Now how is that possible?
(Some may be finite in size. Some may be infinite.
Now how is that possible? )
There are different uses and contexts for the word “infinite.” Infinite is the number of things a two-year-old can make sticky with just one drop of honey. Or, it is the space between a liberal’s ears.
But in this context, I suspect it applies where this universe is for all intents infinite in dimensions we understand, but our universe may exist like the cards in a deck with many other universes. Given what Einstein said about dimensions our universe has at least 11. Seven of those are unknowable to us except as mathematical constructs. The four dimensions we know are called “space-time.” They consist of height, length, width and time. But those things interact with gravity. The speed which an object made of those dimensions is moving also affects our perceptions of the object. The faster the object goes the smaller and more massive an object in those dimensions gets.
The problem perceiving our universe on its own comes in when we start to look at equations and they imply things we can’t see, like the other seven dimensions. If you’ve ever dealt with “imaginary” numbers, which commonly occur and have real world implications in engineering, you start to appreciate what the authors are saying.
Now, how can there be dimensions we can’t experience? Imagine you’re a stick figure man on a two dimensional piece of paper. Somebody sticks a pencil through the paper, the sum total of your universe. What do you, the stick figure man, see of the pencil? You see only the part that appears in your two dimensional world, a line. You then come up with an equation that explains how it starts like a tiny line (the pencil point) and then becomes a line, (the body of the pencil is dynamically moving only in the third dimension), then the line varies in length (the metal clamped eraser passing by) then it changes in texture (the eraser) then it disappears leaving a discontinuity (hole) in your universe.
The other seven dimensions in our universe would be similarly difficult to conceptualize because, like the stick figure man, we have no direct experience with them, only with the mathematics that describes them. We know they do exist, however, because we can conduct tests that show they do. Particle accelerators, for example, can make particles appear and disappear. You have to ask, where did they come from? Where did they go? The answer is in the math describing those other dimensions.
As for the multiple universes, each of which can be finite or infinite, those are elements in repeating equations, one portion of which represents our universe. But other terms which pop out of the equations, like our imaginary numbers which do have real world implications, may or may not represent other universes. Or, they just could be artifacts of the rules we used to construct our math which describes our universe.