What is Time?

Answer 1

We all know that clocks measure time. But clocks are complex physical systems and hence are subject to imperfection, breakage, and disruptions of electrical power. If I take any two real clocks, synchronise them, and let them run, after some time, they will always disagree about what time it is.


So which of them measures the real time? Indeed, is there a single, absolute time which, although measured imperfectly by any actual clock, is the true time of the world? It seems there must be, otherwise, what do we mean when we say that some particular clock runs slow or fast? On the other hand, what could it mean to say that something like an absolute time exists if it can never be precisely measured?


A belief in an absolute time raises other paradoxes. Would time flow if there were nothing in the universe? If everything stopped, if nothing happened, would time continue?
On the other hand, perhaps there is no single absolute time. In that case, time is only what clocks measure and, as there are many clocks and they all, in the end, disagree, there are many times. Without an absolute time, we can only say that time is defined relative to whichever clock we choose to use.


This seems to be an attractive point of view, because it does not lead us to believe in some absolute flow of time we can't observe. But it leads to a problem, as soon as we know a little science.
One of the things physics describes is motion, and we cannot conceive of motion without time. Thus, the notion of time is basic for physics. Let me take the simplest law of motion, which was invented by Galileo and Descartes, and formalised by Isaac Newton: A body with no forces acting on it moves in a straight line at a constant speed. Let's not worry here about what a straight line is (this is the problem of space, which is perfectly analogous to the problem of time, but which I won't discuss here). To understand what this law is asserting, we need to know what it means to move at a constant speed. This concept involves a notion of time, as one moves at a constant speed when equal distances are covered in equal times.


We may then ask: With respect to which time is the motion to be constant? Is it the time of some particular clock? If so, how do we know which clock? We must certainly choose because, as we observed a moment ago, all real clocks will eventually go out of synchronisation with one another. Or is it rather that the law refers to an ideal, absolute time?
Suppose we take the point of view that the law refers to a single, absolute time. This solves the problem of choosing which clock to use, but it raises another problem, for no real, physical clock perfectly measures this imagined, ideal time. How could we truly be sure whether the statement of the law is true, if we have no access to this absolute, ideal time? How do we tell whether some apparent speeding up or slowing down of some body in a particular experiment is due to the failure of the law, or only to the imperfection of the clock we are using?


Newton, when he formulated his laws of motion, chose to solve the problem of which clock by positing the existence of an absolute time. Doing this, he went against the judgements of his contemporaries, such as Descartes and Gottfried Leibniz, who held that time must be only an aspect of the relationships among real things and real processes in the world. Perhaps theirs is the better philosophy, but as Newton knew better than anyone at the time, it was only if one believed in an absolute time that his laws of motion, including the one we have been discussing, make sense. Indeed, Albert Einstein, who overthrew Newton's theory of time, praised Newton's "courage and judgement" to go against what is clearly the better philosophical argument, and make whatever assumptions he had to make to invent a physics that made sense.


This debate, between time as absolute and preexisting and time as an aspect of the relations of things, can be illustrated in the following way. Imagine that the universe is a stage on which a string quartet or a jazz group is about to perform. The stage and the hall are now empty, but we hear a ticking, as someone has forgotten, after the last rehearsal, to turn off a metronome sitting in a corner of the orchestra pit. The metronome ticking in the empty hall is Newton's imagined absolute time, which proceeds eternally at a fixed rate, prior to and independently of anything actually existing or happening in the universe. The musicians enter, the universe all of a sudden is not empty but is in motion, and they begin to weave their rhythmic art. Now, the time that emerges in their music is not the absolute preexisting time of the metronome; it is a relational time based on the developing real relationships among the musical thoughts and phrases. We know this is so, for the musicians do not listen to the metronome, they listen to one another, and through their musical interchange, they make a time that is unique to their place and moment in the universe.


But, all the while, in its corner the metronome ticks on, unheard by the music makers. For Newton, the time of the musicians, the relational time, is a shadow of the true, absolute time of the metronome. Any heard rhythm, as well as the ticking of any real physical clock, only traces imperfectly the true absolute time. On the other hand, for Leibniz and other philosophers, the metronome is a fantasy that blinds us to what is really happening; the only time is the rhythm the musicians weave together.
The debate between absolute and relational time echoes down the history of physics and philosophy, and confronts us now, at the end of the twentieth century, as we try to understand what notion of space and time is to replace Newton's. If there is no absolute time, then Newton's laws of motion don't make sense. What must replace them has to be a different kind of law that can make sense if one measures time by any clock. That is, what is required is a democratic rather than an autocratic law, in which any clock's time, imperfect as it may be, is as good as any other's. Now, Leibniz was never able to invent such a law. But Einstein did, and it is indeed one of the great achievements of his theory of general relativity that a way was found to express the laws of motion so that they make sense whichever clock one uses to embody them with meaning. Paradoxically, this is done by eliminating any reference to time from the basic equations of the theory. The result is that time cannot be spoken about generally or abstractly; we can only describe how the universe changes in time if we first tell the theory exactly which real physical processes are to be used as clocks to measure the passage of time.


