Turtle Theory

The Turtles All the Way Down Theory states that the Earth rests on turtles upon turtles, which however are invisible and thus in no way can be detected. 

The Turtle Theory is experimentally verified by the observation that the Earth does not fall down, as illustrated by the picture.

Convinced? If not, then you may have some difficulty of buying modern physics in the form of of relativity theory, quantum mechanics, the standard model and string theory:

• relativity theory: curved spacetime root of gravitation
  
• experiment: perihelion of Mercury

• quantum mechanics: smallest quantum of energy
• experiment: black-body radiation

• standard model: protons and neutrons made of quarks
• experiment: atomic kernels made of protons and neutrons 

• string theory: quarks made of tiny vibrating strings
• experiment:  not available - no observation of quarks.

The experimental evidence is here indirect/missing: curved spacetime, quantum of energy, quarks and strings are not directly observable, just as invisible turtles are not directly observable. 

The epistemology of modern physics characterized by indirect experimental evidence was laid in a fierce debate between Mach and Boltzmann about matter as atoms and molecules in the late 19th century, before the existence of atoms and molecules was experimentally verified. 

Boltzmann developed a statistical mechanics gas dynamics theory based on an assumption of a gas as a large collection of little elastic spheres representing hypothetical atoms or molecules assumed to have statistically independent velocities before collision. Mach objected claiming that the theory was empty as long as the basic assumption of statistically independent 

elastic collision of atoms/molecules, was not experimentally verified. 

Bolzmann first vigorously defended his theory, despite its lack of direct experimental support, but eventually Mach's criticism was effective: Boltzmann came to doubt his own theory to the degree of taking his own life in 1906.  Shortly thereafter the existence of atoms was experimentally verified by Rutherford, which ended also Mach's career, even if Boltzmann's assumption of statistical independence before collision, was not experimentally verified. 

The curtain of the tragic opera of classical physics thus closed with everybody left dead on the scene, and out of the turmoil modern physics was born with a new modern epistemology: 

• Direct experimental evidence of the basic axioms of a theory is not necessary.
  
• Indirect evidence is enough

motivated by the following argument:

• See how wrong Mach was! 
• Atoms and molecules do exist! 
  

This opened the door to modern physics allowing you:

• to assume any basic laws for quarks, strings, turtles...
  
• escape experimental verification of basic laws,
• consider indirect experimental evidence to be enough.

But there is a weak point in the argument: OK, atoms and molecules apparently do exist, in one form or the other, and thus Mach's criticism missed the point. But this does not imply that all forms of Machian criticism can be refuted: The key question is still if quarks, strings and invisible turtles do exist or not. 

OR?