# Why do Planets Have Gravity

*Most recent answer: 10/22/2007*

- Anonymous

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)*

## Follow-Up #1: gravity and spin

- 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)*

## Follow-Up #2: Planetary mass, gravity, and acceleration.

- michael french

napavine

^{2}= (mc

^{2})

^{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)*

## Follow-Up #3: What is the motion of a falling object in the moon?

- nicol john (age 15)

phlippines

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)*

## Follow-Up #4: fast spinning earth?

- Anonymous

Canada

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)*

## Follow-Up #5: Some philosophical questons

- Graham (age 51)

IOM

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)*

## Follow-Up #6: philosophy of physics

- Graham

IOM

Mike W.

*(published on 07/30/2011)*

## Follow-Up #7: remarks on General Relativity

- Graham

IOM

Mike W.

*(published on 08/10/2011)*

## Follow-Up #8: Why doesn't the Earth crash into the SUn?

- Todd (age 20)

Canada

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)*

## Follow-Up #9: what became of centrifugal force?

- Guido Fernandez (age 81)

Longmont CO Boulder

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)*

## Follow-Up #10: being stationary in a rotating spaceship

- Greg (age 24)

Sydney, Australia

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)*

## Follow-Up #11: Why is there gravity?

- 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)*

## Follow-Up #12: gravity and Higgs

- 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)*

## Follow-Up #13: Mass of the planet mercury?

- 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)*

## Follow-Up #14: spinning Earth and gravity

- 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)*

## Follow-Up #15: solar system center of mass

- 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)*