Here is the first version of the Higgs FAQ. I am sure I will add to it, and I promise over time to make some of the more obscure remarks clearer by adding pages of their own. It’s way too much to do all in one day! Sorry. Please feel free to make comments about what you find unclear or ambiguous in what I’ve said; I will use your responses to improve my answers. (That goes for my other pedagogical pages too, of course.) Here we go…
- What is the Higgs particle?
Do you know what a particle is?
- Not really.
Do you know what a field is?
- Not really.
Okay, let’s back up.
- So, what’s a field?
A field is something that
- is present everywhere in space and time,
- can be, on average, zero or not zero, and
- can have waves in it.
- And if it is a quantum field, its waves are made from particles.
So for example: the electric field is a part of nature that is found everywhere. At any given point in space, and at any particular time, you can measure it. If it’s non-zero on average in some region, it can have physical effects, such as making your hair stand on end or causing a spark. It can also have waves — visible light is such a wave, as are X-rays and radio waves.
- Ok, so, what is a particle?
A quantum field’s waves cannot be of arbitrary intensity. The least-intense possible wave that a field can have is called a particle, and it often behave in rough accordance with your intuitive notion of “particle”, moving in a straight line and bouncing indivisibly off of things, etc., which is why we give it that name. In the case of the electric field, its particles are called “photons”; they represent the dimmest possible flash. Your eye absorbs light one photon at a time (though it typically waits for several photons to arrive before sending a signal to your brain.) A laser produces very intense waves, but if you shield a laser with a screen so that only a tiny fraction of the light gets through, you will find, if you shield it enough, that the light passes through the screen in little blips — single photons — all of them equally dim.
- I kinda get it. So the Higgs particle is the smallest possible Higgs wave, and a Higgs wave is a ripple in the Higgs field.
You got it. Sorry for my way-too-short version of the story. I will try to give a much more detailed and pedagogical treatment of particles and fields, with pictures and analogies and dancing bears, at a later time.
- Why do particle physicists care so much about the Higgs particle?
Well, actually, they don’t. What they really care about is the Higgs field, because it is so important.
- What’s so important about the Higgs field?
The Higgs field has a non-zero average value. And because it does, many particles have mass, including the electron, quarks, and the W and Z particles of the weak interactions. If the Higgs field’s average value were zero, those particles would be massless or very light. That would be a disaster; atoms and atomic nuclei would disintegrate. Nothing like human beings, or the earth we live on, could exist without the Higgs field having a non-zero average value. Our lives truly depend upon it.
- What do we know about the Higgs field?
Almost nothing. Just that it’s there, and that it has a non-zero value. We have some limited information about how it interacts with matter, but not much.
- Then if the Higgs field is so important, why is there so much hype about finding the Higgs particle?
On the one hand, finding the Higgs particle (or whatever takes its place, see below) is the easiest (and perhaps only) way for physicists to learn about the Higgs field — which is what we really want. In that sense, finding the Higgs particle is the first big step toward the main goal: understanding the properties of the Higgs field and why it has a non-zero average value. On the other hand, our modern media world insists on generating hype. And since explaining the Higgs field and its role and its relation to the Higgs particle takes too long for a typical news report or interview, journalists, and people talking to them, typically cut the story short. So the Higgs particle gets all the attention, while the poor Higgs field labors in obscurity, protecting the universe from catastrophe but getting none of its deserved credit…
- Are physicists sure there’s a Higgs field?
Yes, though I should add comments to that “yes”. We are sure, from the results of many experiments and their successful interpretation with mathematical equations, there is some field that has a non-zero average value and makes the electron, the W and Z particles, and many other elementary particles massive, thus permitting our world and our lives to exist. The evidence is more than overwhelming. We call that field the “Higgs field” essentially by definition. However there are many things we don’t know. For instance:
- There might be one Higgs field, or there might be several of them, each with its own type of particle (all collectively referred to as “Higgs particles”.)
- Or the Higgs field may in fact be an agglomeration or “composite” of several other fields. We have examples of such things in nature already — for example, just as a proton is a composite object made from quarks, antiquarks and gluons, the proton field is a composite field made from quark, antiquark and gluon fields — and we don’t know whether the Higgs is an elementary field, as is the electron field, or a composite of more elementary fields, as is the proton field.
The only way to know how many Higgs fields there are, whether they are elementary or not, and how they interact with the particles we know and perhaps ones we don’t yet know, is to run an experiment: the Large Hadron Collider, or LHC.
- What does elementary mean?
Sorry about this, but the answer is circular — it means “not composite”. Can’t be broken apart into more elementary pieces. Or more precisely, it can’t be broken into parts using the technology we have now. (People used to think protons were elementary. Before that they thought atoms were elementary — hence the “Periodic Table of the
- Are particle physicists sure there’s a Higgs particle?
Absolutely not! Don’t Panic!!!!!!!! Just read on, please, carefully. What we know for sure is that either
- there is at least one type of Higgs particle, and we will find it (or them) at the LHC, or
- Higgs particles fall apart too rapidly for us to identify them, but only because they are strongly affected by new particles and forces that we will be able to discover at the LHC instead! At a later time I will explain exactly how new particles and forces can make the Higgs unobservable, and why the particles we know so far cannot do so.
