Where do relativity and quantum mechanics disagree?
It’s not known how relativity and quantum mechanics connect at very small distances, or equivalently, at very high energies. The relevant scale is the “Planck scale”, corresponding to energies of about GeV, or lengths of about 10 m. At such scales, there’s no good (and experimentally tested) theory of particle interactions. This is beyond the scope of this class but you may see it a few courses down the line.
