Davy_Jones

Cf. Ptolemaic vs Copernican cosmology. Both theories work well (i.e. yield accurate predictions) for certain purposes, e.g. navigation. You could navigate your way to Australia using either one, or so I'm told. Given, however, that the two theories make logically incompatible claims, it is not possible that both are true.

@studiot Hey, it works! Thanks for the tip! (Now all I need to do is master the quote function) "You also seem to be more au fait with Philosophy authors or scientists discussing the philosophy of science." Yes, that's right. The bulk of my reading is in the philosophy of science. "As such have you heard of Berkson ? He has written a philosophical book on this subject" No, I don't think I've heard of him. Sounds like the kind of thing I'd enjoy. Alas, I'm not allowed in the local uni library for the time being thanks to that darn virus. Grrr! Re your post box example. My first thought (almost certainly wrong) is that the post box is red at all times. Of course, if you turn off the lights, or cover it with a white sheet, its redness will be concealed. I'd like to think more about this, though. Great example! Re the final section of your post. What you seem to be saying, if I understand correctly, is that in certain circumstances it may be more expedient to bring Newtonian physics to bear on a problem, Einsteinian relativity in others. Not unlike Bohr's principle of complementarity, perhaps? No argument there. But this is to treat theories as mere instruments. And again, there's nothing wrong in that per se. Problems with inconsistency, such as the ones I've been alluding to in this thread, only arise when theories are treated realistically. For example, if Theory A says that space and time are uniform and absolute, and theory B says that they are not, instrumental efficacy notwithstanding (both theories may work very well), it is not logically possible that both theories are true.

Some of you may be familiar with the wonderful lecture series produced by "The Teaching Company" (aka "The Great Courses", I think). Right now, by chance, I'm going through their course entitled "The Evidence for Modern Physics: How We Know What We Know" presented by professor Don Lincoln. https://www.thegreatcourses.com/professors/don-lincoln Near the end of Lecture 15, "The Awesome Evidence for General Relativity", at the 28:30 mark, Prof. Lincoln perorates: "I hope I've convinced you that there are very good reasons that general relativity has been very clearly validated. I mean I know that it's very weird to think of gravity as the bending of space and time, but it's just literally impossible to believe otherwise these days." So, pace other contributors to the thread--perfectly entitled to their opinions--who have told us that no one knows what gravity is, or science cannot answer questions such as what gravity is, clearly, Prof. Lincoln does not share your skepticism. The good professor is not only telling us what gravity is (It's the bending of space and time), but that to believe that gravity is anything other than the bending of space and time is not possible. A tad hyperbolic if you ask me, but hey, I toss it out here for consideration. Edit P.S. Also for everyone's reference, Prof. Lincoln opens the same lecture with this: "Of all of the known forces, gravity is probably the one with which you have the most experience."

@ Studiot What is my understanding of a "force", you ask. What immediately comes to mind are the words of St. Augustine (musing over time rather than force): That said, my own understanding of force--assuming I had anything intelligent to say on the subject at all--doesn't seem particularly relevant here. The reason I started this thread was in the hope of gaining some expert (= you guys) clarification into what strikes my layman's lugs as a pair of mutually inconsistent claims that I frequently hear, namely: 1. Gravity is one of the fundamental forces of nature and 2. Gravity, under general relativity (which I take, perhaps mistakenly, to be current orthodoxy), is not a force at all. (See OP for more) I second that emotion!

I thought it might be fruitful to quote philosopher of science Arthur Fine at some greater length. Assuming our contributors here constitute a representative sample of scientists as a whole, Fine would appear to be right on the mark. i.e., GR, properly understood, is merely a model, a calculating device, not a representation of reality. (from the same source, same page)

Just a slight addendum. The above should read: [Many scientific realists would say] "If a theory is empirically adequate (i.e. saves the phenomena), moreover, yields new and surprising predictions --which are subsequently corroborated--then we have good reason to believe that it is true" And "true" here means literally true. More later. Thanks again to all contributors. [my emphasis in red] Apparently Einstein himself thought so, at least in his later years. - Arthur Fine, "The Shaky Game", p123

