The Vafa-Ellis-Rubenstein debate about the multiverse has reminded me about the extraordinary difficulty that the laymen – such as the host of the debate, David Malone, a Green Party politician and BBC filmmaker – may have with simple enough terms such as the "multiverse". There must be something fatally wrong about the very word, right?
Shouldn't scientists be prevented from using the term "multiverse"?
The word "universe" comes from Latin; "universus" means "turned into one". Note that "uni-" is one, "versus" is a past participle of "vertere", i.e. "turned". Similarly, "multiverse" is a neologism used to represent "many universes". All of being is represented as belonging to several universes.
First of all, a sociological answer. You really don't want to plan a ban of the multiverse in the scholarly papers. Google Scholar lists 22,800 papers that include the word "multiverse". Top cited papers with the word in the title have about 500 citations. You may find papers on physics and cosmology but also those on proteins and the Indian cinema, among other things.
Clearly, these unusual places where people use the word are "derivative" or "experimental" exploitations of the physics terminology and marketing seems to be the main goal of this choice of the words.
In physics, we primarily mean one thing – in eternal inflation – but as e.g. Brian Greene described in his popular book The Hidden Reality, there are almost a dozen of types and generalizations of the "multiverse" in fundamental physics – which have very different degrees of generality, connectedness with the expert literature (or, on the contrary, with pop science or the mass culture), and probability to be correct.
First, our Universe may be very large and regions outside the visible Universe may exist. These regions are just like us but the volume may be huge and allow many extra volumes analogous to the visible Universe. This is just a "large Universe" – incidentally, it's pretty much proven that the total Universe connected to ours is at least 100 times larger than the visible one.
Second, there is the real "multiverse" in the main sense – a collection of universes that are only connected by "parental relationships" – some universes grew as bubbles out of their parent universes – and where the long-distance laws of physics generally differ. One universe may have a heavier charged particle, another one has a lighter one, and this diversity is useful for the understanding of the seemingly random properties of particle physics in our world. This main "multiverse" seems to be "almost certainly" implied by the inflationary cosmology, our main theory about "what preceded the normal power-law Big Bang expansion".
Third, Greene talks about the parallel branes in the braneworlds (also "other universes" in another sense); many worlds interpretation of quantum mechanics (which is in no way a viable complete replacement for the proper Copenhagen rules of quantum mechanics but has some degree of popularity among physicists); the Platonic multiverse of all mathematical constructions, simulated universes, and more.
OK, the "multiverse of the eternal inflation" is the only totally correctly named multiverse that should hold the trademark, and the users of the term "multiverse" in all the other contexts should pay some royalties to the inflationary cosmologists. Is it true? Is it science?
Well, these are two very different questions. We aren't sure it is true. But it is certainly science. It is used in thousands of the physics papers and for a good reason: it is
- clearly a possible scheme of the world in the biggest picture that has logical relationships with various proposed theories of physics; it has emerged from some theories and hasn't been ruled out yet
- the possibility of many or very many universes is clearly on par with the possibility that there is just one Universe we roughly see – and it would be a prejudice to throw away one of the competing, clearly comparably viable a priori, ideas
- its existence or non-existence has consequences for the validity of our theories
- according to some theories, the existence and the detailed structure of the multiverse has statistical consequences (predictions) for properties of our universe
The reason why some of the most ill-informed laymen want to declare "multiverse" a blasphemy is their belief in what philosophers call naive realism. You see, hear, smell, taste, or touch objects, and this perception gives you direct information about the existence and character of all objects, naive realists believe. Everything is divided to real and unreal things and the world is simple.
A problem is that naive realism is just a philosophical belief and it has arguably been increasingly incompatible with the state-of-the-art science in the 20th century (quantum mechanics etc.). But even if you failed to agree that they have been increasingly incompatible, you should be capable of understanding that naive realism is just another metaphysical prejudice and a key condition for science to work well is that no assumptions may be promoted to dogmas in advance. Everything can ultimately be questioned: this is really a defining characteristic of the scientific approach.
We don't see the other universes in the multiverse but that doesn't prove that they don't exist.
