# Principle of Relativity: Classical Physics Example

• B
In summary, the principle of relativity states that the laws of physics are the same in all inertial reference frames. This principle is found in classical physics, which is the physics that Newtonian mechanics is based off of.f

hi everyone
"The principle of relativity: The laws of physics are the same in all inertial reference frames."
Is in classical physics The laws of physics aren't the same in all inertial reference frames!? Give an example in classical physics

Thanks

The principle of relativity holds in all systems of physics since Galileo. Aristotle would have disagreed with it, arguing that (in modern terms) the rest frame of the Earth's surface is special in some sense.

Einstein probably felt the need to state the principle explicitly since dropping it was one approach you could consider to resolve the mismatch between Maxwell and Newton. Relativity, of course, resolves the mismatch without abandoning the principle of relativity.

topsquark and russ_watters
Is in classical physics The laws of physics aren't the same in all inertial reference frames!?
No. The principle of relativity in this form was actually first enunciated by Galileo, and Newtonian mechanics is bulit on it. The difference between Newtonian mechanics and special relativity is the specific form of the transformation between different inertial frames: in Newtonian mechanics it is the Galilean transformation, in SR it is the Lorentz transformation.

topsquark
First of all, I guess with "classical physics" you mean "Newtonian physics". Of course, in Newtonian physics the special principle of relativity must also hold. In both Newtonian physics and special relativistic physics thus Newton's 1st Law is valid, i.e., there exists an "inertial frame of reference", in which a point mass moves with constant velocity, if it's not interacting with anything.

The difference comes with Einstein's additional postulate for special relativity, i.e., that the phase velocity of electromagnetic waves in a vacuum (in short "the speed of light") is independent of the relative motion between source and detector.

Together with the additional assumptions about the symmetries of space and time you find out that you either get the Galilei transformations between two inertial reference frames,
$$t'=t, \quad \vec{x}'=\vec{x}-\vec{v} t, \quad \vec{v}=\text{const}$$
or the Lorentz transformations (making the direction of the relative velocity that in the ##x##-direction),
$$c t'=\gamma (c t-\beta x), \quad \beta=v/c, \quad \gamma=1/\sqrt{1-\beta^2},$$
$$x' = \gamma (x-\beta c t).$$
The Galilei transformations of course belong to Newtonian and the Lorentz transformations to special relativistic physics, and in special relativity, the speed of light, ##c##, is a "limiting speed", i.e., nothing can move faster than the speed of light within an inertial frame of reference. There's no such limiting speed in Newtonian physics, of course.

topsquark