In Einstein's theory of relativity, space and time are not separate entities. They are woven together into a single four-dimensional structure called spacetime. Because an object moves through both space and time at this one unchanging speed, its total "spacetime speed" must be divided between the two.
- Moving through time: When a person is sitting perfectly still in a chair, they are not moving through space at all. Instead, 100% of their spacetime speed is dedicated to moving forward through time.
- Trading time for space: As a person stands up and starts moving, or boards a fast rocket, they begin moving through space. Because their total speed through the fabric of spacetime cannot change, some of their motion through time must be diverted into motion through space. This is the mechanism behind time dilation—time actually ticks slower for objects moving at high speeds because they are sacrificing "time speed" to gain "space speed."
The absolute maximum speed a physical object can achieve through space occurs when it diverts all of its available motion away from time. At that extreme threshold, time stops completely for the object, and 100% of its motion is directed purely through space. This maximum spatial velocity is what people call the speed of light.
A spacecraft cannot go faster than the speed of light through space because it is mathematically and physically impossible to give up more than 100% of its motion through time. Once an object is moving through space at light speed, its clock stops ticking. There is simply no more time left to trade for speed.