Either way, we learn something about what we want to know: how does the Higgs field work? The LHC was designed to be virtually certain of answering this question. So there might not be a Higgs particle, but that is perfectly ok: we will still be able to use the LHC to achieve the real goal, which is understanding the Higgs field. That said, doing so could be quite easy and start happening this year, or it could be very, very difficult and take up to a decade in the worst cases.
- But the press — and even many physicists — say explicitly that the LHC was built to find the Higgs particle! What’s going on?
Well, what can I say? These are white lies, and unfortunate ones. The correct statement is that the LHC was built to figure out what the Higgs field is (or Higgs fields are), how it works (or they work), and whether it is (or they are) elementary or composite. Searching for the Higgs particle(s), or whatever takes its (their) place, is the way to do that; and failing to find the Higgs particle or particles does not mean failure of the endeavor! It only implies that one has to find the particles and forces that make it possible for the Higgs particle to be absent. Let us not confuse the ends for the means! Understanding the field is the end goal! Finding and studying the particle is the means, and other means will do just fine at the LHC if the Higgs particle’s absence forces us to use them.
- I’ve read that the Higgs particle will be found or excluded in the next year or two. Is that true?
Not as stated, no. What is true is that the Standard Model Higgs particle — the particle of the simplest possible Higgs field, which involves one and only one elementary field added to the other types of fields we know — will be found or excluded this year (2011) or next, barring a problem with the LHC. (Already a third of the possible mass range for the Standard Model Higgs particle has been largely excluded, using the LHC data set from the first half of 2011.) But if the Standard Model Higgs particle isn’t found, that just means that the simplest possible Higgs field is not what nature has to offer. It may then be several years more before we figure out why we didn’t find it. It could be that
- there is no Higgs particle (but then we know there are other discoverable new particles and forces that are responsible)
- there are several Higgs particles (in which case they will typically be harder to produce and take longer to find than the Standard Model Higgs particle)
- there’s only one Standard-Model-like Higgs particle, but it is harder to find than expected because previously unknown particles and forces cause it to behaves in unexpected ways.
In all of these cases, the world is a richer place than if the Standard Model Higgs particle is found. So don’t be disappointed if we don’t find the Standard Model Higgs particle. I won’t be! I’ll be thrilled!! Because it means that there’s more to nature’s story than just one simple new field and its one simple new particle. It will take longer, but the LHC should be able to get to the bottom of it (or, at least, well into the thick of it.)
- Are you totally absolutely completely 100% cross-your-heart sure that there is a Higgs field in nature?
Yes, yes, yes. I don’t say absolutely yes very often, but here I do. If you try to take the Higgs field out of the mathematics but keep the W and Z particles and the other heavy particles (such as the top quark) that we have already discovered and know are present in nature, you will find that the mathematics of the Standard Model simply makes no sense. You get a theory that predicts that certain processes (including ones that the LHC can study) occur with a probability bigger than one. Sorry, that can’t happen; it’s logically unsound. The probability of anything obviously cannot be bigger than one or less than zero. It might surprise you that it is very hard to write down logically sound theories. Most theories that you can imagine predict negative probabilities or probabilities bigger than one. Only a very, very few make sense. To restore the theory of the Standard Model to working order, you must add a Higgs field, or something like it, to the fields that we have already discovered experimentally. But there are many possibilities as to how to do this, and the only way to figure out which one is right is to run an experiment — namely, the LHC!
- Why is the Higgs particle often called the “Higgs boson”? (pronounced “boh-zon”)
All the particles in nature — whether elementary or not — can be divided into two classes, fermions and bosons. [There are some weird exceptions inside certain solid materials; I tell you this only to avoid having a brick thrown at my head by some of my colleagues.] It happens that the Higgs particle is a boson. But this isn’t actually very important for what it does or why we want to find it. To understand the distinction between fermions and bosons is kind of off-topic. I’ll add some words about this at a later time, and on a different page.
- Why is the Higgs particle called the “God particle”?
Because the media thinks it sounds cool and gets readers to read their stories. The origin of the nickname is about as non-religious and non-scientific as one could imagine: it was invented as advertising. Professor and Nobel Prize Winner Leon Lederman, a very important experimental physicist who deserves enormous credit for his contributions to the field, deserves some serious demerits for having allowed his book on the Higgs particle to be assigned this attention-getting title… which is somewhere between inappropriate and blasphemous, depending on where you come from. When I first heard him use this moniker in a talk that he gave while I was in grad school, my jaw hit the floor. I knew enough physics even then to know how completely absurd it was. I have never heard or seen a physicist refer to the Higgs particle in this way in the context of a scientific paper, a talk at a conference, or even an informal scientific discussion. There’s nothing in the mathematical equations, in the interpretation of the physics, in any philosophy of which I am aware, or in any religious text or tradition with which I am familiar that connects the Higgs particle or the Higgs field with any notion of religion or divinity. The nickname is pure invention. Personally I think it is not healthy for either science or religion to be pushed around by the need of the publishing industry to sell books, or the media to sell stories. The sooner we drop this notion, the better.