What if our universe was just one in an infinite number of parallel universes; a possible outcome from the specific predictions of quantum mechanics. The idea of multiple universes is common in science fiction, but is there any actual science to back this theory up?
Transcript: MultiversesDownload the transcript
Fraser: Astronomy Cast Episode 166 for Monday November 30, 2009, Multiverses. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville. Hello Pamela…
Pamela: Hey Fraser…
Fraser: In this universe…
Pamela: How’s it going?
Fraser: Good! So, we’re in our single universe, but today, perhaps, we’ll find other universes. Now before we did, I want to embarrass you… no, it’s cool. I encourage anyone with a computer… go check out Wikipedia for “Uranus.” And in there, under the nomenclature, is how to pronounce the name of that planet. Someone has kindly included a quote from Pamela which is quite funny. So, yeah, if you get a chance, check out Wikipedia’s article on Uranus and go down to the nomenclature part… or search for Pamela’s name… and you’ll see this really funny bit where they kept… uh, yeah, anyway, it’s hilarious, so take a look at it. Ok, so… which is very bizarre because it’s sorta like the universe folding in on itself… the fact that you’re there on Wikipedia, and we use Wikipedia… I’m about to go crazy. What if our universe was just one in an infinite number of parallel universes… a possible outcome from the predictions of quantum mechanics. The idea of multiple universes is common in science fiction, but is there any actual science to back this theory up? Alright Pamela, so when I’m thinking of multiple universes, I’m thinking Star Trek… you know, the “mirror episode” where everyone’s got a beard. That is a whole other universe… they’re all evil, and they’ve all got a little goatee…
Pamela: Spock with a goatee…
Fraser: Yeah, that’s how you can tell that they’re different, that they come from a whole other universe where it makes sense for Vulcans to sport facial hair… So what, would you say, is sort of a traditional description of a multiple universe?
Pamela: Well, sadly, just as a multiverse theory predicts multiple universes, there are multiple multiverse theories… so we have multiple…
Fraser: Of course, it predicts multiple multiverses…
Pamela: Yeah, it’s nesting dolls.
Fraser: Right, but when people talk about parallel universes, multiverses, what are they talking about?
Pamela: The two basic schools of thought–and they’re not mutually exclusive–are that on one hand, our universe is just one of many universes out there… each one with their own initial conditions, each one perhaps with their own rules of physics… In some case, maybe two electrons actually attract each other, perhaps somewhere gravity is a repulsive force… perhaps somewhere Spock wears a beard… But then the other multiverse base idea is every time a decision is made, there’s a splitting of the universe and there’s some other universe where that other decision is made.
Fraser: Right. So, it’s like every time there’s a probability… you roll the dice… or, you flip a coin and in our universe the coin comes up heads… and that creates a splitting off of the universe where a coin comes up tails.
Fraser: Ok, well let’s go over the first one first then… So, we could imagine… I guess it’s the equivalent of multiple houses, right? In our house we have two adults, two children– boy and a girl ages 6 and 8. You go over to someone else’s house and maybe there’s a couple in their 60s with the kids all moved out of the house. Each universe is its own separate house, and obviously the rules are very different… the rules of physics would be very different, right?
Pamela: Right. And so of course there’s always the how do you get to those different rules of physics problem. And this is where theorists get involved.
Fraser: And I guess part of the problem is that since we don’t know where our universe came from, it’s very hard to then predict if you could get other ones, right?
Pamela: And even more to the point is that many scientists are very uncomfortable with the idea that we live in a special place, that we live in a special time, how is it that we live in a special universe that is capable of generating life? And so one of the things that we have is what’s called the anthropic principle which says we live in a universe that seems to be finely tuned to life. And there’s three basic escape routes from fine tuning… one is that it’s actually fine tuned… there is a God… we can’t prove this… move on, science loses. This is an uncomfortable one. None of us like anything we can’t test, so we move on. The second part of the anthropic principle is that there is some sort of underlying physics, some set of equations that if we only know that set of equations then the entire universe makes sense. We haven’t found it yet. And we hope that when we do, that set of physics that’s so perfectly fits together that there could be no alternative also says that we have to live in a universe that just happens to have life. Still uncomfortable! The third solution that gets us out of the “how did we end up in a finely-tuned universe” is to say that we’re just one of a whole lot of universes, perhaps even an infinite number of universes. And if you roll the dice enough times, every possibility comes into existence somewhere. And while the majority of the universes out there may lead to death or at least lack of life, the fact that we live in one that does allow life to occur is just a fluke of being the tail end of the number distribution.
