Sure! So, take as a silly example a regular pendulum of mass m and length l swinging on earth where the acceleration due to gravity is g. Suppose we want to f

Sure! So, take as a silly example a regular pendulum of mass m and length l swinging on earth where the acceleration due to gravity is g. Suppose we want to figure out what the period of the pendulum is (i.e. how long it takes for it to make one full swing from left to right back to left). We could go ahead and solve this by using Newton's laws, but at the end of the day whatever formula we find should have within it the parameters mentioned above (as well as regular numbers like 2s, pis, etc.). If we take a look at the units of the parameters above, we see: mass of pendulum (m): kg length of pendulum (l): m acceleration due to gravity (g): m/s^2 Out of these we want to produce something with units of time, and it turns out that there's a unique way of combining the above to get that: 1/g has units of s^2/m, so l/g will have units of s^2. Just take the square root and you find sqrt(l/g). Now, this doesn't necessarily mean that the period of the pendulum is exactly sqrt(l/g), it just means that it needs to be proportional to this. In fact the formula for the period has an extra factor of 2pi in it: 2pi sqrt(l/g). The point is that the general magnitude you'd expect for any timescale associated with the pendulum would be roughly equal to sqrt(l/g). For quantum gravity the story is the same. We have some parameters that we expect should be deeply part of any prediction: Planck's constant, the speed of light, Newton's constant of gravitation. The units are as follows: Planck's constant (hbar): m^2 kg / s Speed of light (c): m/s Gravity (G): m^3 / kg s^2 From these we want to figure out what the relevant energy scale is (i.e. when will this theory start differing from a non quantum mechanical version of gravity). Well, there's only one way to combine these into an expression that has the dimensions of energy. This particular way of combining the constants is huge and is traditionally called the Planck scale: E_Pl ~ sqrt(hbar c^5 / G) ~ 10^19 GeV In contrast, the LHC is currently probing physics on the scale of ~ 10^4 GeV so we're not even close to probing things in the realm of quantum gravity. Note that this analysis has nothing to do with string theory at all and generalizes to any theory of quantum gravity.