Matt Strassler [October 12, 2012]
Here is the new (and very long overdue) version of the Higgs FAQ, intended for those with little or no scientific background. The old version (from long before the Higgs-like particle was discovered in July 2012) is HERE.
If you have no math or physics in your background, you may also find it useful, after you read this FAQ, to read my literary article on Why the Higgs Particle Matters. Or you could read it first, if you like.
If you have a little math in your background (algebra, trig, and calculus through derivatives) and a little physics (you know what energy is, what a ball on a spring does, and have thought at least once about what waves are) then, after reading this FAQ, you may want to follow up by reading my Particles and Fields articles, followed by my explanation of the Higgs field and how it works.
Ok — without further ado, 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. The Higgs field is the key to the story.
- And 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. The electric field can also have waves, in which the size of the field repeatedly becomes larger and smaller — visible light is such a wave, as are X-rays and radio waves, and all the other things we collectively call “electromagnetic waves”.
- Ok, so, what is a particle?
A quantum field’s waves cannot be of arbitrary intensity; they can’t be arbitrarily `dim’, or `quiet’. The least-intense possible wave that a field can have is called a “quantum” or a “particle”. 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 can absorb 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. (Click here for a video [BIG! 284 MB and 23 minutes, unfortunately; and you'll get the point after just 10 seconds] which demonstrates this effect; the screen registers the light one photon at a time. Here’s the webpage it’s from if you want to learn what the whole video is about.)
- I kinda get it. A Higgs wave is a ripple in the Higgs field, and the Higgs particle is the smallest — well, `dimmest’ — such wave.
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, before the end of 2012. A version requiring a little math and physics background, such as one would get from the first few months of university-level physics, is already available here.
- 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 (unlike most of the elementary fields of nature) has a non-zero average value throughout the entire universe. 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. Mostly 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. But the recent discovery of what may be its ripples — the Higgs particle — may soon give us additional insights.
- 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 electric 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?
We didn’t used to be! The only reason we are almost certain they exist is from recent experimental evidence from July 2012. At that time a new particle was discovered, and all the evidence so far suggests strongly that it is a Higgs particle — but results are still not absolutely conclusive. By March 2013 we may well be sure.
In the past, what we knew for sure was 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!
And now we’ve learned something: apparently, option 2 is false. Although there might not have been a Higgs particle in nature, it appears there is one. And now, to learn more about the Higgs field, we need to see whether this is the only type of Higgs particle, and what its properties are.
- The press — and even many physicists — say explicitly that the LHC was built to find the Higgs particle! Since that’s happened, isn’t the LHC done with its task?
These statements that you read in the press are white lies, and deeply 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 and studying the Higgs particle(s) is the way to do that. Let us not confuse the ends for the means! Understanding the field is the end goal! Finding and studying the particle or particles is the means, and there is much left to do at the LHC as far as studying the particle that’s been found and searching for others that might be awaiting discovery.
- I’ve read that the Higgs particle has been found. Is that true?
Not as stated, no. The correct and precise statements are
- Using data collected in 2011 and the first half of 2012, a new particle was discovered at the LHC;
- This particle’s behavior is still little-studied, but it is consistent with the behavior expected of a generic type of Higgs particle;
- It is also still consistent with the behavior of the simplest type of Higgs particle — the so-called Standard Model Higgs
In other words: there’s a new type of particle that might well be a Higgs particle of some form, possibly even the only type of Higgs particle in nature, and perhaps even a Standard Model Higgs. But only additional data and study over the coming few years will clarify its true nature… and allow us to understand more about the Higgs field as a result. And meanwhile we’ll also need to keep looking for other Higgs particles that are more difficult to find; just because we’ve found one so far doesn’t mean there aren’t two or five or twelve of them!
- 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 and study it.
- Why is the Higgs particle called the “God particle”?
Because the media thinks it sounds cool and that it 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.
