Why do Planets Have Gravity

Why do planets have gravity?
- Anonymous

That is quite a question! Gravity is one of the most familiar forces of nature, but also one of the most resistant to a proper, complete, and satisfying explanation. Everyone knew all about gravity on Earth for as long as people knew about anything, but it took Newton to realize that this same force is what keeps the Moon in orbit around the Earth and the planets around the Sun, and was responsible for the tides. Newton’s tremendous achievement, helped by the precise astronomical data and Kepler’s inferences, describes gravity’s behavior for most practical purposes, but fails to answer why there is gravity in the first place; it merely says that the strength of gravity depends on how much mass there is and how far away you are from this mass.

Later work on gravity was done by Einstein. Einstein’s description works in more cases than Newton’s. For example, a collection of energetic particles in a box will have a gravitational effect that depends not only on the mass of the particles but also their energies. And his model, the "general theory of relativity" (which is a theory of gravity, space and time) predicts that light will bend in a gravitational field. Einstein’s theory predicts the existence of black holes (that bend light so much it cannot get out), and is used in descriptions of the entire universe (cosmology), where Newton’s model would be inadequate. Einstein’s theory also predicts that energy can propagate as waves in the gravitational field, and evidence for this has been observed from a pair of compact objects rotating around each other, losing energy at the rate predicted by the loss due to gravitational waves. Experiments of ever-increasing sensitivity are being carried out to detect these waves, but no gravitational waves have yet been observed directly.

The interpretation in Einstein’s viewpoint is that there is no "force" of gravity at all, but rather that space and time are bent in such a way that a particle moving freely with no other forces on it will follow a path that bends along with the local geometry of space and time, and follow the paths that are described by Newtonian gravity in the limits in which Newton’s model applies.

Einstein’s (and Newton’s) theory have as central features that mass (and energy in Einstein’s model) create the effects of gravity, but do not explain (as far as I know) why this must be the case, and why there are not other sources of gravity. For instance, the other forces depend on "charges" -- like electrical charge, to generate the fields and react to them. Gravity’s "charge" is matter and energy.

Once one supposes that it is in fact the case, plus some other assumptions, such as the laws of physics must be the same in inertial frames, and that gravitation is locally indistinguishable from acceleration, then the description of gravitation is very constrained to the models we have now.

One problem with all of this is that it is hard to incorporate gravity into our quantum-mechanical models of the other forces and unify them. Electricity and magnetism, the strong, and the weak nuclear forces all seem to have similar quantum structures and understanding one of them helps us to understand the others. This hasn’t been true with gravity. Quantum theories of gravity predict the existence of "gravitons" which carry gravity just as photons carry the electromagnetic force, but no direct evidence for gravitons has been found. Simple quantum theories of gravity predict nonsense, however. String theory, a more complicated model, attempts a unified explanation of gravity along with the other forces, but it needs to be tested -- some prediction of it which is not a prediction of other models must be verified with an experiment.

All that having been said, we still don’t know "why" matter and energy has gravity, only that it does. Given that as a starting point, we know lots about gravity already but still not as much as we would like to, and it continues to be an active area of research.

Tom

(published on 10/22/2007)

Is it because the Earth spins that we have gravity?
- Anonymous
sydney, NSW, Australia

No, the Earth has gravity just because it has mass. It would have almost exactly the same gravity even if it wasn't spinning at all. The gravitational effects of its spin are extremely subtle, and have not yet been reliably measured.
[update: The tiny spin effect, "frame dragging", has finally been accurately measured, in an extremely difficult  satellite experiment.]
If you get a chance, we'd love to hear how the spin-gravity connection came to mind.

Mike W.

(published on 08/13/2009)

I had a similar thought. In partical accelerators, they have shown that as things speed up, they gain mass. Also, even though we percieve centrifical force as pushing out, the direction of the acceleration is toward the center of the rotation. I felt this could have an overall affect on the mass of a planet. The bigger the planet, or any body in space, the faster it would be moving on its surface, and the more massive it would be. this would assume equal rotational speeds. Could this be true?
- michael french
napavine

Well, not quite.  The complete relativistic relationship is E² =  (mc²)²  +  (pc)² , where E is the energy, m is the rest mass,  p is the momentum, and c is the speed of light. The velocities of the planets are very small compared to the velocity of light, the standard of the industry, so their velocity gives a negligible contribution to the relativistic equation via the momentum.  There are, however, very small relativistic effects on planetary motion.  For example the orbit of Mercury is slightly altered by general relativistic effects.

