# UY1: Principle Of Relativity

If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws hold good in relation to any other system of coordinates K’ moving in uniform translation relatively to K.

– Albert Einstein: The Foundation of the General Theory of Relativity

The Principle of Relativity states that the laws of Physics are the same whether or not you are at rest or in a moving (but not accelerating) box.

• The only way to know that you are in a moving box is to look out of it.
• Looking out of the box tells you your speed RELATIVE to the surroundings.

It is impossible to determine an absolute speed or direction – only in relation to something else.

Ask yourself this question: Are you moving right now?

Arguments for “no”

• Your surroundings are not moving with respect to you.
• You are moving with respect to your surroundings.
• Therefore you are not moving.

Arguments for “yes”

• You know that the Earth is rotating about its axis.
• You also know that you are on the Earth, and the Earth is in orbit around the Sun.
• Therefore, you are moving along with the Earth.

Before we address this “paradox”, we shall look at Einstein’s Postulates of Special Relativity.

Next: Einstein’s Postulates of Special Relativity & Inertia Frames

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