
Ask HN: What is Gravity? - shawndumas
While watching a documentary that mentioned gravity (forgive me the particulars escape me at the moment) my 8yo son reached across my lap and tapped the space-bar pausing the video. He then turned toward me and asked, ‘what <i>is</i> gravity?’<p>Now, in our house we require explanations for actions because we are becoming, sadly too late in life, convinced that being on purpose is paramount. Because it is our opinion that if you do not understand, even at the most base level (epistemological questions aside), why you are doing something then you will never have any power over what you do. When any of us does something that results in negative consequences we try to be in the habit of either tracing down the root of our reasoning or admitting that we had none.<p>To this end we have made much of the distinction between description and explanation. Briefly; all explanations include description but not the reverse. When we ask one another for an explanation and instead receive only description it has become a sort of litmus test for a lack of purpose.<p>So when I began my pedantic diatribe on gravity -- two objects attract each other with a force directly proportional to the product of the two masses and inversely proportional to the square of the distance between them… 32 ft per sec per sec... ad nauseam -- my son put his hand on my arm and said, “That’s a description dad. I wanted an explanation.”<p>It was then that I realized that I don’t have one. Yes, I have analogies but the very definition of an analogy precludes it being the thing it analogizes.<p>And so I ask (with assistance from the great brain augmenter Google): How is gravitational force produced?  Is it associated with an elementary particle, a graviton?  Is it related to strong, weak, and electromagnetic forces?  How is it transmitted through almost-empty space between the earth and moon?<p>Why is the explanation of gravity?
======
mian2zi3
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.

~~~
shawndumas
[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.

~~~
Gormo
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.

------
Elite
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...](http://www.youtube.com/watch?v=695Flhmjmg4&feature=related)

------
sz
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.

~~~
_delirium
> 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...](http://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics)).

~~~
sz
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.

------
cjg
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.

------
ck2
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)

~~~
quux
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.

~~~
ck2
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.

------
profquail
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>

------
ntoshev
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.

------
ostso
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>

------
snikolov
_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).

~~~
shawndumas
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...](http://www.springerlink.com/content/57557324753u6881/fulltext.html)

------
jonjacky
"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.

------
marze
No one knows. Only the effects are well understood.

~~~
acqq
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.

------
jules
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.

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

~~~
Elite
And why does the mass curve spacetime?

~~~
sajid
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.

~~~
shawndumas
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.

~~~
aamar
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).

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

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

------
sentinel
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.

------
shadowfox
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?

~~~
shawndumas
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?

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

~~~
shawndumas
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>

------
scotty79
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

------
davidw
<http://en.wikipedia.org/wiki/Gravitation>

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

I know that sounds crazy, but I am tired.

------
zeynel1
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...](http://en.wikipedia.org/wiki/Scientific_theory#Description_and_prediction)
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

~~~
shawndumas
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>

~~~
zeynel1
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

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

------
korch
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...](http://www.amazon.com/gp/product/B000YJ4ZTE/ref=pd_lpo_k2_dp_sr_2?pf_rd_p=486539851&pf_rd_s=lpo-
top-
stripe-1&pf_rd_t=201&pf_rd_i=0805387323&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0STRP8Z4KE68914D6EMC)

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!).

