Tachyon

A tachyon is a hypothetical particle that always travels faster than light and cannot be slowed to light speed or below. No tachyon has been observed.

Spacetime: The four-dimensional structure combining space and time in relativity. Tachyon motion is defined by how its path sits relative to light cones in spacetime.
Tardyon: A hypothetical or idealized particle with positive rest mass that travels slower than light. Tardyon worldlines remain inside the light cone, opposite to the proposed tachyon's outside it.
Special relativity: Einstein's theory describing space, time, and motion when gravity can be neglected. The invariant light speed and causal structure of this theory define the tachyon hypothesis.
Causality: The principle that causes precede their effects and that physical influences respect a consistent causal order. Usable tachyon signals would threaten ordinary causal ordering between observers in relative motion.
Light cone: The boundary in spacetime separating events connected by light-speed signals from those that cannot be causally connected. A tachyon's path lies outside the light cone, unlike paths of ordinary massive particles.
Luxon: A massless particle that travels at the speed of light in vacuum. Luxons occupy the light-cone boundary, between tardyon and tachyon motion.
Gerald Feinberg: An American theoretical physicist who introduced the term “tachyon” in a 1967 paper. Feinberg developed the name and explored whether such hypothetical particles could be consistent with physics.
Lorentz invariance: The symmetry of physical laws under Lorentz transformations between inertial reference frames. Tachyon models must confront how observers disagree about the order of superluminal emission and reception.
Relativistic energy–momentum relation: The equation relating a particle's energy, momentum, and invariant mass in special relativity. Applying it to a tachyon requires a negative mass-squared parameter and yields counterintuitive energy behavior.
Cherenkov radiation: Light emitted when a charged particle moves faster than light travels through a particular medium. This real effect exceeds light's speed in a medium, not the vacuum speed that defines tachyons.

Linked from 4 pages