user: Write an article on Quantum Gravity

assistant: Quantum Gravity: A Theory of Everything

Quantum gravity is a field of theoretical physics that aims to unify quantum mechanics and general relativity into a single theory of everything. This theory would describe the behavior of matter and energy at the smallest scales and in the presence of strong gravitational fields.

Quantum mechanics and general relativity are two of the most successful theories in physics, but they are incompatible with each other. Quantum mechanics describes the behavior of particles at the subatomic scale, while general relativity describes the behavior of large objects and the behavior of gravity.

In quantum mechanics, particles can exist in multiple states at once and can exhibit quantum entanglement, where the state of one particle can instantaneously affect the state of another particle, regardless of the distance between them. In contrast, general relativity describes gravity as a curvature of spacetime caused by the presence of matter or energy.

Quantum gravity attempts to resolve the incompatibility between these two theories by combining the principles of quantum mechanics and general relativity. It would provide a framework for understanding how gravity behaves at the smallest scales and how it interacts with quantum particles.

One of the main challenges of developing a theory of quantum gravity is that it must be compatible with both the principles of quantum mechanics and general relativity. This has proven to be a difficult task, as the two theories make very different predictions about the behavior of matter and energy.

One of the most promising approaches to quantum gravity is string theory, which suggests that the fundamental building blocks of the universe are not particles but tiny, one-dimensional strings. These strings can vibrate and produce the various particles and forces that make up the universe.

Another approach to quantum gravity is loop quantum gravity, which attempts to quantize the geometry of spacetime itself. In this theory, spacetime is described as a network of interconnected loops, and the quantization of these loops gives rise to a discrete, granular structure of spacetime.

Despite the progress that has been made in the field of quantum gravity, much work remains to be done. The theory of quantum gravity is still in its early stages of development and much of it remains speculative. However, the potential implications of a successful theory of quantum gravity are enormous, as it would provide a complete and consistent description of the behavior of matter and energy at the smallest scales and in the presence of strong gravitational fields.

In conclusion, quantum gravity is a field of theoretical physics that aims to unify quantum mechanics and general relativity into a single theory of everything. The development of a successful theory of quantum gravity would have far-reaching implications for our understanding of the universe, and it is an area of active research and development.

user: I find string theory to be very interesting. 
I read a few years ago that at the time, there was no known experimental data that supports it's validity.  Is this still the case?

assistant: One problem with many current approaches to quantum gravity, including string theory is, that these models are extremely flexible, involving lots of empirically unknown, freely tunable parameters.
As a result, most tests in this domain merely rule out parts of the parameter space, rather than properly ruling out any one of these theories as a whole.
If such a test finds new results that rule out the currently most popular variants of a given theory, usually all that means is, that people tweak the parameters and find a new variation that works after all.
Critics of many of these theories have compared this to Epicycle theory which is an outdated model of planetary motion wherein, at first, all planetary orbits were assumed to be circular, with the Earth as stationary in the center, and in order to correct theoretical orbits to better fit observations, further circles were simply introduced - planets would then orbit on circles which themselves orbit on circles.
This theory was later rejected by a much simpler one that puts the sun at the center of the solar system, and allows for more flexible conic-section-based orbits, able to explain the same observations in much simpler equations.
There was a hope, that the Large Hadron Collider (LHC) in CERN would result in new kinds of particles besides the Higgs boson. Speculative particles predicted by Super Symmetry (SUSY) theories where every regular particle has a super symmetric partner with different kinds of properties. This has not yet happened, narrowing out the simplest versions of these theories. To further test these theories, more powerful colliders might be needed, to probe ever higher energies.