- I hear the Higgs particle decays rapidly, so how can it create or support the Higgs field? What I have read seems to imply that there is this sea of Higgs particles and this somehow sets up the Higgs field. That wouldn’t work if the Higgs particle existed for just an instant.
The Higgs field doesn’t have to be created by a process; it is just *there*, the way the electric field of nature is just there, always and everywhere.
The Higgs field has a non-zero value in nature on average. (The electric field is zero on average). This non-zero value also is just *there*; it doesn’t have to be generated by a process. It is simply the preferred state of our universe for the Higgs field to be non-zero. We don’t know why, but nobody has to do anything to make it that way.
The non-zero value of the Higgs field is not to be thought of as a sea of Higgs particles; that is the wrong intuition. A Higgs particle is a ripple of minimal intensity in the Higgs field; a ripple varies over space and time, just as any wave does. But the non-zero value of the Higgs field is constant over space and time; it does not vary. A pretty good analogy: the density of the air is a field; it has a constant average value; waves in the air are sound waves; and there is no sense in which the constant average density of the air should be thought of as built up from a sea of sound waves, which are evanescent ripples in the air.
Higgs particles are not formed spontaneously. You have to put energy in. You have to use something like a Large Hadron Collider proton-proton collision to whack the Higgs field and make it wiggle, just as you have to clap your hands to make sound, hit the surface of a lake to make a ripple, or pluck a violin string to get it to vibrate. Just as a ripple dies away after a while, and a violin string eventually stops vibrating, a Higgs particle will decay away too. The air, the lake, the violin string, and the Higgs field remain behind after the vibrating dissipates.
- Then Higgs particles don’t normally exist? I think this is why you also mentioned that there are no Higgs particles in the room I am in, yet my electrons have mass. What role, if any, does the Higgs particle play in the mass mechanism? I was thinking they might be a force carrier particle like the W for the weak force, but it doesn’t sound like Higgs particle is supposed to do this. At a recent lecture by Frank Close, I asked him about whether there are Higgs particles in the room and he mentioned that they could bubble into existence by “borrowing” energy for a moment and then dissappearing. So there would be Higgs particles in the room. Do you agree with that picture?
The Higgs particle does not have any role to play in the mass mechanism. It’s the Higgs field — in particular, the fact that its average value is non-zero — which gives mass to the various particles. It’s the field that we really want to understand, not the particle… the particle is a means to an end, not an end in itself.
The Higgs particle is a ripple in the Higgs field, and studying the Higgs particle can tell us something about the Higgs field. For more about this, take a look at my video clips on the matter, from my Secret Science Club talk: http://profmattstrassler.com/videoclips/
There are indeed virtual Higgs particles in the room, but virtual particles are not particles at all, despite the name. Higgs particles are nicely behaved waves in the Higgs field, whereas virtual Higgs “particles” are more general types of disturbances in the Higgs field. Higgs particles have a definite mass; virtual Higgs “particles” do not. See http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/ So Frank Close wasn’t really lying to you, but he wasn’t really being clear either. What he was telling you is the standard “white-lie” most theoretical physicists usually tell the public, but it is so deeply misleading that it confuses people terribly (as I see regularly, through the questions I am asked) so I urge you to disregard it.
- If mass is created by a particle interacting (moving through) the Higgs Field then is the field moving or the particle or both? If a particle is static (not moving) relative to the Higgs Field, can it lose its mass?
- Since gravity pulls on things proportional to their mass, and since the Higgs field is responsible for giving everything its mass, there obviously must be a deep connection between the Higgs and gravity… right?
A very reasonable guess, but — it turns out to be completely wrong. The problem is that this statement combines a 17th century notion of gravity, long ago revised, with an overly simplified version of a late-20th century notion of where masses of various particles comes from. Let me bring out my professorial training and correct the statement above with a red pen:
- Since gravity pulls on things proportional to their
massto a combination of their energy and momentum, and since the Higgs field is responsible of giving everythingnot everything, just the known elementary particles excepting the Higgs particle itself itsmass, there obviously must be a deep connectionbetween the Higgs and gravity… right?wrong.
