why does the curvature of spacetime depend on the matter density of an object, and not just the mass?
466045
Picture two objects, one being a large beach ball, tightly filled with air, the other a fist sized stone.  These two objects will be dropped into a pond from the same height.  Which one makes the bigger splash?  Another way of looking at it would be to picture the same two objects sitting on the middle of a stretched out rubber sheet.  Looking at the indentation fromt he under side of the sheet, the stone will have a much larger one.  The cool thing is when the proportions of mass to density are unusual in that an object has an extremely large density and very small mass.  The resutls to the surrounding fabric of spacetime is noticibly more dramatic the further apart these two measurements become.  Another fun thing to think about is that as speed increases towards the speed of light an object increases in density and becomes extremely heavy.  This means that an object with any density while stationary has a variable potential for manipulation of spacetime dependant upon it's speed.  Read The Elegant Universe.  It's a cool book if you like this sort of stuff.  :)\n\nAnother thing to picture to help understand this concept:  Again go back to a stretched out rubber sheet, this time you are pressing objects into it with the same force.  One is a pencil (eraser side), the other is a basketball.  Remember that you are pressing just as heard on both objects (which represents gravity having the same pull on both) even though they are of different sizes.  This represents one small object with heigh density having the same pull as a larger mass object with lower density.  Now think about what the stretches on the sheet look like from each experiment.  Now take that idea and try to picture it in 3 dimensions (or 4 if you want to get technical :p)
164688
There are ways of calculating the average mass and the size of the universe, but so far nobody there have been several flawless answers that don't agree with one another.\n\nThe answer is going to depend on what shape space is. Gravity is going to work differently in flat space, positively curved space, or negatively curved space. At the tiny, tiny, tiny level (say, a planet or moon), gravity is just going to pull you to the closest physical body. Even on the slightly larger level (a star and its planets), gravity is pretty constant. But when you start looking at the effect of galaxies on one another, and those wacky and inexplicable quasars way the hell out there, suddenly the principles of gravity that seem to simple are breaking apart!\n\nSo is it infinite? Go find an answer you like, and there's probably good evidence to support it. Does the universe stop? That's most likely true, but if you go to the edge of the universe, then the edge of the universe won't be at that point anymore. So again, find an answer you like and be happy with it, because nobody can prove you wrong!
376917
Absolutely!  Gravity bends space and time itself.  Imagine taping all four eges of Saranwrap to an empty box.  Put a baseball on it, and it will sink, just a little, curving the previously flat sheet.  Now roll a marble around the baseball.  If it is fast enough it can roll right on by.  If it is too slow, it can get caught by the curve in the Suranwrap.  This is what mass does to space and this bending is responsible for gravity.  It acts like a sink to any matter or, as Einstein proved, to energy (E=mc^2).   A large mass in space will affect the area around it just as the baseball does.  The larger the mass, the more pull.  Black holes are a prime example of the effects on light.  They have so much gravitational force that light cannot escape.  Astronomers have also noticed light from space being bent on its approach to earth by other large bodies, mostly stars, in space.  As, the guy above me has also indicated.\n\nThe guy below me has no clue.  The acceleration due to gravity on earth is 9.8 m/s^2.  The velocity of light in a vacuum is 3.0*10^8 m/s.  Velocity and acceleration are two different beasts entirely.  In addition, the force of gravity on earth is not the same of the force of gravity everywhere in the universe.  Back to my black hole example.
601513
The basic idea behind “warping” space or time comes from Einstein’s General theory of Relativity… What he proposed was a totally new, geometrical description of space and time that led to a different description of gravity—one that linked space to time and mass to energy in a fundamentally geometrical fashion. In other words, mass(-energy) shapes space(-time) instead of acting like a “force” in the traditional Newtonian sense. This alteration in the fundamental geometry of space-time is often described as the “curvature” or “bending” of space. \n\n(Thus, in Newton’s description of gravity, one would say that all bodies exert an attractive gravitational force on one another; “space” is just the place where it happens. In Einstein’s conception, one would say that all bodies change the underlying geometry of space—and time, too—in a way that causes them to be attracted to one another.)\n\nSo we often talk of space-time being “warped” or “curved” by the presence of mass(-energy). The only we see space-time deformed in the Universe around us is by the presence of a lot of matter—light being bent around the Sun, clocks being affected by Earth’s gravity, that sort of thing. \n\nThe caveat is that curvature or bending really only provides an analogy to what the equations tell us. If you want to imagine the effects of an object on its surrounding space(-time), then you can think of it as changing the shape of space(-time) in its vicinity. The distortion is real—we can measure its effects—but the images that are often presented (of flexible sheets of space-time with bowling balls or marbles rolling around on them) simply present analogous situations.\n\n\nRyan Wyatt\nRose Center for Earth & Space\nNew York, New York
882919
Matter is something that has mass. Mass, according to the general theory of relativity, is caused by the warping of space. A particle that warps space is said to have mass. These particles are known as matter. The terminology is kind of redundant.\n\nNo one knows what the actual particles are made of. Wheeler tried to explain everything as being a warping or curvature of space. But failed.\n\nThe smallest units of matter, that we know of, are leptons (such as electrons) and quarks.
