It’s impossible for humans to truly visualise more than 3 spatial dimensions.
It’s impossible to truly visualise 4D spacetime, because space and time are fused together, i.e., physically combined in some sense. Relativity theory as initially theorised via special relativity has proven that absolute time and absolute length do not exist in our reality, but instead are dependent on moving bodies relative velocities.
The spacetime interval – Presented by: Gabe Perez-Giz (PBS Space Time)
Whilst visualising some physical volume of stuff, it’s virtually impossible to not be able to imagine dividing that stuff into smaller pieces, i.e., trying to visualise something that is microscopic to the naked eye makes no sense unless you're consciously making reference to previously experienced imagery. In contrast, it’s effortless to consider such a value as the Planck length, and just as effortless to acknowledge an even smaller numerical value.
However, the smallest possible unit of both time and space do indeed exist, whether they’re the Planck units of space and time (as in accordance with quantum mechanics), or even smaller values that one day become theorised about or discovered.
Matter, energy, spacetime, empty space, physical (mathematical) laws – all don’t exist.
There is no beyond the physical (spacetime) end of straight-line limits – we’re all locked in our cage, even if that cage exists as multiple so-called bubbles (also known as pocket universes) spread throughout the totality of spacetime (sometimes referred to as the cosmos).
Consider infinite distance for example – it is perhaps quite natural to imagine distance extending forever into outer space. Typically, you’ll hear opposing arguments to the universe being finite such as “what happens if you hit the edge?”, or “what medium is space expanding into?”
Spacetime cannot be infinite as explained in section: “Straight-line distance infinity analysis”. Not being able to truly imagine absolute nothingness is a limitation of our consciousness.
Any conceivable medium that is supposed to exist beyond the boundary (totality) of spacetime is subject to the same impossible (paradoxical) logic that proves infinite straight-line distance cannot exist.
So, what does happen if you hit the edge? ... Consider:
You cannot, because spacetime itself is expanding faster than light speed.
If space is curved as opposed to being flat and you were to travel (theoretically) far enough, you might end up back where you started – like travelling around the Earth but via the strangeness of 4D spacetime.
If the universe is flat as some people suspect and you teleported (hypothetically) a spaceship to the edge, then it seems as if the ship would have to experience some other characteristic of spacetime distortion.
Even though infinity can easily be disproven by using only basic logic, many people still find it uncomfortable to think about, understandably...
The human mind’s ability to visualise things in space is almost impossible to switch off at times – so one obvious difficulty when pondering about absolute nothingness beyond certain finite limits, is that we seem to visually insist that there must always be something more – but our minds are actually deceiving us, i.e., our ability to imagine things visually is so intuitive, that it overwhelms our ability to analyse things logically.
It is impossible to visualise absolute nothingness – that is a contradiction even as a suggestion. Visual consciousness being limited in this way is why it is so difficult to acknowledge nothingness; but our own cognitive limitation in this respect can result in us trying to comprehend logically paradoxical scenarios involving infinitely extending distance – an impossibility.
The spacetime continuum is finite – it belongs to a whole; there’s an all of it; it’s part of a totality.