So, this much being clear, why then do I say that I do not know what time is? The problem is that general relativity is only half of the revolution of twentieth-century physics, for there is also the quantum theory. And quantum theory, which was originally developed to explain the properties of atoms and molecules, took over completely Newton's notion of an absolute ideal time.
So, in theoretical physics, we have at present not one theory of nature but two theories: relativity and quantum mechanics, and they are based on two different notions of time.


The key problem of theoretical physics at the present moment is to combine general relativity and quantum mechanics into one single theory of nature that can finally replace the Newtonian theory overthrown at the beginning of this century. And, indeed, the key obstacle to doing this is that the two theories describe the world in terms of different notions of time. Unless one wants to go backward and base this unification on the old, Newtonian notion of time, it is clear that the problem is to bring the Leibnizian, relational notion of time into the quantum theory. This is, unfortunately, not so easy. The problem is that quantum mechanics allows many different, and apparently contradictory, situations to exist simultaneously, as long as they exist in a kind of shadow or potential reality. (To explain this, I would have to write another essay at least as long as this one about the quantum theory.) This applies to clocks as well, in the same way that a cat in quantum theory can exist in a state that is at the same time potentially living and potentially dead, a clock can exist in a state in which it is simultaneously running the usual way and running backward. So, if there were a quantum theory of time, it would have to deal not only with freedom to choose different physical clocks to measure time, but with the simultaneous existence, at least potentially, of many different clocks. The first, we have learned from Einstein how to do; the second has, so far, been too much for our imaginations.


So the problem of what time is remains unsolved. But it is worse than this, because relativity theory seems to require other changes in the notion of time. One of them concerns my opening question, whether time can begin or end, or whether it flows eternally. For relativity is a theory in which time can truly start and stop.
One circumstance in which this happens is inside of a black hole. A black hole is the result of the collapse of a massive star, when it has burned all its nuclear fuel and thus ceased to burn as a star. Once it is no longer generating heat, nothing can halt the collapse of a sufficiently massive star under the force of its own gravity. This process feeds on itself, because the smaller the star becomes, the stronger the force by which its parts are mutually attracted to one another. One consequence of this is that a point is reached at which something would have to go faster than light to escape from the surface of the star. Since nothing can travel faster than light, nothing can leave. This is why we call it a black hole, for not even light can escape from it.


However, let us think not of this, but of what happens to the star itself. Once it becomes invisible to us, it takes only a short time for the whole star to be compressed to the point at which it has an infinite density of matter and an infinite gravitational field. The problem is, what happens then? The problem, indeed, is what, in such a circumstance, "then" might mean. If time is only given meaning by the motion of physical clocks, then we must say that time stops inside of each black hole. Because once the star reaches the state of infinite density and infinite gravitational field, no further change can take place, and no physical process can go on that would give meaning to time. Thus, the theory simply asserts that time stops.


The problem is in fact even worst than this, because general relativity allows for the whole universe to collapse like a black hole, in which case, time stops everywhere. It can also allow for time to begin. This is the way we understand the big bang, the most popular theory, currently, of the origin of the universe.
Perhaps the central problem that those of us who are trying to combine relativity and quantum mechanics think about is what is really happening inside a black hole. If time really stops there, then we must contemplate that all time, everywhere, comes to a stop in the collapse of the universe. On the other hand, if it does not stop, then we must contemplate a whole, limitless world inside each black hole, forever beyond our vision. Moreover, this is not just a theoretical problem, because a black hole is formed each time a massive enough star comes to the end of its life and collapses, and this mystery is occurring, somewhere in the vast universe we can see, perhaps one hundred times a second.

Answer 2

Time is something wasted by the young and coveted by the elderly.

Time can be a source of idolatry: we wish we had more time, we celebrate the passage of time.

Time is a component of our reality that makes it work and gives it a basis. Stuff gets older...

Imagine the smallest measureable distance and the smallest measureable time: those two measures would define the limits of the physical space. You might say you can always divide those measures in half - yup, but after a while you hit the wall that defines the universe we occupy. How low can you go? Before you leave check into the uncertainty principle...

Answer 3

Physical time is a speed "c". Our physical composition is that of physical time. Were this not so, time would not be of exactly the same value to all people, objects and events. A pure field of time, apart from electrical and magnetic movement, is that of a field of gravity - c2 = E/m. Time is of an exact magnitude for all our universe, thus making the past, present, and future a possibility. Were any of these three values different that the others, that segment of existence would cease to exist. Time appears to be the most basic, and greatest singular value of all creation.

Answer 4

Time can be a state, as in "the meeting starts at 12:45", or the length of an interval, as in "the meeting lasted 3 hours", or a sum of lengths of intervals, as in "I spent 20 hours on this project". In the latter case time is often seen as a commodity ("I have little time.")

Time has long been a major subject of philosophy, art, poetry, and science. There are widely divergent views about its meaning; hence it is difficult to provide an uncontroversial definition of time. Scholars disagree on whether time itself can be measured or is itself part of the measuring system. Many fields use an operational definition in which the only definition attempted is that of the units used.

