# Strong force

1. Jan 16, 2008

### wolram

Is the strong force (carried) by elemenary particles or does this force exsist every where in the universe, so that no matter where particles are the strong force can act on them,
sorry if this is a crapy question but i do not know.

2. Jan 17, 2008

### malawi_glenn

I did not understand, you are wondering if the Strong Force has finite or inifinte range?

Strong Force is between hadrons (colorless composite particles), and there can be no gluon emission bewteen them. So one model/theory is that mesons are the force mediators of the strong force. But this is under theoretical development, the nature of the strong force is (one of) the most challenging problems in physics right now.

The Strong force is indeed short ranged, approx 1,5 - 2 fm (about the size of a proton/neutron). This is one of the reasons why you can build a bound nucleus with more than A' nucleons, there exists one max A' (where A is the sum of nucleons in a nucleus)

The EM force has infinte range, but has force carriers (photons). So there is not a conflict between having force mediators and having infinte range.

3. Jan 17, 2008

### wolram

Thank you Malawi, I am trying to understand how the strong force (finds) particles to act on, if put i some particles in a remote part of space will the strong force find them straight away (strong force is every where in space) or do the particles carry the strong force with them ( the strong force is part of every particle), thanks.

4. Jan 17, 2008

### malawi_glenn

It just works as "ordinary" forces...but is short ranged, it ONLY reaches 2fm from the proton/neutron/hadron.. and an atom is appor 100 000 fm, so the strong force is NOT everywhere.

Around each particle, there is a field, a field of virtual force mediation particles. An electron has a lot of virutal photons, a quark has a lots of gluons and Z and W's etc.

The hadrons (bound quark systems) must have another thing than gluons, since they are colorless. Gluons transfer "color-charge". And as I said in my last post, this is one of the most challenging problems in physics today, and have been for a loooooong time. ;) =)

Nuclei are built under very dense regions, in stellar interiors and in Novae and Supernova explosions, where the nuclei have so high energy that there is high probability that they will meat. Here on earth we must do this experimentally with beams etc.

So out side of stars and SN's we only have the strong force in the very very inner part of the atoms. And since the spacing between the electron "cloud" and the nucleus are so big, there is no interation between the nuclei of different atoms in solids and (low temperature) gases.

5. Jan 17, 2008

### wolram

Thanks Malawi, i understand now.

6. Jan 17, 2008

Question. Suppose two elementary particles at a distance 9.0 x 10^5 km apart in the universe, and they are moving away from each other at "c", the speed of light. How can the EM force carrier, the photon, have effect on them ? Is not the EM force limited by the distance that can be transversed by a photon when moving at "c", ~ 3.0 x 10^5 km/sec, and since it is so limited it must be finite in its range, for the infinite is never limited by anything. Or, are you saying the a photon can move faster than "c" if it must carry the EM force between two elementary particles that are greater than ~3.0 x 10^5 km apart and moving away from each other--this seems to be the only way the range of the photon can be infinite. What am I missing here ?

7. Jan 18, 2008

### malawi_glenn

nope they are not moving away from each other at c, you must the relativistic addition of velocities theorem when adding relativistic velocities.

So what you have missed is just elementary knowledge about Special Relativity :)

That the range of the EM force is infinite, you can see from the Coloumb law, it never gets zero, it converges to zero.

Also since the photon is massless, it moves at c. And not even when you are having one particle moving at -x direction at 0.8c with respect to the coordinate axis, and another moving at 0.8c at +x direction with respect to the coordinate axis, their relative velocity is NOT 1.6c... but 0.976c.

Also the virtual force mediators exists during a time according to the Hesienberg Uncertanty principle:
$$\Delta E \cdot \Delta t \sim \hbar$$
The greater enerrg/mass of the virtual particle, the less time it can exisits, and shorter way it can travel.

Last edited: Jan 18, 2008
8. Jan 19, 2008