Intuitively, all humans know what time is, but to describe it without saying the word "time" is difficult. After all, time is the time it takes for things to happen. Is it not? While time itself may be difficult to explain, in much the same way a color is difficult to explain, its effects can be measured and its components can be discovered. The field of physics has made great strides in using these two to best describe what time is.
The First Attempts to Explain Time
Philosophy is credited with the first scholarly attempt to describe time. Prior to philosophers, mystics and religionists had many descriptions of time, as a spirit or as a god, or as a creation of gods. Hindus and Buddhists regarded time as cyclical, while Judeo-Christians regard time as linear. But with the advent of philosophy, time as a subject fell into two different philosophies, and one of them carries on today in science.
One of the two philosophies of time came from the likes of Immanuel Kant and Gottfried Leibniz, who said that time isn't anything. To these "unrealists", time is just a concept. It isn't real, and since it isn't real, it can't really be measured. They are part of a long line of philosophers who suggested that time, and even space, are simply a priori constructs of the mind made so that humans can utilize their senses and experiences.
The second philosophy, a type of "realism", says that time is just as real as space and matter. Sir Isaac Newton, the famous natural philosopher whose theories on gravity and creation of calculus were among many other Enlightenment ideas that sparked modern science and physics, thought that time was independent of the observer, and always moved forward at a set speed. He believed that time couldn't be perceived except through mathematics, but it does exist as a sort of dimension in which objects move through. (Newton)
Newtonian Time or Classical Physics
To Newton, time was a dimension separate from the observer, but one that the observer (and all matter) moves through. As such, time is an important part of his gravitational theories. In fact, time and space are the foundations of Newtonian physics, also known as 'classical physics'. Classical physics describes accurately the motion of objects in the real world by referencing their mass, velocity, speed, trajectory, acceleration, distance traveled, etc. Classical physics works well with the commonsense world that humans are familiar with, and which they operate most comfortably in. By "common sense world" is meant the world in which human senses operate, i.e. within the visible light spectrum, the size of objects, the speeds of which can be observed by the human eye, etc.
Newtonian physics was the predominant physics until the 20th century, when Albert Einstein found a more accurate way to describe motion. Newtonian physics treated spatial dimensions as variable, while time was a constant.
Einstein's Notion of Time
Einstein's theories of relativity showed that time and space were not constant, but relative to the motion of the observer. Whereas Newton separated time and space as being absolute and separate of each other, Einstein's spacetime merged time and space into one. "Spacetime" is a single continuum which all things are a part of and which all things move in. It is usually interpreted as the three dimensions representing space, and time as a fourth dimension that isn't spatial.
Physicists have found that when they combine space and time into their mathematical constructs, an extraordinary amount of physical theories are simplified. The reason that time cannot be separated is that measurements have shown that the observed rate of time for an object is dependent on its velocity through space relative to the observer, and also the strength of gravitational fields. (Einstein)
Einstein's theory will probably not be the last humanity hears on the concept of time. Physicists are trying to find a way in which discrepancies between general relativity and quantum mechanics (particle physics) can be resolved. As such, additional dimensions are being proposed - most of which are spatial. Of these different theories, string theory, and more specifically M-Theory, is probably the most popular. The addition of additional dimensions will probably affect quantum physics the most.
So what does all this mean? Time is difficult to describe even in common sense terms, as it will only lead to circular reasoning. (Time is the time it takes for things to happen.) But as humans progress in scientific understanding, time has gotten even harder to understand without an understanding of the applicable mathematics. Today, the best description of time is that it is the temporal component of the spacetime continuum, of which the entire universe is made. Mathematically, the image in the photo is one way that time is portrayed using general relativity.
Sources
Burnham, Douglas : Staffordshire University (2006). "Gottfried Wilhelm Leibniz (1646–1716) Metaphysics – 7. Space, Time, and Indiscernibles". The Internet Encyclopedia of Philosophy.
Einstein, A., H. A. Lorentz, H. Weyl, H. Minkowski, The Principle of Relativity, Dover Publications, Inc, 2000
Mattey, G. J. : UC Davis (1997-01-22). "Critique of Pure Reason, Lecture notes: Philosophy 175 UC Davis"
Newton, Isaac. In Philosophiae Naturalis Principia Mathematica See the Principia on line at Andrew Motte Translation
Andrew Perry - Andrew has a B.A. in Philosophy & Journalism. He has worked professionally for newspapers, news sites, and local government.
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