Now let me explain these corrections.
When you first learn about gravity in school, you learn Newton’s law: that the force of gravity between two objects, one of mass M1 and one of mass M2, has a strength proportional to the product M1 M2.
But that was true before Einstein. It turns out that Newton’s law needs to be revised: the Einsteinian statement of the law is (roughly) that for two objects that are slow-moving (i.e. their speed relative to one another is much less than c, the speed of light) and have energy E1 and E2, the gravitational force between them has a strength proportional to the product E1 E2.
How are these two statements, the Newtonian and the Einsteinian, consistent? They are consistent because Einstein and his followers established that for any ordinary object, the relation between its energy E, momentum p and mass M [sometimes called ``rest mass'', but just called `mass' by particle physicists] is
- E2 = (p c)2 + (M c2)2
For a slow-moving object, p ≈ Mv (where v is the object’s velocity) and pc ≈ Mvc is much smaller than Mc2. And therefore
- E2 ≈ (M c2)2 (i.e., E ≈ M c2 for slow objects)
Since planets, moons, and artificial satellites all move with velocities well below 0.1% of c relative to each other and to the sun, the gravitational forces between them are proportional to
- E1 E2 ≈ M1 M2 c4
And since c is a constant, for such objects Einstein’s law of gravity and Newton’s law of gravity are completely consistent; the force law is proportional to the product of the energies and to the product of the masses, because the two are proportional to one another.
But for objects that have high speeds relative to one another, or for objects subject to extremely strong gravitational pulls (which will quickly develop high speeds if they don’t have them already), the Einsteinian law of gravity involves a complicated combination of momentum and energy, in which mass does not explicitly appear. This is why Einstein’s version of gravity even pulls on things like light, which is made from photons that have no mass at all. (And it is why gravitational waves — waves in space and time, massless just like light — can be formed by objects that are orbiting one another.) Simply put, the Einsteinian view of gravity (now reasonably well confirmed by experiment) differs significantly from the Newtonian view, and in particular, it is not mass but energy and momentum which are primary. And all objects, not matter what they are made from or how they are moving from your point of view, have energy — so everything in the universe exerts a gravitational effect on everything else. We say “gravity is a universal force ”(here the term is not referring not to the universe but to the notion of universality — of complete generality.)
What about the Higgs field being the source for all mass in the universe? This statement, though you will often find it in the press or in glib articles written for the public, is false.
What is the true statement? Well, here is a list of the elementary particles that we know about so far. The massless ones are
- photons, gluons, gravitons (the latter presumed to exist)
while the ones with mass are
- W and Z particles
- quarks: top, bottom, charm, strange, up, down
- charged leptons: electrons, muons, taus
- neutrinos: three types (at least two and probably all three with small masses)
- the recently discovered new particle with a mass of 125 GeV/c2 (which I will assume for now is a Higgs particle of some type)
Now it is true that the W and Z particles, the quarks, the charged leptons and the neutrinos must get their mass from a Higgs field. It’s not possible for them to have masses any other way. But this is not true of the Higgs particle itself.
The mass of the Higgs particle does not entirely come from the Higgs field!
Where does its mass come from? Oh, that’s a long story that ends in a question rather than an answer. I will try to explain it someday. For now, suffice it to say that the mass of the Higgs particle does not have a single, simple, understood source, and the curious feature is that its mass is so small — this is one aspect of the enormous puzzle called the hierarchy problem.
But in any case, the Higgs field is not the universal giver of mass to elementary particles. The Higgs particle itself gets its mass, at least in part, from elsewhere. And it probably isn’t alone. It is very possible that dark matter is made from particles, and these too probably get at least part of their mass from another source. Dark matter is believed by most physicists and astronomers to be the majority of the matter in the universe; it is believed to provide the majority of the mass of the Milky Way Galaxy that we inhabit. The Higgs field likely provides little of that mass.
