Talk:Equivalence principle/Archive 1

message: A few problems 
*There is lack of distinction in the article between active gravitational mass that causes gravitational effects through whatever mechanism, and passive gravitational mass that responds to whatever the agent of this mechanism is doing, the mass that is equal to the inertial mass making all the bodies fall with the same acceleration. So basically the mentioned in second paragraph principle of equivalence of masses refers only to passive gravitational mass being equal to inertial mass. An old Newtonian principle.
: (William M. Connolley 20:26, 26 Jul 2004 (UTC)
replies: text: (William M. Connolley 20:26, 26 Jul 2004 (UTC)
thread_title:  A few problems (july 2004) 

message: I have just completed a major revision of this article.  The revision is at User:Ems57fcva/sandbox/Equivalence_Principle.  My objections to the current version are as follows:
* This article uses the Strong Equivalence Principle (SEP) instead of the Weak Equivalence Principle (WEP).  The SEP is not true for the reasons stated in the revision.
* This article brings up a lot of GR-related details which are superfluous to the Equivalence Principle:
** The fact that the Sun bends light rays passing near its surface by 1.75" is due to the full metric treatment of GR.  Only the bending of light in a generic sense is due to the Equivalence Principle.  (The original 1911 article predicting the bending of light was off by a factor of 2 in the amount of the bending!)
** Inertial movement ocurring along geodesics of spacetime, while an important aspect of GR itself, is not relevant to the Equivalence Principle.  To describe geodesics, you need a metric of spacetime, but the Equivalence Principle is independent of any metric.
Comments will be appreciated.  I intend to do the replacement soon.
--EMS 05:25, 30 Mar 2005 (UTC)
thread_title:  Major revision ready 

message: The revision is now in place.
--EMS 16:24, 31 Mar 2005 (UTC)
thread_title:  Changes made 

message: I am removing the following changes for the reasons stated in the more indented text:
:... One way of stating a fundamental principle of general relativity is to say that gravitational mass is identical to inertial mass. 
:: This is another way of expressing of the Equivalence Principle.  It is not a seperate principle of GR.
: Experiments performed in a uniformly accelerating reference frame with acceleration a are indistinguishable from the same experiments performed in a non-accelerating reference frame which is situated in a gravitational field where the acceleration of gravity = g = -a = intensity of gravity field.
:: My objections are as follows:
::* This is a poor attempt to state the strong equivalence principle, which as I state later in the page is not true and is not the currently accepted version of the Equivalence Principle.
::* A reference from where an acceleration due to gravity is locally detectable is not "non-accelerating" at all.  That is the point of the Equivalence Principle.
:One of the implications of the principle is that since photons have momentum and therefore must be attributed an inertial mass, they must also have a gravitational mass. Thus photons should be deflected by gravity. 
:: Photons are massless.  Their deflection is a gravitational field is due to the frame of reference being accelerated, therby making their path look curved (since for an inertial observer it is not curved).
:They should also be impeded in their escape from a gravity field, leading to the gravitational red shift and the concept of a black hole. It also leads to gravitational lens effects.
:: Acceleration is enough to explain the red shifting of light, as I mention later in the article.  Also, black holes and gravitational lenses are irrelevant to the Equivalence Effect, and are best not mentioned here at all.
I will retain the spelling corrections and added links.
--EMS 03:15, 4 Apr 2005 (UTC)
thread_title:  Reasons for reverting changes 

message: I noticed that there were a number of errors in the last rewrite, particularly that the strong equivalence principle is incorrect (it has never been shown to be violated) and I thought the treatment of the weak equivalence principle was unclear. I was out of town while the update was in User:Ems57fcva's sandbox, and didn't get a chance to point this out.
I tried to revise things and make it clear the difference between the three versions of the equivalence principle and the ways in which they can each be tested. My article is a little lacking in discussing the tests of the Einstein and strong equivalence principles, other than looking for variation of the dimensionless fundamental "constants." In particular it would be good to add something about the limits placed by solar system tests of the strong EP and red-shift tests of the Einstein EP. --Joke137 23:54, 4 Apr 2005 (UTC)
thread_title:  new version 

