Philosophy of physics

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In philosophy, philosophy of physics deals with conceptual and interpretational issues in modern physics, and often overlaps with research done by certain kinds of theoretical physicists. Philosophy of physics can be very broadly lumped into three main areas:

Contents

Philosophy of space and time

The existence and nature of space and time (or space-time) are central topics in the philosophy of physics. [1]

Time

Time, in many philosophies, is seen as change. Wooden hourglass.jpg
Time, in many philosophies, is seen as change.

Time is often thought to be a fundamental quantity (that is, a quantity which cannot be defined in terms of other quantities), because time seems like a fundamentally basic concept, such that one cannot define it in terms of anything simpler. However, certain theories such as loop quantum gravity claim that spacetime is emergent. As Carlo Rovelli, one of the founders of loop quantum gravity has said: "No more fields on spacetime: just fields on fields". [2] Time is defined via measurement—by its standard time interval. Currently, the standard time interval (called "conventional second", or simply "second") is defined as 9,192,631,770 oscillations of a hyperfine transition in the 133 caesium atom. (ISO 31-1). What time is and how it works follows from the above definition. Time then can be combined mathematically with the fundamental quantities of space and mass to define concepts such as velocity, momentum, energy, and fields.

Both Newton and Galileo, [3] as well as most people up until the 20th century, thought that time was the same for everyone everywhere. The modern conception of time is based on Einstein's theory of relativity and Minkowski's spacetime, in which rates of time run differently in different inertial frames of reference, and space and time are merged into spacetime. Time may be quantized, with the theoretical smallest time being on the order of the Planck time. Einstein's general relativity as well as the redshift of the light from receding distant galaxies indicate that the entire Universe and possibly space-time itself began about 13.8 billion years ago in the Big Bang. Einstein's theory of special relativity mostly (though not universally) made theories of time where there is something metaphysically special about the present seem much less plausible, as the reference-frame-dependence of time seems to not allow the idea of a privileged present moment.

Time travel

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or certain types of motion in space, may allow time travel into the past and future. Concepts that aid such understanding include the closed timelike curve.

Albert Einstein's special theory of relativity (and, by extension, the general theory) predicts time dilation that could be interpreted as time travel. The theory states that, relative to a stationary observer, time appears to pass more slowly for faster-moving bodies: for example, a moving clock will appear to run slow; as a clock approaches the speed of light its hands will appear to nearly stop moving. The effects of this sort of time dilation are discussed further in the popular "twin paradox". These results are experimentally observable and affect the operation of GPS satellites and other high-tech systems used in daily life.

A second, similar type of time travel is permitted by general relativity. In this type a distant observer sees time passing more slowly for a clock at the bottom of a deep gravity well, and a clock lowered into a deep gravity well and pulled back up will indicate that less time has passed compared to a stationary clock that stayed with the distant observer.

Many in the scientific community believe that backward time travel is highly unlikely, because it violates causality [4] i.e. the logic of cause and effect. For example, what happens if you attempt to go back in time and kill yourself at an earlier stage in your life (or your grandfather, which leads to the grandfather paradox)? Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel a variant of the Fermi paradox, with time travelers instead of alien visitors. [4]

Space

Space is one of the few fundamental quantities in physics, meaning that it cannot be defined via other quantities because there is nothing more fundamental known at present. Thus, similar to the definition of other fundamental quantities (like time and mass), space is defined via measurement. Currently, the standard space interval, called a standard metre or simply metre, is defined as the distance traveled by light in a vacuum during a time interval of 1/299792458 of a second (exact).

In classical physics, space is a three-dimensional Euclidean space where any position can be described using three coordinates and parameterised by time. Special and general relativity use four-dimensional spacetime rather than three-dimensional space; and currently there are many speculative theories which use more than four spatial dimensions.

Philosophy of quantum mechanics

Quantum mechanics is a large focus of contemporary philosophy of physics, specifically concerning the correct interpretation of quantum mechanics. Very broadly, much of the philosophical work that is done in quantum theory is trying to make sense of superposition states: [5] the property that particles seem to not just be in one determinate position at one time, but are somewhere 'here', and also 'there' at the same time. Such a radical view turns many common sense metaphysical ideas on their head. Much of contemporary philosophy of quantum mechanics aims to make sense of what the very empirically successful formalism of quantum mechanics tells us about the physical world.

