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Is Gravity a Force?


Davy_Jones
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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:

 

Quote

As far as we are able to judge at present, the general theory of relativity can be conceived only as a field theory. It could not have developed if one had held onto the view that the real world consists of material points which move under the influence of forces acting between them.

- Einstein, "The Meaning of Relativity" 

 

Quote

By 25 November 1915, Einstein had the final equations. Matter and energy were intimately linked with the geometery of space-time. Planets orbit Suns because the geometry of space-time around these massive bodies is curved, just as two drivers starting at the Earth's equator going north gradually approach each other because the Earth's surface is curved. There is no "force" pulling them toward each other. Einstein showed that a similar, albeit more complex kind of geometrical curvature can explain gravity.

- Jeffrey Crelinsten, "Einstein's Jury", p88

 

Quote

Light, and all other physical phenomena, must travel on locally curved paths in gravity. This point strongly motivates the idea that gravity is an aspect of geometry and does not belong in the menagerie of forces!

- 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!
 

Edited by Davy_Jones
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47 minutes ago, Davy_Jones said:

Anyway, the question in short: Is gravity a force or not? (In layperson's terms, insofar as possible)

Interested to hear any comments. Thanks!

No I'm not a Physicist. What needs to be noted is the fact that we near exclusively use Newtonian mechanics in all Earth based measurements and all space shots as far as I am aware. In Newtonian we treat gravity as a force and get sufficiently accurate "correct" answers to  all everyday problems on Earth and even our space endeavours.

We could though if we chose, use the GR concept if we like. That would give us more accurate answers, but accuracy we do not really require, and in essence a burden due to the far more complicated mathematics involved. So we just use it for those scenarios where that accuracy is required, [such as the perhelion shift of Mercury] or the lense Thirring effect of a spinning Earth.

Both are models that work within their zones of applicability...

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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?

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What is gravity, really ?
Short answer; we don't know.

We know how it behaves in most circumstances ( not hi energy/small separations ), and our most accurate model, Einstein's GR, is a field'model where geometry is the 'field'.
in general, all of the effects that you have termed 'forces', where "material points move under the influence of forces acting between them" have been given the field model treatment, such that we have field theories for Electrodynamics ( QED ) and the color interaction ( QCD ).

It is more the case, that our description of the effect's behaviour, can be modelled in different ways, sometimes with equivalent accuracy,sometimes nearly equivalent accuracy, as in Newtonian vs GR.

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42 minutes ago, Davy_Jones said:

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?

We model gravity as I believe I said. What it actually is, I'm not sure we really know, other then an attraction between masses.

If we check WIKI we get this....

https://en.wikipedia.org/wiki/Gravity

Gravity (from Latin gravitas 'weight'[1]), or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light[2]—are attracted to (or gravitate toward) one another. On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the tides of the oceans. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing and forming stars and caused the stars to group together into galaxies, so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become weaker as objects get further away.

Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915), which describes gravity not as a force, more at link...................

 

So even WIKI says GR "describes" it not as a force but spacetime curvature...still no closer though to revealing what it actually is...and as a "natural phenomenon"

Edited by beecee
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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:

Quote

We're routinely told--by scientists--that there are four fundamental forces of nature, one of which is gravity.

 

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:

Quote

In general relativity, the effects of gravitation are ascribed to spacetime curvature instead of a force.

 

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?

Edited by Davy_Jones
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What is gravity?

As @MigL and @beecee said, ultimately we don't know.

You can model it as force at a distance, à la Newton, but that doesn't work for rapidly changing or relatively strong (stellar) fields. There comes general relativity to the rescue.

You can model it as geometry of space-time, but that leads to a couple of problems:

1) Horizons are entropic (they hide information)

2) Gravity is non-renormalisable at arbitrarily strong fields (ultraviolet limit, high-energy collisions), because it's dimensionally bad-behaved. 

