Time Research

2. How Is Time Related to Mind?Physical time is public time, the time thatclocksare designed to measure. Biological time, by contrast,is indicated by an organism's circadian rhythm or body clock, which is normally regulated by the pattern of sunlight and darkness. Psychological time is different from both physical time and biological time. Psychological time is private time. It is also called phenomenological time, and it is perhaps best understood as awareness of physical time. Psychological time passes relatively swiftly for us while we are enjoying an activity, but it slows dramatically if we are waiting anxiously for the pot of water to boil on the stove. The slowness is probably due to focusing our attention on short intervals of physical time. Meanwhile, the clock by the stove is measuring physical time and is not affected by any persons awareness or by any organism's biological time.When a physicist defines speed to be the rate of change of position with respect to time, the term time refers to physical time, not psychological time or biological time. Physical time is more basic or fundamental than psychological time for helping us understand our shared experiences in the world, and so it is more useful for doing physical science, but psychological time is vitally important for understanding many mental experiences.Psychological time is faster for older people than for children, as you notice when your grandmother says, "Oh, it's my birthday again." That is, an older person's psychological time is faster relative to physical time. Psychological time is slower or faster depending upon where we are in the spectrum of conscious experience: awake normally, involved in a daydream, sleeping normally, drugged with anesthetics, or in a coma. Some philosophers claim that psychological time is completely transcended in the mental state callednirvanabecausepsychological time slows to a complete stop. There is general agreement among philosophers that, when we are awake normally, we do not experience time as stopping and starting.A major philosophical problem is to explain the origin and character of our temporal experiences. Philosophers continue to investigate, but so far do not agree on, how our experience of temporal phenomena produces our consciousness of our experiencing temporal phenomena.With the notable exception ofHusserl, most philosophers say our ability to imagine other times is a necessary ingredient in our having any consciousness at all. Many philosophers also say people in a coma have a low level of consciousness, yet when a person awakes from a coma they can imagine other times but have no good sense about how long they've been in the coma.We make use of our ability to imagine other times when we experience a difference between our present perceptions and our present memories of past perceptions. Somehow the difference between the two gets interpreted by us as evidence that the world we are experiencing is changing through time, with some events succeeding other events. Locke said our train of ideas produces our idea that events succeed each other in time, but he offered no details on how this train does the producing.Philosophers also want to know which aspects of time we have direct experience of, and which we have only indirect experience of. Is our direct experience of only of the momentary present, as Aristotle,Thomas Reid, and Alexius Meinongbelieved, or instead do we have direct experience of what William James called a "specious present," a short stretch of physical time? Among those accepting the notion of a specious present, there is continuing controversy about whether the individual specious presents can overlap each other and about how the individual specious presents combine to form our stream of consciousness.The brain takes an active role in building a mental scenario of what is taking place beyond the brain. For one example, the "time dilation effect" in psychology occurs when events involving an object coming toward you last longer in psychological time than an event with the same object being stationary. For another example, try tapping your nose with one hand and your knee with your other hand at the same time. Even though it takes longer for the signal from your knee to reach your brain than the signal from your nose to reach your brain, you will have the experience of the two tappings being simultaneousthanks to the brain's manipulation of the data. Neuroscientists suggest that your brain waits about 80 milliseconds for all the relevant input to come in before you experience a now. Craig Callender surveyed the psycho-physics literature on human experience of the present, and concluded that, if the duration in physical time between two experienced events is less than about a quarter of a second (250 milliseconds), then humans will say both events happened simultaneously, and this duration is slightly different for different people but is stable within the experience of any single person. Also, "our impression of subjective present-ness...can be manipulated in a variety of ways" such as by what other sights or sounds are present at nearby times. See (Callender 2003-4, p. 124) and (Callender 2008).Within the field of cognitive science, researchers want to know what are the neural mechanisms that account for our experience of timefor our awareness of change, for our sense oftimes flow, for our ability to place events into the proper time order (temporal succession), and for our ability to notice, and often accurately estimate, durations (persistence). The most surprising experimental result about our experience of time is Benjamin Libets claim in the 1970s that his experiments show that the brain events involved in initiating our free choice occur about a third of a second before we are aware of our choice. Before Libets work, it was universally agreed that a person is aware of deciding to act freely, then later the body initiates the action. Libet's work has been used to challenge this universal claim about decisions. However, Libet's own experiments have been difficult