What is mass?

Mass is merely a dimension. Of itself, it has no material existence, although it is one of the defining qualities of the material objects. In a weightless environment mass does not change to zero. When mass is a near large planet, it does not become greater. When an object with mass is accelerated to near the speed of light, it does not increase mass. There is mass in resistance, there is mass in potential, there is mass in energy, there is mass in angular momentum. It is all the same mass, but manifested differently. It might help to realize that there is time in units too. There is time in resistance, there is time in potential, there is time in energy, and there is time in angular momentum. You can perceive time as change, but you cannot isolate time from a unit. You can perceive mass as inertia and length as distance, but you cannot separate the dimensions of mass and length from units. In the same way, you cannot remove the bricks from a brick building without also removing the building.

Once we stop thinking of mass as equal to matter, and realize that mass is neither physical nor is it something convertible, then it becomes easier to see what mass really is and how mass behaves. You cannot truly weigh mass, but you can weigh something that has mass. You cannot make mass turn into energy. The whole issue about converting energy from mass clearly reveals itself when we realize the indestructible and unchangeable nature of dimensions. You cannot convert mass, length, time, or charge. They are absolutes. Mass is always mass. Mass is only a dimension.

Energy is a unit, mass is a dimension
E is composed of the dimensions of mass, length, frequency.
E = M X L^2 X F^2

When it comes down to the truth to the truth of it, mass is a dimension while energy is a unit made up of three dimensions.

In SR, mass converts to energy as an object approaches the speed of light. If this were true, then instead of having an infinite mass as the theory proposes a spaceship should be mass-less at the speed of light. However, if the spaceship were mass-less at the speed of light, then it would have no energy because the mass is zero. No doubt the die hard relativists will come forth with arguments that there is relativistic mass, which is different from rest mass.

Mass is ultimately only a dimension. Mass is not equal to matter or energy. Mass does not rest and does not move as an independent entity. There is no such "thing" as mass that can be converted to energy, of which energy is merely a unit