# How can you determine if a reaction occurs by electromagnetic, strong or weak force

1. Jan 2, 2009

### j-lee00

How can you determine if a reaction occurs by the electromagnetic, strong or weak force by looking at the reaction equation

2. Jan 2, 2009

### humanino

Re: How can you determine if a reaction occurs by electromagnetic, strong or weak for

Which equation are you referring too ?
Do you mean something like "$e\gamma\rightarrow e\gamma$" ?
If so, are you familiar with Feynman diagrams ?

3. Jan 2, 2009

### j-lee00

Re: How can you determine if a reaction occurs by electromagnetic, strong or weak for

For example P + P -> P + P

This is just one example. but is there a set of rules that can be followed to come to the correct outcome each time

4. Jan 2, 2009

Staff Emeritus
Re: How can you determine if a reaction occurs by electromagnetic, strong or weak for

The process you wrote can occur via any of the three interactions.

There is a set of rules - humanino points you to them, in terms of what are called Feynman diagrams. However, I don't think there's a shortcut to figuring out what goes with what processes other than learning the details of each theory.

5. Jan 3, 2009

### clem

Re: How can you determine if a reaction occurs by electromagnetic, strong or weak for

Generally, you would use selection rules.
If the reaction is allowed in strong interactions, it will interact that way.
If it cannot go by strong interactions, then it will go by EM interactions
UNLESS it violates an EM selection rule. Then it would be weak.
You can also judge by reaction time, tau, for decays.
For strong, tau~10^-23 sec.
For EM, tau~10^-20 sec.
For weak, tau~10^-8 sec.
These times are very approximate.