Physics has all but surrendered to mathematics in the last hundred years. I believe this has been detrimental, and nowhere more than in gravitation theory. The general theory of relativity was conceptual in origin, mathematical in its corroboration. The theory has represented gravitation as a product of the “curvature” or deformation of spacetime in the presence of mass, and both the evidence and the supportive mathematics have been entirely adequate to justify its acceptance. Gravitation is nevertheless described in terms of the mathematics of quantum theory as a force and associated with a hypothetical particle, without either an explicit dissension from the geometric conception or empirical evidence of the particle.
Conceptual physics, which I take to be roughly coextensive with pre-quantum physics, involved the initial development of coherent hypotheses and secondarily the employment of mathematics (and/or experiments) to support their plausibility. A mathematical formalism without conceptual coherence would be regarded as irremediably provisional, if not unsatisfactory, in the former methodology. With respect to the former physics, two thought-experiments will be employed here, without resort to mathematics, to demonstrate that the quantum interpretation of gravitation is conceptually flawed and without empirical support.
A description of the first experiment may be unnecessary, but the pre-relativistic association of gravitation with inertia and of inertia with universal mass is still maintained on occasion, if only tacitly, and may be the ultimate basis of the continued identification of gravitation with force. The identification may also be a residue of one of our most familiar experiences on the earth’s surface: The pressure we feel between ourselves and the surface is fundamental to our original concept of gravitation; we tend to regard the pressure as a force (“the force of gravity”) and our surface station as being at rest. The following experiment may therefore be helpful toward more clearly dispelling the identification of gravitation with force and inertia, and also in prefacing the second experiment (actually a thought-investigation) of the force-free continuity between astronomical gravitation and gravitation at the surface of a massive body:
Imagine a spacecraft traveling a uniform path relative to the “fixed stars” which comes under the influence of a stellar object nearby and
begins to deviate toward it, while continuing in uniform motion by the evidence of free-floating objects inside. In order to maintain the
original course a thruster is fired and inertial effects are experienced onboard as the craft accelerates just enough to counter the influence
of the local gravitational field.
Note that in this experiment inertial effects are associated with uniform motion relative to the distant stars, contrary to the pre-relativistic Machian expectation. Aside from the discrimination of inertia from any influence of the overall mass of the universe (an association that is seldom explicitly defended now anyway), the experiment demonstrates what I hold to be most significant, that at least in the situation just described, force becomes evident in conjunction with gravitation only when gravitation is being resisted.*
Now consider an experiment that comprehends the transition from astronomical gravitation along a geodesic to an involvement with force and inertia at the surface of a massive body:
Imagine two test bodies gravitating toward the earth from some considerable distance. For the sake of simplicity, consider the earth to be at
rest and the test bodies to be gravitating directly toward its center of mass. (They appear to be simply “falling” from a perspective on the
earth’s surface.) One body is an immense hollow sphere of negligible mass, the other is relatively small in size — an extra-vehicular scientist,
let’s say — and also of negligible mass. Notice that while the test bodies are falling toward the earth (or more accurately, while the three
bodies are converging) there is among them a purely relative transformation of potential energy to kinetic energy as each moves uniformly in
its own frame of reference — there is, at least as yet, no occasion for an exchange of mass-energy in the form of the supposed gravitational
energy. Let the sphere and the scientist be placed initially close together so that as they approach the earth their geodesics converge enough
to bring their surfaces in contact some time before the larger impact. (It is the fantastic size of the hollow sphere that allows the surfaces of
the two bodies to meet somewhere above the earth’s surface). From the moment the sphere and the scientist come in contact until they reach
the surface of the earth an inertial acceleration between them will intensify as each tries to conform to its own geodesic at an ever greater
angle to the normal. The situation will, if viewed in isolation, come to resemble the gravitation of a small body pressing against a planetary
surface (although the gravitation between them is actually insignificant due to their negligible masses) and the scientist will even be able to
stand upon the sphere. This development of an increasing inertial acceleration between the test bodies is the only aspect of the situation
which changes from the moment they meet; the earthward component of their motion continues as before, a relative gravitation. In a way
similar to the first experiment, force has developed in the resistance to what is in this case a convergent gravitation of two bodies toward
another. And once the two reach the earth the situation remains essentially the same: Each one, now in conjunction with the entire
conglomerate of the earth, presses toward the center of mass with the same sort of conflict of geodesics as was observed between them
when they were gravitating from a distance. Along with the other components of the earth at and below the surface, they are resisted, and
thereby accelerated, by those further below, due to the coincidence of the common inclination toward the center of mass and the
subordinate obstructions.
