The best definition I have found on web is " time is the continuum of experience in which events pass from the future through the present to the past" It's very poetic and permits nice analogy with movie where one cadre follows others in equal intervals of time. Wait a moment. We are using for description of time a intervals of time. The subject can't be described from itself. Time cannot be defined other then from itself. A paradox these are legion when we a talking about time.
Does future exist? No, yet. Does past exist? No, already. Does present exist? Yes. But… Where is it? Present immediately became past as soon it shows up. Nobody can catch it. What is the object that nobody can hold in hand? What is it?
In science time is a third by importance base characteristic of … (let's be most broad and vague here) Everything. Second is a one of base units in any system of measurements. To do measurement one has to compare an object or process with standard. To measure a distance we can use a ruler that is an object. To measure mass we can use weight that is an object too. To measure time we cannot use any object by itself. The object to measure time must be in motion. The standard to measure time is a process not an object. This is the very fundamental statement that is under appreciated in most cases when speaking about of nature of time.
In everyday language we very often are mixing time and velocity. Runners compete to be fastest but we register their records not in velocity units but in seconds. Velocity and time are unconscious the same in our minds. Presume that there are much more reasons in frivol misuse of the terms then just convenience.
Let's try to choose as a base unit in system of measurements a velocity instead of time. We have a natural constant of velocity speed of light in vacuum so we will have at once a primary standard for base unit. The problem will be in common language where the word time played more then just significant role. But advantage to do this replacement will be enormous too. The rate of processes will be measured with the analogous characteristic - velocity not with something artificial ideas like time. If we will dare to go even further we will accept that time is existed only in our mind as a concept of logical development of events.
What in this case to do with theoretical and experimental fact that clocks slow down in the spacecraft moving with high velocities. One can say that in this case a "viscosity" of space metrics is increasing that slows down all processes around. Empty space looks like filled with something that defined behaviour of all material objects.
Time is not existed as an independent material reality. Concept of time is invented by homo sapience reflected of observation of logical picture of nature. If the same circumstances unavoidable produce the same response we will observe stable periodically repeated processes like a changing day and night, a summer and winter, phases of moon. We don't necessary need a concept of time to explain it. Movements of planets will do the job. But we need clocks to meet our date and we have to have a word to set a moment to be in the same place. This word is a time. Just a word. Time machine will never send anybody to the past to fix misfortune.
Initially submitted at Dec. 1, 2004; 14:27
After time passed since initially submitted thinking about related matters bring the suspicious that concept of virtual reality has a lot with time. If time permits I will continue the topic in other articles.
Nov. 27, 2017; 14:52 EST