The Everett interpretation

The Everett, or many-worlds interpretation of quantum mechanics claims that the wave-function of a quantum system is telling us claims about the reality of that physical system. It denies wavefunction collapse, and claims that superposition states should be interpreted literally as describing the reality of many-worlds where objects are located, and not simply indicating the indeterminacy of those variables. This is sometimes argued as a corollary of scientific realism, [6] which states that scientific theories aim to give us literally true descriptions of the world.

One issue for the Everett interpretation is the role that probability plays on this account. The Everettian account is completely deterministic, whereas probability seems to play an ineliminable role in quantum mechanics. [7] Contemporary Everettians have argued that one can get an account of probability that follows the Born Rule through certain decision-theoretic proofs. [8]

Physicist Roland Omnés noted that it is impossible to experimentally differentiate between Everett's view, which says that as the wave-function decoheres into distinct worlds, each of which exists equally, and the more traditional view that says that a decoherent wave-function leaves only one unique real result. Hence, the dispute between the two views represents a great "chasm." "Every characteristic of reality has reappeared in its reconstruction by our theoretical model; every feature except one: the uniqueness of facts." [9]

Uncertainty principle

The uncertainty principle is a mathematical relation asserting an upper limit to the accuracy of the simultaneous measurement of any pair of conjugate variables, e.g. position and momentum. In the formalism of operator notation, this limit is the evaluation of the commutator of the variables' corresponding operators.

The uncertainty principle arose as an answer to the question: How does one measure the location of an electron around a nucleus if an electron is a wave? When quantum mechanics was developed, it was seen to be a relation between the classical and quantum descriptions of a system using wave mechanics.

In March 1927, working in Niels Bohr's institute, Werner Heisenberg formulated the principle of uncertainty thereby laying the foundation of what became known as the Copenhagen interpretation of quantum mechanics. Heisenberg had been studying the papers of Paul Dirac and Pascual Jordan. He discovered a problem with measurement of basic variables in the equations. His analysis showed that uncertainties, or imprecisions, always turned up if one tried to measure the position and the momentum of a particle at the same time. Heisenberg concluded that these uncertainties or imprecisions in the measurements were not the fault of the experimenter, but fundamental in nature and are inherent mathematical properties of operators in quantum mechanics arising from definitions of these operators. [10]

The term Copenhagen interpretation of quantum mechanics was often used interchangeably with and as a synonym for Heisenberg's uncertainty principle by detractors (such as Einstein and the physicist Alfred Landé) who believed in determinism and saw the common features of the Bohr–Heisenberg theories as a threat. Within the Copenhagen interpretation of quantum mechanics the uncertainty principle was taken to mean that on an elementary level, the physical universe does not exist in a deterministic form, but rather as a collection of probabilities, or possible outcomes. For example, the pattern (probability distribution) produced by millions of photons passing through a diffraction slit can be calculated using quantum mechanics, but the exact path of each photon cannot be predicted by any known method. The Copenhagen interpretation holds that it cannot be predicted by any method, not even with theoretically infinitely precise measurements.

History of the philosophy of physics

Aristotelian physics

Aristotelian physics viewed the universe as a sphere with a center. Matter, composed of the classical elements, earth, water, air, and fire, sought to go down towards the center of the universe, the center of the earth, or up, away from it. Things in the aether such as the moon, the sun, planets, or stars circled the center of the universe. [11] Movement is defined as change in place, [11] i.e. space. [12]

Newtonian physics

The implicit axioms of Aristotelian physics with respect to movement of matter in space were superseded in Newtonian physics by Newton's First Law of Motion. [13]

"Every body" includes the Moon, and an apple; and includes all types of matter, air as well as water, stones, or even a flame. Nothing has a natural or inherent motion. [14] Absolute space being three-dimensional Euclidean space, infinite and without a center. [14] Being "at rest" means being at the same place in absolute space over time. [15] The topology and affine structure of space must permit movement in a straight line at a uniform velocity; thus both space and time must have definite, stable dimensions. [16]

Leibniz

Gottfried Wilhelm Leibniz, 1646 – 1716, was a contemporary of Newton. He contributed a fair amount to the statics and dynamics emerging around him, often disagreeing with Descartes and Newton. He devised a new theory of motion (dynamics) based on kinetic energy and potential energy, which posited space as relative, whereas Newton was thoroughly convinced that space was absolute. An important example of Leibniz's mature physical thinking is his Specimen Dynamicum of 1695. [17]

Until the discovery of subatomic particles and the quantum mechanics governing them, many of Leibniz's speculative ideas about aspects of nature not reducible to statics and dynamics made little sense. For instance, he anticipated Albert Einstein by arguing, against Newton, that space, time and motion are relative, not absolute: [18] "As for my own opinion, I have said more than once, that I hold space to be something merely relative, as time is, that I hold it to be an order of coexistences, as time is an order of successions." [19]

Quotes from Einstein's work on the importance of the philosophy of physics

Einstein was interested in the philosophical implications of his theory. Albert Einstein photo 1921.jpg
Einstein was interested in the philosophical implications of his theory.