LeeH

(published on 08/07/2010)

What is the motion of a falling object in the moon?
- nicol john (age 15)
phlippines

Similar to that here on Earth except that the force of gravity is only one sixth as strong as ours.  You may remember seeing film clips of Neil Armstrong bouncing around on the Moon's surface.  
The reason for the Moon's weaker gravity is that the force due to gravity at the surface of a spherical object is proportional to the mass of the object times G, Newton's constant, and inversely proportional to the square of the radius of the object.   Putting it all together you get one sixth of Earth's gravitational force.  

LeeH

(published on 08/09/2010)

Going back to Follow-Up Question #1 somewhat implying that gravity is *due to* the earth's spin, the only connection I am able to make is the one that Newton supplied (http://books.google.com/books?id=GVlpKZ67DscC&pg=PA79#v=onepage&q&f=false): that gravity acts a centripetal force and can be used to explain how two massive objects (e.g. earth and satellite in orbit) interact. It does not entail gravity (centripetal force) to be due to centrifugal force, rather it exploits the relationship of two forces that are simultaneously present. Personally, a careful choice of words is critical when saying gravity is due to... something else. Since gravitational force is present when two objects are sufficiently close, so as not to fall off the 1/d^2 scale, and both not moving, there is gravitational force and no centripetal force. Thus, gravity, in this case, does not "stand in" for centripetal force - depending on the context of relative velocities. I have two main questions: 1. Say we found a way to increase the spin of the earth, what would the outcome be if we spun it fast enough to counterbalance the effect of gravity acting as a centripetal force (centrifugal>centripetal)? What would happen to the loose objects on the crust of the earth? What would happen to the layers of the earth (I assume different rotational speeds for the differences in each of the earth's layers' materials.)? 2. If incereasing rotational spin (or revolution) reaches a speed that breaks centripetal force, can this same phenomenon be carried out in spinning out the contents from blackholes? Theoretically, we'd have to invest a lot of time and resources into *spinning out* a blackhole. But, say we find an optimal angle to shoot our infinite supply of near-speed-of-light space-junk-mass, is it possible to counterbalance the gravitational effects of something as strong as a blackhole to empty its contents? Or is there something about blackholes' gravitational pull that is more... I don't know... sophisticated(?) than our planet scaled gravitational functions that would make this completely impossible?
- Anonymous
Canada

1. Sure, if the earth were spinning much faster then its gravity would not be strong enough to keep things on the surface from flying off.  In a realistic case, stuff spinning that fast would never have managed to lump together as a planet to begin with. Let's say, however, that as in your question it had lumped together and then been set spinning. After a thin layer peeled off, what would happen to the next layer? Let's say (this isn't quite right) that the whole top layer were gone, still leaving a sphere with the same spin rate. The acceleration on the surface due to that spin is proportional to the radius. For an object with uniform density,  the surface gravitational acceleration is also proportional to the radius. So stuff would keep coming off. Probably some one has solved for the full problem, with a changing shape, but I won't try to track that down unless asked.

2. Once a black hole has formed, I don't think you can increase its angular momentum enough to change the qualitative features of its horizon, since any stuff you throw in also increases the total mass. There appear to be solutions ("worm holes") of the General Relativistic equations that allow for peculiar horizons, but even if these could be stable I don't think you can get one started with an ordinary black hole. The book you want to read to get more than just my half-educated guesses is one by Kip Thorne ().

Mike W.

(published on 05/22/2011)

