# Reaching a consensus on what time is:

1. Oct 7, 2006

### kmarinas86

Many often speak of time as if it exists. Consider the following:

The time subset in a period of time
• By choosing a time on a digital clock, such as 2:45 PM, we are chosing one point of time out of a 24-hour period.

Defining the unit of time
• Perceived time is relative to clocks. The standard for the second is rooted in a quantum mechnical phenomenon involving cesium-133 (9,192,631,770 "cesium periods" = 1 second).

Cesium period - my short hand for "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 caesium-133 atom".

(9,192,631,770 cycles/second) / (299,792,458 meters/second) = 30.663319 cycles/meter

A thought experiment
• Two identical highly-acclaimed physics laboratories in the same building independently measure the speed of light. The second one is 20 stories above the first one. Each laboratory has its own atomic clock to determine when a meter has been reached, that is to say, where the light is after 30.663319 "cesium periods" after the light has left a source. They mark this by placing precision markers at this distance. When the experiments are completed, the two laboratories compare their results (the meters they came up with), and they are the same! Then a bystander suggests that the two laboratories decide to synchronize their experiments so that the first laboratory's experiment begins and ends at the same time as the second laboratory's experiment begins and ends. The experimenters, however, point out that this is not possible, because, even though the laboratories are identical, they are located at different floors so the one above cannot possibly end when the one below ends if both begin at the same time. Regardless, the experimenters from both laboratories run each experiment again, but at the same time, and the resulting meters from both laboratories remain the same and that is satisfying to them.

An uneducated peep then suggested that if the clocks run at different rates in each laboratory and if the light still traveled the same "distance" then the laboratory below must take a larger slice out of space time than the one above. His reasoning was that if a meter at the second (upper) laboratory corresponds to 30.663319 cesium periods at the second (upper) laboratory which is briefer than 30.663319 cesium periods at the first (lower) laboratory, then length of the space-time slice for the identical laboratories would be proportional to each's time dilation.

The scientists say the man's reasoning from his proposition to his implication is a mistake, but they do not offer a reason.

Question to those willing to reply: What is this reason?

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Last edited: Oct 7, 2006
2. Oct 7, 2006

### Staff: Mentor

Any respectable scientist would offer a reason.

The reason/answer to the apparent discrepancy is simple: a clock is at rest with respect to itself. The "uneducated peep" is assuming that there exists an absolute state of rest.

3. Oct 16, 2006

### Exitsign

I am just wondering what level of education you start to learn things like that at.. I'm in grade 10.