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It was created as a useful convention, like when we decided to use certain coin-shaped metal objects as money. Money is culturally real but not objectively real because it would disappear if human culture were to disappear, even if the coin-shaped objects were not to disappear. Money and gold both exist, but money's existence depends upon social relations and conventions that gold's existence does not depend upon. Is time's existence more like money than gold in that regard?

Although it would be inconvenient to do so, our society could eliminate money and return to barter transactions. Time is a way to describe the pace of motion or change, such as the speed of a light wave, how fast a heart beats, or how frequently a planet spins…but these processes could be related directly to one another without making reference to time. Earth: , beats per rotation. Light: , kilometers per beat.

Truthmakers : The Contemporary Debate

Thus, some physicists argue that time is a common currency, making the world easier to describe but having no independent existence Callender , p. He said possible empirical tests cannot determine very much about time, so he recommended the convention of adopting whatever concept of time that makes for the simplest laws of physics. Nevertheless, he said, time is otherwise wholly conventional, not objective. One should be cautious about claiming something is merely a mathematical artifact that plays a merely auxiliary mathematical role in our theories of mathematical physics.

When the concept of an atom was first proposed several centuries ago, atoms could properly be said to be merely a mathematical artifact and so not real.

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But when Einstein won a Nobel Prize for showing in that atoms are indispensable for explaining Brownian motion, a phenomenon which was until then unexplained, the scientists generally united in agreement that atoms are real. There are two primary reasons to believe time is not merely conventional: First, there are so many one-way processes in nature. For example, mixing cold milk into hot, black coffee produces lukewarm, brown coffee, but agitations of lukewarm, brown coffee have never turned it back into hot black coffee with cool milk.

Time's arrow is a key feature of time itself. Second, our universe has so many periodic processes whose periods are constant multiples of each other over time. That is, their periods keep the same constant ratio to each other. For example, the frequency of rotation of the Earth around its axis, relative to the "fixed" stars, is a constant multiple of the frequency of swings 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 constant multiple of the frequency of a vibrating quartz crystal.

The relationships do 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 constant relationships—which cannot be changed by convention—makes our system of physical laws much simpler than it otherwise would be, and it makes us more confident that there is some convention-free, natural kind of entity that we are referring to with the time-variable in those physical laws.

Bothered by the contradictions they claimed to find in our concept of time, Parmenides, Zeno , Spinoza , Hegel, and McTaggart said time is not real. Plato's classical interpretation of Zeno's paradoxes is that they demonstrate the unreality of any motion or any other change. Assuming the existence of time requires the existence of change, then Zeno's paradoxes also overturn Greek commonsense that time exists. The early 20th century British philosopher J. McTaggart believed he had a convincing argument for why a single event can acquire the intrinsic properties of being a future event, a present event and also a past event, and that since these are contrary properties, our concept of time is inconsistent.

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It follows for McTaggart that time is not real. The early 20th century absolute-idealist philosopher F.

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Bradley claimed, "Time, like space, has most evidently proved not to be real, but a contradictory appearance…. The problem of change defies solution. Regarding the inconsistencies in our concept of time that Zeno, McTaggart, Bradley, and others claim to have revealed, most philosophers of time will say that there is no inconsistency, and that the complaints can be handled by clarification or by revising the relevant concepts.

For example, Zeno's paradoxes were solved by requiring time to be a linear continuum like a segment of the real number line. If you believe that for time to exist it needs to have certain features of the commonsense image of time, but you believe that science implies time does not have those features, you may be tempted to conclude that science has really discovered that time does not exist. Regarding the claim that our commonsense understanding of time by science is not treated fairly by the science of time, there is no agreement about which particular features of commonsense time cannot be rejected, although not all can be or else we would be rejecting time itself and changing the subject.

But science has not required us to reject our belief that some events happen in time before other events, and our belief that some events last for a while. The word "time" refers to a real or existing entity because the reference helps to explain, understand, and predict phenomena, plus there do not exist alternative, better ways of doing this.

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It is still an open question among physicists and philosophers as to whether time exists below the Planck scale. However, for simplicity of presentation, the attitude taken in the remainder of this article is that time does exist at scales above the Planck scale, that the concept is objective rather than subjective, that it is not primarily conventional or a mathematical artifact, that any inconsistency in time's description is merely apparent or is not essential and can be eliminated , and that time is real regardless of whether it is emergent.

You are much more likely today to meet a time traveler from the past than from the future. Would you yourself like to travel to the future and read about the history of your great grandchildren? You can do it. Nothing in principle is stopping you except some financial difficulties and a better-engineered spaceship that can survive occasional collisions with objects in space. Would you like to travel, instead, to the past? You may have regrets and would like to set things straight. The term "time travel" has now become a technical term. It means physical time travel, not psychological time travel.

You are not a time traveler if you merely dream of living in the past, nor do you time travel for five minutes simply by being alive for five minutes. And you do not travel in time by crossing a time zone, or by having your body frozen. In , the Princeton University metaphysician David Lewis offered this technical definition of time travel:. Time travel occurs when correct clocks get out of synchronization. If you are the traveler, your personal time is shown on your small personal clock that travels with you and that is not frozen if your body is ever frozen. The person not taking the journey is said to be using "external time.

Informally, and perhaps misleadingly, we say time "flows" differently for the time traveler than for the rest of us. If you enter a room, then exit fifteen seconds later according to your personal clock, is the room a time machine that sends you fifteen seconds into the future? No, as can be seen by applying Lewis' definition your clock and a nearby clock that remained outside the room do not disagree on the time.

Lewis' definition is widely accepted, but has been criticized occasionally in the philosophical literature. The definition has no implications about whether, if you travel forward in external time to the year or backward to , you can suddenly pop into existence then as opposed to having traveled continuously during the intervening years.

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Here is a diagram of a traveler's sudden appearance back in external time showing her death before her birth. The machine abruptly transports her back to the way the world was at t 1 , but her personal time is still T 2 , and T 2? Later she dies at T 3 , in a situation where the external time is t 2. Thus, according to external time, she dies before she is born. Suppose at t 1 , you met this time traveler.

Her clock would disagree with yours and would show t 5.

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  • The diagram illustrates why, regardless of whether the backward time travel is instantaneous or requires some travel time, if you were to meet someone whose watch runs ahead of yours, this could be a sign that you've just met a time traveler from the future. If Lewis' definition of time travel is acceptable, any requirement that rules out discontinuous jumps by the traveler in external time and demands spatiotemporal continuity of time travel will have to be supported by an additional argument.

    The argument that the general theory of relativity requires this continuity is such an argument.

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    Time travel to the future does occur very frequently, and it has been observed and measured by scientists. Travel to the past is much more controversial. Relativity theory implies there are two kinds of future time travel—two clocks getting out of synchrony due to their moving relative to each other, and two clocks getting out of synchrony due to their encountering different gravitational forces.

    When you travel to the future, you eventually arrive at some future event having taken less time on your clock than the non-travelers do on their clocks. To them, you zipped through time; to you, they were sluggish. It's all relative. You might travel to the future in the sense that you participate in an event ten years in their future, having taken only two years according to your own clock. That would be an eight-year leap forward in time, judged by their clock.

    However, the Earth observers would notice that you turned the pages in your monthly calendar very slowly. The rate of ticking of your clock differs from that of their clock during the flight, but when the two clocks are reunited and not moving relative to each other, they will tick at the same rate while showing an eight-year difference in times. Reversing your velocity and traveling back to the place you began the trip will not undue this effect.