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Ross PK81
2008-Aug-31, 11:47 AM
If you put two grains of rice close enough together in space, would the gravity they have attract one another together or would it not be strong enough?

Nowhere Man
2008-Aug-31, 12:41 PM
They would attract one another. All matter has mass, and all mass (as far as we know) has gravity.

However, two grains of rice are so small that any other nearby matter would affect the experiment.

Fred

Ken G
2008-Aug-31, 01:53 PM
My feeling would be, we really don't know unless we can detect it. Certainly all successful theories of gravity so far would predict there would be such an attraction, but that's not quite the same as saying there would be. An interesting question is, how far outside the domain of what can be measured do you have to go before you cannot expect to extrapolate your existing physics? Maybe we can confidently expect two grains of rice to attract in the normal way, but what about two atoms? At some point, it just isn't science any more to discuss expectations.

antoniseb
2008-Aug-31, 03:05 PM
The Cavendish experiments have shown gravity working down to a relatively small scale. I don't know what the smallest ever measured was, but I had the impression that it has been done with a single Carbon nanotube as the torsion balance, which means it would have been measured with something smaller than a grain of rice... like a Gold nano-particle if I remember correctly.

Nowhere Man
2008-Aug-31, 03:08 PM
You can calculate the gravitation felt by the two grains using Newton. If there's mass, there is gravity, although it may be an incredibly small amount, beyond the limit of most experimental equipment.

At the scale of atoms, the gravitational effects are swamped by the electromagnetic effects.

Fred

Lepton
2008-Aug-31, 03:24 PM
You can calculate the gravitation felt by the two grains using Newton. If there's mass, there is gravity, although it may be an incredibly small amount, beyond the limit of most experimental equipment.

At the scale of atoms, the gravitational effects are swamped by the electromagnetic effects.

FredI thought the problem with detection would be the sheer amount of problems isolating the "test" from ALL outside forces. I mean the attraction of gravity between two grains of rice is detectable in theory but it would be overwhelmed by the attraction of the Earth's gravity (as well as the EMF and friends).

Ken G
2008-Aug-31, 03:41 PM
The Cavendish experiments have shown gravity working down to a relatively small scale. I don't know what the smallest ever measured was, but I had the impression that it has been done with a single Carbon nanotube as the torsion balance, which means it would have been measured with something smaller than a grain of rice... like a Gold nano-particle if I remember correctly.That is of course interesting, what is the smallest scale on which gravity has been measured. Citation?

Ken G
2008-Aug-31, 03:42 PM
If there's mass, there is gravity, although it may be an incredibly small amount, beyond the limit of most experimental equipment.
Is this a statement of faith, or of science? The key word there is "most"-- versus "all". That distinction is what we are trying to establish here, for our current level of technology. For example, on very large scales, we now speculate that there can be gravity (antigravity, in fact) that does not stem from mass. How do we know that something similar couldn't apply to much smaller scales, normally undetectable, that could interfere with the purely theoretical calculation you suggest is a known reality?

antoniseb
2008-Aug-31, 03:44 PM
I don't have a citation. I'm working on a fading recollection of something I read a few years ago in some news shorts type article in SciAm, or New Scientist, or the like... not the original research. Sorry.

Ken G
2008-Aug-31, 03:57 PM
A quick Google didn't really turn up anything. There are microgravity accelerometers that use the vibrational modes in nanotubes (not quite Cavendish but perhaps still capable of measuring the response to very weak gravities), but the application they seem to have in mind at a quick look is measuring the prevailing field in microgravity environments, not measuring weak gravity effects against a typical gravitational background. So I can't tell if any small gravitational interactions between ultra-low-mass objects has been measured by it-- it might require an ultra-low-gravity environment. Grains of rice seem not too hard, perhaps-- it's hard to say.

To see the scale of the problem, note that the g at the surface of a sphere at roughly Earth density is about r/R times the Earth g, where r is the radius of the sphere and R is the radius of the Earth. Rice is probably lower density, but not too much, and r/R is probably about ten orders of magnitude below unity, so g is ten orders of magnitude below "1 gee". That's a challenge, I'm sure.

