In general relativity, a vanishing stress-energy tensor implies, through Einstein field equations, the vanishing of all the components of the Ricci tensor. Vacuum does not mean that the curvature of space-time is necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces and gravitational waves (technically, these phenomena are the components of the Weyl tensor). The black hole (with zero electric charge) is an elegant example of a region completely "filled" with vacuum, but still showing a strong curvature.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Tidal forces and gravitational waves can produce zero what?
Ah, so.. unanswerable

Some NPOs which are particularly well known, often for the charitable or social nature of their activities performed during a long period of time, include Amnesty International, Oxfam, Rotary International, Kiwanis International, Carnegie Corporation of New York, Nourishing USA, DEMIRA Deutsche Minenräumer (German Mine Clearers), FIDH International Federation for Human Rights, Goodwill Industries, United Way, ACORN (now defunct), Habitat for Humanity, Teach For America, the Red Cross and Red Crescent organizations, UNESCO, IEEE, INCOSE, World Wide Fund for Nature, Heifer International, Translators Without Borders and SOS Children's Villages.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What is a well known NPO that helps people from low incomes become homeowners?
Ah, so.. Habitat for Humanity

From 2004 to 2007, the top five U.S. investment banks each significantly increased their financial leverage (see diagram), which increased their vulnerability to a financial shock. Changes in capital requirements, intended to keep U.S. banks competitive with their European counterparts, allowed lower risk weightings for AAA securities. The shift from first-loss tranches to AAA tranches was seen by regulators as a risk reduction that compensated the higher leverage. These five institutions reported over $4.1 trillion in debt for fiscal year 2007, about 30% of USA nominal GDP for 2007. Lehman Brothers went bankrupt and was liquidated, Bear Stearns and Merrill Lynch were sold at fire-sale prices, and Goldman Sachs and Morgan Stanley became commercial banks, subjecting themselves to more stringent regulation. With the exception of Lehman, these companies required or received government support. Lehman reported that it had been in talks with Bank of America and Barclays for the company's possible sale. However, both Barclays and Bank of America ultimately declined to purchase the entire company.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): How much debt did the top five U.S. investment banks report in fiscal year 2007?
Ah, so..
over $4.1 trillion