Physically, what do positive frequency modes represent?

[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?

 

[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.

 

[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.