The most commonly proposed Fourth Law is the Onsager reciprocal relations, which give a quantitative relation between the parameters of a system in which heat and matter are simultaneously flowing.

According to general relativity, no matter or information can flow from the interior of a black hole to an outside observer, although quantum mechanics may allow deviations from this strict rule.

The shape changes depending on if the black hole is charged or spinning, and where and how much matter is flowing in to it.

In open systems, matter may flow in and out of the system boundaries.

Just as air flows from one room to another if the air pressure is unequal, dark matter might be flowing into our dimension to equalize some sort of imbalance.

This second little matter flows clearly from the first.

A key debate to all interpretations of the series is to decide whether matter flows from thought, or the opposite.

Thus, in Sheena's spacecraft, matter and energy flowed in great loops, sustained by sunlight, regulated by its central machinery as if by a beating heart.

He could cloak anything in an illusion, and if the wish was maintained for long enough the matter underneath would eventually flow into the shape and texture which he was visualizing.

In this model energy and matter flows through an ecosystem: H are herbivores, C are carnivores, TC are top carnivores, and D are decomposers.