It doesn't prove that they're unscientific, either. Take a simple analogy, something that is just 6,000 kilometers away. The Earth's core. Does it exist? Can science talk about it and study its details? You bet. We can't get there. It's virtually impossible in practice to drill through thousands of kilometers of a rock that displays some extreme pressures and rather extreme temperatures. Electromagnetic waves are absorbed long before they get to the center. And it's not much better with the sound waves.
Well, the Earth's core is "untestable" in this sense. If you look more closely, there is some information you may get from the seismic waves but the reliability of that information is poor. Well, for example, almost everyone knows that the Earth's inner core is liquid, some melted iron and nickel. Half a year ago, researchers confirmed that the Earth's inner core is solid, however. Do you believe it? Is the core real at all? Is it science? It clearly is. Well-established laws of physics imply that there can't really be a big vacuum hole or some totally non-Euclidean geometry inside the Earth. We're just not certain about the phase and properties of the material in the core. However, more detailed theories in condensed matter physics tell us something about the properties of the material in the inner core.
It is analogous with the other universes. Although they don't directly influence what the telescopes see – even by definition, the other universes should be causally disconnected from our region in our universe, many of them should be on par with the spacelike-separated regions – they logically influence the truth values of propositions in our theories.
This is really a basic point about science that the "naive realist" laymen don't get at all – and because of that, they pretty much misunderstand most of modern science. They think in terms of "objects" and "physical influences between them". But that's not the fundamental level of concepts with which theoretical physics – or deep enough science – operates. Theoretical physics thinks in terms of "propositions about Nature" which include an "axiomatic framework" and in terms of the "logical relationships between these propositions". As I have argued many times, this switch from "real objects" to "propositions" (from the practical man's life to the thinking of a mathematician) is essential for statistical physics; and especially for quantum mechanics. As Bohr correctly said, physics doesn't study how objects are but what correct statements you can make about the world. Physics of the Newtonian era was basically completely compatible with naive realism, the contemporary science isn't.
This general refocus of the "fundamental concepts" affects the discussions about the multiverse and related questions, too. As an object, a foreign universe doesn't directly affect objects on Earth. However, the statement that "the multiverse exists and has some properties or others" does logically influence some statements about science that is detectable on Earth, or it at least affects some statements that affect the usual statements etc. There is a possibly indirect and long yet connected chain of reasoning that connects "propositions about the multiverse" with "propositions about the apparatuses on Earth" which is why there exists no valid way how someone could possibly "cut" the discussions about the multiverse from the rest of science.
With some extra assumptions about the methods by which the multiverse implies probabilistic distributions, a more detailed theory of the multiverse – which is a very natural and almost unavoidable extension of the inflationary cosmological theories about a single universe – the existence of a multiverse predicts probabilities that our low-energy laws of physics have some properties (or spectrum of particles or values of fundamental constant) or others. Buy Greene's book for some basic possibilities of that kind.
While we're not certain we live in the universe, we can't prohibit the statements related to the multiverse in scientific papers because they're demonstrably connected with science. And it is a very important, fundamental science that simply excites the people who are curious researchers by heart.
There are lots of technicalities to say about the multiverse, what seems right or wrong or avoidable or unavoidable about it, which discussions of the anthropic principle are almost universally vacuous, which of them are almost universally wrong, which of them are promising to tell us something new, and so on. There are many possibilities to turn the mere existence of the string theory landscape – which is basically a scientifically settled fact – into frameworks that imply or don't imply some predictions.
But there is something easier about this whole discussion, something that even the laymen used to understand but they are getting dumbed down again. And it's the statement that you simply cannot declare some words or ideas "taboo" in science. Science can only eliminate possibilities by actual logical and empirical evidence, not by ideological screaming "it is not science" or "it is blasphemous" or any other stupid dehonestation. This tabooization or demonization of cherry-picked "inconvenient" ideas is exactly what has defined the opposition to the emerging science some 400 or more years ago. Paradoxically and arrogantly, many of the people who want to ban self-evidently legitimate scientific terms and theories such as the "multiverse" often say that they are defenders of science.
On the contrary, ladies and gentlemen who smoke crack pot (I liked that pun), you are medieval zombies who want to revive the Inquisition. But your deluded efforts cannot succeed and you should better return to your graves voluntarily.
And that's the memo.