Perhaps there is a world, in a distant galaxy, where I am being paid $6,000 a word to write this introduction. In that world, I’d almost certainly feel compelled to make it as long as possible—to ruminate at length on the philosophical implications of the multiverse, perhaps even write detailed breakdowns of the cultures/landscapes of five or six of them, tracking my divergent selves across multiple cosmic plains. That world, tragically, is not this world. But is it, or something like it, out there somewhere? Is the multiverse made up, or does it have some scientific merit? For this week’s Giz Asks, we reached out to a number of experts to find out.
Postdoctoral Researcher, Astronomy and Cosmology, Western Sydney University, and co-author of The Cosmic Revolutionary’s Handbook (Or: How to Beat the Big Bang) and A Fortunate Universe: Life in a Finely Tuned Cosmos
Let’s start with a dose of reality: We have no unambiguous physical evidence of parallel universes. There is no observation or experiment that requires parallel universes. Sorry to be a buzzkill.
So, why are some scientists talking about parallel universes? Because they might exist, and we won’t know what to look for if we don’t explore the idea. And there are some interesting clues…
Let’s start with the multiverse. The universe that we can see is roughly the same in all directions, so the simplest assumption is that it’s the same everywhere, even the parts we can’t see. But we have some clues from our universe that a process called “cosmic inflation” caused the universe to expand very rapidly, shortly after the Big Bang. Many of our ideas about how cosmic inflation happened predict that our universe isn’t the same everywhere. Other parts of the universe might, for example, have fundamental particles that are heavier than ours, or a stronger force of gravity.
If the multiverse exists, it might explain a puzzle: Why does a universe that is able to support life exist at all? This is a puzzle because, as physicists consider other ways that the universe could have been, many are disastrous for life: too simple, too hot, too chaotic, too short lived. A life-permitting universe (our universe) exists, against the odds, because there are lots of different universes out there. We won the cosmic lottery.
Physicists also talk about the Many Worlds Hypothesis. If the multiverse is about other universes, then Many Worlds is really about parallel universes. Here’s the idea. We have a theory about how matter behaves called quantum mechanics. Its equations explain the evidence fantastically well, but physicists don’t agree on what the equations are telling us about what the world is really like.
One interpretation, in particular, is called the Many Worlds Interpretation. In quantum mechanics, we often write down expressions like “Cat Alive + Cat Dead.” What does the plus mean? In some interpretations, it means “the cat is neither alive nor dead, but if you measure the cat, there’s a 50% chance that you will observe (and in a sense, create) each option.” For the Many Worlds Interpretation, it means “there are two parallel worlds, one in which the cat is alive, and one in which it is dead. When you measure the cat, you find out which world you are in.” (Other interpretations are available.)
For Many Worlds supporters, the advantage of this approach is that it avoids the awkward situation of having “measurement” play a crucial role in your theory, because then you have to decide what really counts as a measurement: an apparatus? A flea? A dog? An undergrad? This doesn’t just look like a hard question: it looks like the wrong sort of question entirely.
Another way to make parallel universes is in other dimensions. Perhaps the three dimensions of space and one dimension of time that we are familiar with aren’t all there is. This idea has some interesting consequences. If you take Einstein’s theory of gravity, and add an extra dimension, you get some extra equations. Those equations, it turns out, are the equations we use to describe electricity and magnetism! Pull that thread, and 100 years later physicists are talking about a universe made of tiny strings that wrap around seven or eight or 23 extra dimensions. Again, we have no unambiguous evidence yet, but we won’t even know what to look for if we don’t explore these ideas.
Astrophysicist at UC San Diego’s Center for Astrophysics & Space Sciences
One version of parallel universes that cosmologists consider is the so-called ‘Multiverse,’ which is a consequence of the dominant theory of cosmic genesis known as inflation. Inflation explains why the Universe has the properties it’s observed to have—the absence of large-scale spatial curvature (also called ‘the flatness problem’), the pattern of fluctuations in the Cosmic Microwave Background, and the origin of vast structures such as galaxy clusters. The major lacuna of inflation theory is a way for inflation’s energy source—the inflaton—to begin inflating and produce the properties of a universe which has observers such as us. The incredible degree of fine-tuning required for life suggests, to some, the Multiverse paradigm wherein all values of the inflaton’s energy are realized…somewhere, in a potentially infinite number of disconnected, parallel Universes.
Physicist Max Tegmark estimates that a volume of spacetime with properties identical to our observable universe—our ‘nearest neighbor’ universe—could be about 10^10115 meters away from us, which is a number so incomprehensibly large it cannot be written in decimal form in a human lifetime, let alone detected with any conceivable technology cosmologists have access to. For now, the question of the reality is purely academic. But if inflationary B-mode polarization of the Cosmic Microwave Background is convincingly detected, it will give credence to the inflationary paradigm, which will indirectly bolster the case for the Multiverse’s version of parallel universes.
Professor, Applied Mathematics, University of Cape Town, South Africa
There are various proposals for parallel universes. There is not the slightest evidence that any one of them is in fact true. Perhaps the most popular one is the many worlds theory of quantum physics, often extravagantly extended to something called a many minds theory. Many people, myself included, think it’s not even a coherent proposal. It does not solve the basic quantum mechanics issue it was meant to solve, namely the measurement problem, because it does not account for how individual measurements obey the statistics of the quantum outcomes. One of the reasons I don’t believe it is that I don’t believe there is a meaningful wave function for individual brains, let alone for the whole universe.
Professor, Physics, Harvard University, whose research connects theoretical insights to puzzles in our current understanding of the properties and interactions of matter
My answer to this question depends very strongly on what we mean by parallel universes. If we simply mean other universes we cannot access, I find that very likely, as there is no reason this should not be the case. Our universe has a finite lifetime and the speed of light is finite, so there might well be inaccessible regions of the universe.
If we mean additional dimensions with other “brane” universes, possibly even within causal contact, I’d say there is a nonzero (but perhaps small) probability. Again, no reason why not, though there would be constraints.
If we mean alternative branches of the quantum mechanical wavefunction in some abstract sense, probably yes. If there are out there all branches, I’m skeptical.
Finally, is there a carbon copy of our universe? I doubt that very much. There is an infinity of infinities when we consider all the connections (or a large number of large numbers), and I find the statistical arguments suspect.
So, yes, stuff out there. But no, nothing to do with us!
Professor of Religion and of Science in Society at Wesleyan University, and the author of Worlds Without End: The Many Lives of the Multiverse, among other books
I’m going to give the Annoying Professor answer and say it depends on what “real” means. Multiverse theorists claim that multiple universes are the only explanation for puzzling phenomena within our own—like the pressure of empty space, the size of the electron, the distribution of matter right after the Big Bang, or even the Big Bang itself. So “parallel” universes are “real” in the sense that they solidify the standard model of cosmology. They’re mathematically real. They’re conceptually real. But are they “out there”? At this point, the question breaks down, because by definition, anything outside our universe lies beyond observation and experimentation—not to mention travel. By definition, we lack the tools to access anything beyond everything-to-which-we-have-access. So in that sense, practically speaking, parallel universes are not real. We can’t see them; we can’t touch them; we can’t get to them. We can’t even know that they’re there. We can’t even know that “there” is there.
But then again, the very fact that our own universe only makes sense as part of a collection of an infinite number of others means that “reality” might be a lot bigger, and a lot less tangible, than we think it is.
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