message: I get the impression that EMS is discussing a spectrum of the volume of space in which the Principle of Equivalence is seen to hold. Weak Principle of Equivalence is then that it only holds in the limit of infinitisimally small volumes of space. (Fortunately, mathematics provides the tools to handle infinitisimals, integrating them back to reality.) A bit stronger assertion of the Principle of equivalence is that it holds in human-sized volumes of space. Formally it doesn't, by reasonable approximation it does. The strongest assertion is that the Principle of Equivalence holds throughout the universe, which is obviously untenable. --Cleon Teunissen | Talk 08:24, 6 Apr 2005 (UTC)
thread_title:  Two independent spectrums of weak vs strong 

message: What I've written below is in line with what User:Cleon_Teunissen has said above, and I think it resolves the misunderstanding. I picked up Ohanian at the library. He states:
: Local experiments can distinguish between a reference frame at rest in a gravitational field and an accelerated reference frame far away from all gravitational fields. Gravitational effects are not equivalent to the effects arising from an observer's acceleration.
He adds in a footnote: 
:It would seem that a perfectly homogeneous gravitational field (zero tidal force) cannot be distinguished from the pseudo-force field of uniform acceleration. This is true, but not very relevant: perfectly homogeneous gravitational fields can only exist under extremely exceptional and unrealistic conditions. It can be shown that uniform fields are only possible in regions (cavities) completely surrounded by a continuous distribution of mass.
Ohnian's statment of the strong EP is:
:All laws of physics [are] the same in a laboratory freely falling in a graviational field, and in another laboratory far away from any field.
And he states, correctly, that it is wrong, because the observer can detect the presence of tidal forces, or, equivalently a curved geometry of space. But he is using the wrong definition of local. The strong EP is restricted to local experiments, where local in this sense means the dimensions of the experiment are much smaller than the curvature radius of spacetime. In Will, section 3.3:
: In any metric theory of gravity, matter and nongravitational field respond only to the spacetime metric g. In principle, however, there could exist other gravitational fields besides the metric, such as scalar fields, vector fields, and so on. If matter does not couple to these fields what can their role in gravitation theory be? Their role must be that of mediating the method by which matter and nongravitational fields generate gravitational fields and produce the metric. Once determined, however, the metric alone interacts with matter as prescribed by EEP.
He goes on:
: Consider a local, freely falling frame in any metric theory of gravity. Let this frame be small enough that inhomogeneities in the external gravitational fields can be neglected throughout its volume. [i.e. a local experiment] However, let the frame be large enough to encompass a system of gravitating matter and its associated gravitational fields. The system could be a star, a black hole, the solar system, or a Cavendish experiment. Call this frame a "quasilocal Lorentz frame". To determine the behavior of the system we must calculate the metric. The computation proceeds in two stagves. First, we determine the external behavior of the metric and gravitational fields, thereby establishing boundary values for the fields generated by the local system, at a boundary of the quasilocal frame "far" from the local system. Second, we solve for the fields generated by the local system. But because the metric is coupled directly or indirectly to the other fields of the theory, its structure and evolution will be influence by those fields, particularly by the boundary values taken on by those fields far from the local system. This will be true even if we work in a coordinate system in which the asymptotic form of g_{\mu\nu} in the boundary region between the local system and the external world is that of the Minkowski metric. Thus, the gravitational environment in which the local gravitating system resides can influence the metric generated by the local system via the boundary values of the auxiliary  fields. [i.e. the vector or scalar fields described in the preceding quotation] Consequently, the results of local gravitational experiments may depend on the location and velocity of the frame relative to the external environment. Of course, local nongravitational experiments are unaffected since the graviational fields they generated are assumed to be negligible, and since those experiments couple only to the metric whose form can always be made locally Minkowskian. Local gravitational experiments might include Cavendish experiments, measurements of the acceleration of massive bodies, studies of the structure of stars and planets, and so on.
Finally, he defines the strong EP:
: (i) WEP is valid for self-gravitating bodies as well as for test bodies (GWEP), (ii) the outcome of any local test experiment is independent of the velocity of the (freely falling) apparatus, and (iii) the outcome of any local test experiment is independent of where and when in the universe it is performed. The distinction between SEP and EEP is the inclusion of bodies with self-gravitational interactions (planets, stars) and of experiments involving gravitational forces (Cavendish experiments, gravimeter measurements).
This is the definition used nearly universally in the literature, as I've demonstrated above. Sorry I had to use such extensive quotation, although I think it qualifies as fair use. --Joke137 18:11, 6 Apr 2005 (UTC)
thread_title:  Ohanian 