Problem 1) is both conceptual: What is this geometry with an entropy/temperature; what are the hidden degrees of freedom? And it is also wanting in mathematical/logical consistency: Since Hawking we know that black holes must evaporate if gravity is a quantum field, so microscopic information disappears ==> Distinctions between trajectories disappear ==> Predictability vanishes at too fundamental a level.

Black holes are so interesting because they are quantum objects and they are general-relativistic objects; so many people hope they will show us eventually what's wrong with the present picture.

A very interesting change in the mind frame of physicists took place in the 20th Century: Renormalisation. What is renormalisation? In very general terms, it's the realisation that physical problems look one way or another depending on the scale and the range of phenomena at which you wish to describe things.

Consider a wooden stick. What is it? Newtonian mechanics considers it as a rigid body, which is described by 6 real variables --typically the position of its centre of mass and three orientation angles. But what if you want to consider situations like shooting at it, deforming it, breaking it into pieces? (higher energies, changes at small spatial scale). Then your parametrisation is no longer useful, and you need to take into account interatomic interactions.   

The motto for this loss of innocence is: My parametrisation of the physical system is not the physical system; it's just my parametrisation of the physical system.

Edited by joigus
minor stylistic correction
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@ 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:

  1. Gravity (weakest force with an infinite range)
  2. Weak nuclear force (next weakest force with short range)
  3. Electromagnetic force (stronger force with infinite range)
  4. Strong nuclear force (strongest force with short range)

 

https://scienceissimple.com/four-fundamental-forces/

Edited by Davy_Jones
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4 hours ago, Davy_Jones said:

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.

 We have two useful models of gravity......Gravity as a force in Newtonian, and gravity as spacetime curvature in GR...both give the same answers in the less accurate domain of Newtonian

If you call gravity a force, you are essentially in the Newtonian domain and the most well known and used domain with everyday run of the mill calculations.

If you call gravity as geometry, specifically spacetime geometry, then you are in the GR domain, and essentially when high accuracy is the goal.

4 hours ago, Davy_Jones said:

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).

Not unless you are ignorant of what Newtonian and GR are doing...essentially modeling the effects we see. In Newtonian we observe an attraction between masses that falls of as the inverse square of the distance...In the far more accurate model of GR, we see that attraction explained by the curvature of spacetime in the presence of mass, spacetime also being a metric model.

In Newtonian, we are unable to say why different masses attract...they just do. In GR, we are unable to say why spacetime geometry in the presence of mass, should curve and warp, It just does.

4 hours ago, Davy_Jones said:

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:"

 Scientific theories/models are based on observational and experimental support. In that respect GR has passed all tests thrown its way, so it is overwhelmingly supported by scientists, and incidently why it was formulated to explain what Newtonian could not. 

2 hours ago, Davy_Jones said:

"We know what gravity is: It's the curvature of spacetime"

Any thoughts?

Yes, agreed, but tell me why does spacetime curve in the presence of mass/energy.

2 hours ago, Davy_Jones said:

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?

We talk of the "force of gravity" all the time, probably firstly most don't understand GR, and secondly, because we all work in the Newtonian system. As GR is the superior [more encompassing] model, it is more correct to say gravity is spacetime curvature, but again essentially, we don;t know why..it just works tremedously well

Edited by beecee
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This is a very interesting question and much like you Davy-Jones the nature of what gravity is, rather than how it is modelled, is one that I often ponder.

Interestingly in QM gravity is modelled as a force;

"In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathematical problem with renormalization in general relativity"

So which is it, the curvature of space time or the exchange of particles between masses, or a field in which mass interacts?

  

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9 hours ago, Davy_Jones said:

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. 

........

Anyway, the question in short: Is gravity a force or not? (In layperson's terms, insofar as possible)

Interested to hear any comments. Thanks!
 

 

Before offering any discussion I would be interested to learn what you understand by a 'Force' ?

I ask this because your question is clear cut black and white yet most things in Nature become more complicated than that when we enquire more deeply into them, often much more complicated.

