I’m still early on in my attempts to explain the “naturalness problem of the Standard Model” and its implications. A couple of days ago I explained what particle physicists mean by the term “natural” — it means “typical” or “generic”. And I described how, at least from one naive point of view, the Standard Model (the equations we use to describe the known elementary particles and forces) is unnatural. Indeed any theory is unnatural that has a
a spin-zero particle (in the Standard Model, the newly discovered Higgs particle), which
is very lightweight in the following sense: it has a very very low mass-energy compared to the energy at which gravity becomes a strong force, and which
isn’t accompanied (in the Standard Model specifically) by other related particles that also have small masses.
But I didn’t actually explain any of this yet; I just described it.
Specifically, I didn’t start yet to explain what causes the Standard Model to be unnatural. This is important to do, because, as many attentive readers naturally complained, my statements about the unnatural aspect of the Standard Model was based on a rather arbitrary-sounding statistical argument, and story-telling, which is hardly enough for scientific discussion. Patience; I’ll get there, not today but probably the next installment after today’s.
To see why the argument I gave is actually legitimate (which doesn’t mean it is right, but if it’s wrong it’s not for a simple reason you’ll think of in five minutes), it is necessary to look in a little bit more detail at one of the most fundamental aspects of quantum field theory: quantum fluctuations, and the energy they carry. So for today I have written an article about this, reasonably complete.
Be prepared — the article runs headlong into the only naturalness problem in particle physics that is worse than the naturalness problem of the Standard Model (the one I wrote about on Tuesday)! I am referring to the “cosmological constant problem”. In a nutshell:
we can calculate that, in any typical quantum field theory with gravity, the amount of energy in empty space (often called `dark energy’) should be huge, and we know of no way to avoid having it in a typical somewhat-realistic theory of the universe,
yet measurements of the cosmos — in fact, the very existence of a large and old universe — assure that, if Einstein’s theory of gravity is basically right, then instead of a huge amount of `dark energy’, there’s just a very small amount — not much more than the total amount of mass-energy [E=mc² energy] found in all the matter that’s scattered thinly throughout the universe.
After you’ve read about quantum fluctuations and the cosmological constant problem, and have a bit of a sense as to why it is so hard to make it go away, we can go back to the Standard Model, and try to understand the naturalness problem that is associated with the Higgs particle and field. It all has to do with another aspect of quantum fluctuations — the fact that their energy depends on, and therefore helps determine, the average value of the Higgs field.
The first day of the SEARCH workshop was focused on current and future measurements of the new Higgs particle discovered in 2012. A lot of the issues I’ve written about before (for instance here and here) and most of the updates were rather technical, so I won’t cover them today. But I thought it useful to take a look at what was said by Raman Sundrum and separately by Nima Arkani-Hamed, whom you’ve heard about many times (for instance, here and here), on the subject of the hierarchy problem and “naturalness”.
First, let me remind you of the issue. The hierarchy problem can be phrased in many ways. Here’s one. Here’s another: for a Standard Model Higgs (the simplest possible type of Higgs particle) to show up, without any other new particles or forces at the Large Hadron Collider, is … well, let’s say it’s completely shocking, with a caveat. Why?
Because every spin-zero particle (or particle-like object) that has ever been observed, in particle physics and in similar contexts within solids and fluids, has been accompanied by new phenomena at an energy scale comparable to the scalar’s mass-energy (E=mc2 energy).
And although we cannot calculate the mass of the Higgs particle using the Standard Model (the equations we use to predict the behavior of the known particles and forces) — the Higgs particle’s mass is something we put in to the equations, which is why we didn’t know, before the LHC, what it would be — there are many speculative theories that go beyond the Standard Model where the Higgs particle’s mass can be computed, or at least estimated. And in all of these cases, the Higgs particle is accompanied by other particles and forces that show up at scales comparable to the Higgs particle’s mass-energy.
This fact — that spin-zero particles like the Higgs are accompanied by other particles and forces at a similar energy range — isn’t a mystery. Particle physicists (and others who use quantum field theory, the type of math used in the Standard Model) understand why this should be true, and have for several decades. The jargon is that it is “natural” (not meaning “from nature”, but rather meaning “generically true”) for spin-zero particles to have other particles and forces around at comparable energy scales. (I’ll explain the argument another time.)
So to discover the Higgs particle at a mass-energy of 125 GeV, and no other new particles or phenomena below, say, 1000-2000 GeV or so, would fly in the face of what we’ve seen again and again in physics, both in past data and in calculations within speculative theories. In this sense, finding nothing except a Standard Model Higgs at the LHC would be shocking. (I say “would be” rather than “is” because the LHC is still young, and no overarching conclusions can yet be drawn from its current data.)
