“Is the multiverse science? How can we test it?”I added “Is the multiverse real” after Google offered it as autocomplete:
This is a timely question, one that has been much on my mind in the last years. Some influential theoretical physicists – like Brian Greene, Lenny Susskind, Sean Carroll, and Max Tegmark – argue that the appearance of multiverses in various contemporary theories signals that we have entered a new era of science. This idea however has been met with fierce opposition by others – like George Ellis, Joe Silk, Paul Steinhardt, and Paul Davies – who criticize the lack of testability.
If the multiverse idea is right, and we live in one of many – maybe infinitely many – different universes, then some of our fundamental questions about nature might never be answered with certainty. We might merely be able to make statements about how likely we are to inhabit a universe with some particular laws of nature. Or maybe we cannot even calculate this probability, but just have to accept that some things are as they are, with no possibility to find deeper answers.What bugs the multiverse opponents most about this explanation – or rather lack of explanation – is that succumbing to the multiverse paradigm feels like admitting defeat in our quest for understanding nature. They seem to be afraid that merely considering the multiverse an option discourages further inquiries, inquiries that might lead to better answers.
I think the multiverse isn’t remotely as radical an idea as it has been portrayed, and that some aspects of it might turn out to be useful. But before I go on, let me first clarify what we are talking about.
What is the multiverse?
The multiverse is a collection of universes, one of which is ours. The other universes might be very different from the one we find ourselves in. There are various types of multiverses that theoretical physicists believe are logical consequences of their theories. The best known ones are:
- The string theory landscape
String theory doesn’t uniquely predict which particles, fields, and parameters a universe contains. If one believes that string theory is the final theory, and there is nothing more to say than that, then we have no way to explain why we observe one particular universe. To make the final theory claim consistent with the lack of predictability, one therefore has to accept that any possible universe has the same right to existence as ours. Consequently, we live in a multiverse.
- Eternal inflation
In some currently very popular models for the early universe our universe is just a small patch of a larger space. As result of a quantum fluctuation the initially rapid expansion – known as “inflation” – slows down in the region around us and galaxies can be formed. But outside our universe inflation continues, and randomly occurring quantum fluctuations go on to spawn off other universes – eternally. If one believes that this theory is correct and that we understand how the quantum vacuum couples to gravity, then, so the argument, the other universes are equally real as ours.
- Many worlds interpretation
In the Copenhagen interpretation of quantum mechanics the act of measurement is ad hoc. It is simply postulated that measurement “collapses” the wave-function from a state with quantum properties (such as being in two places at once) to a distinct state (at only one place). This postulate agrees with all observations, but it is regarded unappealing by many (including myself). One way to avoid this postulate is to instead posit that the wave-function never collapses. Instead it ‘branches’ into different universes, one for each possible measurement outcome – a whole multiverse of measurement outcomes.
- The Mathematical Universe
The Mathematical Universe is Max Tegmark’s brain child, in which he takes the final theory claim to its extreme. Any theory that describes only our universe requires the selection of some mathematics among all possible mathematics. But if a theory is a final theory, there is no way to justify any particular selection, because any selection would require another theory to explain it. And so, the only final theory there can be is one in which all mathematics exists somewhere in the multiverse.
Take Newtonian gravity: Is there a universe for each value of Newton’s constant? Or General Relativity: Do all solutions to the field equations exist? And Loop Quantum Gravity has multiverses with different parameters for an infinite number of solutions like string theory. It’s just that Loop Quantum Gravity never tried to be a theory of everything, so nobody worries about this.
What is new about the multiverse idea is that some physicists are no longer content with having a theory that describes observation. They now have additional requirements for a good theory, like for example that the theory have no ad hoc prescriptions like collapsing wavefunctions; no small, large, or in fact any numbers; or initial conditions that are likely according to some currently accepted probability distribution.
Is the multiverse science?
Science is what describes our observations of nature. But this is the goal and not necessarily the case for each step along the way. And so, taking multiverses seriously, rather than treating them as the mathematical artifact that I think they are, might eventually lead to new insights. The real controversy about the multiverses is how likely it is that new insights will emerge from this approach eventually.
The maybe best example for how multiverses might become scientific is eternal inflation. It has been argued that the different universes might not be entirely disconnected, but can collide, thereby leaving observable signatures in the cosmic microwave background. Another example for testability comes from Mersini-Houghton and Holman who have looked into potentially observable consequences of entanglement between different universes. And in a rather mindbending recent work, Garriga, Vilenkin and Zhang, have argued that the multiverse might give rise to a distribution of small black holes in our universe which also has consequences that could become observable in the future.
As to probability distributions on the string theory landscape, I don’t see any conceptual problem with that. If someone could, based on a few assumptions, come up with a probability measure according to which the universe we observe is the most likely one, that would for me be a valid computation of the standard model parameters. The problem is of course to come up with such a measure.
Similar things could be said about all other multiverses. They don’t presently seem very useful to describe nature. But pursuing the idea might eventually give rise to observable consequences and further insights.
We have known since the dawn of quantum mechanics that it’s wrong to require all mathematical structures of a theory to directly correspond to observables – wave-functions are the best counter example. How willing physicists are to accept non-observable ingredients of a theory as necessary depends on their trust in the theory and on their hope that it might give rise to deeper insights. But there isn’t a priori anything unscientific with a theory that contains elements that are unobservable.
So is the multiverse science? It is an extreme speculation, and opinions widely differ on how promising it is as a route is to deeper understanding. But speculations are a normal part of theory development, and the multiverse is scientific as long as physicists strive to eventually derive observable consequences.
Is the multiverse real?
The multiverse has some brain-bursting consequences. For example that everything that can happen does happen, and it happens an infinite amount of times. There are thus infinitely many copies of you, somewhere out there, doing their own thing, or doing exactly the same as you. What does that mean? I have no clue. But it makes for an interesting dinner conversation through the second bottle of wine.
Is it real? I think it’s a mistake to think of “being real” as a binary variable, a property that an object either has or has not. Reality has many different layers, and how real we perceive something depends on how immediate our inference of the object from sensory input is.
A dog peeing on your leg has a very simple and direct relation to your sensory input that does not require much decoding. You would almost certainly consider it real. On the contrary, evidence for the quark model contained in a large array of data on a screen is a very indirect sensory input that requires a great deal of decoding. How real you consider quarks thus depends on your knowledge of, and trust in, the theory and the data. Or trust in the scientists dealing with the theory and the data as it were. For most physicists the theory underlying the quark model has proved reliable and accurate to such high precision that they consider quarks as real as the peeing dog.
But the longer the chain of inference, and the less trust you have in the theories used for inference, the less real objects become. In this layered reality the multiverse is currently at the outer fringes. It’s as unreal as something can be without being plain fantasy. For some practitioners who greatly trust their theories, the multiverse might appear almost as real as the universe we observe. But for most of us these theories are wild speculations and consequently we have little trust in this inference.
So is the multiverse real? It is “less real” than everything else physicists have deduced from their theories – so far.