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Mar 21 2013

The Higgs Story-Part 2: What ordinary matter is made of

Everyday matter is made up of protons, neutrons, electrons, and something called electron neutrinos. These particles interact with each other via one or more of four forces: gravity, electromagnetic (which is the unified force of electricity and magnetism), strong nuclear, and weak nuclear. Almost all of everyday life could be explained pretty well with just this short list of four particles and four forces.

But science is unreasonable. It is not content to deal with reasonably good explanations of just the easily accessible but pushes its explorations to the limits of accuracy and precision. In particular, it probes those particles and forces in more detail in order to understand them better and in the process we have discovered a whole slew of other particles that we were unaware of before. These are either extremely short-lived, or cannot be isolated because they are always ‘hidden’ inside other particles, or both, which is why we were not aware of their existence for so long.

In order to explain this large menagerie of particles that now run into well over a hundred, we have had to change and expand our understanding of what constitutes elementary particles and how they interact, and the resulting theory that we have now arrived at is what is called the Standard Model.

The idea of mass will play an important role in this series of posts and it is usually expressed in units of energy, ever since Einstein showed that mass (m) and energy (E) are convertible using E=mc2, where c is the speed of light.

The basic unit of energy used in particle physics is called the electron-volt (eV). But because we will be dealing with a huge range of energies, we also use derivatives of this basic unit (like we do with derivatives of the meter such as millimeters, centimeters, and kilometers) such as thousands of eV (KeV), millions of eV (MeV), billions of eV (GeV), and trillions of eV (TeV). For example, the electron has a mass of about 511,000 eV or 0.511 MeV, while the proton is 938.272 MeV and the neutron is 939.566 MeV. In the world of elementary particle physics, 1 TeV is considered a huge amount of energy, although in terms of everyday life it is tiny, corresponding to the amount of energy used by a 60-watt light bulb in about one-billionth (10-9) of a second.

When dealing with a composite object made up of smaller entities, the mass of the composite includes not only the masses of the components but also the interaction energies between the components. So for example, if you have two masses connected by a spring, and you squeeze the masses together by compressing the spring, the total mass is not just the mass of the two objects plus the spring. It will also include the energy stored in the compressed spring. In everyday life, this is a very small effect so we ignore it, but it is the basis for the enormous energies that are released in nuclear reactions. This is why the mass of a hydrogen atom (938.738 MeV) is not exactly the sum of the mass of a proton (938.272 MeV) plus the mass on an electron (0.511 MeV), but slightly less, since some energy is released when the proton and electron come together to form an atom.

In developing the Standard Model of particle physics, we have come to realize that protons and neutrons and many other particles are no longer ‘elementary’ (in that they have no substructure) but are comprised of yet smaller entities called quarks. These quarks are believed to be elementary and have the peculiar property that they are found in nature only in combinations with other quarks inside other non-elementary particles (like protons and neutrons) and never as isolated single entities. Hence they are quite elusive and hard to detect.

So how does one determine the mass of an entity that is part of a bigger entity and cannot be separated from it? It would be like trying to measure just the mass of your arm without separating it from the rest of your body. It cannot be done directly but has to be done using a theoretical framework. In the case of quarks, the mass is inserted as a parameter into a theory and the value that gives the best results with some observable quantity is then taken as the mass. But this is always a theory-dependent value. In the case of quarks, the most commonly used masses in particle data tables are the so-called ‘current quark’ masses that are obtained using lattice gauge theory.

The Standard Model postulates the existence of six kinds of quarks with the following names and approximate masses:

up=2 MeV; down=5 MeV; strange=100 MeV; charm=1 GeV; bottom=4 GeV; top=172 GeV

We also have a further six elementary particles known as leptons consisting of three negatively charged ones (electron=0.5 MeV; muon=106 MeV; tau=1.78 GeV) and three electrically neutral ones (electron neutrino, muon neutrino, and tau neutrino). The three neutrino masses are believed to be non-zero but so small that we have not been able to measure them as yet and can effectively treat them as zero. Of these, the electron is a ‘stable’ particle meaning that it can exist alone and if isolated will live forever without decaying into other particles. The muon and tau are short-lived particles that are produced in reactions but decay soon afterwards into more stable particles like the electron and neutrinos. The neutrinos interact so weakly with matter that they are extremely hard to detect.

The number of fundamental forces has remained unchanged at four but we have deepened our understanding of them so that they are now described as being generated by the exchange of different kinds of elementary particles called ‘gauge bosons’. So the gravity force between two particles occurs when they exchange a particle called the graviton; the electromagnetic force is mediated by the photon; the strong nuclear force by the gluon; and the weak nuclear force by three particles known as the positively charged W+, the negatively charged W-, and the electrically neutral Z.

The masses of these force carriers are:

graviton=0; photon =0; gluon=0; W+= W-=80.4 GeV; and Z=91.2 GeV

The graviton and the photon are also stable particles but the graviton is so weakly interacting that it only plays an important role when interacting with massive objects like planets and stars. For the world of particle physics, we can pretty much ignore the graviton because its effects are negligible. The gluons, just like the quarks, are never found in isolation but always inside non-elementary particles like the proton and neutron. The W+, W-, and Z are very short-lived particles, quickly decaying into other particles.

