Some theorists hve speculated on alternate realities. Particularly when the subject of reverse time travel comes up. If you go back into the past, and begin altering reality, causality is violated, and the universe would make no sense. So they speculate that if you do go back into the past, you would in fact enter a parallel spacetime universe in which events are different. Each instant in time, when an event occurs, such as when we make one decision or another, becomes like a "fork" where multiple realities split off, and only one becomes realized in our experience; the one "chosen" over the others.
So there is a notion that the other, unchosen realities ("counterfactuals") might exist in some way, yet we simply don't have access to them. If you accept that the universe and all its matter, events and space and time itself consists of vibrating loops of string, then it doesn't seem farfetched to believe that all of these other states of them (collective vibrational patterns) exist, yet only one path through them has been chosen. I have seen two articles, using both space and time to access these counterfactuals. One, in Scientific American, proposes an infinite space, and the further out you go, you run out of possible configurations of matter and energy. So all matter can do is to start repeating itself. Eventually you will run across exact and near copies of everything we see around us, including ourselves.
The other one (http://www.exitmundi.nl/eternity.htm) does the same thing using infinite time. After the entire universe burns itself out and cools into nothingness, the quantum uncertainty principle proposes matter will be randomly popping in and out of existence. Given enough time, more complex objects will appear, including eventually, copies of everything we see around us, including ourselves and a new big bang.
One problem I have with infinite space or time, is that matter and events become zero in the overall scale of the universe.
But to me, parallel realities created from being "unchosen" in the here and now (and not cast off into infinite distance or future simply because the universe has run out of possible combinations) creates a new continuum, in addition to space and time. Space is the medium in which we measure the relative locations of matter and events. We use it to get from one location (marked by matter or an event) to another (see http://www.erictb.info/essays.html#space&time). Time is the medium in which we measure the chain of causality or simultaneity between events. We use it as we live and experience one event after another.
So think, what medium would be the one "travelled" by jumping straight from the here and now in our universe, to a parallel universe where we wore red instead of blue in the here and now? You might think time, but time is marked by a causal chain. On event causes the other. Yet an event we are experiencing now did not cause an alternate event in a parallel universe. They are results of a different choice (event) at a point further back in time where the two realities merge. The causal chain lies in the forward time dimension itself, not in the perpedicular dimension in which you jump from one to another.
It is also not space. When we think of "parallel universes", we are usually thinking of space, where one space is embedded in another space with more dimensions, containing other lower dimensional spaces parallel to the first one. You might think that alternate realities would be the temporal counterpart to embedding in higher spaces. But again, in higher dimensions, the medium in which the lower spaces ("branes") are located relative to each other is still space. The medium between counterfactuals is not time, because, once again, the relationship between parallel corresponding events is not causal.
So this is an all new medium. To give it a familiar monosyllabic name like "space" and "time", I would call it "chance".
I always like things like this in threes. So we have space, time and chance. It sort of parallels the Christian concept of the Trinity. And creationist Henry Morris and others have even linked the concept. (Apologize for referencing the most infamous old-school Young Earth creationist. Even as a young Christian struggling between C vs E; I used to be totally offended by a lot of his rhetoric. But this idea of his is truly interesting!) While the Trinity is often thought of as three equal beings sitting side by side, Morris framed them in terms of a reference, a visual form and an experential form. So the Father is what God is, or who we reference when speaking of God. The Son is God made flesh, visible and tangible in the world (the Father cannot be contained in space and time). The Holy Spirit is how God is experienced (in the heart). Morris ultimately still holds the "traditional" symmetrical view of three "equals" side by side. But when I researched all of this, I found that the pre-Nicene church fathers actually held a non-symmetrical view in which the Father was the Godhead from eternity, and the Son and Spirit were manifested from Him in time (i.e. at the birth of Christ). Forms of this were later revived by the likes of Marcellus and Servetus, but the church by then condemned them in favor of the symmetrical view, which has become the official dogma ever since. (more on this at http://www.erictb.info/triune.html)
Morris had linked this tri-unity to the universe, which he said was referenced to space, seen in matter and experienced through time. (and space had its three dimensions; and time had past, present and future, etc).
I did not like making space the "Father-like" element and matter the "Son-like" element. I had already started coming to see space and time as the visible/experential counterparts, and expected the third continuum to be another kind of continuum like space and time, unlike matter. Matter is what occupies space, and that may appear to fit the "visible manifestation" role, but it is not the same sort of thing. (Though mass is often the third measurement next to distance and duration in equations). You can imagine a universe without matter. Measurement of distance and events then becomes irrelevant, but it is still hypothetically possible. But a universe with no space or time is a whole different kind of existence.
So if we look at the universe as the entire set of possibilities, then this new "chance" continuum is what it is "referenced" to, and space and time are manifestations of it, with space as what it is seen in and time what it is experienced in.
Just as you can get "close" to a point in space (with gradual changes as you pass one material object after another. Think of the transition from country to city as you get closer and the density of people and buildings gradually increases), and closer to a point in time (as one event leads to another, and a new "present" takes shape. Think of any transformation in time. A flower or other living thing growing, etc), you can also hypothetically/theoretically move closer to a point in chance by changing things to alternate states.
Like if I chart my position using the four dimensions of spacetime. At such and such time, I and at a particular longitude, latitude and altitude. In an alternate reality, I may have moved to a point five feet away in latitude. Or maybe ten feet away. That would indicate a further "distance" in chance, since five feet away is "closer" to the starting reality than ten feet away. Of course this will affect the choices I would have had to have made in space and time, in order to get to that point. Just like space and time determine where you can go in each other. So this is interchangeable, just like space and time, and can also be measured. While space has three dimensions, and time has only one dimension, the number of dimensions in chance seems to be unlimited.
So what does everyone think of this. Have any of the theorists thought of it? If so, I've never read about it.