But — here’s the caveat — how bad is this shock? After all, somewhat surprising things do happen in nature all the time. Only astonishingly, spectacularly surprising things are very rare. Yes, it would be a very big shock if new particles and forces associated with the Higgs have a mass-energy a trillion trillion times higher than that of the Higgs. But what if they’re just a few times higher than would be natural, let’s say at 10,000 GeV — which would be out of reach of the LHC? Maybe that is a small enough shock that we shouldn’t pay it much attention. Unfortunately, this is a judgment call; there’s no sharp answer to this question.
Raman Sundrum and his three options.
As Sundrum put it, there are (crudely) three logically distinct possibilities for what lies ahead:
No shock: The hierarchy problem is resolved naturally; the associated new particles will soon be seen at the LHC.
Mild shock: The hierarchy problem is resolved in a roughly natural way; most of the associated new particles will be a bit beyond the reach of the LHC, but perhaps one or more will be lightweight enough to be discovered during the lifetime of the LHC.
Severe shock: The hierarchy problem is not resolved naturally; any associated particles may lie far out of reach, though of course other particles (associated, say, with dark matter) might still show up at the LHC.
Arkani-Hamed made a similar distinction, but addressed the third case in more detail, breaking it up into two sub-cases.
The solution to the hierarchy problem is that it results from a bias (= selection effect = a form of the “anthropic principle”) ; the universe is huge, complex and diverse, with particles and forces that differ from place to place [sometimes called a “multiverse”], and most of that universe is inhospitable to life of any sort; the reason we live in an unusual part of that universe, with a lightweight unaccompanied scalar particle, is because this happens to be the only place (or one of very few places) that life could have evolved. A key test of this argument is to show that if the particles and forces of nature were much different from what we find them in our part of the universe, then our environment would become completely inhospitable — perhaps there would be no atoms, or no stars. It is controversial whether this test has been passed; good arguments can be made on both sides.
The solution to the hierarchy problem involves a completely novel mechanism. Easy to say — but got any ideas? Arkani-Hamed gave us two examples of mechanisms which he had studied that he couldn’t make work — but perhaps someone else can do better. One is based on trying to apply notions related to self-organized criticality, but he was never able to make much progress. Another is based on an idea of Ed Witten’s that perhaps our world is best understood as one that
has two dimensions of space (not the obvious three)
is supersymmetric (which seems impossible, but in three dimensions supersymmetry and gravity together imply that particles and their superpartner particles need not have equal masses)
All of this seems completely contradictory with what we observe in our world. But! One of the important conceptual lessons from string theory [this is yet another example of something important that would not have been learned if people hadn’t actually been studying string theory] is that when forces become very strong, making the physics extremely complicated to describe, it is possible that a better description of that world becomes available — and that in some special cases, this better description has one additional dimension of space and weaker forces. In short, Witten’s idea is that our way of understanding our world, with three spatial dimensions, no apparent superymmetry and no extremely strong forces, might actually be simply an alternative and simpler description of a supersymmetric world with only two spatial dimensions with an extremely strong force. Arkani-Hamed, trying to apply this to the hierarchy problem, noticed this idea makes a prediction, but he showed that the prediction is false in the Standard Model, and it seems impossible to add any collection of particles that would make it true.
A well-dressed Nima Arkani-Hamed, waving his hands.
Greetings from Stony Brook’s Simon’s Center, and the SEARCH 2013 workshop. (I reported on the SEARCH 2012 workshop here, here, here and here.) Over the next three days, a small group (about 50) of theoretical particle physicists and experimentalists from ATLAS and CMS (two of the experiments at the Large Hadron Collider [LHC]) will be discussing the latest results from the LHC, and brainstorming about what else should be done with the existing LHC data and with future data.
The workshop was organized by three theorists, Raman Sundrum, professor at Maryland (who has opened the day with a characteristically brilliant and inspirational talk about the status of the field and the purpose of the workshop), Patrick Meade, professor at Stony Brook, and Michele Papucci, soon-to-be professor at Michigan.
Of course we’ll be discussing the newly discovered Higgs particle — that discussion will occupy most of today — but we’ll be also looking at many other types of particles, forces and other phenomena that nature might be hiding from us, and how we would be able to uncover them if they exist. There’ve been many dozens of searches done at both ATLAS and CMS, but the experimentalists certainly haven’t had time to try everything plausible — and theorists haven’t yet thought of everything they might try. Workshops like this are aimed at making sure no stones are left unturned in the existing huge pile of data from 2011-2012, and also that we’re fully prepared to deal with the new data, from higher-energy proton-proton collisions, that will start pouring in starting in 2015.
