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I read the book "A Brief History of Time" By Stephan Hawking. It states that Einstein helped scientists like Pauli etc. in the development of the quantum theory and even shared the Nobel Prize with them for his contributions, but to his dying day, didn't agree with the theory. He even made the quote:

I, for one, do not believe that He plays dice.

If he was so much against the theory, when then he helped in its development?

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  • $\begingroup$ Read my answer. His quote "He doesn't play dice" has been taken out of context. He was responding to the Copenhagen Interpretation of QM and the notion, then believed, that the observer affects the collapse of the wave function. This we know to be false; however, when Einstein was alive this was a common belief (promoted by Bohr himself). $\endgroup$ – Albert Heisenberg Aug 5 '16 at 23:01
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    $\begingroup$ Helping to develop, scientifically speaking, includes doing your absolute best to poke holes in a theory to force the theory to develop further in response and thereby strengthen the theory. Strengthen here means to have the theory be able to explain or take into account what seem to be inconsistencies. $\endgroup$ – Broklynite Aug 6 '16 at 8:56
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Einstein made a number of contributions of momentous importance to quantum theory in the 'early days'. In 1905, his famous annus mirabilis, he published a paper on the photo-electric effect that laid the basis for the modern understanding of photons (i.e. quantized wavepackets).

This was twenty years before the foundations of quantum mechanics were properly formulated by Heisenberg, Schrödinger, Dirac, Born, etc. At that time, the implications of Einstein's work were not quite clear to him, or anyone else for that matter. Another important contribution came in 1924, when Einstein ensured that Bose's work on what later became known as Bose-Einstein statistics were published in a mainstream journal. By that time, however, Einstein already worried much about the foundations of quantum mechanics and the lack of complete determinacy it presents us with.

After that, Einstein did not do much constructive work on quantum mechanics, but his continuous criticism was important in forcing the proponents of quantum mechanics to give shape to their ideas, and consider how they apply in complicated situations. The most famous instance of this occurring is the Fifth Solvay Conference in 1927, when Einstein went head-to-head with Niels Bohr, proposing a number of 'inconsistencies' of the Heisenberg principle, with Bohr coming up with a refutation time and again.

I will not give a serious account of Einstein's later work on the EPR paradox; I think the answer by Logan Maingi already addresses this sufficiently. In conclusion, I would like to point out that most of Einstein's constructive work on quantum theory was done before the theory was well-understood, but his importance in sharpening the minds of the inventors of quantum mechanics cannot be underestimated. I don't think it's fair to say that Einstein was against quantum theory: He simply thought it was not the final theory.

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    $\begingroup$ Two other Einstein contributions should be mentioned. He gave a new and exceptionally clear derivation of Planck's black-body radiation law, using the so-called A and B coefficients. Also, both Heisenberg and Schrödinger credited conversations with Einstein as having decisive influence in helping them formulate their versions of quantum mechanics. Schrödinger referred to Einstein's "short but infinitely far-seeing remarks". Heisenberg recounted a conversation where Einstein remarked that it is one's theory that tells one what is, in principle, observable, not the other way round. $\endgroup$ – Michael Weiss Oct 29 '14 at 17:56
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It's not true that Einstein rejected quantum mechanics completely. He acknowledged that it gave numerically accurate predictions in a wide variety of cases, as did any competent physicist by 1935. In that year, he introduced the EPR paradox, which shows that quantum mechanics doesn't respect locality in special relativity. In particular, if one considers an entangled state of two spins at spacial separation, and measures the spin of one of them, the state of the other must immediately change in order to accommodate the measurement of the first. He considered this sort of faster-than-light action at a distance to be at odds with any "reasonable definition of the nature of reality". This was his best objection to quantum mechanics as it was interpreted at the time. Of course, Einstein also did some early work on quantum mechanics before these issues were apparent to him.

Thus, Einstein chose to reject not quantum mechanics outright, but it's conventional interpretation. He favored a theory in which all physical measurements were determined, but could not all be measured. This would be a so-called hidden-variable theory, arguing that quantum mechanics was not complete and that additional local degrees of freedom exist which would give a theory that was essentially classical. However, these additional "hidden variables" couldn't hope to be measured in practice, and so quantum mechanics is what we end up seeing. This philosophical position is sometimes known as "local realism".

Local realism, while not necessary to explain any physical measurement, was still on good experimental standing until 1964. Up until that point, the prevailing view was that any quantum mechanical theory could be made into a local hidden variable theory, though no one knew exactly how. In that year, Bell derived his now-famous inequalities showing that quantum mechanics predicts smaller correlations between certain measurements than could ever be accommodated by any classical hidden variable theory. This gave rise to actual measurements which definitively showed that local hidden variables aren't what we have in nature. By that point, one had to either settle for non-local hidden variables, which wouldn't have satisfied Einstein very much, or just quantum mechanics. Einstein didn't live long enough to have to make that decision though, as he died in 1955.

