Editing Curved spacetime (section)

=== Definitions: Active, passive, and inertial mass ===
Bondi distinguishes between different possible types of mass: (1) {{nowrap|1=active mass (<math>m_a</math>)}} is the mass which acts as the '''''source''''' of a gravitational field; (2){{nowrap|1=passive mass (<math>m_p</math>)}} is the mass which '''''reacts to''''' a gravitational field; (3) {{nowrap|1=inertial mass (<math>m_i</math>)}} is the mass which reacts to acceleration.<ref name=Bondi />
* <math>m_p</math> is the same as {{nowrap|1=gravitational mass (<math>m_g</math>)}} in the [[#Basic propositions|discussion of the equivalence principle]].

In Newtonian theory,
* The third law of action and reaction dictates that <math>m_a</math> and <math>m_p</math> must be the same.
* On the other hand, whether <math>m_p</math> and <math>m_i</math> are equal is an empirical result.

In general relativity,
* The equality of <math>m_p</math> and <math>m_i</math> is dictated by the equivalence principle.
* There is no "action and reaction" principle dictating any necessary relationship between <math>m_a</math> and <math>m_p</math>.<ref name="Bondi">{{cite book |last1=Bondi |first1=Hermann |url=http://www.edition-open-sources.org/sources/5/24/index.html |title=The Role of Gravitation in Physics: Report from the 1957 Chapel Hill Conference |date=1957 |publisher=Max Planck Research Library |isbn=978-3-86931-963-6 |editor1-last=DeWitt |editor1-first=Cecile M. |location=Berlin, Germany |pages=159–162 |access-date=1 July 2017 |editor2-last=Rickles |editor2-first=Dean |archive-url=https://web.archive.org/web/20170728121700/http://www.edition-open-sources.org/sources/5/24/index.html |archive-date=28 July 2017 |url-status=live}}</ref>