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Clarification of distinctness of fundamental forces/interactions

  1. Mar 16, 2012 #1

    my questions are regarding the fundamental interactions. I don't have much physics knowledge beyond high school but am trying to form my own way of understanding these fundamental interactions. So I have some questions. They may be vague/strange so I will try to clarify if so (though I don't know for certain what exactly I'm looking for as an explanation anyway - leads will do :) )

    Basic theme to questions: how do we know that each of the fundamental interactions is indeed a single thing, and what does it mean to unify seemingly different forces?

    - the strong interaction is framed as the force holding quarks in a nucleon. But it is also the force which binds nucleons together in the nucleus. In this sense it is said to be 'residual' from the former use. How do we know that the force holding a nucleon together is the same force holding different nucleons near each other, and what does it mean for this effect to be 'residual' (is this like van der waals forces from chemistry)?

    - the weak interaction supposedly has two types: a charged and a neutral interaction. How do we know that (or in what sense are) these are the same force?

    - the electromagnetic force is said to be unified with the weak interaction as the electroweak force. What does this mean and why are the forces still treated as distinct then? It says something
    Last edited: Mar 16, 2012
  2. jcsd
  3. Mar 16, 2012 #2


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    Welcome to PF!

    That is an excellent analogy: the strong force is described by a theory known as "Quantum Chromodynamics", or QCD, which describes the interactions of quarks with the gluon force carrier. These are the forces that keep quarks bound in nucleons and other similar particles. The "residual" force holding nucleons together can be thought of as the van der Waals-QCD force (where the the van der Waals force of chemistry is really electromagnetic in origin).

    To some extent there is some semantics here. The "Weak Nuclear Force" of Enrico Fermi's day only had a charged interaction, mediated by a W boson (W for "weak"). Over the years, however, we realized that there was another neutral interaction mediated by a Z boson. This did not come easily but after years of study and debate. The issue was finally resolved with the LEP machine at CERN (the predecessor to the LHC) which positively identified these interactions.

    It is hard to explain without some extra knowledge the extent to which these two interactions are grouped into a single "weak force". It has to do with something called "Gauge Symmetries". At first glance it might make sense to think of these as two SEPARATE forces ("charged-weak" and "neutral-weak"), which have some funny relationships between the "charges" of particles under each force.

    At very low energies, the electromagnetic and (both) weak forces are really different forces. But when you start cranking up the energy of the interactions (particles colliding at very close to the speed of light, for example), something funny happens: the charges of the weak and electromagnetic forces become equal, and the W and Z bosons, which are usually VERY heavy, behave as though they were massless, just like the photon. It is in this sense that we say that weak and electromagnetic forces "unify at high energies." But as long as we are at low energies (like in the everyday world) these forces are quite distinct.

    Hope that helps. Keep asking questions!
  4. Mar 23, 2012 #3
    hi, thanks for your reply!

    good to know i had the right idea with the van der waals analogy (actually it does mention this on the wikipedia page for strong nuclear force, maybe i read that and forgot i had read it >.>)

    also, very fascinating notion that there are two variants of the same basic mechanism with the weak interaction.

    and even more fascinating that at high energies the weak and EM force coincide.

    -what else is happening in such extreme conditions? do other strange things happen to masses or flavors or such? what is a good resource to learn about what happens at high energies?
    Last edited: Mar 23, 2012
  5. Mar 23, 2012 #4
    Exactly what happens at high energies is still being researched. This is what we built the Large Hadron Collider for.

    You've probably heard about the searches for the Higgs boson, which is another particle predicted by the electroweak unification theory. Latest experimental results are compatible with this existing and having a mass of about 125GeV, but we don't yet have sufficient data to say for sure - at this stage the results could still be explained by a statistical fluke, a bit like that of throwing two consecutive double sixes in Monopoly.

    In fact, the term "unified electroweak force" is a slight misnomer, because this theory actually starts out with two forces, whose corresponding particle charges are called "weak isospin" and "weak hypercharge", and (at the energy scale at which the forces become unified) still have different strengths. The standard model cannot predict these force strengths, nor why the weak isospin force only affects left-handed particles, so there may well be further phenomena at even higher energies.

    We do know some things, for example masses change incrementally through a process we call "renormalisation". But the maths of this are quite advanced and, personally, I'm no expert.

    Various candidate theories have predicted that new particles will appear at higher energies, most notably "superpartners" of the existing particles. These are now being looked for at the LHC, though no indications of any of these have yet been seen. The results of these searches will in due course let us know which of these theories are or aren't compatible with the observed facts.

    There is also the unresolved question of what constitutes the "cold dark matter" that cosmologists have estimated makes up about 20% of the contents of the universe. If superpartners exist then one/some of them would be good candidates for this.
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