Forces and how they operate, what other physical quantities they affect or need and so on fall into the category of being much more complicated.

Even when considered in a newtonian manner, gravity operates differently from say electromagnetism, although there are similarieties as well.

 

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11 hours ago, Davy_Jones said:

But that's not the question. The question is: According to our current understanding, what is gravity really

Science cannot answer this. It’s a metaphysical question.

One can’t affirm that GR is reality, only that it models observed behavior very well. In that model, gravity isn’t a force. In the Newtonian model, it is.

3 hours ago, Intoscience said:

This is a very interesting question and much like you Davy-Jones the nature of what gravity is, rather than how it is modelled, is one that I often ponder.

Interestingly in QM gravity is modelled as a force;

"In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathematical problem with renormalization in general relativity"

So which is it, the curvature of space time or the exchange of particles between masses, or a field in which mass interacts?

  

You’re obviously quoting someone. What is the source of this quote?

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First,  welcome @Davy_Jones, who I know as another contributor of my now-defunct sciencechatforum.   

While I agree that the whole question of what forces and fields really are crosses into the land of metaphysics,  I am also one who wonders at how tenuous our grasp of what gravity is.  If we're allowed to free-fall in a gravitational field,  then we don't experience a force.   We only experience a force when we land on something and our body's atom's outer electrons experience electrostatic forces from,  say,  a chair's outer electrons.  Without that repulsive force,  we would just continue our freefall through the chair,  the floor,  the earth's crust and so on  along a space-time geodesic.   (and be in pretty bad shape,  no doubt?)   So,  is gravity then construed there as a pseudoforce?  Were we only following a line of curved space until a real electrostatic force made itself known to us?  It's in such perspectives that gravitons seem especially incoherent as anything but a math toy.   

John Wheeler always comes to mind... "Spacetime tells matter how to move; matter tells spacetime how to curve."  From a meta perspective, it's awfully hard to say what this "telling"  process is.   

 

 

Edited by TheVat
word choices that smelled bad
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Would it help if we thhrew out the concept of a 'force' in this context, and said that there are four fundamental interactions ?
We don't know the exact nature of these nteactions, but we have various ways of modelling them.
None of the models tell us what they are, but they make good predictions where they are applicable.

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19 minutes ago, MigL said:

Would it help if we thhrew out the concept of a 'force' in this context, and said that there are four fundamental interactions ?
We don't know the exact nature of these nteactions, but we have various ways of modelling them.
None of the models tell us what they are, but they make good predictions where they are applicable.

 

I like the idea of fundamental interactions, as opposed to fundamental forces. +1

Davy specifically asked  about forces so it would be a better idea to find out what we mean by a force before throwing it out, lest we throw the dog out in mistake for the cat.

Edited by studiot
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14 minutes ago, MigL said:

And why is it OK to throw out the cat, but not the dog ?

It's what you do every night.

🙂

1 hour ago, swansont said:

One can’t affirm that GR is reality, only that it models observed behavior very well. In that model, gravity isn’t a force. In the Newtonian model, it is.

It is interesting to note that in a Newtonian analysis centrifugal force is not a force. But in a D'Alembertian analysis of the same situation, it is.

Edited by studiot
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I'd like to point out that in lieu of modern spacetime formalisms and mathematical knowledge it is also the case that even in a classical context that gravity should be treated as a force could be considered dubious. Versions of Newton-Cartan gravity exist which is basically a geometrical realization of classical gravitation or, if you are avoidant of such terminology, is a version of newtonian gravitation/Classical physics in which what an inertial reference frame is or what path they follow is found to be dependent on the local/global mass distributions. So if you placed an object at rest or give it some initial velocity it wouldn't remain at rest or follow merely a straight line but a curved one in the presence of some mass distribution while all local experiments would convince you as to it being an inertial reference. 