This second experiment demonstrates that it is only in the inertial conflict of geodesics (or as in the first experiment, in a singular inertial acceleration) that force can be observed in association with gravitational phenomena. The intersection of geodesics and the consequent inertial effects constitute the interruption of gravitation, and what is commonly conceived as “the force of gravity” at a surface would be more accurately described as anti-gravitation.
Gravity has to be considered absolute in the aspect that a geometric vertex exists at a center of mass that cannot be transformed, either conceptually or mathematically. But unless the geodesic of a body brings it to a massive obstruction, such as the surface of a planetary body, gravitation involves uniform motion with only relative accelerations — no force can be attributed.
There remains a most significant aspect of the situation disclosed in the second experiment to be comprehended, although its full implications must be left outside the scope of this discussion. The energy expressed in the continuous static acceleration of bodies at or below a surface toward a center of mass is rendered inexplicable in purely geometric terms when gravitation is finally discriminated from force. If there is no “force of gravity”, what accounts for the persistent energy of the inertial acceleration at a surface after a body has come to a relative state of rest? Recall that in the initial appearance of force in the second experiment only a conflict of geodesics is present and resistant against the otherwise uniform motion of the test bodies. No extrinsic source of energy can be identified, yet there is a static acceleration between the two, while the gravitation with the earth remains relative. I believe the only available explanation is that motion as-such, the motion of matter in general, must be regarded as absolute, although relative in the various incidental trajectories between individual bodies. I wish to maintain that the source of the energy usually identified as gravitational energy must be attributed to an intrinsic and ceaseless dynamic of mass-energy, independent of gravitation but uniquely revealed by its coincidence.
Having briefly acknowledged the implications of a consistent geometric theory of gravitation, that gravitation and motion in general are each in their own way both relative and absolute, that mass-energy is somehow intrinsically dynamic and the source of the energy disclosed in the opposition of gravitation and its occasional resistance, I will consolidate the findings with regard to quantum theory in the following summation:
Gravitation is evidently a deformation of spacetime in the presence of mass, its effects the product of the concentration of spacetime at vertices, at centers of mass. As such, gravitation cannot be a force, and cannot therefore be mediated by a particle. The assimilation of gravitation by quantum theory and its derivatives (e.g., string theory) as a field of force, and the positing of a gravitational quantum of action where none is apparent, theoretically necessary, or conceptually coherent is entirely without justification.
This is admittedly an unsettling proposition, but in consolation its acceptance would make one of the principal projects of quantum theory less complicated, as gravitation with all its peculiarities could be disregarded in the pursuit of a unified field theory. I hope that it might also signal the need to rely more upon conceptualization, and not so heavily on mathematical formalisms, in the development of physical hypotheses.
Endnote
* Incidentally, there may also be evidence of force if the gradient of a gravitational field is severe enough to produce tidal stresses to a body’s molecular binding energies.
Jim,
We are indeed going in circles here. I see no point in continuing. This is not personal, just a recognition that my part of discussion has ended.
The same is true of my earlier discussion of gravitational waves. Tidal phenomena are not related to gravitational waves but to gravity itself. A gravitational wave is a disturbance in spacetime that propagates (presumably at the speed of light) from a change in the distribution of mass/energy.
I haven’t done the calculations, nor it seems has the anonymous contributor, but from my reading about the LIGO and LISA projects, it seems apparent that the Moon’s motion produces a much weaker gravitational wave, despite its proximity, than the merging of two supermassive black holes at galactic distances.
I have nothing more to say about this as well, since my knowledge is limited to the links noted earlier, but I think you’re stuck in a misunderstanding of what a gravitational wave is.
David has been silent. Perhaps he agrees with me that you seem stuck in your misconceptions and is too kind to say so. Anyway, this is not personal. Just a recognition that we have reached the end of the road for any productive interchange.
Sorry.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Anonymous,
You wrote: “The gravitational waves produced by the Moon’s motion are too weak to detect. To test the theory, we need to look for ripples in spacetime caused by humongous events like the collisions of supermassive black holes.â€
If you consider the energy expressed in the motion of the ocean tides, the moon’s gravitational wave, though weak compared to galactic events, is LOCAL, and consequently, obviously, extremely powerful in the earth’s vicinity.
It may not be exotic, it may be disappointing, but the gravity wave of the moon renders untenable the idea of energy-bearing gravitational waves, and ultimately, the association of gravitation with inertial acceleration.
Jim
Jim wrote, and I reply in my only message for today.
Nope. It applies to the whole thought experiment. Nowhere does Einstein leave the realm of uniform acceleration/gravity.
