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Michio Kaku's explanation of universes in hyperspace in this youtube video gives a metaphor of our universe being the surface of a hyperspace bubble that's currently becoming larger. He also says these bubbles colliding would produce the creation of new universes and the destruction of involved universes.

But this makes me think there's a lot of room in hyperspace for a universe to float for a very long time without encountering another universe.

  • Is there anything significant to note about areas of hyperspace that don't currently have a universe?

  • Is it highly entropic?

  • Does hyperspace change in any way as a universe approaches?

  • Is it similar to how a galaxy will warp the space-time around it, in that hyperspace follows the same mechanics, but with more dimensions?

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I would guess that Kaku is referring to the brane world scenario, though he has had to simplify it for popular consumption to the point where it is barely recognisable.

String theory is most naturally formulated in ten spacetime dimensions, one time dimension and nine space dimensions, so the question is why don't we see all these dimensions. The brane world idea is that we are confined to four out of the ten dimensions. When I say we I mean all fundamental particles (except for gravitons) so as far as we are concerned the other six dimensions are undetectable. Technically our universe is a three dimensional D-brane.

But there is nothing special about the three spatial dimensions we occupy. The nine spatial dimensions are all the same, and the three dimensions we experience are just a random outcome of the angle our D-brane happens to be at. Other D-branes can exist that are at different angles so their three spatial dimensions wouldn't correspond to our three spatial dimensions. Kaku uses the word hyperspace to refer to the space outside our brane, but there is no special significance to the term. It's not hyperspace in the science fiction sense of some alternate world.

I will leave the final word on this to the Wikipedia article I mentioned:

There is no experimental evidence for this hypothesis, nor is there any definite need for the brane multiverse in M-theory or string theory.

Response to comment:

You say:

I meant that in order for a universe to expand there has to be somewhere to expand into

and in fact Kaku does say something like that in the video. But it is most definitely not true. This is such a beginner's error that Kaku must mean something else by his statement, though I'm not sure what.

The expansion is intrinsic not extrinsic, and the universe isn't expanding into anything. I won't go into this here, because it's covered in some detail in the questions What is the universe 'expanding' into? and If the universe is expanding, what is it expanding into?.

So there aren't any unused bits of spacetime that the universe is expanding into. The universe fills all of spacetime. It's just that some parts of the universe are undergoing inrinsic expansion.

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John Rennie
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