"--Gravity causes gas to fall towards the central baby star, but the measured speed is much less than the free-fall speed. Something must be slowing the gas down.--" this is a paste from a recent article in "science news".( ALMA reports Baby str's blanket) perhaps the 1 km/sec "infall" speed is due to the anemic gravity in the inside, thought to be the outside.
Where is most "gravity", inside or out?
- Thread starter nebel
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the disk is a disk, not a sphere, In a disk there would be even less gravity on the inside, because the area of the ideal disk declines with the square of the radius from the surface toward the center. Origin's graph would be modified to show a slumping curve toward the zero gravity at center. So, refute this: In an ideal disk inside gravity declines at the square of the distance toward the center, but on the outside diminishes in linear progression toward zero at infinity. I have never heard that caveat from orbital velocity researchers since Vera Rubin, or?
yes, and remember the fringes might have more substance than we suspect. so refute the assertions of last two posts above, preferable with relevant graphics.
There is a dinstinct maths for Gravitational Calculations of Disk type dictribution of mass.
You are right if the radius of a disk is r and maximum thickness (bulge) is t (< r), then Gravity at a point x from the centre (where r > x > t) will be different if x is inside the disk ( r side) or if the x is on the bulge side.
Astrophysical Journal, 2016; 822 (1): 21 DOI: 10.3847/0004-637X/822/1/21 from Science news; "--dizzying spin in the galactic halo--"
as expected. there is no gravity at the center of it all, and the most action in in the periphery and 'from here to eternity.'
as expected. there is no gravity at the center of it all, and the most action in in the periphery and 'from here to eternity.'
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Tha area is not important the volume/density is important.
I misunderstood the question and so the graph is for a "solid" object such as a planet or a star.
At some distance a disc will appear to be a point source so the gravity will decrease at 1/r^2. Refuted.
The area is part of the gravity equations for it is a function of the spatial measurements. Pack the area/volume with more matter, and the gravity will still fall to zero at the center, but greatly increase in the distant distance infinity. The more matter the better for that matter. see post 84
how would you define some distance? and do not forget: for every km added to the radius in/or outside a "perfect" disk, you add only ~1.6 km , one mile to the circumference, regardless of size, even across the universe. so:--?
and: Scientists looking for invisible dark matter can't find any. so: may be there is more gravity outside than the inside --ders think.
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in spherical, circular entities, any symmetrically distributed mass , a ring, a shell, or variation thereof, will gravitationally affect only it's outside; so: expect higher orbital velocities there; do not expect invisible heavenly halos to behave as being heavy, having a mass/gravity effect on the entity they enclose. case closed, or?
Fair enough. I prefer volume but it is of little consequence.
What does "but greatly increase in the distant distance infinity" mean?
I do not know what that is suppose to mean. How can something be 'better' for matter? Since we are on post #48, I am not sure how to see post #84.
The rule of thumb for the inverse square law is: when the distance from a source is >10x the dimension of the largest dimension of the source the source can be treated as a point source.
So what.
The problem is that there is no evidence or reason to believe that there is something strange going on with gravity. It is possible, but not very likely. There is MUCH more evidence that there is some sort of matter that does not typically interact with 'normal' matter or photons.[/QUOTE]
How did you get this ~1.6 km ?
Janus58
Valued Senior Member
Don't confuse the common everyday image of a "halo" as being a ring shape, with what "halo" means in astronomy. The dark matter halo is not ring or torus that encircles the galaxy, but is instead a spherical mass in which the disk of the galaxy is embedded. (dark matter exists between the stars of the galaxy as well as "above", "below" and surrounding it.) This means that for any particular star, there is a spherical volume of dark matter that is closer to the center of the galaxy than it is and effects its orbit.
