Question about equivalence principle

[Dale]

Of course a measurable change in G implies a corresponding change in a relevant dimensionless quantity. But that is not the point.

Well, that was the only point that I was making, so it seems that we agree.

 

[DrGreg]

What if instead of redefining the ratio and adjusting G, we actually make the ratio slightly different. Would that mean that our measured value of G would be slightly different?

I'm not really sure what you think the difference is between "redefining the ratio" and "actually make the ratio slightly different". Seems like the same thing to me.

But, yes, I'm saying that if the ratio was different, we'd measure a different value of G.

All the equations of physics that have "active gravitational mass" in them always contain a GM term rather than M on its own. That's true in General Relativity as well as Newtonian theory. So any constant of proportionality is accounted for within the value of G.


Technicality: relativists often choose to measure mass in units such than G = 1 (instead of kg) so their equations seem not to have a G in them. But if you switch to other units, what I said above is true.

 

[TurtleMeister]

I'm not really sure what you think the difference is between "redefining the ratio" and "actually make the ratio slightly different". Seems like the same thing to me.

I was just trying to make the point that the equivalence of active gravitational mass and passive gravitational mass is a testable hypothesis. (I admit I did not do a very good job.)

This is the of type of experiment I have been looking for:
http://www.iop.org/EJ/article/1742-6596/189/1/012026/jpconf9_189_012026.pdf?request-id=69a8b2bc-bdaa-4b3a-939b-daae73156a2a"
On page 4 beginning with 3. Newton-I Experiment, is a test of the composition dependence of the gravitational constant. I believe this is in effect a test for the equivalence of active gravitational mass and passive gravitational mass for different compositions of matter.

 

[TurtleMeister]

So I would say the equivalence of active and passive mass isn't a testable hypothesis, it's a definition. And any "test of active mass" is really just another name for "measuring the value of G".

Yes, I think I understand what you are saying. And you are correct. But it is based on the assumption that gravitational mass and inertial mass are the same for all compositions of matter. And indeed, that is what EP postulates. So I am not disputing your claim. However, I do believe that EP should be tested.

So let me ask my second question again, being more precise.

There is a test of the equivalence principle which is sometimes referred to as a test for composition-dependent forces. This is a valid test, because EP states that all gravitational forces are independent of mass composition. Many of these tests involve the http://www.npl.washington.edu/eotwash/intro/intro.html" . While this is a perfectly good test of the EP, it does not address the question of composition dependence based on active gravitational mass. The test masses in the composition dipole are essentially responding to a source gravitational field. In other words, they are the passive mass. There are, and have been, many other similar types of tests, such as STEP. But they are all the same in that the test masses are passive.

Now take a look at the experiment (for composition dependence of gravitational constant) linked in my previous post. Notice that the test masses are the attractors. In this experiment the test masses are the "source" of the gravitational field. So this is a test for composition dependence based on active gravitational mass.

So my question is: Why so many EP tests for composition dependent gravity based on Mgp, and so very few EP test for composition dependent gravity based on Mga?

Thank you to everyone who has responded. And thank you to those who have PM me with links and information.

Turtle

 

[TurtleMeister]

Here is the equation for measuring the value of G:

G = Fr² / (M_target M_attractor)

My third question: Does it make any difference whether the test mass is the target or the attractor? Keep in mind that the attractor is the source (large mass), and that the experiment is to measure the value of G using different compositions of mass as the test mass. This is done by measuring G with a test mass of composition "a", and then repeating the experiment with a test mass of composition "b". The two values of G are then compared. A null experiment is G_a - G_b = 0.