Multiversing around

multiverse [noun]

From L. multus “much, many” and L. versare “to busy oneself,” lit. “to turn to.” Tech: A hypothetical collection of different variants of -> universes. Colloq: A large collection of no apparent purpose. Expl: “She has a whole multiverse of shoes,” “His essay received a multiverse of comments.”

I am considering to consider to read Brian Greene’s new book The Hidden Reality on the multiverse. On the pro side there’s a likely readable synopsis of an interesting topic. On the con side there’s two of Greene’s books in my shelf that I never finished reading. A writing therapy exercise I thought might be useful.

First, let’s get over with the terminology. Yes, multiverse is a disingenuous nomenclature. If the universe is by definition all that exists, then anything to the multiverse more than the universe does by the same definition not exist. But it’s moot to complain about terminology that has already become common use. What exactly the multiverse is depends on the context, but in either case it’s something that exists in addition to what the presently most widely accepted theories let physicists expect to observe. Some theories seem to imply the existence of “more,” of a multiverse of “more,” and that in other ways than “more of the same.”

Multi versus uni

The central question is what does it mean “to exist?” As a particle physicist I’d say something that can’t be observed doesn’t exist. (Observation doesn’t necessarily mean a direct interaction.) Talking about the “existence” of something that can’t be observed opens the door to fairy tales. Though my invisible friend disapproves, from a scientific point of view I am interested in the multiverse only if it’s observable. And even then my interest is very limited since I find the presently discussed possibilities of observation remote and implausible. But yes, there are versions of the multiverse that may have observable consequences. Eg. we recently discussed a paper on signatures of bubble collisions in eternal inflation, one possible multiverse scenario, and there’s Laura Mersisni’s superhorizon entanglement giving rise to the giant void, and related stories.

For the more entertaining part I’ll now take off my physicist’s hat (okay, it’s an Einstein wig really) and put on my hobby philosopher hat (if you really want to know, it’s actually a tea pot lid).

multiversal [adj]

From noun -> multiverse. Colloq: Of confusing variety. Expl: “By the year 2010, social networking had become multiversal,” “The promises during the election campaign were multiversal.”

A lot of effort has been spent on the search for a “Theory of Everything.” Commonly meant to be a theory unifying General Relativity with the Standard Model of particle physics, it is another misnomer in common use: It is unlikely that a reductionist approach will ever be able to actually explain everything, not in practice and maybe not even in theory. I will however refer here to a TOE in the more general sense as a theory that leaves us with no “Why” questions and reduces all of science to a question of “How” and, knocking on the teapot lid, I’ll refrain from pointing out that we can never know if we’ve found it.

What may such a TOE look like? None of the currently pursued approaches to grand unification or quantum gravity comes even close. Even if string theory or something similar would allow us to compute all the parameters in the Standard Model and in the ΛCDM model, and so on, Nobelprizes would be handed out for certain, but it would just move the Why’s elsewhere, for all these theories have other unexplained assumptions: Why are strings/ loops/ E8/ networks fundamental? Why causality? Why these initial conditions? Why quantization? Why a semi-classical or classical limit? Why matter? Why in fact anything instead of nothing?

For this TOE we cannot use an assumption that constrains the theory to reproduce observation. The only guidance eventually left is mathematical consistency. Most occurrences of the multiverse actually still have additional assumptions, but already the problem is the same: too many possibilities. If you don’t want to settle for a “just because,” a question without answer, an unexplained final cause, you have to swallow that all that can exist, according to current theories, does exist. That doesn’t answer the question, but it removes the need for an answer. That is, in a nutshell, the reason for the recurrence of the multiverse in various branches of theoretical physics: Mathematical consistency just isn’t enough.

(A probabilistic approach for the multiverse with the assumption that our universe is one of the common ones, trying to derive some features of our universe at least as probable, is a reentry into the question-room through the backdoor. It just rephrases the question why our universe is special and what theory allows us to derive the details, to the question why our universe is especially unspecial and what measure allows us to derive the details, and it makes additional assumptions about how to compute probabilities rspt. about the logic used etc. I’m not dismissing the attempts to define a probability measure on the multiverse as useless since sometimes looking at an old problem from a new direction is fruitful. But the attempt in itself isn’t actually progress.)

Mathematical consistency is not a strong requirement. The complex plane and holomorphic functions on it for example are mathematically consistent (unless you insist on some wrong theorem), so is linear algebra on n-dimensional vector spaces. What sort of a universe is that, you might ask. But if mathematical consistency is all that you’re left with, that’s what you get: Everything that’s mathematically consistent “exists” in the same sense as the world around us, a notion of “existence” not in agreement with that put forward by the strange person with the funny wig. This thought then brings us straight to Tegmark’s Mathematical Universe: All of mathematics is real, and all that’s real is mathematics. There is no distinction because there’s no other meaning to “existence.”

My problem with the Mathematical Universe is not that I dislike the idea of being made of math (whatever that might mean). In fact, I quite like the idea (up to a face factor). My problem is that for all I can tell it’s not of use for anything (oops, lid slipped off) and it is based on an assumption I don’t find particularly plausible: That humans in the 21st century have already found the language to describe the fundamental nature of reality.

Formal mathematics is a quite recent achievement in mankind’s evolution. Sure, its precision and usefulness in the description of nature is vastly superior to that of, say, the English language. But 50,000 years ago our ancestors have thought of their precise spoken language as the ultimate tool to describe nature, vastly superior to grunting and waving with paws. So how sure really can we be mathematics is so intimately connected to nature that nature is mathematics?

Versus multi

Now let us turn the argument around. Searching for a TOE we were ultimately left with mathematical consistency as only guidance and it’s not enough of a constraint. It offers too many possibilities and eventually doesn’t explain anything. Unless, that is, mathematical consistency is not the only requirement. (There is of course the requirement to reproduce observation, but that’s too pragmatic for my tea pot.) The only way to avoid a multiverse then seems to be that mathematics is not sufficient to describe the fundamental nature of reality.

So the options are: a) Accept a final cause. b) Accept the multiverse. c) Accept that there’s a way to describe nature better than with mathematics.

If you don’t like a) and b) and therefore have to sympathize with c) you are however left wondering what may describe nature even better than mathematics? Well, you can. Tegmark’s Mathematical Universe irks people because they believe there is a distinction between reality and mathematics, between platonic ideas and the world out there. The common point of view is that the math used in theoretical physics is a description of nature, but humans provide the map between reality and the math. It is possible that this mapping is itself a purely mathematical process. I can imagine there to be an algorithm that searches for mathematical definitions whose properties fit to observed data. Yet presently there is no answer to the question whether there is in fact such an algorithm able to do science like a human. Gödel’s incompleteness theorem is happily waving its tail, waiting for a chance to pee on your leg.

In summary this means if there is neither a final cause nor a multiverse, there likely won’t be any Singularity in 2045 either since no computer algorithm will be able to go beyond math. And vice versa, if a computer algorithm, coded in the language of math, is able to map every aspect of reality to a mathematical structure, then you’re likely stuck with the multiverse, Tegmarkian version, subsuming all other versions. It might then just be that the next revolution in physics comes from neuroscience.

multiverse [verb]

From noun -> multiverse. From L. multus “much, many” and versus, pp. of vertere “to turn.” To make many turns. Colloq: To act or talk incoherently. “She spent the afternoon multiversing around,” “His job interview was a disaster; he totally multiversed it.”

I’ll finish with a quotation from a wise physicist, who wants to remain unnamed but reportedly reads this blog: “The multiverse, the simulation hypothesis, modal realism, or the Singularity –it’s all the same nonsense, really.”