The density of this dark matter doesn't even have to very large to have a significant effect. In the vicinity of the Solar system, the estimated density of dark matter is 1e-22 kg/m^2. Over the entire volume of the solar system this works out to be the equivalent to the total mass of a small asteroid, it is also ~1/100 the density of the interplanetary medium, which can be considered a better vacuum than any we can form on Earth.
But even at that low density, in the volume encircled by a star 50,000 light years from the center of the galaxy, and which contains any mass that effects its orbit, it adds up to a mass of ~ 20 billion solar masses, or a significant fraction of the mass of the visible matter of the galaxy.
The formula for the circumference for a circle any size is: diameter x 3.14, so it would be radius x 1.5713 . so, the galaxie's circumference would would increase by one mile for every kilometer increase in radius. or?
sorry, could not read the small number, it is post 46, about the unexpectedly high velocities in the halo. and: the more matter you pack into the space occupied by an entity, the shorter the proportional fall to zero in the center, the greater the gradual fall of gravity to the distant infinity. or?
thank you for that precise definition, the graph in post 2 shows the same amount of gravity is reached at less than 2r, so there is plenty of scope to have a linear, strictly area-based gravity situation in a thin disk. or?
I should have phrased that better. In your graph post 2, the maximum gravity is correctly shown at the "surface" on an entity, What I meant by "place" was the sum gravity of all locations in the area, comparing the inside with the outside. If you measured all the gravity forces that a body experienced, and measured by the free fall/orbital velocities, The inside fall would be short and zero-acceleration at 1r in but long and at max near the surface (get a parachute or a net, pile of cartons to catch you). or?
PS: If you recorded the strength of gravity and the time it acts, on a constant velocity body, would you not register much more gravity action during the outside recording session?
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suppose the idea of the circumference (the curved line that the central disk's mass/gravity acts on),- increasing only by a mile for every km of increased distance from the center of the galaxy, or cluster, then the attractive effect would be diluted very little, at those scales. and: on the outside the scaling goes on forever, whereas on the inside it comes to a relieving zero after only r. or?
There is a recent news item (I think I tried to post on the next "gravity turned inside/out" thread), that highlighted that attempts to corral "dark" matter by other than velocity measurements failed. If you got the application of gravity wrong, You could be looking for a "red herring" warping space. but, agreed, the estimate, computation of matter distribution in large bodies is complicated, and simple me might be missing something there.
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I thought you would get the hint. You missed the radius based formula.....it is actually ...Radius * 2 * 3.14.
So if you increase the radius by 1 meter of any circle, then circumference will go up by 2 * 3.14 = 6.28 meters irrespective of the original radius...
PS : May be you interchanged diameter with radius or vice versa.
Thank you for the correction. apart from this glaring error, what it means though, is, that the circumference of any size disk entity would increase only be 4.6 meter more for that 1 meter increase in radius, or reach of gravity. In other words: the effect of gravity increases with greater and greater distance, because the ratio of radius (reach) and affected arch becomes increasingly insignificant. --- yeah, divided instead of multiplied by 2. --ok so what is 1.6 vs 6,28 metric units between friends talking about thousands of light year halos? or?
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so here is my question: how would that play out, not in an ideal disk, but a sphere? , which is acting as a point mass close by, not only at a distance? could we have mis-interpreted gravity because overlooking (until now) this simple Ancient Greece arithmetic? Gravity's effects becoming more pronounced with distance?
so, are you saying that dark matter is evenly distributed everywhere, even appearing in the expanding enclosed space of the universe? would that not mean that it would have zero effect on the existing gravity gradients ? , but only shift it to higher values ?
I am referring to the shell theorem, where an empty shell has zero gravity inside, only projecting it into the infinite distance, but with ever increasing effect, as opined in the thread on "gravity inside/outside". this because of the ever diminishing effects of linear expansion on the expanded perimeter. do not astrophysicists consider a distinct halo, wimpy or macho, as the source of the higher outer orbit velocities? PS : would ubiquitous dark matter not render the shell theorem invalid? dark matter not detected in recent experiments.
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