Possibilities

We can't undo anything we've done. We can make amends and express regrets, but done is done.

If you could go back in time, would you be able to undo? Would you want to? You don't know how you'd affect the future, as a myriad of trite movies and TV shows illustrate. Does the question matter? If we are ever able to travel back in time, the travelers must be with us now, in their past. Maybe the travelers are so clever we can't detect them. Or maybe they can only travel to a point in their own lives without knowledge of the future, which is equivalent to doing nothing. I'll assume that such travel is nonexistent or its equivalent, at least for our universe.

When the universe came into being (by whatever means you choose to believe), so did time, for time has no meaning without existence. We wind our threads forward through time. We can remember the past, revel in it, regret it, wax nostalgic, and reinvent it in our minds though not in reality. The past is set, but the future appears to be nebulous, becoming the past as we move in the present.

Because we can't travel to the past we assume that it is gone... nonexistent. We can't travel to a distant star system, but it exists. We live our individual lives in a brief, finite thread of time. We only experience the present, a speck on the thread, but the thread exists in the context of the immense, though finite, fabric of space-time. Our threads would be insignificant to anyone who could view the entire fabric, but they're significant to us.

Why should we only have one thread? Because the moment we diverge from one thread to create a new one, we are different people living in a different universe from that of the other thread. Our abilities, aspirations, environment, memories and a host of other factors define us. We're part of the universe and what we do defines it at a microscopic level.

So one thread per customer per universe. How many universes? Our universe has always existed in the sense that "always" only has meaning where there is time. But the discontinuity at the time origin for the fabric gives us an "absolute zero" for time, an epoch. We can imagine a time before that, though it remains imaginary.

Existence of one universe implies that others must exist - probability and improbability of something ever occurring mean nothing outside of time (which defines "ever"). We can think of them as parallel universes, but since each has its own time axis, it's just a convenience. They could as well be serial universes. Since any universe cannot, by definition, detect or affect another, we are free to choose whatever constructs our feeble brains need to comprehend a small bit of the immensity.

If we could step outside of space-time and see our huge but finite universe in all its glory, we could see the minute threads of our lives. But this doesn't mean our futures are cast. For there are other universes, similar but not duplicates (and there may indeed be duplicates but they converge to the same for practical purposes). The futures are different in them. Indeed, so is the past in many of them. "We" exist in only one universe, but we also have a huge number of analogs that, if we could see them, we would identify as "us". There is no reason to require that all space-planes along the time-axis of multiple universes be unique. "Now" has many intersections.

The cosmic view may be of no comfort, or it may be of great comfort. What we believe doesn't really matter. Truth matters. And faith is not truth.