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One perspective is that physics at this level is purely descriptive. Hawking, in his response to Penrose in the book The Large, the Small, and the Human Mind, writes: "[I believe] that physics theories are just mathematical models we construct, and that it is meaningless to ask if they correspond to reality, just whether they predict observations." It is a common mistake for people to confuse a physical theory like Newtonian mechanics or general relativity for reality itself. I feel like an explanation of gravity at this point is a metaphysical question that is outside the realm of science. It may be that science ultimately comes up with an explanation, but it will be in terms of something else and you will naturally be led to ask, what is an explanation of THAT thing?

As for your description "... two objects attract each other ...," this is an outdated, Newtonian way of thinking about gravity. It involves a "spooky" action at a distance. As I understand it, it is generally thought that the best theories are purely local ones. General relativity is the proper local formulation of gravity: masses curve space, and all objects move in "straight" lines (that is, geodesics) through curved space. Newton was right about inertia. There might be more sophisticated ways of understanding gravity in quantum gravity, but that's beyond me.

[I believe] that physics theories are just mathematical models we construct, and that it is meaningless to ask if they correspond to reality, just whether they predict observations." --Hawking

That's where I'm heading... I am slowly coming to the point, which is hard to take-in emotionally, that that’s all the justification we have for any scientific theory.

I know what you're going through. I was deeply troubled (still am, really) when I first read Hume and started thinking about the Problem of Induction. You might enjoy taking a look at Hume's Inquiry on Human Understanding. (I'm sure you can find it online.) I also found Wesley Salmon's essay "An Encounter with David Hume" useful. I originally read it here:

http://www.amazon.com/Reason-Responsibility-Readings-Problem...

(This book has several other good essays in philosophy of science.) Salmon's essay seems to be reproduced here:

http://kslinker.com/An_Encounter_with_David_Hume.htm

That's pretty bog-standard empiricist epistemology: reality is what it is, irrespective of our understanding. But it's very hard for the human mind to fully comprehend that, because the mind can only operate on ideas, not the reality that those ideas represent.

We are incapable of distinguishing the world itself from our understanding of it, except through a kind of cognitive dissonance: tentatively accept certain ideas as "true" in order to apply them to useful ends, while maintaining a conscious skepticism of any of them actually being "true".

This is itself a very hard balance to maintain. Many people tend to gravitate toward the polar extremes, becoming either ideologues who thoroughly conflate the entirety of reality with their subjective ideas, or "just the facts" cynics who dismiss anything remotely theoretical.

Scientific theories aren't so different from explanations of any other sort. None of them (whether science or otherwise) are descriptions of "reality"; they are all models. But anyway that seems off to the side of what you were asking for, which is an explanation (i.e. reasons or causes), not direct correspondence to reality, which is a different issue.

Scientific theories, like other theories, do have "explanations", in the sense of broader or more general theories. It's true that there are some scientific theories (arguably including gravity) that do not yet have good explanations, but I'd guess that in all such cases, people are still looking.

If you're son already understands the distinction between a description and an explanation I'd say he's off to an excellent start.

That being said, I'm not a physicist, but my understanding is that no one know exactly what gravity is. We understand it's effects pretty well from both classical physics as well as via the curvature of space. But we don't know what causes it.

You should go back to your son and tell him humbly that we don't know. I would explain to him the curvature of spacetime using a heavy ball in a table cloth as an example. This unravels one layer of the mystery but leaves other unanswered questions. WHY does an object curve spacetime?? Unanswered questions do wonders to inspire curious children like your son.

BTW, Watch this video from Richard Feynman. He tells an amazing story about him asking his father a similar question about "inertia". http://www.youtube.com/watch?v=695Flhmjmg4&feature=relat...

I'm not sure that what you're asking is possible. An "explanation" is just a description of a deeper concept that implies what you're trying to explain. Physics is pretty much as deep as you can go. Put another way, physical theories are descriptions of the universe that agree with observation. You can explain other things in terms of physics, but it's hard to explain the nature of the universe in terms of something deeper without religion.

You might want to explain to your son how science works instead; how theories are formed, accepted, invalidated, and replaced. Looking up Feynman interviews on YouTube would be a good start.

