General relativity is only one of the pillars of modern physics. The other is quantum mechanics, which describes what happens at the atomic and subatomic scale. Its modern incarnation, quantum field theory, has been spectacularly successful at describing and predicting the behaviour of fundamental particles and forces.
The main challenge now is to combine the two ideas into one overarching theory, to be known as quantum gravity. Such a theory would be crucial for explaining the first moments of the big bang, when the universe was dense, hot and small, or what happens near the singularity at the cores of black holes, where the effects of quantum physics may compete with those of general relativity.
Although there is as yet no final theory of quantum gravity, there are several candidate theories being actively explored. One is string theory, which describes the fundamental constituents of matter not as point-like particles but as microscopic vibrating strings. Depending on how they vibrate, the strings will be perceived as different particles 鈥� including the graviton, the particle thought to carry the gravitational force.
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Another possibility is that space-time is not smooth but built up of discrete building blocks that interact with each other. As a result, if we were able to peer at its fine structure, it might look like a frothy space-time foam. In such theories, what we perceive as the space-time that bends and warps smoothly in the presence of matter is merely an emergent phenomenon masking more radical behaviour on small scales.
The quest for the theory of quantum gravity is arguably the biggest challenge facing modern physics. One of the difficulties is that it only really manifests itself at extremely high energies, well beyond our experimental reach. Physicists now face the task of devising experiments and astronomical observations that can test candidate theories of quantum gravity in the real world.
Read more: Instant Expert: General relativity