Effect at close range and effect at a distance
Action at close range and action at a distance denote two competing historical concepts of classical physics in relation to the question of how forces spread.
In the case of an action at a distance , it is assumed that the physical action acts over any distance without a mediating medium , in Newton's theory of gravity even instantaneously (i.e. without a time delay). The classical physical theories of mechanics - d. H. the Newtonian gravity , the electrostatic and magnetostatic - have a long-range effect on the foundation. This is expressed, for example, in Newton's third law of actio and reactio : two bodies act on each other at any moment, regardless of how far they are from each other and how they move, with oppositely equal forces. From a physical point of view, the way in which such forces are transmitted cannot be explained in more detail, and their emergence has been one of the “occult” properties of the body since Aristotle .
The concept of proximity effect was of Galileo Galilei made the medieval notions of the occult properties contrary. René Descartes justified it philosophically in such a way that a body is already completely defined by nothing other than its spatial expansion and an effect on another body can therefore only take place when it is touched. Nevertheless, due to the success of classical mechanics, the idea of action at a distance largely prevailed in the 18th century. But in 1838 Michael Faraday discovered that the electrostatic force between two bodies depends on the type of matter between them. Further developed by James Clerk Maxwell , this discovery led to a new picture of the close-up effect: the electrical charge first changes the surrounding ether , and as soon as this change reaches the location of another charge, this other charge experiences the electrostatic force. This also results in a finite speed of propagation of the effect.
Faraday introduced the term field for the transmission of power across space . The term field is also valid in modern physics.
Action at a distance in Newton's law of gravitation
The law of gravity by Isaac Newton describes an instantaneous action at a distance. If z. B. if the sun were suddenly shifted, then the earth would immediately feel the changed gravity and react with a corresponding change in its orbit. The mutual orbital disturbances of the planets are also calculated according to Newton using the forces that are determined from the current positions of the planets.
Hence, Newton's theory was generally understood to mean that the gravitational force he introduced could penetrate absolutely empty space without delay. Even his contemporaries, including Christiaan Huygens and Gottfried Wilhelm Leibniz , accused him of reintroducing occult forces if he ascribed the ability to act at a distance to matter, understood only as inert and passive , and Leonhard Euler still saw this as an absurd assumption.
But Newton himself also strictly rejected this interpretation of his calculations and even the mere possibility of such a long-range effect:
"It is unconceivable that inanimate brute matter should (without the mediation of something else which is not material) operate upon and affect other matter without mutual contact; as it must if gravitation in the sense of Epicurus be essential and inherent in it. And this is one reason why I desired you would not ascribe innate gravity to me. That gravity should be innate inherent and essential to matter so that one body may act upon another at a distance through a vacuum without the mediation of any thing else by and through which their action or force may be conveyed from one to another is to me so great an absurdity that I believe no man who has in philosophical matters any competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws, but whether this agent be material or immaterial is a question I have left to the consideration of my readers. "
“It is inconceivable that inanimate, raw matter (without the mediation of something else that is not material) should act on other matter without direct contact, [...]. That the gravitation of matter should be innate, inherent and essential, so that one body should act on another over a distance through a vacuum and without the mediation of anything else, is such a great absurdity for me that I don't believe anyone who has a thought ability trained in philosophical matters can ever join this. Gravitation has to be caused by a mediator who works constantly and according to certain laws. But I left the question of whether this mediator is material or immaterial to my readers. "
Against the criticism expressed, he specified that he only deduced obvious laws from observations of nature, and that only their causes possibly remained occult.
After all, it was the success of Classical Mechanics , based on Newton's laws , which established the action at a distance as an accepted model in many areas of natural science for a long time. The philosopher Immanuel Kant also understood attraction as an effect at a distance. He writes (1786): "The attraction essential to all matter is an immediate effect of the same on others through empty space." As a result, attempts, going back to Descartes, to create a mechanical explanation of gravitation with the help of a vortex movement of an ether , were hardly heard.
Electricity and the theory of close effects
Influenced by the success of Newton's theory of action at a distance, in the 18th and 19th centuries the laws of attraction and repulsion for bodies with electrical charges of the same name or of the same name were initially understood as forces acting at a distance. In the 19th century, however, there was a growing belief that forces are mediated by fields and thus by physical quantities.