The elevator is irrelevant to the convergence. In a free-falling elevator in a resistance-free shaft, everything seems weightless. Whether objects converge slowly without falling down, or fall downward and converge slightly has no relevance here.
[Terminology note: I use the terms up/down to correspond with the direction of the apparent gravity. Thus objects gravitate (fall) downward.]
Einstein envisions a closed chest in which you make measurements. One way to interpret the measurements is that you are in an inertial frame of reference (i.e. in uniform motion with respect to all other inertial frames) with a uniform gravitational field.
Another way to interpret the measurements is that you are in a gravity-free accelerated frame of reference. Both interpretations are equally valid, as is any other combination of acceleration and gravity. (You could be in a frame accelerating upward at g/2 and gravity downward would be g/2, producing a net apparent gravity of g downward.)
The point of the thought experiment is that no measurement can distinguish between being in an accelerated frame of reference and a uniform gravitational field. The uniformity is critical to his analysis and Einstein’s explanation of the principle of equivalence.
I was afraid you meant that, because we have now gone full circle to the previous discussion, where you described gravitational fields as roughly “concentric,” and I pointed out that seems to reflect an inability to give up the idea of a privileged frame of reference.
To address that one more time, I’ll ask this question: How do you define “local”?
For instance, I suspect you are defining our “local center of mass” as the center of the Earth, and including in that “local region” everything within Earth’s atmosphere (or some further distance, but not as far as the Moon).
From a person’s perspective on Earth, the geodesic points roughly (but not exactly) toward the Earth’s center. That person’s frame of reference is not inertial. It is rotating with Earth, accelerated toward the Sun as Earth orbits the Sun, and accelerated toward the galactic center as the Sun orbits that center. (We could go further to include the local cluster or the super-cluster of galaxies that includes our local cluster.)
We call the direction of our local geodesic “down,” but in an inertial frame–let’s choose the frame of the galactic center as the closest nearby approximation of an inertial frame, that direction is constantly changing as Earth rotates and moves along its path through the galaxy.
So I am back to where I was in our previous discussion, thinking that you have either a fundamental misunderstanding of frames of reference or an inability to break away from the idea of a privileged frame of reference. If it isn’t that, then we are having major semantic difficulties.
In any case, that’s why I viewed the situation as “hopeless” when I ended the previous discussion. We are now going around in circles and even arguing about the centers of those circles. Don’t you agree it is time to bring this to an end?
If David Halliday is reading this, perhaps he can see your comments in a different light than I do. He may have taught students who get entangled in incorrect assumptions that they do not see and helped them find their way out.
That is, in fact, my view of your situation. I wish I could help you, but I can’t.
Helplessly yours,
Fred
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
I should really sleep on a comment before pressing the button.
“Gravitation” is fine as the verb, “a field of gravity” or “a gravitational field” is the noun.
Fred, your quote from Einstein only applies to the one aspect of the thought experiment, where he wanted to set up an inertial acceleration independent of gravitation. The other aspect, where the observer is standing in an elevator in a gravitational field is obviously in – a gravitational field. That’s where two objects will converge if dropped.
You ask “The center of mass of what system?”
The center of mass of the local system. The gravitational field of the earth is far more powerful at the surface than any other field or combination of fields. The universe is made up of a virtually infinite centers of mass, and combined centers of mass, but we can certainly abstract from vanishing influences to illustrate a principle. The difficulty here isn’t mine.
Jim
Jim asks: “And given that the gravitational field of our moon moves across the earth in a wave, and provokes massive kinetic energies, why isn’t it a prime object for study, rather than some distant black hole or binary system?”
The gravitational waves produced by the Moon’s motion are too weak to detect. To test the theory, we need to look for ripples in spacetime caused by humongous events like the collisions of supermassive black holes.
Before I disappear for today, you also write;
“It [gravity] causes the geodesics of bodies moving freely to curve toward the center of mass.”
The center of mass of what system?
If the problem isn’t semantics, it may lie in your difficulty in accepting the idea that there is no “center” of the universe except in the sense that any object is in the center of its own frame of reference and its own observable universe.
Now I’m off for the day.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Jim states: “I’m not aware of Einstein making that qualification.”
He did indeed. I cited it in our previous discussion in Don Hamilton’s thread. Don had posted a chapter of Einstein’s writings from bartleby.com on the principle of equivalence.
