Real Scalar Field Fourier Transform

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SUMMARY

The Fourier transform of the quantized real scalar field \(\phi(x)\) is expressed as \(\phi(x) = \int \frac{d^3k}{(2\pi)^3 2k_0} \left( a(k)e^{-ik \cdot x} + a^{\dagger}(k)e^{ik \cdot x} \right)\) in texts such as Peskin and Schroeder and Ryder's QFT. This formulation includes both the annihilation operator \(a(k)\) and the creation operator \(a^{\dagger}(k)\), which together ensure that the field remains real. The presence of both terms is essential as the second term acts as the complex conjugate of the first, resulting in a real-valued field when combined.

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Homework Statement



Silly question, but I can't seem to figure out why, in e.g. Peskin and Schroeder or Ryder's QFT, the Fourier transform of the (quantized) real scalar field \phi(x) is written as

\phi (x) = \int \frac{d^3k}{(2\pi)^3 2k_0} \left( a(k)e^{-ik \cdot x} + a^{\dagger}(k)e^{ik \cdot x} \right) .

It's just weird because the Fourier transform usually only has one term, and one coefficient.

Thanks for any help.

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The Attempt at a Solution


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It just makes the field real. Note that the second term is the complex conjugate (or the hermitian conjugate once the coefficients have been promoted to operators) of the first term and in general, adding an expression to its complex conjugate gives something real.
 
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