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Encyclopedia > Arrow of time

In the natural sciences, time’s arrow, or arrow of time as it is also known, is a term coined in 1927 by British astronomer Arthur Eddington used to distinguish a direction of time on a four-dimensional relativistic map of the world; which, according to Eddington, can be determined by a study of organizations of atoms, molecules, and bodies. The term natural science as the way in which different fields of study are defined is determined as much by historical convention as by the present day meaning of the words. ... One of Sir Arthur Stanley Eddingtons papers announced Einsteins theory of general relativity to the English-speaking world. ...


Physical processes at the microscopic level are either entirely or mostly time symmetric, meaning that the theoretical statements that describe them remain true if the direction of time is reversed; yet when we describe things at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time. An arrow of time is anything that exhibits such time-asymmetry. The first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density. ... A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. ... T-symmetry is the symmetry of physical laws under a time-reversal transformation— The universe is not symmetric under time reversal, although in restricted contexts one may find this symmetry. ... Macroscopic is commonly used to describe physical objects that are measurable and observable by the naked eye. ... Two distinct views exist on the meaning of time. ...

Contents

History of term

From the 1928 book The Nature of the Physical World, which helped to popularize the term, Eddington states:

   
Arrow of time
Let us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing which cannot be undone. I shall use the phrase ‘time’s arrow’ to express this one-way property of time which has no analogue in space.
   
Arrow of time

Eddington then gives three points to note about this arrow: Image File history File links Cquote1. ... The first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density. ... Image File history File links Cquote2. ...

  1. It is vividly recognized by consciousness.
  2. It is equally insisted on by our reasoning faculty, which tells us that a reversal of the arrow would render the external world nonsensical.
  3. It makes no appearance in physical science except in the study of organization of a number of individuals.

Here, according to Eddington, the arrow indicates the direction of progressive increase of the random element. Following a lengthy argument into the nature of thermodynamics, Eddington concludes that in so far as physics is concerned time's arrow is a property of entropy alone. ‹ The template below has been proposed for deletion. ... In thermodynamics, entropy, symbolized by S, is a state function of a thermodynamic system defined by the differential quantity , where dQ is the amount of heat absorbed in a reversible process in which the system goes from the one state to another, and T is the absolute temperature. ...


Overview

The symmetry of time can be understood by a simple analogy: if time were perfectly symmetric then it would be possible to watch a movie taken of real events and everything that happens in the movie would seem realistic whether it was played forwards or backwards.


For example, a movie showing a cup falling off a table seems realistic when run forwards, but seems unrealistic if run backwards. On the other hand, a movie of the planets orbiting the sun would look equally realistic run forwards or backwards; either way the orbital motions would appear to conform to physical laws. Major features of the Solar System (not to scale): The Sun, the eight planets, the asteroid belt containing the dwarf planet Ceres, outermost there is the dwarf planet Pluto (the dwarf planet Eris not shown), and a comet. ... The Sun is the star of our solar system. ...


An example of irreversibility

Main article: irreversibility

Consider the situation in which a large container is filled with two separated liquids, for example a dye on one side and water on the other. With no barrier between the two liquids, the random jostling of their molecules will result in them becoming more mixed as time passes. However, if the dye and water are mixed then one does not expect them to separate out again when left to themselves. A movie of the mixing would seem realistic when played forwards, but unrealistic when played backwards. Irreversibility is that property of an event which makes reverting back to the state before the occurrence of the event impossible. ... Containers in the port of Kotka (Finland) on the Baltic Sea. ... A liquid will usually assume the shape of its container. ... In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces. ...


If the large container is observed early on in the mixing process, it might be found to be only partially mixed. It would be reasonable to conclude that, without outside intervention, the liquid reached this state because it was more ordered in the past, when there was greater separation, and will be more disordered, or mixed, in the future.


Now imagine that the experiment is repeated, this time with only a few molecules, perhaps ten, in a very small container. One can easily imagine that by watching the random jostling of the molecules it might occur — by chance alone — that the molecules became neatly segregated, with all dye molecules on one side and all water molecules on the other. That this can be expected to occur from time to time can be concluded from the fluctuation theorem; thus it is not impossible for the molecules to segregate themselves. However, for a large numbers of molecules it is so unlikely that one would have to wait, on average, longer than the age of the universe for it to occur. Thus a movie that showed a large number of molecules segregating themselves as described above would appear unrealistic and one would be inclined to say that the movie was being played in reverse. The second law of thermodynamics stands in apparent contradiction with the time reversible equations of motion for classical and quantum systems. ...


