Being lonely is unnatural, at least if you are a slim Higgs particle (with mass on the order of the type recently discovered)–according to an intriguing statistical argument given by particle physicist Matt Strassler (sketched below). Strassler sets out “to explain the scientific argument as to why it is so unnatural to have a Higgs particle that is “lonely” — with no other associated particles (beyond the ones we already know) of roughly similar mass.
This in turn is why so many particle physicists have long expected the LHC to discover more than just a single Higgs particle and nothing else… more than just the Standard Model’s one and only missing piece… and why it will be a profound discovery with far-reaching implications if, during the next five years or so, the LHC experts sweep the floor clean and find nothing more in the LHC’s data than the Higgs particle that was found in 2012. (Strassler)
What’s the natural/unnatural intuition here? In his “First Stab at Explaining ‘Naturalness’,” Strassler notes “the word ‘natural’ has multiple meanings.
The one that scientists are using in this context isn’t “having to do with nature” but rather “typical” or “as expected” or “generic”, as in, “naturally the baby started screaming when she bumped her head”, or “naturally it costs more to live near the city center”, or “I hadn’t worn those glasses in months, so naturally they were dusty.” And unnatural is when the baby doesn’t scream, when the city center is cheap, and when the glasses are pristine. Usually, when something unnatural happens, there’s a good reason……
If you chose a universe at random from among our set of Standard Model-like worlds, the chance that it would look vaguely like our universe would be spectacularly smaller than the chance that you would put a vase down carelessly at the edge of the table and find it balanced, just by accident.
Why would it make sense to consider our universe selected at random, as if each one is equally probable? What’s the relative frequency of possible people who would have done and said everything I did at every moment of my life? Yet no one thinks this is unnatural. Nevertheless, it really, really bothers particle physicists that our class of universes is so incredibly rare, or would be, if we were in the habit of randomly drawing universes out of a bag, like blackberries (to allude to C.S. Peirce). Anyway, here’s his statistical argument:
I want you to imagine a theory much like the Standard Model (plus gravity). Let’s say it even has all the same particles and forces as the Standard Model. The only difference is that the strengths of the forces, and the strengths with which the Higgs field interacts with other particles and with itself (which in the end determines how much mass those particles have) are a little bit different, say by 1%, or 5%, or maybe even up to 50%. In fact, let’s imagine ALL such theories… all Standard Model-like theories in which the strengths with which all the fields and particles interact with each other are changed by up to 50%. What will the worlds described by these slightly different equations (shown in a nice big pile in Figure 2) be like?
Among those imaginary worlds, we will find three general classes, with the following properties.
- In one class, the Higgs field’s average value will be zero; in other words, the Higgs field is OFF. In these worlds, the Higgs particle will have a mass as much as ten thousand trillion (10,000,000,000,000,000) times larger than it does in our world. All the other known elementary particles will be massless …..
- In a second class, the Higgs field is FULL ON. The Higgs field’s average value, and the Higgs particle’s mass, and the mass of all known particles, will be as much as ten thousand trillion (10,000,000,000,000,000) times larger than they are in our universe. In such a world, there will again be nothing like the atoms or the large objects we’re used to. For instance, nothing large like a star or planet can form without collapsing and forming a black hole.
- In a third class, the Higgs field is JUST BARELY ON. It’s average value is roughly as small as in our world — maybe a few times larger or smaller, but comparable. The masses of the known particles, while somewhat different from what they are in our world, at least won’t be wildly different. And none of the types of particles that have mass in our own world will be massless. In some of those worlds there can even be atoms and planets and other types of structure. In others, there may be exotic things we’re not used to. But at least a few basic features of such worlds will be recognizable to us.
Now: what fraction of these worlds are in class 3? Among all the Standard Model-like theories that we’re considering, what fraction will resemble ours at least a little bit?
The answer? A ridiculously, absurdly tiny fraction of them (Figure 3). If you chose a universe at random from among our set of Standard Model-like worlds, the chance that it would look vaguely like our universe would be spectacularly smaller than the chance that you would put a vase down carelessly at the edge of the table and find it balanced, just by accident.
