Changes

Since speed is relative, it follows that two observers in different inertial reference frames may disagree on the mass and kinetic energy of a body. Since all inertial reference frames are treated on an equal footing, it follows that mass and energy are interchangeable.

Since speed is relative, it follows that two observers in different inertial reference frames may disagree on the mass and kinetic energy of a body. Since all inertial reference frames are treated on an equal footing, it follows that mass and energy are interchangeable.

There is a logical difficulty, however, to an increase in relativistic mass.  Such increase would only exist in the direction of motion, and the rest mass would remain intact with respect to a force applied in a direction orthogonal to velocity.  But mass is not a vector, and the notion of the mass of an object having different values depending on the direction of an applied force is unacceptable.  Accordingly, most physicists today avoid Einstein's original reliance on relativistic mass and his suggestion that mass increases.  Instead, most physicists today teach that F=<math>\gamma</math>ma where <math>\gamma</math> varies with velocity as mass m remains constant. Force F is a vector and thus can handle the directional aspect of the relativistic effects better than the concept of relativistic mass can.

There is a logical difficulty, however, to an increase in relativistic mass.  Such increase would only exist in the direction of motion, and the rest mass would remain intact with respect to a force applied in a direction orthogonal to velocity.  But mass is not a vector, and the notion of the mass of an object having different values depending on the direction of an applied force is unacceptable.  Accordingly, most physicists today avoid Einstein's original reliance on relativistic mass and his suggestion that mass increases.  Instead, most physicists today teach that  

 

<math>F=\frac{d}{d\tau} p</math>

 

where <math>p</math> is the momentum defined by <math>\gamma m v</math>, <math>\gamma</math> is the standard Lorentz factor, and <math>\tau</math> is the proper time. Force F defined this way is a vector and thus can handle the directional aspect of the relativistic effects better than the concept of relativistic mass can.

==Evidence for Relativity==

==Evidence for Relativity==