The shift of a quantum energy level in an atom owing to the atom not residing in an environment at absolute zero temperature. Objects emit radiation whose wavelength depends on the object's temperature. At room temperature, the wavelengths emitted are in the infrared region of the electromagnetic spectrum. Even in the most remote section of intergalactic space, an isolated atom is still subject to radiation in the form of the cosmic microwave background. Uncertainties in the blackbody radiation shift impose limits on the accuracy of the best atomic clocks.