Albert Einstein was extremely interested in the philosophical conclusions of his work. He writes:

"I fully agree with you about the significance and educational value of methodology as well as history and philosophy of science. So many people today—and even professional scientists—seem to me like somebody who has seen thousands of trees but has never seen a forest. A knowledge of the historic and philosophical background gives that kind of independence from prejudices of his generation from which most scientists are suffering. This independence created by philosophical insight is—in my opinion—the mark of distinction between a mere artisan or specialist and a real seeker after truth." Einstein. letter to Robert A. Thornton, 7 December 1944. EA 61–574.

Elsewhere:

"How does it happen that a properly endowed natural scientist comes to concern himself with epistemology? Is there no more valuable work in his specialty? I hear many of my colleagues saying, and I sense it from many more, that they feel this way. I cannot share this sentiment. ... Concepts that have proven useful in ordering things easily achieve such an authority over us that we forget their earthly origins and accept them as unalterable givens. Thus they come to be stamped as 'necessities of thought,' 'a priori givens,' etc."

"The path of scientific advance is often made impassable for a long time through such errors. For that reason, it is by no means an idle game if we become practiced in analyzing the long-commonplace concepts and exhibiting [revealing, exposing? -Ed.] those circumstances upon which their justification and usefulness depend, how they have grown up, individually, out of the givens of experience. By this means, their all-too-great authority will be broken." Einstein, 1916, "Memorial notice for Ernst Mach," Physikalische Zeitschrift 17: 101–02.

See also

Related Research Articles

The Copenhagen interpretation is an expression of the meaning of quantum mechanics that was largely devised from 1925 to 1927 by Niels Bohr and Werner Heisenberg. It is one of the oldest of numerous proposed interpretations of quantum mechanics, and remains one of the most commonly taught.

Quantum mechanics Branch of physics

Quantum mechanics, including quantum field theory, is a fundamental theory in physics describing the properties of nature.

Space General framework of distances and directions according to a physical observer in its proper time

Space is the boundless three-dimensional extent in which objects and events have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum known as spacetime. The concept of space is considered to be of fundamental importance to an understanding of the physical universe. However, disagreement continues between philosophers over whether it is itself an entity, a relationship between entities, or part of a conceptual framework.

Theory of relativity physical theory

The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity. Special relativity applies to all physical phenomena in the absence of gravity. General relativity explains the law of gravitation and its relation to other forces of nature. It applies to the cosmological and astrophysical realm, including astronomy.

An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics "corresponds" to reality. Although quantum mechanics has held up to rigorous and extremely precise tests in an extraordinarily broad range of experiments, there exist a number of contending schools of thought over their interpretation. These views on interpretation differ on such fundamental questions as whether quantum mechanics is deterministic or stochastic, which elements of quantum mechanics can be considered "real", and what is the nature of measurement, among other matters.

Eternalism is a philosophical approach to the ontological nature of time, which takes the view that all existence in time is equally real, as opposed to presentism or the growing block universe theory of time, in which at least the future is not the same as any other time. Some forms of eternalism give time a similar ontology to that of space, as a dimension, with different times being as real as different places, and future events are "already there" in the same sense other places are already there, and that there is no objective flow of time. It is sometimes referred to as the "block time" or "block universe" theory due to its description of space-time as an unchanging four-dimensional "block", as opposed to the view of the world as a three-dimensional space modulated by the passage of time.

In physics, hidden variable theories are proposals to provide deterministic explanations of quantum mechanical phenomena, through the introduction of unobservable hypothetical entities. The existence of indeterminacy for some measurements is assumed as part of the mathematical formulation of quantum mechanics; moreover, bounds for indeterminacy can be expressed in a quantitative form by the Heisenberg uncertainty principle.

