# Two Competing Theories Could Solve Physics' Greatest Paradox

## Метаданные

- **Канал:** Arvin Ash
- **YouTube:** https://www.youtube.com/watch?v=VD4oHv5kO2Q
- **Источник:** https://ekstraktznaniy.ru/video/25623

## Транскрипт

### Segment 1 (00:00 - 05:00) []

At large scales, gravity is described by Einstein's general relativity. At small scales, the universe is described by quantum mechanics. But when we try to apply both at once, the math breaks and even spacetime itself becomes uncertain. In this video, we'll build the problem of quantum gravity step by step, then explore the two leading approaches physicists are using to solve it. That's coming up right now. On a dark night away from city lights, you can get a glimpse of the vast universe around us. It's amazing to think that every star, planet, or structure that you can see is controlled by the same force, the force of gravity. And it is this force that binds stars to galaxies, planets to stars, and us to the Earth. We've been aware of this force for a long time. And thanks to Isaac Newton over 350 years ago, we got the first mathematical model for describing it. His equation contained a constant now called Newton's gravitational constant G subn. His theory allowed us to model the solar system fairly accurately, but over time we found that it was incomplete because it gave us some wrong results. Classical gravity works extraordinarily well until we push it into the regimes where quantum effects can no longer be ignored. In 1915, Einstein gave us its successor, general relativity, which fixed the issues with Newton's universal law of gravitation. It allowed us to describe new exotic phenomena such as black holes, gravitational lensing, and even build a model for the entire universe called the lambda CDM models. sometimes colloally referred to as the big bang theory. However, we find today the general relativity is also incomplete. Since Einstein's great insight, we have come to find that the universe at its core seems to be quantum, not classical. But general relativity is classical. So we find that if we try to use Einstein's theory to make a meaningful quantum model of gravity gives us nonsense results. The issue isn't just technical. It's conceptual. Gravity behaves unlike any other force we've ever quantized. General relativity unique is that it is a theory of spacetime itself, not stuff happening in spaceime, which is the case with the other three fundamental forces. Gravity is what results because of the geometry and curvature of spacetime. The other forces describe events happening within this background geometry, not the background itself like gravity does. Remember that Einstein's crucial insight was that spacetime is not a static background where physics takes place. It's dynamic and can change with energy and mass. Since gravity is a feature of spacetime, if we believe a quantum theory of gravity exists, it is probably going to have to be a quantum theory of spacetime. This is where the clash becomes unavoidable. Quantum mechanics needs a fixed stage, but general relativity says the stage itself evolves. We found that quantum mechanics, which is at least as proven, ruins them. For example, if you consider the simplest atom, the hydrogen atom, consisting of one proton and one electron, quantum theory says that the electron is in a superp position. This means that it is in multiple locations at various distances from the nucleus at the same time. We only know the probability of finding it at a particular radius if we were to measure it. This is a problem because since the electron has mass, according to general relativity, it must curve spaceime. You'll notice that one force is conspicuously missing from this picture and that is gravity. This is because according to our best model for gravity, Einstein's general relativity, gravity is not a force but is a result of the curvature of the background, the curvature of spacetime. But the equations of general relativity treat spacetime as a bending of the continuous background of spaceime, not as a result of discrete particles that confer a force. To fix this, physicists take two radically different paths. One changes particles, the other changes spacetime. The interesting thing is that if you treat the universe as strings vibrating in nine dimensions, a particular closed string vibration results in a unique gravitton particle. This appears naturally in the mathematics of string theory. If the graviton exists as predicted by string theory, then gravity could be treated as a force where virtual gravitons act as the mediating particle which transmits the force of gravity between two objects. The mathematics of this force

### Segment 2 (05:00 - 08:00) [5:00]

could be then put into the quantum field theory framework and be treated similarly to the mathematics of for example the electromagnetic force where virtual photons act as the mediating particles between two electrically charged objects. Unlike electromagnetism though where only electrically charged particles are affected by the force of electromagnetism. In the case of the graviton, anything with mass or energy would be affected by the force of gravity. Loop quantum gravity takes the opposite approach. Instead of quantizing forces, it quantizes geometry itself. LQG says that there are about 10^ the 99 quant of volume in every cubic centimeter of space. This quantum of volume is so tiny that there are more such quanta in a cubic cm than there are cubic centimeters in the entire visible universe. And time itself has a minimum quantity as well which is about 10^ the -43 seconds or close to plank time. And what do these spac-time quanta look like? Well, spacetime is basically made up of finite loops with nodes connecting them. The nodes that intersect is where the quant volume of space reside. It has a volume that is a multiple of the plank volume 10 to the 999 cm. The loops in between these nodes represent two-dimensional areas and large quantities of these loops and nodes are called spin networks because their properties are related to a particle quality called spin. Space is defined by the geometry of this spin network and time is defined by the moves that rearrange this spin network. The spin network when combined with these quantum movements of time is called a spin foam. Time does not flow like a river and the spin foam it ticks like a digital clock. And each quantum tick or movement of the spin network is about 10^ the - 43 seconds. Every location in the spin network where a quantum move takes place, time has ticked once. How do particles traverse this quantized space when mass and energy are added to the spin foam? The shape of the volumes of the spin network is distorted. This distorts space and time because any movement of these quanta also affects the time quanta. Time is essentially movement of these volume quanta. And this distortion of space and time is what we perceive as gravity. The exciting thing about LKG is that it makes some predictions that could be tested. For example, it predicts that the speed of light has a small dependence on energy. Photons of high energy travel slightly slower than lower energy photons. The effect is very small, but it amplifies over time. So two photons produced by a gammaray burst for example billions of years ago if they are of different frequency should arrive on earth at slightly different times and this time delay should be large enough to be detected by our instruments but sadly this has not been observed. It is possible that either the theory is wrong or our instruments are not sensitive enough. Quantum gravity forces us to rethink what spacetime itself is. String theory suggests gravity emerges from quantum strings. Loop quantum gravity suggests space itself is made of a discrete structure and may replace the big bang singularity with a bounce. But whichever idea is correct, nature will decide. And the only way to find out is through observation and experimentation. I'll see you in the next video, my friend. —