Tim Thompson
2008-Aug-31, 03:58 PM
That is of course interesting, what is the smallest scale on which gravity has been measured. Citation?
See Improved constraints on non-Newtonian forces at 10microns (http://adsabs.harvard.edu/abs/2008PhRvD..78b2002G); Michael Garaci, et al., Physical Review D 78(2): 022002, July 2008. They place strong constraints on non-Newtonian gravity at a range about 10 microns. Also see The Eöt-Wash Group webpage (http://www.npl.washington.edu/eotwash/) from the University of Washington. This group has a regular program of research into short-range gravity (i.e., Schlamminger, et al., 2008 (http://adsabs.harvard.edu/abs/2008PhRvL.100d1101S); Kapner, et al., 2007 (http://adsabs.harvard.edu/abs/2007PhRvL..98b1101K); Hoyle, et al., 2004 (http://adsabs.harvard.edu/abs/2004PhRvD..70d2004H)). I think about 5-microns is the minimum distance we can say anything seriously experimental about gravity.

Ken G
2008-Aug-31, 04:05 PM
Thanks. Well, 10 microns is pretty small! Densities on that scale are still Earth-like, so that's a g that is down from ours by about 14 orders of magnitude.

parallaxicality
2008-Aug-31, 04:07 PM
I think any gravitational attraction between the two grains would be vastly outweighed by the force used to put them in place to begin with.

cosmocrazy
2008-Aug-31, 04:14 PM
Is this a statement of faith, or of science?

I would say both. Faith in the current theories behind the scientific evidence and math.


The key word there is "most"-- versus "all". That distinction is what we are trying to establish here, for our current level of technology. For example, on very large scales, we now speculate that there can be gravity (antigravity, in fact) that does not stem from mass. How do we know that something similar couldn't apply to much smaller scales, normally undetectable, that could interfere with the purely theoretical calculation you suggest is a known reality?

The bottom line is until we can successfully define gravity at the quantum level then all we have is the classical GR and Newtonian theories of gravity. Both would suggest that yes the 2 grains of rice are massive enough to attract each other from the influence of gravity.

Ken G
2008-Aug-31, 04:24 PM
The bottom line is until we can successfully define gravity at the quantum level then all we have is the classical GR and Newtonian theories of gravity. Both would suggest that yes the 2 grains of rice are massive enough to attract each other from the influence of gravity.But the issue is not what our current best models "suggest", what they suggest came up already. It is what reality is doing-- and there's no reason to feel we know that two grains (or maybe one has to go even smaller to get well beyond our experimental capabilities), don't repel each other gravitationally.

Ken G
2008-Aug-31, 04:27 PM
I think about 5-microns is the minimum distance we can say anything seriously experimental about gravity.
Come to think of it, the 14 orders of magnitude I quoted above would seem way too small to isolate from other forces in the experiment. Furthermore, those experiments use a particular model of short-range gravity, involving a Yukawa term (which is exponentially decreasing with distance). Hence, I strongly suspect that those experiments don't say anything at all about the actual attraction/repulsion of gravity on a scale of 5 microns-- other than to rule out a particular form that would have the gravity vastly increase on those scales. That's my guess, anyway-- if so, it means that yes there are some constraints on that scale, but none sufficient to actually say what the force is.

cosmocrazy
2008-Aug-31, 04:28 PM
But that simply isn't true-- there's no reason at all to feel we know that two grains, or maybe one has to go even small, don't repel each other gravitationally.

Ok i'll run with you on this Ken. Can you explain to me why they could repel?

thanks david

hhEb09'1
2008-Aug-31, 04:28 PM
The bottom line is until we can successfully define gravity at the quantum level then all we have is the classical GR and Newtonian theories of gravity. Both would suggest that yes the 2 grains of rice are massive enough to attract each other from the influence of gravity.Yes, definitely the mainstream interpretation :)

Ken G
2008-Aug-31, 04:40 PM
Ok i'll run with you on this Ken. Can you explain to me why they could repel?
That's easy. How could galaxies at very large distances repel? (It seems they do.) The answer is: reality can do whatever it wants, it is our job to get theories that explain it-- not the other way around.

Ken G
2008-Aug-31, 04:41 PM
Yes, definitely the mainstream interpretation See my last post for why that is only the "mainstream interpretation" of our own theories-- not the mainstream interpretation of how we should expect reality to work. The latter is a very complex question involving how far one can really expect to extrapolate one's physics outside of what has been observed, before significant modifications are likely to appear. I would say the "mainstream" is quite well aware of this, if it is aware of any amount of history of science.

The bottom line is, I haven't the least idea of how to assess a "probability" that matter attracts gravitationally on any scale that is far smaller than anything we can constrain observationally. Presumably, the probability increases as we approach what has been constrained, but how quickly? I have never seen anyone even attempt an answer. Moreover, if someone builds a machine that can fly you to Paris if atoms attract gravitationally, but will kill you if they don't, do you put your family in it before that particular aspect has been tested?

hhEb09'1
2008-Aug-31, 05:02 PM
See my last post for why that is only the "mainstream interpretation" of our own theories-- not the mainstream interpretation of how we should expect reality to work. The mainstream interpretation of mainstream theories is what appears outside of ATM, on this board, right?

