Arrow of time

The arrow of time, or time's arrow is the concept positing the "one-way direction" or "asymmetry" of time. It was developed in 1927 by the British astronomer Arthur Eddington, and is an unsolved general physics question. This direction, according to Eddington, could be determined by studying the organization of atoms, molecules, and bodies, and might be drawn upon a four-dimensional relativistic map of the world ("a solid block of paper").[1]

Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time.

Overview

The symmetry of time (T-symmetry) can be understood by a simple analogy: if time were perfectly symmetrical, a video of real events would seem realistic whether played forwards or backwards.[2] An obvious objection to this notion is gravity: things fall down, not up. Yet a ball that is tossed up, slows to a stop, and falls into the hand is a case where recordings would look equally realistic forwards and backwards. The system is T-symmetrical, but while going "forward", kinetic energy is dissipated and entropy is increased. Entropy may be one of the few processes that is not time-reversible. According to the statistical notion of increasing entropy, the "arrow" of time is identified with a decrease of free energy.[3]

Eddington

In the 1928 book The Nature of the Physical World, which helped to popularize the concept, Eddington stated:

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.

Eddington then gives three points to note about this arrow:

  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. (By which he means that it is only observed in entropy, a statistical mechanics phenomenon arising from a system.)

According to Eddington the arrow indicates the direction of progressive increase of the random element. Following a lengthy argument upon the nature of thermodynamics he concludes that, so far as physics is concerned, time's arrow is a property of entropy alone.

Arrows

Thermodynamic arrow of time

The arrow of time is the "one-way direction" or "asymmetry" of time. The thermodynamic arrow of time is provided by the second law of thermodynamics, which says that in an isolated system, entropy tends to increase with time. Entropy can be thought of as a measure of microscopic disorder; thus the second law implies that time is asymmetrical with respect to the amount of order in an isolated system: as a system advances through time, it becomes more statistically disordered. This asymmetry can be used empirically to distinguish between future and past, though measuring entropy does not accurately measure time. Also, in an open system, entropy can decrease with time.

British physicist Sir Alfred Brian Pippard wrote, "There is thus no justification for the view, often glibly repeated, that the Second Law of Thermodynamics is only statistically true, in the sense that microscopic violations repeatedly occur, but never violations of any serious magnitude. On the contrary, no evidence has ever been presented that the Second Law breaks down under any circumstances."[4] However, there are a number of paradoxes regarding violation of the second law of thermodynamics, one of them due to the Poincaré recurrence theorem.

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.

Harold Blum's 1951 book Time's Arrow and Evolution[5] "explored the relationship between time's arrow (the second law of thermodynamics) and organic evolution." This influential text explores "irreversibility and direction in evolution and order, negentropy, and evolution."[6] Blum argues that evolution followed specific patterns predetermined by the inorganic nature of the earth and its thermodynamic processes.[7]

Cosmological 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.

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 thermodynamic arrow of time and the second law of thermodynamics are thought to be a consequence of the initial conditions in the early universe.[8] Therefore, they ultimately result from the cosmological set-up.

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 accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves,[9] 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.

Causal arrow of time

A cause precedes its effect: the causal event occurs before the event it affects. Birth, for example, follows a successful conception and not vice versa. Thus causality is intimately bound up with time's arrow.

An epistemological problem with using causality as an arrow of time is that, as David Hume maintained, 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, or to define the events to which they refer. However, it does seem evident that dropping a cup of water is a cause while the cup subsequently shattering and spilling the water is the effect.

Physically speaking, the perception of cause and effect in the dropped cup example is a phenomenon of the thermodynamic arrow of time, a consequence of the second law of thermodynamics.[10] Controlling the future, or causing something to happen, creates correlations between the doer and the effect,[11] and these can only be created as we move forwards in time, not backwards.

Particle physics (weak) arrow of time

Certain subatomic interactions involving the weak nuclear force violate the conservation of both parity and charge conjugation, but only very rarely. An example is the kaon decay.[12] 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.