Do note that together with the curvature based formulation of General Relativity there is also a version of General Relativity known as tele-parallel gravity that would replicate most if not all of General Relativities predictions but seems to appear to be a force field. This is done by assuming that the curvature is absent from determining the interaction and rather its the torsion (how much your world line bends) in spacetime that determines how the gravitational interaction takes place. So, is gravity then a force once again?

As was showcased in the first paragraph in the classical situation perhaps this is all but arbitrary and what you are really looking for has less to do with force. Perhaps, it has more to do with what you consider to be inertial or non-inertial and how much you're willing to apply occam's razor. 

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8 hours ago, Davy_Jones said:

Thanks for the response (which largely went over my head LOL).

Re-reading myself I realise I didn't explain at least a couple of technical terms, so I'm sorry.

Horizons: Surfaces in space-time that separate regions of space-time that are inaccessible to observers on one side of those surfaces

Renormalization: Treatment of a physical problem taking into account how it depends on the scale at which you study it

Entropy: A variable that measures lost information; physical information that gets scrambled

Gravity is peculiar on all of these accounts. Other 'forces' don't have horizons associated with them. They're not scale-dependent, like gravity is. This is the meaning of 'bad-behaved'. Other forces don't have an "intrinsic" entropy.

9 hours ago, Davy_Jones said:

Gravity (weakest force with an infinite range)

Gravity is not so much a weak force as it is a scale-dependent force. That is, whether it's weak or strong depends a lot on the scale at which you look at it. It's actually the dominant force at scales that approach a Planck's length worth of distance.

At stellar distances gravity becomes relevant again, but not because of scale-dependence. Rather, because gravity cannot be screened.

Gravity also has a cosmologically-relevant component, which is the vacuum energy.

Gravity is peculiar in many senses. So, whatever a force is --I'm with other users here that whether it really is this or that verges on metaphysical--, gravity is very different to the other bunch. It's the odd one out.

I really hope that was helpful, but it's a difficult topic. Other users express themselves more eloquently than me.

And welcome to the forums. :) 

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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 . . .

Quote

What does not satisfy me in that theory, from the standpoint of principle, is its attitude towards that which appears to me to be the programmatic aim of all physics: the complete description of any (individual) real situation (as it supposedly exists irrespective of any act of observation or substantiation). Whenever the positivistically inclined modern physicist hears such a formulation his reaction is that of a pitying smile. He says to himself: “there we have the naked formulation of a metaphysical prejudice, empty of content, a prejudice, moreover, the conquest of which constitutes the major epistemological achievement of physicists within the last quarter-century. Has any man ever perceived a ‘real physical situation’? How is it possible that a reasonable person could today still believe that he can refute our essential knowledge and understanding by drawing up such a bloodless ghost?” 


-- Albert Einstein, "Reply to Criticisms"

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9 minutes ago, Davy_Jones said:

I leave you with the arch-realist himself . . .

Well if this really is the long goodbye, perhaps you will at least try to answer questions from your next victims.

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@ 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".

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13 minutes ago, Davy_Jones said:

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?

How do you test it? How do determine if that’s the “truth”? All you have are observations. If you don’t have that, all that’s left is philosophy (points at metaphysics)

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37 minutes ago, swansont said:

How do you test it? How do determine if that’s the “truth”? All you have are observations. If you don’t have that, all that’s left is philosophy (points at metaphysics)

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.

Edited by Davy_Jones
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44 minutes ago, swansont said:

How do you test it? How do determine if that’s the “truth”? All you have are observations. If you don’t have that, all that’s left is philosophy (points at metaphysics)

So,  have to ask,  is string theory physics?  Or,  because we don't know to what degree it might describe the real world,  is it a theoretical framework that lies on the borderlands of metaphysics? 

String theory seems to describe a landscape of possible universes,  most of which intelligent life can never behold.   At least, with Davy's long-fingered lemurs,  we can go to Madagascar and look at quite a few of them.  (careful,  they will steal your glasses)

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