See also another example. Unsolved problems in physics: Arrow of time: Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Entropy is the only quantity in the physical sciences that picks a particular direction for time, sometimes called...


The thermodynamic arrow of time

The thermodynamic arrow of time is provided by the Second Law of Thermodynamics, which says that in an isolated system entropy will only increase with time; it will not decrease with time. Entropy can be thought of as a measure of disorder; thus the Second Law implies that time is asymmetrical with respect to the amount of order in an isolated system: as time increases, a system will always become more disordered. This asymmetry can be used empirically to distinguish between future and past. Unsolved problems in physics: Arrow of time: Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Entropy is the only quantity in the physical sciences that picks a particular direction for time, sometimes called... The second law of thermodynamics states that which is equivalent to this scientific statement: The Second Law is a statistical law and thus applicable only to macroscopic systems. ... In thermodynamics, an isolated system, as contrasted with a closed system, is a physical system that does not interact with its surroundings. ... In thermodynamics, entropy, symbolized by S, is a state function of a thermodynamic system defined by the differential quantity , where dQ is the amount of heat absorbed in a reversible process in which the system goes from the one state to another, and T is the absolute temperature. ... Disorder may refer to : A disease, in medicine Randomness (lack of order), in information theory This is a disambiguation page — a list of pages that otherwise might share the same title. ... ... Look up Future in Wiktionary, the free dictionary. ... PASTa building located at Zielna 37 street - Built in 1904-10. ...


The Second Law does not hold with strict universality: any system can fluctuate to a state of lower entropy (see the Poincaré recurrence theorem). However, the Second Law seems accurately to describe the overall trend in real systems toward higher entropy. The Poincaré recurrence theorem states that a system having a finite amount of energy and confined to a finite spatial volume will, after a sufficiently long time, return to an arbitrarily small neighborhood of its initial state. ...


This arrow of time seems to be related to all other arrows of time and arguably underlies some of them, with the exception of the weak arrow of time (see below).


The cosmological arrow of time

See also: Entropy and Entropy (arrow of time)

The cosmological arrow of time points in the direction of the universe's expansion. It may be linked to the thermodynamic arrow, with the universe heading towards a heat death (Big Chill) as the amount of usable energy becomes negligible. Alternatively, it may be an artifact of our place in the universe's evolution (see the Anthropic bias), with this arrow reversing as gravity pulls everything back into a Big Crunch. In thermodynamics, entropy, symbolized by S, is a state function of a thermodynamic system defined by the differential quantity , where dQ is the amount of heat absorbed in a reversible process in which the system goes from the one state to another, and T is the absolute temperature. ... Unsolved problems in physics: Arrow of time: Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Entropy is the only quantity in the physical sciences that picks a particular direction for time, sometimes called... The heat death is a possible final state of the universe, in which it has reached maximum entropy. ... Anthropic bias is the bias arising when your evidence is biased by observation selection effects, according to philosopher Nick Bostrom. ... Gravity is a force of attraction that acts between bodies that have mass. ... In physical cosmology, the Big Crunch is a hypothesized collapse of the universe upon itself after its expansion eventually stops — a counterpart to the Big Bang. ...


If this arrow of time is related to the other arrows of time, then the future is by definition the direction towards which the universe becomes bigger. Thus, the universe expands - rather than shrinks - by definition.


The radiative arrow of time

Waves, from radio waves to sound waves to those on a pond from throwing a stone, expand outward from their source, even though the wave equations allow for solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments which have created convergent waves, so this arrow probably follows from the thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it, making the latter contradictory to the Second Law of Thermodynamics in usual circumstances. Radio frequency, or RF, refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. ... This article is about compression waves. ... The wave equation is an important partial differential equation that describes a variety of waves, such as sound waves, light waves and water waves. ... The second law of thermodynamics states that which is equivalent to this scientific statement: The Second Law is a statistical law and thus applicable only to macroscopic systems. ...


The causal arrow of time

Causes are ordinarily thought to precede effects. The future can be controlled, but not the past.


