The Order of Time

* The route by which Einstein arrived at this conclusion was a long one: it didn’t end with the writing of the equations of the field in 1915 but continued in his tortuous efforts to understand its physical significance, causing him in the process to change his ideas repeatedly. He was confused in particular regarding the existence of solutions without matter, and by whether gravitational waves were real or not. He achieves definitive clarity only in his last writings and, in particular, in the fifth appendix, “Relativity and the Problem of Space,” which was added to the fifth edition of Relativity: The Special and General Theory (Methuen: London, 1954). This appendix can be read at http://www.relativitybook.com/resources/Einstein_space.html. For copyright reasons, this appendix is not included in most editions of the book. A more in-depth discussion can be found in chapter 2 of my Quantum Gravity (Cambridge, UK: Cambridge University Press, 2004).