In physics, all physical quantities are inherently relative in the sense that they are defined and measured relative to a chosen frame of reference. The concept of relativity is fundamental in modern physics and is captured by theories like special relativity and general relativity.
Special relativity, developed by Albert Einstein, showed that the laws of physics are the same for all observers in inertial (non-accelerating) frames of reference. However, measurements of physical quantities such as distance, time, mass, energy, and velocity depend on the observer's reference frame. For example, the length of an object, the duration of an event, or the velocity of an object can vary depending on the relative motion between the observer and the observed.
Similarly, general relativity extends this principle of relativity to include gravity, where the curvature of spacetime is influenced by mass and energy. In this framework, physical quantities are also dependent on the distribution of matter and the geometry of the spacetime manifold.
Therefore, in the context of these theories, there is no physical quantity that is absolute or independent of the reference frame or the conditions of the observed system. All physical quantities are subject to relativistic effects and are measured relative to a particular frame of reference.
However, it's worth noting that in certain practical situations or for certain purposes, certain quantities may be considered approximately constant or invariant. For example, fundamental physical constants, such as the speed of light in a vacuum (c) or the Planck constant (h), are often treated as constants with high precision, but even they can exhibit slight variations under certain extreme conditions or in different physical contexts.
In summary, while the laws of physics are consistent across different reference frames, all physical quantities are fundamentally relative and dependent on the chosen frame of reference or the conditions of the observed system.