Going back to Jones and his lemur fixation, let us suppose for the sake of argument that no one knows why aye-ayes are nocturnal while many other lemurs are diurnal. Does it follow from this that Jones does not know what an aye-aye is? I don't think so. We'd just say Jones knows what an aye-aye is alright; he just doesn't know why they are nocturnal. Let us now suppose no one knows why mass/energy makes spacetime curve. Does it follow from this that no one knows what gravity is? I don't think so. Two points to note: 1. It may be the case that no one knows what gravity is. It does not seem to me, however, that this follows from the fact (if it is) that no one knows why mass/energy makes spacetime curve. 2. It seems to me there is no shortage of people claiming to know what gravity is. Anyone, say, who believes the general theory of relativity to be literally true (i.e. a more-or-less faithful representation of reality, as opposed to a mere model or calculating instrument) is effectively claiming to know what gravity is. ("It is the curvature of spacetime . . . etc., etc.") Whether they do know or not is another matter, of course.

One standard realist response to that one would be appeal to "Inference to the Best Explanation" (IBE) Roughly (it comes in various forms): from a set of candidate explanations we are licenced to infer to the truth, or approximate truth, of the best among them. The antirealist will have none of this, of course. Edit P.S. Other fairly standard realist responses would look something like the following: "If a theory is empirically adequate (i.e. saves the phenomena), moreover, yields new and surprising predictions then we have good reason to believe that it is true" William Whewell, meanwhile, would speak of a "consilience of inductions" conferring epistemic warrant on a theory, i.e., good reason to believe that it is true.

@ Studiot (above) Sir/madam, I've been slightly overwhelmed here, struggling to understand most of it. Grateful nonetheless for so many informed responses. If you're referring to your question "What is a force?" that's an easy one. My answer: "I dunno".

Well, as a wise man once said, "For every complex issue there's a simple answer . . . and it's almost certainly false". Clearly, as other posters (e.g. Studiot) have pointed out, my expecting a simple answer to a deceptively simple, but apparently complex, question was a little overly sanguine. Thanks to all who have contributed. Just a few (no doubt incompetent) thoughts for now: Supposing Jones were asked "What is an aye-aye?" and he responds . . . "The aye-aye is a long-fingered lemur, native to Madagascar with rodent-like teeth that perpetually grow and a special thin middle finger. It is the world's largest nocturnal primate." No doubt there are many things Jones does not know about aye-ayes, indeed, doubtless there are many things about aye-ayes unknown to anyone. Nonetheless, is it not reasonable to say that Jones has answered the question, that Jones knows what an aye-aye is? Even supposing he is quite wrong, he has still given an answer to the question; he has attempted to answer the question. Now, certain posters (e.g. Swansont) have expressed the opinion that science cannot answer the question "What is gravity - really?" on the grounds that it's metaphysical. First of all, how metaphysics is to be demarcated from physics, or simply science in general, remains obscure, assuming it can be done at all. Perhaps the most celebrated attempt to do so was from the logical positivists armed with their 'verifiability criterion of meaning' (that which cannot be verified constitutes metaphysics and is literally meaningless). The project failed for reasons that needn't be rehearsed here. But let's suppose, for the sake of argument, it is a metaphysical question. Why should that preclude science (à la Jones), whether right or wrong, from providing an answer to the question? The existence and nature of atoms, say, might well have been regarded as a metaphysical question once. Science now tells us a great deal about atoms, even if not all their properties are known, à la Jones once again with his incomplete knowledge of aye-ayes. It seems to me that the Newtonian realist (not necessarily the great man himself) of yesteryear who pronounced that "Gravity is an attractive force which acts instantaneously over any distance . . . etc., etc." is answering the question "What is gravity?" even if her answer is no longer widely accepted nowadays. Similarly, the GR realist of today who asserts "Gravity is the curvature of spacetime. It increases around massive bodies . . . etc., etc." is answering the question "What is gravity?" Finally, I think we must recognize that scientists are a heterogeneous bunch; they do not all speak with one voice. Of course there are scientists of a more antirealist (positivist, instrumentalist, empiricist, William Tell, whateverist) persuasion who echo sentiments expressed by certain contributors here to the effect that science is not in the business of telling us what gravity is, science just constructs models which more or less save the phenomena, just shut up and calculate, etc., etc. Not all scientists feel this way. Many--those of a more realist bent--feel it is the business of science to answer such questions; it is the business of science to provide us with a complete and faithful representation of reality. (cf. "One can’t affirm that GR is reality, only that it models observed behavior very well." - Swansont) I leave you with the arch-realist himself . . . -- Albert Einstein, "Reply to Criticisms"

