Heisenberg’s starting point was the philosophical judgment, that a physical theory should not concern itself with things like electron orbits in atom… - Steven Weinberg

Electrons are believed to be absolutely stable, and protons and neutrons (when bound...) live at least 10³⁰ years. With few exceptions, all other particles have very short lifetimes...

As a special case of Newton's Second Law, a body... when acted on by zero force, will experience zero acceleration—that is, it will move with constant velocity. Newton listed this... as the First Law... The Third Law... action equals reaction: If one body exerts a force on another... the second... exerts an equal force in the opposite direction on the first.

Consider the geometry of a three-dimensional homogeneous and isotropic space. ...[G]eometry is encoded in a metric <math>g_{ij}(\mathbf{x})</math> (with i and j running over the three coordinate directions), or equivalently a line element <math>ds^2 \equiv g_{ij} dx^i dx^j</math>, with summation over repeated indices... <math>ds</math> is the proper distance between <math>\mathbf{x}</math> and <math>\mathbf{x}+\mathbf{dx}</math>, meaning... the distance measured by a surveyor who uses a... Cartesian [coordinate system] in a small neighborhood of... point <math>\mathbf{x}</math>.) One... homogeneous isotropic three-dimensional space with positive definite lengths is flat space, with line element<math>ds^2=d\mathbf{x}^2</math>...The coordinate transformations that leave this invariant are... ordinary three-dimensional rotations and translations. ...Another ...possibility is a four-dimensional with some radius <math>a</math>, with line element<math>ds^2=d \mathbf{x}^2+dz^2,\;\;z^2 + \mathbf{x}^2 = a^2</math>,...Here the transformations that leave the line element invariant are four-dimensional rotations; the direction of <math>\mathbf{x}</math> can be changed to any other direction by a four-dimensional rotation that does not change <math>z</math>. ...[T]he only other possibility (up to a coordinate transformation) is a hyperspherical surface in four-dimensional , with line element<math>ds^2 = d\mathbf{x}^2 - dz^2,\;\;z^2 - \mathbf{x}^2 = a^2</math>,...where <math>a^2</math> is (so far) an arbitrary positive constant. The coordinate transformations that leave this invariant are four-dimensional pseudo-rotations, just like s, but with <math>z</math> instead of time.