# Conclusion

The combination of relativity and quantum theory has led to some of the greatest triumphs of theoretical physics, but also to its most persistent unsolved questions.

The marriage of special relativity and quantum concepts leads to so-called relativistic quantum field theories – the basis of modern particle physics – and to a variety of predictions that have passed rigorous experimental tests.

On the other hand, the question of how a union of general relativity and quantum theory should look like – a theory of quantum gravity – has not yet found a complete and convincing answer. And as long as this question remains unanswered, so do a number of the most fundamental questions physics can ask: What happens inside a black hole? What happened at the very beginning of the universe as we know it?

Physicists have found promising candidates for a theory of quantum gravity, from loop quantum gravity with its discrete space-time structure to string theory with its promise to describe not only gravity, but all forces and matter in the same unified quantum way. However, so far none of these candidates give a complete answer to the question of quantum gravity.

This ends our brief tour of the relativistic world.

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