A problem with using causality as an arrow of time is that, as David Hume pointed out, the causal relation per se cannot be perceived; one only perceives sequences of events. Furthermore it is surprisingly difficult to provide a clear explanation of what the terms "cause" and "effect" really mean. It does seem evident that dropping the plate is the cause, the plate shattering is the effect. However, it may be that the asymmetry that the observer relies upon in such cases is really the thermodynamic one. If the thermodynamic arrow were reversed then one would regard the gathering shards as the cause and the fused plate jumping up into our hands as the effect. David Hume (April 26, 1711 – August 25, 1776)[1] was a Scottish philosopher, economist, and historian, as well as an important figure of Western philosophy and of the Scottish Enlightenment. ...


The weak arrow of time

Certain subatomic interactions involving the weak nuclear force violate the conservation of parity, but only very rarely. According to the CPT Theorem, this means they should also be time irreversible, and so establish an arrow of time. Such processes should be responsible for matter creation in the early universe. The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... CPT-symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity and time simultaneously. ... Baryogenesis is the generic designation for the physical processes that generate matter (more specifically, a class of fundamental particle called baryon) from an otherwise matter-empty state (such as it is generally believed to be the state of the Universe at its onset, the so-called Big Bang). ...


This arrow is not linked to any other arrow by any proposed mechanism, and if it would have pointed to the opposite time direction, the only difference would have been that our universe would be made of anti-matter rather than from matter. More accurately, the definitions of matter and anti-matter would just be reversed.


That parity is broken so rarely means that this arrow only "barely" points in one direction, setting it apart from the other arrows whose direction is much more obvious.


The quantum arrow of time

Quantum evolution is governed by the Schrödinger equation, which is time-symmetric, and by wave function collapse, which is time irreversible. As the mechanism of wave function collapse is still obscure, it's not known how this arrow links to the others. While at the microscopic level, collapse seems to show no favor to increasing or decreasing entropy, some believe there is a bias which shows up on macroscopic scales as the thermodynamic arrow. According to the theory of quantum decoherence, and assuming that the wave function collapse is merely apparent, the quantum arrow of time is a consequence of the thermodynamic arrow of time (also see Entropy (arrow of time)). In physics, the Schrödinger equation, proposed by the Austrian physicist Erwin Schrödinger in 1925, is the definition of energy of a quantum system. ... In quantum mechanics, quantum decoherence is the process by which quantum systems in complex environments exhibit classical behavior. ... In quantum mechanics, quantum decoherence is the process by which quantum systems in complex environments exhibit classical behavior. ... Unsolved problems in physics: Arrow of time: Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Entropy is the only quantity in the physical sciences that picks a particular direction for time, sometimes called...


The psychological/perceptual arrow of time

Psychological time is, in part, the cataloguing of ever increasing items of memory from continuous changes in perception. The ancient method of comparing unique events to generalized repeating events such as the apparent movement of the sun, moon, and stars provided a convenient grid work to accomplish this. The consistent increase in memory volume creates one mental arrow of time. Another arises because one has the sense that one's perception is a continuous movement from the unknown (Future) to the known (Past). Anticipating the unknown forms The psychological future which always seems to be something one is moving towards, but, like a projection in a mirror, it makes what is actually already a part of memory, such as desires, dreams, and hopes, seem ahead of the observer.


Even the association of (behind = past) and (ahead = future) may be culturally conditioned. It was reported during 2006 that the Aymara people associated (ahead = past) and (behind = future).[1] The Aymara are a native ethnic group in the Andes region of South America; about 2. ...


The psychological arrow of time is thought to be reducible to the thermodynamic arrow: it has deep connections with Maxwell's demon and the physics of information; In fact, it is easy to understand its link to the Second Law of Thermodynamics if we view memory as correlation between brain cells (or computer bits) and the outer world. Since the Second Law of Thermodynamics is equivalent to the growth with time of such correlations, then it states that memory will be created as we move towards the future (rather than towards the past). Unsolved problems in physics: Arrow of time: Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Entropy is the only quantity in the physical sciences that picks a particular direction for time, sometimes called... The second law of thermodynamics states that which is equivalent to this scientific statement: The Second Law is a statistical law and thus applicable only to macroscopic systems. ... Unsolved problems in physics: Arrow of time: Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Entropy is the only quantity in the physical sciences that picks a particular direction for time, sometimes called...