Next: The basic elements of the Standard Model

16 comments

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  1. 1
    Somite

    What an excellent series Dr. Singham! Here is a question. When physicists say that a “force is generated by the exchange of particles”; What is the best way to visualize this? Is it force particles coming back and forth? Is it a one time exchange?

    Looking forward to the rest of the series.

  2. 2
    The Lorax

    Compressing a string is adding mass…

    A “force” is an exchange of particles…

    I know these are fundamental concepts of quantum mechanics, but I must say, my brain is delightfully twisted from those statements. I feel as though I understand just a little bit more.

  3. 3
    Mano Singham

    Like most questions in science, the answer begins: It depends”.

    In this case it depends on the range of the force, the strength of the force, and the time the two particles spend in each other’s vicinity. So for example, if you have a high speed particle whizzing by a target particle (as happens in reactions that occur in accelerators), there may be an exchange of just one particle. But if the two particles are bound together (like the quarks inside a proton), then there will be multiple exchanges.

  4. 4
    Mano Singham

    As a minor note, I was talking about compressing springs, not strings.

  5. 5
    The Lorax

    Bah! That was a typo on my part. Sorry!

  6. 6
    MNb

    Let this be a nice long series of say 12, 16 or 20 posts … the more the better.
    May I assume you will address the question how the graviton functions given the huge distances in the Universe and the speed of light being the max?

  7. 7
    Mano Singham

    The graviton will play almost no role in this series simply because gravity is not a significant factor in particle physics except right after the Big Bang, which is why we need, but do not yet have, a theory of quantum gravity in order to properly address the physics of the very early universe.

    However, I can say something here about your question. Since the graviton is massless, it travels at the speed of light. So for the effects of gravity to be felt at large distances, it takes time. For example, we know that light takes about 8 minutes to get here from the Sun. If the Sun were to disappear right now, we would not notice the absence of light for eight minutes but the Earth’s orbit would also not be affected for eight minutes, because the change in the gravitational field would take that amount of time to get here.

  8. 8
    DonDueed

    Well, the same would be true for a string, too, only they’re pretty hard to compress.

    Let’s see, maybe we could thread the string into a little tube, seal one end, and apply force on a tiny piston at the other end…

    Regarding the force particles, how would we visualize the forces of gravity between large masses at very large distances — say, between the Sun and the core of the Milky Way? It would seem like there must be either a very large number of such particles permeating all of space, or else they’re pretty darned smart to find their way so precisely to their target (given how empty most of space is). I suddenly feel a little sorry for all the poor gravitons that stream on and on for eons without encountering anything to interact with… but then, if they’re moving at C they won’t experience any passage of time. Hmm.

    Maybe this will be clarified when you get into the Higgs particle/field in more detail.

  9. 9
    DonDueed

    Ah, we crossed in the mail, Mano.

    I have read somewhere that your statement about not feeling the loss of gravity from the Sun for 8 (or 9) minutes is incorrect — that we would in fact begin to fly off tangentially as soon as the Sun vanished. Unfortunately I don’t remember where I read this, and the explanation seemed a bit dodgy, but apparently it’s important that it be true for the physics to work out. Sorry I don’t understand it better.. As you are not addressing the subject of gravity in the series, it’s probably not worth further discussion here.

  10. 10
    Rob Grigjanis

    In interactions, the mediating force-carrying particles are virtual, so the speed of light is not a limit (thanks to the Heisenberg Uncertainty Principle).

  11. 11
    Mano Singham

    Think about how we get light from the Sun as an analogy. The Sun radiates photons in all directions and what we see are only the photons that happened to come in our direction. It is not that these photons were smart enough to find us, it is just by dumb luck that they started out on their journey in a direction that ended up here. All the other photons went elsewhere.

  12. 12
    Mano Singham

    I can’t see how that could happen. If changes in the gravitational field were transmitted instantaneously, then we could use that to send signals faster than the speed of light and that would violate the laws of relativity.

  13. 13
    MNb

    Thanks. Excellent thought experiment – the kind that sounds so simple that I should have thought of it myself.

  14. 14
    DonDueed

    As I recall, the argument was that if gravity acted like it propagated at light speed, orbits would not work as we observe them to because the force vector would have to point to where the object *was* (e.g. nine minutes ago), but in fact the vector points to the actual (instantaneous) position of the body. Again, I didn’t understand the finagle that causes this to occur — I never saw the math and it may be “above my pay grade” anyway.

    Regarding the issue of FTL communication, that doesn’t work because the premise is impossible in the real world — you can’t just make mass vanish Harry Potter style. It wouldn’t be the only idea for superluminal information transfer that was foiled by that pesky reality thing.

  15. 15
    DonDueed

    So also by analogy, every object that has mass must be constantly radiating gravitons uniformly in all directions, with flux proportional to the amount of mass. Otherwise (in my example) there wouldn’t be any gravitons for the Sun to intercept to keep it in orbit around the galactic core.

    And of course, this all somehow must be reconciled with the classical interpretation of gravity as a field or spacetime manifold. Perhaps that relates to wave-particle duality somehow? Mmm… my brain hurts.

    I am often impressed by the amount of unseen “stuff” that is continually passing around and through us all the time. Imagine trying to explain such things as solar neutrinos to Plato.

  16. 16
    Mano Singham

    You don’t have to make the mass vanish to signal. All you would have to do is move the mass at one location and then detect the effect of that move at a distant location.

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