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:
The existence and nature of space and time (or space-time) are central topics in the philosophy of physics.
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". 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, as well as most people up until the 20th century, thought that time was the same for everyone everywhere. Our 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.
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.
These effects are to some degree similar to hibernation, or cooling of live objects (which slow down the rates of chemical processes in the subject) almost indefinitely suspending their life thus resulting in "time travel" toward the future, but never backward. They do not violate causality. This is not typical of the "time travel" featured in science fiction (where causality is violated at will), and there is little doubt surrounding its existence. "Time travel" will hereafter refer to travel with some degree of freedom into the past or future of proper time.
Many in the scientific community believe that backward time travel is highly unlikely, because it violates causality 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.
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.
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: 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 a lot of our 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, 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, 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. Contemporary Everettians have argued that one can get an account of probability that follows the Born Rule through certain decision-theoretic proofs.
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."
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.
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.
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. Movement is defined as change in place, i.e. space.
"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. Absolute space being three-dimensional Euclidean space, infinite and without a center. Being "at rest" means being at the same place in absolute space over time. 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.
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.
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: "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."
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.
"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.
...the existence and nature of space and time (or space-time) is a central topic.
In philosophy, A series and B series are two different descriptions of the temporal ordering relation among events. The two series differ principally in their use of tense to describe the temporal relation between events. The terms were introduced by the Scottish idealist philosopher John McTaggart in 1908 as part of his argument for the unreality of time, but since then they have become widely used terms of reference in modern discussions of the philosophy of time.Abstract object theory
Abstract object theory is a branch of metaphysics regarding abstract objects. Originally devised by metaphysicist Edward Zalta in 1999, the theory was an expansion of mathematical Platonism.
Abstract Objects: An Introduction to Axiomatic Metaphysics (1983) is the title of a publication by Edward Zalta that outlines abstract object theory.On Zalta's account, there are two modes of predication: some objects (the ordinary concrete ones around us, like tables and chairs) "exemplify" properties, while others (abstract objects like numbers, and what others would call "non-existent objects", like the round square, and the mountain made entirely of gold) merely "encode" them. While the objects that exemplify properties are discovered through traditional empirical means, a simple set of axioms allows us to know about objects that encode properties. For every set of properties, there is exactly one object that encodes exactly that set of properties and no others. This allows for a formalized ontology.Ansatz
In physics and mathematics, an ansatz (; German: [ˈʔanzats], meaning: "initial placement of a tool at a work piece", plural ansätze ; German: [ˈʔanzɛtsə] or ansatzes) is an educated guess that is verified later by its results.Bas van Fraassen
Bastiaan Cornelis van Fraassen (; born 5 April 1941) is a Dutch-American philosopher. He is a Distinguished Professor of Philosophy at San Francisco State University and the McCosh Professor of Philosophy Emeritus at Princeton University, noted for his seminal contributions to philosophy of science.Causality (physics)
Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science – especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics. From the perspective of physics, causality cannot occur between an effect and an event that is not in the back (past) light cone of said effect. Similarly, a cause cannot have an effect outside its front (future) light cone.Classical physics
Classical physics refers to theories of physics that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major paradigm shift, then the previous theories, or new theories based on the older paradigm, will often be referred to as belonging to the realm of "classical physics".
As such, the definition of a classical theory depends on context. Classical physical concepts are often used when modern theories are unnecessarily complex for a particular situation. Most usually classical physics refers to pre-1900 physics, while modern physics refers to post-1900 physics which incorporates elements of quantum mechanics and relativity.Cosmology (philosophy)
Philosophical cosmology, philosophy of cosmology or philosophy of cosmos is a discipline directed to the philosophical contemplation of the universe as a totality, and to its conceptual foundations. It draws on several branches of philosophy—metaphysics, epistemology, philosophy of physics, philosophy of science, philosophy of mathematics, and on the fundamental theories of physics. The term cosmology was used at least as early as 1730, by German philosopher Christian Wolff, in Cosmologia Generalis.Eternalism (philosophy of time)
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.Foundations of Physics
Foundations of Physics is a monthly journal "devoted to the conceptual bases and fundamental theories of modern physics and cosmology, emphasizing the logical, methodological, and philosophical premises of modern physical theories and procedures". The journal publishes results and observations based on fundamental questions from all fields of physics, including: quantum mechanics, quantum field theory, special relativity, general relativity, string theory, M-theory, cosmology, thermodynamics, statistical physics, and quantum gravity
Foundations of Physics has been published since 1970. Its founding editors were Henry Margenau and Wolfgang Yourgrau. The 1999 Nobel laureate Gerard 't Hooft was editor-in-chief from January 2007. At that stage, it absorbed the associated journal for shorter submissions Foundations of Physics Letters, which had been edited by Alwyn Van der Merwe since its foundation in 1988. Past editorial board members (which include several Nobel laureates) include Louis de Broglie, Robert H. Dicke, Murray Gell-Mann, Abdus Salam, Ilya Prigogine and Nathan Rosen. Carlo Rovelli was announced as new editor-in-chief in February 2016.Harvey Brown (philosopher)
Harvey R. Brown, FBA (born April 4, 1950 in the United Kingdom) is a philosopher of physics. He is emeritus professor of philosophy at the University of Oxford and a Fellow of Wolfson College, Oxford, as well as a Fellow of the British Academy.
