Questions tagged [gravity]

Questions regarding the attractive force which exists between any two bodies of matter.

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Is the dark matter/baryonic matter ratio the same in galaxies with supermassive black holes?

My hypothesis is that if the ratio of dark matter/matter in galaxies with supermassive(weighing billions of suns) black holes are higher the black hole itself would be made of a large portion of dark ...
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1 vote
2 answers
113 views

How can we see black holes collapse if time inside the event horizon is infinite?

My understanding is that if you fell into a black hole, i.e. crossing the event horizon time would be speed up infinitely fast. This means I could see my whole galaxy die, new blackholes develop and ...
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4 votes
1 answer
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Why are what end up being meteorites not in stable orbit as planets and large asteroids are?

We don't have to worry about Mars, etc. hitting Earth. Is it that, being smaller, small asteroids have less inertia and so are more affected by, I guess, the gravity of various planets?
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Is it coincidence that the change in GPE when falling from deep space into the largest know neutron star is approximately mc squared Joules?

We can calculate the change in GPE in falling from a very long distance to the surface of a star or planet from $$\Delta{GPE} = \int_{r}^{\infty}mg\,dh $$ But taking into account that g will change ...
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4 votes
1 answer
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Why is gravity constrained to speed of light of gravity itself affects space time?

Gravity(gravitational waves)they say travels at the speed of light. But speed is distance/time. If gravity affects space-time or “time” then how can we be sure that gravity indeed travels with a ...
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0 votes
1 answer
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Is a black hole really a neutron star with a gravitational time dilation factor of 1.0

Here's why I'm asking. The gravitational time dilation factor at the surface of a star or planet can be expressed as $$gr/c^2$$ In theory, the same equation should apply to the gravitational length ...
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  • 191
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Could the forces that hold the galaxies together already have changed?

Since the theory is, that effect of gravity travels at the speed of light, could the forces that hold galaxies together already have changed as we are 20k light years are so away from the centre ? And ...
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5 votes
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Do any equivalent of Lagrange points exist between galaxies?

I'm wondering whether there is a similar effect between Galaxies as we have between a planet and it's star. I know there are tidal waves between galaxies and that gravity seems to always be attractive ...
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2 votes
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154 views

By dint of some 'reverse engineering' I have produced the equation below. Does anyone recognise it? Does it come from general relativity?

The equation is $$a = \frac{\sqrt{2} \cdot c^2}{k}$$ where a is the acceleration due to gravity, c is the speed of light and k is the radius of curvature of the geodesic. The equation is dimensionally ...
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2 votes
1 answer
99 views

If Earth was a moon of Jupiter orbiting at the same distance as Metis (Jupiter's innermost moon), then how much lighter would a 100 kg person feel?

Metis is Jupiter's innermost moon, orbiting at a semi-major axis of 128,000 km. If Earth were to magically replace Metis, then how much lighter would a human weighing 100 kg be? There's no need to ...
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17 votes
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How could low-gravity dust in the early solar system have formed hard stones?

I wonder how very low-gravity dust could make bind together and make hard and dense stones that later on will merge together and make some of the planets? I read some meteorites as hard as a hard ...
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3 votes
1 answer
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How does general relativity explain gravity?

General relativity explains that gravity is the curving of spacetime by massive objects. This makes sense, like when I throw a ball, it follows the curve of spacetime, which is towards the center of ...
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6 votes
1 answer
111 views

Declaring of 3C273 as a quasi-stellar object

I am currently reading A Brief History of Time and chapter 6 about black holes. Now, here it says He found it too large to be caused by a gravitational field: if it had been a gravitational redshift, ...
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2 votes
1 answer
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How does this Toomre GI criteria have the period in the denominator?

I saw this equation in a literature review recently talking about the Toomre criterion for gravitational instability: Given here in section 2.1.1: https://arxiv.org/pdf/1801.06117.pdf, viz. But I am ...
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4 votes
2 answers
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Can planetary rings distort the shape of a planet?

I was watching this Kurzgesagt video on what would happen if the moon crashed into the Earth, and it mentioned that at the very end the tidal forces would tear the moon apart and create planetary ...
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7 votes
4 answers
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Units for orbital period and gravitational constant

Using this site for binary system orbital period calculations: https://www.omnicalculator.com/physics/orbital-period The formula given there is $$T_{binary} = 2 \pi \sqrt{\frac{a^3}{G (M_1+M_2)}} $$ ...
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3 votes
2 answers
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Time in 0 gravity points

If being close to a supermassive body like a black hole makes time pass more slowly for us than for an observer from a point of view with a weaker gravitational field, if we get to be at a point in ...
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4 votes
3 answers
237 views

Great Attractor's gravity vs Universe Expansion

I would like to know if the trajectory of our galaxy has been calculated because it is usually said that the cosmos is emptying due to the expansion of the universe but at the same time there is a ...
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10 votes
1 answer
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Why is the L1 point (Lagrange) almost 1 million miles from Earth? Shouldn't it be closer to us?