to repeat because he drilled through the skull and inserted electrodes to shock the underlying brain tissue. See (Damasio 2002) for more discussion of Libet's experiments.Neuroscientists and psychologistshave investigated whether they can speed up our minds relative to a duration of physical time. If so, we might become mentally more productive, and get more high quality decision making done per fixed amount of physical time, and learn more per minute. Several avenues have been explored: using cocaine, amphetamines and other drugs; undergoing extreme experiences such as jumping backwards off a tall bridge with bungee cords attached to one's ankles; and trying different forms of meditation. So far, none of these avenues have led to success productivity-wise.Any organismssenseof time is subjective, but is the time that is sensed also subjective, a mind-dependent phenomenon? Throughout history, philosophers of time have disagreed on the answer. Without minds in the world, nothing in the world would be surprising or beautiful or interesting. Can we add that nothing would be in time? Philosophers disagree on this.The majority answer is "no." The ability of the concept of time to help us make sense of our phenomenological evidence involving change, persistence, and succession of events is a sign that time may be objectively real. Consider succession, that is, order of events in time. We all agree that our memories of events occur after the events occur. If judgments of time were subjective in the way judgments of being interesting vs. not-interesting are subjective, then it would be too miraculous that everyone can so easily agree on the ordering of events in time. For example, first Einstein was born, then he went to school, then he died. Everybody agrees that it happened in this order: birth, school, death. No other order. The agreement on time order for so many events, both psychological events and physical events, is part of the reason that most philosophers and scientists believe physical time isobjectiveandnot dependent on being consciously experienced.Another large part of the reason to believe time is objective is that our universe has so many different processes that bear consistent time relations, or frequency of occurrence relations, to each other. For example, the frequency of rotation of the Earth around its axis is a constant multiple of the frequency of oscillation of a fixed-length pendulum, which in turn is a constant multiple of the half life of a specific radioactive uranium isotope, which in turn is a multiple of the frequency of a vibrating violin string; the relationship of these oscillators does not change as time goes by (at least not much and not for a long time, and when there is deviation we know how to predict it and compensate for it). The existence of these sorts of relationships makes our system of physical laws much simpler than it otherwise would be, and it makes us more confident that there is something objective we are referring to with the time-variable in those laws. The stability of these relationships over a long time makes it easy to create clocks. Time can be measured easily because we have access to long-term simple harmonic oscillators that have a regular period or regular ticking. This regularity shows up in completely different stable systems: rotations of the Earth,a swinging ball hanging from a string (a pendulum), a bouncing ball hanging from a coiled spring, revolutions of the Earth around the Sun, oscillating electric circuits, and vibrations of a quartz crystal. Many of these systems make good clocks.The existence of these possibilities for clocks strongly suggests that time is objective, and is not merely an aspect of consciousness.The issue about objectivity vs. subjectivity is related to another issue: realism vs. idealism. Is time real or instead just a useful instrument or just a useful convention or perhaps an arbitrary convention? This issue will appear several times throughout this article, including in the later section onconventionality.Aristotleraised this issue of the mind-dependence of time when he said, Whether, if soul (mind) did not exist, time would exist or not, is a question that may fairly be asked; for if there cannot be someone to count there cannot be anything that can be counted (Physics, chapter 14). He does not answer his own question because, he says rather profoundly, it depends on whether time is the conscious numbering of movement or instead is just the capability of movements being numbered were consciousness to exist.St. Augustine, adopting a subjective view of time, said time is nothing in reality but exists only in the minds apprehension of that reality. The 13th century philosophers Henry of Ghent and Giles of Rome said time exists in reality as a mind-independent continuum, but is distinguished into earlier and later parts only by the mind. In the 13th century,Duns Scotusclearly recognized both physical and psychological time.At the end of the 18th century,Kantsuggested a subtle relationship between time and mindthat our mind actually structures our perceptions so that we can know a priori that time is like a mathematical line. Time is, on this theory, a form of conscious experience, and our sense of time is a necessary condition of our having experiences such as sensations. In the 19th century, Ernst Mach claimed instead that our sense of time is a simple sensation, not an a priori form of sensation. This controversy took another turn when other philosophers argued that both Kant and Mach were incorrect because our sense of time is, instead,an intellectual construction (see Whitrow 1980, p. 64).In the 20th century, the philosopher of science Bas van Fraassen described time, including physical time, by saying, There would be no time were there no beings capable of reason just as there would be no food were there no organisms, and no teacups if there were no tea drinkers.The controversy in metaphysics between idealism and realism is that, for the idealist, nothing exists independently of the mind. If this controversy is settled in