Time is not universal: how speed and gravity bend it
October 6, 2026 · 16 min read
Michelle Thaller, Brian Cox and Sean Carroll each explain how relativity makes time run differently depending on motion and gravity, and Cox goes on to what that means for time travel. Three explanations of the same counterintuitive fact make it easier to grasp and to separate real physics from science fiction.
The rules, numbers and steps worth keeping in front of you.
- There is no universal clock; elapsed time depends on speed and gravity.
- Moving fast makes less time pass for you.
- Being deeper in a gravitational field slows your clock; being higher speeds it.
- A clock raised two feet runs measurably differently.
- GPS corrects for both effects; uncorrected, positions drift about six miles a day.
- A year on the space station leaves an astronaut about a hundredth of a second younger.
- Nothing with mass can reach light speed; the energy required grows without limit.
- From a fast traveler's view, distances shrink.
- Cox: a near-light-speed ship could reach Andromeda in a minute of its own time.
- Travel to the future is allowed; travel to the past appears forbidden but is unproven.
- Carroll: space and time are one spacetime, and time is the length of a path through it.
- The straight path takes the longest time, so the traveling twin ages less.
- Gravity is the curvature of spacetime.
- Relativity says how time behaves, not what time is.
This is the short form. The full insight has every expert's reasoning, their own words and the sources.