Fraser: Right. I think of this as, well, imagine this nothingness… and you wait a very, very, very, very, very long time, and perhaps particles can pop into being every now and then. A particle pops into being and disappears… perhaps something bigger a trillion years later pops into being, or an infinite number of years later and eventually, if you’ve got enough time, everything will… with an infinite amount of time, anything’s possible. And so you’ll eventually get a universe. And then maybe as you said, there was an infinite number of universes that came before us, and an infinite number of universes that will come after us. And it was just a matter of time before the one that we ended up with happened. So those other ones might still be out there expanding or contracting… or gravity’s repulsive and whichever laws get written into the program or into the blueprint of the universe are what everything has to deal with from that point on.
Pamela: And one of the ways that this gets described is as a cosmic foam with all the universes bubbling up and sometimes quickly ending, sometimes living a few trillion years, sometimes maybe living much, much longer. But there’s this cosmic foam of bubbling boiling universes popping in and out of existence.
Fraser: But when you think of it as a cosmic foam, you kind of think of it being spatially connected, right? You know, you’ve got this foam, and this bubble’s on top of that bubble, and that bubble’s over there… But I think that that is maybe not very helpful because with that whole concept of multiple universes, as long as you can open up an interdimensional portal to get from one to the other, they are beyond all reach… in theory.
Pamela: Right, and this is where it gets ugly… we have all these universes and we have this wonderful theory, or at least this theory that gets rid of the necessity of us living someplace special, and all these universes are, as far as we can tell, completely out of reach of one another. Now, there are some ideas that maybe they can gravitationally touch. There’s the rather disturbing idea that if they touch a little bit too violently, everything ceases to exist, and we hope that one doesn’t happen.
Fraser: Right, so I guess that’s where there’s two schools of thought. One is that each universe is its own separate entity… completely and totally cut off from everyone else… it is a single moment of creation and expansion or contraction or whatever… whatever it does… ice cream doesn’t make you fat… you know, whatever you need to be in that universe that happens… But, it is completely and totally cut off from the other one, and that one is completely and totally cut off from the other one, and so scientifically, there is no way to tell if these other universes exist because they are in all ways detached from us.
Fraser: And so this is what you’re saying is that there are those people who think there is some playground where all these universes sit within, and they do interact.
Pamela: Right. And the question is how do they interact? Where we keep landing is that they interact by occasionally eating each other. But potentially they interact in a much friendlier way which is they interact by touching and merging and you have one universe essentially rippling across the other, replacing the other one’s physics, which if you’re in the winning universe is a good thing, and if you’re in the losing universe it’s the Neverending Story’s big Nothing coming to consume you. It’s interesting to think about. We’re not sure how to test it, though.
Fraser: Right, and so where does that come from? That comes from string theory, right?
Pamela: It comes in part from string theory and in part from other forms of theoretical physics. In string theory, there’s what we call M-Theory. This is where we consider universes as arising from two different branes colliding with one another.
Fraser: Not brain like human brain, but brane like membrane.
Pamela: Right. And so what they’re saying, and I have to admit that a lot of this is just words to me, the math is extremely complicated and they don’t have good ways to visualize it yet, because it’s kinda hard to visualize something that is eleven dimensions if you’re in one way, or twenty-six dimensions if you’re in another.
Fraser: Right. And forget about testing it.
Pamela: Yeah. And so the words are that when two p-branes… which I just love the idea of naming something a p-brane… when two p-branes collide, it creates a d-brane and a universe is on the surface of this d-brane.
Fraser: So can you… so then what would a p-brane be? A proto-universe? Like not a universe yet? You know, like if I put a wheel and handles and shifters and a seat… and when I put them together I get a bike…
Pamela: Right, so this is where I have to admit that branes are one of those things that is a mathematical concept of basically the idea that particles exist in more dimensions than those that we experience. We experience three spatial dimensions and time. When you start expanding the way you view particles into these extra dimensions, you can end up with membranes across these multiple dimensions. Here, a membrane is a 2-dimensional surface that may have a non-flat geometry to it in our reality. Well, a p-brane is one of these multidimensional membranes that expands through the same space that strings occupy.
Fraser: And I guess this is… one of the proofs of this… one of the ways to test this, maybe, is… isn’t this one of the reasons why gravity is so strangely weak?
Fraser: The gravity is actually extending out of our universe…
Pamela: Across all these different… right… and so that gets kind of, well, ugly.
Fraser: Well, sure… so, ok.. so I can kind of imagine then that you’ve got these membranes floating around in some cosmic sandbox, and it’s the interactions of them which are creating universes with different rules. So you might have one set of rules… it’s almost like DNA, right? You have one set of rules coming from one membrane and a different set of rules coming from another membrane, and it’s the mixture creates a universe and its rules.
Pamela: Yes, exactly.
