The time in science
The concept that science has of the time has undergone several changes throughout history. We can say that the first great theory is the one that is denominated habitually physical classic, and that Isaac Newton enunciated in his book Philosophiae Naturalis Principia Mathematica. According to it, the time determines the relation between the speed and the crossed space. The following formula may be well known by all of us:
Speed = d/t
A trip at the speed of light would run a variable space (for a traveller placed on the light beam the distance would appear different than for one off it) in a well defined time range. Hence the speed is different for each observer. Einstein claimed that, the speed of light is absolute and therefore it does not rely on the own observant's perception of it. This leads us to conclude that, since space is relative and different for every observer, time shall be likewise.
Special relativity, did not seem consistent with the theory of the gravitation of Newton. This problem challenged Einstein during some years, until in 1915 he was able to reconcile them in his theory of general relativity. This theory discovered that the space and the time intimately are bound. In fact the gravity has a direct influence in the time. Thus we could say that the parabolic movement of planets is equivalent to a rectilinear movement in the half-framed space-time, the planets follow what is denominated as a geodesic one.
Following this line of reasoning we reached the conclusion that time passes slowlier the closer to a massive object we are. In fact it is possible to be verified empirically that it exists a deviation between a clock located to a terrestrial surface level and one that is remote from it. This effect has repercussions in GPS systems. The satellites clocks are faster with respect to the ones located at terrestrial level according to the parameters defined by the theory of general relativity.