That the combination of parity and charge conjugation 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. This arrow had not been linked to any large scale temporal behaviour until the work of Joan Vaccaro, who showed that T violation could be responsible for conservation laws and dynamics.[13]

Quantum arrow of time

Question, Web Fundamentals.svg Unsolved problem in physics:
What links the quantum arrow of time to the thermodynamic arrow?
(more unsolved problems in physics)

According to the Copenhagen interpretation of quantum mechanics, 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 philosophically obscure, it is not completely clear how this arrow links to the others. Despite the post-measurement state being entirely stochastic in formulations of quantum mechanics, a link to the thermodynamic arrow has been proposed, noting that the second law of thermodynamics amounts to an observation that nature shows a bias for collapsing wave functions into higher entropy states versus lower ones, and the claim that this is merely due to more possible states being high entropy runs afoul of Loschmidt's paradox. According to one physical view of wave function collapse, the theory of quantum decoherence, the quantum arrow of time is a consequence of the thermodynamic arrow of time.

Relational quantum mechanics proposes that there is no such thing as an absolute wave function collapse, and that what an observer sees as wave function collapse is in fact the observer becoming entangled with the measured state. The thermodynamic arrow is an increase in entanglement over time; in this way, relational quantum mechanics relates the quantum arrow to the thermodynamic arrow.

In 2019, a team of Russian scientists reported the reversal of the quantum arrow of time on an IBM quantum computer.[14] By observing the state of the quantum computer made of two and later three superconducting qubits, they found that in 85% of the cases, the two-qubit computer returned into the initial state.[15] The state's reversal was made by a special program, similarly to the random microwave background fluctuation in the case of the electron.[15] However, according to the estimations, throughout the age of the universe (13.7 billion years) such a reversal of the electron's state would only happen once, for 0.06 nanoseconds.[15] The scientists' experiment led to the possibility of a quantum algorithm that reverses a given quantum state through complex conjugation.[14]

Quantum source of time

Physicists say that quantum uncertainty gives rise to entanglement, the putative source of the arrow of time. The idea that entanglement might explain the arrow of time was proposed by Seth Lloyd in the 1980s. Lloyd argues that quantum uncertainty, and the way it spreads as particles become increasingly entangled, could replace human uncertainty in the old classical proofs as the true source of the arrow of time. According to Lloyd, "The arrow of time is an arrow of increasing correlations."[16]

Psychological/perceptual arrow of time

A related mental arrow arises because one has the sense that one's perception is a continuous movement from the known (past) to the unknown (future). Anticipating the unknown forms the psychological future, which always seems to be something one is moving towards. However, 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.

The association of "behind ⇔ past" and "ahead ⇔ future" is itself culturally determined. For example, the Aymara language associates "ahead ⇔ past" and "behind ⇔ future".[17] Similarly, the Chinese term for "the day after tomorrow" 後天 ("hòu tiān") literally means "after (or behind) day", whereas "the day before yesterday" 前天 ("qián tiān") is literally "preceding (or in front) day."[18]

The words "yesterday" and "tomorrow" both translate to the same word in Hindi: कल ("kal"),[19] meaning "[one] day remote from today."[20] The ambiguity is resolved by verb tense. परसों ("parsoⁿ") is used for both "day before yesterday" and "day after tomorrow", or "two days from today".[21] नरसों ("narsoⁿ") is used for "three days from today."[22]

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. From the Rubaiyat:

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 (translation by Edward Fitzgerald).

