Is gravity inherent in mass?

What is gravity? I know it’s a natural force but does it act independently of mass or does gravity reside in mass?

What started me thinking about this was the analogy of gravity ‘warping’ space used by Sagan. The bowling ball on the trampoline. When less massive tennis ball is rolled near it, it becomes trapped by the distortion caused by the bowling ball. But rather than creating “gravity” this experiment relies on existing gravity. The tennis ball is really attracted to the earth not the bowling ball.

Other questions:
Why do the large gas planets have so much gravitational pull when they apparently don’t have much mass?
Where does gravity go when a star explodes?

I’ll take this one. Of course, they have a lot of mass. Jupiter, for example, is about 318 times the mass of the Earth.

>What is gravity? I know it’s a natural force but does it act independently of mass or does gravity reside in mass?

I’ll take this one. Yes, gravity and inertia and mass are all tied together. Mass is anything that exhibits inertia and gravity. As Newton put it, the relation between mass and force is that force is the time derivative of velocity multiplied by mass. He wondered if this necessarily was always tied to gravitational force, but now we know it is. Gravitational force is just the consequence of mass causing the space around it to accelerate radially, which is what is often called “warping space”. Fundamentally, mass is made of information.

I should have been more emphatic, to wit: gravity never acts independently of mass in any way. It is strictly a consequence of mass.

I should have been more clear, is gravity in mass itself (like a magnet) or does mass cause a deformation of space that other mass “falls” into?

The latter.

You can’t really separate those two things. Insofar as anyone knows, you cannot have a warping of spacetime without the presence of mass. Gravity is an inevitable and constant result of the presence of mass; given a particular mass, you will always have the same amount of gravity, regardless of where in the universe you put it. Therefor, it’s just as proper to say that gravity originates from mass as it is to say that mass warps spacetime which results in gravity.

Isn’t the concept of “falling into” something a consequence of gravity in the first place? Falling into a hole won’t happen in the absence of gravity will it? So does this make this a circular argument?


No, it’s not a circular arghument. The illustrative example of a ball on a rubber sheet subjected to earth’s gravitation is just that, an illustrative example. It in no way is the proper explanation of the effect of gravity on the coordinates of space-time.

Gravity can be defined as the forces that masses exert on each ofther by virtue of their mass. It is more accurately defined as the distortion of the coordinate system resulting from the presence of mass.

>I should have been more clear, is gravity in mass itself (like a magnet) or does mass cause a deformation of space that other mass “falls” into?

I don’t understand the question, or at least don’t see that there is a physical distinction between the two choices. Magnets are a clear case, because the force they manipulate is just electrostatic force as a consequence of Einsteinian special relativistic shortening along the travel axis of moving charge. You could say magnets change the space around them, or that they interact by exchanging photons, the particles that mediate the electromagnetic force. You could also say that masses change the space around them, or (in my understanding) that they interact by exchanging gravitons. Perhaps we think of what masses do as more fundamental or more essential to space itself because of what we call “warping”, but this is somewhat circular because we use the resistance of mass to velocity change as our test of inertial frames. It might take a debate about what we imply by “space”, “force”, and so on, to figure this out. And this all might be a sort of semantic reflection of how our language and ways of thinking evolved to deal with the world we typically grew up in as a species.

Can you think of some test cases that would distinguish between your two options as you see them?

And by mass, we mean mass/energy, given that energy and mass are equivalent via E = m c[sup]2[/sup].

My stab at answering the OP: mass and energy affect the space-time they reside in, in a way described by the General Theory of Relativity. (Special Relativity describes the relation between mass and velocity inside inertial frames of reference.) The way mass-energy changes space-time causes other mass-energy to experience an attractive force,* called gravity. Gravity is an inherent property of mass-energy, just like inertia.

  • Now that I’m thinking about, is gravity a force or a pseudo-force? Since it’s just an effect of the coordinate system, I’m thinking it’s actually more of a pseudo-force (like the Coriolis force, for example).

What about during acceleration. In physics they told us that as we speed up our “mass” increases, but that doesn’t make much sense to me. How does mass increase? Or is it just that mass appears to increase? Does something that has negligible mass cause gravitational distortions when accelerating very quickly?

It takes energy to speed up and energy is the equivalent of mass. We can see this in dramatic form every time we speed particles around an accelerator. We can measure the great increase in mass that results.

That increased-mass particle will have exactly the same gravitational effect as a stationary particle of equivalent mass. (A speeding electron may have the same effective mass as a proton, say.) Of course, any particle, even a heavy one, has essentially negligible gravity. But it’s there.

All right. You guys lost me.

  1. Pleonast stated that energy and mass are related through *E = mc2
  2. Gravity is a function of mass.

But if I understand your example, Exapno, (particle at rest versus the particle at high speed), this change in energy state has no effect on it’s gravitational properties…

That sounds contradictory…

No, I think he means that a particle of increased-mass A will have the same gravitational effect as a stationary particle of mass A.

:smack: Thanks.

:confused: What is the difference between a Force and a Property of Mass, if Mass is a Force itself?

Mass isn’t a force. Mass is Force divided by acceleration.

Since nobody’s addressed this question yet…

The gravity is associated with the matter that makes up the star. If a portion of the star’s mass is ejected, the remaining mass that makes up the white dwarf star or black hole that results is much less than that of the star that exploded (presumably due to a supernova). The resulting white dwarf or black hole will therefore have less gravitational attraction for other objects because you’re now dealing with a smaller stellar object.

However, that portion of the star that was ejected, though spread out in all directions, also has mass associated with it, and the total amount of mass (and associated gravity) is very near the same as that of the original parent star. (“Very nearly” because there may be some mass-energy conversion issues going on in a supernova, not to mention production of metals/high atomic number elements).

Hmm. How fast are these changes in gravity felt outside the exploding star? Does that move with lightspeed too?