Fraser: And so would that then define that there is an underlying set of rules that you can play with… you don’t have an infinite number of things that are possible, you have a finite based on… just like one of my children isn’t going to have wings because I can’t bring wings to the table in my DNA, and neither can my wife.
Pamela: Well, one of the crazier notions out there that is as legitimate as any other idea out there, is that anything you can describe mathematically… anything you can describe mathematically… any theory of anything can happen in a multiverse. This is a theory that comes originally from Tegmark. It basically throws out the notion of… if you can mathematically describe your child having wings, which I think you could do… it would be a bit strange looking, but might make for a good genetically-induced Halloween costume…. that can happen. So, this is the ultimate ensemble hypothesis. Tegmark basically points out that there’s really no reason to limit anything, and that it’s conceivable that parallel universe theories can basically sum up any of the other multiverse ideas… can go in new directions… and we’re only limited by the fact that in a given geometry, two plus two should equal four.
Fraser: So if the math doesn’t work, then you wouldn’t be able to get that to be physically possible. Like when I was in computer science there was this… some of you are going to know what I’m talking about… there was this mathematical problem where there’re these bridges in these parts of the city and you have to be able to walk across the bridges to different parts of the city. It’s not possible, it’s not solvable… it can’t be done. There’s a certain way that you can’t go back on you’re own route. So a city where that was possible… where you could follow one path and go over the bridges… it can’t exist, because that is mathematically not possible.
Pamela: It’s sort of like trying to fly from Washington D.C. to Venice, I’ve discovered recently.
Fraser: Right, it’s not possible, yeah… And so, if two plus two equals four, so we cannot have a universe where two plus two equals five. But we can have a universe as long as those underlying mathematical requirements are kept, then everything is fine…
Pamela: And we even have theories that help us understand–as much as you can understand anything that’s not testable scientifically–where these multiple universes might have arisen. This is where we start looking at some of the work by Andrei Linde, looking at inflationary theory. Where we can imagine that while inflation stopped in our universe at a specific moment that led to the universe where we live and breathe and experience and observe, we’re only one pocket. And there are other pockets of universe out there that either stopped inflating sooner or kept inflating and maybe spawned one off of another. This is where we start getting into a chaotic inflation theory, cosmic inflation theories. Different ideas of space expanding into different places with different physical parameters inside of them. You can envision it almost as the splitting of eggs that leads to twins in embryonic expansion.
Fraser: Right. And the only way to figure that out, I guess, is to be able to look right out to the very edge of the visible universe, and maybe see the edge of some place where the laws of the physics are going all screwy.
Pamela: Well, the neat part about this idea is just like the splitting of eggs leads to twins leads to two–hopefully–completely separate bodies, you can actually end up with these pockets of universe breaking away and forming different bubbles of different sets of physical rules that perhaps have symmetry-breaking occurring in different ways where perhaps the ratio of matter to anti-matter is radically different, where all these other things that we rely on occur in slightly different ways and the space just broke off and continued in its own reality.
Fraser: So its possible that our own universe began when we broke off…
Pamela: Dropped like an acorn…
Fraser: Dropped like an acorn from that inflationary foam somewhere… ok, so what does quantum mechanics tell us about multiple universes?
Pamela: This is a completely different way of getting at it. With quantum mechanics, it’s at every moment that a decision is made, the other decision is made, too. If any of you have read Kelly McCullough’s WebMage series, he actually has this idea tied up in his books and I highly recommend the Webmage series… they’re a quick, easy holiday read. Imagine that in one universe you got out of bed, and coffee was already made by your loving partner. In another universe, you get up and the cat has left you a dead mouse beside the bed. Both these possibilities, if you own a cat and have a significant other, are a reality every day in some universe. Anything that could happen, does happen somewhere. This means that if in this universe I spilled my coffee on myself on my way to work, there’s another universe where I spilled my coffee but missed my clothing, in another universe where I didn’t spill my coffee at all, and another universe where I left my coffee on the counter… anything that could happen does happen.
Fraser: Right, and I guess if we could zero right in all the way to the simplest situation, you have a particle… a radioactive particle… that could decay, and if it does decay… in one universe it does decay and in another universe it doesn’t decay… and then you’ve got two universes. In one it did, and in one it didn’t. You can build all of these different probabilities… coins tossing, dice rolling, getting a raise, spilling your coffee… all that kinda stuff… all of these things that seem like it could go either one way or another. It’s kinda strange, once again, as a whole other physical universe being created, what’s the mechanism that makes that happen?
Pamela: And where does the energy come from? Where does the mass come from? It’s this idea of everything just sort of splitting…. well, right… where did it go?
Fraser: So why then did anyone even come up with this idea? Doesn’t it play into some of the questions that quantum mechanics comes up with?
