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Special Relativity

Published online by Cambridge University Press:  26 June 2023

James Read
Affiliation:
University of Oxford

Summary

This Element presents the philosophy of special relativity, from the foundations of the theory in Newtonian mechanics, through its birth out of the ashes of nineteenth-century ether theory, through the various conceptual paradoxes which the theory presents, and finally arriving at some of its connections with Einstein's later theory of general relativity. It illustrates concepts such as inertial frames, force-free motion, dynamical versus geometrical understandings of physics, the standard hierarchy of classical spacetimes, and symmetries of a physical theory; it also discusses specific topics in the foundations of special relativity such as Einstein's 1905 derivation of the Lorentz transformations, the conventionality of simultaneity, the status of frame-dependent effects, and the twin paradox.
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Online ISBN: 9781009300599
Publisher: Cambridge University Press
Print publication: 20 July 2023

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References

Acuña, P. (2016). Minkowski spacetime and Lorentz invariance: The cart and the horse or two sides of a single coin? Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 55, 112. https://doi.org/10.1016/j.shpsb.2016.04.002.CrossRefGoogle Scholar
Anderson, R., Vetharaniam, I. & Stedman, G. (1998). Conventionality of synchronisation, gauge dependence and test theories of relativity. Physics Reports, 295(3), 93180. https://doi.org/10.1016/S0370-1573(97)00051-3.Google Scholar
Barbour, J. B. (1989). The discovery of dynamics: A study from a Machian point of view of the discovery and the structure of dynamical theories. New York: Oxford University Press.Google Scholar
Barrett, T. W. (2015). Spacetime structure. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 51, 3743. https://doi.org/10.1016/j.shpsb.2015.06.004.Google Scholar
Bell, J. S. (1992, Sep). George Francis FitzGerald. Physics World, 5(9), 31–5. https://doi.org/10.1088/2058-7058/5/9/24.CrossRefGoogle Scholar
Bell, J. S. (2004). How to teach special relativity. In Speakable and unspeakable in quantum mechanics: Collected papers on quantum philosophy (2nd ed., pp. 6780). Cambridge: Cambridge University Press. https://doi.org/10.1017/CBO9780511815676.011.Google Scholar
Belot, G. (2000). Geometry and motion. British Journal for the Philosophy of Science, 51(4), 561–95. https://doi.org/10.1093/bjps/51.4.561.Google Scholar
Berzi, V. & Gorini, V. (1969). Reciprocity principle and the Lorentz transformations. Journal of Mathematical Physics, 10, 1518–24. https://doi.org/10.1063/1.1665000.CrossRefGoogle Scholar
Bridgman, P. W. (1967). A sophisticate’s primer of relativity. London: Routledge and Kegan Paul.Google Scholar
Brown, H. R. (1997). On the role of special relativity in general relativity. International Studies in the Philosophy of Science, 11(1), 6781. https://doi.org/10.1080/02698599708573551.Google Scholar
Brown, H. R. (2005). Physical relativity: Space-time structure from a dynamical perspective. Oxford: Oxford University Press.CrossRefGoogle Scholar
Brown, H. R. & Pooley, O. (2001). The origin of the spacetime metric: Bell’s ‘Lorentzian pedagogy’ and its significance in general relativity. In Callender, C. & Huggett, N. (eds.), Physics meets philosophy at the Planck scale (256–72). Cambridge: Cambridge University Press.Google Scholar
Brown, H. R. & Pooley, O. (2004). Minkowski space-time: A glorious non-entity. In Dieks, D. (ed.), The ontology of spacetime (pp. 6789). Amsterdam: Elsevier.Google Scholar
Brown, H. R. & Read, J. (2016). Clarifying possible misconceptions in the foundations of general relativity. American Journal of Physics, 84, 327. https://doi.org/10.1119/1.4943264.CrossRefGoogle Scholar
Brown, H. R. & Read, J. (2021). The dynamical approach to spacetime theories. In Knox, E. & Wilson, A. (eds.), The Routledge companion to philosophy of physics (pp. 7085). London: Routledge.CrossRefGoogle Scholar
Cajori, F. (1934). Sir Isaac Newton’s mathematical principles of natural philosophy and his system of the world (A. Motte, trans.). Berkeley: University of California Press.Google Scholar