message: As I see it, there are three main versions of the equivalence principle in use by physicists. In order of their strength:
* the weak equivalence principle (WEP) or the universality of free fall (UFF)
* the Einstein equivalence principle (EEP) or simply the equivalence principle
* the strong equivalence principle (SEP)
I think the article should be kept as one, even if it gets long, because all the principles are so closely related. It's better to have one really good article than six confusing articles on closely related subjects. However, the WEP is clearly the most important, and I think it is the principle that should be stated prominently in the introduction to the article and discussed first. I would also like to see discussions of tests of the EP.
WEP: I think we agree on what the WEP is, but perhaps not how it ought to be stated. I can see that UFF implies the WEP as presently stated in the article, but I'm a little uncomfortable with how it is stated: to test the WEP you don't even have to know what force, inertial mass or an accelerated frame of reference are. You don't have to know anything about physics, really, you just need to drop different objects and see what happens.
: One of your references included the quote from Einstein that "we will now assume the physical equivalence of the gravitational field and an accelerated frame of reference".  To me, that is the essense of the WEP.  The UFF can be confirmed by observation, but WEP is a very non-obvious concept which is essential to GR.  To leave out references to frames of reference is IMO to lose what WEP is all about.
It should also be stated in the simplest possible form: the gravitational motion of objects depends only on their initial position and velocity, not on their constitution, and give an example, perhaps from Galileo. We should note, that the object must be small (compared to the radius of curvature of the gravitational field), light (does not appreciably perturb the gravitational field), and uncharged (i.e. not interacting through other forces).
: I would like to cover this in the History section.
This is the most important, and probably the most profound form of the EP, because in Newtonian mechanics it seems like a miracle that inertial and gravitational mass are the same.
Incidentally, although it has been said above that gravity is not a force, it depends largely on your definition. In GR, perhaps not, but a particle physicist would certainly call gravity a force: it has a coupling constant and is mediated by the graviton.
:See my response to Cleon - Gravitions are not emitted by static or oscillating masses, whereas photons are emitted by static and oscillating charges.  Gravitons seem to mediate curvature change, not gravitation itself.  
EEP: This is the statement that in addition to the WEP, non-gravitational physics in local Lorentz frames is independent of your velocity and location. People do not often consider theories that depend on your velocity (these are called prior geometric theories, and are not Lorentz invariant) but theories may depend on your position, usually through variation of the fundamental "constants" (i.e. the marginal detection of the variation of the fine-structure constant of electromagnetism) and variation of particle mass ratios.
Schiff's conjecture says that WEP imples EEP. It is not proven, but seems credible. Nonetheless, this equivalence is not obvious, plenty of theories that violate both the EEP and WEP are most easily tested by looking for variations of the fine-structure constant.
SEP: The SEP is the least-often considered principle, because it is used to differentiate between theories of gravity, and most people believe Einstein's theory is the correct one. However, new theories of particle physics have plenty of light scalar fields which, if not stabilized, result in long-range, gravitational-strength forces which would manifest themselves as violations of the SEP or EEP. The unfortunate thing about Ohanian is that he is completely dismissive of these ideas and doesn't seem to see why they're worth bothering with. After all, he correctly points out that the idea of a uniform gravitational field is a mathematical idealization, and doesn't seem to worry that it holds with extraordinary accuracy over most of the universe. The book was written at just the right time, before the discovery of dark energy and dark matter, and before the rise of string theory and its menagerie of friends.
Of course, SEP implies EEP but the converse is false. Brans-Dicke theory has a varying gravitational constant, for example. The SEP is tested by fifth-force experiments (i.e. looking for Yukawa-type deviations from the inverse square law of gravity), by looking for specific deviations in celestial orbits with rangefinding experiments (the Nordtvedt effect) or by looking for cosmological variation in Newton's constant G.
I think the literature is consistent on what the statement of the SEP is, except that Ohanian and Synge disagree about what local means. Since there is considerable interest in this principle, including a number of important and expensive experiments to test it, I would like to focus on the common version, although we should also mention that there has been disagreement about what exactly the principle is.
Joke137 19:27, 7 Apr 2005 (UTC)
replies: text: I think that overall we have a game plan here.  If you are happy to work within a single article, then so am I.  As I see it, we can break it up later if that is needed.  My focus then will be the WEP, along with the intro and the history.  You can take care of the EEP and SEP.  Also, I think I want Ohanian's straw-man to be covered since his point is valid even if his dismissal of the SEP using it is not.  BTW - I think that the EEP is covered by the WEP if the special principle of relativity is assummed.
--EMS 20:00, 7 Apr 2005 (UTC)
thread_title:  My thoughts 