The "mainstream interpretation of how we should expect reality to work"? Is that mainstream philosophy?
The latter is a very complex question involving how far one can really expect to extrapolate one's physics outside of what has been observed, before significant modifications are likely to appear. I would say the "mainstream" is quite well aware of this, if it is aware of any amount of history of science.I am confident that the "mainstream" is aware of some amount of history of science :)

Moreover, if someone builds a machine that can fly you to Paris if atoms attract gravitationally, but will kill you if they don't, do you put your family in it before that particular aspect has been tested?I don't put my family in it before the price drops far enough that they can fly economy. For now. :)

cosmocrazy
2008-Aug-31, 05:03 PM
That's easy. How could galaxies at very large distances repel? (It seems they do.) The answer is: reality can do whatever it wants, it is our job to get theories that explain it-- not the other way around.

Well no one really knows why galaxies seem to repel but one of the current theories is space-time is expanding fast enough to counter act the gravitational effects.

But i can see what you are getting at regarding what is happening in reality. I was just pointing out that to answer the OP directly based on classical assumptions it is most likely that yes 2 average grains of rice are massive enough to attract each other through the influence of gravity.

Ken G
2008-Aug-31, 05:21 PM
The mainstream interpretation of mainstream theories is what appears outside of ATM, on this board, right?
Hmm, I have no clue why you are talking about interpretations of theories. No one has any confusion about that, the question in the OP is about reality (by my reading of it, anyway). But I did already say what the theories predict, above.

Ken G
2008-Aug-31, 05:28 PM
Well no one really knows why galaxies seem to repel but one of the current theories is space-time is expanding fast enough to counter act the gravitational effects. Not quite-- the current leading theory is that it is itself a gravitational effect. Imagine if someone in 1980 had said "gravity must attract on all scales, just look at our theories of gravity"-- they would not have been considered a terribly good cosmologist.

But i can see what you are getting at regarding what is happening in reality. I was just pointing out that to answer the OP directly based on classical assumptions it is most likely that yes 2 average grains of rice are massive enough to attract each other through the influence of gravity.Yes, it is not so obvious what a question like the OP is really asking. A point I've been trying to make lately is that we really need to distiguish, both in the questions and the answers, whether what is being asked is "what does such-and-such a theory say about such-and-such a situation", versus, "what happens in reality in such a situation". The Q&A section is really riddled with that confusion, and it leads to a lot of tangential threads. In this case, I thought the OP was more like the latter type of question, to which the answer seems to be, "it has not yet been determined", though some sensitive experiments have been done and maybe to some degree it has, that's unclear at the moment.

cosmocrazy
2008-Aug-31, 05:37 PM
Yes, it is not so obvious what a question like the OP is really asking. A point I've been trying to make lately is that we really need to distinguish, both in the questions and the answers, whether what is being asked is "what does such-and-such a theory say about such-and-such a situation", versus, "what happens in reality in such a situation". The Q&A section is really riddled with that confusion, and it leads to a lot of tangential threads. In this case, I thought the OP was more like the latter type of question, to which the answer seems to be, "it has not yet been determined", though some sensitive experiments have been done and maybe to some degree it has, that's unclear at the moment.

Yes it depends on how we each interpret the question and the following answers. :)

hhEb09'1
2008-Aug-31, 05:38 PM
Hmm, I have no clue why you are talking about interpretations of theories. No one has any confusion about that, the question in the OP is about reality (by my reading of it, anyway). But I did already say what the theories predict, above.Questions about reality, in the Questions and Answers forum of BAUT, are answered with the mainstream interpretation of mainstream theories. Other speculation about possibilities is restricted to the Against The Mainstream forum.

Such speculation is certainly a common (and IMO necessary) pursuit of scientists, but that alone does not make it mainstream.

cosmocrazy
2008-Aug-31, 05:55 PM
Ok guys can we safely assume based on the current mainstream theory of gravity that the 2 grains of rice are likely to come together under the influence of gravity. But because we don't really know for sure due to the lack of experimental evidence we can keep an open mind until a better more accurate theory of gravity is discovered?

Lepton
2008-Aug-31, 05:59 PM
Ok guys can we safely assume based on the current mainstream theory of gravity that the 2 grains of rice are likely to come together under the influence of gravity. But because we don't really know for sure due to the lack of experimental evidence we can keep an open mind until a better more accurate theory of gravity is discovered?