Pamela: It comes from the fact that we observe particles… electrons… if you fire a single electron at a pair of slits and watch where it lands on the other side, it doesn’t behave the way a BB would. It’s somehow able to interact with itself, and if you do this enough times, you get the exact same result as if you fire a thousand electrons all at once at these two slits. Part of the way this is explained is each electron does every single possible thing every single time it goes through, it’s just the universe splitting and we’re only able to see one of the ways the electron goes through the pair of slits. And that’s just a little creepy.
Fraser: Ok, so then what evidence is there that any of this is true?
Pamela: Yeah, we’re kinda missing the evidence thing…
Fraser: Right… so there is no evidence whatsoever… no hard evidence right now that either the quantum foam… the multiple universes… brane theory… quantum theory creating multiple universes… none of that… there’s no evidence that’s been found…
Pamela: There’s no evidence… there’s no test… at least there’s no believable evidence… there’s a few people that claim that a cold spot in the cosmic microwave background is evidence, but it’s not a mainstream idea.
Fraser: Right. But there are some experiments, some tests… like with gravity, right? Some people have talked about the fact that you might be able to detect how the branes are transferring gravity…
Pamela: But right now, there’s no way to run those tests and differentiate the results from well… gravity’s just not that strong… deal with it.
Pamela: So, it’s going to take more tests than we can currently perform, and that’s where it gets problematic.
Fraser: Yeah, yeah… absolutely.
Pamela: Problematic is probably the understatement of the year…
Fraser: Right, so then what do you think? I almost never ask you this… I think we have opinions on aliens, we have opinions on humans vs. robotic space flight… what does your gut tell you about all this? Which is of course worthless.. don’t trust your gut… I’m going to completely ignore anything you say and mock you about it later… But for now, what do you think?
Pamela: I don’t think the constant splitting of the universe with every decision is happening.
Fraser: Right… so you sort of feel that quantum mechanics, as bizarre as it is, is perfectly fine within the physics of our existing universe… there’s no need to manufacture other universes to explain quantum weirdness…
Pamela: It’s just probability functions…
Pamela: Now, when it comes down to “Are we the only universe out there?”… that one… my gut is torn in half… which isn’t a good thing to do to a gut…
Pamela: No… smells kinda bad….
Fraser: Right… so where are the two parts of you coming down on?
Pamela: I’m really torn between the chaotic inflation theory… the idea that inflation caused a variety of bubble universes with different physical parameters to exist out there. And the idea that there’s some underlying physics so perfect that it doesn’t allow the universe to exist in any other way…. I don’t see an intermediate solution of… well we have physics that works… it’s ugly, but it works. Yeah, something else could’ve happened, but this is the way it happened and this is the only universe. That, I don’t see happening. But, the idea that it’s either some form of chaotic inflation with multiple universes out there, or some form of as yet unattainable meshing of equations that doesn’t allow for other possibilities… those are the two things that have me torn in half. Depending on the mood that I’m in, you’ll get more favor of one over the other.
Fraser: I think that for me, and this is an argument that I read a little while ago, that the universe is actually not very suitable for life… there are tiny little portions of the universe that are possibly suitable for life… and we’ve found one… and that the vast amount of the universe is completely unsuitable for life and actually quite hostile to life. So it more makes me feel like that this is a universe that was just barely good enough for life that life was able to take hold… that we happened to have gravity… it was strong… or weak, but not too weak and not too strong… and I think about, as I sit here chilly in my downstairs office, that I could live in Hawaii right now… that would be nice… you know, if everywhere on Earth was more like Hawaii… that would be a universe that was more conducive to life… as opposed to me living in Canada which is right on the edge of being conducive to life, and I think that’s the theme across the whole universe… that we do… those of you in Hawaii, whom I’m very jealous of… are like… yeah, this is the perfect universe…. we couldn’t ask for anything more… I’m going to eat another breadfruit… but for us here in Canada it’s right on the ragged edge of it being comfortable for life. I think that for me I kinda like the idea of universes popping into existence, and each one having some random mixture of laws and of physics and eventually you get one that sticks, and that’s the one we happen to end up in. I like that. And as you often say, this is as much a philosophy question as it is a science question… probably more.
Pamela: Yes. Now there are scientists that are working on this… Larry Susskind, Andrei Linde, there’s some really great work out there.
Fraser: Brian Greene, Michio Kaku… there’s a lot of… and popularization of it… I wouldn’t be surprised if in a couple of years someone says hey, I figured out a test for detecting multiple universes… or string theory, or whatever… we just need better instruments… Alright, well in this universe, Pamela, I thank you very much for covering it and we’ll talk to you next time.
Pamela: Sounds good, Fraser, I’ll talk to you later.
This transcript is not an exact match to the audio file. It has been edited for clarity.