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Real World Physics Problems Newsletter - Syncing Clocks, Issue #41
April 26, 2017

Synchronizing Clocks for Precise Time Measurement

A few years ago in the news, it was reported that in an experiment, a beam of neutrinos (which are tiny sub-atomic particles) traveled faster than light. This however violates the well established law of physics stating that nothing can travel faster than light. It was later determined that part of the error in the calculations was due to a master atomic clock running too fast. Atomic clocks are used for very precise time measurements, such as on GPS satellites and when calculating the speed of light. As it turns out, the master atomic clock was running too fast by 125 nanoseconds per second.

When the speed of light was calculated in the neutrino experiment, it was based on the time measurements of two atomic clocks located in two different geographic positions - with each located on one end of the neutrino beam. In order for the calculation to be done correctly, the time on the two atomic clocks must be synchronized exactly. This is actually more difficult to do than you might think.

When an atomic clock, or any clock, is transported from one place to another, time dilation takes place. The effect of special relativity (caused by speed), and general relativity (caused by changes in gravitation, and acceleration - experienced by something when it moves) must be accounted for in the case of atomic clocks requiring high precision. If two clocks are initially synchronized, and one of the clocks is moved to another location, the two clocks will no longer by synchronized. This is caused by the effects of relativity, as mentioned. The time difference between the clocks will be extremely small, but this small difference will introduce an unacceptable amount of error in calculations requiring high precision.

In the neutrino experiment, at the instant the beam was emitted, the time on the clock at that location was noted. At the instant the beam was received at the other end (731 kilometers away), the time on the other clock at that location was noted. The time difference between the two clocks was recorded as the travel time of the beam. The clocks needed to be very precisely synchronized, and read the exact same time, in order for the travel time, and therefore beam speed, to be determined with high accuracy. But it turns out that the travel time of the neutrino beam, over the 731 kilometer travel distance, was measured to be 60 nanoseconds faster than if that same distance had been traversed by a beam of light - hence the faulty conclusion that the neutrinos traveled faster than light. The source of the time measurement error was due to a master clock running too fast, and a faulty cable connection was the other source of error.

To get a better understanding of the concept involved, consider a GPS satellite. Due to the effects of relativity, an atomic clock placed on board a GPS satellite must deliberately be made to run slightly slower than an earth-based atomic clock, so that when the satellite is orbiting the earth it will tick at the exact same rate as the earth clock, and be synchronized with it. This will allow both clocks to always read the exact same time, which is crucial for precise position measurements using GPS, which involves using the time readings on both the satellite clock and the earth clock.

There are basically two methods that can be used to synchronize two atomic clocks located in two different places on earth. The first way is to determine the exact motion the clocks experience as they travel to their two different locations, and then apply physics calculations to offset the ticking rates, so that they will be in sync afterwards.

The second method, which is what was used in the neutrino experiment and is perhaps the easiest way, is to use a master atomic clock which synchronizes the two clocks.

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