A useful reddit comment

0x10000 comments on A fifty pence coin I got as change explains the offside rule:

• When you point your flashlight at the wall and turn it on, it seems
  the light ray reaches the wall instantaneously, and so "speed of light for everyone watching" = c = ∞. Or equivalently c⁻¹ = 1/c = 1/∞ = 0.

But careful observation of Nature shows us that in fact: c⁻¹ ≠ 0, c⁻¹ = 1 (or something else if you prefer another unit system, but never 0). Speed of light is finite and the same for everyone watching. Surprising, yes, but it works for really fast things and that's proven. No biggie.

• When you cut a stick in half, then cut the half in half, then get a
  knife, cut the rest into something even smaller, sharpen the knife, and
  so on, it feels like you can do this for ever, and with infinite time
  "it makes sense to have something infinitely small to almost size zero" and we note this idea ħ = 0.

But careful observation of Nature shows us that in fact: ħ ≠ 0, ħ = 1 (or something else if you prefer another unit system, but never 0). It doesn't make sense
to have laws governing something infinitely small. Surprising, yes, but
it works for really small things and that's proven. No biggie.

• When a needle falls to the ground, it is pulled down by the gravity
  from entire Earth (and that's really really big). But you can pick it up
  with a tiny magnet. So gravity seems so ridiculously small compared to
  other forces that in this perspective, we are almost tempted to ignore
  it as if it didn't exist: G = 0.

But observation of Nature shows us that it obviously does exist: G ≠ 0, G = 1 (or something else if you prefer another unit system, but never 0).This is not so surprising since you see an apple fall. No biggie.



So we thought that c⁻¹ = 0, ħ = 0 and G = 0, but we now know that c⁻¹ = 1, ħ = 1 and G = 1 and we want one consistent theory starting from there.



The theory behind c⁻¹ = 1, ħ = 0 and G = 0 is called Special Relativity.



The theory behind c⁻¹ = 0, ħ = 1 and G = 0 is called Quantum Mechanics.



The theory behind c⁻¹ = 0, ħ = 0 and G = 1 is called Newton's law of universal gravitation.



The theory behind c⁻¹ = 1, ħ = 0 and G = 1 is called General Relativity.



Other combinations have different theory names, but the one you asked for was:



The theory behind c⁻¹ = 1, ħ = 1 and G = 0 is called Relativistic Quantum Mechanics.



We have no consistent theory for c⁻¹ = 1, ħ = 1 and G = 1, even
though we know it is true. This problem is called "quantization of
gravity". A candidate theory is Loop Quantum Gravity. Another, more famous one is String Theory.