Theory of everything differ both from gravity and from electromagnetism. A further hurdle was the acceptance that in a TOE, quantum mechanics had to be incorporated from the start, rather than emerging as a consequence of a deterministic unified theory, as Einstein had hoped. Gravity and electromagnetism could always peacefully coexist as entries in a list of classical forces, but for many years it seemed that gravity could not even be incorporated into the quantum framework, let alone unified with the other fundamental forces. For this reason, work on unification, for much of the twentieth century, focused on understanding the three "quantum" forces:
Unified growth theory take-off from an epoch of stagnation to the modern era of sustained growth. The testable predictions of this evolutionary theory and its underlying mechanisms have been confirmed empirically and quantitatively. Unified growth theory contributes to Macrohistory. It sheds light on the divergence in income per capita across the globe during the past two centuries. It identifies the factors that have governed the transition from stagnation to growth and have thus contributed to the observed worldwide differences in economic development. It highlights the persistent effects that variations in historical and prehistorical conditions have had on the composition of human capital and