Personal and professional activities require me to take a short break from posting. But I hope, whether you’re a novice with no knowledge of physics, or you’re a current, former, or soon-to-be scientist or engineer, or you’re somewhere between, that you can find plenty of articles of interest to you on this site. A couple of reminders and pointers:
* If you haven’t yet seen my one-hour talk for a general audience, “The Quest for the Higgs Boson”, intended to explain accurately what the Higgs field and particle are all about, while avoiding the most common misleading short-cuts, it’s available now, along with a 20-minute question and answer session.
* If you want a slightly more technical and written discussion of the Higgs field and particle, complete with animated images, and suitable for people who may once have had a semester or two of university physics and math, try this series of articles first, and then go to this series.
* If you’d like to better understand the language of “matter”, “mass”, and “energy” that is everywhere in popular explanations of science, but eternally confusing because of how different authors choose to talk about these subjects, you might find some useful tips in these articles: #1, #2, #3, #4.
* If you need a reminder about what “ordinary matter” (i.e. things like pickles, people and planets) is made of, try this series, which goes all the way from molecules down to quarks.
Hopefully there’s something on that list that interests you, and many links within those articles to other things that may even interest you more. Have fun exploring! And stay tuned; I’ll be writing more in the near future…
One year ago today, I arrived, bleary-eyed from my overnight flight, at the CERN laboratory in Geneva, Switzerland, host of the Large Hadron Collider. Everyone at the lab was very excited, anticipating what promised to be the biggest event during my career in particle physics — the announcement of the discovery, or at least strong evidence, of something resembling a type of Higgs particle. The following day did not disappoint, nor did the ensuing weeks of thinking and discussion and hard work. A year later, we no longer wonder whether this is a type of Higgs particle; instead we have moved on to ask which type it is, and whether it has cousins — other types of Higgs particles still waiting to be found.
Since that time, I’ve been working to find new methods of explaining particle physics, and specifically the Higgs field and particle, to a variety of audiences, with a diversity of backgrounds and with different amounts of time to spare.
Then I wrote a long sequence of articles — actually two sequences, one about fields and particles, and one specifically about how the Higgs field works — intended for people who have had the equivalent of first-year university physics.
I recently gave a set of four 90-minute classes intended for highly interested non-experts, assuming little or no background in math or science.
And I developed a new one-hour public talk (see below), entitled “The Quest for the Higgs Boson”, for a general audience, in which I tried to explain, as accurately as possible but with no math at all, what fields and particles are, how a Higgs field can give mass to the known elementary particles, and what finding and studying Higgs particles is all about.
That one-hour talk was first delivered a few months back, as part of the Nick and Maggie DeWolf Public Lecture Series, at the Wheeler Opera House in Aspen, Colorado. It was filmed by a local TV station, GrassRoots Community Television. And they have made this film available online. Click here to reach the GrassRoots TV page, then click “Watch Now” on the right-hand side. [It’s a .wmv file that should, after a little delay, begin streaming; if it doesn’t, it will laboriously download, which may take quite a while. In any case you’ll want a good internet connection. And if it is super-slow, try again another day; their server could easily get overloaded, I suspect.]
By the way, the talk is preceded by about 5 minutes of introductory remarks by Professor Howard Haber (a Higgs-particle expert who has been mentioned before on this blog), and concludes with about 20 minutes of questions from the audience, so altogether the film is almost 90 minutes long.
It is well-known in science that if the title of a post or paper begins with a question, the answer is always “NO”, or at best, “probably not”. Today we’re working with “very probably not”.
Yes, we’ve found one type of Higgs particle, but there might be two, three, or even more types of Higgs particles in nature. Such particles might well be discovered eventually at the Large Hadron Collider [LHC] or at future experiments hardly dreamt of today. But at present, there’s no evidence yet for a second Higgs particle. And all the hullabaloo we’re hearing right now is about old news, reprocessed into new news, which actually wasn’t news anyway and really isn’t now either. (more…)
I’m back, after two weeks of teaching non-experts in a short course covering particle physics, the Higgs field, and the discovery of the Higgs particle. (The last third of the course, on the politics and funding of particle physics and science more broadly, is wisely being taught by a more disinterested party, an economist with some undergraduate physics background.) And I’ve been reminded: One of the great joys (and great secrets) of teaching is that the teacher always learns more than the students do.
