Physically, what do positive frequency modes represent?

1. Oct 16, 2012

jarod765

I know this is a basic question, but why are positive frequency modes so important when expanding a field operator. Furthermore, what do they represent?

2. Oct 18, 2012

Demystifier

A mode with a positive (negative) frequency corresponds to a particle with positive (negative) energy. Obviously, in nature we only see particles with positive energy.

3. Oct 18, 2012

Bill_K

A positive frequency mode of a field operator represents the creation of a positive energy particle. A negative frequency mode of a field operator represents the annihilation of a positive energy antiparticle.