# Measurement of time

1. Jan 10, 2014

### sydneybself

According to "Physics for Engineers and Scientists by Ohanian and Markert, a second is the time needed for 9,192,631,770 vibrations of a cesium atom.

In 1971, Hafele and Keating put four cesium-beam atomic clocks aboard commercial air liners which were flown around the world - two eastward, two westward and compared their times with two clocks which remained at the airport. After the test, the three pairs of clocks disagreed with one another. These results are supported by the experiments done with muons.

How long is a second, really? (if the question has a real[?] meaning)?
Wouldn't clocks on Mars keep different times from those on earth?
What about the length of a meter, which is based on the measurement of the speed of light?

2. Jan 10, 2014

### Student100

To quote wiki

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom—pretty much verbatim from various texts.

Last edited: Jan 10, 2014
3. Jan 10, 2014

### Staff: Mentor

Yes. That's because a different number of seconds elapsed on the various journeys, and the scientists traveling along with the clocks aged by (imperceptibly) different amounts one the journey. This despite the fact that they started at the same place at the same time and ended at the same place at the same time.

So....

Just as you said.... long enough for a cesium atom under your nose to undergo 9,192,631,770 oscillations. And therefore:

Yes. Not very different, but occasional corrections would be needed if you wanted to keep a clock on Mars and on earth synchronized. For that matter, we have to worry about this effect with the GPS satellites in orbit around earth.

The definition of the meter is subtly different from what you've said: it is defined as the distance that light travels in a particular time (which is of course given in seconds so is rooted in the cesium atom definition of the second). Thus, I can construct a one-meter measuring rod without knowing the speed of light, just by seeing how far light travels in a given time.