Page "Apache Point Observatory Lunar Laser-ranging Operation" Paragraph 29

Finally, even if the experiments show no effect, there is a tiny theoretical loophole.

The measurements show the sum of the WEP and SEP violations.

If the experiments show no effect, the most natural explanation is that neither WEP or SEP are violated.

But it is conceptually possible that both are violated, and by equal and opposite amounts.

This would be an incredible coincidence since WEP and SEP depend on very different and arbitrary properties — the exact composition of the Earth and the Moon, and their self-energies.

But this unlikely case cannot be completely ruled out until either other solar system bodies are measured to similar precision, or laboratory experiments reduce the bounds on WEP violations alone.