# Exchange Forces

All four of the fundamental forces involve the exchange of one or more particles. Even the underlying color force which is presumed to hold the quarks together to make up the range of observed particles involves an exchange of particles labeled gluons.
Such exchange forces may be either attractive or repulsive, but are limited in range by the nature of the exchange force. The maximum range of an exchange force is dictated by the uncertainty principle since the particles involved are created and exist only in the exchange process - they are called "virtual" particles. Such exchange forces are often pictured with Feynman diagrams.

### graviton

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# Range of Forces

If a force involves the exchange of a particle, that particle has to "get back home before it is missed" in the sense that it must fit within the constraints of the uncertainty principle. A particle of mass m and rest energy E=mc2 can be exchanged if it does not go outside the bounds of the uncertainty principle in the form

A particle which can exist only within the constraints of the uncertainty principle is called a "virtual particle", and the time in the expression above represents the maximum lifetime of the virtual exchange particle. Since this exchange particle cannot exceed the speed limit of the universe, it cannot travel further than c times that lifetime. The maximum range of the force would then be on the order of

### Calculation

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# Range of Exchange Force

Using the approximate range expression arising from the uncertainty principle and the speed of light, an exchange particle of mass

m = x10^ kg = MeV/c2 = GeV/c2
 m = =

 could give a range of :

Range = x10^ m = fermi* = proton radii**.

The range expression can be explored numerically by entering masses or rest mass energies, like those of the pions below. A range can be entered to calculate the exchange particle mass associated with such a range.

* 1 fermi = 10-15 meters.
**calculated radius using nominal nuclear density 2.3 x 1017 kg/m3.
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# Pion Range of Strong Force

An estimate of the range of the strong force can be made by assuming that it is an exchange force involving neutral pions. When the range expression

### Calculation

is used with a pion mass of

p0 mass = 264 me = 135.0 MeV/c2
the range obtained is
Range = 0.73 x 10-15 m = 0.61 x classical proton radius.

This range is in the neighborhood of one fermi. When Hideki Yukawa was working of a theory of the strong force, he judged that the range of the nuclear force was about a fermi, and calculated that the exchange particle should be in the neighborhood of 100 MeV in mass equivalent. This triggered the search which led to the discovery of the pion. When the nuclear particles are very close together, other heavier particles must also be included in this type of model of the strong force. The current view is that the strong force is fundamentally an interaction between quarks, and that the strong force is really a residual color force.

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