Other things get their masses from sources other than the Higgs particle. The majority of the mass of an atom is its nucleus, not its lightweight electrons on the outside. And nuclei are made from protons and neutrons — bags of imprisoned or “confined” quarks, antiquarks and gluons. These quarks, antiquarks and gluons go roaring around inside their little prison at very high speeds, and the masses of the proton and neutron are as much due to those energies, and to the energy that is needed to trap the quarks etc. inside the bag, as it is due to the masses of the quarks and antiquarks contained within the bag. So the proton’s and neutron’s masses do not come predominantly from the Higgs field. [Experts: There is a subtlety here, having to do with how the Higgs field affects the confinement scale; but even when it is accounted for, the statement remains essentially true.] So the mass of the earth, or the mass of the sun, would change, but not enormously, if there were no Higgs field… assuming they could hold together at all, which would not be true of the earth.
And black holes, which are some of the most massive objects in the universe, holding court at the centers of most galaxies, can in principle be made entirely from massless things. You can make a black hole entirely out of photons, in principle. In practise most black holes are made from ordinary matter, but ordinary matter’s mass is mostly from atomic nuclei, and as we just noted, that doesn’t come entirely from the Higgs field.
No matter how you view it, the Higgs field is not the universal giver of mass to things in the universe: not to ordinary atomic matter, not to dark matter, not to black holes. To most known fundamental particles, yes — and it is crucial in ensuring that atoms exist at all. But there would be just as much interesting gravitational physics going on in the universe if there were no Higgs field. There just wouldn’t be any atoms, or any people to study them.
Finally, you can ask more technically whether, in the equations that physicists study, there is any mathematical connection between gravity and the Higgs field. The answer is no. Gravitational fields have spin 2 and are described as part of space and time; they interact with all particles and fields in nature. The Higgs field, which has spin 0, only interacts directly with elementary particles and fields that also participate in the electromagnetic and weak nuclear forces.
So — the guess that the Higgs has something to do with gravity is natural for a non-expert, but I am afraid it is naive; it comes from misunderstanding both
- the Higgs field, which is not universal: it gives masses to most of the known elementary particles but not to the Higgs particle itself, and not to protons and neutrons, dark matter (most likely), or black holes,
- and Einstein’s gravity, which is universal and has to do with energy and momentum but not mass directly, and most certainly does pull on protons and neutrons, dark matter and black holes even though their masses don’t come entirely from the Higgs field.
It’s really true: despite appearances at first glance, the relation between gravity and the Higgs is just skin deep.
- Since it makes sense to seek a fundamental explanation for the values of the *masses* of elementary particles, why do we not also seek explanations for the particular values of the *charge* and *spin* of these particles?
We do. But in quantum field theory (the type of equations used in particle physics) mass turns out to be very different from charge and spin. The charge and spin of a particle are fixed; once specified, they are determined. But mass can be changed dynamically from zero to non-zero, and once non-zero the precise value of a particle’s mass is determined, in a very complex quantum mechanical way, by the strength and nature of that particle’s interactions with all of the other types of particles. [A similar complexity affects the strengths of forces.] So the question of where the masses (and strengths of forces) come from turns out to be of a very different nature from the question where the charges and spins come from.
- Has the Higgs field always been non-zero?
This depends on the history of the universe, which we don’t know well enough yet. It is quite possible that there was an extremely short time when the universe was very hot and the Higgs field’s value was close to zero; it is even possible there was an extremely short time when all of the fields we know about were rearranged beyond recognition (as might happen in a different vacuum of the landscape of fields, sometimes called the “string theory landscape” but this need have nothing to do with string theory.) Or maybe it was a long time. The history of the universe before the Big Bang may have been very short or it may have been very long; we really have no idea.
However, the Higgs field has been non-zero ever since the current universe-as-we-know-it has been cooler than a few million billion degrees… since a tiny fraction of a second after the current Big Bang is naively thought to have begun.
- Why do the equations of the Standard Model of particle physics not yield a prediction of exactly what mass the Higgs particle will have?