Einstein’s description of the thought experiment begins thus:
Whether or not our disagreement derives from semantics, I need to opt out of this discussion long enough to do a good day’s work. Perhaps there is enough meat here for David to continue.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Fred wrote: “The gravitational field (whether you characterize it as a force field akin to electromagnetism or as a geometric effect) is still present when you ‘interrupt’ the motion of an object that would otherwise follow a geodesic.â€
Yes, I admit there’s room for misinterpretation, but I’m using “gravitation†in this context as a verb. “Gravitation” the noun is a distortion or curvature of spacetime; “gravitation” the verb describes the motion of a body as affected by a gravitational distortion or curvature. I’m not sure a perfectly unproblematic statement distinguishing the latter from the former can be constructed, but an interruption of the gravitational field would, I agree, be nonsense. Maybe as a convention, “gravity†should be reserved for the noun, “gravitation†for the verb.
Regarding the Principle of Equivalence: “The presence of a massive body like Earth creates a nonuniform gravitational field and thus violates the conditions of the thought experiment.â€
I’m not aware of Einstein making that qualification. It’s reminiscent of the later “weak principle†of equivalence, where it’s said that in a sufficiently small region of spacetime gravitational distortions can be ignored for many experimental purposes. The interpretation you’re describing says, in effect, there’s an equivalence between gravitation and inertial acceleration if the curvature of the gravitational field is undetectable. Drawing again on a classic aphorism, it’s alot like saying all cows are black if the night is sufficiently dark.
“In other words, we may not be disagreeing about anything except vocabulary.
Unfortunately, if that’s the case, there’s nothing novel in your discussion.â€
Even if a distinction between gravitation (the verb) being resisted by the surface of a massive body and a mechanical or force-induced inertial acceleration can be rendered undetectable in the special case of a hypothetical uniform, or flat gravitational field (is it physically possible for a “uniform field†to be intense enough for an observer to stand in it?), there remains the distinction in the typical case of a non-uniform field, and also between free gravitation (the verb) and inertial acceleration. Free gravitation (the verb) is equivalent to motion along a geodesic, inertial acceleration is not. And the transition from free gravitation (the verb) to obstructed (“staticâ€) gravitation (the verb) at the surface of a massive body can’t be comprehended consistently except as a circumstance where gravitation (the verb) is being resisted, with the resistance identifiable as the source of the force involved.
“By the way, I haven’t been tracking the attempts to detect gravitational waves…. And to be detected, they would have to carry energy.â€
According to geometrodynamics, gravity (the noun) is a distortion of spacetime in the vicinity of mass. It causes the geodesics of bodies moving freely to curve toward the center of mass. The said free-moving bodies continue to move uniformly along force-free geodesic paths. Where is the energy? And given that the gravitational field of our moon moves across the earth in a wave, and provokes massive kinetic energies, why isn’t it a prime object for study, rather than some distant black hole or binary system?
I believe there’s more than semantics involved here.
Jim
Fred wrote: “Nowhere does Einstein leave the realm of uniform acceleration/gravity.†But Fred, you’re referring to Einstein on the equivalence of inertial and gravitational mass, an equivalence that presumes an association of gravitation with inertial acceleration – the issue I’m contesting. The original, fundamental principle as formulated by Einstein in 1907 referred to the equivalence “of a gravitational field and the corresponding [inertial] acceleration of [a] reference frame” (Einstein, A. (1907), “ueber das Relativitatsprinzip und die aus demselben gezogenen Folgerungen”, Jahrb. Radioakt. Elektr. 4:411-462, trans, pp134-5, Relativity and Geometry, Torretti, R., New York: Pergamon Press, 1983)
I hesitate to follow Fred on a derivative issue, but it seems necessary in order for me to assert that it’s derivative. It doesn’t matter if you agree – my point here is that IF gravitation and inertial acceleration are entirely distinct, the equivalence of “gravitational mass†and inertial mass is tautological: If a body is moving freely, and is observed to accelerate (gravitate) from another coordinate system, the “force†said to be acting upon it to produce the acceleration (ignoring geometrodynamics and the actual geodesic motion which makes the acceleration relative and force-free) will necessarily be calculated as equivalent to its inertial mass. If a body is “at rest†at the surface of a massive body, the inertial acceleration necessary to prevent it from moving toward the center of mass (more on that below) will necessarily be equivalent to its inertial mass. Thus, the reason “gravitational mass†is equivalent to inertial mass according to a consistent geometrodynamics is that both are actually measures of inertial mass.
In a sense it doesn’t matter whether Einstein’s original statement of the Equivalence Principle involved a comparison of a gravitational field (non-uniform, non-zero curvature) with an inertial acceleration. In my attempt to demonstrate that gravitation and inertial acceleration are distinct, and that the two situations alleged to be equivalent (at “rest†in a “static†gravitational field vs. being accelerated out in space) are not in fact equivalent, I pointed out that their non-equivalence is proven by the fact that in the former situation, two objects will converge if dropped by the observer, and they will converge according to the coordinate system of any observer of the observer dropping the objects. The point stands on its own, regardless of anyone’s statement or modification of the equivalence principle.
