# What are “Force Carriers”?

The concept of "force carriers" is hard for me to understand. I can understand "energy carriers". I can understand mass x acceleration but I can't see how this applies. Does anyone have a descriptive definition? Something more classical?

Perhaps "force carrier" is QM shorthand for a more complex concept.

Gravity is a force that can be described in terms of geometry. I can understand that.

In the standard model of physics, the SU(3)xSU(2)xU(1), this is the table of particles that enter into the Lagrangian: The red column has the "carrier of the forces", gluon the strong force, gamma the electromagnetic,Z and W the weak. They are the gauge bosons which have a mass of zero in the unified theory , before symmetry breaking by the Higgs field.

They are exchanged in the first order Feynman diagrams of the interactions and in this sense characterize most of the crossections attributed to that specific fundamental force, that is why they are called "force carriers", they transfer momentum between interacting particles to first order and force is ~dp/dt.

A classical analogue is the boomerang and the boat analogy, see my answer to this question.

• Thank you (and others) for the answers. I'm studying anna v's answer and will get to the others later. As of now, the term Force Carrier is still hard for me to grasp. I think my confusion is that "force" as I know it is not the kind of entity that can be "carried." I promise to work hard to get it. – Incredible II Feb 17 '16 at 3:10
• I would swear that the "red column" is actually orange. – ohwilleke May 1 '19 at 5:47
• @ohwilleke it depends on your laptop pixel response to the color codes. I think they wanted red blue green, i get pink violet green., symbolic upload.wikimedia.org/wikipedia/commons/thumb/2/2c/… – anna v May 1 '19 at 5:49
• @IncredibleII if You are familiar with Green functions and understand classical field theory, You can try to read the first two chapters (I don't remember exactly) of Zee, "QFT in a nutshell. Zee derives how photon exchange creates the force between two charges. – Artem Alexandrov May 1 '19 at 7:56

When particles experience a mutual force, it can be thought of in a couple of different, but equivalent ways. The first is in terms of a field that produces a force depending on the coordinates in space and time. The second is in terms of the exchange of other particles that mediate the force.

For example, scattering between two electrons can be understood as one electron emitting a photon which is absorbed by the other, transferring momentum.

Force carriers are the fundamental particles that describe these interactions. In general they are bosons, i.e. particles with integer spin.

For example the gauge bosons are force carriers in the standard model of particle physics:

Particle          Corresponding force
-------------------------------------
Photon            Electromagnetic
Gluon             Strong nuclear
W and Z Bosons    Weak


Force carriers is a concept from quantum field theories. There are two basic classifications for fundamental particles: fermions, which are those which obey Fermi-Dirac statistics, have spin that is an odd multiple of 1/2; these contrast with the bosons, which obey Bose-Einstein statistics, and have integral spin.

Examples of fermions are electrons, neutrinos, quarks - all spin 1/2.

Examples of bosons are photons and gluons - all spin 1; they also have no mass.

The bosons are also known as force carriers, while the fermions are used to build up material systems.

The both carry energy and momentum. You can find endless amounts of information for bosons and fermions at www.theparticleadventure.org

• So it sounds like the term "force carriers" is not at all related to classical mechanics, is purely a quantum concept, and that I shouldn't worry about it. I'm good with that. Thanks. – Incredible II Feb 14 '16 at 2:05
• Actually, you can introduce 'classical' force carriers too, as long as statistical fluctuations of the effective forces mediated by these particles are small. In a sense, water molecules can be thought of as force carriers in a fluid (large fluctuations would then correspond to statistical turbulence). Heavy particles in the fluid would then interact through induced flow. – TotallyRhombus Feb 14 '16 at 2:26

Materia consist from particles. The energy exchange between materia happens by the emission and absorption of photons. This you can prove for all physical and chemical processes except processes inside the nucleus, where the quarks interact through gluons.

But how to deal with an electrostatic field and with a magnetic field? Both fields exist without any energy transfer (if suppose that both phenomenon are intrinsic properties and exist without our observation or any other interaction). Approaching two equally charged particles together, the energy content of the electrostatic fields is not changed, but the fields get deformed and by this they store energy (as one see when release them, they immediately repel each other).

To explain this phenomenon it was introduced the force carrier. No energy exchange but force exchange. For magnetic fields this is much harder to explain such exchange since the field is closed in circles because magnetic monopoles do not exist. The concept of force carrier seems to be a little bit helpless since every force is based on energy exchange.

Fields have to be understood as having an inner structure and consist of quanta. When instead of photon exchange there would be a changing inner structure of quanta clusters. Photons are nothing else than emitted clusters from this quanta and arise when the inner structure of the fields is not able to transform itself. But it seems that to think about the inner structure of electric and magnetic fields and the electromagnetic radiation is not worth. Anyway, it happen to me and I was free to elaborate it: Complex one-dimensional structures of space.