Understanding time

Flow of time

In physics, the treatment of time is a central issue. It has been treated as a question of geometry. One can measure time and treat it as a geometrical dimension, such as length, and perform mathematical operations on it. It is a scalar quantity and, like length, mass, and charge, is usually listed in most physics books as a fundamental quantity. Time can be combined mathematically with other fundamental quantities to derive other concepts such as motion, energy and fields. Time is largely defined by its measurement in physics. Timekeeping is a complex of technology and science issues, and part of the foundation of recordkeeping.

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The flow of time

Currently, the standard time interval (called conventional second, or simply second) is defined as 9,192,631,770 oscillations between the two hyperfine levels of the ground state in the 133Cs (caesium) atom. The UTC timestamp in use worldwide is an atomic time standard. The relative accuracy of such a time standard is currently on the order of 10-15 (corresponding to 1 second in about 1015 seconds, or an accuracy of 1 second in approximately 30 million years). The smallest time step considered observable is called the Planck time, which is on the order of 5*10-44 seconds - many orders of magnitude below the resolution of current time standards. Both Galileo and Newton and most people up until the 20th century thought that time was the same for everyone everywhere. This is the basis for timelines, where time is a parameter. Our modern conception of time is based on Einstein's theory of relativity, in which rates of time run differently depending on relative motion, and space and time are merged into spacetime, where we live on a world line rather than a timeline. Thus time is part of a coordinate, in this view. Physicists believe the entire Universe and therefore time itself began about 13.7 billion years ago in the big bang. (See #Time in cosmology below) Whether it will ever come to an end is an open question.

In 1583, Galileo Galilei (1564-1642) discovered that a pendulum's harmonic motion has a constant period, which he learned by timing the motion of a swaying lamp in harmonic motion at mass at the cathedral of Pisa, with his pulse. In his Two New Sciences (1638), Galileo used a water clock to measure the time taken for a bronze ball to roll a known distance down an inclined plane; this clock was "a large vessel of water placed in an elevated position; to the bottom of this vessel was soldered a pipe of small diameter giving a thin jet of water, which we collected in a small glass during the time of each descent, whether for the whole length of the channel or for a part of its length; the water thus collected was weighed, after each descent, on a very accurate balance; the differences and ratios of these weights gave us the differences and ratios of the times, and this with such accuracy that although the operation was repeated many, many times, there was no appreciable discrepancy in the results." Galileo's experimental setup to measure the literal flow of time, in order to describe the motion of a ball, preceded Isaac Newton's statement in his Principia: I do not define time, space, place and motion, as being well known to all. The Galilean transformations assume that time is the same for all reference frames.

Understanding Time

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Understanding time

Astronomer Carl Sagan had it right when he said that time is "resistant to simple definition." Lots of us think we know what time is, but it is hard to define. You can not literally see or touch time, but you can see its effects. The evidence that we are moving through time is found in everything -- our bodies age, buildings weather and crumble, trees grow. Most of us feel the pressure of time as we are pushed to meet deadlines and make appointments. Our lives are often dictated by what time we need to be somewhere. Ask most people to define time and they are likely to look at their watch or a clock. We see time as the ticking of the hands on these devices. We know that there are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day and 365 days in a year. These are the basic numbers of time that we all learned in grade school. Time is also defined as being the fourth dimension of our universe. The other three dimensions are of space, including up-down, left-right and backward-forward. Time cannot exist without space, and likewise, space cannot exist without time. This interconnected relationship of time and space is called the spacetime continuum, which means that any event that occurs in the universe has to involve both space and time. According to Einstein's theory of special relativity, time slows as an object approaches the speed of light. This leads many scientists to believe that traveling faster than the speed of light could open up the possibility of time travel to the past as well as to the future. The problem is that the speed of light is believed to be the highest speed at which something can travel, so it is unlikely that we will be able to travel into the past. As an object nears the speed of light, its relativistic mass increases until, at the speed of light, it becomes infinite. Accelerating an infinite mass any faster than that is impossible, or at least it seems to be right now. But time travel in the other direction is not as difficult, and the future may one day be a possible destination.


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