Do tidal forces mean the Equivalence Principle is BS?

[Ikoro]

who extended einstein's work? you?

Am done helping...go read, if you have a problem.

 

[Dickfore]

Am done helping...go read, if you have a problem.

Maybe you should learn that not all questions are asked in need of 'help'. The question was intended at the OP who apparently bailed out his own thread.

 

[my_wan]

No, tidal forces do not say anything about EP. Tidal forces are the result of different parts of a body traveling in two different direction, while each part is doing exactly what EP said it should do.

Consider two masses on each side of the world. They will both inertially accelerate toward the Earth in opposite directions, just as EP demands. Now if you move them only a few kilometers apart they will still inertially accelerate toward the center of the Earth, meaning they will keep getting closer together the closer they get to the center of the Earth. Now, even if you put them right next to each other, as though they were one object, they are still far enough apart that inertial motion still tries to press them closer together as they approach the center of Earth. Hence their internal pressure must push them back apart in opposition to EP. This force can break up meteors, but not because of any problem with EP, but because EP is trying to require the parts of the meteor to come closer together when they cannot.

Saying this invalidates EP is like saying a rocket motor invalidates inertial forces, or inertial motion is invalidated by two meteors hitting each other. It is just not so.

 

[A.T.]

What about tidal forces?

The EP about inertial vs. gravitational mass. The example about an uniform gravitational field compared to an accelerated frame, is correct and applies in reality. Just because most gravitational fields in nature are not uniform, doesn't mean that statements about uniform gravitational fields are wrong.

doesn't hold in reality if you want to think about physics fundamentally?

Reality and fundamental principles are different things. Reality is always more complex than the idealized example meant to explain the application of a certain fundamental principle.

 

[kmarinas86]

When you compare two masses of equal caliber obviously it won't fall at the same rate. All test particles fall the same in a gravitational field and the stated principle restricts itself to test particles.

This "restriction" is what I and the OP have a problem with. I'd rather deal with what logicians call "universals".

It seems evident that the Equivalence Principle sits somewhere between a "singular existential statement" and a "universal statement". This is not settling for those who see their "unlearning" of previous teachings as an obstruction against their ability to learn. This is bad pedagogy in my opinion.

 

[my_wan]

This "restriction" is what I and the OP have a problem with. I'd rather deal with what logicians call "universals".

It seems evident that the Equivalence Principle sits somewhere between a "singular existential statement" and a "universal statement". This is not settling for those who see the "unlearning" previous teachings as an obstruction against their ability to learn. This is bad pedagogy in my opinion.

They are falling at the same rate, just not in the same direction because separate points in the same object are trying to go to the same point at the center of the planetary mass. Hence they bump into each other and often even break in the process.

 

[Phrak]

Well, the whole point in this exercise is the that gravitational field is constant, and in the same direction, which isn't the case for a spherical body. Take a body of uniform density and carve a spherical cavity in it. Now you have a uniform gravitational field in the weak field limit. Where are the objections to this model?

Really, Einstein's notion of equivalence is something he latter tries to clarify, calling it general covariance. This notion is better solidified mathematically as diffeomorphism invariance which is a scary way to say that things like vectors and stuff are independent of the coordinate system in which they are, or can be expressed. So if you go from Cartesian to polar coordinates, say, the vector you have doesn't get longer or shorter or change direction.

 

[atyy]

The two ingredients of the EP within GR are:

1) the gravitational field is the metric field - this geometrical notion ensures that when the metric is curved, things are still everywhere "locally" flat -where "local" means we don't look at curvature.
2) matter, conceived as all non-gravitational fields, is universally and minimally coupled to the metric - this means that there are laws of physics that are "local" and don't "directly" probe the curvature of the metric.

Curvature is non-local in the sense that it is defined using second derivatives of the metric, and higher order derivatives are more non-local than lower order ones, since they involve more differences between spacetime separated quantities (technically, derivatives are local since they are limits and exist at a point, but I'm not using that jargon). The EP does fail for experiments that are non-local.

http://www.pma.caltech.edu/Courses/ph136/yr2006/text.html, section 24.7