At the very beginning of the universe, quantum mechanics and gravity were inseparable — the same force, the same description. Our two greatest theories of physical reality, general relativity and quantum field theory, both work with stunning precision within their domains but are fundamentally incompatible with each other. A true theory of quantum gravity would not merely advance physics — it would complete our description of reality itself.
Where the Very Small Meets the Unimaginably Large
Quantum mechanics is responsible for making stars shine. Inside the Sun's core, protons approach each other with enough energy to overcome their mutual repulsion — but classical physics says even the Sun's core temperature is not quite hot enough for fusion. The resolution is quantum tunnelling — the ability of a quantum particle to tunnel through an energy barrier it classically cannot surmount. Without quantum mechanics, the Sun would not shine, and neither would any other star.
Hawking's most famous theoretical prediction combined quantum mechanics and general relativity in a startling way. He showed in 1974 that black holes should slowly radiate energy due to quantum effects near the event horizon. This 'Hawking radiation' means that black holes have a temperature and will gradually evaporate over enormous timescales. The implication, still hotly debated, is that when a black hole evaporates completely, all information about the matter that fell in might be lost forever.
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