Regarding the issue of gravitational fields being, as I said, “roughly concentric†and having a center of mass, Fred wrote that it “seems to reflect an inability to give up the idea of a privileged frame of reference.â€
I’m sorry, that’s incorrect. I have referred to a body’s involvement with inertial acceleration at the surface of massive body (e.g., the earth) as the interruption of that body’s (non-inertial) gravitational acceleration toward the earth’s center of mass. The gravitational center of mass of a body at a massive surface can be determined by a plumb bob. A plumb bob held by any observer at a gravitational surface will point to that observer’s gravitational center of mass, and its orientation will be agreed upon from any reference frame. Similarly, for any body in orbit, the center of mass determining its orbit can be calculated by means of the field equations. You seem to be referring to a situation where a body is moving freely and not orbiting another, or at least not orbiting anything in the region being considered. That’s not the situation I was describing.
I googled “detection of gravitational waves” and found this “Physical Review Focus” story.
The apparent detection of gravitational waves I remembered has been discredited. However, additional googling pointed out that a detector called LISA will be searching for the gravitational waves resulting from the merger of Supermassive black holes.
In other words, finding the evidence to support, overturn, or modify prevailing theory is not an easy process, but the observations are underway or under development.
The absence of evidence is not evidence of absence in any case; but in this case, we may not yet have sensitive enough detectors.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Jim,
Thanks for clarifying what you mean by “interrupted.” I am now convinced we are arguing over semantics and not physics.
The reason I have trouble with your nonstandard terminology of “interrupted gravitation” is this:
The gravitational field (whether you characterize it as a force field akin to electromagnetism or as a geometric effect) is still present when you “interrupt” the motion of an object that would otherwise follow a geodesic.
The physics works out fine whether you consider gravity a force or a geometric effect, although the language you use to describe it may be different.
When saying that being in a uniform gravitational field is equivalent to being in an accelerated frame of reference, Einstein may have been saying the same thing that you are saying about gravity not being a force.
The presence of a massive body like Earth creates a nonuniform gravitational field and thus violates the conditions of the thought experiment.
In other words, we may not be disagreeing about anything except vocabulary.
Unfortunately, if that’s the case, there’s nothing novel in your discussion.
By the way, I haven’t been tracking the attempts to detect gravitational waves, but I seem to remember that they have indeed been detected and more sensitive detectors are being built as a result.
And to be detected, they would have to carry energy.
If anyone reading this has been following that work more carefully, perhaps they can chime in with a link to the relevant observational evidence.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Jim,
I added my perspective on the previous discussion because I thought it might help your conversation with David. Your response may have done that. Here’s some additional commentary that may continue the process.
Note that I am not responding to your physics, but to semantics. I’m not arguing semantics, because that is, as David notes, counterproductive. I’m just trying to clarify where there seem to be semantic differences so the discussion can move forward.
If people use different terminology, they often talk past one another.
The first area is “frame of reference.” As for the “astonishment” (my characterization of your response–and that may be too loaded a word), I’ll quote what you wrote. That may clarify why I think you are having trouble with that term.
You wrote:
I’ll leave it to David to try to explain why your disbelief (as implied by “Otherwise… could… be said to”) suggests a problem in understanding the standard definition of “frame of reference.”
We could go round and round on your use of “interrupted,” because I just don’t understand what you mean by that.
Perhaps David can also help you with the body on the table example. In this case, there is explicitly another body in the discussion, namely Earth. The conditions of Einstein’s thought experiment would eliminate Earth, so the discussion would be different.
I just chose that example to explain why I think you are coming up with the idea of interrupting a force. Again, the problems I see with this are in the language. No standard physics literature that I am familiar with uses that terminology or has a need for it.
Perhaps David can figure out what you mean by the term. Your attempts to explain it to me have not succeeded.
Thus I remain frustrated by an inability to speak a common language with you. For that reason, it is better that you and David pick things up from here. By stating where I am having difficulty communicating with you, I may be helping others who are trying to read along as well.
That’s why I prefer that you address David’s comments and leave mine as a sidelight that may illuminate the main thread.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Fred,
You asked me to not respond to you, then you wrote again, so I assume I’m ‘within bounds’ in taking this liberty….
