# Whadda ya call it when ?

## Main Question or Discussion Point

The motion of a particle in a uniform constant force field is equivalent to that of a free-particle measured from an accelerating reference frame. What is this principle of equivalence known as? I thought it might be the Galilean equivalence principle but, no, that is the equivalence between inertial and gravitational mass.

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It is known as the Principle of Equivalence of Gravitation and Inertia (or EP for short). Einstein called it "His Happiest Thought".

My problem is this: EP is a principle so does that mean that I can ignore it if I choose?

I believe GR would be obtainable without the EP (according to some lecture notes I read by Prof. J. J. Binney), but the road to the field equations is far more difficult and dangerous than with the EP.

DaveC426913
Gold Member
Oxymoron said:
It is known as the Principle of Equivalence of Gravitation and Inertia (or EP for short). Einstein called it "His Happiest Thought".

My problem is this: EP is a principle so does that mean that I can ignore it if I choose?
I think what it means is: all known real and theoretical instances adhere to this principle - but nonadherance is not explicitly ruled out.

Thanks for the answers. Let's restate it one more time be sure. The equivalence between gravitational mass and intertial is "the Galilean principle of equivalence" whereas the eqivalence between constant force fields and accelerated frames is the "Principle of Equivalence of Gravitation and Inertia". Do I have that right?

I also heard the first called the "weak equivalence principle" and the second called the "strong equivalence principle".

EDIT: A very cursory look at http://en.wikipedia.org/wiki/Equivalence_principle makes me worry I may be misunderstanding the second one. So can someone verify or say otherwise about my usage of these terms?