In other words, if the Standard Model (plus gravity) describes everything that exists in our world, then among all possible worlds, we live in an extraordinarily unusual one — one that is as unnatural as a vase balanced to within an atom’s breadth of falling off or settling back on to the table. Classes 1 and 2 of universes are natural — generic — typical; most Standard Model-like theories are in those classes. Class 3, of which our universe is an example is a part, includes the possible worlds that are extremely non-generic, non-typical, unnatural. That we should live in such an unusual universe — especially since we live, quite naturally, on a rather ordinary planet orbiting a rather ordinary star in a rather ordinary galaxy — is unexpected, shocking, bizarre. And it is deserving, just like the balanced vase, of an explanation. One certainly has to suspect there might be a subtle mechanism, something about the universe that we don’t yet know, that permits our universe to naturally be one that can live on the edge.
Does it make sense to envision these possible worlds as somehow equally likely? I don’t see it. How do they know that if an entity of whatever sort found herself on one of the ‘natural’ and common worlds that she wouldn’t manage to describe her physics so that her world was highly unlikely and highly unnatural? Maybe it seems unnatural because, after all, we’re here reporting on it so there’s a kind of “selection effect”.
An imaginary note to the Higgs particle:
Dear Higgs Particle: Not long ago, physicists were happy as clams to have discovered you –you were on the cover of so many magazines, and the focus of so many articles. How much they celebrated your discovery…at first. Sadly, it now appears you are not up to snuff, you’re not all they wanted by a long shot, and I’m reading that some physicists are quite disappointed in you! You’re kind of a freak of nature; you may have been born this way, but the physicists were expecting you to be different, to be, well bigger, or if as tiny as you are, to at least be part of a group of particles, to have friends, you know, like a social network, else to have more mass, much, much, much more … They’re saying you must be lonely, and that– little particle–is quite unnatural.
Now, I’m a complete outsider when it comes to particle physics, and my ruminations will likely be deemed naïve to the physicists, but it seems to me that the familiar intuitions about naturalness are ones that occur within an empirical universe within which we (humans) have a large number of warranted expectations. When it comes to intuitions about the entire universe, what basis can there possibly be for presuming to know how you’re “expected” to behave, were you to fulfill their intuitions about naturalness? There’s a universe, and it is what it is. Doesn’t it seem a bit absurd to apply the intuitions applicable within the empirical world to the world itself?
It’s one thing to say there must be a good explanation, “a subtle mechanism” or whatever, but I’m afraid that if particle physicists don’t find the particle they’re after, they will stick us with some horrible multiverse of bubble universes.
So, if you’ve got a support network out there, tell them to come out in the next decade or so, before they’ve decided they’ve “swept the floor clean”. The physicists are veering into philosophical territory, true, but their intuitions are the ones that will determine what kind of physics we should have, and I’m not at all happy with some of the non-standard alternatives on offer. Good luck, Mayo
Where does the multiverse hypothesis come in? In an article in Quanta by Natalie Wolchover
Physicists reason that if the universe is unnatural, with extremely unlikely fundamental constants that make life possible, then an enormous number of universes must exist for our improbable case to have been realized. Otherwise, why should we be so lucky? Unnaturalness would give a huge lift to the multiverse hypothesis, which holds that our universe is one bubble in an infinite and inaccessible foam. According to a popular but polarizing framework called string theory, the number of possible types of universes that can bubble up in a multiverse is around 10500. In a few of them, chance cancellations would produce the strange constants we observe.[my emphasis].
Does our universe regain naturalness under the multiverse hypothesis? No. It is still unnatural (if I’m understanding this right). Yet the physicists take comfort in the fact that under the multiverse hypothesis, “of the possible universes capable of supporting life — the only ones that can be observed and contemplated in the first place — ours is among the least fine-tuned.”
God forbid we should be so lucky to live in a universe that is “fine-tuned”![i]
[i] Strassler claims this is a purely statistical argument, not one having to do with origins of the universe.