favor of idealism, then physical time, too, would have that subjective feature.It has been suggested by some philosophers that Einsteins theory of relativity, when confirmed, showed us that physical time depends on the observer, and thus that physical time is subjective, or dependent on the mind. This error is probably caused by Einsteins use of the term observer. Einsteins theory implies that the duration of an event depends on the observersframe of referenceor coordinate system, but what Einstein means by observers frame of reference is merely a perspective or coordinate framework from which measurements could be made. The observer need not have a mind. So, Einstein is not making a point about mind-dependence.To mention one last issue about the relationship between mind and time, if all organisms were to die, there would be events after those deaths. The stars would continue to shine, for example, but would any of these events be in the future? This is a controversial question because advocates ofMcTaggarts A-theorywill answer yes, whereas advocates of McTaggarts B-theory will answer no and say whose future?For more on the consciousness of time and related issues, see the article Phenomenology and Time-Consciousness. For more on whether the present, as opposed to time itself, is subjective, see the section called "Is the Present, the Now, Objectively Real?"3. What Is Time?Physical timeseems to be objective, whereas psychological time is subjective. Many philosophers of science argue that physical time is more fundamental even though psychological time is discovered first by each of us during our childhood, and even though psychological time was discovered first as we human beings evolved from our animal ancestors. The remainder of this article focuses more on physical time than psychological time.First, some attention needs to be paid to terminology. Physical time is all the times or all the ordered times. Times are the temporal coordinates of our coordinate system. Think of a time as a point on a line. A moment is not an event [think of the event of feeling a specific pain] but rather is either a time [as in, "He called me the moment he felt the pain"] or the duration of a brief event [as in, "The pain lasted for only a moment]. The terms instant and moment are often used interchangeably, but when used as a duration in physics then theduration is exactly zero seconds. When the term "instant" is used as a time rather than a duration, then we can say time is composed of the instants.Instantaneous events, on the other hand, are not instants; they are events that last onlyforan instant [in the sense of duration] and onlyatan instant [in the sense of a single time coordinate]. Although instants are not events, too often an author will conflate the terms "instantaneous event" and "instant" and say, "There is an ordering of instants that forms the totality of all the events throughout history" when the author really means instantaneous events and not instants. Another terminological difficulty occurs when an author says, "Time is a series of instants." This is a misuse of the word"series." A series is a sum of discrete terms; time, however, is a linear continuum of instants, not a series of instants.Any definition of the word "instant" that ties instants to events will presuppose the controversialrelational theoryof time. For example, in 1929 Bertrand Russell offered these precise definitions of instant and of occurring at an instant:X is aninstantiff X is an exhaustive class of mutually overlapping events.Event E isat instantX iff E is a member of X.On Russell's definition, an instant is neither a time, nor an event, nor an event's duration, but rather is a class.Now, back to the question, "What is time?"Time is what we use aclockor calendar to measure. But measurements depend onframes of reference.Before the creation of Einstein's special theory of relativity, it might have been said that time fixes these four features of reality: (1) For any event, it fixes when it occurs. (2) For any event, it specifies its durationhow long it lasts. (3) For any event, it fixes what other events occur simultaneously with it. (4) For any pair of instantaneousevents that are not simultaneous, it specifies which happens first. With the creation of the special theory of relativity in 1905, it was realized that these four features of time can be different in different reference frames. Nevertheless, within a reference frame, these are still four key parts of the answer to the question, "What is time?"Relativity theory implies that in any reference frame over short durations, that is, locally, time can be embedded in the mathematician's real line, so the temporal coordinates have the structure of the real numbers rather than merely the structure of the integers or the fractions. It is because of what time is that we can succeed in this embedding.All of the above are important features of time, but they do not tell us all of what time itself is.a. The Variety of AnswersBothered by the contradictions they claimed to find in our concept of time,Zeno,Plato,Spinoza, Hegel, andMcTaggartgave a radical answer the question, What is time? by replying that it is nothing because time does not exist (see LePoidevin and MacBeath 1993, p. 23). In a similar vein, the early 20th century English philosopher F. H. Bradley argued, Time, like space, has most evidently proved not to be real, but a contradictory appearance.The problem of change defies solution. In the mid-twentieth century, Gdel argued for the unreality of time because Einstein's equations allow for physically possible worlds in which events precede themselves. In the twenty-first century some physicists such as Julian Barbour say that in order to reconcile general relativity with quantum mechanics either time does not exist or else it is not fundamental in nature; see (Callender 2010) for a discussion of this. However, most philosophers agree that time does exist. They just cannot agree on what it is.We cannot trip over time, so what exactly is it? Is time human-made in analogy to how, according