Special relativity indicates that, for an observer in an
inertial frame of reference, a clock that is moving relative to them will be measured to tick slower than a clock that is at rest in their frame of reference. This case is sometimes called special relativistic time dilation. The faster the
relative velocity, the greater the time dilation between one another, with the rate of time reaching zero as one approaches the
speed of light (299,792,458 m/s). This causes
massless particles that travel at the speed of light to be unaffected by the passage of time.
Theoretically, time dilation would make it possible for passengers in a fast-moving vehicle to advance further into the future in a short period of their own time. For sufficiently high speeds, the effect is dramatic. For example, one year of travel might correspond to ten years on Earth. Indeed, a constant 1
g acceleration would permit humans to travel through
the entire known Universe in one human lifetime.