@ joigus (post above) Thanks for the response (which largely went over my head LOL). What you're telling us, I think, is that scientists sometimes treat gravity as if it were an attractive force (à la Newton), and sometimes treat it as if it were the geometry of space-time (à la Einstein). All well and good, though this fails to address the question of what we are to say of these scientists or textbooks that routinely tell us simpliciter that gravity is one of the fundamental forces of nature. These scientists are not saying "Gravity is (sometimes) treated as if it were a fundamental force of nature"; they are saying "Gravity is a fundamental force of nature". Are they wrong? Are they simplifying? Are they claiming knowledge of that which is not known? Or what? E.g. First hit I got on google right now (admittedly a site aimed at children by the looks of it) . . . What are the four fundamental forces? There are four fundamental forces of physics, each of which varies in terms of its strength and the range it works on. The strong nuclear force is the strongest force but only works on the quantum (very small, subatomic) level. Gravity is the weakest force but has the largest range – the entire universe. The four forces are: Gravity (weakest force with an infinite range) Weak nuclear force (next weakest force with short range) Electromagnetic force (stronger force with infinite range) Strong nuclear force (strongest force with short range) https://scienceissimple.com/four-fundamental-forces/

Thanks to the two posters above for attempts at clarification, though I'm still puzzled . . . In response to the question, "What is gravity really?" answers proffered include: "Short answer; we don't know." - MigL and "What it actually is, I'm not sure we really know, other then an attraction between masses." - beecee In my opening post, I noted: Presumably, we've all heard this before (if not, I'll try to find quotes). For now, if you'll allow me, let's just accept that scientists do routinely say such things. Granting the above, is it not the case--contra MigL and beecee--that these scientists are saying we do know what gravity is? It is a force, they are telling us, indeed, one of the fundamental forces in nature. But then in the Wiki article that beecee linked, much like the three authors I quoted in the OP, including Einstein himself, we are told in no uncertain terms: So, two points: 1. Assuming that spacetime curvature is not a force, then gravity cannot be both a force and spacetime curvature. So the aforementioned scientists who tell us that gravity is a force are wrong, unless they are denying general relativity and Newton had it right after all. 2. To say "we don't know", as MigL and beecee do, seems to me tantamount to a denial that we have any good reason to believe Einstein's general relativity as being literally true (as opposed to simply a useful tool). Is this your position? After all, knowledge is standardly taken to be (at least) justified, true belief. Thus, all persons who believe Einstein's theory of gravity to be true, and assuming GR constitutes a good reason (= justification) for belief, are in a position to at least claim knowledge. They are in a position to say: "We know what gravity is: It's the curvature of spacetime" Any thoughts?

Beecee, The instrumental efficacy of Newtonian mechanics is not disputed, at least not by anyone I know. That is to say, Newtonian mechanics works, indeed works very well. The same goes for GR, to an even greater degree. But that's not the question. The question is: According to our current understanding, what is gravity really? In other words, if we read Einstein literally (as opposed to instrumentally), is gravity a force or not?

This question is for the physicists mainly, I suppose, though there may also be a philosophical element to it. (Mods may wish to relocate the thread as appropriate). I ask as an interested layman. We're routinely told--by scientists--that there are four fundamental forces of nature, one of which is gravity. This is so commonly heard that I assume quotations are unnecessary. Gravity construed as a force seems entirely unproblematic under the erstwhile Newtonian paradigm. But times have moved on . . . Much of the lay reading I've done in this area seems to suggest that general relativity--if read literally--treats gravity not as a force at all; rather, it is to be identified with the curvature/geometry of spacetime. Here are a few examples: - Einstein, "The Meaning of Relativity" - Jeffrey Crelinsten, "Einstein's Jury", p88 - John B. Kogut, "Special Relativity, Electrodynamics, and General Relativity", p198 Would it be accurate to say that contemporary physicists continue to speak of gravity as if it were a force, even though (assuming Einstein got it right) it is not . . . perhaps out of deference to their scientific forebears, or to engender a sense of continuity? Anyway, the question in short: Is gravity a force or not? (In layperson's terms, insofar as possible) Interested to hear any comments. Thanks!