Philosophy of physics

Philosophy of physics

Philosophy of physics is the study of the fundamental, philosophical questions underlying modern physics, the study of matter and energy and how they interact. The main questions concern the nature of space and time, atoms and atomism. Also the predictions of cosmology, the results of the interpretation of quantum mechanics, the foundations of statistical mechanics, causality, determinism, and the nature of physical laws. Classically, several of these questions were studied as part of metaphysics (for example, those about causality, determinism, and space and time). Today, the philosophy of physics is very close to the philosophy of science.

Philosophy of space and time


Time is a fundamental quantity (that is, a quantity which can not be defined via other quantities because at present we don't know anything more basic). 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). Time then can be combined mathematically with other fundamental quantities (like space and mass) to derive other concepts such as velocity, momentum, energy and fields. Physicists measure and use theories to predict measurements of time. What exactly time "is" and how it works follows from the above definition. Coupled with the current definition of space (via length) this definition of time makes special relativity absolutely correct. Both NewtonRoger Penrose, The Road to Reality: A Complete Guide to the Laws of the Universe, Jonathan Cape, London, 2004, ISBN 0-224-04447-8 (hardcover), ISBN 0-09-944068-7 (paperback)] 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, in which rates of time at separate places run differently, and space and time are merged into spacetime. It is possible that time may be quantized, with the theoretical smallest time, 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 thirteen to fourteen billion years ago in the big bang. Whether it will ever end is an open question.

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.


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, while determinism was the fundamental presupposition of post-Newtonian physics, it quickly led philosophers to a tremendous problem: 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 it be accepted 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 advent of quantum mechanics gave thinkers alternatives to the strictly bounded possibilities outlined above. It became possible to believe in a universe that followed universal laws 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. Heisenberg 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 who believed in fate and determinism and saw the common features of the Bohr-Heisenberg theories as a threat. Within the widely but not universally accepted Copenhagen interpretation of quantum mechanics (i.e. it was not accepted by Einstein or other physicists such as Alfred Lande), the uncertainty principle is 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 potentials. 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.

If one goes even further to the "direct" interpretation that classical physics and ordinary language are "only" approximations to a completely quantum reality, then the probabilities are assigned to these approximations and are no longer fundamental. The equations of quantum mechanics themselves specify the progression of the "quantum" state of any isolated system uniquely.


The idea of complementarity is critical in quantum mechanics. It says that light can be both a particle and 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 complementary came along. Quantum mechanics allows things that are completely opposite intuitively to each other to exist without problem.

The importance of philosophy of physics

Albert Einstein was extremely interested in the philosophical conclusions of his work, and the following two quotes explain a few of the more important reasons why this subject knowledge is important.

" "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 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, from 'Ernst Mach. ' Physikalische Zeitschrift 17 (1916): 101, 102 - A memorial notice for the philosopher, Ernst Mach.

Subjects in the philosophy of physics

* Interpretation of quantum mechanics
* Philosophy of space and time
* Philosophy of thermal and statistical physics
* Philosophical interpretation of classical physics

See also

* Bohr-Einstein debates
* Reductionism
* Holism


External links

* [ Stanford Encyclopedia of Philosophy] :
** [ Experiments in Physics]
** [ Holism and Nonseparability]
** [ Intertheory Relations in Physics]
** [ Reichenbach's Common Cause Principle]
** [ Structuralism in Physics]
** [ Symmetry and Symmetry Breaking]
** [ Uncertainty principle]
** ['s_sword Occam's sword]

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