How Physics Connects Our Universe: From Classical Mechanics to Quantum Gravity and the Double Copy

In this engaging lecture, Professor Chris White introduces the foundational ideas of physics, emphasizing the distinction between matter and forces and the progression from Newtonian mechanics to the modern frameworks of special relativity and quantum mechanics. He details how Newtonian mechanics, while applicable to everyday life and planetary motion, fails when describing phenomena at very high speeds or at atomic scales, necessitating more advanced theories.

The talk covers electromagnetism as the first comprehensively understood force, explaining electric and magnetic fields and their wave-like solutions—electromagnetic waves—that describe light. This insight led to the development of Maxwell's equations and paved the way to recognizing the speed of light's constancy, a cornerstone of special relativity, which altered our classical understanding of space, time, and motion.

Quantum mechanics is introduced next as a revolutionary but deeply counterintuitive theory explaining atomic and subatomic phenomena. Its probabilistic nature challenges classical determinism and underlies much of modern chemistry, biology, and technology, despite ongoing debates about its interpretation. Professor White presents quantum field theory as the unifying framework that blends quantum mechanics with special relativity, describing all matter and forces via fields and their quanta, such as photons for electromagnetism.

The lecture then shifts focus to gravity, the weakest of the four fundamental forces, best explained by Einstein's general relativity as the curvature of spacetime caused by mass. This theory elegantly describes macroscopic phenomena including black holes, the Big Bang, and gravitational waves—the ripples in spacetime recently detected, opening a new window for astronomical observations beyond electromagnetic signals.

Addressing the challenge of reconciling general relativity with quantum mechanics, Professor White introduces the concept of quantum gravity and the difficulties in its calculation. Here, he highlights the Double Copy correspondence discovered around 2010, which reveals a remarkable relationship between the complex mathematics of gluon interactions in quantum field theory and gravity, allowing previously intractable gravitational calculations to be simplified by leveraging analogous gluon results.

Finally, Professor White describes the broader landscape of quantum field theories connected by a web of correspondences reminiscent of the Double Copy, suggesting an underlying mathematical structure yet to be fully understood. This emerging framework holds promise for breakthroughs in fundamental physics and illustrates the ongoing quest to unify the fundamental forces, deepen our grasp of the universe’s origins, and enhance practical applications like gravitational wave astronomy.