Chen, L. & Fritz, T. (2021). An algebraic approach to physical fields. Studies in History and Philosophy of Science Part A, 89(C), 188201. https://doi.org/10.1016/j.shpsa.2021.08.011.Google Scholar
Cheng, B. & Read, J. (2021). Why not a sound postulate? Foundations of Physics, 51(3), 120. https://doi.org/10.1007/s10701-021-00479-0.CrossRefGoogle ScholarPubMed
Dasgupta, S. (2016). Symmetry as an epistemic notion. British Journal for the Philosophy of Science, 67(3), 837–78. https://doi.org/10.1093/bjps/axu049.Google Scholar
Debs, T. A. & Redhead, M. L. G. (1996). The twin ‘paradox’ and the conventionality of simultaneity. American Journal of Physics, 64, 384–92.CrossRefGoogle Scholar
Dewar, N. (2019). Sophistication about symmetries. British Journal for the Philosophy of Science, 70(2), 485521. https://doi.org/10.1093/bjps/axx021.CrossRefGoogle Scholar
Dewar, N. (2020). General-relativistic covariance. Foundations of Physics, 50(4), 294318. https://doi.org/10.1007/s10701-019-00256-0.CrossRefGoogle Scholar
Dewar, N., Linnemann, N. & Read, J. (2022). The epistemology of spacetime. Philosophy Compass, 17(4). https://doi.org/10.1111/phc3.12821.Google Scholar
Earman, J. (1989). World enough and spacetime. Cambridge, MA: MIT Press.Google Scholar
Earman, J. & Friedman, M. (1973). The meaning and status of Newton’s law of inertia and the nature of gravitational forces. Philosophy of Science, 40(3), 329–59. https://doi.org/10.1086/288536.CrossRefGoogle Scholar
Eddington, A. (1924). The mathematical theory of relativity. Cambridge: Cambridge University Press.Google Scholar
Eddington, A. (1966). Space, time and gravitation: An outline of the general theory of relativity. Cambridge: Cambridge University Press.Google Scholar
Einstein, A. (1905a, January). Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig? Annalen der Physik, 323(13), 639–41. https://doi.org/10.1002/andp.19053231314.Google Scholar
Einstein, A. (1905b, January). Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen. Annalen der Physik, 322(8), 549–60. https://doi.org/10.1002/andp.19053220806.CrossRefGoogle Scholar
Einstein, A. (1905c, January). Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt. Annalen der Physik, 322(6), 132–48. https://doi.org/10.1002/andp.19053220607.CrossRefGoogle Scholar
Einstein, A. (1905d, January). Zur Elektrodynamik bewegter Körper. Annalen der Physik, 322(10), 891921. https://doi.org/10.1002/andp.19053221004.Google Scholar
Einstein, A. (1907, January). Bemerkungen zu derNotiz von Hrn. Paul Ehrenfest: Die Translation deformierbarer Elektronen und der Flächensatz. Annalen der Physik, 328(6), 206–8. https://doi.org/10.1002/andp.19073280616.Google Scholar
Einstein, A. (1919). What is the theory of relativity? The Times. Friday, 28 November.Google Scholar
Einstein, A. (1921). Geometrie und erfahrung: Erweiterte fassung des festvortrages gehalten an der preussischen akademie der wissenschaften zu berlin am 27.januar 1921. Berlin: J. Springer.CrossRefGoogle Scholar
Einstein, A. (1935). Elementary derivation of the equivalence of mass and energy. Bulletin of the American Mathematical Society, 41(4), 223–30.CrossRefGoogle Scholar
Einstein, A. (1954). The fundamentals of theoretical physics. In Ideas and opinions (pp. 323–35). New York: Bonanza Books.Google Scholar
Einstein, A. (1969). Autobiographical notes. In Schilpp, P. A. (ed.), Albert Einstein: Philosopher-scientist (Vol. 1, pp. 194). Chicago, IL: Open Court.Google Scholar
Einstein, A. (1995). Letter to Arnold Sommerfield, January 14, 1908 Klein, M. J. (, Knox, A. J. & Schulmann, R., eds.). Princeton, NJ: Princeton University Press.Google Scholar
Fine, K. (2005). Tense and reality. In Modality and tense: Philosophical papers. Oxford: Oxford University Press.Google Scholar
FitzGerald, G. F. (1889). The ether and the Earth’s atmosphere. Science, 13, 390.Google Scholar
Friedman, M. (1974). Explanation and scientific understanding. Journal of Philosophy, 71(1), 519.CrossRefGoogle Scholar
Friedman, M. (1983). Foundations of space-time theories. Princeton, NJ: Princeton University Press.Google Scholar
Galilei, G. (1967). Dialogues concerning the two chief world systems (S. Drake, trans.). Berkeley: University of California Press.CrossRefGoogle Scholar