message: I have also written a sandbox version of the 'Principle of Equivalence' article.<BR> 
User:Cleon_Teunissen/sandbox/Equivalence_Principle <BR>
For the time being, I have no intention to replace the wikipedia version of the article with my sandbox version.
The mathematics of a theory is exact science. On the other hand, there is often room for a multiplicity of interpretations of what the theory means. In quantum physics there is agreement on the mathematics, and there are several mutually incompatible interpretations: The Copenhagen interpretation, the Many-worlds interpretation, Quantum decoherence interpretation. Each of these interpretations is self-consistent and consistent with the observations. -
The corpus of observations that a theory is inferred from can be presented in a form that is as interpretation-free as possible. Beyond that I think that the ideal of a canonical interpretation of the theory must be relinquished, much as a wishful ideal like 'absolute space' had to be relinquished. ---Cleon Teunissen | Talk 11:53, 11 Apr 2005 (UTC)
thread_title:  Another sandbox version of the article 

message: I finally have an edit avaialable in user:ems57fcva/sandbox/Equivalence_Principle.  Please feel free to comment.  Hopefully this will be a fairly non-controversial product that can be used as a basis for continued improvement by those interested over time.
(In any case, I see the "disputed" tag as being joke137's to remove.  Hopefully this edit will be adequate for that.)
--EMS 19:29, 11 Apr 2005 (UTC)
thread_title:  Have updated this article in my sandbox 

message: Source: <BR> arxiv.org/abs/physics/0204044<BR>
Physics, abstract physics/0204044<BR>
Einstein's gravitational field<BR>
Authors: Peter M. Brown<BR>
Subj-class: General Physics<BR>
There exists some confusion, as evidenced in the literature, regarding the nature of the gravitational field in Einstein's General Theory of Relativity. It is argued here the this confusion is a result of a change in interpretation of the gravitational field. Einstein identified the existence of gravity with the inertial motion of accelerating bodies (i.e. bodies in free-fall) whereas contemporary physicists identify the existence of gravity with space-time curvature (i.e. tidal forces). The interpretation of gravity as a curvature in space-time is an interpretation Einstein did not agree with. 
[quote from the article]<BR>
Another example of a gravitational field with zero space-time curvature is that
of a vacuum domain wall. A domain wall is two a dimensional structure
characterized by a fixed energy per unit area \sigma. A surface tension equal to \sigma (in appropriate units) is required in order for \sigma to remain constant.<BR>
[end quote]
Cleon Teunissen | Talk 07:05, 12 Apr 2005 (UTC)
thread_title:  The crucial importance of tidal effects 

