Time is not universal: how speed and gravity bend it
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.
What two things change the rate at which time passes for you? Show answer
How fast you are moving, and how deep you are in a gravitational field.
How sensitive are modern clocks to this effect? Show answer
They can detect time running differently when a clock is raised just two feet.
Why does GPS need relativity? Show answer
Satellite clocks are slowed by speed and sped up by weaker gravity. Uncorrected, positions would be off by about six miles in a day.
How much younger is an astronaut after a year on the space station? Show answer
About a hundredth of a second, because their speed is the dominant effect.
Why can nothing with mass reach the speed of light? Show answer
The energy needed to accelerate it grows more and more as it nears light speed, and becomes infinite at the limit.
What do protons in the Large Hadron Collider show? Show answer
At their speed distances shrink about 7,000 times, so the 27-kilometer ring is about four meters across to them.
What is Brian Cox's Andromeda example? Show answer
A ship near light speed could cross two million light years in a minute of its own time, while millions of years pass on Earth.
Is time travel possible? Show answer
Into the future, yes, by moving very fast. Into the past appears forbidden, though that is a conjecture.
What was Minkowski's insight, as Sean Carroll tells it? Show answer
Space and time blend into one four-dimensional spacetime with no fixed division between them.
Why does the traveling twin come home younger? Show answer
Time is the length of a path through spacetime, and the straight path of the twin who stays takes the longest time.
What is gravity in general relativity? Show answer
The curvature of spacetime.
What does relativity not tell us? Show answer
What time is. It only says how elapsed time depends on motion and gravity.