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Philosophy of physics
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What is the philosophy of physics?
The philosophy of physics involves philosophical speculation regarding the concepts, methods, and theories of the physical sciences, especially physics. It is an intellectual frontier where theoretical physics and modern Western philosophy have been informing and unsettling each other for over 400 years.
Many deep intellectual commitments of Western culture have been challenged since the start of modern science. These include the character of matter, the nature of space and time, the question of determinism, the meaning of probability and chance, and the possibility of knowledge.
The influence of physics extends to other scientific fields, philosophy, and the worldview of the developed world. Physics elucidates the fundamental questions in nature, allowing it to have a profound impact on other sciences.
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How is physics different from philosophy?
Physics and philosophy differ in their approaches and focuses, though the philosophy of physics uses philosophical speculation to examine the concepts, methods, and theories of the physical sciences, especially physics. Physics seeks to understand the fundamental laws of nature through observation, experimentation, and mathematical modeling. It aims to provide precise, quantitative descriptions of physical phenomena. Philosophy, on the other hand, explores abstract concepts, values, and the nature of reality through critical reasoning and analysis. While physics focuses on the "how" of the universe, philosophy delves into the "why".
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Why do scientists think about the meaning and methods of physics?
Scientists delve into the meaning and methods of physics because it serves as an intellectual frontier where theoretical physics and modern Western philosophy intersect. This exploration allows for examination of the logical structures, metaphysical motivations, and epistemological implications of physical theories.
Since the emergence of modern science, many concepts central to Western culture have been challenged, including the character of matter, the nature of space and time, determinism, the meaning of probability and chance, and the possibility of knowledge. Examining physics allows for a continued conversation regarding humanity's perception and understanding of nature.
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What are laws of nature in physics, and why do philosophers question them?
In physics, a law of nature is a consistent regularity in the world's phenomena under specific conditions. These laws can be universal, applying in all known instances, or probabilistic, indicating a stated fraction of occurrences displaying a particular condition.
Philosophical questioning arises around scientific laws regarding explanation, prediction, control, and distinguishing genuine laws from accidental generalizations. Some philosophers focus on identifying laws as generalizations that would feature in the best systematization of all natural phenomena. Others are beginning to abandon the notion that laws are central to science, focusing instead on the search for symmetries in physics.
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What is determinism, and how does it relate to physics and philosophy?
Determinism is a philosophical and scientific concept that posits all events, including human decisions, are causally inevitable given prior events and the laws of nature. It suggests that if the state of the universe at any given time and the governing natural laws are known, then every subsequent state is completely determined.
In physics, the Newtonian conception of the universe is deterministic, meaning that knowing the position and properties of every particle at a moment in time would allow predicting the entire future history of the universe. However, quantum mechanics introduces indeterminacy, suggesting some events at the quantum level are random and unpredictable.
Philosophically, determinism is often seen as incompatible with free will, leading to the view known as "hard determinism," which denies moral responsibility. Compatibilists, or "soft" determinists, argue that determinism and free will can coexist by redefining free will. The extreme opposite of determinism is indeterminism, which states that some events occur randomly or by chance.
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How do different interpretations of quantum mechanics show the importance of philosophy in physics?
Different interpretations of quantum mechanics highlight the deep connections between physics and philosophy. Quantum mechanics introduces randomness, challenging determinacy, as the theory can only predict the probability of a result.
The Copenhagen interpretation emphasizes basing theory on observations and measurements, rejecting hidden variables. The theory has two categories of physical laws: one for when a system is not observed and another when it is, yet the distinction between the two is vague.
Interpretations of quantum mechanics have prompted a reexamination of determinism and the meaning of measurement. Niels Bohr's Copenhagen interpretation introduces complementarity, suggesting that a phenomenon depends on the experimental setup, requiring consideration of contradictory aspects for a complete explanation.
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Philosophy of physics, philosophical speculation about the concepts, methods, and theories of the physical sciences, especially physics.
The philosophy of physics is less an academic disciplineāthough it is thatāthan an intellectual frontier across which theoretical physics and modern Western philosophy have been informing and unsettling each other for more than 400 years. Many of the deepest intellectual commitments of Western cultureāregarding the character of matter, the nature of space and time, the question of determinism, the meaning of probability and chance, the possibility of knowledge, and much else besidesāhave been vividly challenged since the inception of modern science, beginning with the work of Galileo (1564ā1642). By the time of Sir Isaac Newton (1642ā1727), a lively conversation between physics and a distinctly modern Western philosophical tradition was well under way, an exchange that has flourished to the present day. That conversation is the topic of this article.
This article discusses the logical structures of the most general physical theories of modern science, together with their metaphysical and epistemological motivations and implications. For treatment of the elements of scientific inquiry from a philosophical perspective, see science, philosophy of.
