einstein

CHAPTER TWENTY: QUANTUM ENTANGLEMENT
1. Hoffmann 1972, 190; Rigden, 144; Léon Rosenfeld, “Niels Bohr in the Thirties,” in Rozental 1967, 127; N. P. Landsman, “When Champions
Meet: Re-thinking the Bohr–Einstein Debate,”Studies in the History and Science of Modern Physics 37 (Mar. 2006): 212.
2. Einstein 1949b, 85.
3. Ibid.
4. Einstein to Max Born, Mar. 3, 1947, in Born 2005, 155 (not in AEA).
5. Einstein to Erwin Schrödinger, June 19, 1935, AEA 22-47.
6. New York Times , May 4 and 7, 1935; David Mermin, “My Life with Einstein,” Physics Today (Jan. 2005).
7. Albert Einstein, Boris Podolsky, and Nathan Rosen, “Can Quantum-Mechanical Description of Physical Reality Be Regarded as
Complete?,” Physical Review, May 15, 1935 (received Mar. 25, 1935); www.drchinese.com/David/EPR.pdf.
8. Another formulation of the experiment would be for one observer to measure the position of a particle while at the “same moment” another
observer measures the momentum of its twin. Then they compare notes and, supposedly, know the position and momentum of both
particles. See Charles Seife, “The True and the Absurd,” in Brockman, 71.
9. Aczel 2002, 117.
10. Whitaker, 229; Aczel 2002, 118.
11. Niels Bohr, “Can Quantum-Mechanical Description of Physical Reality Be Regarded as Complete?,”Physical Review , Oct. 15, 1935
(received July 13, 1935).
12. Greene 2004, 102. Note that Arthur Fine says that the synopsis of EPR used by Bohr “is closer to a caricature of the EPR paper than it is
to a serious reconstruction.” Fine says that Bohr and other interpreters of Einstein feature a “criterion of reality” that Einstein in his own
later writings on EPR does not feature, even though the EPR paper as written by Podolsky does talk about determining “an element of
reality.” Brian Greene’s book is among those that do emphasize the “criterion of reality” element. See Arthur Fine, “The Einstein-
Podolsky-Rosen Argument in Quantum Theory,”Stanford Encyclopedia of Philosophy , plato.stanford.edu/entries/qt-epr/, and also: Fine
1996, chapter 3; Mara Beller and Arthur Fine, “Bohr’s Response to EPR,” in Jann Faye and Henry Folse, eds., Niels Bohr and
Contemporary Philosophy (Dordrecht: Kluwer Academic, 1994), 1–31.
13. Arthur Fine has shown that Einstein’s own critique of quantum mechanics was not fully captured in the way that Podolsky wrote in the EPR
paper, and especially in the way that Bohr and the “victors” described it. Don Howard has built on Fine’s work and emphasized the issues
of “separability” and “locality.” See Howard 1990b.
14. Einstein to Erwin Schrödinger, May 31, 1928, AEA 22-22; Fine, 18.
15. Erwin Schrödinger to Einstein, June 7, 1935, AEA 22-45, and July 13, 1935, AEA 22-48.
16. Einstein to Erwin Schrödinger, June 19, 1935, AEA 22-47.
17. Erwin Schrödinger, “The Present Situation in Quantum Mechanics,” third installment, Dec. 13, 1935, www.tuharburg.de/rzt/rzt/it/QM/cat.html.
18. More specifically, Schrödinger’s equation shows the rate of change over time of the mathematical formulation of the probabilities for the
outcome of possible measurements made on a particle or system.
19. Einstein to Erwin Schrödinger, June 19, 1935, AEA 22-47.
20. I am grateful to Craig Copi and Douglas Stone for helping to compose this section.
21. Einstein to Erwin Schrödinger, Aug. 8, 1935, AEA 22-49; Arthur Fine, “The Einstein-Podolsky-Rosen Argument in Quantum
Theory,”Stanford Encyclopedia of Philosophy , plato.stanford.edu/entries/qt-epr/. Note that Arthur Fine uncovered some of the Einstein-
Schrödinger correspondence. Fine, chapter 3.
22. Erwin Schrödinger to Einstein, Aug. 19, 1935, AEA 22-51.
23. Erwin Schrödinger, “The Present Situation in Quantum Mechanics,” Nov. 29, 1935, www.tu-harburg.de/rzt/rzt/it/QM/cat.html.
24. Einstein to Erwin Schrödinger, Sept. 4, 1935, AEA 22-53. Schrödinger’s paper had not been published, but Schrödinger included its
argument in his Aug. 19, 1935, letter to Einstein.
25. en.wikipedia.org/wiki/Schrodinger’s_cat.
26. Einstein to Erwin Schrödinger, Dec. 22, 1950, AEA 22-174.
27. David Bohm and Basil Huey, “Einstein and Non-locality in the Quantum Theory,” in Goldsmith et al., 47.
28. John Stewart Bell, “On the Einstein-Podolsky-Rosen Paradox,”Physic 1, no. 1 (1964).
29. Bernstein 1991, 20.
30. For an explanation of how Bohm and Bell set up their analysis, see Greene 2004, 99–115; Bernstein 1991, 76.
31. Bernstein 1991, 76, 84.
32. New York Times , Dec. 27, 2005.
33. New Scientist , Jan. 11, 2006.
34. Greene 2004, 117.
35. In the decoherent-histories formulation of quantum mechanics, the coarse graining is such that the histories don’t interfere with one
another: if A and B are mutually exclusive histories, then the probability of A or B is the sum of the probabilities of A and of B as it should
be. These “decoherent” histories form a tree-like structure, with each of the alternatives at one time branching out into alternatives at the
next time, and so forth. In this theory, there is much less emphasis on measurement than in the Copenhagen version. Consider a piece of
mica in which there are radioactive impurities emitting alpha particles. Each emitted alpha particle leaves a track in the mica. The track is
real, and it makes little difference whether a physicist or other human being or a chinchilla or a cockroach comes along to look at it. What
is important is that the track is correlated with the direction of emission of the alpha particle and could be used to measure the emission.
Before the emission takes place, all directions are equally probable and contribute to a branching of histories. I am grateful to Murray Gell-
Mann for his help with this section. See also Gell-Mann, 135–177; Murray Gell-Mann and James Hartle, “Quantum Mechanics in the Light
of Quantum Cosmology,” in W. H. Zurek, ed., Complexity, Entropy and the Physics of Information (Reading, Mass.: Addison-Wesley,
1990), 425–459, and “Equivalent Sets of Histories and Multiple Quasiclassical Realms,” May 1996, www.arxiv.org/abs/gr-qc/9404013.
This view is derived from the many-worlds interpretation pioneered in 1957 by Hugh Everett.
36. The literature on Einstein and realism is fascinating. This section relies on the works of Don Howard, Gerald Holton, Arthur I. Miller, and
Jeroen van Dongen cited in the bibliography.
Don Howard has argued that Einstein was never a true Machian nor a true realist, and that his philosophy of science did not change much