In this week's "Ask a Physicist" we figure out whether there's a universe out there with another version of you, and whether you could kill her and take her place.


For this week's "Ask a Physicist" I'll answer a question put to me by our own Annalee Newitz who wants to know:

Do we live in a multiverse or not? And if yes, what is the likelihood that other universes will have versions of us in them, the way they do on Fringe or in Iain Banks' novel Transition?


I'll go out on a limb from the outset and say with as much definiteness as we're able: "probably," but I'm also going to add some lawyer-talk. Even if your double exists, you can't ever meet alternate Annalee, or kill her and take her place. Not even with a heaping dose of septus. Those of you who are only reading this in the hopes of making out with your double can stop reading now, though I should point out that it's worth staying tuned for the carnage that's likely to ensue in the comments section alone. You should also feel free to follow up with your own question, or check out my book.

But before doing any of that, let's start with a bit of nerdly taxonomy. There are lots of different kinds of multiverses. Fortunately for us, Max Tegmark at M.I.T. came up with a nice hierarchy of them. Without any further ado:

Level 1: The universe is big, very big, but in a normal sort of way.

It's so big, in fact, that we can't see to the ends of it, even with light traveling toward us since the beginning of time. We can only see to a distance of about 50 billion light years, a distance known as the horizon. Some smarty-pants on the internet is no doubt going to note that since the universe is only about 14 billion years old, we shouldn't be able to see further than that many light years. True, except that the universe has been expanding this whole time, which changes the calculation somewhat.


Regardless of the exact distance, the universe has a horizon, and nothing outside the horizon could possibility have affected what happens here on earth. You might think of everything within the horizon as being our "universe."

But what's beyond the horizon?

The first instant or so of the multiverse was pretty active, and the first 10-35 seconds, especially so. The current theory (for reasons I'll get into another time if someone has the burning desire to ask) is that at for a brief instant, the universe underwent a tremendous exponential expansion known as inflation making small patches of the universe about 10100 times larger than they started out.


Are there an infinite number of these patches? Or is it more like a Pacman universe, where if you went far enough in one direction, in principle you'd get back to where you started? We honestly don't know, but the point is that even if we do live in a Pacman universe, the size of the screen is humungous.

So we do live in a multiverse. There are other regions of space with the exact same laws of physics as ours, but which lie outside of our horizon just as we lie outside of theirs. For all intents and purposes, these are different universes than our own.


How big does a Level 1 Multiverse have to be before we start getting exact duplicates of everybody? Pretty damn big. Tegmark estimates it at around 1010^29 meters from here. This is the biggest number that's going to come up in this discussion outside of infinity itself. What this means is that every atom in the duplicate universe is in precisely the same spot and moving with the same speed (up to the limits of quantum uncertainty) as in our own universe. That means that even if alternate Annalee didn't have exactly the same history as our own, her brain is configured so that she thinks she did.

Of course, if the universe were infinite (and I'm not convinced that it is) then it would be plenty big enough to accommodate not only a duplicate of you, but an infinite number of them.


It's humbling, and also a bit creepy. It's like you have an infinite number of stalkers.

But if our Level 1 Multiverse is anything less than infinite, it's not clear that it's as large as it needs to be to have duplicates, although there might be "close enough" worlds. A conservative theoretical estimate puts the minimum size of our Multiverse at around 1075 meters, which seems huge until you realize that it's only a tiny, tiny fraction of the space required for duplicates. The observational limits, primarily from the WMAP satellite are even smaller than that.


Level 2: Other universes popped out of inflation.

Our bit of the universe grew out of one tiny patch of the very early multiverse, but it's possible that others did as well. What's more, some, perhaps all, of those patches may have physics just a wee bit different from our own. Electricity might be a bit stronger or weaker; the strong force (the one that holds protons and neutrons together) could be a bit different; there could be more than 3 dimensions.


Let me make a couple of things clear:

1. It's not obvious that this model is correct. It may be that the fundamental forces really are hardwired in nature, and that all universes have the same underlying physics. We don't really know, but I kind of hope that this is the case, in which case there's no Level 2 Multiverse.

2. If there really is a Level 2 Multiverse, with universes each with different physics, these universes aren't going to look much like our own. No io9, no Annalee, no humans, and possibly no life or complex structure at all. It turns out that physics needs to be very finely tuned in order to make things like stars or heavy elements, and most universes simply don't cut mustard. This is the origin of the "anthropic principle" (the weak version for all of you pedants out there), which says that there may be lots of universes with different physics, and we're very lucky to be in one which supports life — but of course we couldn't be anywhere else and still have the conversation.

The weak anthropic principle pisses off most physicists.

I will say that while most universes in the Level 2 Multiverse are completely uninhabitable, if you wanted to get extra-fancy in your scifi writing, this is the only option where you get to change the fundamental physics.


Level 3: Many Worlds.

Admit it; this is the one you've been waiting for. If you've read Transition or, more shamefully, if you were a fan of Sliders, or for that matter, if you've read any science or science fiction in the last 50 years, you almost certainly know what I'm talking about. In 1957, Hugh Everett, fed up with the uncertainty in the standard model of quantum mechanics devised the "Many Worlds Interpretation," which is the idea that at each instant, as different quantum mechanical "choices" are made on the subatomic scale, universes split off from one another. In one universe, you've got a dead Schrodinger's Cat and in another, you have you a live one.


People love this idea, because it allows us to think about all the possible counterfactuals in history. If I were a grumpier sort, I'd point out that Many Worlds is simply an interpretation of quantum mechanics and it doesn't make any concrete predictions at all beyond what ordinary quantum mechanics says. I'll let this go for now and simply point out that even if your double does exist there's no way to visit her without invoking magic.


Besides, the Many Worlds are unlikely to be as similar to our own as we'd like for scifi purposes. As the tagline from "Butterfly Effect" wisely points out, "Change one thing. Change everything."

Incidentally, Tegmark has a 4th level of multiverse in his hierarchy, but the other universes would be even stranger than in Level 2.


In the end, there are lots of possible ways to get duplicate yous, but it would be far more expedient to pursue the cloning option.

Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (Like us on facebook.) He is an Associate Professor of Physics at Drexel University. Feel

free to send him your questions about the universe.