Hawking: The Grand Design

"If the total energy of the universe must always remain zero, and it costs energy to create a body, how can a whole universe be created from nothing?  That is why there must be a law like gravity.  Because gravity is attractive, gravitational energy is negative: One has to do work to separate a gravitationally bound system, such as the earth and the moon.  This negative energy can balance the positive energy needed to create matter, but its not quite that simple.  The negative gravitational energy of the earth, for example, is less than a billionth of the positive energy of the matter particles the earth is made of.  A body such as a star will have more negative gravitational energy, and the smaller it is (the closer the different parts of it are to each other), the greater this negative gravitational energy will be.  But before it can become greater than the positive energy of the matter, the star will collapse to a black hole, and black holes have positive energy.  That's why empty space is stable.  Bodies such as stars or black holes cannot just appear out of nothing.  But a hole universe can.

Because gravity shapes space and time, it allows space-time to be locally stable but globally unstable.  On the scale of the entire universe, the positive energy of the matter can be balanced by the negative gravitational energy, and so there is no restriction on the creation of whole universes.  Because there is a law like gravity, the universe can and will create itself from nothing in the manner described in Chapter 6.  Spontaneous creation is the reason there is something rather than nothing, why the universe exists, why we exist.  It is not necessary to invoke God to light the blue touch paper and set the universe going.

The Grand Design.  Bantam.  2012. 180.

1.  Has gravity always existed?
2.  What is gravity?  It it a thing?  A relation of things?
3.  What is matter?  What is energy?
4.  What is nothing?  What is non-being?
5.  What is causation?  What does it mean to have a cause?