This was a statement given in my class:

"Strangeness is conserved in the strong and electromagnetic interactions, but not in a weak interaction "

But could someone please tell me how we differentiate between a weak and a strong interaction? An example would be helpful.

  • $\begingroup$ Can you clarify what you are asking. Are you asking how we distinguish between the interaction in a scattering experiment, or are you asking how the fundamental mechanisms differ? $\endgroup$ – John Rennie Jun 11 '14 at 9:18
  • $\begingroup$ There is a sense in which this is like asking how we differentiate gravity from electromagnetism. We differentiate them by the different laws they follow and the different effect that arise from them. $\endgroup$ – dmckee Jun 11 '14 at 20:19

This link is useful generally





We differentiate them by the strength. The strength of the interactions has been measured experimentally and experiments are the basis on which we classified them as weak and strong, and fitted them with the coupling constants seen in the table.

Relative to the strong interaction the weak one is $10^{-6}$ in coupling strength. The couplings enter squared when calculating probabilities from the state functions.

If a strong reaction is allowed, it will flood out a weak one. Thus we see the effect of the weak interaction only where a strong one is not allowed.

Neutron decay is the classical example and where the weak interaction first was observed. In general, the typical lifetime for the strong interactions is less than $10^{-23}$ seconds, whereas that for the weak interactions can be anything from $\sim 10^{-11}$, to years, depending on the isotopes.


The way you distinguish between different interactions in the universe is by looking at the mediator bosons thereof, which appear when scattering two particles under this or that interaction.

Take two electrons as example and scatter them up: if you identify that photons take part to the process then it is an electromagnetic interaction; if you find out that gravitons or weak vector bosons do, then it is a gravitation or weak one, respectively. In general all the interactions will be present at the same time and just by looking at the strength does not in principle allow to distinguish amongst them all (you may always create a very "strong" weak interaction and a very "weak" strong interaction by changing the parameters at your disposal).


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