Hi there. I'm no physicist, I'm just a curious civilian that has been studying the thorny problem of gravitation for some 25 years (and therefore may be Not Even Wrong :) In so doing I have tried to examine as many as possible of the concepts which have been posited since antiquity. In my examination of "phusikoi" I noted that a number of ideas proposed by Ancient Greek natural philosophers have survived into C21st science; in particular the atomist ideas of Leucippus, Democritus and Epicurus. Broadly speaking, their hypotheses stated that: - matter was indivisible below a certain scale ("άτομος") (Leucippus?) Bear in mind that an atom of gold cannot be made of gold. - everything that happened or came to be in the Universe was a result of the random* interaction of these particles (e.g Nuclear Physics, Chemistry) (*Democritus?) - the particles were endowed with a natural, innate motion which tended toward a straight line but which could be caused to 'swerve' (the latter described by Epicurus as "clinamen") The Greeks do appear to have noticed that things almost invariably travel straight towards the Earth's surface when you let go of them; and they also observed that things didn't necessarily fall straight down if your hand was moving at the point of release; a phenomenon which I agree deserves a better explanation than "Things just is the way they is". However, their explanations of these observations were less than satisfactory. Aristotle and his rather incoherent concept of mechanics seem to have held the field to such an extent that no plausible theory of mechanics was available until the Avicenna-Buridan hypothesis.(?) Somewhere along the line, atomism was rediscovered and set on a modern scientific basis. It appears on examination that the third postulate above was rejected; for instance the correspondence between Newton and Bentley refers to 'brute, inanimate matter' and postulates that Epicurus must be wrong about "gravity [being] essential and inherent in [matter]" and that gravitational attraction must be the result of some external agency (If I have understood "ye Dense C17th Prose" correctly) When the Great Names of C19th physics did their work the particle zoo contained only two exhibits; there are now considerably more particles and we enjoy a deeper understanding of what actually makes matter. It seems to me that, as we now understand matter to be anything but brute and inanimate, we should seriously re-examine Epicurus' above third postulate. In examining (and possibly validating) a concept of matter - which is capable of self-motivation (action) through a greater statistical ordering of its energetic internal processes and - which responds to external signals by increasing said ordering in such a fashion as to produce a net translation or rotation (for example restricting the freedom of tumbling "4D donuts" from a hypersphere to a hyperplane, in which the toroidal axis of rotation could conceivably indicate the position of the gravitational source). In the case of gravity all atoms accelerate at the same rate; the "energy" is not diluted - which similarly responds to external impulse (including EM) by an increase of order which persists after the impulse is removed. (However, physical impulse is transmitted electromagnetically through matter, which induces material stress. Gravity is not EM and generally does not induce such stresses other than tidal in cases where the volume in which a "significant" dG takes place is of a similar scale to the object attracted) - in which notwithstanding any tidal effects, said ordering takes a finite amount of time during which the object resists a change of its momentum through what amounts to a "drag" against the fabric of spacetime - in which the interchange of information and energy is bidirectional we may notice that a good many of the issues related to gravitation, mass, inertia, momentum etc simply vanish. We may also note that we require more degrees of freedom than the four currently offered by spacetime to explain colour charge or the kinetic effects that are brought about by the gravitational force. We might come to understand that some of these non-Euclidean degrees of freedom support only rotation. We might also consider the location of mass within nuclei (the lion's share of which is said to reside in QCBE) and wonder what the relationship between this energy, gravity and inertia might be We might even create a technology which could engineer this hypothetical action to produce a net translation - or within a region reduce the response of matter to an ambient gravitational field. We might even be able to define an absolute rest state for particles. In summary, I would hazard that all objects produce and receive the gravitational effect through their inner dynamics and the interaction of these dynamics with the fabric of spacetime at both the smallest and longest scales This post does not constitute a statement; rather it is a pondering and the question marks are implicit. It is a notion which I find rather compelling, which is why I'm happy to expose myself to the ridicule and violent oppostion of my peers : ) Any thoughts or responses on the plausibility of this suggestion would be most gratefully received
- Graham (age 51)
IOM

That covers a lot of material, and may serve as a nice intro to some of the history for some of our readers. On specifics:
1."we should seriously re-examine Epicurus' above third postulate." In effect, quantum mechanics has done that. Even ignoring the geometrical issues of General Relativity, nothing propagates in a simple straight line, thanks to wave mechanics.
2. "all objects produce and receive the gravitational effect through their inner dynamics and the interaction of these dynamics with the fabric of spacetime at both the smallest and longest scales" I guess that description would be consistent with current efforts in string theory.
3. At least on distances large compared to the Planck scale, GR already specifies the relation between energy, momentum, and gravitational effects.
4.  "We might even create a technology which could engineer this hypothetical action to produce a net translation - or within a region reduce the response of matter to an ambient gravitational field.  We might even be able to define an absolute rest state for particles." This sounds very far-fetched. The postulate that there are no preferred frames has had a spectacular run of successes in guiding the development of GR and quantum field theory. Maybe that run won't go on for ever, but I'd want to see some more definite argument before looking for a true rest frame.

I may have missed some other implicit questions, but perhaps that can get you started.

Mike W.