Michael Faraday (1791–1867) was the first to come to an idea of the field through his experiments on electricity and electromagnetic induction . In his opinion, the space is excited by the field-generating arrangement, so that another body experiences a force. He introduced the field as an independent variable in physics. Forces are therefore transmitted indirectly with the help of a field which, according to the original view, spreads instantaneously in space. The force acts on a charged body in the sense of the concept of close-up effect with the field strength prevailing at the relevant location . Faraday was already convinced in 1852 that this assumption is correct not only for magnetic and electric fields, but also for gravitational fields.
In 1865, James Clerk Maxwell (1831–1879) then presented a complete field theory of electromagnetism . His hypothesis of the displacement currents makes it necessary to extend this theory of close effects to the vacuum as well. The Maxwell equations show that electric and magnetic fields that vary over time generate each other and therefore have to be combined to form an electromagnetic field that spreads overall at a finite speed. The forces on a charged body then result as a Coulomb force and Lorentz force from the field strengths prevailing at its location. Furthermore, there is the possibility that an electromagnetic field, once generated, continues to exist independently of its source and propagates through space as an electromagnetic wave .
In order to distinguish the field standpoint from the action at a distance, it was called the action at close range. An explanation for the word “close-up effect” arises from the idea that the body experiences the force through the space that surrounds it. Each effect is carried over from a point P 1 to an "adjacent" point P 2 . It is not known who used the term “close-up effect” for the first time.
As long as the arrangements are static , there is no difference between distance and close-up effects, but there are dynamic problems. The experimental breakthrough finally took place in 1887 with the detection of electromagnetic waves by Heinrich Hertz .
For the propagation of electromagnetic waves, the theory of a mechanical propagation medium, the ether, was favored well beyond the end of the 19th century. Even the leading theorists of electrodynamics at the beginning of the 20th century Hendrik Antoon Lorentz , Henri Poincaré , and Joseph Larmor assumed the existence of an ether. The experimental physicist Albert A. Michelson , whose zero result in the Michelson-Morley experiment made a significant contribution to overcoming the idea of an ether, was not convinced of the non-existence of an ether until his death.
While the field concept and the near-action theory prevailed in general, there are also formulations of classical electrodynamics about a direct particle-particle interaction (though not instantaneous, but with a time delay that corresponds to the speed of light barrier) by Karl Schwarzschild , Adriaan Fokker and Hugo Tetrode , what was taken up in the 1940s by John Archibald Wheeler and Richard Feynman (Absorber Theory). It uses advanced and retarded potentials equally and is thus time-symmetrical. Furthermore, it has the advantage that the self-interaction of charged point-like particles can be avoided with the associated divergences. Fred Hoyle and Jayant Vishnu Narlikar used this theory in a cosmology that implements Mach's principle . Carl Friedrich Gauß had the idea of a long-range action with finite propagation speed for electrodynamics in 1845, but was not yet able to formulate it precisely at that time.
Gravitation and relativity
Parallel to the developments in electrodynamics (Maxwell already assumed that fields spread with finite speed), various physicists tried between 1870 and 1910 to describe gravity as a close-up effect. Finally Albert Einstein succeeded in formulating such a theory with the general theory of relativity , which includes the speed of light as the speed of propagation of the fields. He showed that no effective effects, including fields and forces, can spread faster than light. An instantaneous effect over any distance is therefore impossible.
In the hypothetical example with the shifted sun, the gravitational effect on the earth would only change after approx. 8 minutes - that is, after the time that the light also needs from the sun to the earth. In particular, we would therefore not feel the shift until we saw it.
The calculation of planetary orbits around the sun with this retarded potential does not result in an exact ellipse , but a spiral that ends after many revolutions in the sun. In general relativity, the largest part of this effect is compensated for by the gravitomagnetic effect ; the remaining energy loss of the planet is explained by the emission of gravitational waves . These would be too weak for planets to be observed, but with sufficiently massive objects they should be observable. An indirect proof of the gravitational waves by Russell Hulse and Joseph Taylor shows exactly this effect: The pulsars of the double pulsar PSR 1913 + 16 circle around each other in a spiral path, which leads to a measurably increasing orbital frequency . In 2016, the existence of gravitational waves was experimentally proven by a signal recorded on September 14, 2015 in the USA.
