I frequently hear people say that the universe is based off of a 4th-dimensional space-time continuum. As a result, it could hypothetically do things like loop on itself if you went in one direction long enough. However, people often point out that the 4th dimension is not time, but rather an independent spacial dimension. At the same time though, they might talk about how the 4th dimension actually is time in the space-time continuum, which makes no sense to me at all. Is the 4th dimension time in the space-time continuum, or something else?
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Dimensions are just ways to measure things at their core. You can define a 3D object in terms of length, width and height. The universe has a rather obvious 4th dimension in time- to properly place something in the universe, you have to specify the time, because it changes over that axis.
There is also rather compelling mathematical and observational reason to think that there are other spatial dimensions as well. For one the math on string theory demands it. These dimensions are rather small and usually tightly roll themselves up, but they should be there if string theory is right. The math itself is a bit above the paygrade of eli5 but the observational reasons are useful as well: gravity is a fundamental force, but it’s dwarfed by the other 3 fundamental forces. Like you wouldn’t even see it if placed on a graph at the same scale as the others. This has bothered scientists for ages. I mean, really think about it: you can resist the gravitational force of an entire planet with a 2 cent fridge magnet. That’s pretty weird. A popular, plausible hypothesis is that gravity as a force is much more pronounced (i.e. ‘leaking’) into those other dimensions. There’s still no consensus about that, I don’t want to paint it as fact, but it gives you an idea of the thought process.
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