(published on 07/29/2011)

Hi Mike. I was hoping to get you guys started and just sit back :) Thank you so much for answering my post in such a polite and considered fashion; I pressed the send button with a certain feeling of dread, as some mainstream sites are less than kind to "Weird Science Prophets" and anything that resembles them. I hope you don't mind if I address your points in separate posts, as the backend strips off my formatting and bullet lists 1) Let me reduce the "third postulate" to what I believe to be the essential part: "...particles [are] endowed with a natural, innate motion". Here's a metaphor I constructed: Imagine you are a tribesman from the Amazon rainforest and you have no experience of the world at large; one day you stumble upon C21st civilization in the form of a man with a remote control airplane. Being an intelligent human being, you soon notice the relationship between the movements of the aircraft and the manipulation of the black box with the silver stick on top. This intrigues you and you begin to study the phenomenon. Your great opening insight is establishing the relationship between RC unit and plane. Your greatest error might well be to assume that the energy that keeps the craft in flight is being supplied by the remote control; it will take a little while longer for you to grasp that the large part of that energy resides within the craft itself and the RC unit merely supplies a relatively tiny control signal. Well, that's how I am coming to see gravitation (and other 'field-produced' effects).
- Graham
IOM

Names like "energy" or "field" or "metric tensor" are not among the primitive concepts we start with. Rather, they denote concepts developed as part of an overall explanatory system. You seem to be pointing toward some entirely new explanatory system. I'm not sure what it is, and not sure why you would expect, say, the concept of metric tensor to not remain in it but the concept of energy to translate untouched.

Mike W.

(published on 07/30/2011)

Hi Mike, I don't think g[mu,nu] is going anywhere any time soon. It's much more likely that I'm talking drivel; but let's take a fibreglass cast of Einstein's rubber sheet and invert it to form a positive gradient. Now let's take our cast to the barycentre of the universe so all gravitational terms are balanced and summed to zero. Let's replace the ball bearing with a clockwork car with movable (passively steerable) front wheels. So, the hump in the middle is the posited "signal"; and for a certain family of release trajectories and velocities the car climbs to the top and stays there. (Of course, this is still a "Batman and Robin" scenario rather than a mutual interaction mediated by the metric tensor). I would suggest that my idea is related more to T[mu,nu] on the RH side of EFE. What is your interpretation of the metric tensor and the Ricci terms in the LH side of the EFE?
- Graham
IOM

Hi Graham- I'm not educated enough to have coherent interpretive thoughts on Einstein's field equation. I do have a remark on the rubber-sheet analogy, or your fiberglass cast version. The usual picture in which there's some external gravity pulling a heavy thing down and stretching the sheet to make a downward depression is highly misleading. The "up" or "down" for the distortion of the sheet is meaningless, since gravity only concerns geometrical relations within the sheet, not relations to some hypothesized other dimension. About all I know about those tensors is that they are indeed tensors, not vectors. Turning things "upside down" doesn't make any difference.

Mike W.

(published on 08/10/2011)

I'm sorry if my question is not as complex as others but here goes. my understanding is that gravity on earth is caused by earths disruption of the space time fabric as described by Einstein's general theory of relativity and our rotation around the sun is cause by its disruption of space time and our solar system spins around our galaxy because of the gravity from the black whole in the center......... if that's even correct but what stops gravity from completely sucking us in why do we rotate around the sun rather then get sucked right into it. why does the moon rotate the earth rather then crash right into it. What's that counter force keeping us spinning
- Todd (age 20)
Canada

 This is an interesting question that we get often in many different forms.

The answer is actually simpler to answer without trying to imagine the "space time fabric."  It's a complex metaphor that tends to be misleading.

The simple answer to why the Earth doesn't crash into the Sun, is because the Earth is moving relative to the sun.  If we sort of freeze frame the process

We have the Sun on the left, and we'll say it's stationary.  The Earth, on the right has some velocity going up, with a force pulling it to the left.  A force on an object will cause an acceleration, which is a change in velocity. That's Newton's Second law of motion.  In this case, since the force is at a right angle to the velocity, the magnitude of the velocity isn't going to change, but the direction will.  So after a little bit of time the direction of the Earth's velocity will be up, and a little to the left, but during that time the earth had already been moving upward so it will be further up than it was before (an object in motion tends to stay in motion the same way, Newton's First Law).  The direction of the force is always directly toward the sun so since the earth moved up the force is now pointing left, and a little bit down, relative to the earth.  So again, the force is at a right angle to the velocity.  If we keep doing this one small step at a time we will slowly watch the Earth trace a circular path around the sun.