See also

References

  1. ^ Weinert, Friedel (2005). The scientist as philosopher: philosophical consequences of great scientific discoveries. Springer. p. 143. ISBN 978-3-540-21374-1., Chapter 4, p. 143
  2. ^ David Albert on Time and Chance
  3. ^ Tuisku, P.; Pernu, T.K.; Annila, A. (2009). "In the light of time". Proceedings of the Royal Society A. 465 (2104): 1173–1198. Bibcode:2009RSPSA.465.1173T. doi:10.1098/rspa.2008.0494.
  4. ^ A. B. Pippard, Elements of Chemical Thermodynamics for Advanced Students of Physics (1966), p.100.
  5. ^ Blum, Harold F. (1951). Time's Arrow and Evolution (First ed.). ISBN 978-0-691-02354-0.
  6. ^ Morowitz, Harold J. (September 1969). "Book review: Time's arrow and evolution: Third Edition". Icarus. 11 (2): 278–279. Bibcode:1969Icar...11..278M. doi:10.1016/0019-1035(69)90059-1. PMC 2599115.
  7. ^ McN., W. P. (November 1951). "Book reviews: Time's Arrow and Evolution". Yale Journal of Biology and Medicine. 24 (2): 164. PMC 2599115.
  8. ^ Susskind, Leonard. "Boltzmann and the Arrow of Time: A Recent Perspective". Cornell University. Cornell University. Retrieved June 1, 2016.
  9. ^ Mathias Fink (30 November 1999). "Time-Reversed Acoustic" (PDF). Archived from the original (PDF) on 31 December 2005. Retrieved 27 May 2016.
  10. ^ Physical Origins of Time Asymmetry, chapter 6
  11. ^ Physical Origins of Time Asymmetry, pp. 109–111.
  12. ^ "Home". Physics World.
  13. ^ Vaccaro, Joan (2016). "Quantum asymmetry between time and space". Proceedings of the Royal Society A. 472 (2185): 20150670. arXiv:1502.04012. Bibcode:2016RSPSA.47250670V. doi:10.1098/rspa.2015.0670. PMC 4786044. PMID 26997899.
  14. ^ a b G. B. Lesovik, I. A. Sadovskyy, M. V. Suslov, A. V. Lebedev, V. M. Vinokur (13 March 2019). "Arrow of time and its reversal on the IBM quantum computer". Nature. 9. doi:10.1038/s41598-019-40765-6. Retrieved 13 March 2019.CS1 maint: Uses authors parameter (link)
  15. ^ a b c "Physicists reverse time using quantum computer". Phys.org. 13 March 2019. Retrieved 13 March 2019.
  16. ^ Wolchover, Natalie (25 April 2014). "New Quantum Theory Could Explain the Flow of Time" – via www.wired.com.
  17. ^ For Andes tribe, it's back to the future — accessed 2006-09-26
  18. ^ mbdg.net Chinese-English Dictionary — accessed 2017-01-11
  19. ^ Bahri, Hardev (1989). Learners' Hindi-English Dictionary. Delhi: Rajpal & Sons. p. 95. ISBN 978-81-7028-002-6.
  20. ^ Alexiadou, Artemis (1997). Adverb placement : a case study in antisymmetric syntax. Amsterdam [u.a.]: Benjamins. p. 108. ISBN 978-90-272-2739-3.
  21. ^ Hindi-English.org Hindi English Dictionary परसों — accessed 2017-01-11
  22. ^ Shabdkosk.Raftaar.in Hindi English Dictionary नरसों — accessed 2017-01-11

Further reading

External links

A Brief History of Time

A Brief History of Time: From the Big Bang to Black Holes is a popular-science book on cosmology (the study of the universe) by British physicist Stephen Hawking. It was first published in 1988. Hawking wrote the book for nonspecialist readers with no prior knowledge of scientific theories.

In A Brief History of Time, Hawking writes in non-technical terms about the structure, origin, development and eventual fate of the universe, which is the object of study of astronomy and modern physics. He talks about basic concepts like space and time, basic building blocks that make up the universe (such as quarks) and the fundamental forces that govern it (such as gravity). He writes about cosmological phenomena such as the Big Bang and black holes. He discusses two major theories, general relativity and quantum mechanics, that modern scientists use to describe the universe. Finally, he talks about the search for a unifying theory that describes everything in the universe in a coherent manner.

The book became a bestseller and sold more than 10 million copies in 20 years. It was also on the London Sunday Times bestseller list for more than five years and was translated into 35 languages by 2001.

Arrow of Time (Numbers)

"Arrow of Time" is the 11th episode of the fifth season of the American television show Numb3rs. In the episode, Federal Bureau of Investigation (FBI) agents track an escaped prisoner who is seeking revenge against one of their own. Written by series writer/executive producer Ken Sanzel, the episode continued two storylines. The first storyline continued one that began with two episodes shown very early in season three. The other was originally launched in the season four season finale "When Worlds Collide".

"Arrow of Time" first aired in the United States on January 9, 2009.

Big Crunch

The Big Crunch is one of the theoretical scenarios for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero or causing a reformation of the universe starting with another Big Bang.

Some experimental evidence casts doubt on this theory and suggests that the expansion of the universe is accelerating, rather than being slowed down by gravity. However, more recent research has called this conclusion into question.

Condition Hüman

Condition Hüman is the fourteenth studio album by American heavy metal band Queensrÿche. It was released on October 2, 2015 through Century Media. It is the second studio album recorded with vocalist Todd La Torre, after Geoff Tate was fired from the band and gave up the rights to the name after a two-year lawsuit regarding the rights to the Queensrÿche brand was amicably settled.The album's first single, "Arrow of Time", became available for streaming on July 20, 2015, the day the album's title and release date were announced. It was recorded in early 2015 at Uberbeatz Studios in Washington and produced by Chris "Zeuss" Harris.

