No one really knows, and perhaps we never will because all black holes are surrounded by an event horizon, which means that anyone traveling in could never get back out again, and certainly not to the same time that they left Earth!

Some theoreticians think that there may be some kind of bridge between universes in the heart of each black hole, but others feel that the dynamical nature of space-time under these conditions would prohibit any information from passing between the connected universes. The exception may occur at the quantum scale of 10^-33 centimeters, some 10^20 times smaller than the nucleus of an atom. Here, the quantum mechanical influences of many universes might interfere with each other through quantum wormholes. No information would be passed, but the universes would, nevertheless, quantum mechanically affect each other just like the ripples on a pond's surface cause each ripple to be changed individually.

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