to some constructivist philosophers, mathematical objects are created by humans, and once created then they have well-determined properties some of which might be difficult for humans to discover? Or is time a Platonic idea existing outside of the physical world where it is independent of human activity? Or is time an emergent feature of physical changes, in analogy to how a sound wave is an emergent feature of the molecules of a vibrating tuning fork, with no single molecule making a sound? When we know what time is, then we can answer all these questions.Oneanswer to our question, What is time? is that time is whatever the time variabletis denoting in the best-confirmed and most fundamental theories of current science. Time is given an implicit definition this way. Nearly all philosophers would agree that we do learn much about physical time by looking at the behavior of the time variable in these theories; but they complain that the full nature of physical time can be revealed only with a philosophical theory of time that addresses the manyphilosophical issuesthat scientists do not concern themselves with.Lets briefly explore other answers that have been given throughout history to our question, What is time?Aristotleclaimed that time is the measure of change (Physics, chapter 12). He never said space is a measure of anything. Aristotle emphasized that time is not change [itself] because a change may be faster or slower, but not time (Physics, chapter 10). For example, a specific change such as the descent of a leaf can be faster or slower, but time itself cannot be faster or slower. In developing his views about time, Aristotle advocated what is now referred to as therelational theorywhen he said, there is no time apart from change. (Physics, chapter 11). In addition, Aristotle said time is not discrete or atomistic but is continuous. In respect ofsizethere is no minimum; for every line is dividedad infinitum. Hence it is so with time (Physics, chapter 11).RenDescarteshad a very different answer to What is time? He argued that a material body has the property of spatial extension but no inherent capacity for temporal endurance, and that God by his continual action sustains (or re-creates) the body at each successive instant. Time is a kind of sustenance or re-creation ("Third Meditation" inMeditations on First Philosophy).In the 17th century, the English physicist Isaac Barrow rejected Aristotles linkage between time and change. Barrow said time is something which exists independently of motion or change and which existed even before God created the matter in the universe. Barrows student, Isaac Newton, agreed with thissubstantival theoryof time. Newton argued very specifically that time and space are an infinitely large container for all events, and that the container exists with or without the events. He added that space and time are not material substances, but arelikesubstances in not being dependent on anything except God.GottfriedLeibnizobjected. He argued that time is not an entity existing independently of actual events. He insisted that Newton had underemphasized the fact that time necessarily involves anorderingof events. This is why time needs events, so to speak. Leibniz added that this overall orderistime. He accepted a relational theory of time and rejected a substantival theory.In the 18th century, ImmanuelKantsaid time and space are forms that the mind projects upon the external things-in-themselves. He spoke of our mind structuring our perceptions so that space always has a Euclidean geometry, and time has the structure of the mathematical line. Kants idea that time is aformof apprehending phenomena is probably best taken as suggesting that we have no direct perception of time but only the ability to experience things and eventsintime. Some historians distinguish perceptual space from physical space and say that Kant was right about perceptual space. It is difficult, though, to get a clear concept of perceptual space. If physical space and perceptual space are the same thing, then Kant is claiming we know a priori that physical space is Euclidean. With the discovery of non-Euclidean geometries in the 19th century, and with increased doubt about the reliability of Kants method of transcendental proof, the view that truths about space and time area priori truthsbegan to lose favor.In the early 20th century, Alfred North Whitehead said time is essentially the form of becominga cryptic, but interesting philosophical claim.By contrast, a physics book will say time is locally a linear continuum of instants.Michael Dummetts model of time implies instead that time is a composition of intervals rather than of instants. His model is controversial for a second reason. It is constructive in the sense that it implies there do not exist any times which are not detectable in principle by a physical process.The above discussion does not exhaust all the claims about what time is. And there is no sharp line separating a definition of time, a theory of time, and an explanation of time.b. Time vs. TimeWhatever time is, it is not time. Timeis the most common noun on the Internet; time is not.Nevertheless, it might help us understand time if we improved our understanding of the sense of the word time. Should the proper answer to the question What is time? produce a definition of the word as a means of capturing its sense? No. At least notif the definition must be some analysis that provides a simple paraphrase in all its occurrences. There are just too many varied occurrences of the word: time out, behind the times, in the nick of time, and so forth.But how about narrowing the goal to a definition of the word time in its main sense, the sense that most interests philosophers and physicists? That is, explore the usage of the word time in its principal sense as a means of learning what time is. Well, this project would require some consideration of the grammar of the word time. Most philosophers today would agree with A. N. Prior who remarked that, there are genuine metaphysical problems, but I think you have