And if they are discrete, what are the “spaces” between the elemental units of space and time? What is the fundamental unit of spacetime? Is it discrete or contunous and if it is discrete what would its aforementioned “spaces” be?
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Basic answer is that the universe is believed to be quantum in nature. In very ELI5 terms, it is not infinitely divisible. In slightly not so ELI5 terms, as we deal with the very small, the classical ideas of space and time no longer hold.
At a quantum level “where is x” and is no longer a perfectly answerable question. This is pretty hard to get an intuitive sense of. Your question of “what is between” doesn’t really make sense because it isn’t even possible to say where an object is and isn’t at any particular “moment of time” – it appears probabilistic. So even the idea of “between” really cannot be described.
Perhaps a more fair description is that we don’t exactly know. At the levels we are currently able to experiment and observe with, the evidence is that the universe is discretized but there are theories out there, string theory for example, that might have a more fundamental explanation that could be more “continuous”.
No one knows. This is one of the fundamental questions that physics and the Standard Model still can’t answer. Anyone who is able to answer that question will be placed up with Einstein and Newton and will win the Nobel Prize.
We have good reasons to think that the fundamental units of space time may be discrete (or quantized, as physicists like to say), but we also have good reasons to think otherwise.
The biggest reason to think that spacetime might be quantized is that several very thorny math problems with our current models of physics (such as the prediction of a singularity of infinite density in the center of a black hole) vanish if space has a fundamental size limit. Quantization of energy was how we solved a similar problem back in the late 1800s/early 1900s, known as the Ultraviolet Catastrophe, and led to the development of quantum mechanics.
However, while we have mathematical reasons to believe that time and space are discrete, we don’t have any actual physical evidence for it. Personally I believe that spacetime is discrete, but we don’t know.
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