Giovanelli, M. (2014). ‘But one must not legalize the mentioned sin’: Phenomenological vs. dynamical treatments of rods and clocks in Einstein’s thought. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 48(1), 2044. https://doi.org/10.1016/j.shpsb.2014.08.012.Google Scholar
Giovanelli, M. (2021). Nothing but coincidences: The point-coincidence and Einstein’s struggle with the meaning of coordinates in physics. European Journal for Philosophy of Science, 11(2), 164. https://doi.org/10.1007/s13194-020-00332-7.Google Scholar
Griffiths, D. J. (2013). Introduction to electrodynamics (4th ed.). Boston, MA: Pearson.Google Scholar
Grünbaum, A. (2001). David Malament and the conventionality of simultaneity: A reply. Foundations of Physics, 40(9–10), 1285–97. https://doi.org/10.1007/s10701-009-9328-3.Google Scholar
Heras, J. A. (1994, October). Electromagnetism in Euclidean four space: A discussion between God and the Devil. American Journal of Physics, 62(10), 914–16. https://doi.org/10.1119/1.17681.Google Scholar
Hertz, H. (1894). Die prinzipien der mechanik. Leipzig: J. A. Barth.Google Scholar
Huggett, N. (2000). Reflections on parity nonconservation. Philosophy of Science, 67(2), 219–41. https://doi.org/10.1086/392773.Google Scholar
Huggett, N. (2006). The regularity account of relational spacetime. Mind, 115(457), 4173. https://doi.org/10.1093/mind/fzl041.Google Scholar
Huggett, N. (2009). Essay review: Physical relativity and understanding space-time. Philosophy of Science, 76(3), 404–22. https://doi.org/10.1086/649814.CrossRefGoogle Scholar
Huggett, N., Hoefer, C. & Read, J. (2022). Absolute and relational space and motion: Post-Newtonian theories. In Zalta, E. N. (ed.), The Stanford encyclopedia of philosophy (Spring 2022 ed.). Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/archives/spr2022/entries/spacetime-theories.Google Scholar
Jackson, J. D. (1998). Classical electrodynamics (3rd ed.). New York: Wiley.Google Scholar
Jammer, M. (2006). Concepts of simultaneity: From antiquity to Einstein and beyond. Baltimore, MD: Johns Hopkins University Press.Google Scholar
Janis, A. (2018). Conventionality of simultaneity. In Zalta, E. N. (ed.), The Stanford encyclopedia of philosophy (Fall 2018 ed.). Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/archives/fall2018/entries/spacetime-convensimul.Google Scholar
Janssen, M. (2009). Drawing the line between kinematics and dynamics in special relativity. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 40(1), 2652. https://doi.org/10.1016/j.shpsb.2008.06.004.Google Scholar
Kitcher, P. (1989). Explanatory unification and the causal structure of the world. In Kitcher, P. & Salmon, W. (eds.), Minnesota studies in the philosophy of science (Vol.13, pp. 410503). Minneapolis: University of Minnesota Press.Google Scholar
Knox, E. (2013). Effective spacetime geometry. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 44(3), 346–56. https://doi.org/10.1016/j.shpsb.2013.04.002.Google Scholar
Knox, E. (2014). Newtonian spacetime structure in light of the equivalence principle. British Journal for the Philosophy of Science, 65(4), 863–80. https://doi.org/10.1093/bjps/axt037.Google Scholar
Lange, M. (2007). Laws and meta-laws of nature: Conservation laws and symmetries. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 38(3), 457–81. https://doi.org/10.1016/j.shpsb.2006.08.003.Google Scholar
Larmor, J. (1900). Aether and matter. Cambridge: Cambridge University Press.Google Scholar
Lehmkuhl, D. (2014). Why Einstein did not believe that general relativity geometrizes gravity. Studies in History and Philosophy of Modern Physics, 46, 316–26.Google Scholar
Lehmkuhl, D. (2021). The equivalence principle(s). In Knox, E. & Wilson, A. (eds.), The Routledge companion to philosophy of physics (pp. 125–44). London: Routledge.Google Scholar
Linnemann, N. & Read, J. (2021). Constructive Axiomatics in Spacetime Physics Part I: Walkthrough to the Ehlers–Pirani–Schild Axiomatisation. (Unpublished manuscript.)Google Scholar
Linnemann, N. & Salimkhani, K. (2021). The Constructivist’s Programme and the Problem of Pregeometry. (Unpublished manuscript.)Google Scholar
Lipman, M. A. (2020). On the fragmentalist interpretation of special relativity. Philosophical Studies, 177(1), 2137. https://doi.org/10.1007/s11098-018-1178-4.Google Scholar
