Even though the guts of General Relativity are obtusely mathematical, and for decades was relegated to math departments rather than proper physics, you get to experience the technological gift of relativity every time you navigate to your favorite restaurant. GPS, the global positioning system, consists of a network of orbiting satellites constantly beaming out precise timing data. Your phone compares those signals to figure out where you are on the Earth. But there is a difference in spacetime between the surface of the Earth and the orbit of the satellites. Without taking general relativity into account, your navigation would simply be incorrect, and you’d be late for dinner.
As revolutions go, general relativity is a big one. And as unifications go, it’s a warning. To make this union happen Einstein had to radically, permanently alter not just our conceptions of gravity as a force acting through space and time, but our conceptions of space and time itself. It took no less than a complete overhaul of our entire philosophical understanding of the relation between space and time to bridge the gap.
And as Einstein would find in the ensuing decades, all the way to the time of his death, that bringing other forces like electromagnetism into the same unified fold would be all but impossible. Electromagnetism, and the other forces, cannot be conceptualized in the same way. Instead we have to use quantum probabilities to make predictions, and when we apply the same technique to gravity we just get infinities.
This is Einstein’s waking nightmare. This is the demon haunting the equations. This is the wedge driven between the force of gravity and electromagnetism. This is the intractable thorn that prevents us from reaching the nirvana of unification. And just as in unifications past, from Newton and Maxwell and Einstein, to solve this, to crack this infernal puzzle, to move around or through or between the infinities, will require a complete revolution in our understanding of the very fabric of reality.
Thankfully, we will not be alone in our quest. The dream of unifying the great powers of physics – the quantum and the gravitational – did not end with Einstein’s death, and since his passing hundreds of physicists have taken up the mantle, contributed to the cause in some way minor or significant, and passed on their learning to the next generation.
That long-dreamt search is not yet over, but over the course of the past near-century a contender has risen to the top, a collection of remarkable ideas that one day hopes to solidify into a verified, complete theory of the physical universe: string theory. And string theory has created a surprising idea: that our universe is not as it seems. Dubbed the holographic principle, it states that our universe is not the familiar three dimensions of our existence, but really just an impossibly thin two-dimensional membrane. In this framework, all the information contents of the universe can be casted onto its surface, without losing a single bit, and once in place gravity itself disappears, replaced with much more familiar quantum forces.
This is holography in action. The mapping from the universe to its boundary preserves all the necessary information. The primary goal of holography has been achieved. That itself is an astonishing and unexpected delight, that nature should allow such a mapping to take place in the first place. But then it goes one step further. The best way to solve quantum gravity, it seems, is to apply holography so that gravity itself disappears. No more tangled mess of mathematics, no more piling of infinities, no more convulsions of spacetime. Just a normal quantum theory that we have become quite adept at solving with normal quantum tools.
To date this holographic idea is mere conjecture. But it does hold an enormous amount of promise.