Ever rethink a decision? When we say would've, might've, and should've we enter what I call the Domain of the Apostrophe: the uncertain and unprovable world of what might have been, if only. With 20-20 hindsight we can often this see clearer than what actually did happen.
In writing, there is a specific genre for this kind of speculation: alternative history fiction. What if Nixon were still president? (This is part of the plot line of Watchmen). What if Hitler or Napoleon had thought better and sought peace before their ill-fated Russian incursions? What if the South had won the American Civil War? If you can conceive of it, probably somebody has written a piece on it.
This genre is a kind of mash-up between sci-fi and historical fiction.
Yet another genre along these lines is time-travel fiction. What if you went back in time and shot Hitler? What if you went back in time and shot your own grandfather? This kind of fiction is interesting because it deals with paradox. Lots of movies and stories feature this device: the Terminator series is the most famous example. Alternate realities are created and a common theme is a time traveler going back into the past to change something to gain an advantage or, more importantly, to prevent an adversary from becoming a problem. This sort of plot is featured in Time Cop. Traveling back in time to prevent a global calamity is the main theme of Terry Gilliam's 12 Monkeys. Traveling back in time and accidentally changing history with disastrous results is the theme of the Harlan Ellison-originated screenplay for the famous Star Trek episode The City On The Edge Of Forever. The Doctor Who series is loaded with time travel as well.
The world line theory says that, when you make a decision, you are actually charting a course through a series of realities. If you had made a different decision, you would be in a different reality. The theory says both realities exist, but the one you are actually in is determined by the decisions made. So the world line you are on charts a course along the decisions you make. But what if you could travel between world lines?
Some authors call each world line an alternate reality. Others call them alternities, or just alternate earths. The Sliders series, with Jerry O'Connell, featured this specific kind of travel.
Is there any physics to these stories or is it really only the stuff of fiction?
Consider dark matter. This is matter with mass, or at least gravitational effects, but with no discernible interaction with the electromagnetic spectrum. In other words, it is matter that is not directly observable, possibly phased out of existence, but which can still be detected. It can be detected by its gravitational lensing effect, for instance.
Estimates point to a majority of the mass of our universe coming from dark matter. What if dark matter were a manifestation of mass from a parallel universe? An alternate dark reality? Knowledge of dark matter could forever change the ways we think about our universe, and the ways we interact with it.
Consider dark energy. This is the energy and inter-workings of space itself. When we talk about a photon or even gravity as a wave, there must be something in space to transport it. The notion of a Bose-Einstein Condensate, which is theorized to permeate the fabric of space itself, helps us to comprehend what is meant by dark energy: energy contained in the vacuum of space. Extracting such energy is an opportunity that we should concentrate on: imagine plentiful free energy and the effect it could have on the world.
But when we figure out how to tweak the internal workings of space itself, then something new and wonderful can happen. We can fully understand it and probe the secrets of dark matter and once and for all satisfy ourselves whether alternate realities and mirror universes can actually exist.
We can never access alternate realities without a fuller understanding of the universe.
Perhaps the most interesting and singular scientific development lies in the concept of quantum entanglement, sometimes known as quantum teleportation. In this property of physics, two particles, such as photons, can be entangled and they can share quantum states. But, once entangled, they can nonetheless be separated by arbitrary distance and they still share a quantum state. When you change one of them, the quantum state of the other also changes. This has been verified by experimenters over kilometers. And the state change propagates out at the speed of light.
Were the state change instantaneous, it might indicate that somehow the particles are really somehow next to each other in some topology of space. But, since the state change propagates outward at the speed of light, it indicates that information inherently takes time to propagate through the fabric of the universe.
It doesn't seem to make any difference what matter is in between the separated paired particles. This state change happens anyway.
The really cool thing is that this property clearly may be exploited for quantum communications. And, of course, it could be used to make messages secure and unblockable.
My point here is that there are properties of the universe that we still do not fully comprehend. And we need to know why these properties exist and exactly how they function. Once we know this, our discoveries and knowledge will lead us to greater things.
Perhaps alternate realities will never be possible. Perhaps causality, the deterministic nature of cause-and-effect, will never be successfully undone. And perhaps we will all be safer for it.