message: I took the liberty of replacing the article with the version from User:Ems57fcva's sandbox, and edited it, merging in changes from my old version in the hope that we can reach some sort of consensus. I am pretty happy with this version, although I believe it could still do with a lot of work. Feel free to update it and/or remove the disputed tag. I hope we can accept this version as at least the "working draft" from which to make changes from.
I've been quite busy at work these past few days, and unfortunately haven't had much time to visit the talk page, but I wanted to put this version on the main page to get the ball rolling. --Joke137 21:48, 12 Apr 2005 (UTC)
replies: text: The Einstein Equivalence Principle then states: (i) WEP is valid, (ii) the outcome of any local nongravitational test experiment is independent of the velocity of the (freely falling) apparatus, and (iii) the outcome of any local nongravitational test experiment is independent of where and when in the universe it is performed.
It looks like you agree with the first two parts, but disagree with the third. The second condition is really a very weak condition, because it is satisfied by any Lorentz invariant theory of gravity. The review on variation of the constants by Uzan uses the same definition (p.4).
The problem with measuring the fundamental constants, like the speed of light, is you can never really be sure what it means for such a constant to vary except as expressed by dimensionless quantities. This is because we could simply define out units differently at each point in space time, and claim that the fundamental "constants" were varying. The numerical values of dimensionful fundamental constants are more a matter of convention than anything else, and it is the dimensionless constants in which real physical changes will show up (e.g. the fine structure constant is the perturbation parameter in quantum electrodynamics, and reducing it will affect different processes, such as Thomson scattering). Particle physicists often avoid this issue entirely with Planck units.
The strong equivalence principle measures variation of the (dimensionful) gravitational constant. What is really being looked for is the change in the ratio of particle masses to the Planck mass. --Joke137 19:12, 15 Apr 2005 (UTC)
thread_title:  Replaced article, again 

message: I have a copy of Clifford Will's "Was Einstein Right?" lying around, and started looking at it yesterday.  On of the chapters is called "the straight road to curved spacetime", and it does a very good job of straightenning out the variations of the Equivalence Principle.
The first thing it did was to confirm something that I have gotten more and more suspiscious of:  The EEP and SEP are in fact two names for the same thing.
Secondly, it gave a coherent (for me) definition of the SEP:  The underlying laws of physics when written in covaraint form are the same in all inertial frames of reference.  ("covariant form" refers to a type of tensor notation.)  This is a saner statement that the "all experiments ..." one, which is obviously bogus:  Not only can tidal effects be detected in curved spacetimes, but any experiment involving measurements of the cosmic microwave background will have results that vary based not only by where and when in the universe you are but also by on how you are moving with respect to other observers!
Finally, there is something that I have encountered elsewhere called Schiff's Conjecture.  The gist of Schiff's Conjecture is that the WEP implies the SEP.
I will worry about editting this page a little later, but wanted this much to be known.
--EMS 18:30, 18 Apr 2005 (UTC)
thread_title:  "The straight road to curved spacetime" 

message: In his 1907 and 1911 papers, Einstein does not discuss kinematics. Instead Einstein offers calculations based on the assumption of complete validity of the equivalence principle. In his 1907 and 1911 papers, Einstein discusses the influence of gravitation on the propagation of light. Einstein assumes from the outset that it is all about the rate of time. 
In his 1913 paper together with Grossmann, Einstein offers the following reasoning: in radio-active decay binding energy is released. The inertial mass of the nucleus of an atom is not just the addition of the inertial masses of the constituting particles, this binding energy contributes to inertial mass. Remarkably, the binding energy contributes equally to gravitational mass. That suggests a profound equivalence in the physical natures of acceleration and gravitation. Hence Einstein's wording:
:[...}we shall therefore assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system. 
External link: The genesis of general relativity<BR>
--Cleon Teunissen | Talk 08:08, 23 Apr 2005 (UTC)
thread_title:  Einstein did not propose the Weak Equivalence Principle 

message: :all the fine print is hidden in the word local, which implies that the experiment is not trying to measure anything external (such as the CMB, which sets a preferred frame for the universe)  --Joke137 01:30, 19 Apr 2005 (UTC)
thread_title:  Hidden fine print 

message: I have decided to insert a version of the equivalence principle article. The   main goal is to have an article that is in conformance with the wikipedia article on gravity. A lot of what I write is derived from that article. The basis of the equivalence principle is Einstein's 1907 vision that gravitational interaction is mediated by changing the rate of time, by curving space-time geometry. Then and only then can the effects of acceleration cancel the effects of gravitation. Everything else follows from that. Every interaction has a mediator. In the case of gravitation curvature of space-time is that mediator. --Cleon Teunissen | Talk 16:38, 25 Apr 2005 (UTC)
thread_title:  Rewrite of the article