From 1978 to 1984, he was assistant professor at the University of São Paulo. In 1984 he became university lecturer in philosophy of physics at the University of Oxford, where he was promoted to reader in philosophy in 1996 and professor of philosophy of physics in 2006.Inverse-square law
The inverse-square law, in physics, is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space (see diagram).
Radar energy expands during both the signal transmission and also on the reflected return, so the inverse square for both paths means that the radar will receive energy according to the inverse fourth power of the range.
In order to prevent dilution of energy while propagating a signal, certain methods can be used such as a waveguide, which acts like a canal does for water, or how a gun barrel restricts hot gas expansion to one dimension in order to prevent loss of energy transfer to a bullet.Karen Barad
Karen Michelle Barad (; born 29 April 1956) is an American feminist theorist, known particularly for her theory of agential realism. She is currently Professor of Feminist Studies, Philosophy, and History of Consciousness at the University of California, Santa Cruz. She is the author of Meeting the Universe Halfway: Quantum Physics and the Entanglement of Matter and Meaning. Her research topics include feminist theory, physics, twentieth-century continental philosophy, epistemology, ontology, philosophy of physics, cultural studies of science, and feminist science studies.
Barad earned her doctorate in theoretical physics at Stony Brook University. Her dissertation presented computational methods for quantifying properties of quarks, and other fermions, and in the framework of lattice gauge theory.
Barad serves on the advisory board for the feminist academic journals Catalyst: Feminism, Theory, Technoscience and Signs: Journal of Women in Culture and Society.Model-dependent realism
Model-dependent realism is a view of scientific inquiry that focuses on the role of scientific models of phenomena. It claims reality should be interpreted based upon these models, and where several models overlap in describing a particular subject, multiple, equally valid, realities exist. It claims that it is meaningless to talk about the "true reality" of a model as we can never be absolutely certain of anything. The only meaningful thing is the usefulness of the model. The term "model-dependent realism" was coined by Stephen Hawking and Leonard Mlodinow in their 2010 book, The Grand Design.Nothing comes from nothing
Nothing comes from nothing (Latin: ex nihilo nihil fit) is a philosophical expression of a thesis first argued by Parmenides. It is associated with ancient Greek cosmology, such as is presented not just in the works of Homer and Hesiod, but also in virtually every internal system—there is no break in-between a world that did not exist and one that did, since it could not be created ex nihilo in the first place.Philosophy of computer science
The philosophy of computer science is concerned with the philosophical questions that arise with the study of computer science, which is understood to mean not just programming but the whole study of concepts and methods that assist in the development and maintenance of computer systems. There is still no common understanding of the content, aim, focus, or topic of the philosophy of computer science, despite some attempts to develop a philosophy of computer science like the philosophy of physics or the philosophy of mathematics.
The philosophy of computer science as such deals with the meta-activity that is associated with the development of the concepts and methodologies that implement and analyze the computational systems.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?Quantum clock
A quantum clock is a type of atomic clock with laser cooled single ions confined together in an electromagnetic ion trap. Developed in 2010 by National Institute of Standards and Technology physicists, the clock was 37 times more precise than the then-existing international standard. The quantum logic clock is based on an aluminium spectroscopy ion with a logic atom.
Both the aluminium-based quantum clock and the mercury-based optical atomic clock track time by the ion vibration at an optical frequency using a UV laser, that is 100,000 times higher than the microwave frequencies used in NIST-F1 and other similar time standards around the world. Quantum clocks like this are able to be far more precise than microwave standards.Synchronicity
Synchronicity (German: Synchronizität) is a concept, first introduced by analytical psychologist Carl Jung, which holds that events are "meaningful coincidences" if they occur with no causal relationship yet seem to be meaningfully related. During his career, Jung furnished several different definitions of it. Jung defined synchronicity as an "acausal connecting (togetherness) principle," "meaningful coincidence", and "acausal parallelism." He introduced the concept as early as the 1920s but gave a full statement of it only in 1951 in an Eranos lecture.In 1952 Jung published a paper "Synchronizität als ein Prinzip akausaler Zusammenhänge" (Synchronicity – An Acausal Connecting Principle) in a volume which also contained a related study by the physicist and Nobel laureate Wolfgang Pauli, who was sometimes critical of Jung's ideas. Jung's belief was that, just as events may be connected by causality, they may also be connected by meaning. Events connected by meaning need not have an explanation in terms of causality, which does not generally contradict the Axiom of Causality but in specific cases can lead to prematurely giving up causal explanation.
Jung used the concept in arguing for the existence of the paranormal. A believer in the paranormal, Arthur Koestler wrote extensively on synchronicity in his 1972 book The Roots of Coincidence. The idea of synchronicity as extending beyond mere coincidence (as well as the paranormal generally) is widely rejected in the academic and scientific community.Temporal finitism
Temporal finitism is the doctrine that time is finite in the past. The philosophy of Aristotle, expressed in such works as his Physics, held that although space was finite, with only void existing beyond the outermost sphere of the heavens, time was infinite. This caused problems for mediaeval Islamic, Jewish, and Christian philosophers, who were unable to reconcile the Aristotelian conception of the eternal with the Genesis creation narrative.Modern cosmogony accepts finitism, in the form of the Big Bang, rather than Steady State theory which allows for a universe that has existed for an infinite amount of time, but on physical rather than philosophical grounds.