> Physics is pretty much as deep as you can go.

I don't think all physicists actually think that, or there wouldn't be nearly as much interest in and effort expended on things like interpretations of quantum mechanics, i.e. attempts to explain what quantum mechanics means at some level deeper than merely "here is quantum mechanics, and it makes testable predictions" (http://en.wikipedia.org/wiki/Interpretations_of_quantum_mech...).

I should have said philosophy instead of religion in my comment. You're right, there isn't a perfect line between physics and philosophy (cf. the whole debate on whether string theory is science). When theories about the universe are no longer testable (pure mathematical speculation), there's not much to distinguish physics from philosophy-with-equations. But all of our theories about the universe up to the limit of reasonable certainty I would consider physics.

"Physics is pretty much as deep as you can go." ... http://imgs.xkcd.com/comics/purity.png

Our current best theory of gravity is Einstein's General Relativity.

The model in this theory is that mass (and energy) curves spacetime. A body that doesn't have a force applied to it follows a straight line, but in curved spacetime, a straight line might correspond to, for example, a planet's orbit.

This is, however, just another level of description, and you could ask why this happens. We don't yet know the next level of description. It is hoped that some future theory will reconcile quantum mechanics with general relativity and that might give us some further insight into "why".

It's worth pointing out that any theory isn't necessarily a description of how something actually works. It is merely a model that fits experimental results. Often a model can be reformulated using different, but ultimately equivalent, mathematical language and it would give the same predictions.

In a similar vein, you could ask why objects continue to move in a straight line if no force is applied (Newton's first law). That's something I would like to know.

Hold a blanket up horizontally by four corners so it's flat parallel to the ground.

Drop a basketball in the middle.

See the sag? That represents gravity in this analogy.

Space/time is the blanket, the sag is gravity because of the mass of the basketball.

Gravity is the sag in space/time because mass came into existence.

(matter came from the energy @ big bang when it cooled)

'[I]n this analogy[...]'

I have analogies but the very definition of an analogy precludes it being the thing it analogizes.

Right, but what's actually special about matter/energy that it causes the spacetime to bend?

In your analogy the basketball is accelerated through the blanket by gravity, and thus deforms the blanket, but in the real world we don't know what the mechanism is that causes spacetime to bend.

Im not sure "mechanism" is the best approach here.

If you start at special relativity (c is constant), you can use the equivalence principle to show that time runs differently in gravitational fields. Then its only a small step to light bending & all paths bending.

Does energy cause space/time to sag? Or just matter?

If the universe had no matter, only "empty space", there would be no gravity.

I see it as space/time WANTS to be perfectly flat but any existence of mass causes it to warp/sag. Blackholes cause super-sags to the point of infinity (some say it can tear space/time at the final point) and wormholes in theory can connect two points of the space/time "blanket".

The problem is people want to see gravity as some kind of particle or wave like light but it's not, it's an effect, a result of mass itself.

It's not that we are being pulled, we are actually "falling" into the sag itself because we are supported by the evenness of space/time and without it, we "fall".

In empty space where there are no big bodies of mass close enough, there are no nearby sags in space/time, hence no gravity, hence no "falling into space/time" or at least the effect is very very weak.

There was another explanation that came up recently, called "emergent gravity". The author of the paper (linked below) believes that gravity is an entropic force (i.e., it is a result of entropy in the universe, but not a fundamental force in it's own right).

You can read the author's blog for more info -- specifically, the post titled "Essential points of the paper".

Blog: http://staff.science.uva.nl/~erikv/page20/page18/page18.html

Paper (PDF): http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.0785v1.pdf

Feynman's "Messenger lectures" @ Cornell begin with the example of the law of gravitation, and it's a really excellent lecture (for you, not for your son).

I think your son doesn't need answers to the questions you ask here. He wants a simpler explanation allowing him to understand the concept and relate it to things he sees in his daily life. Start with something counter-intuitive to hold his attention, e.g. with the fact that things with different mass fall with equal speed. Then talk about the Earth and the moon, tides, what an orbit is.

Here is Richard Feynman's answer to a similar question (this isn't about gravity but about this type of question): http://www.youtube.com/watch?v=wMFPe-DwULM

Yes, I have analogies but the very definition of an analogy precludes it being the thing it analogizes.
...

