I'm just here to give my $0.02 on a common notion that has been bugging me for a while.
Time is not "the fourth dimension". Let's just establish that right now. It has dimensionality, sure enough. But it does not obey the same laws as the three established dimensions. If you want to call time a dimension, it would be more accurately called the first temporal dimension, and in turn, our three primary dimensions would be best categorized as the (first) three spatial dimensions. Each set of dimensions would be a conductor - spatial dimensions conduct matter, and temporal dimensions conduct change.
But why does this matter, you ask? Because there is likely to be a fourth spatial dimension.
Consider this. A zero-dimensional concept would be an infinitely small point in space time. When you place an infinite number of these tiny points directly on top of each other, you create the first dimension. When you put an infinite number of these first dimensions on top of each other, you create the second dimension. When you put an infinite number of these second dimensions on top of each other, you create the third dimension. Simple, right?
But then, what happens when you place an infinite number of these three dimensions on top of each other? You'd get the fourth dimension, correct? This leads to my hypothesis, and the subject matter in this thread - layered space. When most people think of infinite 3-dimensional spaces layered on top of each other, the image conjured is probably one of alternate universes - commonly regarded as a multiverse. But I dislike this term, as it carries the implication that every realm (a tentative term I will use for alternate 3-dimensional spaces in the 4th dimension) might contain its own universe and be anti-social to each other. This doesn't quite stand to reason.
Consider how the first three spatial dimensions interact with each other. Each additional dimension interacts with itself in multiple layers. One dimensional lines interact in the second dimension. Two dimensional planes interact in the third dimension. So it would stand to reason that three dimensional realms would interact in the fourth dimension.
In my hypothesis, this fourth dimension contains an infinite number of realms which contribute as an extension of our 3-dimensional view of the universe. Say you've got a door. Next to the door is a switch. When you flip the switch up, the door leads to one room. When you flip the switch down, the door leads to a completely different room. In both situations, the door would open the same way.
This is what I mean by layered space. In the example, the door is a simple 4-dimensional barrier, and the switch is the control. One room would exist in your realm, the other room would exist in another realm at the exact same point in euclidian space. Both exist in the same exact spot, and both are equally real, they simply hold different spots in the fourth dimension.
Obviously it would not be this simple. An open gate is potentially dangerous given how much of the universe is a vacuum (which therefore means that any random spot in another universe has a high probability of being within a vacuum). A gate such as the one I described is also excessively idealistic as it requires the two rooms to have the same orbital speed and trajectory in euclidian space, as well as the same location. If they were moving in different directions or at different speeds, the gate would only last a moment at that exact spot before the alternate realm room fell out of the range of the gate.
If anyone has any thoughts on this hypothesis, I'd love to hear them.