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Soon after Max Planck introduced the Planck constant h in 1899 to account for the spectrum of blackbody radiation, he realized that the only way to construct a unit of length out of h = 6.625x10^{27} ergsec, the velocity of light c = 3x10^{10} cm/sec, and the gravitational constant G = 6.67x10^{8} cm^{3}/sec^{2}gm, is L_{PL} = (Gh/c^{3})^{1/2}. This is the now famous Planck length. Since h is related to Quantum Theory, c and G emerge from the Special and General Relativity respectively, it implies that any length scale in a Quantum Gravity Theory would involve L_{PL}, which is extremely short at 4x10^{33} cm. 
 
Figure 1532 Realm of Planck Scale [view large image] 

paircreation with large quantum fluctuation in momentum (~ mc) resulting in position uncertainty (x/p) about the order of a Compton wavelength. Individual object does not exist in region below that line as the single particle description is no longer applicable. The straight line on the right is a plot of mass against the Schwarzschild radius r_{s}=2Gm/c^{2}. Objects cannot be accessed in region below that line as it would be wrapped inside the event horizon (since r < r_{s}). All objects exist only within the region bound by these two lines. Table 1503 lists some of the objects within or at the border. The two lines converge at a point where both quantum effect and gravity become important.  
Figure 1533 Quantum Gravity Convergence _{} 
Figure 1533 also shows some characteristic scales of the universe. 
Object  Mass (gm)  Compton Wavelength (cm)  Size (cm) 

Electron  9.1x10^{28}  3.8x10^{11}  ~ 10^{16}, Pair Creation at High Energy 
Proton  1.67x10^{24}  2x10^{14}  ~ 10^{13}, Composite Particle 
Higgs Boson  1.25x10^{22}  10^{16}  Excitation from Higgs Field, Unstable 
Buckyball (C_{60})  1.2x10^{21}  2.8x10^{17}  ~ 10^{7} 
Protein  ~ 10^{19}  3.2x10^{19}  ~ 10^{6} 
Virus  ~ 10^{8}  3.5x10^{30}  ~ 10^{5} 
Object  Mass (gm)  Schwarzschild Radius (cm)  Size (cm) 
Earth  6x10^{27}  0.9  6x10^{8} 
Sun  2x10^{33}  4x10^{4}  2x10^{11} 
Neutron Star  10^{34}  2x10^{5}  10^{6} 
Cygnus X1  2x10^{34}  4x10^{5}  Stellar Binary + Black Hole 
Sgr A*  8x10^{40}  1.2x10^{13}  ~ 6.4x10^{12} Milky Way Black Hole 
3C273  4x10^{42}  6x10^{14}  Quasar + Black Hole 
Observable Universe  4x10^{55}  6x10^{27}  10^{28} (very close to form a Black Hole) 
 
Figure 1534 White Hole _{} 
Figure 1535 Evolution of Cosmic Entropy _{} 
Since the Schwarzschild Radius r_{s} = 2GM_{PL}/c^{2} = 2L_{PL} for a Planck scale entity, it is a black hole, while the distance to horizon D_{H} = c t_{PL} = L_{PL}, and the energy density ~ 10^{114} erg/cm^{3}. 
Another example : The direction of cosmic evolution of entropy always tends to increase or at least to remain constant as shown in Figure 1535. According to the scenario of backward tracing, the universe would be a dense pack of particles with maximum entropy at the beginning. It is not clear then what makes this trend of increasing entropy if it was at its maximum already. It turns out that there would not be any problem to explain the origin of low entropy at Big Bang if it starts from a black hole (see "Origin of Time"). It increases upward forever since then as expected by the 2nd law of thermodynamics.  
Figure 1536 _{}Quantum Gravity 
Figure 1536 shows the relationship between quantum gravity and the other branches of physics at the limit of the various universal constants. In other word, the various theories at the perimeter are just the special cases of the more general theory of quantum gravity  hence it is often referred to as the Theory of Everything. 