How is gravitational force produced? Is it associated with an elementary particle, a graviton?

You seem to suggest that analogies get in the way of real understanding. But I think at the root of understanding (as opposed to just proving empirically) is a set of nested analogies at an increasing level of detail. For example, if we knew that gravity was caused by say, gravitons, our understanding would be little more than a set of analogies about the gravitons (but ones that have been made rigorous and proven empirically).

If I gave the impression that good analogies are anything other than a good thing then I apologize.

A good analogy can be crucial to understanding. But understanding an analogy in and of itself is not the same thing as understanding the thing analogized. At some point demonstrating the ability to move beyond analogy to explaining the real thing is necessary.

In other words using an analogy to augment an actual explanation is one thing; substituting an analogy for an explanation is another. It's actually an argumentative fallacy and has a name; 'Retroductive Analogy' (Retroductive as opposed to Inductive, or Deductive.)

Some more info: "This fallacy derives from the retroductive or abductive form of reasoning described by Charles Sanders Peirce. According to Peirce’s account, retroduction can provide good reasons to pursue a hypothesis but does not, by itself, provide good reasons to believe the hypothesis. In successful applications of retroduction, pursuit leads to the accumulation of evidence. In retroductive analogy, comparison with other successful hypotheses is substituted for the genuine pursuit of evidence."[1]

[1]: http://www.springerlink.com/content/57557324753u6881/fulltex...

"That’s a description dad. I wanted an explanation."

It might be helpful to explain that a description at one level is an explanation at the next level up. For example with gravity, the description of the force between masses (etc.) is (part of) the explanation of the observed motions of the planets (etc.). At the very bottom, at the most fundamental level, we might have only description.

No one knows. Only the effects are well understood.

And it must be acceptable answer -- it's not the only thing we don't know. Almost all of the things in physics are known only as observations, and as "laws" which fit with observations and allow us to predict the results of the similar observations. Then we intentionally try to find some observation that makes us refining the rules. Like Lorentz's formulas are the refinement of some Newtonian "laws." That's how science works.

One valid viewpoint may be It doesn't matter. The only thing there is is the effect of gravity. Whether you explain it one way or another way doesn't matter. What matters is the result. Two theories that predict exactly the same things in different ways are for all purposes equivalent and indistinguishable. Of course one theory might be easier to use.

A mass curves the spacetime around it. The curved spacetime then affects the path of nearby objects. That's gravity.

And why does the mass curve spacetime?

I'm not sure it makes sense to ask why mass curves spacetime in GR. That would be like asking why the speed of light is invariant in SR.

Ok, why is the speed of light a constant? The answer is that we say that light travels at constant speed because it’s an observation that works with all the currently excepted theories.

We are stuck saying that light travels at a constant speed because that’s an observation that fits in a coherent theory with all sorts of other detailed observations of diverse phenomena.

I am slowly coming to the point, which is hard to take in emotionally, that that’s all the justification we have for any scientific theory.

Your distinction between "description" and "explanation" is interesting. It seems like "explanation" is very much what you describe in your second sentence: describing how an observation is a consequence of a broader array of forces from which other, diverse phenomena also emerge.

So you can explain a theory in much the same way by embedding it in a yet broader basis that explains multiple theories. For example, the theory of magnetism was long ago explained as a special case of electromagnetism, and now of the electroweak force. In the same way, gravity could be explained by quantum field theory or string theory, but more work needs to be done to do so.

Your feeling of disappointment might come from the issue of "intuition," which can make explanations feel satisfying. Many explanations for everyday things feel intuitive; they "make sense" because they fit patterns that we're familiar with. Many physics theories are deeply unintuitive unless you spend time with the detailed math. That doesn't mean the explanations aren't explanations; it just means that they don't feel satisfying (without intense studying).

see my response[1] to a very similar question above

[1]: http://news.ycombinator.com/item?id=1630769

Eistein's original formulation of SR required two postulates: that the laws of physics are indistinguishable between any two inertial reference frames, and that the speed of light is constant in all inertial reference frames. Both of these are experimentally motivated.

