Sum Omnia
Sum Omnia, or "I Am All" is the concept that when you see another sentient being, you are seeing what you have been and will be again at a different point in your Infinite Journey.
The eternal throwing of dice from between lives will eventually get you into every life that has proven itself to exist by existing. The implications of Sum Omnia on morality are discussed in Cosmic Justice.
The One-Electron Universe
The One-Electron Universe is a postulate half-seriously proposed by John Wheeler to Richard Feynman that all electrons appear identical because they are, in fact, a single electronmoving back and forth through time. What we perceive as a positron, he posited, is simply that same electron traveling backward in time.
Now this is an absurd idea for many reasons, particularly because if it were true, we would see many more positrons than we do. Still, the concept stuck with Feynman. It later influenced his development of Feynman diagrams, a foundational tool in quantum electrodynamics (QED), which incorporate particles moving both forward and backward in time.
In relativistic quantum mechanics, particularly in solutions to the Dirac equation, particles traveling backward in time can be reinterpreted as antiparticles traveling forward in time. This reinterpretation is consistent with charge conjugation and time-reversal symmetry in quantum field theory. Thus, an electron moving backward in time does behave mathematically like a positron moving forward in time.
Of course, this doesn't mean there is only only one electron, however the odd uniformity that all electrons appear identical with the same mass, charge, and spin lends a kind of philosophical charm to Wheeler’s imaginative suggestion. What began as a wild idea, the one-electron universe ended up illuminating deep truths and proving conceptually useful in the development of modern physics.
Implications
But what does the one-electron universe have to do with Sum Omnia? Well, in a strange way it is conceptually useful in a totally different manner than it was originally intended.
Let's try abstracting the Infinite Journey in a way that can be visualized. Imagine we have a large room full of air molecules. Now imagine your egonaut is one of these molecules and your current life is your position in this room. So your molecule moving from one place to another corresponds to you in two different lives.
The Second Law of Thermodynamics tells us that the entropy and disorder of this system is more likely to increase as time goes on. Therefore, you should expect that, given eternal time, you should occupy all spaces within this hypothetical room.
So, you should expect that, given eternal lives and entropy, you will live at some point as everyone and every thing capable of experiential existence.
Like with the concept of the One-Electron Universe, it may not literally be the case that someone else is you at a different point in the Infinite Journey, but it is a useful concept for understanding the grander eternal picture.
The One-Sentience Universe
While the "One-Sentience Universe" isn't implied here to be impossible, Doctrina Ultra tells us that without a mechanism to ensure only one, we should presume more than one.
If the "One-Sentience Universe" did exist, it should exist infinitely with a different ultimate sentient entity for each and thus when abstracted would be understood as less about cosmic lonesomeness and more about cosmic equality between infinite beings across infinite universes, just structured in a more bizarre manner.
See Also
➤ Infinite Journey: Cosmic Justice, Infinite Journey, Doctrina Ultra, Ultraverse
➤ Wikipedia: One-Electron Universe, Quantum Electrodynamics