But before we venture too far into alternative/parallel universe conjectures, I would first like to examine the intersection of the subject of parallel universes with that of alien contact. While the idea of alien contact is plausible in our universe, it may be rather implausible between two universes.
Some hypotheses, like the Many Worlds Interpretation (MWI), or quantum multiverse, state that different universes arise "merely" from the random fluctuations that happen on a quantum level. Chance and true randomness exists, and creates multiple universes. Einstein did not much like the idea, did not like what the math, what the theories were suggesting. He could not believe that God plays with dice.
In a random event, there are more than one possible result. In the MWI, for each random event, each possible result occurs: one occurs in our universe, and the others occur in newly created parallel universes. Thus, those universes have the same physical laws as our universe, just with some small change in a quantum level random event. If aliens were somehow able to travel from one of those parallel universes, they would enter a space time continuum that would function the same as it did in their home universe. Though MWI states that no communication between the universes would be possible, thus no travel, Although there could be quantum interferences between them, though I doubt much could be communicated that way.
Other hypotheses predict changes in physical laws for each universe. This would have major consequences for any alien explorer from another universe. If parallel universe A's gravitational constant, for instance, is stronger than universe B's, then everything in universe A would come to existence, work toward equilibriums, and evolve within this stronger gravity. And since gravity is part of the space-time continuum, the very fabric of space, if aliens from universe A somehow finds a way to physically pop into universe B, they would find themselves in a new space-time continuum - one with weaker gravity. Their bodies, their evolution, their spacecraft and the materials within it were all grew, lived, and evolved in and/or was designed for stronger gravity.
This would affect them at both the atomic level and the macro level. It would affect every particle of their beings, and every particle of their ships. Some very small changes to physical laws could have very large changes for the physical structures (both living and inorganic). It would be very dangerous, even lethal, to pop into a parallel universe without checking the conditions; the aliens would need to send over a M.A.L.P. (Mobile Analytic Laboratory Probe - yes, I'm a Stargate fan, though I doubt that would surprise anyone). Though it raises the question that even if they were able to send over something like a M.A.L.P. first, could they truly trust the readings they get back since they are using sensors that would be affected in unknown ways by the unknown differences between the universe being investigated and the home universe?
It would be best to find a means of communicating without actually physically setting foot in another universe. One possible way of communicating may actually be via gravity - some hypothesize that gravity may leak out of universes and or otherwise be felt by other universes. If so, that may be the most logical way to carry on a inter-universal communication. Wouldn't that be wild - to communicate with a sentient species in another universe?
Maybe the safest way too - if it turns out the other sentient race are war mongering conquerors, we would have little to worry about (as long as each universe is incompatible to life from the other); though I wonder, could they eventually figure out how to use a gravity wave weapon, but what would it profit them to do so? Of course we've seen on our own planet some people just can not tolerate anyone different from themselves, and that is reason enough for them to murder.
This topic will, of course, be discussed in more detail future posts.