Yes, energy and momentum are also relative to the observer in the framework of special relativity. In classical physics, energy and momentum were considered to be absolute quantities, but in special relativity, they are found to be observer-dependent.
According to the principles of special relativity, the energy and momentum of an object are part of a four-vector called the energy-momentum four-vector. This four-vector combines energy and momentum into a single entity that transforms under Lorentz transformations, which are the mathematical transformations that describe how measurements change between different inertial reference frames.
The energy and momentum of an object are different for observers in relative motion, and their values depend on the relative velocity between the object and the observer. This is due to the time dilation and length contraction effects predicted by special relativity.
The relationship between energy, momentum, and velocity is described by relativistic equations, such as the relativistic energy-momentum equation:
E² = (pc)² + (mc²)²
where E is the energy, p is the momentum, c is the speed of light, and m is the rest mass of the object. This equation shows that the energy and momentum of an object are interconnected, and their values change depending on the object's velocity relative to the observer.
Therefore, energy and momentum are not absolute quantities but are instead observer-dependent in the framework of special relativity.