So, it wasn't so much that Einstein believed quantum mechanics was incorrect. Rather, he thought it was incomplete. His later frustrations with it were more that no one had managed to figure out how to realize it with hidden variables (and few people were even trying). When he said things like "God doesn't play dice" and the like, he wasn't saying that quantum mechanics was wrong, so much as incomplete, and was upset that no one was doing what he thought was necessary to make it complete. Through the lens of history, we can see that he was wrong, but at the time this was a seemingly reasonable stance to take.

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  • $\begingroup$ This was an interesting answer. I'd say you have a deep understanding of EPR. I find it frustrating that people don't understand the argument clearly, see my question here, which is more of a commentary than a question: physics.stackexchange.com/questions/114651/… $\endgroup$ – user7348 Nov 30 '14 at 0:58
  • $\begingroup$ Why do people write about "local hidden variables", when the argument is clear and simple: Either measured properties are pre-determined, or there is "spooky action at a distance" as Einstein remarked. Bell showed pre-determined properties don't work. You seem to get this argument very clearly. However, I have to ask: don't you feel that this is at odds with special relativity? $\endgroup$ – user7348 Nov 30 '14 at 1:02
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A bit more about Einstein's indirect contributions to quantum theory.

In Heisenberg's autobiographical essay "Theory, Criticism, and Philosophy", in the section "Einstein on Theory and Observation", Heisenberg recounts a conversation he had with Einstein shortly after Heisenberg had proposed his version of QM (called matrix mechanics).

Einstein asked me to come to his flat and discuss the matters with him. The first thing he asked me was: "What was the philosophy underlying your kind of very strange theory? The theory looks quite nice, but what did you mean by only observable quantities?"

Here Einstein alludes to Heisenberg's claim that physics should deal only with observable quantities; this justified discarding the idea of electron trajectories. Heisenberg responded,

I felt that one should go back to those quantities which can really be observed and I also felt that this was just the kind of philosophy which he had used in relativity; because he also had abandoned absolute time... Well, he laughed at me and then he said, "But you must realize that it is completely wrong." I answered: "But why, is it not true that you have used this philosophy?" "Oh yes", he said, "I may have used it, but still it is nonsense!"

Einstein explained to me that it is really the other way around. He said, "Whether you can observe a thing or not depends on the theory which you use. It is the theory which decides what can be observed.

Heisenberg explains that this conversation set him on the train of thought which culminated in his Uncertainty Principle.

Turning to Schrödinger, in a footnote to his paper "On the Relation of the Heisenberg-Born-Jordan Quantum Mechanics to Mine", he wrote:

My theory was inspired by L. de Broglie and by brief but infinitely far-seeing remarks of A. Einstein (Berl. Ber. 1925, p.9ff)

I believe the cited paper is Einstein's second one on Bose-Einstein statistics.

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While the answers provided so far are both good, their authors forget to mention what is the starting point of Einstein's disagreement with quantum mechanics - the Copenhagen interpretation. This (up to date) most spread interpretation of quantum mechanics states that observable quantities do not have a specific value prior to a measurement, after which the quantum state of the system randomly collapses into one of possible measurement eigenstates. It is this inherent randomness which bothered Einstein (and not only him, another famous example is Schroedinger's cat that is dead and alive at the same time until one measures its state). This also gives rise to the super-luminal effects in EPR paradox and led to the quote about god not playing dice.

Einstein's major contribution to quantum physics - the explanation of the photoelectric effect is about 20 years older than the Copenhagen interpretation and was formulated in the very early years of quantum mechanics. The EPR paradox, on the other hand is older than the Copenhagen interpretation and was designed primarily to show its apparent inconsistency. There is thus no inconsistency in Einstein helping develop a theory he did not agree with.

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    $\begingroup$ "The EPR paradox, on the other hand is older than the Copenhagen interpretation". Should that be "younger"? $\endgroup$ – Faheem Mitha Aug 3 '17 at 9:03
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  1. His Law of the Photoelectric Effect (a misnomer - it should REALLY be called, the Quantization of the Radiation Field).

  2. His Paper on the Specific Heat of Solids (1906)

  3. His Paper on Quantum Vibrations (1907) ...Mind you he was the ONLY physicist in the world seriously working on quantum theory - even Bohr thought his idea of quantized energy (i.e. photons) was silly, given how great thinkers like Poissant and Maxwell had "proven" that light was a wave. Not a SINGLE notable scientist believed in Einstein's 1905 paper until at least the First Solvay Conference of 1911, and even then the vast majority were 'quantum skeptics.'

4.In 1909 Einstein was the first to show that statistical fluctuations in thermal radiation fields display both particlelike and wavelike behaviour; his was the first demonstration of what would later become the principle of complementarity.

  1. 1916/1917 marks Einstein's most underrated paper. After finishing his magnum opus, General Relativity, he turned to the interplay of matter and radiation to create a quantum theory of radiation. He once again based his arguments on statistics and fluctuations. Bohr introduced a crucial new concept called stationary states in his 1913 paper on hydrogen but major features of Bohr's model could be interpreted as absolute nonsense because, according to electromagnetic theory, the electron would radiate intensely, emitting a broad spectrum as it crashed into the nucleus. Here we see contradictions in classical laws, and yet major properties of Bohr's hydrogen model rested on those laws.

