If you could enlarge the proton of a hydrogen atom to the size of a baseball, the electron of that atom would be about the size of a BB rotating around the proton ten miles away at the speed of 10,000 miles a second. Now if the earth were a proton and the moon an electron and the space between them had the same ratio that exists in a hydrogen atom, the moon would be 1,000 times farther away than it is now. I know, it can be a bit difficult to wrap your brain around it.
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So, knowing that all this space exists in an atom, many experts in the field of atomic energy have come to the conclusion that it is theoretically possible for parallel worlds to coexist at the same time in the same space, both passing through the other, unaware of the each others existence.
Yeah, but none of them have ever BEEN there.