No. Theory tells us there is an interaction so until a competing theory overturns that with valid science, believing anything other than there is an interaction (including keeping a mind so open that brains leak out) is really very foolish.

hhEb09'1
2008-Aug-31, 06:07 PM
No. Theory tells us there is an interaction so until a competing theory overturns that with valid science, believing anything other than there is an interaction (including keeping a mind so open that brains leak out) is really very foolish.Hey, that part about brains leaking out wasn't in there before, was it? :)

I was about to say, keeping an open mind is never foolish, but if you want to expand the meaning of the term to include trepanning, then I'll acquiesce that it might be in some circumstances.

Ken G
2008-Aug-31, 06:10 PM
Questions about reality, in the Questions and Answers forum of BAUT, are answered with the mainstream interpretation of mainstream theories. No, certainly not, that's my whole point. They are answered by experimental results, that's science-- and got us into the evidence available. But we can agree that a questioner needs to be clear if they want an answer about a theory, or about experimental evidence. That's why I gave both answers.

Ken G
2008-Aug-31, 06:13 PM
Ok guys can we safely assume based on the current mainstream theory of gravity that the 2 grains of rice are likely to come together under the influence of gravity. But because we don't really know for sure due to the lack of experimental evidence we can keep an open mind until a better more accurate theory of gravity is discovered?That certainly seems acceptable to me, except I wouldn't focus on the need for a "better more accurate theory", but simply, experimental verification. The former will proceed from the latter, and might be a difficult problem that takes a long time.

hhEb09'1
2008-Aug-31, 06:19 PM
No, certainly not, that's my whole point. They are answered by experimental results, that's science-- and got us into the evidence available.Experimental results are included in the mainstream interpretation of mainstream theories :)

Ken G
2008-Aug-31, 06:20 PM
No. Theory tells us there is an interaction so until a competing theory overturns that with valid science, believing anything other than there is an interaction (including keeping a mind so open that brains leak out) is really very foolish.Theory never tells us there is an interaction, that's a logically backward description of what theory is for (but it is a widespread misconception, which is why I'm taking it on). Observing the interaction is what tells us that there is an interaction, and theory never does anything but organize and unify the experimental results in a way that allows us to make predictions and gain understanding of behavior of similar systems-- where "similar" is a virtually completely subjective concept. Thus it's a hard problem to know how many times and in how many ways we need to observe a phenomenon before we can start drawing conclusions about it, versus what is really a new domain that we don't know about. It's a very gray area of science, but it at least needs to be recognized and appreciated, or we end up with silly questions like "does the universe have an edge" or "what happened before the universe began".

Ken G
2008-Aug-31, 06:24 PM
Experimental results are included in the mainstream interpretation of mainstream theories Surprisingly, I am aware of that. It hardly closes the logic of your argument, because it never excuses us from the need to have experimental support for our expectations. See my comment about cosmologists in 1980.

hhEb09'1
2008-Aug-31, 06:26 PM
Theory never tells us there is an interaction, that's a logically backward description of what theory is for (but it is a widespread misconception, which is why I'm taking it on). I'm not sure where you are going with this, so maybe this question would help: I have two 100kg iron balls in my garage, do they attract each other gravitationally?

Ken G
2008-Aug-31, 06:29 PM
As I said above, it is a subjective element of science to decide when past observations apply to future situations. We all do it, since we were babies, that's what "prediction" means. So my answer is, I can certainly make a prediction about your question, and experience gives me a high degree of certainty my prediction is correct. That situation is precisely what is lacking in the issue that this thread is actually about, as asked in the OP.

Ken G
2008-Aug-31, 06:31 PM
As a general remark, it may surprise some on this thread that scientists have, in the past, made predictions that they were virtually certain would hold true-- and didn't. Perhaps everyone should have that experience-- it would help us all understand science better.

Ross PK81
2008-Aug-31, 07:27 PM
I think any gravitational attraction between the two grains would be vastly outweighed by the force used to put them in place to begin with.

Just imagine them in deep space with nothing else close to them.

Now, I've just been thinking, that the two grains of rice must attract each other, even two atoms, as the particles of gases in space must attract each other to create planets and stars. Or am I missing something there?

cosmocrazy
2008-Aug-31, 07:33 PM
Just imagine them in deep space with nothing else close to them.

Now, I've just been thinking, that the two grains of rice must attract each other, even two atoms, as the particles of gases in space must attract each other to create planets and stars. Or am I missing something there?