to talk about grammar at least a little bit in order to solve most of them. However, do we learn enough about what time is when we learn about the grammatical intricacies of the word? Ordinary-language philosophers have studied time talk, whatWittgensteincalled the language game of discourse about time. Wittgensteins expectation is that by drawing attention to ordinary ways of speaking we will be able to dissolve rather than answer our philosophical questions. But most philosophers of time are unsatisfied with this approach; they want the questions answered, not dissolved, although they are happy to have help from the ordinary language philosopher in clearing up misconceptions that may be produced by the way we use the word in our ordinary, non-technical discourse.When chemists made their great breakthrough in understanding water by finding that it is essentially H2O, this wasn't a discovery about the meaning of "water," but about what water is. Don't we want something like this for time?c. Linear and Circular TimeIs time more like a straight line or instead more like a circle? If your personal time were circular, then eventually you would be reborn. With circular time, the future is also in the past, and every event occurs before itself. If your time is like this, then the question arises as to whether you would be born an infinite number of times or only once. The argument that you'd be born only once appeals toLeibnizs Principle of the Identity of Indiscernibles:each supposedly repeating state of the world would occur just once because each state would not be discernible from the state that recurs, so counting the recurrences wouldn't make sense. The way to support the idea of eternal recurrence or repeated occurrence seems to be to presuppose a linear ordering in some "hyper" time of all the cycles so that each cycle is discernible from its predecessor because it occurs at a different hyper time.During history (and long before Einstein made a distinction betweenproper timeand coordinate time), a variety of answers were given to the question of whether time is like a line or, instead, closed like a circle. The concept of linear time first appeared in the writings of the Hebrews and the Zoroastrian Iranians. The Roman writer Seneca also advocated linear time. Plato and most other Greeks and Romans believed time to be motion and believed cosmic motion was cyclical, but this was not envisioned as requiring any detailed endless repetition such as the multiple rebirths of Socrates. However, thePythagoreansand someStoic philosopherssuch as Chrysippus did adopt this drastic position. Circular time was promoted in the Bible in Ecclesiastes 1:9: "That which has been is what will be, That which is done is what will be done, And there is nothing new under the sun." The idea was picked up again by Nietzsche in 1882. Scholars do not agree on whether Nietzsche meant his idea of circular time to be taken literally or merely for a moral lesson about how you should live your life if you knew that you'd live it over and over.Many Islamic and Christian theologians adopted the ancient idea that time is linear. Nevertheless, it was not until 1602 that the concept of linear time was more clearly formulatedby the English philosopherFrancis Bacon. In 1687, Newton advocated linear time when he represented time mathematically by using a continuous straight line with points being analogous to instants of time. The concept of linear time was promoted by Descartes, Spinoza, Hobbes, Barrow, Newton, Leibniz, Locke and Kant. Kant argued that it is a matter of necessity. In the early 19th century in Europe, the idea of linear time had become dominant in both science and philosophy.There are many other mathematically possible topologies for time. Time could be linear or closed (circular). Linear time might have a beginning or have no beginning; it might have an ending or no ending. There could be two disconnected time streams, in two parallel worlds, and perhaps one would be linear and the other circular. There could be branching time, in which time is like the letter "Y", and there could be a fusion time in which two different time streams are separate for some periodbut then merge into one stream. Time might be two dimensional instead of one dimensional. For all these topologies, there could be discrete time or, instead, continuous time. That is, themicro-structureof time's instants might be analogous to a sequence of integers or, instead, analogous to acontinuumof real numbers. For physicists, if time were discrete or quantized, their favorite lower limit on a possible duration is the Planck time of about 10-43seconds.d. Does Time Have a Beginning or End?In ancient Greece, Plato and Aristotle agreed that the past is eternal. Aristotle claimed that time had no beginning because, for any time, we always can imagine an earlier time. The reliability of appealing to our imagination to tell us how things are eventually waned, thanks in large part to the influence of Aquinas. In Medieval times, Aquinas' contemporary St. Bonaventure said there was a first motion and thus a first time. Martin Luther estimated the world to have begun in 4,000 B.C.E. Johannes Kepler estimated it to have begun in 4,004 B.C.E. The Calvinist James Ussher calculated that the world began on Friday, October 28, 4,004 B.C.E. Advances in the science of geology eventually refuted all these small estimates, and advances in astronomy eventually refuted the idea that the Earth and the universe were created at about the same time.Isaac Newton believed future time is infinite and that, although God created the material world some finite time ago, there was an infinite period of past time before that.Contemporary physicists generally agree that future time is infinite, but it is an open question whether past time is finite or infinite.Many physicists believe that past time is infinite, but many others believe instead that time began with the Big Bang about 13.8 billion years ago, that is, 13,800,000,000 years ago.In the most well-accepted version of the Big Bang Theory in the field