The other side of the psychological passage of time is in the realm of volition and action. We plan and often execute actions intended to affect the course of events in the future. Hardly anyone tries to change past events. Indeed, in the Rubaiyat it is written (sic):

The Moving Finger writes; and, having writ,
  Moves on: nor all thy Piety nor Wit
Shall lure it back to cancel half a Line,
  Nor all thy Tears wash out a Word of it.
- Omar_Khayyám Rubaiyat is a common shorthand name for the collection of Persian verses known more formally as the Rubaiyat of Omar Khayyam. ... Fictional drawing of Omar Khayyám For other people, places or with similar names of Khayam, see Khayyam (disambiguation). ...


On the personal level, the process of aging is more than just the accumulation of memories, but, historically, there has been more hope of reversing ageing than of time travel or of reversing the arrow of time. See Fountain of youth. The Fountain of Youth by Lucas Cranach the Elder The Fountain of Youth is a legendary spring that reputedly restores the youth of anyone who drinks of its waters. ...


See also

Anthropic bias is the bias arising when your evidence is biased by observation selection effects, according to philosopher Nick Bostrom. ... Loschmidts paradox states that if there is a motion of a system that leads to a steady decrease of H (increase of entropy) with time, then there is certainly another allowed state of motion of the system, found by time reversal, in which H must increase. ... Maxwells demon is a character in an 1867 thought experiment by the Scottish physicist James Clerk Maxwell, meant to raise questions about the second law of thermodynamics. ... Two distinct views exist on the meaning of time. ... T-symmetry is the symmetry of physical laws under a time-reversal transformation— The universe is not symmetric under time reversal, although in restricted contexts one may find this symmetry. ... The British Royal Institution Christmas Lectures have been held annually since 1825. ...

References

  1. ^ For Andes tribe, it's back to the future — accessed 2006-09-26

2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... September 26 is the 269th day of the year (270th in leap years) in the Gregorian Calendar, with 96 days remaining. ...

Further reading

  • Halliwell, J.J. et.al. (1994). Physical Origins of Time Asymmetry. Cambridge. ISBN 0-521-56837-4. (technical).
  • Boltzmann, Ludwig (1964). Lectures On Gas Theory. University Of California Press. Translated from the original German by Stephen G. Brush. Originally published 1896/1898.
  • Peierls, R (1979). Surprises in Theoretical Physics. Princeton. Section 3.8.
  • Feynman, Richard (1965). The Character of Physical Law. BBC Publications. Chapter 5.
  • Penrose, Roger (1989). The Emperor's New Mind. Oxford University Press. ISBN 0-19-851973-7. Chapter 7.
  • Penrose, Roger (2004). The Road to Reality. Jonathan Cape. ISBN 0-224-04447-8. Chapter 27.
  • Price, Huw (1996). Time's Arrow and Archimedes' Point. ISBN 0-19-510095-6. Website
  • Wehrli, Hans (2006). Metaphysik - Chiralität als Grundprinzip der Physik. ISBN 3-033-00791-0.
  • Zeh, H. D (2001). The Physical Basis of The Direction of Time. ISBN 3-540-42081-9. Official website for the book

Ludwig Eduard Boltzmann (Vienna, Austrian Empire, February 20, 1844 – Duino near Trieste, September 5, 1906) was an Austrian physicist famous for his founding contributions in the fields of statistical mechanics and statistical thermodynamics. ... Richard Phillips Feynman (May 11, 1918 in Queens, New York – February 15, 1988 in Los Angeles, California) (surname pronounced FINE-man; in IPA) was an influential American physicist known for expanding greatly on the theory of quantum electrodynamics, particle theory, and the physics of the superfluidity of supercooled liquid helium. ... Sir Roger Penrose, OM, FRS (born 8 August 1931) is an English mathematical physicist and Emeritus Rouse Ball Professor of Mathematics at the University of Oxford. ... Sir Roger Penrose, OM, FRS (born 8 August 1931) is an English mathematical physicist and Emeritus Rouse Ball Professor of Mathematics at the University of Oxford. ...

External links

  • The Ritz-Einstein Agreement to Disagree Electrodynamic arrow of time, origin of second law of thermodynamics.

  Results from FactBites:
 
time: Definition, Synonyms and Much More from Answers.com (7003 words)
Time units are the intervals between successive recurrences of phenomena, such as the period of rotation of the Earth or a specified number of periods of radiation derived from an atomic energy-level transition.
Julian Barbour believes time to be an illusion which we interpret through what he calls "time capsules," which are "any fixed pattern that creates or encodes the appearance of motion, change or history." One example of this is the arrow of time.
Time travel is the concept of moving backward or forward to different points in time, in a manner analogous to moving through space.
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