Methods of keeping track of time date back to ancient days, but it wasn't until the 19th and 20th centuries that the science behind more accurate timekeeping and how that relates to the universe around us was more understood -- or at least investigated. Key players in science and mathematics also played an important role in trying to understand how time works. Prominent scientific figures such as Galileo and Dutch astronomer Christian Huygens led the understanding of time in the 16th and 17th centuries, but it was Albert Einstein, Carl Neumann, and the like who looked at absolute and mathematical time at the end of the 19th and beginning of the 20th centuries and then questioned those theories even further to try to explain the workings of modern timekeeping.

Sir Isaac Newton (1643-1727) was an English mathematician upon whose theory more investigation into time was based. Newton's law of inertia, which meant that one particle in space, not acted upon by other forces, continued to move in the same direction and would remain at the same speed, was the basis of Carl Neumann's (1832-1925) development of the inertial clock in 1883. He believed, however, that absolute time didn't rely upon absolute space. This meant the law of inertia given intervals which could be measured would measure absolute time.

At the beginning of the 20th Century it was Albert Einstein (1879-1955) who refuted the idea that time is absolutely defined and hypothesized that time is connected to the speed of light, thus giving birth to the theory of special relativity. Einstein reportedly said, "My solution was really for the very concept of time, that is, that time is not absolutely defined but there is an inseparable connection between time and the velocity of light."

Gravity, Einstein discovered also has an impact on time. He observed that the passage of time is slower where there is more gravitational pull compared to where there is not as much. The Earth's gravitational pull and rotation meant that a clock on Earth will loose about 1 one-billionth of a second per hour.

In the 20th Century we also have the discovery of quantum mechanics. Quantum mechanics had an impact on the understanding of time and introduced the idea that there are several dimensions that impact time.

As many scientific theories as there are related to time and how it is measured, it is still the basis of the quartz clock that has standardized how time is measured today. It relies on the theory that a quartz crystal will vibrate in an electric field at the same standardized frequency. These findings utilized the principles related to the way a mechanical and free pendulum works. (Link to article #3 on the pendulum.)

Even today, new ideas are emerging about time and its relationship to the universe. What is "now" and how does that impact the "future" and measure the "past?" Those were questions posed by Einstein himself and still at the forefront of research. Perhaps an understanding of "now" is "outside the realm of science," as suggested by Einstein, and we must simple rely on the modern interpretation set forth by organizations like the National Institute of Standards and Technology (NIST) and their double pendulum clock to measure time.