Thought experiments and a-priori physical principles
Main article: Thought experiment A thought experiment , sometimes called a Gedanken experiment, is a proposal for an experiment that would test or illuminate a hypothesis or theoryEinstein's thinking underwent a transformation in 1905. He had come to understand that quantum properties of light mean that Maxwell's equations were only an approximation. He knew that new laws would have to replace these, but he did not know how to go about finding those laws. He felt that guessing formal relations would not go anywhere.
So he decided to focus on a-priori principles instead, which are statements about physical laws which can be understood to hold in a very broad sense even in domains where they have not yet been shown to apply. A well accepted example of an a-priori principle is rotational invariance In mathematics, a function defined on an inner product space is said to have rotational invariance if its value does not change when arbitrary rotations are applied to its argument. For example, the function f = x2 + y2 is invariant under rotations of the plane around the origin. If a new force is discovered in physics, it is assumed to be rotationally invariant almost automatically, without thought. Einstein sought new principles of this sort, to guide the production of physical ideas. Once enough principles are found, then the new physics will be the simplest theory consistent with the principles and with previously known laws.
The first general a-priori principle he found was the principle of relativity In physics, the principle of relativity is the requirement that the equations, describing the laws of physics, have the same form in all admissible frames of reference, that uniform motion is indistinguishable from rest. This was understood by Hermann Minkowski to be a generalization of rotational invariance from space to space-time. Other principles postulated by Einstein and later vindicated, are the principle of equivalence In the physics of general relativity, the equivalence principle refers to several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually the same as the pseudo-force experienced and the principle of adiabatic invariance In thermodynamics, an adiabatic process is a change that occurs without heat flow and slowly compared to the time to reach equilibrium. In an adiabatic process, the system is in equilibrium at all stages. Under these conditions the entropy is constant of the quantum number. Another of Einstein's general principles, Mach's principle is fiercely debated, and whether it holds in our world or not is still not definitively established.
The use of a-priori principles is a distinctive unique signature of Einstein's early work, which has become a standard tool in modern theoretical physics.
<<Table of Contents Albert Einstein (pronounced /ˈælbərt ˈaɪnstaɪn/; German: [ˈalbɐt ˈaɪ̯nʃtaɪ̯n] ; 14 March 1879 – 18 April 1955) was a theoretical physicist. He is best known for his theories of special relativity and general relativity. Einstein received the 1921 Nobel Prize in Physics "for his services to Theoretical Physics, and especially | Next>> | Show All>>
