From the Internet Encyclopedia of Philosophy

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From the Internet Encyclopedia of Philosophy

Time has been studied by philosophers and scientists for 2,500 years, and it is much better understood today than long ago. Nevertheless, many questions remain to be resolved. Philosophers discuss whether time travel is possible; whether time had a beginning; whether time is an objective feature of reality or only a product of subjective experience; and whether it is sensible to speak of time flowing. Philosophers also investigate and debate about whether the future and past are real or, instead, only the present is real; whether time exists when nothing is changing; what gives time its arrow one way and not the other way; and whether time is infinitely divisible or instead composed of discrete moments. This article explores both what is known about time and what is controversial and unresolved. The article is structured so that it discusses answers to the following questions about time:

What should a philosophical theory of time do? 1

How is time related to mind? 1

Definitions of time 2

What science requires of time 2

Time travel 3

Is the relational theory of time preferable to the absolute theory? 5

The flow of time 5

The arrow of time 6

Is the future real? 7

Temporal indexicals and essentially tensed facts 7

The symbolic logic of time 8


What is a reference frame? 9

What is spacetime? 9

What is an event? 9

Does the theory of relativity imply time is partly space? 9

Is time the fourth dimension? 10

Is time infinite? 10

Is there more than one kind of physical time? 10

How is time relative to the observer? 10

What are the relativity and conventionality of simultaneity? 10

What is the difference between the past and the absolute past? 11

What is time dilation? 11

How does gravity affect time? 11

What happens to time near a black hole? 12

What is the solution to the twins paradox? 12

What is the solution to Zeno's paradoxes? 12

How do time coordinates get assigned to points of spacetime? 13

How do dates get assigned to actual events? 14

What is essential to being a clock? 14

What is our standard clock? 14

Why are some standard clocks better than others? 15

Suggestions for Further Reading: 15

What should a philosophical theory of time do?

Can we begin with a short definition of time? Yes, but there are two considerations that must be faced. First, the definition will not be able to to define time in terms of more primitive, yet familiar, notions. Second, succinct definitions of time are rarely helpful unless they are backed up with a more elaborate and systematic treatment of time. The brief definitions that stand alone are either trivial (Time is what keeps everything from happening all at once) or too imprecise (Time is the dimension of causality) or circular (Time is the collection of instants) or simply cryptic (Time is the flow of events past the stationary I). When philosophers ask, "What is time?", they normally are asking for a philosophical theory designed to answer many of the philosophical questions about time such as whether the past-present-future distinction is objective and how we should understand the flow of time. A succinct definition of time will be adequate only insofar as it is backed up by this more elaborate theory

Consider what a more systematic theory of time should do. It should reveal, among other things, the relationship between time and the mind. It is easy to confuse time itself and the perception of time. Does time exist for beings that have no minds?

A theory of time should reveal what physical science presupposes and implies about time. Does it imply the possibility of time travel, for instance? What does it assume about the relationship between time and spacetime? Physicists say that, locally, time is made of a linear continuum of instants, with each instant lasting for zero seconds. Being a continuum implies that between any two instants there is another instant. No time measurement is so fine grained that it could detect whether this is true for instants that are extremely close together in time. If so, then on what grounds do scientists know that time is a continuum?

A philosophical theory of time should describe the relationship between instants and events. Does the instant that we label as "11:01 AM" for a certain date exist independently of the events that occur then? In other words, can time exist if no event is happening? This question raises the thorny metaphysical issue of absolute vs. relational theories of time

A theory of time should address the question of time's apparent direction. If the projectionist in the movie theater shows a film of cream being added into black coffee but runs the film backwards, we in the audience can immediately tell that events couldn't have occurred this way. We recognize the arrow of time because we know about the one-directional processes in nature--that brown coffee never unmixes into black coffee and cream, for example. This arrow becomes less and less apparent to the viewer as the film subject gets smaller and smaller and the time interval gets shorter and shorter. Philosophers disagree about the explanation of this arrow. The arrow appears to be very basic for understanding nature, yet it is odd that asymmetries in time don't appear in most of the basic dynamical laws of physics. Philosophers also wonder what life would be like in some far off corner of the universe if the arrow of time were reversed there. Would our counterparts walk backwards up steps while remembering the future?

Another philosophical problem about time concerns the questions, "What is the present moment and why does it move into the past?" Present events seem to flow by, receding ever farther into the past. Many philosophers are suspicious of this notion of the flow of time. They doubt whether it is a property of time as opposed to being some feature of human perception. There are also suspicions about the present, the feature that is referred to by the indexical word 'now.' If the present is real, then why isn't there a term for it in the laws of science?

For a last example of a philosophical problem regarding time, some philosophers argue that the future is not real, but the present is. These philosophers have a problem with the apparent implication that, if the future were real, then it would be fixed now, and we would not have the freedom to affect that future. Other philosophers disagree

A full theory of time should address this constellation of philosophical issues and paradoxes about time

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