No you're not missing anything. Based on our current mainstream understanding of gravity and how it interacts with matter, we would assume that the rice would attract each other just like the gas atoms & molecules do so to form stars and so on. :)

cosmocrazy
2008-Aug-31, 07:41 PM
As a general remark, it may surprise some on this thread that scientists have, in the past, made predictions that they were virtually certain would hold true-- and didn't. Perhaps everyone should have that experience-- it would help us all understand science better.

It does not surprise me at all Ken. In fact it would surprise me more if all expected certainties came true!

You are quite correct, we can not be certain that the rice in this thought experiment would in fact attract each other. But for the purpose of the OP our best answer would need to be based on mainstream expectations rather than speculations of different possibilities. My understanding of mainstream gravity is an attractive interaction between matter/energy (setting aside antimatter and so forth).

Ken G
2008-Aug-31, 07:51 PM
Now, I've just been thinking, that the two grains of rice must attract each other, even two atoms, as the particles of gases in space must attract each other to create planets and stars. Or am I missing something there?Actually, you are missing something there. The forces that drew pairs of grain-sized material, and smaller, together in our understanding of planet formation is not gravity, but electrostatic forces of the type alluded to by parallaxicality above. That's one of the reasons we don't have complete experimental constraints on the gravitational interaction between two such isolated particles on the scales of them close to touching (as in the OP).

Ken G
2008-Aug-31, 07:55 PM
But for the purpose of the OP our best answer would need to be based on mainstream expectations rather than speculations of different possibilities.That depends on what was being asked. I presume the OPer already knows the basic idea behind Newtonian gravity, or there would be no point in distinguishing rice grains from planets and moons. All I'm trying to do is demonstrate the difference between a question about a theory and a question about observations that were used to construct a theory. I'll settle for that.

My understanding of mainstream gravity is an attractive interaction between matter/energy (setting aside antimatter and so forth).But it now appears that your above-stated understanding is seriously incomplete on cosmological scales-- what makes you think it could not also be incomplete on atomic scales, or maybe even grain scales? Isn't that what the OP is asking?

cosmocrazy
2008-Aug-31, 08:00 PM
Actually, you are missing something there. The forces that drew grain-sized material, and smaller, together in our understanding of planet formation is not gravity, but electrostatic forces of the type alluded to by parallaxicality above. That's one of the reasons we don't have experimental constraints on the gravitational interaction between two such isolated particles on the scales of them close to touching (as in the OP).

I was not aware that electrostatic forces would be the main proponent for the mutual attraction between small particles as mentioned (although it makes sense to me for it to do so), so i do apologies for making any statement that refutes that fact. But would gravity still play a part in the interaction anyway. And also would this still not answer the OP? The grains of rice would mutually attract each other.

cosmocrazy
2008-Aug-31, 08:05 PM
But it now appears that your above-stated understanding is seriously incomplete on cosmological scales-- what makes you think it could not also be incomplete on atomic scales, or maybe even grain scales? Isn't that what the OP is asking?

Yes i would say thats true. I have limited understanding of such theories so that is why I'm asking the question of why do i need to understand that gravity interacts in different ways other than attraction?

Thanks David

Ken G
2008-Aug-31, 08:05 PM
I was not aware that electrostatic forces would be the main proponent for the mutual attraction between small particles as mentioned (although it makes sense to me for it to do so), so i do apologies for making any statement that refutes that fact. But would gravity still play a part in the interaction anyway. And also would this still not answer the OP? The grains of rice would mutually attract each other.It's experimentally true that huge clouds of grains must experience an ensemble-averaged gravitational attraction that is strong enough to form stars and so forth. But that is a huge ensemble average on very large scales-- it's not clear what role the kinds of weak and short-term effects inquired about in the OP might have. For example, we know that atoms stick together well enough to make a brick-- but they also repel so you can't squash a brick. Physics offers a lot of possibilities on various scales.

Ken G
2008-Aug-31, 08:08 PM
I have limited understanding of such theories so that is why I'm asking the question of why do i need to understand that gravity interacts in different ways other than attraction? No one thought they needed to understand that up until a decade ago or so. But we keep an open mind anyway-- we've been surprised so many times. In this case, it appears that masses at really huge separations will start to accelerate away from each other, for no reason other than gravity (at least, you can model it that way).

cosmocrazy
2008-Aug-31, 08:15 PM
No one thought they needed to understand that up until a decade ago or so. But we keep an open mind anyway-- we've been surprised so many times. In this case, it appears that masses at really huge separations will start to accelerate away from each other, for no reason other than gravity (at least, you can model it that way).

