Physically, what do positive frequency modes represent?

jarod765
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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?
 
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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.
 
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.
 

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