This just occurred to me and I've never read anything in the popular press about it:

We start with a piece of matter, which we transform all or partly into energy via E=MC^2. What happens to the lost matter's gravity? Does it just stop? ie: gravity waves, or whatever gravity turns out to be - simply ceases to radiate out into the universe?

Is our sun's gravity becoming weaker as it loses 15 billion metric tons of mass each hour? It must. I don't suppose we could ever measure that, could we? Say by measuring the distance to the sun and comparing that to the theoretical distance of a sun that is not losing mass.

Are there any practical gravity experiments that can be conducted regarding this principle?

RBG

No. Gravity's still there. The problem is how to keep the energy from spreading.

The Sun's mass is decreasing, although the anoint is extremely small compared to the mass of the star. Sun's gravity decreases, because its energy is radiated into the space. But that amount is negligible compared to the amount that is lost due solar wind. That form of mass loss will speed up dramatically when the Sun turns into a red giant. Finally, of course, the Sun loses most of its mass that way.

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Okay, that is interesting. You are then saying that "energy" has and retains the gravity. Presumably no-loss proportional to the originally transformed mass. That's not what I would have expected.

Sorry for these basic & naive questions but I'm compelled by the ghost of Galileo to continue.

So if energy has gravity, can you then have huge pockets or reserves of pure "energy" in space perhaps invisible to telescopes but never-the-less tugging on the rest of the universe?

RBG

No, you cannot have huge reserves, at least not in the way you seem to be thinking of it. The Sun is a huge reserve of energy, but when it is manifested it takes off at the speed of light in all directions.

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T. Anderson

i think they are called "naked singularity's"

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RBG,

Mass already is energy. Mass is just one form energy can take.

However, mass is special in that it is the most concentrated form of
energy. All other forms of energy are less compact. Light, as an
extreme example, always spreads out because it is always in motion.
So it isn't practically possible to have a large enough amount of light
in a small enough volume to detect its gravity. The total amount of
energy in the cosmic background radiation is very large, but is spread
out very thinly throughout an enormous volume of space. The same
is true of all the light from all the stars that have ever been.

-- Jeff, in Minneapolis

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Kaptain K: If the energy manifests itself at the speed of light in all directions, then the implication, of course, is that a star's own gravity is decreasing, and that the gravity is being "released" in an ever-expanding speed-of-light sphere around the star.

Can this "store" of progressively weakening, invisible gravity from an extinguished or dying star in any way account for the dark matter that appears to be inexplicably tugging at the rest of the universe?

RBG

Yes, that makes sense. Thanks Jeff.

As a very crude analogy think of a rubber sheet with a heavy ball placed in the middle. Replace this heavy ball with a pile of sand, where the sand is most concentrated the sheet is depressed the deepest. As you spread the sand across the sheet the depression becomes shallower until once spread evenly across the whole sheet the depression disappears. I guess you could say that energy works in a similar way. In the universe very dense lumps of energy form matter, as this congregates to larger lumps the curvature of space becomes more apparent. As you know very massive objects concentrated in a very small volume curve space-time to such an extreme that even light cannot escape, hence the existence of BH's. Spread this energy/matter over a large area the effects of curvature become less apparent.

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No. Dark matter is something different. We're not exactly sure what dark matter is, but it does have significant mass and it interacts with ordinary matter (and itself) by gravity. It neither emits nor absorbs EM radiation, that's why we can't see it.

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T. Anderson

Someone, the late John Wheeler I believe got interested in whether the EFE (Einstein Field Equations, the governing equation of General Relativity) would allow basically a "ball of pure energy" to hold itself together by its own gravity. That is, for instance, could some self sustaining region of EM radiation hold itself together gravitationally without spreading out to the four winds. And besides that, how about a ball of *gravitational radiation* (which would be a really odd thing, pure self-sustaining gravity).

The answer turned out to be no, such a self-sustaining ball was not possible, it would fly apart.

-Richard

Ah, the term Wheeler coined was "geon" for this. His hope was that "geons" could explain fundamental particles purely in terms of geometry. A geon would be a little "packet" of self-sustaining curvature.

It turns out that is has NOT been definitely proven that no stable geon configuration is possible, but most everyone thinks they are not.

-Richard

Ditto publius.

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

Would you consider this the reason (based on the current BB model) that at the beginning of the universe the immense energy of the initial state of the universe caused the rapid expansion and then on to cool further to create the forces and the matter (in its original simplest form) that we see today? At this initial state would there not be a ball of pure energy? I realize that gravity is considered to have come into force when the original hypothetical "single force" split into the 4 we consider today. Its all interesting stuff!

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We are not alone, but we may as well be!