# What is time? (simple explanation)

Time is something that we all experience, although it is not easy to define and understand. This experience is often explained as the perception of a progressive succession of irreversible eventsor at least apparently irreversible.

But in physics, time is treated as a magnitude. It is not explained as a perception but as a measurable property of material reality. Thus, time would be the physical magnitude that measures the duration of an event that occurs within a system, or the interval that elapses between two events.

Thus, time allows in turn to measure the rate or speed of change of the system.

Throughout history we have used different systems and units of time measurement. Currently, the base unit in the International System is the secondof symbol sand defined as the duration of 9 192 631 770 oscillations of the radiation emitted in the transition between the two hyperfine levels of the ground state of the isotope 133 of the cesium atom (133Cs) at a temperature of 0 K or absolute zero.

As we see, time is defined by reference to a known rate of change, and a rate of change is a measure of speed. By comparing this known speed to the speed of other events, we can measure time. And it is that speed and time are so intimately related that the measurement of one implies the measurement of the other.

## the direction of time

Measuring time allows us to order the succession of events that occur in a system. From the future, to the present and the past. If a system does not change, it is a timeless system, since if there are no events there is no time.

The mathematical equations that try to explain physical reality work the same regardless of the direction in which this sequence of events that measures time is ordered. In the mathematical equations used in physics, time can be negative..

However, in the natural world, time goes on. one way and one way. The explanation of why time passes irreversibly, or at least that is how we experience it, is one of the great questions for which science does not have an answer.

## The relativity of time

The fact that time measures the succession of events that occur in a system introduces a concept that helps to understand the relativity of time. Time becomes relative to the observer's frame of reference.

Until Einstein reinterpreted the concepts of time and space from Newtonian mechanics, time was considered the same throughout the universe. But from Einstein's theory of relativity, time came to be considered a measure relative to frame of reference.

The relativity of time is determined by the speed of light, which according to Einstein's postulates is a universal constant and independent of the observer. In the theory of relativity, two observers can obtain different measurements of time for the same event. but observe from the same frame of reference.

For example, if one observer is moving faster than the other, and both are observing the same event, the faster moving observer will get longer time measurements. The different measurements of each observer find their relationship if the reference frame of each observer is extrapolated to the speed of light, which is constant for both and would constitute an equivalent system for both.

If an observer were able to reach the speed of light, all the events observed would be simultaneous, there would be no succession of events. Time would have dilated indefinitely to the point of stopping. Because the speed of light cannot be exceeded, an observer traveling at this speed would not be hit by any light, no event would be able to catch up with him, and time would stand still for him.

Time dilation as the observer's speed increases has been recorded in various experiments. For example, in time measurement systems placed in orbit at high speed or in measurements of the decay speed of muons and other elementary particles. One of the first and most famous experiments on the relativity of time was the Michelson and Morley experiment.

## start and end of time

In theory, the timeline could have a beginning and an end. If we observe the succession of events that have occurred in the Universe, which is the largest reference frame that we can study, both the mathematical equations and the observations in the radiation of space, reach a point of zero time. According to the Big Bang theory, this point would have occurred 13.799 million years ago.

It is not known what was there before the Big Bang, it is not even known that it happened just at the time of the Big Bang. In fact, in the theory of relativity, time would start right at the Big Bang, before there would be no time or space, a difficult concept to understand. The human mind generates a constant succession of thoughts that follow a time line. Imagining the absence of time is really difficult for us.

It is also not known if time will have an end. If the Universe continues to expand as it is now, time would never end. If a new Big Bang takes place, our timeline could be truncated and a new one could begin.

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