So the answer to the question "What's that counter force keeping us spinning" is that there is none.  Its the fact that we have initial velocity perpendicular (or nearly perpendicular) to the direction of the force of gravity.

There are a few things to note about this.  First you don't usually get circles in reality.  A circular orbit requires a perfect balance between the velocity and force, in reality you typically get elliptical orbits.  Next, a misconception that a lot of people have is that the Earth orbits the Sun. Period.  This isn't wrong but it doesn't describe the whole picture.  If we look more carefully we see both the Sun and the Earth would orbit their mutual center of gravity.  We say the Earth orbits the Sun because since the Sun is so much more massive than the Earth, this center of gravity is going to be much closer to the center of the Sun than the center of the Earth.  This follows Newton's Third Law of motion, which tells us that the total momentum of the combined system doesn't change. And in fact every object in the solar system orbits around the whole system's center of gravity, which again is much closer to the sun than anything else. The main other object in the solar system, besides the sun, is not the Earth but massive Jupiter.

We can extend this description to the whole galaxy.  Yes there is a super massive black hole at the center.  Most things don't fall into it because they have a high enough tangential velocity to avoid doing so.  However, even if there wasn't a black hole at the center of the galaxy, the galaxy could still orbit around its center of gravity.  There doesn't actually have to be an object at the center.

We can also translate the same picture to the Moon-Earth combination.

Mike Boehme

(published on 04/24/2012)

re. your answer to Question #8: Why doesn't the earth crash into the sun? I understand your answer but what became of "centrifugal" force? Is the acceleration of earth (due to its change of direction)equal to the centrifugal force? Thanks.
- Guido Fernandez (age 81)
Longmont CO Boulder

There really isn't any centrifugal force, at least from the point of view of a standard "inertial" frame. If you try to use a rotating frame, like a merry-go-round, you'll find an odd tendency of things to accelerate even when there's nothing around exerting any force on them. Centrifugal force is one of those sourceless pseudo-forces that arise from using strange accelerating coordinates.

At a very deep level gravity is also a pseudo-force, but to the extent that we can use Euclid's geometry and Newton's physics gravity is just a regular force, the one Newton described. Unlike "centrifugal force",  gravity has obvious sources.

Mike W.

(published on 02/10/2013)

If I were on a large space station that rotated to simulate gravity, and I were to jump in a specific way, would it be possible to overcome the centrifugal, coriolis and whatever other forces to "float freely" as I would when jumping off the deck of an "ordinary" space station?
- Greg (age 24)
Sydney, Australia

That's a neat question. Let's look from the point of view of an inertial frame, say based on the center of mass of the ship. The person who jumped just right to become stationary in that frame would experience no accelerations, other than a tiny bit of gravity from the ship. So the ship would just keep rotating around them as they stayed suspended between the walls.

The description in the rotating frame is messier. If the ship is rotating say clockwise, in its frame the person is rotating counterclockwise, accelerating inward. There's an outward centrifugal pseudo-force, and (since there's a tangential instantaneous velocity in this frame) an inward Coriolis pseudo-force, twice as big. The net result gives the correct acceleration.

Mike W.

(published on 02/14/2013)

Why do objects have gravitational pull. My teacher says even people have a gravitational pull, but why and how?
- Mitch (age 14)
Strathmore, Alberta, canada

Mitch- The short answer is that we (at least those of us doing the answering here) don't know. Tom discusses some of what's known above.

Mike W.

(published on 06/12/2013)

If gravity comes from mass and mass comes from the Higgs field, couldn't we say that gravity arises because of the Higgs field?
- Anonymous

Gravity comes from all forms of energy/momentum. The Higgs field contributes the rest mass (rest energy) of the massive elementary particles of ordinary matter (electrons, quarks) , so it plays a big role in familiar gravity. Other terms in the mass of composite particles (protons, neutrons, nuclei) come from the interactions between the more elementary ones. Since the type of bound states the more elementary ones form depends on their masses, and hence on the Higgs contribution, it's important for all ordinary gravity. Gravity would exist without it, however.

Mike W.

(published on 09/25/2014)

why Mercury have less gravity as compared to earthwhile its mass is almost same as earth.is it because of lower spin speed
- Imran (age 33)
Lahore,Punjab,Pakistan

Hello Imra,

I think you have your numbers wrong.  According to the latest measurements the mass of Mercury is only about one twentieth that of the earth.   Its density is about the same as the earth's.