However, that's not the only way to obtain SR. You can, for example, assume Maxwell's equations (or any of the higher-level electrodynamic field formulations) and postulate #1, and obtain the invariant speed of light as the only valid solution for self-propagating EM waves in free space. I rather prefer this formulation, as the EM field formulation feels more "fundamental" than a fixed speed of light.

You can also derive SR from postulate #1 and some properties of space: isotropy, homogeneity, and memorylessness.

This is a pretty good explanation: http://news.ycombinator.com/item?id=1631206

You've got two problems, though: 1) We don't yet have a consistent explanation for what gravity is. General Relativity is good, but breaks down at small distances. 2) At some point, physics becomes math, and at some point after that there are no explanations. For example, why does 1=1? Basically, the answer is "by definition".

It's a perfectly reasonable question to ask. As we learn more about science we are able to unravel layers of mystery from physical phenomena and explain observations that were previously taken to be axioms.

Although the statement "mass curves spacetime" is true, I don't think it is simple enough to be considered an axiom. We'll have to dig deeper. Who knows, maybe the OP's son will do it.

I believe speed of light in vacuum is invariant because universe has maximum speed and light can't go faster. If it wasn't invariant then in some cases it would have to.

Then again why universe has speed limit? Does it have anything to do with causality?

Everything curves space-time. Even the gravitational field itself curves space-time.

little kid mode

Why?

scram little kid mode :)

Any scientist who can't explain to an eight-year old what he is doing is a charlatan.

  —Kurt Vonnegut, Cat's Cradle

Because the hyperbolic metric Einstein created allows us to define how much space-time can change the amount of mass(and vice versa!). And this only works if the speed of light is constant—that's actually the God-like conceptual leap Einstein took when realizing the Michelson-Morley experiment implied much bigger things about space, time, mass and gravity, and not just mere light speed.

Also, it keeps several other super-important constraints in place too. That's the problem with gravity—it's more like an infinite Jenga puzzle and anything you add to the explanation must not contradict dozens of other old physics experiments about things having nothing to do with gravity. We can't throw out all the other stuff we do know after all. Like the constancy of the speed of light in any reference frame. And the conservation of energy. And Maxwell's electrodynamic equations. And a bunch of other specific experiments I don't even know much about.

I don't know how much of thermodynamics has been "relativized", but gravity can't contradict the laws of thermodynamics either. It's actually comforting that the world of thermodynamics is so tiny, cold, deals with such a large number of "particles"(ensembles), and is built from insanely complex mathematical machinery that we probably have no hope of ever devising an experiment that could say anything meaningful about gravity in terms of thermodynamics. (I so hope that I'm very wrong here.)

However, Gravity is famously in contradiction with Quantum Mechanics, though I haven't yet progressed far enough in either to be able to say specifically what the contradiction is. I suspect it's just the lack of a math "bridge" between Relativity, which is built up from calculus, differential forms, tensors, Rienmannian geometry, the calculus of variations and weird ways QM uses linear algebra, eigenvalues, infinite bounded operators and insane algebraic concepts like von Neumann algebras to setup infinite matrices of probabilities that map back to the periodic, bounded solutions of the Schrödinger wave equation that tell us what the energy levels of some physics experiment must be.

Oh and the model of gravity has to be able to fit in well with any new stuff we discover too. Being able to pull off all of that is why I place Einstein in the pantheon of geniuses! He's kind of like a one-armed man who holds off an entire army of swordsmen using only a pocket knife.

> And why does the mass curve spacetime?

Because that's exactly what means to be a mass. You can't call something a mass if it does not curve spacetime.

Not sure, but this awesome video might help:

http://www.youtube.com/watch?v=D3AfNXdg2Tk

It's about black holes, which are objects with immense gravitational fields. About 4 minutes into the movie, they start explaining gravity.

I must say I didnt quite understand how explanation differs from description (other than that the former encompasses the latter, but not vice-versa). What should be added to a description to make it an explanation?

If the former always encompasses the latter but not vice-versa then, tautologically, the latter cannot be equivalent to the former. For example since all dogs are animals but not all animals are dogs then dogs and animals are not coterminous. So at the very least we seem to agree that description and explanation are not coterminous.

