Time Dilation Near Black Holes
The basic idea (time depends on the path through spacetime)
In relativity, “time” is not a universal global clock that everyone shares. Each observer measures their own proper time along their worldline. Gravity changes how proper time accumulates compared to far-away clocks.
That’s what “time dilation” means here: two observers following different paths (different heights in a gravitational field, or different velocities) will generally accumulate different amounts of proper time between the same two events.
Different observers can disagree—and both be right
A classic example is comparing a far-away observer’s coordinate time to the proper time experienced by someone hovering closer to a massive object. The details depend on the spacetime metric and the observer’s motion.
This is why statements like “time stops at the horizon” need context: which observer? which definition of time?
What happens near the event horizon?
In many coordinate systems used for distant observers, signals from an infalling object can appear increasingly redshifted and delayed as the object approaches the horizon. But the infalling object’s own proper time is not “frozen” in the same way.
This is one reason black holes are such a good teaching tool: they force you to be precise about what is being measured.
Common misconceptions
- “Time literally stops at the event horizon.” In many cases this is a statement about a particular coordinate time for a distant observer, not a universal fact.
- “Time dilation means you can automatically time travel.” Time dilation affects elapsed time comparisons; it doesn’t provide a simple mechanism for arbitrary time travel.
- “A movie number must be physically real.” Films often choose dramatic values; real physics depends on mass, spin, orbit, and assumptions.
Learn more on this site
- FAQ section (includes an Interstellar section; treat movie numbers as storytelling unless derived and justified)
- Minkowski spacetime basics
- Deriving the geodesic equations
- Event horizon context in the black hole chapter