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The philosophy of space and time
The Newtonian conception of the universe
According to Newton, the physical furniture of the universe consists entirely of infinitesimal material points, commonly referred to as particles. Extended objects, or objects that take up finite volumes of space, are treated as assemblages of particles, and the behaviours of objects are determined, at least in principle, by the behaviours of the particles of which they are composed. The properties of particles include mass, electric charge, and position.
The Newtonian conception is both complete and deterministic. It is complete in the sense that, if it were possible to list, for each moment of past time, what particles existed, what their masses, electric charges, and other intrinsic properties were, and what positions they occupied, the list would represent absolutely everything that could be said about the physical history of the universe; it would contain everything that existed and every event that occurred. The Newtonian conception is deterministic in the sense that, if it were possible to list, for a particular moment of time, the position and other intrinsic properties of each particle in the universe, as well as how the position of each particle is changing as time flows forward, the entire future history of the universe, in every detail, would be predictable with absolute certainty. Many thinkers, however, have regarded this determinism as incompatible with deep and important ideas about what it is to be a human being or to lead a human lifeāideas such as freedom and responsibility, autonomy, spontaneity, creativity, and the apparent āopennessā of the future.
The logical structure of Newtonian mechanics
The rate at which the position of a particle is changing at a particular time, as time flows forward, is called the velocity of the particle at that time. The rate at which the velocity of a particle is changing at a particular time, as time flows forward, is called the acceleration of the particle at that time. The Newtonian conception stipulates that force, which acts to maintain or alter the motion of a particle, arises exclusively between pairs of particles; furthermore, the forces that any two particles exert on each other at any given moment depend only on what sorts of particles they are and on their positions relative to each other. Thus, within Newtonian mechanics (the science of the motion of bodies under the action of forces), the specification of the positions of all the particles in the universe at a particular time and of what sorts of particles they are amounts to a specification of what forces are operating on each of those particles at that time.
According to Newtonās second law of motion, a certain very simple mathematical relation invariably holds between the total force on any particle at a particular time, its acceleration at that time, and its mass; the force acting on a particle is equal to the particleās mass multiplied by its acceleration: F = ma
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The application of this law (hereafter āNewtonās law of motionā) can be illustrated in detail in the following example. Suppose that one wished to calculate, for each particle i in a certain subsystem of the universe, the position of that particle at some future time t = T. For each particle at some initial time t = 0, one is given the particleās position (x0i), velocity (v0i), mass (mi), electric charge (ci), and all other intrinsic properties.
One way of performing the calculation is by means of a succession of progressively better approximations. Thus, the first approximation might be to calculate the positions of all the particles at t = T by supposing that their velocities are constant and equal to v0i at t = 0 throughout the interval between t = 0 and t = T. This approximation would place particle i at x0i + v0i(T) at t = T. It is apparent, however, that the approximation would not be very accurate, because in fact the velocities of the particles would not remain constant throughout the interval (unless no forces were at work on them).
A somewhat better approximation could be obtained by dividing the time interval in question into two, one interval extending from t = 0 to t = T/2 and the other extending from t = T/2 to t = T. Then the positions of all the particles at T/2 could be calculated by supposing that their velocities are constant and equal to their values at t = 0 throughout the interval between t = 0 and t = T/2; this would place particle i at x0i + v0i(T/2) at T/2. The forces acting on each of the particles at t = 0 could then be calculated, according to the Newtonian conception, from their positions at t = 0 together with their masses, charges, and other intrinsic properties, all of which were given at the outset.
The velocities of the particles at T/2 could be obtained by plugging the values of these forces into Newtonās law of motion, F = ma, and assuming that, throughout the interval from t = 0 to t = T/2, their accelerations are constant and equal to their values at t = 0. This would make the velocity of particle i equal to v0 + a0i(T/2), where a0i is equal to the force on particle i at t = 0 divided by particle iās mass. Finally, the position of particle i at t = T could be calculated by supposing that i maintains the new velocity throughout the interval between t = T/2 and t = T.
Although this approximation would also be inaccurate, it is an improvement over the first one because the intervals during which the velocities of the particles are erroneously presumed to be constant are shorter in the second calculation than in the first. Of course, this improvement can itself be improved upon by dividing the interval further, into 4 or 8 or 16 intervals.
As the number of intervals approaches infinity, the calculation of the particlesā positions at t = T approaches perfection. Thus, given a simple-enough specification of the dependence of the forces to which the particles are subjected on their relative positions, the techniques of integral calculus can be used to carry out the perfect calculation of the particlesā positions. Because T can have any positive value whatsoever, the positions of all the particles in the system in question at any time between t = 0 and t = ā (infinity) can in principle be calculated, exactly and with certainty, from their positions, velocities, and intrinsic properties at t = 0.