Quantum gravity (QG) is a theoretical physics field aiming to describe gravity according to the principles of quantum mechanics, unifying it with the other three fundamental forces. This endeavor is crucial because Albert Einstein's highly successful general theory of relativity, which explains gravity, breaks down in extreme environments such as within black holes or the moments after the Big Bang, where both quantum and gravitational effects are significant. Scientists are exploring various approaches, most notably M-theory (part of string theory) and Loop Quantum Gravity, some seeking a "theory of everything" while others focus solely on quantizing gravity.
A major hurdle is the lack of experimental data, as quantum gravitational effects are predicted to manifest at the incredibly tiny Planck scale (around 10⁻³⁵ meters), far beyond current particle accelerator capabilities. Consequently, physicists rely on thought experiments to explore potential resolutions to open questions, such as how elementary particle spin sources gravity. Nevertheless, new experiment designs in "phenomenological quantum gravity" suggest indirect testing may become feasible within the next few decades, offering hope for unraveling this profound mystery.