# The relativity of time

Naoshi Sugiyama: didacticism to explain why time is relative |

Naoshi Sugiyama, a physicist from the Nagoya University, spoke about time according to the Einstein's special and general theories of relativity on March 9, at the Physics Workshop of the second phase of the Intercontinental Academia (ICA), in Nagoya.

Sugiyama's approach has been didactic and simplified for a proper understanding of the audience, with several participants from the humanities and the social sciences.

He explained that the special theory of relativity (1905) is based on two principles:

- Principle of Relativity, which states that all inertial frames of reference (moving at a constant speed) are equal;
- Principle of Invariant Light Speed, which is the same for all inertial frames of reference.

According to him, the understanding of these principles makes it easy to understand why time is relative and not absolute, as considered before Einstein's theories, which he quoted: "If the observer is still, the clock of a moving system beats more slowly." This is called dilution, Sugiyama said.

He added that in the General Theory of Relativity (1915) Einstein included the effect of gravity in the theory ("with the presence of strong gravity, time is also retarded") and established the equivalence principle, in which gravity and inertial strength can not be distinguished.

Regarding practical life, Sugiyama demonstrated how this dilution of time needs to be considered in the operation of a Global Positioning System (GPS). "It takes at least four satellites to determine x, y, z and t (the three spatial dimensions and time), and to calculate the distance from them by means of very precise measuring of time."

This precision is important because as the speed of light is 300,000 km / s if there is an error of a second the determined location will be at a distance of 300,000 km from where it actually is.

For the location to be identified with a margin of error of 10 cm, time needs to be measured with a maximum tolerance of 3/10 of a billionth of a second.

The effect of dilution by relativity implies the identification of spots on the Earth's surface outside their actual location: "In the case of special relativity, as satellites travel at high speed (4 km / s), the estimated location gets 25 cm away from the actual position at each second. In the case of general relativity, as gravity at 20,000 km high is weaker than that on the Earth's surface, the difference between the assumed location and the actual one is 160 cm."

To be precise, the GPS has to deal with the dilution of time caused by the satellite's speed and the weak gravity at the height of its orbit, said Sugiyama.