You wrote: “here’s where I think there may be semantic differences and areas where I think Jim’s understanding is incomplete…. Frame of reference — I am not sure Jim has yet grasped the idea that an object can’t move in its own frame of reference. If so, he probably isn’t grasping relativity as well as he thinks.â€
I think this is a question of both semantics and misinterpretation. I wrote in another blog [http://www.scienceblog.com/cms/black-hole-myth-14427.html#comments] “a body moving freely in a gravitational field is always moving uniformly from its own frame of referenceâ€, not that “a body can move in its own frame of reference.†The former statement just means that if an observer considers himself to be moving (not at rest), which of course he is as justified in doing as he is to regard himself as not-moving, then his motion is uniform (along a geodesic) and without evidence of force. This was in context of a response to your contention that an (inertially) accelerated frame of reference is a state of rest.
Here’s where I think we agree and disagree:
You wrote “The equivalence [principle] is between two interpretations of a measurement: an accelerated frame of reference or the presence of gravity yield the same result, and there is no way to distinguish between them.â€
Agreed, it is (in part) about measurement, but as I pointed out, there is an identifiable difference of measurement, and moreover, the equivalence of measurement is intended to indicate an equivalence of gravitational acceleration with inertial acceleration. The non-equivalence of measurement occurs when, in the non-gravitationally-accelerated elevator, the observer drops two objects, they fall in parallel. When in the elevator located in a ‘static gravitational field’ an observer drops two objects, they converge toward the center of mass.
The conflation of an equivalence of measurement and a physical equivalence can be identified in your statement (again, in the other blog): “[an accelerated] observation can be interpreted in either of two equivalent ways. You are in a gravitational field pointing the backwards direction, or you and your frame of reference are accelerating forward.†I agree, and without the “astonishment†you’ve attributed to me, that the measure of the acceleration is equivalent either way (although I can’t think of any practical utility to reversing the measurement), but to interpret, by inference, a physical equivalence of gravitation and inertial acceleration would be a mis-interpretation. Do you agree?
“Obstructed force — Jim uses this term and tries to define it, but I just can’t make sense of his language.â€
I haven’t actually used the term “obstructed force†– I’ve asserted that gravitation is only associated with force when it is obstructed by a force. So when you say: “People often think that an object sitting on a table on Earth is at rest because its action force, gravity, is balanced by a reaction force called the normal force†you’re clearly referring to gravitation as a “forceâ€, and if you mean to do so, we disagree. I’ve already discussed, in other comments, various cases where gravitation is either force-free or interrupted and obstructed. If you or anyone can describe a situation where gravitation acts as a force (in Einstein’s geometrodynamic paradigm) I’d be very interested – and “astonished†– to see it.
Jim
David,
You wrote: “agreement provide points around which additional areas of agreement may form. :-)â€
Yes – I love to agree!
“So the point of departure is exclusively in the interpretations of GR, not in the fundamentals of the theory or its predictions. Is this correct?â€
Yes, with the exception of what I consider derivative issues, the association of gravitation with force and predictions of energy-bearing grav waves, I don’t think there’s anything of practical significance that I disagree with.
“The hard part, of course, is identifying the concept[s] that need modification. (Ultimately, of course, one or more falsifiable/testable claims/predictions need to be madeâ€
Please reconsider my previous points, where I claimed that gravitation can be distinguished from inertial acceleration and force, from any coordinate system. I believe they are falsifiable. Do you agree?
Jim
David and Jim,
To move the discussion along, here’s where I think there may be semantic differences and areas where I think Jim’s understanding is incomplete.
Frame of reference — I am not sure Jim has yet grasped the idea that an object can’t move in its own frame of reference. If so, he probably isn’t grasping relativity as well as he thinks.
Obstructed force — Jim uses this term and tries to define it, but I just can’t make sense of his language. I think there he may have a basic misunderstanding of Newton’s Third Law, which I often explain thus: “Forces always exist in pairs.”
The term action-reaction often confuses people into thinking one member of the pair causes the other. Neither is the cause and neither is the effect. They both exist in mutuality. People often think that an object sitting on a table on Earth is at rest because its action force, gravity, is balanced by a reaction force called the normal force.
In fact, the force paired with the gravitational force on the object is the gravitational force it exerts on Earth, and the force paired with the normal force the table exerts on the object is the normal force the object exerts on the table.
I’m sure David understands this perfectly, but I have the sense that Jim does not, and that may be the source of his use of the term “obstructed force.”
I hope this helps you both move in a productive direction.
P.S. for Jim: You’re right that I am too busy to get into a long discussion on this. But I hope I can help move this along by reflecting on our earlier interaction — before it reached the point of mutual frustration.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Jim:
To begin, I’m very glad we can agree on this:
I think that finding points of agreement can be very important in a discussion: It allows for the discussion to move on, so effort doesn’t have to be continually expended hashing such things out.