Lorentz, H. A. (1892). De relative beweging van de aarde en den aether. Koninklijke Akademie van Wetenschappen te Amsterdam, Wis-en Natuurkundige Afdeeling, Versalagen der Zittingen, 1, 74–9. (Reprinted in English translation, ‘The relative motion of the Earth and the ether’. In Zeeman, P. and Fokker, A. D. (eds.), Collected papers, pp. 219–23, The Hague: Nijhjoff, 1937.)Google Scholar
Lorentz, H. A. (1895). Versuch einer thoerie der electrischen und optischen erscheinungen in bewegten körpern. Leiden: Brill.Google Scholar
Luminet, J.-P. (2011). Time, topology, and the twin paradox. In Callender, C. (ed.), The Oxford handbook of philosophy of time (pp. 528–45). Oxford: Oxford University Press.Google Scholar
Malament, D. (1977). Causal theories of time and the conventionality of simultaneity. Noûs, 11(3), 293300. https://doi.org/10.2307/2214766.CrossRefGoogle Scholar
Malament, D. (2012). Topics in the foundations of general relativity and Newtonian gravitation theory. Chicago, IL: University of Chicago Press.CrossRefGoogle Scholar
Martens, N. C. M. & Read, J. (2020). Sophistry about symmetries? Synthese, 199(1–2), 315–44. https://doi.org/10.1007/s11229-020-02658-4.Google Scholar
Maudlin, T. (2012). Philosophy of physics: Space and time. Princeton, NJ: Princeton University Press.Google Scholar
Menon, T. (2019). Algebraic fields and the dynamical approach to physical geometry. Philosophy of Science, 86(5), 1273–83. https://doi.org/10.1086/705508.Google Scholar
Mercati, F. (2018). Shape dynamics: Relativity and relationalism. Oxford: Oxford University Press.Google Scholar
Michelson, A. A., & Morley, E. (1887). On the relative motion of the Earth and the luminiferous ether. American Journal of Science, 34(203), 333–45.Google Scholar
Miller, A. I. (1981). Albert Einstein’s special theory of relativity. Reading: Addison–Wesley.Google Scholar
Minkowski, H. (1909). Raum und zeit. Physikalische Zeitschrift, 10, 104–11.Google Scholar
Myrvold, W. C. (2019). How could relativity be anything other than physical? Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 67, 137–43. https://doi.org/10.1016/j.shpsb.2017.05.007.Google Scholar
Norton, J. D. (1992). Philosophy of space and time. In Introduction to the philosophy of science (pp. 179231). Englewood Cliffs, NJ: Prentice-Hall.Google Scholar
Norton, J. D. (1993). General covariance and the foundations of general relativity: Eight decades of dispute. Reports on Progress in Physics, 56(7), 791858.Google Scholar
Norton, J. D. (2008). Why constructive relativity fails. British Journal for the Philosophy of Science, 59(4), 821–34. https://doi.org/10.1093/bjps/axn046.Google Scholar
Norton, J. D. (2018). Einstein for Everyone. Pittsburg, PA: Nullarbor Press.Google Scholar
Norton, J. D. (2022). The hole argument. In Zalta, E. N. & Nodelman, U. (eds.), The Stanford encyclopedia of philosophy (Winter 2022 ed.). Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/archives/win2022/entries/spacetime-holearg.Google Scholar
Pais, A. (1982). Subtle Is the Lord: The Science and the Life of Albert Einstein. New York: Oxford University Press.Google Scholar
Pauli, W. (2000). Relativitätstheorie. Berlin: Springer. (Originally published by B. G. Teubner in 1921; new annotation by Domenico Giulini.)CrossRefGoogle Scholar
Pelissetto, A. & Testa, M. (2015). Getting the Lorentz transformations without requiring an invariant speed. American Journal of Physics, 83, 338–40.Google Scholar
Pitts, J. B. (2012). The nontriviality of trivial general covariance: How electrons restrict ‘time’ coordinates, spinors fit into tensor calculus, and of a tetrad is surplus structure. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 43(1), 124. https://doi.org/10.1016/j.shpsb.2011.11.001.Google Scholar
Pooley, O. (2013). Substantivalist and relationist approaches to spacetime. In Batterman, R. (ed.), The Oxford handbook of philosophy of physics (pp. 522–86). Oxford: Oxford University Press.Google Scholar
Pooley, O. (2021). The hole argument. In Knox, E. & Wilson, A. (eds.), The Routledge companion to philosophy of physics. London: Routledge.Google Scholar
Quine, W. V. O. (1951). Two dogmas of empiricism. Philosophical Review, 60(1), 2043.CrossRefGoogle Scholar
