Book contents
- Frontmatter
- Contents
- Preface and acknowledgements
- Part I Einstein's revolution
- 1 From Aristotle to Hiroshima
- 2 Einstein in a nutshell
- 3 The twin paradox
- 4 How to build an atomic bomb
- 5 The four-dimensional universe
- 6 Time travel is possible
- 7 Can the mind understand the world?
- Part II Philosophical progress
- Part III Frontiers
- Appendix A Spacetime diagrams
- Appendix B Symmetry and Lorentz's minority interpretation
- Appendix C Simple formulas for special relativity
- Appendix D Websites
- Appendix E Guide to further reading
- Index
3 - The twin paradox
from Part I - Einstein's revolution
- Frontmatter
- Contents
- Preface and acknowledgements
- Part I Einstein's revolution
- 1 From Aristotle to Hiroshima
- 2 Einstein in a nutshell
- 3 The twin paradox
- 4 How to build an atomic bomb
- 5 The four-dimensional universe
- 6 Time travel is possible
- 7 Can the mind understand the world?
- Part II Philosophical progress
- Part III Frontiers
- Appendix A Spacetime diagrams
- Appendix B Symmetry and Lorentz's minority interpretation
- Appendix C Simple formulas for special relativity
- Appendix D Websites
- Appendix E Guide to further reading
- Index
Summary
Symmetry
Among physicists, the word “symmetry” means “sameness across difference”. The prefixes “sym” and “syn” mean “same”, so “symphony” means “many musicians making the same sound” and “synchrony” means “same time”. “Metry” comes from the Greek word for “measure” (as in “metric”) and here means “size” or “shape”. Thus a face has a symmetry when it has the same shape on different sides, but the charm of a human face often lies in its slight asymmetries.
One of the most outrageous aspects of Einstein's theories is their unexpected symmetries. Suppose that two identical spaceships, A and B, are approaching each other and will pass each other in empty space, and each is moving inertially at a steady speed along a straight line. Spaceship A will find that that spaceship B's lengths are contracted and hours are dilated. Everything in stubby spaceship B happens in slow motion. But, Einstein said, spaceship B is also moving inertially and it can also make measurements. According to its rulers and clocks, spaceship A is contracted and slowed. There is a perfect democracy among sets of rulers and clocks. That is, according to Einstein, spaceship A is shorter than spaceship B and spaceship B is shorter than spaceship A. Hours on spaceship A are longer than those on spaceship B and hours on spaceship B are longer than those on spaceship A. Time dilation and length contraction are symmetric.
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- Information
- Space, Time and EinsteinAn Introduction, pp. 31 - 39Publisher: Acumen PublishingPrint publication year: 2002