In addition, and perhaps more importantly, such agreement provide points around which additional areas of agreement may form. :-)
I’m also very glad you have explicitly expressed where you believe you and others diverge:
So the point of departure is exclusively in the interpretations of GR, not in the fundamentals of the theory or its predictions. Is this correct?
This is consistent with your original post, though it expands the issue slightly. (Although there were certainly ample clues in some of your comments, the problem was that the additional comments also left open the possibility that you actually disagreed with more fundamental aspects and predictions of GR. Just as I believe that finding points of agreement is very important and helpful in a discussion, so, I believe, is narrowing the focus on the actual points of departure.)
On another point of agreement (though I certainly cannot speak for [or against] Dr. Bortz’s position, thoughts, motivations, etc.):
As I say, I agree that there must be advance (and, historically, in physics at least, it has had its greatest advances in “scientific revolutions” [“overturning of well-established theories”], which, typically, involve leaving behind some “well-established” [often “well loved”] assumption[s] of previous theories). The hard part, of course, is identifying the concept[s] that need modification. (Ultimately, of course, one or more falsifiable/testable claims/predictions need to be made—like unto Dr. Bortz’s challenge before any such divergent theory can be accepted as scientific, at least in the sense of providing a better match with reality.)
I’ll go on to more of the “gray” areas a little later. There are a few points where either you’re or I’m having a little trouble determining the boundaries between agreement and divergence. In fact, the issues may simply be ones of semantics, but such are often the cause of the worst nonproductive arguments.
‘Till then… Take care.
David
This comment is not directed to Fred, per his request, although it addresses some of his most recent comments. In an important sense it’s not a personal issue anyway. The “force†of gravity is one of our most primal experiences. When we first asked a parent what it is that presses us against the earth, it was the “force of gravity.†And the experience identified as gravity has always FELT like a force, before and after we ever took up a physics book. So it’s not surprising that it’s extremely difficult to distance oneself from that original association, and to consider gravity instead as a non-force, even for physicists devoted to studying the field.
Accordingly, it was pointed out by a previous commenter that he’d never heard the phrase “interrupting a force†in “standard physics literature.†He hasn’t heard it from me either. I’ve referred to “interrupted gravitationâ€, but my basic point has been that gravitation is not a force. Even Einstein, who discovered what’s come to be known as geometrodynamics (Wheeler), never seemed to fully and consistently divest himself of the association of gravitation with force. So it’s not surprising that the idea of “interrupted gravitation†might be novel. My point is that it should finally be recognized, and familiar.
There was a reference in the previous comment to a “body on the table.†I recently watched “Rising Sun†again, on DVD, but other than the one scene in the movie, I’m drawing a blank. (Sorry, but it’s true.) I suspect the objection is to the elevator thought experiment I cited, and my contention that an observer dropping two objects in each of the two situations would measure their fall differently. If so, I do need help in understanding my error.
Regarding the comment that my “disbelief [of reversing frames of reference] suggests a problem in understanding…â€: I can explain why I have a criticism of the idea (not a “disbeliefâ€) that the universe can be said to be accelerating in the opposite direction of a body that is otherwise the one considered to be accelerating. The equivalence is entirely mathematical, valid in that sense and that sense only, and should have no bearing on one’s concept of the relationship of gravitation to inertial acceleration. To consider acceleration in one direction as gravitation in the other is essentially Machian, and in any case it’s untenable unless gravitation is identified as a force.
“Moving the discussion along” means to me finally addressing my original point, that gravitation cannot legitimately be considered a force. I’m unable to imagine a situation where it appears force-like, except when a body’s geodesic motion (with a non-zero Reimann Tensor curvature if you like) is interrupted by a force like the resistance of the surface of a massive body.
Jim
David,
You wrote: “It appears we can both agree that non-geodesic motion implies a force besides gravity is acting on a body. It also appears that you would agree that geodesic motion with a non-zero Riemann curvature tensor implies that the body is moving under the influence of gravity. (Am I right?)â€
Yes.
“So the crux of this matter, then, is whether you would consider that geodesic motion when the Riemann curvature tensor is zero implies that the body is moving under the influence of gravity, or whether this implies the absence of gravity….â€
I think it may be more useful in general to treat the tensor as approaching, but never actually equaling, zero (anywhere), so that geodesic motion is always under the influence of gravity, but I’m not sure what the relevance of the question to our prior discussion might be.