Quine, W. V. O. (1966). The ways of paradox. New York: Random House.Google Scholar
Read, J. (2020a). Explanation, geometry, and conspiracy in relativity theory. In Beisbart, T. S. C. & Wuthrich, C. (eds.), Thinking about space and time: 100 years of applying and interpreting general relativity (Vol.15, pp. 173206). Basel: Birkhäuser.Google Scholar
Read, J. (2020b). Geometrical constructivism and modal relationalism: Further aspects of the dynamical/geometrical debate. International Studies in the Philosophy of Science, 33(1), 2341. https://doi.org/10.1080/02698595.2020.1813530.Google Scholar
Read, J. (2022). Geometric objects and perspectivalism. In Read, J. & Teh, N. J. (eds.), The philosophy and physics of Noether’s theorems (pp. 257–73). Cambridge: Cambridge University Press.CrossRefGoogle Scholar
Read, J. (2023). Background independence in classical and quantum gravity. Oxford: Oxford University Press.Google Scholar
Read, J., Brown, H. R. & Lehmkuhl, D. (2018). Two miracles of general relativity. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 64, 1425. https://doi.org/10.1016/j.shpsb.2018.03.001.Google Scholar
Read, J. & Cheng, B. (2022). Euclidean spacetime functionalism. Synthese, 200(6), 122. https://doi.org/10.1007/s11229-022-03951-0.Google Scholar
Reichenbach, H. (1958). The philosophy of space and time. Berkeley: University of California Press.Google Scholar
Reichenbach, H. (1969). Axiomatization of the theory of relativity. Berkeley: University of California Press.Google Scholar
Rigden, J. S. (1987). Editorial: High thoughts about Newton’s first law. American Journal of Physics, 55(4), 297. https://doi.org/10.1119/1.15191.Google Scholar
Ryckman, T. (2017). Einstein. London: Routledge.CrossRefGoogle Scholar
Salmon, W. C. (1977). The philosophical significance of the one-way speed of light. Noûs, 11(3), 253–92.Google Scholar
Salmon, W. C. (1984). Scientific explanation and the causal structure of the world. Princeton, NJ: Princeton University Press.Google Scholar
Saunders, S. (2013). Rethinking Newton’s Principia. Philosophy of Science, 80, 2248.CrossRefGoogle Scholar
Stevens, S. (2020). Regularity relationalism and the constructivist project. British Journal for the Philosophy of Science, 71(1), 353–72.Google Scholar
Todd, S. L. & Menicucci, N. C. (2017). Sound clocks and sonic relativity. Foundations of Physics, 47, 1267–93.CrossRefGoogle Scholar
Todd, S. L., Pantaleoni, G., Baccetti, V. & Menicucci, N. C. (2021). Particle scattering in analogue-gravity models. Physical Review D, 104 (064035).CrossRefGoogle Scholar
Torretti, R. (1983). Relativity and geometry. New York: Pergamon.Google Scholar
Naval Observatory, US. (2022). Introduction to calendars. https://aa.usno.navy.mil/faq/calendars.Google Scholar
Van Camp, W. (2011). Principle theories, constructive theories, and explanations in modern physics. Studies in History and Philosophy of Modern Physics, 42, 2331.Google Scholar
von Ignatowsky, W. (1911). Das relativitätsprinzip. Archiv der Mathematik und Physik, 17, 124.Google Scholar
Wallace, D. (2019). Who’s afraid of coordinate systems? An essay on representation of spacetime structure. Studies in History and Philosophy of Modern Physics, 67, 125–36.Google Scholar
Wallace, D. (2020). Fundamental and emergent geometry in Newtonian physics. British Journal for the Philosophy of Science, 71(1), 132.Google Scholar
Weatherall, J. O. (2018). A brief comment on Maxwell(/Newton)[–Huygens] spacetime. Studies in History and Philosophy of Modern Physics, 63, 34–8.CrossRefGoogle Scholar
Weatherall, J. O. (2021). Two dogmas of dynamicism. Synthese, 199, 253–75.Google Scholar
Weeks, J. R. (2001). The twin paradox in a closed universe. American Mathematical Monthly, 108(7), 585–90.Google Scholar
Winnie, J. (1970). Special Relativity without One-Way Velocity Assumptions: Part I. Philosophy of Science, 37, 8199.Google Scholar

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Special Relativity
  • James Read, University of Oxford
  • Online ISBN: 9781009300599
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Special Relativity
  • James Read, University of Oxford
  • Online ISBN: 9781009300599
Available formats
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