952077
Its a general rule that things with mass will curve space time. This is through the theory of general relativity and universal gravitation. However, what we do not know is if gravity is responsible for frame dragging (where the mass actually twists space time) or just making a dent in it, or a combo of both. Additionally, many scientists are uncomfortable with relativity because it doesnt agree with quantum mechanics and the math is useless when applying it to black holes, both of which are equally real and can be proven. But why? Im afraid that that is a philosphical question that can be answered simply: they just do. And dont go there because that is a pandora's box, just keep things simple.
180507
Imagine two ships at the opposite ends of the equator moving towards the equator. They move closer to each other as reach the poles. For a person on the ships this is puzzling since there is no force pushing them towards each other. But for an observer in the third dimension it is obviously due to the curvature of the earth.\nAccording to Einstein attraction due to gravity is not an independent force but it exists due to curvature of space.\nThat is the curvature of Space-Time.
734528
It has been proven that space is curved, and there's two observations that stand out in my mind.\n\nOne is the precession of Mercury's orbit. This was something that could not be explained using Newton's classical mechanics, but fit perfectly with Einstein's General Theory of Relativity (which predicts that space-time is curved).\n\nThe second is based on an observation made some years ago during a total solar eclipse. A star which would have appeared near the edge of the disc of the sun was seen within the sun-moon disc (or at least measurably closer). In other words, as the light beam traveled near the sun was bent toward the sun (this is known as the "Gravitational Len's Effect"). Since the photon is a massless particle, the gravitational effect can't affect it. Therefore, the conclusion is that space-time is warped around massive bodies (even around the Earth, but the effect is barely noticeable, if at all).
228461
Large bodies of matter, like planets, have a strong gravitational field.  The gravity, created by the mass of the object tends to pull the surface into a shape with the least amount of entropy.  This shape is a sphere.
285273
The Universe IS Infinite !!! \n\nThe shape of the universe is determined by a struggle between the momentum of expansion and the pull of gravity. \nThe rate of expansion is expressed by the Hubble Constant, Ho, while the strength of gravity depends on the density and pressure of the matter in the universe. If the pressure of the matter is low, as is the case with most forms of matter we know of, then the fate of the universe is governed by the density. If the density of the universe is less than the "critical density" which is proportional to the square of the Hubble constant, then the universe will expand forever. If the density of the universe is greater than the "critical density", then gravity will eventually win and the universe will collapse back on itself, the so called "Big Crunch". However, the results of the WMAP mission and observations of distant supernova have suggested that the expansion of the universe is actually accelerating which implies the existence of a form of matter with a strong negative pressure, such as the cosmological constant. This strange form of matter is also sometimes referred to as the "dark energy". If dark energy in fact plays a significant role in the evolution of the universe, then in all likelihood the universe will continue to expand forever.  \n\n\nThe density of the universe also determines its geometry. If the density of the universe exceeds the critical density, then the geometry of space is closed and positively curved like the surface of a sphere. This implies that initially parallel photon paths converge slowly, eventually cross, and return back to their starting point (if the universe lasts long enough). If the density of the universe is less than the critical density, then the geometry of space is open, negatively curved like the surface of a saddle. If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper. Thus, there is a direct link between the geometry of the universe and its fate.
918848
According to general relativity, the matter inside a black hole collapses to a single point: a "singularity".  At this point, the mass density (ρ = m/V) is undefined (because the volume of a single point is zero). \nWe have a lot to learn about extreme cases such as this.
