Why is a gravitational field "negative energy"? The idea about the universe having zero net energy, as explained for instance in http://en.wikipedia.org/wiki/Zero-energy_universe, seems to be that gravity has 'negative energy', which offsets the positive energy of all the matter and radiation. This may be the idea behind Lawrence Krauss's 'A Universe from Nothing', although I haven't read his book so I don't know. Can anybody explain what it means to say that a gravitational field has negative energy, and why that is true? It seems very counterintuitive to me, but that may be just my being dense. Thank you.