What is the principle of equivalence in physics?

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Discussion Overview

The discussion centers on the principle of equivalence in physics, specifically its definitions and implications within the context of gravitational and inertial mass, as well as its role in general relativity. Participants explore different interpretations and terminologies related to the principle.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that the motion of a particle in a uniform constant force field is equivalent to that of a free particle in an accelerating reference frame, questioning the specific name of this principle.
  • Others identify this principle as the Principle of Equivalence of Gravitation and Inertia (EP), noting Einstein's reference to it as "His Happiest Thought."
  • One participant raises a question about the nature of principles, asking if the EP can be ignored.
  • Another participant suggests that general relativity (GR) could theoretically be developed without the EP, but acknowledges that it would be significantly more complex.
  • There is a clarification attempt regarding the distinction between the Galilean principle of equivalence and the Principle of Equivalence of Gravitation and Inertia, with some participants confirming this differentiation.
  • One participant introduces the terms "weak equivalence principle" and "strong equivalence principle," expressing uncertainty about their correct usage and seeking verification.

Areas of Agreement / Disagreement

Participants express differing views on the implications and interpretations of the principle of equivalence, with no consensus reached on the terminology or the necessity of the principle in the context of general relativity.

Contextual Notes

There are unresolved questions regarding the definitions of the weak and strong equivalence principles, as well as the implications of ignoring the EP. Some assumptions about the principles' applications remain unaddressed.

pellman
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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.
 
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?
 

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