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In order to calculate odds in a multiverse, a certain conceit has to be thrown down which 'ends time' for certain universes. This means that, conceptually at least, universes are destroyed. And so a group of intrepid theorists set out to save universes by saving time, and may have discovered a way to do it.

A friend makes a bet with you. If you roll a die and get an even number, you'll take a nap and he will wake you up in a minute. If you roll and get an odd number, you'll take a nap and he'll wake you up in an hour. You can bet twenty bucks on either one of these outcomes when you wake, but you don't see the number on the die. Which would you choose? It doesn't matter, right? You've got fifty-fifty odds either way.

Turns out, though, that you don't. When you're rolling you have a fifty-fifty shot of either kind of number. When you wake, though, you have a better shot at having slept only a minute. Because if you slept an hour, the world might have ended.

This is the kind of logic that comes up in the multiverse, when people need to calculate odds. If the universe is popping out new little universes whenever some event happens that could split the universe, then the odds of anything happening are as likely as anything else happening. It just depends on what universe you're in. To make life a little easier on themselves, theorists calculating odds give a cut-off time for any given event. That cut-off time, which is essentially the end of the universe, might be anywhere, but it's more likely to happen during the span of an hour than the spand of a minute, skewing the odds in your favor if you choose to fork over the twenty after an hour and collect it after a minute. One universe is more likely to have ended while you slept.

Some theorists, presumably those who wanted to sleep soundly, found a new way of keeping those odds without ending time. If the universe keeps popping out new universes exponentially, then new universes will outnumber old ones. Think of a tree in which each branch, after a year, splits off into three new branches. The first year will have one new branch, the second three new and one old, the third nine new and four old, and so on. Once you've woken up, you know two things; that the experiment took place and that you've woken up afterwards. Since there will be more 'new' universes in which only a minute has passed than there will be 'old' universes in which an entire hour has passed, you are more likely to be in the new one.

Via New Scientist and Arxiv.

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