of astrophysics, at the beginning of our Big Bang, our universe had an almost infinitesimal size and an almost infinite temperature and gravitational field. Our universe has been expanding and cooling ever since.In the more popular version of the Big Bang theory, the Big Bang theory with inflation, our universe once was an extremely tiny bit of explosively inflating material. About10-36second later, this inflationary material underwent an accelerating expansion that lasted for 10-30seconds during which the universe expanded by a factor of 1078. Once this brief period of inflation ended, the volume of our universe was the size of an orange, and the energy causing the inflation was transformed into a dense gas of expanding hot radiation. This expansion has never stopped. But with expansion came cooling, and this allowed individual material particles to condense and eventually much later to clump into stars and galaxies. The mutual gravitational force of the universes matter and energy decelerated the expansion, but seven billion years after our Big Bang, our universes dark energy became especially influential and started to accelerate the expansion again, despite the mutual gravitational force, although not at the explosive rate of the initial inflation. This more recent inflation of the universe will continue forever at an exponentially accelerating rate, as the remaining matter-energy becomes more and more diluted.The Big Bang Theory with or without inflation is challenged by other theories such as a cyclic theory in which every trillion years the expansion changes to contraction until our universe becomes infinitesimal, at which time there is a bounce or new Big Bang. The cycles of Bang and Crunch continue forever, and they might or might not have existed forever. For the details, see (Steinhardt 2012). A promising but as yet untested theory called "eternal inflation" implies that our particular Big Bang is one among many other Big Bangs that occurred within a background spacetime that is actually infinite in space and in past time and future time.Consider this challenging argument from (Newton-Smith 1980, p. 111) that claims time cannot have had a finite past: As we have reasons for supposing that macroscopic events have causal origins, we have reason to suppose that some prior state of the universe led to the product of [the Big Bang]. So the prospects for ever being warranted in positing a beginning of time are dim.The usual response to Newton-Smith here is two-fold. First, our Big Bang is a microscopic event, not a macroscopic event, so it might not be relevant that macroscopic events have causal origins. Second, and more importantly, if a confirmed cosmological theory implies there is a first event, we can say this event is an exception to any metaphysical principle that every event has a prior cause.e. Does Time Emerge from Something More Basic?Is time a fundamental feature of nature, or does itemerge from more basic timeless featuresin analogy to the way the smoothness of water flow emerges from the complicated behavior of the underlying molecules, none of which is properly called "smooth"? That is, is time ontologically basic (fundamental), or does it depend on something even more basic?We might rephrase this question more technically by asking whether facts about time supervene on more basic facts. Facts about sound in the air supervene on, or are a product of, facts about changes in the molecules of the air, so molecular change is more basic than sound. Minkowski argued in 1908 that we should believe spacetime is more basic than time, and this argument is generally well accepted. However, is this spacetime itself basic? Some physicists argue that spacetime is the product of some more basic micro-substrate at the level of the Planck length, although there is no agreed-upon theory of what the substrate is, although a leading candidate is quantum information.Other physicists say space is not basic, but time is. In 2004, after winning the Nobel Prize in physics, David Gross expressed this viewpoint:Everyone in string theory is convincedthat spacetime is doomed. But we dont know what its replaced by. We have an enormous amount of evidence thatspaceis doomed. We even have examples, mathematically well-defined examples, where space is an emergent concept. But in my opinion the tough problem that has not yet been faced up to at all is, How do we imagine a dynamical theory of physics in whichtimeis emergent? All the examples we have donothave an emergent time. They have emergent space but not time. It is very hard for me to imagine a formulation of physics without time as a primary concept because physics is typically thought of as predicting the future given the past. We have unitary time evolution. How could we have a theory of physics where we start with something in which time is never mentioned?The discussion in this section about whether time is ontologically basic has no implications for whether the word time is semantically basic or whether the idea of time is basic to concept formation.f. Time and ConventionalityIt is an arbitrary convention that our civilization designs clocks to count up to higher numbers rather than down to lower numbers as time goes on. It is just a matter of convenience that we agree to the convention of re-setting our clock by one hour as we cross a time-zone. It is an arbitrary convention that there aretwenty-four hours in a day instead of ten, that there are sixty seconds in a minute rather than twelve, that a second lastsas long as it does, and that the origin of our coordinate system for time is associated with the birth of Jesus on some calendars but the entry of Mohammed into Mecca on other calendars.According to relativity theory, if two events couldn't have had a causal effect on each other, then we analysts are free to choose a reference frame in which one of the events happens first, or instead the other event happens first, or instead the two events are simultaneous. But once a frame is chosen, this fixes the time order of any pair of events. This point is discussed