But, and here is the interesting part, if that response was not quite satisfactory then reflect for a moment why. I'll wait. (Done!? Wow you're quick!) That response was a description of one aspect of the difference between a description and an explanation and not an explanation.

As to an explanation it actually consists of, in part, descriptions so sometimes it's a little difficult to see at times. So it's understandable to be somewhat confused.

Let's say you come home and a friend that is staying over, unbeknown to you, has a tea kettle on and it's whistling. You hear a whistling sound and give you're friend a curious look and they reply, 'It's coming from the tea kettle.' That would be a description.

An explanation would go something like this: Heat excites the molecules of water increasing the space between them, causing the steam to take up more space than the water and cooled air did. That builds pressure... etc.

The short answer is that a description has detail and an explanation also has reason.

Does that help?

Hmm. Interesting. I need to think about this :)

If you can though, please let me know what you come up with either way.

I frequently find that some people (intelligent people mind you) struggle with this concept. I am always looking for better ways to explain it and your input might help.

In the meantime read my other response[1] to the same question in this very thread.

Thanks.

[1]: http://news.ycombinator.com/item?id=1631652

Presence of mass [1] shapes space time so that standing still and moving undisturbed looks like traveling over conic curves.

[1] also presence of energy and movement of mass and energy

http://en.wikipedia.org/wiki/Gravitation

I have a feeling that it is opposite of entropy. A kind of legacy information.

I know that sounds crazy, but I am tired.

i read the explanation of explanation http://en.wikipedia.org/wiki/Explanation and the description of description http://en.wikipedia.org/wiki/Scientific_theory#Description_a... and vice versa and i still dont understand the difference - can the original poster give a few simple examples of explanation and description to clarify - thanks

A description is a statement that represents something in words in order to re-create a mental picture that represents a person, place, event, or action so that the reader may picture that which is being described. In short, to describe is to represent in words. (The etymological origin is from French and literally means 'I copy off, transcribe'.)

An explanation is a statement that makes something comprehensible by describing the relevant structure or operation or circumstances. Further, to give a sufficiently detailed report about (a) the reason for something, about why something happened, about a causal chain of events; about (b) how something works, about how elements in a system interact; about (c) how to do something, about the steps which need to be accomplished in order to accomplish a certain goal. (From French as well and means 'to flatten, or spread out'.)

So, for example, saying that, 'that combustion engine is 4 feet high, 150 pounds, shiny gray, loud, and hot' would be descriptive.

Saying the following explanatory bit would be a good start at describing how it works: air is initially introduced into a combustion chamber. The air is then compressed. This high compression heats the air. At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber. The fuel injector ensures that the fuel is broken down into small droplets, and that the fuel is distributed evenly. The heat of the compressed air vaporizes the fuel. The vapor is then ignited by the heat from the compressed air in the combustion chamber. As the vapor reaches ignition temperature this causes an abrupt increase in pressure above the piston. The rapid expansion of combustion gases then drives the piston downward, supplying power to the crankshaft.

As was said: all explanation includes description but not vice-versa. You describe the components of an explanation. You may be making a fallacy of composition (the fallacy of inferring from the fact that every part of a whole is some-thing that the whole also is that same-thing [or something like that]). Just because an explanation is made up of descriptions does not mean that an explanation is only a description. (Just try that with an average American girl; here babe, I brought you a loose pile of pure carbon. What? A diamond! It's the same thing, no?)

Also, check this response out http://news.ycombinator.com/item?id=1630769

this is why i asked a few example - you just gave 1 example

from your example what i understand is that

description is a statement about qualities of something - you give measurements in standard units - color - and its temperature

explanation is a description of a process - because then you describe the order of how a combustion engine works

because you said -heat- in your description - you think you said something other than -temperature- you talked about in your description - but temperature of something was part of its quality - that is - description according to you

so you are not at all clear what type of answer you need for gravity

dont forget that you asked -what is gravity- so you can only expect an answer which says -gravity is x- then you will have to ask what is x

For clarification; is it correct to say that you recognize no difference between a description and an explanation?