Ok i see what you are getting at. And don't get me wrong I'm a firm believer of taking an open minded approach. :)

The misunderstanding i have with gravity having a repelling interaction as well as an attractive one is the inconsistency?

Lepton
2008-Aug-31, 08:22 PM
Theory never tells us there is an interaction, that's a logically backward description of what theory is for (but it is a widespread misconception, which is why I'm taking it on). Observing the interaction is what tells us that there is an interaction, and theory never does anything but organize and unify the experimental results in a way that allows us to make predictions and gain understanding of behavior of similar systems-- where "similar" is a virtually completely subjective concept. Thus it's a hard problem to know how many times and in how many ways we need to observe a phenomenon before we can start drawing conclusions about it, versus what is really a new domain that we don't know about. It's a very gray area of science, but it at least needs to be recognized and appreciated, or we end up with silly questions like "does the universe have an edge" or "what happened before the universe began".
Theory doesn't predict an interaction according to G((m1m2)/r2)? Since when are valid theories unable to make predictions? Theories can make predictions BEFORE experimental results. Do you have theory and hypothesis confused?

kleindoofy
2008-Aug-31, 09:16 PM
Swaying away on a tangent which might interest the OP, I remember an experiment the Nasa did concerning minute *differences* in gravitational pull.

[The following is quoted from very dusty memory cells, so please correct as necessary.]

During the Gemini flights, they did an experiment in which the Gemini capsule rendezvoused with an object that was already in orbit. A tether was connected to both the capsule and the object. The Gemini capsule then maneuvered itself away from the object to a point leaving the object between itself and the Earth, with the tether still slack.

The theory was that although the object was only about 20 meters closer to the Earth than the Gemini capsule, the gravitational pull on the object would be greater, being enough to pull the tether taut.

Guess what. It worked like a charm.

(A similar experiment almost got Niel Armstrong & Co. killed when Newton took over the stearing wheel.)

parallaxicality
2008-Aug-31, 09:47 PM
Just imagine them in deep space with nothing else close to them.


Problem is that everything in space is in constant motion, thanks to the big bang and supernovae.

Ken G
2008-Aug-31, 10:04 PM
The misunderstanding i have with gravity having a repelling interaction as well as an attractive one is the inconsistency?Yeah, the issue is, Newton had a model for gravity that was attractive on all scales, and worked beautifully, yet was only tested on intermediate scales from, say, bowling balls to stars. Better measurements extended it down to somewhat smaller scales, and also to galaxy scales (if one introduces dark matter), but it still seems to fail to be attractive on larger scales than that. So what about smaller scales? The jury is always out until there is data-- all we can say is that any given model make predictions, not statements about reality-- what is our basis for trusting that model is quite subjective in the absence of testing.

Ken G
2008-Aug-31, 10:08 PM
Theory doesn't predict an interaction according to G((m1m2)/r2)? Since when are valid theories unable to make predictions?Um, if you read my posts again, you will find that "prediction" is a perfectly valid application of a theory, as I mentioned in one of the first posts. So the question really is, is the OP asking for a prediction? It's hard to say-- that's what I have been clarifying.
Do you have theory and hypothesis confused? No.

Lepton
2008-Aug-31, 10:22 PM
Um, if you read my posts again, you will find that "prediction" is a perfectly valid application of a theory, as I mentioned in one of the first posts. So the question really is, is the OP asking for a prediction? It's hard to say-- that's what I have been clarifying.No.

Then what exactly did you find wrong with my saying theory tells us (predicts) that there will be an interaction?

Ken G
2008-Aug-31, 10:28 PM
The identification with "tells us" and "predicts". Those are importantly different, and a lot of science lives in that difference.

Lepton
2008-Aug-31, 10:31 PM
The identification with "tells us" and "predicts". Those are different in important ways, relevant to the issue at hand.

Oh, your quibble is with semantics.

Ken G
2008-Aug-31, 11:14 PM
If it is a "quibble" to differentiate the meanings of words, then I'm guilty as charged. (I guess it's better to just think all words mean the same thing.) But if the nuance of the distinction is lost on you, call it a nitpick.

Lepton
2008-Sep-01, 12:36 AM
If it is a "quibble" to differentiate the meanings of words, then I'm guilty as charged. (I guess it's better to just think all words mean the same thing.) But if the nuance of the distinction is lost on you, call it a nitpick.
It's a semantical quibble that I thought was beneath you but I was wrong.