further in the next section.In 1905, the French physicistHenri Poincarargued that time is not a feature of reality to be discovered, but rather is something we've invented for our convenience. He said possible empirical tests cannot determine very much about time, so he recommended the convention of adopting the concept of time that makes for the simplest laws of physics. Opposing this conventionalist picture of time, other philosophers of science have recommended a less idealistic view in which time is an objective feature of reality. These philosophers are recommending an objectivist picture of time.Turning now from the question of whether time is objective, let's consider whether the the measure of time is objective. Can our standard clock be inaccurate? Yes, say the objectivists about the standard clock. No, say the conventionalists who say that the standard clock is accurate by convention; if it acts strangely, then all clocks must act strangely in order to stay in synchrony with the standard clock that tells everyone the correct time. A closely related question is whether, when we change our standard clock, from being the Earth's rotation to being an atomic clock, or just our standard from one kind of atomic clock to another kind of atomic clock, are we merely adopting constitutive conventions for our convenience, or in some objective sense are we making a more correct choice?Consider how we use a clock to measure duration to measure how long an event lasts. We always use the following metric, that is, method: Take the time of the instant at which the event ends, and subtract the time of the instant when the event starts. For example, to find how long an event lasts that starts at 3:00 and ends at 5:00, we subtract and get the answer of two hours. Is the use of this method merely a convention, or in some objective sense is it the only way that a clock should be used? That is, is there an objective metric, or is time "metrically amorphous," because there are alternatively acceptable metrics?There is also an ongoing dispute about the extent to which there is an element of conventionality in Einsteins notion of two separated events happening at the same time. Einstein said that to define simultaneity in a single reference frameyou must adopt a convention about how fast light travels going one way as opposed to coming back (or going any other direction). He recommended adopting the convention that light travels the same speed in all directions (in a vacuum free of the influence of gravity). He claimed it must be a convention because there is no way to measure whether the speed is really the same in opposite directions since any measurement of the two speeds between two locations requires first having synchronized clocks at those two locations, yet the synchronization process will presuppose whether the speed is the same in both directions. The philosophers B. Ellis and P. Bowman in 1967 and D. Malament in 1977 gave different reasons why Einstein is mistaken. For an introduction to this dispute, see theFrequently Asked Questions. For more discussion, see (Callender and Hoefer 2002).4. What Does Science Require of Time?This article has already said quite a bit about what science requires of time, but in this section we recollect some of those points and add a few more.Physics, including astronomy, is the only science that explicitly studies time, although all sciences use the concept. Yet different physical theories place different demands on this concept.At the beginning of the 20thcentury, the appearance of both the general theory of relativity and the Big Bang theory transformed the investigation of whether time is finite or infinite from a primarily speculative and metaphysical investigation into one that occupied scientists in their professional journals.The Big Bang theory of cosmology places demands on the amount of past time. In 1611, Irish Bishop James Ussher declared that the implication of his reading of the Bible reveals that the beginning of time occurred on October 23, 4004 B.C.E. Today's science disagrees; it implies that our universe is at least as old as the beginning of the Big Bang, which was about 13.8 billion years ago. Other astronomical observations suggest the amount of future time is not finite but very probably apotential infinity(in Aristotle's sense of the term) as opposed to an actual infinity.Physical theories treat time as being another dimension, analogous to a spatial dimension, and they describe an event as being located at temporal coordinate t, where t is a real number rather than a rational number. Each specific temporal coordinate is called a "time." An instantaneous event is a moment and is located at just one time, or one temporal coordinate, say t1. It is said to last for an "instant." If the event is also a so-called "point event," then it is located at a single pointlocation, say .The fundamental laws of science do not pick out a present time. This fact is often surprising to a student who takes a science class and notices all sorts of talk about the present. Scientists frequently do apply some law of science while assigning, say, t0to be the name of the present moment, then calculate this or that. This insertion of the fact that t0is the present is an initial condition of the situation to which the law is being applied, and is not part of the law itself. The basic laws themselves treat all moments equally.Science does not require that all its theories have symmetry under time-translation, but this is a goal that physicists do pursue. If a theory has symmetry under time-translation, then the laws of the theories do not change. The law of gravitation in the 21stcentury is the same law that held one thousand centuries ago.Physics also requires that almost all the basic laws of science be time symmetric. This means that a law, if it is abasiclaw, must not distinguish between backward and forward time directions. The second law of thermodynamics is therefore not considered to be a basic law.Science also places requirements on the structure of time. For instance, in physics we