Gravity just might be the most interesting thing out there, second only to Prime Numbers, IMNSHO. As a fledgling "amateur-mathematician" (don't ask), I am astonished that centuries after the Newtonian Revolution kicked off all of science, we still don't know what underlies the very basis of Newton's discovery. Even today, measure the amount of "force" of gravity on very large or small scales is damned near impossible. But we must believe in it nonetheless!

We sure take a lot of our hindsight knowledge for granted.

What is gravity? We sure think we have pinned it down because we can measure its effects on very precise scales. But measuring a thing is not at all the same as understanding a thing, in a teleological sense. I am not a physicist, but then again, I'm pretty sure most physicists are equally in the dark about what gravity is, so there's little harm to be done firing blindly in the dark.

The way I understand it so far, the explanation of gravity is intimately tied into the way we measure, model and predict the behavior of gravity. And all of that is equivalent to our mathematical formulation of gravity.

Newton was fortunate enough that the precision of measurements in his era were orders of magnitudes worse than now. So he could create calculus to explain his notion of gravitational force, assuming all space and time in the universe were a single, absolute "reference frame", and not run into any real problems. I've always seen the creation of the calculus and the formulation of the central force field hypothesis as being flip sides of the same coin—without the notion of one, you would never arrive at the idea of the other. So that's one way of explaining it.

In Netwon's day, the primary objection to gravity was that you had to believe in "action-at-a-distance", and that the force of gravity "travels" instantaneously across the entire Universe. I don't recall, but there was some Aristotilean or Scholastic hang-up on this point. And we still have this problem—it'sonly slightly alleviated by General Relativity sweeping it under the rug of "things-we-can't-hope-to-measure-until-we-get-intergalactic-starships"

Einstein's genius lay in being a great connector of existing ideas. He managed to take Newton's gravity, a dash of Bolztmann's atomic hypothesis & statistical mechanics, a pinch of Riemann's non-euclidean metrical ideas, and notice the very staggering pattern that Lorentz transforms and Minkowski space on paper exactly match how gravity appears to work, even with relaxing Newton's assumption of a single, universe-wide, unchanging space-time. It does seem far, far too convenient that of all the possible models the Universe could take, it "chose" to make gravity appear locally Euclidean, yet globally Minkowski, and that the hyperbolic metric of non-euclidean geometry can be made to exactly model the behavior of gravity in non-inertial-frames. All of that explanation exists only on paper too.

As an aside, I cannot recommend this book enough for a not-too-difficult, motivated-sophomore-level calculus based explanation of gravity—just 60 pages in and you will "get it", as least the bigger picture of gravity: The Geometry of Spacetime: An Introduction to Special and General Relativity http://www.amazon.com/gp/product/B000YJ4ZTE/ref=pd_lpo_k2_dp...

I also highly recommend Westfall's definitive biography of Newton, "Never at Rest." He spends a lot of time picking apart Newton's conceptual problems with gravity. It's like looking over Newton's shoulder, with added commentary, so you can glimpse a lot more about the weird nature of gravity without being drowned in mathematical equations.

So again, the explanation is almost entirely mathematical. While this may turn off a lot of folks who (incorrectly) believe they can never understand enough calculus to even glimpse what Newton & Einstein were saying, I personally believe the opposite—wanting to know what gravity really is serves as a great motivation for learning more math Which everyone ought to do!

It was about 6 months ago that a paper was published(and linked to on HN) saying no separate effect of gravity could be detected even right down to the Planck scale(the smallest theoretical "distances" we can ever hope to measure!). Gravity, being the weakest of the 4 forces, is even weaker than we thought, and yet it is the "prime mover" of the entire Universe. If gravity cannot be easily detected even on the Planck scale, this is going to be a huge, huge problem in figuring out gravity. What the gist of the experiment seemed to suggest is that we have no hope of ever detecting "gravitons." I am not a physicist, so I may not fully understand this new research, but I think that's what it was implying if you read between the lines.

We may never know what gravity is. But I would say it's all about the journey anyways, and never the destination. So all of the mathematics you have to pick up along the way to grok that we don't know what gravity is, is itself of more possible value than knowing what gravity really is. (Of course I might be wrong—a gravity manipulation machine would be most excellent!).

Ask HN: What is Gravity?