Ken G
2008-Sep-01, 01:49 AM
Look, this is not a semantic quibble, this is a serious problem that we have on the Q&A-- questioners, and answerers, are not taking the trouble to distinguish between questions about reality, and questions about the predictions made by particular theories. That might not normally be an issue for, say, a homework problem in a textbook, but since the Q&A questions tend to be ontological questions right on the edge of the frontiers of not only what science has done, but what science can ever do, it turns out to be a huge source of confusion and a source of a lot of just plain wrong answers.

Let me give you an example by picking out a few of the current threads on Q&A:

Basically can light travel at all wavelengths at the same time and we only detect the one that is in sync with our perception of the flow of time?
Now, is this a question about some theory of light, or is it a question about the outcome of an imagined experiment? Who can tell, it hasn't been made clear-- inviting answers that are either unresponsive or dead wrong.


But I'm curious, on an atomic/subatomic level, what is vacuum?Again, is this a question about one of several theories that use a concept of a vacuum (in completely different ways, I might add), is it a question about the outcome of some undisclosed experiment involving vacuum, or is it an invitation to wax profound on purely philosophical issues of nothingness? Not clear, but the answers will wallow in the possibilities for pages without even noticing these are three different things.


If you put two grains of rice close enough together in space, would the gravity they have attract one another together or would it not be strong enough?Yes, that's this thread-- but there's the same problem. It's totally unclear if this is a question about predictions made by either of the two gravity theories, or a more speculative gravity theory, or about the status of experimental evidence on the topic, or a philosophical question about what we imagine gravity even is. So the answers are all over the place, which at its best can "cover the bases", but at its worst lead to a tower of Babel of what people are actually talking about.

You can certainly not think there's an important confusion there, but I do. Therefore, I do not count it a quibble, I count it an opportunity for the general level of appreciation of what physics actually is to be better appreciated on the whole by people on this forum. But hey, it's just semantics-- we don't need to have the slightest idea what we are doing here.

ETA: if it isn't clear, the fundamental problem there is that when an answer that is just the prediction of some theory is mistaken for a verified statement about reality in any situation where it is no such thing, that's when science falls into sheer dogma, and we become guilty of what nonscientists often like to accuse us of. Probably because they are more aware of the history of science than we sometimes seem to be ourselves.

Lepton
2008-Sep-01, 03:20 AM
Look, this is not a semantic quibble, this is a serious problem that we have on the Q&A-- questioners, and answerers, are not taking the trouble to distinguish between questions about reality, and questions about the predictions made by particular theories. That might not normally be an issue for, say, a homework problem in a textbook, but since the Q&A questions tend to be ontological questions right on the edge of the frontiers of not only what science has done, but what science can ever do, it turns out to be a huge source of confusion and a source of a lot of just plain wrong answers.

Let me give you an example by picking out a few of the current threads on Q&A:

Now, is this a question about some theory of light, or is it a question about the outcome of an imagined experiment? Who can tell, it hasn't been made clear-- inviting answers that are either unresponsive or dead wrong.

Again, is this a question about one of several theories that use a concept of a vacuum (in completely different ways, I might add), is it a question about the outcome of some undisclosed experiment involving vacuum, or is it an invitation to wax profound on purely philosophical issues of nothingness? Not clear, but the answers will wallow in the possibilities for pages without even noticing these are three different things.

Yes, that's this thread-- but there's the same problem. It's totally unclear if this is a question about predictions made by either of the two gravity theories, or a more speculative gravity theory, or about the status of experimental evidence on the topic, or a philosophical question about what we imagine gravity even is. So the answers are all over the place, which at its best can "cover the bases", but at its worst lead to a tower of Babel of what people are actually talking about.

You can certainly not think there's an important confusion there, but I do. Therefore, I do not count it a quibble, I count it an opportunity for the general level of appreciation of what physics actually is to be better appreciated on the whole by people on this forum. But hey, it's just semantics-- we don't need to have the slightest idea what we are doing here.

ETA: if it isn't clear, the fundamental problem there is that when an answer that is just the prediction of some theory is mistaken for a verified statement about reality in any situation where it is no such thing, that's when science falls into sheer dogma, and we become guilty of what nonscientists often like to accuse us of. Probably because they are more aware of the history of science than we sometimes seem to be ourselves.

All that boils down to is a semantical quibble.

Ken G
2008-Sep-01, 03:47 AM
Then it's for whoever sees value in it, not for you.

Paul Leeks
2008-Sep-01, 04:18 AM
If I remember

F=Gm1m2/d2

it came to me "without thinking about it"....see you don't forget anything..it's always there!!

PL

Lepton
2008-Sep-01, 04:20 AM
Then it's for whoever sees value in it, not for you.

ok but it's still a semantical quibble.