Einstein, always the original thinker, didn't take as his starting point the well-known field for thermal radiation given by the Planck radiation law. Instead, he assumed that the atoms are in thermal equilibrium and then deduced the properties of the radiation field required to maintain the equilibrium. Guess what? The field turned out to be given precisely by the Planck radiation law. He manages to create quantum effects (stimulated and spontaneous emission) from most classical principles. He uses Wien's displacement law, the canonical Boltzmann distribution, Poynting's theorem, and microscopic reversibility - all classical. The sole quantum idea was the concept of stationary states. And yet from these elements, he's the first to create a complete description of the basic radiation processes and a full description of the general properties of the photon. In his 1917 paper, he creates novel and elegant derivations of Planck's radiation law as well as a proof of Bohr's frequency rule. In it, among many other things, he answers the question of how a gas of atoms maintain the populations of its stationary states in equilibrium with a radiation field.

The aforementioned novel concept of spontaneous emission, which embodies the FUNDAMENTAL interaction of matter with the vacuum, is a brilliant, Nobel-Prize worthy achievement. Why? Spontaneous emission sets the scale for ALL radiative interactions. The rates of absorption and stimulated emission, for instance, are proportional to the rate for spontaneous emission. Spontaneous emission can be viewed as the ultimate irreversible process and the fundamental source of noise in all of nature. With the development of cavity quantum electrodynamics - the study of atomic systems in close-to-ideal cavities - in the 1980s, the phystical situation was profoundly altered. In such cavities, spontaneous emission evolves into spontaneous-cavity oscillations. Although the dynamical behavior is totally altered, the atom-vacuum interaction that causes spontaneous emission sets the time scale for that evolution. It is first in Einstein's 1917 paper that the photon is demonstrated to possess all the properties of a fundamental excitation, and therefore it is quite clear that his radiation paper played a seminal role in the eventual creation of quantum electrodynamics.

Apropos the second brilliant creation of his 1917 paper, stimulated emission of radiation, we see the first genesis of the laser. Stimulated emission underlies the basic mechanism of the laser and, by extension, laser cooling; his analysis of momentum transfer in a thermal radiation field can be immediately applied to atomic motion in a laser field. If the spectral width of a thermal field is replaced by the natural linewidth of the atom, Einstein's viscous damping force would give rise to the phenomenon known as optical molasses. This fundamental process of laser cooling was rediscovered by the atomic community in the 80's. Of course, you need Quantum Mechanics for a full realization of all the mechanisms of radiation, but papers like this are seminal contributions to what would eventually become QM.

Einstein's theory of radiation provided a complete characterization of the particlelike properties of the light quantum and, in retrospect, he was within an arms grasp of working out the statistical mechanics of these particles. Given that his 1905 proposal for the energy quantization of radiation was based on the analogy between entropies of thermal radiation and a system of particles, it is surprising that Einstein didn't extend his method of reasoning to derive the Planck law by treating photons as indistinguishable particles. He was VERY close, and it is quite apparent that Bose himself did not realize he had done anything novel.

  1. On the Quantization of Chaos (1919): In it, Einstein was the first to point out the fundamental problems that arise when one applies classical chaos theory to quantum states (a paper 50 years ahead of its time as this is a problem we have only now began to fully grasp): http://boulderschool.yale.edu/sites/default/files/files/Einstein_chaos.pdf

  2. Fast forward to 1924 and Einstein, not Bose, applied the reasoning in Bose's treatment of photons as indistinguishable particle to a gas of indistinguishable atoms thereby creating Bose-Einstein statistics, and later, Bose-Einstein Condensation. Subsequently, Einstein theorized Bose-Einstein condensation, a work for which 6 Nobel Prizes have been given. Einstein was 45% of the way to the Schrodinger Equation. It was only after Schrodinger had read Einstein's paper that he derived his equations governing the wave function.

  3. Einstein was the first to conceive of ghost fields as probability densities, a concept which he applied to a gas of photons (i.e. probability waves). Max Born essentially took the idea verbatim and applied it to electrons. Born always acknowledged this.

  4. EPR Paradox Paper: the first paper to show how quantum entanglement emerges from the QM's equations.

*Einstein's work on wave-particle duality led directly to De Broglie's thesis on matter waves, and it seems unlikely De Broglie would have conceived of it without Einstein.

Einstein is pretty much the father of early quantum theory, and is one of the co-founders of modern Quantum Mechanics. The three major statistical systems governing the microscopic realm are: Fermi-Dirac Statistics, Einstein-Bose Statistics, and the Boltzmann Statistics. He would rightly be regarded as a legend for his work on BEC alone, and yet he contributed massively to Quantum Mechanics. Kindly check out his paper on the quantization of chaos, it's absolutely brilliant and shows how indispensable his thinking was to the development of QM.

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