Philosophy of physics: Wikis


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In philosophy, the philosophy of physics studies the fundamental philosophical questions underlying modern physics, the study of matter and energy and how they interact. The philosophy of physics begins by reflecting on the basic metaphysical and epistemological questions posed by physics: causality, determinism, and the nature of physical law. It then turns to questions raised by important topics in contemporary physics:

Centuries ago, the study of causality, and of the fundamental nature of space, time, matter, and the universe were part of metaphysics. Today the philosophy of physics is essentially a part of the philosophy of science.


Philosophy of space and time



Time, in many philosophies, is seen as change.

Time is a fundamental quantity (that is, a quantity which cannot be defined in terms of other quantities, because at present we don't know anything more basic than time). Thus 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 exactly time "is" and how it works follows from the above definition. Physicists use theory to predict how time is measured. Time then can be combined mathematically with the fundamental quantities of space and mass to derive concepts such as velocity, momentum, energy, and fields.

Both Newton and Galileo,[1] 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 Hermann Minkowski's spacetime, in which rates of time at separate places run differently, and space and time are merged into spacetime. Time may be quantized, with the theoretical smallest time being 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 13.7 billion years ago in the big bang. Whether and how the universe will ever end are open questions.

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".

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 time travel is highly unlikely, because it violates causality - logic of cause-effect sequence. What happens if you try to go back in time and kill yourself (or your grandfather, leading to the grandfather paradox)? Also, there are no experimental evidences of time travel. 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.


Spacetime, according to general relativity, is bent by objects with mass, causing time dilation.

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 meter or simply meter, is defined as the distance traveled by light in a vacuum during a time interval of 1/299792458 of a second (exact). This definition coupled with the present definition of time (see above) makes our space-time to be Minkowski space and makes special relativity theory to be absolutely correct by definition.

In classical physics, space is a three-dimensional Euclidean space where any position can be described using three coordinates. Special and general relativity uses spacetime rather than space; spacetime is modeled as a four-dimensional space (with the time axis being imaginary in special relativity and real in general relativity, and currently there are many theories which use more than 4-dimensional spaces, both real and complex).

Before Einstein's work on relativistic physics, time and space were viewed as independent dimensions. Einstein's work has shown that due to relativity of motion our space and time can be mathematically combined into one symmetric object - spacetime, in which the time axis (multiplied by ic) is indistinguishable from space axes. (Distances in space or in time separately are not invariant versus Lorentz coordinate transformations, but distances in such so called Minkowski spacetime are - which justifies the name).

Philosophy of quantum mechanics

Quantum mechanics has provided much controversy in philosophical interpretations. As it developed its theories began to contradict many of the accepted philosophies. However, all its mathematical predictions coincide with observations.

In most cases accepted philosophies are based on the everyday experience of the average human - which is extremely limited as it does not include observation of ultra-small systems, or motion with high speeds, or experimenting with high energies, strong gravity, etc. Thus, common-sense "theories", "intuitions" or "feelings" cannot be relied upon when it comes to descriptions or explanations of the behavior of many systems and objects in nature.


The 18th century saw many advances in the domain of science. After Newton, most scientists agreed on the presupposition that the universe is governed by strict (natural) laws that can be discovered and formalized by means of scientific observation and experiment. This position is known as determinism. However, determinism precludes the possibility of free will. That is, if the universe, and thus the entire world, is governed by strict and universal laws, then that means that human beings are also governed by natural law in their own actions. In other words, it means that there is no such thing as human freedom (except as defined in compatibilism). Conversely, if we accept that human beings do have (libertarian or incompatibilist) free will, then we must accept that the world is not entirely governed by natural law. Some have argued that if the world is not entirely governed by natural law, then the task of science is rendered impossible. However, the development of quantum mechanics gave thinkers alternatives to these strictly bound possibilities, proposing a model for a universe that follows general rules but never had a predetermined future.

Uncertainty principle

The Uncertainty Principle is a mathematical principle that follows from the quantum mechanical definition of the operators of momentum and position (namely, the lack of commutativity between them) and that explains the behavior of the universe at atomic and subatomic scales.

The Uncertainty Principle was developed 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 1926, 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 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.[2]

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 Lande) 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.[citation needed] 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.[citation needed] The Copenhagen interpretation holds that it cannot be predicted by any method, not even with theoretically infinitely precise measurements.


The idea of complementarity is critical in quantum mechanics. It says that light can behave both like a particle and like a wave. When the double slit experiment was performed, light acted in some cases as a wave, and some cases as a particle. Physicists had no convincing theory to explain this until Bohr and complementarity came along. Quantum mechanics allows things that are completely opposite intuitively to each other to exist without problem.

Einstein on the importance of the philosophy of physics

Einstein was interested in the philosophical implications of his theory.

Albert Einstein was extremely interested in the philosophical conclusions of his work, and the following two quotes set out some of the reasons why he felt this way.

"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.

See also

Further reading


  1. ^ 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).
  2. ^ Niels Bohr, Atomic Physics andHuman Knowledge, p. 38

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