system
Given the following post, respond as if you're commenting on Reddit:
human
In a theorized multiverse, how are "universes" separated from one other and can they influence each other?
gpt
Which 'multiverse' are you talking about?

One view of the multiverse is that the big bang was a point origin of our universe, but there may be other point origins far removed from ours.  This is the **Ergodic** multiverse.  These universes are separated from each other by immense distances.  They can never influence each other.

In the **Many-Worlds** multiverse theory, the universe branches off every time a quantum effect settles on a random number. (Or possible only certain quantum effects.)  This is where you find your parallel universes where anything that could have happened somewhere did happen.  They are not really separated from each other; in this theory, all possibilities exist somewhere, but we humans are only able to follow and understand one of time's infinite threads.  There may be faint interactions between these universes, and we can speculate that may lead to one possible universe being able to influence another.

The **Brane** theory leads to a multiverse separated by distance in one of the ten dimensions of space-time, as described by certain quantum string theories.  They do influence each other, sharing some gravitational wells and possibly other phenomena -- recently it's been speculated that dark matter is a brane interaction.  But these other universes are alien and probably without the same physical laws, so it's not as if you'll see people living there.

And then there are theories where all these possibilities (as well as other multiverses, such as the holographic universe) are all true *simultaneously*.  When the theory gets that complicated it's impossible to say what can and cannot interact.

tldr; We don't know whether there is a multiverse, or which of several theoretical multiverses may actually exist.  Some allow interaction between universes and some don't.