Since Einstein’s introduction of his theory of relativity, this theory has been regarded as one of the most famous scientific theories of mankind. This theory that was formulated and presented to the scientific community as two complementary theories, i.e., special theory of relativity in 1905 and general theory of relativity in 1917, drastically changed the concept of space and time as compared to the old Newtonian classical mechanics’ assumption of the absoluteness of time and location. Since then, there has been a tremendous amount of effort by researchers all over the world to investigate and understand its profound meanings as well as its complicated implications. For about 100 years, the theory with its revolutionary predictions such as time dilation, length contraction, space curvature, the possibility of travel to the future, etc. has always been regarded as a highly technical topic with far-reaching concepts for the public. Well, this description of the theory is simply not true; indeed, there are many ways that the theory of relativity could be actually sensed and seen in our normal life.
For example, one of the most famous predictions of the theory is a phenomenon called time dilation. To make it simple, time dilation claims that an astronaut in a spaceship moving very fast relative to the Earth will measure the seconds ticking at a slower rate as compared to an earthbound observer, and, as a result, time essentially slows down for the astronaut. The other prediction is that gravity affects time too; i.e., any object in a strong gravitational field too will also experience time dilation.
This may seem somewhat weird, but indeed nowadays these predictions are basics to space agencies. They have to use relativity if they want to dispatch artificial satellites to Mars, Venus or comets. Without considering its effects, the orbit of planets could not be calculated correctly and, as a result, satellites would miss their aimed spots or even the whole planet; but what about our normal life?
Well, General Relativity is essential for the so-called Global Positioning System or GPS. This system is composed of 24 satellites flying in different orbits around the Earth. In order to determine positions, the satellites send their signals to the ground stations as well as GPS units such as those that you may have in your vehicle for positioning. Due to the difference of their speed as well as the difference of gravity on their orbit relative to an observer living on Earth, time dilation becomes a crucial factor for a precise determination of the position. Without getting into detailed calculations and just to have a sense of the accuracy involved in determining the position, it could be proved that only a one-microsecond mismatch of clocks between the satellites and ground stations would lead to a position error of one kilometer! As a result, the clocks in the satellites should be attuned to run more slowly by the calculated amount.
These adjustments are well explained and confirmed by the theory of relativity. Indeed, time dilation due to the combined effect of speed difference of satellites with observers on the ground, as well as the difference of gravity, reaches up to about seven microseconds. If no relativistic effects are taken into account, then your GPS unit that specifies your coordinate as half a mile (0.8 km) to your home would be five miles (8 km) off in only one day!