Check the latest NASA numbers: 

LeeH

 

328.5E21 kg (0.055 Earth mass)

(published on 03/31/2015)

I'm really confused! I had a read through of the questions and answers above but don't think it's quite covered. So, the earth has a force of gravity due to it's mass, of 10m/s2 "downwards". As mentioned above, the spinning of the earth does give some small centripetal force upwards which is negligible. If the earth spun fast enough, could gravity and the centripetal force of it's spinning cancel out? I'm really struggling to get my head around this, surely a satelite in orbit isn't affected at all by the spin of the earth, has no centripetal force exerted on it and therefore the only force affecting it is the gravity of earth (ignoring other planets & sun). I can't quite get my head around where the boundary is for the centripetal force - when we jump in the air, would we become satelites in a way because we're going so fast around the planet that we're constantly falling towards it like a satelite? I suppose if that was the case, when a rocket were to exit the earth's atmosphere, it wouldn't be travelling through space in a "straight line" instead it'd also be moving sideways consistent with the spinning of the earth? Maybe I'm really over complicating this! Am I on the right tracks or completely got lost along the way? Also, I suppose the centripetal force and gravity could only equal on the equator, the closer to the poles you get, the less and less centripetal force would apply.
- Chris Ingham (age 23)
Liverpool

Just to avoid confusing other readers, I'll mention that at points you use "centripetal" when you seem to mean "centrifugal".

Yes, if the Earth were spinning fast enough then its gravity would not be strong enough to hold things at the equator down and some would fly off. That effect already causes the equator to bulge a bit, so that the Earth isn't quite spherical.

A satellite in orbit isn't affected by the Earth's spin, except for a tiny, barely measurable, General Relativistic effect. The satellite stays in orbit because gravity is just strong enough to counteract the tendency to fly away in simple inertial motion. If you want to look at that in a frame rotating around with the satellite, you'd say that the centrifugal force just canceled the gravitational force.

When you jump, you do briefly become a satellite. The velocity you pick up from the Earth's rotation is much smaller than you'd need to have an orbit large enough to avoid bumping back into the Earth. It is enough, however, to help a rocket give you enough sideways velocity to get a big orbit. So it's easier to launch a satellite into an eastward-going orbit than into a westward-going one.

So you are on the right track. The one place where I think you went wrong is to think of some sort of "boundary". The net velocity is all that matters, regardless of whether it comes from the Earth's spin or from a rocket or from something else.

Mike W.

(published on 01/05/2016)

My question is in regards to question #8. My question is with regards to the final statements of the response, which I will provide:"There doesn't actually have to be an object at the center.We can also translate the same picture to the Moon-Earth combination.Mike Boehme"This confused me. I was enjoying the read, picturing a sun that wobbled around it's own center of gravity, rather than the middle school class display of our system which shows a hard-pinned position for the sun. 1) However from everything I have read here, there is no gravity without mass or energy. How then could there be a center of gravity that does not contain mass?My next question regards the frame of thought that brought me on my journey to your amazing page, where anyone has access to expert information (thank you).While staring at stars last night I found myself curious towards gravity. Like previous posters, I found myself intuitively considering rotation to be a central part of gravity's existence. However, the moon does not rotate, and we still see the effects gravity there. What if though the moon were not rotating around Earth? (I almost said "the earth", but I bet that is a pet peeve of some in your department.)2) If an object in space is completely stationary by what % would gravitation be reduced?3) Outside of finding a perfectly stationary object relative to the universe, is the speed of light necessary to calculate gravitational effects on a completely stationary object?
- Curtis S. (age 28)
Warrensburg, Mo, USA

1) The center of mass is kind of an abstraction. For a donut, the center of mass (or center of gravity) is right in the hole, where there's no donut. Something like that can happen with all sorts of collections of masses, with no general rule about whether the center of mass is right where some of the mass sits.

Gravity is what makes it possible for things to rotate without flying away, so I guess that's why people associate rotation and gravity. Rotation, however, isn't necessary for gravity.

2 and 3) Any object is completely stationary from its own point of view. Relativity says that its own point of view is ok, meaning it gets to use our usual laws of physics. For smallish gravitational fields, like those in the solar system, you can calculate gravitational effects pretty well without mentioning the speed of light. More precise calculations, like those used by GPS systems, require full relativistic expressions that do involve c.

Mike W.

(published on 07/24/2016)