Ken G
2008-Sep-01, 04:38 AM
Then I'll guess you'll get tired of hearing it if you stick around Q&A, because it is a point I will have ample opportunity to return to I'm sure. Perhaps you should just put me on ignore now and save yourself the "quibbling," I'm sure I have nothing to tell you anyway.

Neverfly
2008-Sep-01, 04:43 AM
Then I'll guess you'll get tired of hearing it if you stick around Q&A, because it is a point I will have ample opportunity to return to I'm sure. Perhaps you should just put me on ignore now and save yourself the "quibbling," I'm sure I have nothing to tell you anyway.

True, especially on this board.
Oftentimes, what appears to be a 'semantical quibble' is important to helping a person understand a concept.

hhEb09'1
2008-Sep-01, 05:21 AM
I have two 100kg iron balls in my garage, do they attract each other gravitationally?
As I said above, it is a subjective element of science to decide when past observations apply to future situations. We all do it, since we were babies, that's what "prediction" means. So my answer is, I can certainly make a prediction about your question, and experience gives me a high degree of certainty my prediction is correct. That situation is precisely what is lacking in the issue that this thread is actually about, as asked in the OP.

ETA: if it isn't clear, the fundamental problem there is that when an answer that is just the prediction of some theory is mistaken for a verified statement about reality in any situation where it is no such thing, that's when science falls into sheer dogma, and we become guilty of what nonscientists often like to accuse us of. Probably because they are more aware of the history of science than we sometimes seem to be ourselves.Would it be fair to say, then, that your position is that there would be a predicted force between the two iron balls, but it has not been verified in their particular instance?

Ken G
2008-Sep-01, 06:11 AM
That would be completely fair. However, in the case of the iron balls, there have been experiments on very similar systems. Again, as I said, that is a purely subjective determination-- there is no science that tells us how to decide that. We just have to do the best we can to decide when we think we have a "similar" situation to something observed. We have never seen gravity depend on the substance, so iron appears to be irrelevant. We have seen larger and smaller masses, without a hitch. So we are confident the experiments that inform our theory inform your balls. However, the OP is expressly not that situation, ergo my post #3. It's all about the difference between a theory that is a kind of shorthand for organizing existing experimental data, and thinking that a theory is something that constrains reality. The latter is backward logic, pure and simple-- a theory is a proxy for the data that it explains and the other data that we can reasonably expect are in the same group, not the other way around.

ETA:
Here's an analogy. Let's say we are entymologists studying ants, trying to determine the "laws" they obey as they crawl around their anthill. To do that, all we can do is observe their habits and interactions. When we infer those laws, based on their behavior, are not the laws simply a proxy for their behavior, organized in a simple way? Why would we conclude they really do obey laws just the way we infer them (they're ants)? And if there's a chamber much smaller than the rest in the anthill, and we've never seen inside it, how can we be confident those same laws will apply inside that chamber, when we don't really know why those laws were in play in the first place?

hhEb09'1
2008-Sep-01, 06:19 AM
We just have to do the best we can to decide when we think we have a "similar" situation to something observed. To tell you the truth, I don't see much difference between a grain of rice and a 100kg iron ball :)

Ken G
2008-Sep-01, 06:48 AM
Netiher do I, but it's just a matter of degree. At what scale do you see much difference? How do you determine that? What is the science of deciding what "different" means? I'm confident that cosmologists in 1980 did not see much difference between a galaxy cluster and a bunch of galaxy clusters. Newton probably didn't see much difference between 3000 km/s, and 300,000 km/s. Planck might not have at first thought there was much difference between 100 h, and h, when looking at E/nu.

Lepton
2008-Sep-01, 01:54 PM
Then I'll guess you'll get tired of hearing it if you stick around Q&A, because it is a point I will have ample opportunity to return to I'm sure. Perhaps you should just put me on ignore now and save yourself the "quibbling," I'm sure I have nothing to tell you anyway.

And you will hear from me whenever you bring it up that it is still a semantical quibble.

Ken G
2008-Sep-01, 02:23 PM
And you will hear from me whenever you bring it up that it is still a semantical quibble.

That will achieve a new height of immaturity-- I'm embarassed for you.

Lepton
2008-Sep-01, 02:26 PM
That will achieve a new height of immaturity-- I'm embarassed for you.

It will, yet you just said you would bring it up every chance you get. Same as me so I guess it would be the same for you but it would still be a semantical quibble.

ToSeek
2008-Sep-01, 03:54 PM
Aren't both of you better than this? Thread closed.