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What is time?
1. This is the question that scientists dread to be asked especially in a public setting by extremely inquisitive young students. We have learned to measure and calculate it …relative to the natural arrangement of known and visible celestial bodies. This is the ultimate question you ask a theoretical astrophysicist if you want to see them shake in their pants. To this day time can only be described by the measure of elapsed period using the SI unit, Seconds. It is the only SI unit that is freely allowed to shift depending on subject topic. For example in astrophysics, to measure distance between galaxies it would be impractical to use seconds as the measure of time it takes light to travel between the two galaxies 2. One problem is that time to a philosopher is something different from time to a physicist. The reason being that a philosopher is likely to want to articulate what we feel about time and, if possible, create a usable definition of all those woolly ideas we have about time "flowing" or "transcending" or, if the philosopher is not of an analytic bent, paint a picture in half-formed concepts - to the despair of physicists who prefer to characterize time by objective experiments. The simple fact is that time is whatever clocks measure and this definition applies to our everyday concept of time as well as the deepest scientific ideas. Events happen, usually in a predictable order (the laws of nature), and the "direction" in which these events follow each other is called time. In some respects this resembles space. Physics now knows, through many, many experiments, that time is indeed another dimension. Take the event known as the Battle of Hastings. You can ask where it happened, the answer needs 3 dimensions - latitude, longitude and (easily overlooked) altitude. It also has a date, time. But this similarity to space extends even into the realm where space itself is distorted due to relativity. Time behaves the same way, so much so that the two are best thought of as a single 4 dimensional "continuum". Which poses a problem. The whole of the universe, spread out in space and the whole of history together form a single static object. It cannot change, things change within it - a child gets bigger as you look further along the time axis, but there is no movement, just progression, an infinite succession of slightly different states, spread through time. This is in contrast to the way we perceive time, which is something that flows at 60 minutes an hour carrying events with it. Unfortunately this is a pretty incoherent idea - rivers "flow" because the water moves from place to place in time, but if time itself flows, what does it flow in? A second kind of time? And if so, what does that flow in? This is where we have to be ruthless and ignore our subjective impressions, or, even better, actually explain them away. This has an easy side and a difficult one. We only ever experience the present. Another mystery? Not at all, nothing can ever interact with anything from a different time, we cannot influence the past and although the past may determine the present, someone in the past can't suddenly decide to change us now in the present. So of course we only experience the present. But at every "present" moment in the time dimension we experience that particular moment. Naturally it moves forward at 60 minutes per hour. That was the easy bit. The difficult bit is why time only moves forward, why does the future unfold as we move forward into it? Why can we not remember the future? Here we have to turn to real science. The difference between how we experience past and future is easily explained in that nature itself is different in the two directions of time. Certain events tend to follow others, not the other way round. This is a direct result of the universe being in an unstable state, slowly working its way to final equilibrium. Which is a big mystery in itself, but it does cast doubt on the intuitive idea that time is infinite in both directions. Currently it seems that time may very well go on for ever into the future but kind of stop in the past. What actually happens is a bit complex. From the moment of the big-bang, time has been pretty well as it is now, but for the first 10^-33 seconds it may well have been "smeared out" - and also been indistinguishable from space. This means that time would have no particular direction for that first moment. There are other possibilities but that's one that's been around a while and is still popular. Interestingly, it's still not clear what time is made of. It may be that spacetime and superstrings are so intimately connected that there is a kind of primordial fibrous substance that accounts for everything. That may not be terribly helpful though, as it's quite impossible to imagine such a material - after all, it's all around us and is the stuff we are made of. 3. Time is a specific point of a chronological measurement. mm says - Define "chronological" without using the word "time". 4. There is of course the fourth theory that suggests that time is an abstract concept devised by the Swiss to help them sell wrist watches. 5. Time is the way in which the human mind perceives change; change in particular as it relates to entropic processes, which define the realm of experience we exist in. 6. Imagine a world where nothing moves. No clocks, no sound (vibrations in air), no wind, no light (motion of electromagnetic radiation through space). Without motion, there is no time. Nothing can age if it is frozen in place. Time, ultimately, boils down to the ratio of distance to motion. T=D/R. You learn this in high school algebra, but most people prefer to think of time as something far more abstract, metaphysical, or philosophical. Cosmologists would suggest that time began with the creation of the universe--the moment of the big bang, some 13.7 billion years ago. In this regard, time would not be infinite in both directions. It would be meaningless to refer to time before the beginning of time--"before" there was anything to measure. Just because we can conceive of something, such as "before the big bang" or "flying pink unicorns" does not mean such things must exist. There is a small problem with defining time in terms of rate, which itself includes the concept of time. This makes the algebraic definition of time somewhat self referential. It is this paradox that wedges open the door to the appeal to metaphysical mumbo-jumbo. There are lots of things that are not well understood, and time admittedly is near the top of the list. Imagine light, for example. To a particle of light, the universe lacks the dimension of length. It takes, in effect, zero time for a particle of light to cross the breadth of the universe, from the perspective of light itself. The Lorentz transform for relativistic time is: To = Ts /[ (1-v2/c2)^0.5] Where T=Time, o=objective, s=subjective, and v=velocity. The faster one moves, the slower time passes for the objective observer. (Time is constant only for the subjective observer--you look at your watch, the same number of seconds seem to pass as always). As velocity approaches the speed of light, c, the ratio between v2 and c2 approaches one, and the denominator, the tau factor, approaches zero. As tau approaches zero, the ratio of Ts to tau grows large. This is also described by the twin paradox. Take identical twins, born the same day. Send one on a journey near light speed to Alpha Centauri, the star closest to our sun. When he returns he will be little older than when he left, but the twin that remained on earth will be about nine years older. It all boils down to time is motion. Whether you're talking oscillations in some crystal, water draining from a clepsydra, the periodic swinging of a pendulum, or the ticking of a second hand, time is motion. 7. The definition of time is the specific minute or hour that can be measured using a clock. Units of time include seconds, minutes, hours, days, and years. 8. Time is a scale of the movement of matter, sub-matter, and Anti-matter and the structure of the matter has everything to do with the effect. (MORE)