**Einstein’s Main Ideas:**

**Special Relativity**

*Coordinate spacetime is not absolute, the simultaneity of events is observer-dependent, speed of light is invariant.*

*Many of the individual elements of SR had already been developed by Lorentz, Abraham, and Poincare. *

Their work, however, was burdened with being an elaborate extension of classical ideas whose meaning seemed to become more obscure as it proceeded.

*Einstein’s revolutionary contribution was in starting afresh and giving an entirely new physical interpretation to the symbols involved.*

*At first, the limited data on fast electrons contradicted SR, but Einstein was so sure of the theory that he was unperturbed. After several years, new and better data proved him right.*

**General Relativity**

*Gravitational fields are manifestations of the curvature of spacetime. The curvature of spacetime originates in the stress-energy of the material contained within the spacetime.*

*In 1915, Einstein showed that GR would have three measurable effects that differed from the predictions of classical physics:
(i) precession of the perihelion of the planet Mercury’s orbit
(ii) deflection of starlight passing close to the sun
(iii) red shift of the spectral lines of light radiated by a massive body.
*

*(i) resolved a standing problem in physics. *

*(ii) and (iii) were later empirical verified, (ii) most famously by Eddington’s observations during a 1919 eclipse.*

*Mass is a form of energy, E=mc2
*

*Understanding of gravitation, electromagnetism, and other interactions should be sought in a unified-field theory.*

**Quantum Mechanics**

*In one of his 1905 papers, Einstein argued that light itself comes in discrete quanta of energy. *

In 1915, experiments by R.A. Millikan provided strong evidence that Einstein’s explanation was correct.

*However, many physicists remained unconvinced of the literal reality of photons until the discovery of the Compton effect in 1923.*

*Einstein was one of the first to develop the description of atomic processes in probabilistic terms. He early on (1916) expressed discomfort at the element of randomness, and he never fully accepted this aspect of quantum mechanics.*

*In 1935, he wrote “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” with Podolsky and Rosen which discussed the extent to which quantum mechanics was a complete description of reality.*

**Incompleteness**

*Einstein believed that quantum mechanics was incomplete. *

He always hoped to find a new theory which would give a more satisfying account of atomic behavior.

*This theory would not be a mere appending of hidden variables to quantum theory but would establish new concepts from which the quantum theory would emerge as only a statistical approximation of the truth.*

**Einstein’s Philosophical Views**

*Einstein was motivated by simplicity in theory choice. *

*What warrant is there for this? One reason (borrowed from Schlick) is that simpler theories generally contain fewer arbitrary elements and only non-arbitrary elements are likely to correspond to reality.*

**Positivism vs. Realism**

Einstein was attached to the 19th century view on causality.

Summary of the Einstein-Bohr dialogue: complimentary vs. objective reality; “It became clear to me from listening to them both that the advent of quantum mechanics in 1925 represented a far greater break with the past than had been the case with the coming of special relativity in 1905 or of general relativity in 1915 […] how wrong I was in accepting a rather widespread belief that Einstein simply did not care anymore about the quantum theory. On the contrary, he wanted nothing more than to find a unified field theory which not only would join together gravitational and electromagnetic forces but also would provide the basis for a new interpretation of quantum phenomena.”

*Einstein, influenced by Ernst Mach, began his philosophical life as a positivist. *

*Later on (post wide acceptance of quantum theory), he became a defender of Realism*

*Einstein argued that measuring B should not effect elsewhere located object A: “If one renounces the assumption that what is present in different parts of space has an independent, real existence, then I do not at all see what physics is supposed to describe”.*

*Einstein was a realist about determinism. This required him to be an anti-realist about quantum mechanics. *

*Or, as Einstein, tended to put it, it required him to endorse that quantum mechanics was incomplete. The complete description will be deterministic.
This is where the famous, “God doesn’t play dice with the universe” quote comes from.*

**Einstein on the Scientific Method:**

“We now know that science cannot grow out of empiricism alone, that in the constructions of science we need to use free invention which only *a posteriori* can be confronted with experience as to its usefulness. This fact could elude earlier generations, to whom theoretical creation seemed to grow inductively out of empiricism without the creative influence of a free construction of concepts. The more primitive the status of science is the more readily can the scientist live under the illusion that he is a pure empiricist. In the nineteenth century, many still believed that Newton’s fundamental rule ‘hypotheses non fingo‘ should underlie all natural science.” (p14)

“Newton, forgive me; you found the only way which in your age was just about possible for a man with the highest powers of thought and creativity. The concepts which you created are guiding our thinking in physics even today, although we now know that they will have to be replaced by others farther removed from the sphere of immediate experience, if we aim at a profounder understanding of relationships.” (p14, 15)

**Einstein’s Contribution:**

“His special relativity includes the completion of the work of Maxwell and Lorentz. His general relativity includes the completion of Newton’s theory of gravitation and incorporates mach’s vision of the relativity of all motion. In all these respects, Einstein’s oeuvre represents the crowning of the work of his precursors, adding to and revising the foundations of their theories. In this sense he is a transitional figure, perfecting the past and changing the stream of future events. At the same time he is a pioneer, at first Planck, then he, then Bohr founded a new physics without precursors – the quantum theory.” (p15)

In 1905 Einstein produced 6 papers:

1) The light-quantum and the photoelectric effect, completed March 17. This paper, which led to his Nobel Prize in physics, was produced before he wrote his PhD thesis.

2) A new determination of molecular dimensions, completed April 30. This was his doctoral thesis, which was to become his paper most often quoted in modern literature.

3) Brownian motion, received May 11. This was a direct out growth of his thesis work.

4) The first paper on special relativity, received June 30.

5) The second paper on general relativity, containing the *E = mc2* relation, received September 27.

6) A second paper on Brownian motion, received December 19.

These papers mark the entry of Einstein’s genius into the world of physics. These papers concern two central, early 20th century problems of physics. First, molecular reality. “How can one prove (or disprove) that atoms and molecules are real things? If they are real then how can one determine their size and count their number?” (p19)

The second problem was the molecular basis of statistical physics: “If atoms and molecules are real things, then how does one express such macroscopic concepts as pressure, temperature, and entropy in terms of the motion of these submicroscopic particles?” (p 19)

Reference:

*Subtle Is the Lord: The Science And the Life of Albert Einstein* by Abraham Pais Oxford University Press

*italics* are from Melbourne University Lecture Slides