In special relativity, a faster-than-light particle would have spacelike four-momentum, unlike ordinary particles that have time-like four-momentum. While some theories suggest the mass of tachyons is imaginary, modern formulations often consider their mass to be real, with redefined formulas for momentum and energy. Additionally, since tachyons are confined to the spacelike portion of the energy–momentum graph, they cannot slow down to subluminal (slower-than-light) speeds.