Try to follow my simple logic: The Sun is almost exactly 333,000 times as massive as Earth, and gravitational strength increases linearly with mass, so the Sun's gravity is about 333,000 times ours. ...
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19 votes
1 answer
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Is it possible to have a stable 3 body system that orbits in a perfect circle?

I.e. a system that has 3 objects of equal mass, rotating around the system's center of gravity like so: Please excuse the crude drawing, but I've just been reading The Three-Body Problem book by Liu ...
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6 votes
1 answer
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What's the difference between the Roche lobe and Roche sphere?

I am just beginning to look into this topic, so apologies if there are any striking misconceptions in the following. From Wikipedia, the Roche lobe is "the region around a star in a binary system ...
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1 vote
2 answers
124 views

Are there negative masses that attract each other but repel positive masses? [closed]

Is there law/rule in physics that precludes the existence of a negative mass, wherein similar negative masses attract but positive and negative masses repel? To keep this on-topic in Astronomy SE: Has ...
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1 vote
1 answer
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Do Einstein's ten field equations use 20 or 40 variables? (2 or 4 for each tensor equation?)

One site I came across says Einstein's 10 Field Equations use 20 variables, while another said 40. There are four variables in spacetime - three for space and one for time, right? But there are two ...
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1 vote
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Does the Schwarzschild metric solution require two (A and B, or g11 and g44) or four components (g11, g22, g33 and g44)?

Different places on the web imply that Schwarzschild's metric uses four components or separate equations, similar to how Einstein's full set requires 6 or 10, or they say only two are needed. Is it 2 ...
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11 votes
1 answer
783 views

How close to escape velocity are most Oort-cloud comets?

User @antlersoft wrote a nice answer to my question on the difference between barycentric and heliocentric models of the solar system when applied to comets (edge cases of the systems). In a comment, @...
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1 vote
1 answer
137 views

Is it true that the 3 body problem can't be solved using the four basic functions, radicals, and integrals?

The two-body problem can be completely solved via two one-body problems, which only uses the four basic binary functions. However, the three-body problem cannot be solved with these functions and ...
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0 votes
2 answers
102 views

How is light unable to escape a black hole if photons are massless? [duplicate]

I understand light will follow the curved space that the BH is causing due to its mass. I also understand that mass attracts other mass but then photons are massless. So 0(photon) x m2 (the BH) is ...
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0 votes
1 answer
98 views

Are Gravitational Waves Themselves Affected By Gravity?

Are gravitational waves themselves subject to gravity? That is, if a gravitational wave were to pass by a galaxy cluster, would its form get distorted (even though the wave, itself, is a distortion of ...
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1 vote
1 answer
80 views

Which factor most affects the accuracy of Kepler's Third Law?

We know that Kepler's 3rd Law is not perfectly accurate due to gravitational perturbations of other planets, moons, etc., but which factors most affect the accuracy? Is it the semi-major axis, mass of ...
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1 vote
0 answers
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Can the Dark Matter / Energy problem be plausibly reframed as an ignorance of gravity problem?

I understand Dark Matter was originally proposed to explain the rotation rate of galaxies. The rate was not explained by observed matter, ergo there must be unobserved matter. An alternate explanation ...
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6 votes
1 answer
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Physical Meaning of the Principal Tidal Fourier Modes and Their Integers

I'm trying to understand tides evolution of planets, and I often come across the Prinicipal Tidal Fourier Modes expressed as: $$ \omega_{lmpq}$$ For example, see the discussion after Eq. A15 in ...
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10 votes
2 answers
2k views

Can the gravitational redshift of our sun be measured?

How much of a wavelength change does our sun’s gravity cause in the light it emits? I imagine this could be measured by the shift in its spectral lines. And by extension, does the earth’s ...
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0 votes
1 answer
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Using Kepler's 3rd law to calculate $\mu$ (standard gravitational parameter)

I'm trying to calculate $\mu$ (the standard gravitational parameter) with this formula $\mu = 4\pi^2a^3/T^2$ I'm using Earth's orbit is $a=1$ and $T = 365.2564$ for $a^3/T^2$ I'm getting $\approx7.496$...
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0 votes
1 answer
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Why do planets' orbital velocities drop off by less than a third when twice as distant from the sun, rather than three-quarters? If gravity is $1/4$?

