Version Date: 17 May 1998

Usenet Relativity FAQ

Albert Einstein E=mc^2 Spiral Galaxy M100

This is the web version of the Usenet Relativity FAQ (Frequently Asked Questions). Its purpose is to provide good answers to questions which have been discussed often in sci.physics.relativity and related usenet newsgroups. The articles in this FAQ are based on those discussions and on information from good reference sources. That does not mean that they are always perfect and complete. If you have corrections, updates or additional points to make please send E-mail to me, the editor, Philip Gibbs at You can also raise the subject in the newsgroups if you really think you have something new to say. If you want to write up an article following a discussion about a subject not yet covered here then feel free to send it to me, but please do not use this address to ask new questions. Ask them in the appropriate newsgroup instead.

If you are new to this newsgroup, please read "Welcome! to sci.physics.relativity" and "An Introduction to the Physics Newsgroups". They will help you find the right newsgroup for your questions and will give you other crucial advice about appropriate netiquette for these groups. Above all you should check carefully to see if your questions are answered in this FAQ before posting.

This FAQ was originally part of the Physics FAQ which was created by Scott I. Chase in 1992. The web version was created, maintained and enlarged by Michael Weiss. The present editor is Philip Gibbs. Others who have written for the FAQ are credited at the top of the items they submitted while many more who have made smaller contributions have been thanked privately.

This FAQ is currently available at these web sites:


Thanks go to John Baez, Alan Cairns, Dave Edsall, Chung-rui Kao, Matthew Parry, Jon Butterworth, Harald Falkenberg, Johan Wevers and the various organisations for hosting us! If any other non-commercial sites would like to mirror this FAQ in countries not listed above, please contact

The full Physics and Relativity FAQ in HTML form can be downloaded as a single large archive file by FTP from You can install it on your local disk and view it from there with a suitable web browser.

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Index of Subjects

Administrivia and Reference

Speed of Light

Special Relativity

General Relativity and Cosmology

Black Holes

If you do not find your question answered here you may find it in one of these related FAQs.