What are anti-particles?

Anti-particles are often made out to be a lot more mystical and mysterious than they actually are, thanks to science fiction, and other fiction such as Dan Brown’s Angels and Demons.  I emphasize, fiction.

Every type of particle has an anti-particle.  Usually this is a distinct type of particle, but it can happen that the anti-particle and the particle are the same. Only particles satisfying certain conditions (for example, if they are electrically neutral) may be their own antiparticles. The only examples so far from the list of elementary particles are photons, Z particles, gluons and gravitons… and possibly the three neutrinos. Every other particle has a distinct anti-particle, with the same mass but opposite electric charge.  [The neutron is an example of an electrically neutral particle that is not its own antiparticle; like the proton, the neutron contains more quarks than anti-quarks, whereas the anti-neutron contains more anti-quarks than quarks.]

For those particles that differ from their anti-particles, the names of the anti-particles are usually pretty obvious (up anti-quark, anti-neutrino, anti-tau) with the exception of the anti-electron, which is usually called the positron.

What made anti-matter so famous and thence so mysterious-sounding? The statement that “matter and anti-matter annihilate into pure energy.” This statement, though it sounds cool, is glib. It isn’t entirely false, but it certainly isn’t true either. The reality is both more complex and less astonishing-sounding. I’ll explain that elsewhere.

Often, to keep things simple and short, physicists drop the “anti” prefix when it is obvious from context.  Here are two of many examples:

  • Many processes produce a muon and an anti-muon; physicists will sometimes call this a “muon pair”.
  • A W particle decaying to an up quark and a down anti-quark may be said to decay “to quarks”.

I mostly try to avoid such shorthand on this site, since it is confusing if you’re not used to it, but it does add to the verbiage. Other common shorthand used by particle physicists can be found here (soon).

20 responses to “What are anti-particles?

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  2. is dark matter linked with anti matter..?? what is the significance of selectron and squark in antiparticle physics.??

    • 1) anti-particles are not mysterious to physicists. Since the 1930s, we make them and use them and measure them (in small quantities, mind you) all the time… we even make beams out of them. Some of them rain down onto earth from outer space on a regular basis. Dan Brown and his books make anti-particles sound very weird and scary, but they are standard stuff, completely understood, and not at all dangerous (in small quantities).

      2) dark matter’s nature is unknown. However, it cannot not made from known particles, or from the anti-particles of any known particles. Most known particles are unstable, and those that are stable would not be “dark” (i.e., very difficult to observe). The same is true for their anti-particles. The only known stable particles that are dark, in this sense, are neutrinos, but we know enough about them, and about dark matter, to know that dark matter cannot be made from the known neutrinos, or from the known anti-neutrinos.

      We do not know if dark matter is made from one type of particle or many, and we do not know if it contains particles and anti-particles, or just particles, or perhaps particles that are their own anti-particles.

      3) the selectron, squark and other superpartners of known particles may not even exist; but if they do, then there must also be anti-selectrons and anti-squarks, which are the anti-particles of the selectrons and squarks, and the superpartners of anti-electrons (also called “positrons”) and of anti-quarks.

  3. I am confused as to why a anti-particle would have the opposite overall charge.. love this site but that is one answer I simply cant find easily on the web.

    • Because of the way the particle-antiparticle relationship arises in the mathematics, a particle and anti-particle always have the same properties as nothing at all, except energy. So their charges of any type — electric charge, or more exotic charges that arise in other forces — must cancel. Their masses, however, must be equal.

      I’ll try to think of a more intuitive answer, but for now this will have to do.

  4. But how the antiparticle theory came out i.e, how and who kept forward the proposal of existance of antiparticle

    • Dirac developed a theory of the electron consistent with special relativity; he discovered that the equations had some odd features, which he eventually interpreted as the requirement of the existence of an anti-electron (a particle with the same mass and opposite charge as the electron.)

      Later people understood that the existence of anti-particles followed from even deeper principles. I am not quite sure of the history here.

  5. Thanks for the great articles on particle physics. In re-reading the article about interaction energy, the following question occurred to me.

    Do a particle and its anti-particle arise from the same field or are they different fields. For example is there an anti-electron field that is separate from the electron field? Or are the electron and anti-electron two different solutions to the wave equations in a single (electron) field?

    • It’s a single field. This was Dirac’s insight, viewed in modern language; you couldn’t have an electron field without both having ripples in it that are electrons AND ripples in it that are positrons [i.e. anti-electrons].

  6. “he discovered that the equations had some odd features,”
    ->Could you explain/point out what odd features?

    “which he eventually interpreted as the requirement of the existence of an anti-electron”
    ->which is “you couldn’t have an electron field without both having ripples in it that are electrons AND ripples in it that are positrons”?

    “Later people understood that the existence of anti-particles followed from even deeper principles.”
    ->which deeper principles is this referring to?

    • The equations could be constructed so that the universe was filled with a ‘sea’ of electrons of lowest energy, imperceptible due to being everywhere. Promoting an electron to a higher energy level from this ‘sea’ would produce a noticeable electron we could detect, but would also leave behind a ‘hole’, an absence in the electron sea that would have a positive charge and behave very similar to an electron.

      This is actually valid in solid state physics, especially semiconductors which can be considered to have a sea of electrons bound to atoms. Impurities can produce p(ositive) and n(egative) semiconductors for, say, transistors. These holes behave like ‘real’ antimatter, having, for example, negative mass.

      Later on it was realized that many fields can have waves in them that ‘mirror’ other waves. In a rope for example you can have an ‘M’ wave (Two peaks, one trough) that has an ‘anti-wave’ (The ‘W’, two troughs and a peak.) Indeed in many fields any possible wave has an ‘anti-wave’ opposite to it in all respects except one, both waves and anti-waves carry energy.

  7. I has a question that “What is a charge” ? or what is the nature of charge and its existence?

    • Charge is a physical property.
      Asking “what is a charge” is like asking what is mass…which is actually indefinite.

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  9. I always thought that in particle – antiparticle collisions both particles (temporarily) cease to exist and that their energies simply create new particles of same family. So in gluon anti-gluon collision, (since gluons and anti gluons are the same particle) you could say its about energies blinking in and out of existence in incredibly short intervals that we probably could never could detect with any instruments and never know this was happening if we didn’t go by equations. True or, false?

  10. We are not charged doesnot mean we are neutral. Its something else. We are having our anti in some other universe which could be an anti universe of our universe. Everything here is due to seperation of particle from its anti particle. But the energy to seperate them i dnt know where it comes from.

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  13. Is there a property common to all anti-matter particles? If not, how do physicists decide which particle is matter and which is anti-matter? For the electron, it’s more abundant than the positron and was discovered first – so I understand how the electron came to be considered matter and the positron anti-matter.

    But what about the more exotic particles?

  14. I had a qusetion..if the anti matter exists which surely does, where could we locate it? take the case of a positron..where could it be found most probably or how could we create it??

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