Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I just recently got into particle physics and the quantum world and I love it.

So my first big question is. I watch all these videos and people explain the quarks (up, down, top, bottom, strange, charm). And they all say there are 6 quarks. But every so often someone speaks of an anti-quark. What is this anti-quark if there are only 6 quarks? Is it anti-matter? Is it still a quark? (if so that means there are 12 quarks?)

Second question, more for clarification. So there are force carriers and particles. Force carriers are bosons, they carry the strong weak and electromagnetic force and the gravity force carrier is still a mystery as to what carries it (mystery as in we just have not observed the gravity force carrying particle; i.e. higgs/gravitron)? Non-force carriers are leptons and are comprised of only quarks? And quarks can then make more massive particles like protons (uud)?

share|cite|improve this question
Just a tip for future reference: we prefer that you ask each question separately, so e.g. this would have been better as two separate questions. (Don't worry about splitting this question up, since it's already been answered, but just keep this in mind for the future.) – David Z Apr 1 '13 at 19:50
up vote 11 down vote accepted

Yes, the 6 antiquarks are antiparticles of the 6 quarks – in other words, they're particles of "antimatter". The word "antimatter" sometimes represents just a relative label – antimatter of something (antimatter of antimatter is matter again), sometimes it means the antimatter of the particles we routinely see in the world around us.

Because the 6 antiquark flavors – anti-up, anti-down etc. – have the same properties as the quarks (up to the opposite signs), they're not counted as "independent types of elementary particles". Quite generally, we don't consider antiparticle species to be "independent species" because it's a completely general fact that every particle type has an antiparticle (although, in some cases such as the photon, Z-boson, or Higgs boson, they coincide with the original particle).

No one would ever say that there are "12 types of quarks" because of the antiquarks. We either consider antiquarks "not to be quarks" when we talk about "quarks" in a strict sense, or we do include antiquarks among quarks but the antiparticles are considered to be pretty much the same thing as the original quarks (despite the sign flip in all quantum numbers) which is why we still have just 6 quark flavors (the types are called flavors; each of them also has 3 colors and 2 spin polarizations).

Leptons are not composed of quarks. Leptons and quarks are two equally large but mutually disjoint sets of elementary particles – leptons plus quarks are known as "elementary fermions".

The four forces are mediated by the photons (electromagnetic), W-bosons and Z-bosons (weak nuclear force), gluons (strong force), and gravitons (the gravitational force). Physics is pretty much equally sure about all four or five of them. The only way in which gravitons differ is that gravity is such an extremely weak force that individual gravitons are pretty much undetectable. But they're detectable if they're coming in sufficiently strong beams or packages – gravitational waves – and the 1993 physics Nobel prize was given out for the evidence that gravitational waves existed exactly as predicted by Einstein's general theory of relativity.

The Higgs boson is a boson (i.e. not fermion) but it's the only boson in the list that doesn't mediate a fundamental force. It's still very important in the scheme of the Universe because it guarantees that W-bosons, Z-bosons, (charged) leptons, and quarks are massive – via the Higgs/BEH mechanism. The Higgs boson was discovered last July.

enter image description here

Quarks differ by their carrying a color - interacting via the strong force (one mediated by gluons and described by QCD). Leptons don't carry any color so they don't interact by the strong force – which is the reason why their name, "leptons", is related to words like "skinny" in Greek.

share|cite|improve this answer
Nice chart! I especially like the spookiness of the Higgs field. :) And does it mediate a force or not...? Semantics really. – Michael Brown Apr 1 '13 at 15:01
So the two main types of elementary particles (force carriers and not) are bosons and fermions? Then people have just started to group them specially for certain things, like leptons for non-QCD-carrying particles? – KDecker Apr 1 '13 at 15:04
Dear Michael, it's a chart from Wikipedia although I would draw it very similarly if it were my job. ;-) I agree that to some extent, we could say that the Higgs field also mediates "forces" - by the Yukawa interactions etc. Also, it interacts with the W-bosons although these interaction terms are fully dictated by the electroweak symmetry, so they're a part of the electroweak force. But these "electroweak terms" still have new consequences and this new force - exchange of the Higgs - is what keeps the probability of WW scattering in the (0,100%) interval. – Luboš Motl Apr 1 '13 at 15:20
BumSkeeter: yes, there are 2 basic groups of elementary particles, bosons (integer spin, like each other) and fermions (half-integer spin, dislike each other as seen via the Pauli exclusion principle). But the number of particle types and their groups and subgroups and categories is huge and a short answer only sketches a very small portion of this issue. Here we're talking about particles that are elementary according to the state-of-the-art theories. But composite particles of today used to be thought of as fundamental ones in the past etc. – Luboš Motl Apr 1 '13 at 15:21
@BumSkeeter: note that fermions = matter particles and bosons = force carriers is somewhat misleading; we normally don't think of composite bosons as force carriers (except when we do, of course ;)) - iirc (and please correct me if I'm wrong), it's just that if we want to end up with a nice classical potential like the Yukawa one, the mediator must be bosonic, but there are also boson-boson interactions with fermionic mediators; a priori, the labels fermion and boson just tell us something about the particle's spin and their bulk behaviour (Fermi-Dirac vs Bose-Einstein statistics) – Christoph Apr 1 '13 at 16:39

Just as an add to Lubos Motl's answer:

The graviton is extremely hard to detect because the graviton - theorists suggest - exists in the 5th dimension. When the gravitons are in the 5th dimension, gravity is probably as strong as the other fundamental forces. However, on the graviton's 'journey' to our personally observable dimensions it loses this strength, which is why gravity is such a weak force. So really, the reason why gravitons are so hard to detect is because they exist in the 5th dimension naturally, and so far we've had no luck detecting them at particle accelerators and the such because the indirect observations that would suggest their existence are so small that they're hard to clarify as an observation of the graviton.

If you want to know why gravity gets weaker as it travels through the dimensions, see this question that I asked a while ago:

Why does the force of gravity get weaker as it travels through the dimensions?

share|cite|improve this answer
Why has this been downvoted? – Olly Price Apr 7 '13 at 16:17
I just downvoted (I'm sure the first downvoter also downvoted for the same reason. ) . Let me explain, "Gravitons exist in the 5th dimension.". Really? There is no specific "5th dimension", in the first place. There can be 5, or 7, or 8, or 6, or 10, or 11, or 26, or 248 (wait, what?!) dimensions, but there is no "5th dimension". Gravitons are not stuck on to any axis. Nor are they everywhere except our 4-d cross-section. I.e. this entire answer is incorrect from the premises. And graviton fields [($g_\mu\nu-\eta_{\mu\nu}$)], are very easily detected. Simple experiment: Jump out of the window. – centralcharge Jul 15 '13 at 16:42

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.