Quantum foam or spacetime foam is the fluctuation of spacetime on very small scales due to quantum mechanics. The idea was devised by John Wheeler in 1955.

In physics, action at a distance is the concept that an object can be moved, changed, or otherwise affected without being physically touched by another object. That is, it is the nonlocal interaction of objects that are separated in space.

Digital philosophy is a direction in philosophy and cosmology advocated by certain mathematicians and theoretical physicists, including: Edward Fredkin, Konrad Zuse, Stephen Wolfram, Rudy Rucker, Gregory Chaitin, Seth Lloyd. and Paola Zizzi.

Philosophy of space and time is the branch of philosophy concerned with the issues surrounding the ontology, epistemology, and character of space and time. While such ideas have been central to philosophy from its inception, the philosophy of space and time was both an inspiration for and a central aspect of early analytic philosophy. The subject focuses on a number of basic issues, including whether time and space exist independently of the mind, whether they exist independently of one another, what accounts for time's apparently unidirectional flow, whether times other than the present moment exist, and questions about the nature of identity.

Absolute space and time theoretical foundation of Newtonian mechanics

Absolute space and time is a concept in physics and philosophy about the properties of the universe. In physics, absolute space and time may be a preferred frame.

<i>The Fabric of the Cosmos</i> Book

The Fabric of the Cosmos: Space, Time, and the Texture of Reality (2004) is the second book on theoretical physics, cosmology, and string theory written by Brian Greene, professor and co-director of Columbia's Institute for Strings, Cosmology, and Astroparticle Physics (ISCAP).

In physics, complementarity is both a theoretical and an experimental result of quantum mechanics, also referred to as principle of complementarity. Formulated by Niels Bohr, a leading founder of quantum mechanics, the complementarity principle holds that objects have certain pairs of complementary properties which cannot all be observed or measured simultaneously.

The deductive-nomological model, also known as Hempel's model, the Hempel–Oppenheim model, the Popper–Hempel model, or the covering law model, is a formal view of scientifically answering questions asking, "Why...?". The DN model poses scientific explanation as a deductive structure—that is, one where truth of its premises entails truth of its conclusion—hinged on accurate prediction or postdiction of the phenomenon to be explained.

The relational approach to quantum physics is an alternative approach to and interpretation of quantum mechanics. It asserts that the physical world can only be studied accurately in terms of relationships between systems, as all experimentally verifiable facts about the world result explicitly from interactions. According to the relational approach, the assumption that objects possess absolute properties inevitably leads to ambiguities and paradoxes when these objects are studied closely. The approach was adopted, in a time span of 1992-1996, by Q. Zheng, S. Hughes, and T. Kobayashi in the University of Tokyo. As early as in 1985, S. Kochen suggested that the paradoxes of quantum physics could be overcome by developing a relational approach, which was needed at one time to solve the paradoxes of relativistic physics of space and time. It is also hoped that this entry will serve as a complement to Rovelli’s relational quantum mechanics (RQM).

Branches of physics sub-field of study of physics

Physics deals with the combination of matter and energy. It also deals with a wide variety of systems, about which theories have been developed that are used by physicists. In general, theories are experimentally tested numerous times before they are accepted as correct as a description of Nature. For instance, the theory of classical mechanics accurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These "central theories" are important tools for research in more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.

Tim William Eric Maudlin is an American philosopher of science who has done influential work on the metaphysical foundations of physics and logic.

In theoretical physics, the problem of time is a conceptual conflict between general relativity and quantum mechanics in that quantum mechanics regards the flow of time as universal and absolute, whereas general relativity regards the flow of time as malleable and relative. This problem raises the question of what time really is in a physical sense and whether it is truly a real, distinct phenomenon. It also involves the related question of why time seems to flow in a single direction, despite the fact that no known physical laws seem to require a single direction.

A hallmark of Albert Einstein's career was his use of visualized thought experiments as a fundamental tool for understanding physical issues and for elucidating his concepts to others. Einstein's thought experiments took diverse forms. In his youth, he mentally chased beams of light. For special relativity, he employed moving trains and flashes of lightning to explain his most penetrating insights. For general relativity, he considered a person falling off a roof, accelerating elevators, blind beetles crawling on curved surfaces and the like. In his debates with Niels Bohr on the nature of reality, he proposed imaginary devices intended to show, at least in concept, how the Heisenberg uncertainty principle might be evaded. In a profound contribution to the literature on quantum mechanics, Einstein considered two particles briefly interacting and then flying apart so that their states are correlated, anticipating the phenomenon known as quantum entanglement.