Quantum physics
The debate about close and distant effects continued into the 20th century. The postulate of relativity, all effects that a maximum of speed of light spread, resulting in quantum mechanics to the EPR paradox : if one of two mutually entangled particles changes its state, it must be according to quantum mechanics also instantaneously the other change, which is not in accordance with the theory of relativity. This is only resolved by the relativistic quantum field theory , in which a causal connection is always limited to the forward light cone .
Current state
Today it is assumed that three of the four fundamental forces are transmitted through bosons as exchange particles:
- in the case of the electromagnetic force it is the photons
- in nuclear power the gluons
- in the weak interaction the W bosons and Z bosons
If the exchange particles are massless (photons and gluons), their effect is transmitted at the speed of light, for exchange particles with mass at a lower speed.
Exchange particles of the gravitational force, the gravitons , could not (so far) be determined. However, Einstein's general theory of relativity includes the speed of light as the speed of propagation of the gravitational force.
This means that no effective effects, including fields and forces, can spread faster than light. An action at a distance, i.e. H. an instantaneous effect over any distance is therefore impossible.
criticism
In physics, the term close-up effect is used reluctantly as the opposite of the long-distance effect of forces, because it is misleading and unnecessary. The assumption on which the standpoint of remote action is based, that forces act immediately and immediately at any distance, ultimately also includes a close-up effect. It also seems paradoxical that of all things the large ranges of gravitation and electromagnetic waves are called close-up effects.
It is superfluous to introduce the term close-up effect for the field standpoint, as an extra explanation is required as to why it should be synonymous with the simpler field term. In principle, preference should always be given to the clearer term. Suffice it to say: the remote control standpoint has been replaced by the field standpoint.
literature
- Caspar Isenkrahe : About long-distance power and the third Ignorabimus established by Paul du Bois-Reymond . Leipzig 1889.
- Paul Drude : About remote effects . In: Supplement to the annals of physics and chemistry . tape 62 , 1, new series, 1897, p. I – XLIX (Presentation for the 69th Assembly of German Naturalists and Physicians in Braunschweig, 1897; Physics Section). ; Correction of S. XXXIX: Annals of Physics and Chemistry . tape 62 , no. 12 , 1897, pp. 693 .
- Jonathan Zenneck : Gravitation . In: Encyclopedia of Mathematical Sciences, including its applications . V. 1. Leipzig 1903, p. 25-67 .
- Mary Hesse : Forces and Fields: The Concept of Action at a Distance in the History of Physics , Nelson 1961, Dover 2005
Individual evidence
- ↑ Andreas Kleinert: Enlightenment through physics . In: Walter Schmitz / Carsten cell (ed.): Innovation and Transfer . Eckard Richter, Dresden 2004, ISBN 3-933592-37-2 , p. 11-20 . If remote effects are possible, according to Euler, one must fear that the herbs that grow on Saturn will cause digestive problems even without having eaten them.
- ↑ John Fauvel, Raymond Flood, Michael Shortland, Robin Wilson (Eds.): Newton's work: The foundation of modern science . Springer-Verlag, 2013, p. 325 ( "Newton's Active Principles" in Google Book Search).
- ↑ Immanuel Kant, Metaphysical Beginnings of Natural Science , Riga 1786, Chapter 2 Dynamics , Proposition 7.
- ^ M. Faraday: On the physical character of the lines of magnetic force . In: The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science . 4th Series, Volume 3, Taylor & Francis, London 1852, pp. 401-428, online .
- ↑ James Clerk Maxwell, A Treatise on Electricity & Magnetism , Dover Publications, New York 1873, ISBN 0-486-60636-8 and ISBN 0-486-60637-6 .
- ^ Wheeler, Feynman, Interaction with the absorber as the mechanism of radiation, Reviews of Modern Physics, Volume 17, 1957, p. 157
- ↑ Also presented in Feynman Lectures on Physics, Chapter 28, Volume 2
- ↑ Hoyle, Narlikar, Cosmology and action at a distance electrodynamics, Reviews of Modern Physics, Volume 67, 1995, p. 113