Entropy

In statistical mechanics, entropy is an extensive property of a thermodynamic system. It is closely related to the number Ω of microscopic configurations (known as microstates) that are consistent with the macroscopic quantities that characterize the system (such as its volume, pressure and temperature). Under the assumption that each microstate is equally probable, the entropy is the natural logarithm of the number of microstates, multiplied by the Boltzmann constant kB. Formally (assuming equiprobable microstates),

Macroscopic systems typically have a very large number Ω of possible microscopic configurations. For example, the entropy of an ideal gas is proportional to the number of gas molecules N. Roughly twenty liters of gas at room temperature and atmospheric pressure has N6×1023 (Avogadro's number). At equilibrium, each of the Ω ≈ eN configurations can be regarded as random and equally likely.[citation needed]

The second law of thermodynamics states that the entropy of an isolated system never decreases over time. Such systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy. Non-isolated systems may lose entropy, provided their environment's entropy increases by at least that amount so that the total entropy increases. Entropy is a function of the state of the system, so the change in entropy of a system is determined by its initial and final states. In the idealization that a process is reversible, the entropy does not change, while irreversible processes always increase the total entropy.

Because it is determined by the number of random microstates, entropy is related to the amount of additional information needed to specify the exact physical state of a system, given its macroscopic specification. For this reason, it is often said that entropy is an expression of the disorder, or randomness of a system, or of the lack of information about it[citation needed]. The concept of entropy plays a central role in information theory.

Boltzmann's constant, and therefore entropy, have dimensions of energy divided by temperature, which has a unit of joules per kelvin (J⋅K−1) in the International System of Units (or kg⋅m2⋅s−2⋅K−1 in terms of base units). The entropy of a substance is usually given as an intensive property—either entropy per unit mass (SI unit: J⋅K−1⋅kg−1) or entropy per unit amount of substance (SI unit: J⋅K−1⋅mol−1).

Entropy (arrow of time)

Entropy is the only quantity in the physical sciences (apart from certain rare interactions in particle physics; see below) that requires a particular direction for time, sometimes called an arrow of time. As one goes "forward" in time, the second law of thermodynamics says, the entropy of an isolated system can increase, but not decrease. Hence, from one perspective, entropy measurement is a way of distinguishing the past from the future. However, in thermodynamic systems that are not closed, entropy can decrease with time: many systems, including living systems, reduce local entropy at the expense of an environmental increase, resulting in a net increase in entropy. Examples of such systems and phenomena include the formation of typical crystals, the workings of a refrigerator and living organisms, used in thermodynamics.

Much like temperature, despite being an abstract concept, everyone has an intuitive sense of the effects of entropy. For example, it is often very easy to tell the difference between a video being played forwards or backwards. A video may depict a wood fire that melts a nearby ice block, played in reverse it would show that a puddle of water turned a cloud of smoke into unburnt wood and froze itself in the process. Surprisingly, in either case the vast majority of the laws of physics are not broken by these processes, a notable exception being the second law of thermodynamics. When a law of physics applies equally when time is reversed it is said to show T-symmetry, in this case entropy is what allows one to decide if the video described above is playing forwards or in reverse as intuitively we identify that only when played forwards the entropy of the scene is increasing. Because of the second law of thermodynamics entropy prevents macroscopic processes showing T-symmetry.

When studying at a microscopic scale the above judgements can not be made. Watching a single smoke particle buffeted by air it would not be clear if a video was playing forwards or in reverse and in fact it would not be possible as the laws which apply show T-symmetry, as it drifts left or right qualitatively it looks no different. It is only when you study that gas at a macroscopic scale that the effects of entropy become noticeable. On average you would expect the smoke particles around a struck match to drift away from each other, diffusing throughout the available space. It would be an astronomically improbable event for all the particles to cluster together, yet you can not comment on the movement of any one smoke particle.

By contrast, 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. This, however, is neither linked to the thermodynamic arrow of time, nor has anything to do with our daily experience of time irreversibility.

Gold universe

A Gold universe is a cosmological model of the universe. In these models, the universe starts with a Big Bang and expands for some time, with increasing entropy and a thermodynamic arrow of time pointing in the direction of the expansion. After the universe reaches a low-density state, it recontracts, but entropy now decreases, pointing the thermodynamic arrow of time in the opposite direction, until the universe ends in a low-entropy, high-density Big Crunch.

