When physicist Stephen Hawking was 21, he received a death warrant. His doctors told him he would waste away with ALS. He would eventually lose complete control of his body and die within a few short years.

Previously, Hawking was a bored, indifferent student. But this fatal diagnosis suddenly gave his life new meaning and direction. In his few remaining years, he was galvanized to tackle the biggest problem in all of physics: to create a theory of everything. The Oscar-winning Hollywood movie, *The Theory of Everything*, chronicles his historic search for this great theory.

But the question remains: How successful was Hawking in completing this fabled theory? Unfortunately, it is still unfinished.

It was Albert Einstein’s dream to create a single theory that would explain all the forces of nature, which would allow us to “read the mind of God.” To do this, physicists have created two great theories: the theory of the very big (Einstein’s theory of general relativity), which describes black holes and the Big Bang, and the theory of the very small (quantum mechanics), which describes atomic physics.

The goal of physics is to find a single theory that unifies these two theories into a single coherent framework. The problem is that these two theories are incompatible. They are based on different mathematics, different assumptions, and different physical pictures.

Hawking made the first great stride toward finding this theory by applying the quantum theory to black holes. Normally, physicists believe that black holes are so massive that even light itself cannot escape them, and hence they are black. But Hawking overturned this cherished belief by showing that, when quantum mechanics is applied to black holes, they actually emit a faint radiation, which is now called Hawking radiation. Eventually, the black holes lose so much energy that they decay and even explode. So the first attempt to apply quantum mechanics to relativity theory led to a sensational discovery in physics.

But this calculation was only partially successful. It ignored the contributions from gravitons, or tiny pockets of gravity.

According to quantum mechanics, all forces are described by tiny particles, called quanta. The quantum of light, for example, is the photon. The quantum theory, which applied to photons, has been one of the greatest breakthroughs in all of physics, making possible the laser, transistor, the Internet, and all the marvels of modern high technology we see in our living rooms.

Similarly, when we apply quantum mechanics to gravity, we postulate a particle of gravity, called the graviton. But here is the problem: When we calculate the interactions of these gravitons, we find that the calculations blow up. We find that the naive application of the quantum theory to gravity yields useless mathematical nonsense. Usually, the additional terms introduced by the quantum theory are very tiny, but when applied to gravity, they become infinite, which is unacceptable.

Some of the greatest minds in physics have tried to tackle this problem, but have failed.

So far, the leading theory to describe a quantum theory of gravity is called string theory, which says that all particles of nature, including gravitons and sub-atomic particles, are nothing but different vibrations or musical notes on a microscopic string. So the universe is a symphony of vibrating strings. But even the largest machine of science, called the Large Hadron Collider, a gigantic tube 17 miles long outside Geneva, is not capable of a direct test of string theory. The hope, however, is that the LHC might be able to discover a new set of particles (called dark matter) which may shed light on quantum gravity.

So at present, there is no universal consensus in the scientific community about which direction to take. Hawking himself became pessimistic toward the latter part of his life that the final theory would be discovered soon.

But perhaps another bored, indifferent 21 year old may find focus and direction in his or her life to tackle and complete this fabulous quest pioneered by Einstein and Hawking.

*Dr. Michio Kaku is a professor of physics at the City University of New York, and author of The Future of Humanity: Terraforming Mars, Interstellar Travel, Immortality, and our Destiny Beyond Earth (Doubleday).*