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I am new to high energy physics and string theory. I keep reading the terms infrared and ultraviolet in papers. I assume they aren't talking about electromagnetic radiation. For example, one paper talks mentions "infrared physics". What do these terms mean in general?

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Infrared = Low-energy/momentum

Ultraviolet = High-energy/momentum

You're right, it's not only used to describe EM radiation in this context. So "infrared physics" will be physics which is only valid at low energies (typically, in scattering with low-energy incoming particles in the center of momentum frame).

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