Massive objects and information cannot travel locally through space at or faster than the speed of light in a vacuum, . This limit follows from the two postulates of special relativity and the structure of spacetime. HL only: A.5 Galilean and special relativity.
Einstein's first postulate states that the laws of physics are identical in all inertial reference frames. The second states that every inertial observer measures the same vacuum light speed, , regardless of the motion of the source or observer.
To preserve these postulates, space and time coordinates must be related by the Lorentz transformations, which contain the Lorentz factor
As a massive object approaches , increases without limit. At , its denominator becomes zero, so no valid inertial frame exists in which a massive object is stationary at light speed. Reaching would also require an unbounded input of energy. Light can travel at because photons have no rest frame.
This is not merely a technological limitation: increasing engine power can bring a massive object closer to , but each further increase in speed demands disproportionately more energy. The barrier arises from spacetime itself, rather than from friction or imperfect machinery.
Relativistic velocities also do not add using ordinary arithmetic. If a spacecraft travels at and launches an object forward at relative to itself, Earth measures
not . The relativistic velocity addition equation ensures that combining velocities below still gives a result below .
A common misconception is that absolutely “nothing” can exceed . Distant galaxies may have recession speeds greater than because space expands, but they are not moving locally through space faster than light; information still cannot be transmitted locally faster than .
Exam technique: For an IB HL Explain question, state both postulates, use or relativistic velocity addition, and connect the mathematics explicitly to why is an invariant limiting speed.