The measurement of time has also occupied scientists and technologists, and was a prime motivation in astronomy. Time is also a matter of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in our lives. Units of time have been agreed upon to quantify the duration of events and the intervals between them. Regularly recurring events and objects with apparent periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, and the swing of a pendulum.

Time has historically been closely related with space, most obviously with spacetime in Einstein's General Relativity.

Answer 5

A universal way of denoting a period from one event to the other.
Earliest measures of time probably included one phase of moon to the next, or sunrise to sunrise. The term "month" is a derivitive of moon. Annual, (once a year) annular (ring like) are from the same root, hence one ring around the sun to where we come back to the same place is our year (annual). The one thing that is true about time, it passes, waits for no one, and it marches on and we cant stop it.
In the short time you have read this, there have been several: births, deaths, and a lot of human choices made.

Answer 6

After all that's been said about time, you would think
there's not one more thing. But I thought of a philosophical
difference between The East and West. In The West we
say "Time is money". The Chinese say "Time is free".

Answer 7

# an instance or single occasion for some event; "this time he succeeded"; "he called four times"; "he could do ten at a clip"
# an indefinite period (usually marked by specific attributes or activities); "he waited a long time"; "the time of year for planting"; "he was a great actor is his time"
# a period of time considered as a resource under your control and sufficient to accomplish something; "take time to smell the roses"; "I didn't have time to finish"; "it took more than half my time"
# a suitable moment; "it is time to go"
# the continuum of experience in which events pass from the future through the present to the past
# clock time: the time as given by a clock; "do you know what time it is?"; "the time is 10 o'clock"
# clock: measure the time or duration of an event or action or the person who performs an action in a certain period of time; "he clocked the runners"
# fourth dimension: the fourth coordinate that is required (along with three spatial dimensions) to specify a physical event
# assign a time for an activity or event; "The candidate carefully timed his appearance at the disaster scene"
# a person's experience on a particular occasion; "he had a time holding back the tears"; "they had a good time together"
# set the speed, duration, or execution of; "we time the process to manufacture our cars very precisely"
# meter: rhythm as given by division into parts of equal duration
# regulate or set the time of; "time the clock"
# prison term: the period of time a prisoner is imprisoned; "he served a prison term of 15 months"; "his sentence was 5 to 10 years"; "he is doing time in the county jail"
# adjust so that a force is applied and an action occurs at the desired time; "The good player times his swing so as to hit the ball squarely"

Answer 8

Time is something that allows a cause to have an effect. Something that allows movement in a certain dimension. We who see things in 3D also see the 1D and the 2D movement all at the same time, that movement is static so it doesn't need time to happen in 3D but we need time to see things moving in a 3D world. That is if the multidimensional theory is true!

Answer 9

A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.
An interval separating two points on this continuum; a duration: a long time since the last war; passed the time reading.
A number, as of years, days, or minutes, representing such an interval: ran the course in a time just under four minutes.
A similar number representing a specific point on this continuum, reckoned in hours and minutes: checked her watch and recorded the time, 6:17 A.M.
A system by which such intervals are measured or such numbers are reckoned: solar time.

An interval, especially a span of years, marked by similar events, conditions, or phenomena; an era. Often used in the plural: hard times; a time of troubles.
times The present with respect to prevailing conditions and trends: You must change with the times.
A suitable or opportune moment or season: a time for taking stock of one's life.

Periods or a period designated for a given activity: harvest time; time for bed.
Periods or a period necessary or available for a given activity: I have no time for golf.
A period at one's disposal: Do you have time for a chat?
An appointed or fated moment, especially of death or giving birth: He died before his time. Her time is near.

One of several instances: knocked three times; addressed Congress for the last time before retirement.
times Used to indicate the number of instances by which something is multiplied or divided: This tree is three times taller than that one. My library is many times smaller than hers.

One's lifetime.
One's period of greatest activity or engagement.
A person's experience during a specific period or on a certain occasion: had a good time at the party.

A period of military service.
A period of apprenticeship.
Informal. A prison sentence.

The customary period of work: hired for full time.
The period spent working.
The hourly pay rate: earned double time on Sundays.
The period during which a radio or television program or commercial is broadcast: “There's television time to buy” (Brad Goldstein).
The rate of speed of a measured activity: marching in double time.
Music.
The meter of a musical pattern: three-quarter time.
The rate of speed at which a piece of music is played; the tempo.
Chiefly British. The hour at which a pub closes.
Sports. A time-out.
adj.
Of, relating to, or measuring time.
Constructed so as to operate at a particular moment: a time release.
Payable on a future date or dates.
Of or relating to installment buying: time payments.

Answer 10

the continuum of experience in which events pass from the future through the present to the past.
but This is not the correct definition.

Answer 11

Time having a definite speed and it traveling from infinitely year ago and will travel infinitely year. it will never stop. If all particles of universe will die. But, time will never die.