# Proper time: confusion over what it means

1. Jan 24, 2013

### quellcrist

Can anyone explain proper time with a simple example analogous to real life please? I am having a problem understanding it.

Thank you

2. Jan 24, 2013

### ghwellsjr

Be glad to. It's very, very simple. It's the time on any clock.

EDIT: By the way, it has to be an actual clock that has a built-in timing mechanism like a watch or battery-operated wall clock, not a clock that you plug into the wall which derives its time from the power-line frequency or like the "clock" on your cell phone which gets time sent to it by a cell phone tower or the "clock" on your GPS receiver which gets time sent to it by a satellite or the "clock" on your computer which gets time sent to it over the internet.

Last edited: Jan 24, 2013
3. Jan 24, 2013

### PAllen

Proper time is the time read by an (ideal) clock. More specifically, you have some world line representing the path through spacetime of some object or observer. If there is a clock having this history, proper time will describe the clock's readings.

The other common type of time in relativity is coordinate time. This may be made physically by imagining that someone has set up a system of clocks, synchronized by some chosen method. Then, for a given clock moving in some way past this system of reference clocks, coordinate time along the path is given by readings on the reference clocks that are passed. Proper time is the readings on the given moving clock.

4. Jan 24, 2013

### Chronos

Simply put, proper time is the time measured by an observer's clock. In general relativity proper time has no absolute meaning because it passes at different rates for different observers due differences in gravity and motion of the observers' clocks. The differences are usually vanishingly small, except in special cases - such as the presence of exceptionally powerful gravitational fields or involving extreme velocities [relativistic speeds].

5. Jan 24, 2013

### PAllen

I would say that proper time is the only type of time meaningful in GR. It is the reading on an actual instrument. No observers will disagree on what a particular clock reads. What becomes less meaningful in GR is coordinate time, because the process for assigning it is inherently much more arbitrary than for SR (flat spacetime).

6. Jan 24, 2013

### Staff: Mentor

proper time is invariant, so it does have absolute meaning. Coordinate time is the frame variant thing with no absolute meaning.

As far as which passes at different rates I think that is meaningless. The ratio of the rate of coordinate time to proper time changes from place to place, but you could just as easily attribute that to changes in the rate of coordinate time instead of proper time.

7. Jan 24, 2013

### robphy

"proper time" was formulated by Minkowski.
"proper".. in the sense of "ownership" (not correctness)

As others have said, it is the time read on a clock carried by an observer.
To drive the point home, Taylor&Wheeler call it "wristwatch time".
Bondi makes reference to a route-dependent "private time" (as opposed to a universal "public time").

Just as
an odometer measures a path-dependent distance (an arc-length) between two points in a plane,
a wristwatch measures a path-dependent proper-time between two events in spacetime.

8. Jan 24, 2013

### pervect

Staff Emeritus
Let me add that because Newtonian theory has a universal time, it's very tempting and common to attempt to import this concept into Special Relativity (SR). Unfortunately, this leads to numerous contradictions and confusions, due to the relativity of simultaneity.

While "proper time" depends on the exact path one traces through space-time, it turns out to be a fundamental thing that all observers can agree on once the path is specified. They won't share any common notion of "universal time", but given a specific path, they will agree on what the "proper time" is.

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