References

  1. Maudlin, Tim (2012). Philosophy of Physics: Space and Time. Princeton University Press. p. xi. ISBN   978-0691143095 . Retrieved 3 October 2017. ...the existence and nature of space and time (or space-time) is a central topic.
  2. Rovelli, C. (2004). Quantum Gravity. Cambridge Monographs on Mathematical Physics. p. 71.
  3. Roger Penrose, 2004. The Road to Reality: A Complete Guide to the Laws of the Universe. London: Jonathan Cape. ISBN   0-224-04447-8 (hardcover), 0-09-944068-7 (paperback).
  4. 1 2 Bolonkin, Alexander (2011). Universe, Human mmortality and Future Human Evaluation. Elsevier. p. 32. ISBN   978-0-12-415810-8. Extract of page 32
  5. BristolPhilosophy (19 February 2013). "Eleanor Knox (KCL) – The Curious Case of the Vanishing Spacetime" . Retrieved 7 April 2018 via YouTube.
  6. David Wallace, 'The Emergent Multiverse', pp. 1–10
  7. David Wallace, 'The Emergent Multiverse', pp. 113–117
  8. David Wallace, 'The Emergent Multiverse', pg. 157–189
  9. Omnès, Roland (2002). "11". Quantum philosophy : understanding and interpreting contemporary science (in French) (First paperback printing, 2002, translated by Arturo Spangalli. ed.). Princeton: Princeton University Press. p. 213. ISBN   978-1400822867.
  10. Niels Bohr, Atomic Physics and Human Knowledge, p. 38
  11. 1 2 Tim Maudlin (2012-07-22). Philosophy of Physics: Space and Time: Space and Time (Princeton Foundations of Contemporary Philosophy) (p. 3). Princeton University Press. Kindle Edition."Because it is a sphere, Aristotle's universe contains a geometrically privileged center, and Aristotle makes reference to that center in characterizing the natural motions of different sorts of matter. “Upward,”“downward,” and “uniform circular motion” all are defined in terms of the center of the universe."
  12. Tim Maudlin (2012-07-22). Philosophy of Physics: Space and Time: Space and Time (Princeton Foundations of Contemporary Philosophy) (p. 4). Princeton University Press. Kindle Edition. "Aristotle adopts the concept of space, and the correlative concept of motion, that we all intuitively employ."
  13. Tim Maudlin (2012-07-22). Philosophy of Physics: Space and Time: Space and Time (Princeton Foundations of Contemporary Philosophy) (pp. 4–5). Princeton University Press. Kindle Edition. "Newtonian physics is implicit in his First Law of Motion: Law I : Every body perseveres in its state either of rest or of uniform motion in a straight line, except insofar as it is compelled to change its state by impressed forces. 1 This single law smashes the Aristotelian universe to smithereens."
  14. 1 2 Tim Maudlin (2012-07-22). Philosophy of Physics: Space and Time: Space and Time (Princeton Foundations of Contemporary Philosophy) (pp. 5). Princeton University Press. Kindle Edition.
  15. Tim Maudlin (2012-07-22). Philosophy of Physics: Space and Time: Space and Time (Princeton Foundations of Contemporary Philosophy) (pp. 9–10). Princeton University Press. Kindle Edition. "Newton believed in the existence of a spatial arena with the geometrical structure of E3. He believed that this infinite three-dimensional space exists at every moment of time. And he also believed something much more subtle and controversial, namely, that identically the same points of space persist through time."
  16. Tim Maudlin (2012-07-22). Philosophy of Physics: Space and Time: Space and Time (Princeton Foundations of Contemporary Philosophy) (p. 12). Princeton University Press. Kindle Edition. "...space must have a topology, an affine structure, and a metric; time must be one-dimensional with a topology and a metric; and, most importantly, the individual parts of space must persist through time.
  17. Ariew and Garber 117, Loemker §46, W II.5. On Leibniz and physics, see the chapter by Garber in Jolley (1995) and Wilson (1989).
  18. Rafael Ferraro (2007). Einstein's Space-Time: An Introduction to Special and General Relativity. Springer. p. 1. ISBN   978-0-387-69946-2.
  19. See H. G. Alexander, ed., The Leibniz-Clarke Correspondence , Manchester: Manchester University Press, pp. 25–26.

Further reading