There are two models of the universe which support the possibility of a reversed direction of time. The first begins with a state of low entropy at the Big Bang which continually increases until the Big Crunch. The second, a Gold Universe, posits that entropy will increase only until a moment of contraction, then gradually decrease. This latter model suggests the universe will become more orderly after the moment of contraction. The Gold model has been linked to the possibility of retrocausal change, questions concerning the preservation of information in a time-reversed universe (states of decreasing entropy), and causation in general. The Gold Universe is named after the cosmologist Thomas Gold, who proposed the model in the 1960s.

Heat death of the universe

The heat death of the universe, also known as the Big Chill or Big Freeze, is an idea of an ultimate fate of the universe in which the universe has evolved to a state of no thermodynamic free energy and therefore can no longer sustain processes that increase entropy. Heat death does not imply any particular absolute temperature; it only requires that temperature differences or other processes may no longer be exploited to perform work. In the language of physics, this is when the universe reaches thermodynamic equilibrium (maximum entropy).

If the topology of the universe is open or flat, or if dark energy is a positive cosmological constant (both of which are consistent with current data), the universe will continue expanding forever, and a heat death is expected to occur, with the universe cooling to approach equilibrium at a very low temperature after a very long time period.

The hypothesis of heat death stems from the ideas of William Thomson, 1st Baron Kelvin (Lord Kelvin), who in the 1850s took the theory of heat as mechanical energy loss in nature (as embodied in the first two laws of thermodynamics) and extrapolated it to larger processes on a universal scale.

Intercalation (timekeeping)

Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.

Loschmidt's paradox

Loschmidt's paradox, also known as the reversibility paradox, irreversibility paradox or Umkehreinwand, is the objection that it should not be possible to deduce an irreversible process from time-symmetric dynamics. This puts the time reversal symmetry of (almost) all known low-level fundamental physical processes at odds with any attempt to infer from them the second law of thermodynamics which describes the behaviour of macroscopic systems. Both of these are well-accepted principles in physics, with sound observational and theoretical support, yet they seem to be in conflict; hence the paradox.

Negative mass

In theoretical physics, negative mass is matter whose mass is of opposite sign to the mass of normal matter, e.g. −1 kg. Such matter would violate one or more energy conditions and show some strange properties, stemming from the ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass. It is used in certain speculative hypotheses, such as on the construction of traversable wormholes and the Alcubierre drive. Initially, the closest known real representative of such exotic matter is a region of negative pressure density produced by the Casimir effect.

General relativity describes gravity and the laws of motion for both positive and negative energy particles, hence negative mass, but does not include the other fundamental forces. On the other hand, the Standard Model describes elementary particles and the other fundamental forces, but it does not include gravity. A unified theory that explicitly includes gravity along with the other fundamental forces may be needed for a better understanding of the concept of negative mass.

In December 2018, the astrophysicist Jamie Farnes from the University of Oxford proposed a "dark fluid" theory, related, in part, to notions of gravitationally repulsive negative masses, presented earlier by Albert Einstein, that may help better understand, in a testable manner, the considerable amounts of unknown dark matter and dark energy in the cosmos.

Philosophy of thermal and statistical physics

The philosophy of thermal and statistical physics is that part of the philosophy of physics whose subject matter is an amalgam of classical thermodynamics, statistical mechanics, and related theories. Its central questions include: What is entropy, and what does the second law of thermodynamics say about it? Does either thermodynamics or statistical mechanics contain an element of time-irreversibility? If so, what does statistical mechanics tell us about the arrow of time? What is the nature of the probabilities that appear in statistical mechanics?

Present

The present (or here and now) is the time that is associated with the events perceived directly and in the first time, not as a recollection (perceived more than once) or a speculation (predicted, hypothesis, uncertain). It is a period of time between the past and the future, and can vary in meaning from being an instant to a day or longer. In radiocarbon dating, the "present" is defined as AD 1950.

It is sometimes represented as a hyperplane in space-time, typically called "now", although modern physics demonstrates that such a hyperplane cannot be defined uniquely for observers in relative motion. The present may also be viewed as a duration (see specious present).

Quantum entanglement

Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles are generated, interact, or share spatial proximity in ways such that the quantum state of each particle cannot be described independently of the state of the others, even when the particles are separated by a large distance.