Looking at the speeds of the planets in our solar system when traveling around the sun and their distances from it (or, using Kepler's third law,), it seems that when a planets distance is doubled, it'...
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-7 votes
1 answer
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In relativity why does matter bend spacetime in a downward direction [closed]

A major flaw , I think it is in relativity.
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1 vote
0 answers
27 views

List of J₂ values for solar system bodies beyond the planets; are there any prolates?

J₂ values for the planets can be found in several sources including each planet's planetary fact sheet. Without looking I'm going to assume they are all positive (oblate) because each planet rotates ...
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1 vote
0 answers
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Why does Titan's (relatively) rapid migration away from Saturn cause the planet to wobble faster, and eventually (maybe) tip onto its side?

Over the past year and a half, multiple articles in the popular press have come out talking about how unexpectedly rapidly Titan is moving away from Saturn, and how this is causing Saturn to wobble ...
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1 vote
1 answer
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How will be changed Hill's radius and Kepler's formulas in case of 2D space instead of 3D?

Imagine that Flatland habitats go into space. All stars and planets are the circles instead of spheres and gravity law is $F=Gm_1m_2/R$ instead of $F=Gm_1m_2/R^2$. How the other formulas will be ...
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0 votes
1 answer
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How to predict future position of body in solar system (without Kepler's equations, N-body problem)?

I have a solar system where each body interacts with each other by force $F = \frac{Gm_1m_2}{R^2}$ Is it possible to predict future position of some random body after t seconds since bodies velocity ...
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6 votes
1 answer
81 views

How could a captured asteroid enter a circularized orbit?

Ok, the simple answer would be through gravity assists from moons, taking a ballistic capture orbit into a circular orbit. I am wondering the chances of a ballistic orbit becoming a stable (on the ...
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0 votes
2 answers
155 views

Is gravity affected by time dilation? [closed]

I just thought about what time realy is but I can't put my head around this so I was thinking that if time dilation happens at the edge of a black hole that just everything gets slower. But does ...
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2 votes
1 answer
55 views

What is the best visual-aide model that most accurately depicts objects moving through warped spacetime?

so far, the best model I've seen, to help me visualize objects moving through a warped spacetime is this video... A new way to visualize General Relativity. Is there a better visual representation to ...
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0 votes
0 answers
37 views

Density and Velocity profiles of a self gravitating explosion

Suppose an asteroid explodes, into a spherically symmetrical cloud, of radius R, mass M, and surface receding velocity V (normal to the edge of the cloud), initially the density is radially uniform. ...
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3 votes
0 answers
68 views

How much mass to make a shperical object?

I was googling around for info for D&D campaign I am going to run. I was wondering about the mass required for a object to pull itself into a shperical shape and it seems that the answer varies ...
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0 votes
1 answer
113 views

Is a black hole's gravity strongest at its event horizon or at least near it?

I know black holes are extraordinarily powerful when it comes to the gravitational pull. But I wanted to get something clarified - is the gravity stronger at the event horizon (like, on the very edge ...
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14 votes
2 answers
2k views

Is there a better formula for gravitation, besides Newton's?

Many experiments (e.g. changes in Mercury's orbit) show that Newton's Law of Gravity is not exact. According to the theory of relativity, what is the exact formula?
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4 votes
1 answer
157 views

Gravitational influence of other planets?

I have a friend who believes in astrology and she insists that the gravitational influence of the planets has an effect on the child when it's born. The last time she was going on about this a thought ...
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1 vote
2 answers
142 views

How slow would you age on a double gravity planet?

How slow would you age on a double gravity planet? Gravity of the planet A is 10 units. If you are taken from planet A to planet B where gravity is double that of A, i.e. 20 units. How slowly would ...
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2 votes
1 answer
73 views

Does the Barycenter shift/combine when 2 planets line up with the sun?

On August 1st-2nd, Earth & Saturn lined up together in relation to the sun. Here is a website that discusses it: https://www.healththoroughfare.com/science/saturn-will-line-up-with-earth-and-the-...
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  • 1,519
13 votes
2 answers
4k views

Why do gravitational waves travel "only" at the speed of light but the gravitational scalar potential is instantaneous?

To my old Space SE question Besides retarded gravitation, anything else to worry about when calculating MU69's orbit from scratch? @DavidHammen's excellent answer replies that one should not add a ...
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