again, this is not true. Negative is simply a concept to denote something lower or less than an arbitrarily decided upon zero point on a scale. One could argue that a balloon has a negative weight when considered as a system and filled with a gas that is lighter than the air around it and causes a force that counteracts and is greater than the earths pull on it. I think you mean negative mass cannot be physically demonstrated, as negative "weight" of a body easily can.