"Combine general relativity and quantum theory into a single theory that can claim to be the complete theory of nature. This is called the problem of quantum gravity."

“String theory seems to be incompatible with a world in which a cosmological constant has a positive sign, which is what the observations indicate.”

“We are the result of processes much more complicated than the small aspects of our lives and societies over which we have some control.”

“There is no meaning to space that is independent of the relationships among real things of the world. ...Space is nothing apart from the things that exist. ...If we take out all the words we are not left with an empty sentence, we are left with nothing.”

“Time is described only in terms of change in the network of relationships that describes space.”

“In quantum theory, distance is inverse to energy, because you need particles of very high energy to probe very short distances. The inverse of the Planck energy is the Planck length.”

“To be human is to be suspended between danger and opportunity. ...The challenge of life is to choose wisely, from the enormous number of possible dangers, what's worth worrying about. It is also about choosing, from all the opportunities... always in the face of incomplete knowledge of the consequences.”