It could just be a natural phenomenon, like we believe sun spots or storms on Jupiter to be. The appearance of Harey and the other simulacra could be the result of the interaction of some form of energy, like a frequency of a wave of energy that somehow captures the information in the characters' brains and then projects those bits of information that can sustain themselves by inertia via subatomic reactions into a seemingly real copy. It could be that Solaris is alive and doing the same thing, but not fully sentient. Solaris could have the mind of a tree, or a sponge. Solaris could also be sentient, but so different from people that it doesn't understand us, how we work or what we are about. It could also be that Solaris does understand us, that it understands us perfectly, and it is trying desperately to get someone to take it back to Earth to destroy humanity. We don't know.