Faster-than-light

Lore & Background

Particles whose speed exceeds that of light (tachyons) have been hypothesized, but their existence would violate causality and would imply time travel. The scientific consensus is that they do not exist. According to all observations and current scientific theories, matter travels at slower-than-light (subluminal) speed with respect to the locally distorted spacetime region. Speculative faster-than-light concepts include the Alcubierre drive, Krasnikov tubes, traversable wormholes, and quantum tunneling. Some of these proposals find loopholes around general relativity, such as by expanding or contracting space to make the object appear to be travelling greater than c. Such proposals are still widely believed to be impossible as they still violate current understandings of causality, and they all require fanciful mechanisms to work (such as requiring exotic matter).

Reader's Guide

Superluminal travel of non-information does not violate special relativity. Some processes propagate faster than c but cannot carry information, such as daily sky motion (objects in the sky complete one revolution around Earth in one day, giving apparent speeds many times c), light spots and shadows (a laser beam swept across a distant object can seem to move faster than c, but no information travels faster), and closing speeds (two particles approaching each other at near-light speed in opposite directions have a closing speed approaching twice c, but special relativity gives the correct velocity-addition formula). Proper speeds (distance in Earth's frame divided by traveler's clock time) have no limit, but do not represent a speed in a single inertial frame. Phase velocity and group velocity can exceed c in some circumstances, but neither conveys information faster than light. Cosmic expansion causes distant galaxies to appear to recede faster than c, but this is a coordinate effect, not a relativistic velocity.

Did You Know?

• Only particles with zero rest mass (photons) may travel at the speed of light; nothing may travel faster according to special relativity.
• Tachyons, if they existed, would violate causality and imply time travel; scientific consensus is they do not exist.
• Closing speeds of two particles approaching each other at near-light speed can approach twice the speed of light, but special relativity prohibits this from being a relative velocity.
• Cosmic expansion causes distant galaxies to appear to recede from Earth faster than light, but this is not a velocity in a relativistic sense.