I don't think there is a problem with the level of participation here anymore than there is one with the Math and Chemistry sites.
The problem lies with the attitude of some people who started out thinking that Physics Stack Exchange would somehow be the next Math Overflow. No, it's ended up ticking over nicely just like other popular academic sites on the Stack Exchange network, open to a wide range of people including enthusiasts.
However, if you want to "improve" participation more towards Math Over Flow level, then alienate those at the lower level by excluding them from asking questions, and banning homework questions. That way you'll improve participation of professionals , while reducing questions/day and therefore overall participation:
- Mathematics Q&A for people studying math at any level and professionals in related fields 494 questions/day
- MathOverflow MathOverflow Q&A for professional mathematicians 42 questions/day
We need to face facts, physics isn't as popular as mathematics, and the last thing we want to be doing right now is pushing it too far in a direction that will promote its demise, just as for the failed Theoretical Physics site.
- Physics Q&A for active researchers, academics and students of physics 51 questions/day