So, according to my understanding, m= m_o/√(1-(v^2/c^2 )) gives the mass of an object in respect to the object's original mass and its velocity. I wondered what happened if the mass of an object became lower than the rest mass? [I have no idea how this would happen, but it was a, what if it did? kind of question] I made the substitution m=m_o-a for some arbitrary amount lower than the rest mass. After solving the equation for velocity, i got v=c√(1-(m_o^2/(m_o-a)^2 ) in the equation, you can see that, (m_o-a)^2 < m_o^2 thus, (m_o^2/(m_o-a)^2 > 1 so we would end up with the square root of a negative number, giving the object with an imaginary velocity. This matched my predictions because the reason I wondered this was because I wanted to know what was so special about the rest mass of an object. Why is that amount of energy in that amount of space a particle, and why does additional energy cause what we call velocity? So i assumed that a lower amount of energy would do somehow the opposite of velocity, but what is that?