“On another matter (of not especially great import, but has an affect on people’s perception of whether you have a full understanding)… You say that ‘it wasn’t until GR was developed that a space-time geodesic was conceived’…. In a sense you are right, in that before the concept of curved space(-time) manifolds developed (by Riemann, or maybe before—certainly before GR)….“
You’re probably thinking of Gauss. Yes, I understand, and in any case I think my point stands on its own, referring as I did to spacetime, not the pre-relativistic concept of space.
“…one should have a consistent description regardless of the complexities of a given situation. One should not be dependent upon a simple subset of circumstances (like an isolated, compact gravitating mass). This is why I refer to more general concepts, like geodesic motion and whether the Riemann curvature tensor is non-zero in the vicinity of the test body or experiment….â€
I don’t understand your point here. Have I provided an inconsistent description? I’ve shown by a simple experiment that the effect of a non-zero Riemann curvature allows us to discriminate, from any coordinate system, between a purely inertial acceleration and an obstructed geodesic. I don’t see how complicating the experiment can rescue their supposed equivalence, or render their equivalence/non-equivalence relative or arbitrary.
“To move on to another (perhaps somewhat minor) issue… The question, I suggest, still remains as to whether gravity wave “travel” through space(-time), and whether they can carry or convey energy.â€
I believe gravity waves “travelâ€, but based on the issue I’ve been trying to discuss I can see no reason why they might be thought to convey energy. A geometric distortion or curvature of spacetime so-defined is not a force, it has no intrinsic energy, although as the tides make clear, a large moving mass produces a moving distortion, or curvature, that can flux geodesics, stress molecular binding energies, and especially with fluids, result in massive kinetic energies.
“In addition, may I suggest that the lunar (and solar) tides are to gravity what near field effects are to electromagnetism? So we do have this large ‘near field’ effect. What of long range “transmission” or propagation? Is this not worthy of testing?â€
I suppose so, although it seems implausible in the absence of any evidence for the existence of such a dichotomy, and for its apparent basis in the implicit association of gravitation with energy. If a massive local gravity wave (the moon’s orbit, for example) doesn’t produce detectable energy, and (one would expect from its effects on the oceans) a shower of related particles, why should a distant source be expected to do so?
You wrote to Dr Bortz: “I prefer not to jump to the conclusion that he is challenging this well established theory until I have some proof (like a direct statement on his part), since if I judge incorrectly (or even jump to the defense of this theory too strongly) the whole discussion degrades rapidly.â€
I hope it is clear by now that I’m challenging one aspect of gravitation theory, the association of gravitation with a “force of gravityâ€, or “gravitational energy†– and I’m challenging it based on its inconsistency with the overall theory, its absence of theoretical establishment. Dr. Bortz may just be too busy (he hasn’t done well in reflecting my positions), but he seems to be of the more-or-less conscious opinion that any part of a “well-established theory†is beyond question, and must of necessity derive from silliness, a gross lack of understanding. That in-itself would of course be in violation of a most fundamental and well-established theory (a principle) – the theory that science progresses, and derives its effectiveness, from the modification and sometimes overturning of well-established theories.
I’ve tried to show by concrete examples that uniform motion/gravitation, and inertial acceleration are absolutely distinct principles, from any coordinate system. I don’t question what I hold to be the essence of Einstein’s theory of gravitation, in particular, the concept of gravity as geometry, and of geodesic motion in spacetime. But I do contend that the association of gravitation with force or energy undermines the overall theory and sabotages related research, especially the search for a unified field theory, and in some cases, leads to pointless research, as in the search for energy-bearing gravity waves. If I’m wrong, the examples I’ve offered should be refutable on their own terms, not by recourse to the interpretations I’m calling into question. If gravitation is force-like, or energy-like, for example, there should be an experiment one could describe or perform that would comprehend it as such, absent the interruption of gravitation (geodesic motion) by (typically) mechanical or electromagnetic energy.
Jim, if you track the previous discussion, you will note that I took pains to say that my comments about Don were not personal, but his ideas were misguided.
I also did not get personal about your ideas, although you did get rather derisive about me at the end because I could not accept what you were saying.
Finally, I can’t see any substantial distinction between your quote, “a body moving freely in a gravitational field is always moving uniformly from its own frame of reference†and my interpretation of it, “a body moves with respect to its own frame of reference,†especially with your later follow up with astonishment about the fact that other objects seem to be accelerated in the opposite direction when viewed by an observer in an accelerated frame.
I don’t have time to go round and round on this, so please don’t respond further to me. Sorry. it’s not personal, and perhaps David sees something in your writing that I miss. I’ll leave the rest of the discussion to him.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)