need to speak of one event happening pi seconds after another, and of one event happening the square root of three seconds after another. In ordinary discourse outside of science we would never need this kind of precision. The need for this precision has led to requiring time to be a linearcontinuum, very much like a segment of the real number line. So, onerequirement that relativity, quantum mechanics and the Big Bang theory place on any duration is that it be a continuum. This implies that time is not quantized, even in quantum mechanics. In a world with time being a continuum, we cannot speak of some event being caused by the state of the world at the immediately preceding instant because there is no immediately preceding instant, just as there is no real number immediately preceding pi.Einstein's theory of relativity has had the biggest impact on our understanding of time. He saidtime is relative, meaning it is relative to the chosenreference frameor the chosen coordinate system.Galileo and Newton both would have said speed is relative to reference frame. Einstein would agree but would add that both the durations of events and the times when they occur are relative to the chosen reference frame.For example, any observer fixed to a moving railroad car in which you happen to be seated will say your speed is zero [in their reference frame in which the car has zero speed], whereas an observer fixed to the train station will say you have a positive speed. But as Galileo and Newton understood relativity, both observers will agree about the time you had lunch on the train. Einstein would say they are making a mistake about your lunchtime; they should disagree about when you had lunch.For Newton, the speed of anything, including light, would be different in the two frames that move relative to each other [say, one frame is fixed to the train and one frame is fixed to the train station], but Einstein disagreed and said Maxwells equations require the speed of light to be invariant in all frames. This implies that Newton's so-called "Galilean equations of motion" are incorrect. Einstein, in his special theory of relativity, figured out how to change the equations; the consequence is theLorentz transformationsin which two observers in relative motion will have to disagree also about the durations and occurrence times of events.What is happening here is that Einstein is requiring a mixing of space and time; Minkowski said it follows that there is a spacetime which is more fundamental than either time or space alone, and spacetime divides into its space part and time part differently for different observers, that is, different reference frames or different coordinate systems. So, time is relative in the sense that the duration of an event depends on the reference frame used in measuring the duration. Specifying that an event lasted three minutes without giving even an implicit indication of the reference frame is like asking someone to stand over there and not giving any indication of where there is.One implication of this is that it becomes more difficult to defendMcTaggart's A-theorywhich says that properties of events such as "happened twenty-three minutes ago" and "is happening now" are basic properties of events and are not properties relative to chosen reference frames.Another profound implication of relativity theory is that accurate clocks do not tick the same for everyone everywhere. Each object has its ownproper time,and so the correct time shown by a clock depends on its history (in particular, its history of speed and gravitational influence). Relative to clocks that are stationary in the reference frame, clocks in motion in the frame run slower, as do clocks in stronger gravitational fields. In general, two synchronized clocks do not stay synchronized if they move relative to each other or undergo different gravitational forces. Clocks in cars driving by your apartment building run slower than your apartments clock.Suppose there are two twins. One stays on Earth while the other twin zooms away in a spaceship and returns ten years later according to the spaceships clock. That same arrival event could be twenty years later according to an Earth-based clock, provided the spaceship went fast enough. The Earth twin would now be ten years older than the spaceship twin.According to relativitytheory, the order ofevents intimeis only a partial order because for any evente, there is an eventfsuch thateneed not occur before or afterf, nor simultaneous withf. These pairs of events are said to be in each others absolute elsewhere, which is another way of saying that neither could causally affect each other because even a light signal could not reach from one event to the other. Adding a coordinate system or reference frame to spacetime will force the events in all these pairs to have an order and so force the set of all events to be totally ordered in time, but what is interesting philosophically is that there is a leeway in the choice of the frame. For any two specific eventseandfthat could never causally affect each other, the analyst may choose a frame in whicheoccurs first, or choose another frame in whichfoccurs first, or instead choose another frame in which they are simultaneous. Any choice of frame will be correct. Such is the surprising nature of time according to relativity theory.General relativity places other requirements on events beyond the requirements of special relativity. Unlike in Newton's physics and the physics of special relativity, according to general relativity our spacetime is dynamic in the sense that any change in the amount and distribution of matter-energy will change the curvature of spacetime itself. Gravity is a manifestation of the warping of spacetime. In special relativity, its Minkowski spacetime has no curvature. In general relativity a spacetime with no mass or energy might or might not have curvature, so the geometry of spacetime is not always determined by the behavior of matter and energy. This point has been interpreted by many philosophers as a good reason to reject relationism.