us.