When is a Principle not a Principle?

[oldman]

In a current thread Why Expanding Space where a discussion of the Equivalence Principle (EP) of General Relativity (GR) would be off-topic, I said (perhaps unwisely) that the EP is something not understood. Even more unwisely, I said that it is statement raised to the status of a principle, so that one could avoid explaining why it is true.

In reply atyy makes a nice distinction, namely that:

The EP is not a principle principle, it is a heuristic principle...

. I take this to mean that the EP is not 'a truth used as a basis for a theory' (GR) but that it is a convenient and sufficient, but not necessary, assumption to be made for the purpose of developing GR. Or that if GR were to be developed without leaning on the EP as a kind of crutch, the EP would then emerge naturally as part of the structure of GR. I hope I haven't read into this distinction more than was meant -- apologies if I have, atyy.

I'm wary of statements called Principles: however reasonable, they seem to me to have an ex cathedra flavour. For instance the EP is eminently reasonable (Einstein elevator logic) and is fully justified a posteiori by the predictive success of GR and by the excellence of that theory's internal logic. But couldn't there be more direct reasons for the truth of the EP, reducing its status to only a 'heuristic principle' rather than a 'principle principle', as atyy put it? Another example is the Copernican Principle: that the universe is everywhere much the same, which underlies modern cosmology. Why is this so? Well, it could be that everything --- perhaps physical laws included --- had a common origin, as many cosmologists postulate. In which case this also becomes just a well-supported-by-observation heuristic principle.

I'd rather drop the label "principle" entirely rather that distinguish between different varieties of principle. Shouldn't one rather ask why these principles are the truths they appear to be?

 

[Chalnoth]

The equivalence principle is a statement that the gravitational "charge" is the stress-energy tensor, and that none of the other properties of matter affect it. For example, an assemblage of protons and electrons that have the same mass as a different assemblage of neutrons will, as long as the mass is distributed the same in both instances, gravitate identically.

This statement appears to hold up extremely well to experiment, and if there is any violation of the equivalence principle, it is very small. The Wikipedia entry on the equivalence principle has a few of the experimental tests that have been done to date.

 

[nutgeb]

Apparently Einstein never accepted the current standard interpretation of the EP, that it applies in the context of an infintessimally small local space where curved spacetime could be treated as flat - in other words, where tidal effects could be ignored. He argued that if the local space was infintessimal, any accelerations occurring there would be as well, so the principle would be meaningless as a basis for analysis.

Einstein insisted that the EP applies literally only to a homogeneous, static gravitational field, e.g. a field with a rectolinear geometry and no gradient. Of course such a field is an artificial construct that does not exist in the physical world. In this sense, Einstein viewed the EP more as a mental tool or analogy than as a tangible characteristic of the physical world.

 

[wangxinxincum]

I always hold that all the principles are just the descriptions of the various stages of the Existence . Maxwell's ghost is allowed. We need not to worry about it at all.
And I hold it that the universe is a Evolving Universe ,which is just as Darwin's theory of evolution.I love this king of picture of our universe and our existence!

 

[wangxinxincum]

The Magic Existence is our exclusive God.