R-ATPHY-33

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Inertial reference frame - Newton's 1st law is valid
Not accelerating. (or not much)
Non-inertial reference frames are when we are accelerating. This is where General relativity comes in instead.

Newton: Motion
Maxwell: Electromagnetism
- Maxwell believed that the speed of light was $3\times 10^{8}ms^{-1}$ for all observers.
  - This leads to some issues. If an object moving releases light, shouldn't the relative speed of this light to someone else be faster than $3\times 10^{8}ms^{-1}$?
- He also thought light travelled through a special medium, called the ether. (Didn't think light was self-propagating)
Michelson - Morley experiment
There are a bunch of mirrors here.
They hypothesised that the speed of light would vary. They thought the Earth moving through space would create some difference to the speeds if the apparatus was aligned with the Earth's direction.
They got a null result - indicating that the speed of light doesn't vary with the motion of the medium.
- Hence, ether doesn't exist.

Suggested that the motion causes contraction, which explains the results.
Proposed that objects become short by a certain factor (known as the Lorentz or Gamma factor):

$$\begin{align} \gamma = \frac{1}{\sqrt{ 1-\frac{v^2}{c^2}}} \end{align} $$

Hence, we move from classical (Newton, Maxwell) theory to modern (Einstein, relativity, quantum mechanics) theory.

However, classical theory is still good for everything but the extremes (lets be real, we won't be going at 0.9c any time soon)

The laws of physics have the same form in all inertial frames of reference, i.e. there is no absolute inertial frame of reference. They are all valid.
The speed of light is the same in all inertial frames of reference. The speed of light is independent of the speed of its source or its observer.
- Hence. the speed of light is constant for all observers.

For example, if a dude on a rocket (somehow) moving at 0.5c shines a torch, an external observer measures the speed of light to be 1.0c, not 1.5c.