#### Quantum gravity has been formulated to tackle one of the greatest problems in all of physics: the need to unite the two great theories of the 20th century—general relativity and quantum mechanics.

The former presents a framework for understanding the world in terms of space and time, and it covers behavior over large distances. General relativity introduces the notion that time is relative and that gravity itself exists because of a curved space-time. As Einstein first realized, a ball does not fall to the Earth because it is attracted to its mass, as Newton told us; it falls because of the existence of a space-time field that permeates the Universe and curves around large objects.

Quantum mechanics is a mysterious yet incredibly accurate theory that describes the world of the very small. It tells us that both particles and fields exist in discrete units that, because of uncertainty, can only be described probabilistically. The theory also describes entanglement, the bewildering phenomenon in which physical systems can be so intertwined with one another that they lose their independent, individual reality and start obeying rules that apply to a collective.

As far as we can tell, these two theories are both right—and in conflict. Their simultaneous existence generates a paradox, meaning physics is, in a sense, in disarray. While quantum mechanics deals with reality in discrete, granular fashion, relativity tells us that space-time, and therefore gravity, is continuous and non-discrete.

One way to deal with this is to give one of the theories precedence. Since we know the world is quantum, general relativity must be an approximation of an underlying quantum description of space-time itself. And this suggests that any unification of the theories requires that gravity become discrete.

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