Measure the difference between space-time events
Two stars go supernova while four kids watch.
There is a difference in time (dt).
There is a difference in space (dR).
Together they make a difference in space-time.
Each kid measures different values for time and space.
Yet the walkers agree on something they can calculate called the interval. This is the standard physics of Special Relativity, the physics of moving.
This site promotes a new proposal for gravity called Space-times-time invariance for Gravity or Quaternion Gravity (QG) where the kids above or below agree on a different value they calculate, space-times-time. In this standard approach to gravity known as General Relativity, this is almost, but not quite true.
A brief explantion of this page.
Special relativity is special because it is restricted to inertial observers. To cover more cases requires the machinery of differential geometry. A tensor can be added to another tensor or multiplied by a scalar. An interval is formed by contracting two rank 1 contra-variant tensors with a symmetric, rank-2 covariant metric tensor. A connection is needed to describe how the metric changes in space-time. There are many technical choices one makes along the way to calculating an interval in curved space-time.
The site issues a formal challenge to the algebraic standards of differential geometry used today by physics. In place of tensors, metrics, and connections, only quaternions will be used. For those trained in the craft of differential geometry, that should sound wildly inadequate. It is a great challenge to do more with less.