# How do you tell the difference between fundamental forces?

1. Jul 21, 2004

### sexwish

I wonder how do you distinguish the fundamental forces?

After all, they are each only a bunch of Newtons.

Last edited: Jul 21, 2004
2. Jul 21, 2004

### mathman

Gravity affects everything and is quite weak. Electromagnetic affects only charged particles and is quite strong. Both of these obey inverse square laws. The strong and weak forces do not obey inverse square laws. Strong force holds nuclei together, but is not normally observed on a macroscopic scale, except for neutron stars. Weak force shows up only in certain radioactive decays. Another way of looking at it is that they are all quite different.

3. Jul 25, 2004

there are four fundimental forces: gravity - extremely weak, electromagnetic - avg., stong and weak nuklear force. To distinguish these would be only separating them by strength. Otherwise, forces are all a branch of the same effect.

4. Jul 25, 2004

### mathman

There are a lot of other differences. What they act on, how they manifest themselves, and the behavior as a function of distance.

5. Jul 27, 2004

Yes, but the actual force is the same. Im talking about the actual force that can be felt.

6. Jul 27, 2004

### FZ+

They can be usually distinguished by the carrier of the force. Virtual photons for EM, gluons for strong, W for weak, and the elusive gravitons for gravity. The hope is, however, that under certain circumstances, it is not possible to distinguish between them, that they are all aspects of a single, fundamental force.

7. Jul 27, 2004

### daveed

8. Jul 27, 2004

### FZ+

If I am understanding you right, the color force is the same thing as the nuclear strong force.

9. Jul 28, 2004

### zefram_c

Hi all, this question is right up my alley.
This is a very Newtonian way of looking at the fundamental forces, and has to be replaced by quantum field theory A much better term for them is the fundamental interactions , for reasons I will explain in a moment.
Now the question is, what is an interaction? We postulate one whenever we see that a particle is affected by another particle in any way, shape or form, or when a particle spontaneously converts to other particles. Let's assume we start from zero knowledge. We carefully catalogue all observed interactions, and we look for similarities, patterns, etc (you'll see how in a second) Whenever we see such an effect that cannot be explained by our existing theories (which are all quite clear about what can go through and what cannot), we write down a new theory. We postulate the particle(s) involved carry some sort of CHARGE, that due to that charge they interact with some sort of MEDIATORS, and that such interactions can be expressed in terms of dimensionless COUPLING CONSTANTS. Sometimes the particles can carry arbitrary units of charge (electric charge is still not properly quantized, our theories do not explain why there should be fundamental charges of e or e/3 for quarks), sometimes those charges are fixed (quarks cannot carry arbitrary amounts of color charges). The coupling constant sets the inherent strength of an interaction relative to others, the individual charges allows the interaction to proceed at different rates for different particles.
Moving on, the best way to differentiate the theories is by what kind of interactions are allowed to proceed. The Newtonian picture of the interactions resulting in a push or a pull (ie forces) is only valid at low energies. At high energies, you get stuff like pair production/annihilation, higher order corrections and so on. Furthermore, some of those interactions allow particles to convert from one type to another: this definitely can't be expressed in terms of Newtons!

So here are the currently known four fundamental interactions:
-The EM force acts on electric charges and is mediated by massless photons. It permits production or annihilation of particle/antiparticle pairs from/to photons. The lowest order process in which conversions do not occur is the "single virtual photon exchange" (I can't get into Feynman diagrams now), and it results in Coulomb's laws etc. It is of medium strength.
-The strong nuclear force (aka color force) has the highest inherent strength and couples to color charges. Mediated by massless gluons, it keeps quarks bound within baryons (ie protons), and the remnant of the force binds baryons within atoms (like the core EM force binds electrons to nuclei and the remnant of the force binds atoms into molecules). It also allows particle/antiparticle production and annihilation via gluons. Since colored particles (quarks) are predicted to be confined to colorless composites unless you are at very high energies (billions of GeV which is HUGE), it results in forcing quarks into many types of baryons and mesons.
-The weak nuclear force has strength roughly equal to the EM force, but it appears much weaker due to the high mass of the mediators (W and Z). The W allows for quarks to change flavor or electrons to become neutrinos or vice versa, or various other neat stuff. The Z functions like a heavy photon; it can couple to anything the photon does, as well as neutrinos (which the photon does not). All known particles participate in this interaction.
-There are attempts to unify the three above forces into a Grand Unified Theory - personally I think this can and should be done, to explain some remarkable interplays between the interactions (which are highly technical and I cannot get into at this level), and to properly explain the quantization of electric charge.
-Gravity is different from all of the above. Our current theory (Gen Rel) has discarded the Newtonian concept of force (though it can be approximated as such), and cannot be properly quantized as the other three. GR explains gravity in terms of curvature of spacetime - something hard even for a professional physicist to picture. Particles always take the shortest paths through spacetime, and when spacetime is curved by the presence of matter, those paths lead to the particle falling towards the source of curvature. This is very different than all other theories. Huge attempts are underway to express this in a quantum form, and I think this can also be done. String theory is one way of going about it, but it's virtually impossible to test experimentally and comes with a lot of theoretical baggage like extra dimensions that we don't observe.

Anyway, the main point: the interactions are much more than a simple expression in terms of Newtons. All this is usually explained at the introductory level in first year textbooks, that's a good place for anyone interested to become acquainted to this fascinating field.

Last edited: Jul 28, 2004