Measurements of physical properties such as position, momentum, spin, and polarization, performed on entangled particles are found to be correlated. For example, if a pair of particles is generated in such a way that their total spin is known to be zero, and one particle is found to have clockwise spin on a certain axis, the spin of the other particle, measured on the same axis, will be found to be counterclockwise, as is to be expected due to their entanglement. However, this behavior gives rise to seemingly paradoxical effects: any measurement of a property of a particle performs an irreversible collapse on that particle and will change the original quantum state. In the case of entangled particles, such a measurement will be on the entangled system as a whole.

Such phenomena were the subject of a 1935 paper by Albert Einstein, Boris Podolsky, and Nathan Rosen, and several papers by Erwin Schrödinger shortly thereafter, describing what came to be known as the EPR paradox. Einstein and others considered such behavior to be impossible, as it violated the local realism view of causality (Einstein referring to it as "spooky action at a distance") and argued that the accepted formulation of quantum mechanics must therefore be incomplete.

Later, however, the counterintuitive predictions of quantum mechanics were verified experimentally in tests where the polarization or spin of entangled particles were measured at separate locations, statistically violating Bell's inequality. In earlier tests it couldn't be absolutely ruled out that the test result at one point could have been subtly transmitted to the remote point, affecting the outcome at the second location. However so-called "loophole-free" Bell tests have been performed in which the locations were separated such that communications at the speed of light would have taken longer—in one case 10,000 times longer—than the interval between the measurements.According to some interpretations of quantum mechanics, the effect of one measurement occurs instantly. Other interpretations which don't recognize wavefunction collapse dispute that there is any "effect" at all. However, all interpretations agree that entanglement produces correlation between the measurements and that the mutual information between the entangled particles can be exploited, but that any transmission of information at faster-than-light speeds is impossible.Quantum entanglement has been demonstrated experimentally with photons, neutrinos, electrons, molecules as large as buckyballs, and even small diamonds. The utilization of entanglement in communication and computation is a very active area of research.

Roger Highfield

Roger Ronald Highfield (born 1958 in Griffithstown, Wales) is an author, science journalist, broadcaster and director of external affairs at the Science Museum Group.

Sean M. Carroll

Sean Michael Carroll (born October 5, 1966) is a theoretical physicist specializing in quantum mechanics, gravity, and cosmology. He is a research professor in the Department of Physics at the California Institute of Technology. He has been a contributor to the physics blog Cosmic Variance, and has published in scientific journals such as Nature as well as other publications, including The New York Times, Sky & Telescope, and New Scientist.

He has appeared on the History Channel's The Universe, Science Channel's Through the Wormhole with Morgan Freeman, Closer to Truth (broadcast on PBS), and Comedy Central's The Colbert Report. Carroll is the author of Spacetime And Geometry, a graduate-level textbook in general relativity, and has also recorded lectures for The Great Courses on cosmology, the physics of time, and the Higgs boson. He is also the author of three popular books: one on the arrow of time entitled From Eternity to Here, one on the Higgs boson entitled The Particle at the End of the Universe, and one on science and philosophy entitled The Big Picture: On the Origins of Life, Meaning, and the Universe Itself. He began a podcast in 2018 called Mindscape, in which he interviews other experts and intellectuals on a variety of science-related topics.

Second law of thermodynamics

The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. The total entropy of a system and its surroundings can remain constant in ideal cases where the system is in thermodynamic equilibrium, or is undergoing a (fictive) reversible process. In all processes that occur, including spontaneous processes, the total entropy of the system and its surroundings increases and the process is irreversible in the thermodynamic sense. The increase in entropy accounts for the irreversibility of natural processes, and the asymmetry between future and past.Historically, the second law was an empirical finding that was accepted as an axiom of thermodynamic theory. Statistical mechanics, classical or quantum, explains the microscopic origin of the law.

The second law has been expressed in many ways. Its first formulation is credited to the French scientist Sadi Carnot, who in 1824 showed that there is an upper limit to the efficiency of conversion of heat to work, in a heat engine.

Time

Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past, through the present, to the future. Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars.

Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.Time in physics is unambiguously operationally defined as "what a clock reads". See Units of Time. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities. Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms (see below). Time is also 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 human life spans.

Tomorrow (time)

Tomorrow is a temporal construct of the relative future; literally of the day after the current day (today), or figuratively of future periods or times. Tomorrow is usually considered just beyond the present and counter to yesterday. It is important in time perception because it is the first direction the arrow of time takes humans on Earth.

Key concepts
Measurement and
standards
Clocks
  • Religion
  • Mythology
Philosophy of time
Human experience
and use of time
Time in
Related topics

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