２７ Loop Quantum Gravity(LQG)【What is the Universe？】Dialogue with the Universe
[Electromagnetic force + Weak interaction＋Strong interaction ＋ Gravity]
・LQG is an attempt to merge quantum mechanics and general relativity. 
・Quantum gravity (QG) is a field of theoretical physics that seeks to describe the force of gravity according to the principles of quantum mechanics.
○ Overview 
・Elementary particles for each type, the rotational speed takes the value of the discrete, as the basic number of Planck constant. For example, an electron rotates at half the speed of Planck constant.
＜Decide the direction of the space from the spin direction of the particle＞
・In the 1950s, by utilizing this principles of the spin, "the Spin network" theory which defines the direction of each point in space from the direction of the electron spin published by Penrose.
・As a specific method, the theory represents a material at a point, and represents the direction at a line, and make a constant volume by planes perpendicular to the line.
・The predicted size of this structure is the Planck length, which is approximately 10−35 meters.
・Space can be viewed as an extremely fine fabric or network "woven" of finite loops. These networks of loops are called spin networks.
・This change in spin network indicates the presence of gravity and elementary particles.
＜Loop quantum gravity theory creates space, gravity and substance at a time＞
・The evolution of a spin network over time is called a spin foam.
・Spin form changes discretely.
• The time difference is 1 Planck seconds of the change (10-43 seconds), and the time that people feel is what this very short time is stacked.
・The main output of LQG is a physical picture of space where space is granular. It has the same nature as the granularity of the photons in the quantum theory of electromagnetism and the discrete levels of the energy of the atoms.
・In string theory( a quantum theories of gravity) one generally starts with quantized excitations on top of a classically fixed background. This theory is thus described as background dependent. In contrast, loop quantum gravity, like general relativity, is manifestly background independent. 
・LQG is formulated in 4 dimensions and without supersymmetry. 
・Quantum gravity effects are notoriously difficult to measure because the Planck length is so incredibly small. However recently physicists have started to consider the possibility of measuring quantum gravity effects. 
・In 2015, the Advanced LIGO team announced that they had detected gravitational waves from a pair of black holes merging.  
1. Wikipedia: Loop quantum gravity
2. Wikipedia:Quantum gravity (QG)
3. Wikipedia：Gravitational wave
4. Ｎｏ．３０ Gravitational Wave
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