46750
It depends on who you ask (sort of).\n\nAccording to Newton gravity is a force proportional to the mass of the two objects involved, and inversly proportional to the square of their distance.\n(To see the relevant equations go here: \nhttp://scienceworld.wolfram.com/physics/Gravity.html )\n\nFrom our perspective we typically drop things much less massive than the Earth (apples, bicycles, pizzas) and very close to the surface of the Earth (that is, almost at the same distance from the Earth's center) the result is that things fall with the same acceleration 9.8 m/s^2 regardless of their mass. In other words, drop an apple and a bulldozer from the same distance and (if air resistance is ignored) they will hit the ground at the same time.\n\nHowever, if you ask Einstein then the nature of gravity is more complex (although in the limit of apples and bulldozers near the surface of the Earth, the same 9.8 m/s^2 pops out of Einstein's theory).\n\nAccording to Einstein's Theory of General Relativity gravity is a "warping" of space time. Mass will bend space-time, and this bending will influence the way in which other objects move.It is sort of summed up in the expression: "Mass tells space what to do, and space tells mass where to move."\n\nSo for Einstein, in some sense, there is no force of gravity (although it certainly seems that way if you fall out of a building).\n\nIf you ask leading physics people today they will say that Einstein was on the right track all right, but General Realtivity stills needs to be work nicely with the other major idea of modern physics, Quantum Mechanics.\n\nThere is still a long way to go in making QM and GR part of a single consistent theory. But it is a very powerful goal since it would unite gravity with the forces which are explained quite well with QM (electromagnetism, the strong nuclear force, and the weak nuclear force.)\n\nIn QM forces are carried by force particles. Photons, for instance, are the force carrying particles of electromagnetism. The force carrying particle of gravity - something which has not been discovered - is called the graviton. So, in modern physics, if you have a graviton then you are back to gravity being a force. \n\nOf course efforts to successfully unite GR and QM are still going on. String theory, is one way that is popular these days.\n\nSo the answers are:\nNewton - gravity is a force based on mass and distance\nEinstein - gravity is the warping of space-time\nUltra-Modern - gravity is a force carryed by gravitons
366338
Since hydrogen atom is considered the basic of matter formation.\nThe space-time curvature is the same both inside and outside the hydrogen atom.
87660
Any matter with mass also has the property of gravity. There are two basic schools of thought about what gravity itself is. One idea is that the attractive properties of gravity result from two or more chunks of matter exchanging an elementary particle called the "graviton." One example would be the gravitational attraction between the Earth and the Moon---each planet exchanges gravitons. So far, these graviton particles have not been observed.\n\nAnother idea, this one from Einstein's theory of relativity, says that matter (mass) actually bends the fabric of space, sort of like what would happen if you placed a bowling ball on a sheet of thin rubber. The resulting deformation of space by mass results in other objects to follow the shape of the deformed space. In the Earth/Moon example, the Moon orbits around Earth because it's simply following the shape of the wrinkle in space made by Earth's mass.
401680
The theory of dark matter has been developed to try and explain the observed characteristics of large-scale celestial objects, like galaxies. Mass (matter) causes gravity which in turn effects the shape and motions of things like spiral galaxies. However, the observed shape and motions of spiral galaxies can't be explained by the amount of matter known to be there. Some other kind of matter must be present, and that's called "dark matter."\n\nDon't confuse "dark matter" with "dark energy." The latter is another mysterious feature of the universe that is thought to be causing the expansion rate of the universe to be increasing.\n\nHere's the theorized percentages of dark matter, dark energy, and regular matter for the universe --\nDark matter.....23%\nDark energy.....73%\nRegular matter..4%\n\nYou could almost say that 97% of the universe appears to be missing :)
773187
Nobody really knows.  Einstein's special theory of relativity posits gravity as a warp of space-time caused by the object's mass, but that only pushes the question one layer further down.  All we know for sure is that gravity exists, is pervasive, and can be important.
5021
yes, space-time gets warped/bent with mass.  This has been used to try and explain gravity somewhat, but with objects like planets, it doesn't happen that much.  The 'strings' from string theory and black holes bend space-time considerably.  black holes are basically singularities the size of a baseball that are as heavy as a planet or star... that's how dense they are.  It's basically so dense that it's just protons and neutrons smashed together, and considering that most of atoms are usually empty space, this is very very tightly packed, which is why there's so much gravity and the escape velocity to get out of a black hole is so great that even light isn't fast enough to get out.  Cosmic strings (the ones froms string theory) are also this dense and they are also said to bend space-time around themselves.  If you're interested in this, I would read "The Elegent Universe" by Brian Greene.  It will explain it a lot better than I can here.