So here’s how it is.
You have a computer and on this computer runs a program, right? It can be any program – word processor, iTunes, a chat client or even Duke Nukem. Now, you start up another program. For the sake of the argument, these programs are not of the kind that can “talk” to each other. It’s not Photoshop and Flash or something like that. Let’s say it’s iTunes and a spreadsheet thing. These two programs are operating in the same confined space, that is – your computer. They, however, have no knowledge of each other. Each operates on its own, unaware of other programs which may be running right “next” to it.
This is much like our universe. We each run on our own “program” or planet, there may be other planets running next to us (“next” being a relative term, of course) and we will never know. Programs start being aware of each other either when you add some feature that enables them to do so, a plugin or something like that, or when they start competing for system (i.e. natural) resources. So that is how we’ll eventually become aware of other sentient beings – once we improve our capabilities to the point where we’re able to properly communicate with them, or once we start fighting for resources.
It doesn’t end there, however. Cause your tiny little computer is not the only computer out there. You have iTunes running on your machine, but your buddy also has it. As does that chick who works at the Starbucks. You all have your own, special copy of iTunes. Hell, you and the Starbucks chick may, by some weird coincidence, have the exact same songs in your music library. Hers, however, are arranged by album name, while yours are by artist. That’s a tiny difference that affects and amazingly huge list of things. This is what is known as alternate universe. The same program (or planet), with the same data (people), but something tiny that’s a little different. Your buddy’s iTunes may be completely different, he may listen to death metal while you’re a jazz type – that’s how we get an Earth where Hitler won the war.
If you want your computer, your little universe, to talk to other universes, you need to connect them, hook them up together somehow. What you need, is some sort of protocol. For computer we have one, we have several actually. For trans-universal communication we don’t. Or rather, we didn’t. Not until this thing I’m holding right here. TUCP – TransUnivsersal Communication Protocol. Let’s make a phone call, shall we?