At least, this is generally true for a new class that the teacher hasn’t taught before. In many university physics departments, and elsewhere, there is an informal requirement that professors teach a class no more than three years in a row. [Let us ignore for the moment that all of this will be overturned in the coming years by the on-line revolution; we can discuss the possible consequences later.] After the third year, they are expected to switch and teach something else. Now you might think that the benefits of the division of labor would suggest a different approach; after all, shouldn’t each professor perfect a course, become the expert, and teach it year in, year out? This usually doesn’t work (though there are exceptions) because each professor’s interaction with a new course has a natural life cycle. (more…)
This week and next, I’m very busy preparing and delivering a new class (four lectures, 1.5 hours each), for a non-technical audience, on the importance of and the discovery of the Higgs particle. I’ll be giving it in Western Massachusetts (my old stomping grounds). If it goes well I may try to give these lectures elsewhere (and please let me know if you know of an institution that might be interested to host them.) Teaching a new class for a non-technical audience requires a lot of concentration, so I probably won’t get too much writing in over that period.
Now — a few words on the flap over the suggestion that math Ph.D. and finance expert Eric Weinstein, in his mid-40s, may be the new Albert Einstein. I’ve kept my mouth shut about this because, simply, how can I comment usefully on something I know absolutely nothing about? (Admittedly, the modern media, blogosphere and Twitter seem to encourage people to make such comments. Not On This Blog.) There’s no scientific paper for me to read. There’s no technical scientific talk for me to listen to. I know nothing about this person’s research. All I know so far is hearsay. That’s all almost anyone knows, except for a few of my colleagues at Oxford — trustworthy and experienced physicists, who sound quite skeptical, and certainly asked questions that Weinstein couldn’t answer... which doesn’t mean Weinstein is necessarily wrong, only that his theory clearly isn’t finished yet. (However, I must admit my expert eye is worried that he didn’t have ready answers to such basic questions.)
What I do know is that the probability that Weinstein is the new Einstein is very low. Why? Because I do know a lot about how very smart people with very good ideas fail to be Einstein. It’s not because they’re dumb or foolish. (more…)
Particle physicists, cataloging the fundamental forces of nature, have named two of them the strong nuclear force and the weak nuclear force. [A force is simply any phenomenon that pushes or pulls on objects.] More generally they talk about strong and weak forces, speaking of electromagnetism as rather weak and gravity as extremely weak. What do the words “strong” and “weak” mean here? Don’t electric forces become strong at short distances? Isn’t gravity a pretty strong force, given that it makes it hard to lift a bar of gold?
Well, these words don’t mean what you think. Yes, the electric force between two electrons becomes stronger (in absolute terms) as you bring them closer together; the force grows as one over the square of the distance between them. Yet physicists, when speaking their own language to each other, will view this behavior as what is expected of a typical force, and so will say that “electromagnetism’s strength is unchanging with distance — and it is rather weak at all distances.
And the strength of gravity between the Earth and a bar of gold isn’t relevant either; physicists are interested in the strength of forces between individual elementary (or at least small) particles, not between large objects containing enormous numbers of particles.
Clearly there is a language difference here… as is often the case with words in English and words in Physics-ese. It requires translation. So I have now written an article explaining the language of “strong” and “weak” forces used by particle physicists, describing how it works, why it is useful, and what it teaches us about the known forces: gravity, electromagnetism, the strong nuclear force, the weak nuclear force, and the (still unobserved but surely present) Higgs force. (more…)
Dark Matter, Dark Matter, Everywhere! It’s in your shoes, it’s in your coffee, it’s in the stars and even in your favorite cheese… at least, it’s widely believed to be wandering all about, mostly unnoticed. Still it’s not quite as inscrutable as its reputation would lead you to believe. It’s responsible for a galactic glow, an abundance of anti-matter, and now — three quiet little taps in an underground mine.
Or is it?
Apparent effects of dark matter have been “discovered” so many times in the last decade that you may by now feel a bit jaded, or at least dispassionate. Certainly I do. Some day, some year, one of these many hints may turn out to be the real thing. But of the current hints? We’ve got at least six, and they can’t all be real, because they’re not consistent with one other. It’s certain that several of them are false alarms; and once you open that door a crack, you have to consider flinging it wide open, asking: why can’t “several” be “all six”? All of the dark-matter search experiments are difficult, as they involve pushing the technological envelope. And as anyone with experience in science knows, most of the exciting-sounding results emerging from forefront experiments don’t survive the test of time. Never underestimate the challenge of science at the frontier of knowledge!
