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Questions tagged [gravity]

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

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How strong would gravity need to be to prevent star formation

This question got me thinking. In another universe, with a different value for the gravitational constant G, how much bigger would G need to be for star formation to be prevented? In other words in ...
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5answers
8k views

Why does gas form a star instead of a black hole?

When a space gas gets pulled together a star is formed. On the other hand, when a massive star dies, it collapses to a black hole. You would think that the initial mass of the gas would be bigger ...
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1answer
223 views

How would the barycenter of a double-planet system affect the two planets' surface gravity?

Assuming two similarly sized, tidally locked planets orbit a common barycenter, how would the gravity of the barycenter affect the surface gravity? For example, on the inner sides of the planets, what ...
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1answer
129 views

How strong is the gravity from half of the distant universe?

I would like to know, how strong would the gravity be from the distant universe if we were somehow able to remove half of it. For example, if everything behind us did not exist, how strongly would ...
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1answer
209 views

How strong is the gravitational stretch we experience from the edge of the universe?

How much gravitational effect do we experience (e.g. maybe -.00001 G or smaller) from the edge of the visible universe? By edge of the visible universe, I am talking about the region of the cosmic ...
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2answers
145 views

Could Dark Energy be a “Cosmic Gravity Background”

Could dark energy (the accelerating expansion of the universe) be the result of gravity from the dense past universe "pulling" outward from all directions? The universe theoretically started out ...
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2answers
246 views

Can gravity cause a Big Crunch, if the cosmological principle holds?

Say we have a closed universe that follows the cosmological principle. As a result, mass is distributed equally and, if as far as I know, gravity should have no net effect on the movements of galaxies ...
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1answer
82 views

Effect of gravitation [closed]

When the sun reaches close to the centre of the Milky Way galaxy while revolving around it (like the earth closes down on the sun while revolving around it, perihelion), will the gravitational effect ...
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1answer
112 views

What is “superficial gravity”

I have seen the term superficial gravity used and it seems to be equivalent to surface gravity seen, e.g., here http://arxiv.org/pdf/1701.02295 Is there any difference between superficial and ...
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1answer
336 views

Is there a better explanation of Hawking radiation?

I'm writing a piece on Hawking radiation, and find I have something of a problem. The "given" explanation which I find on Wikipedia and elsewhere is unsatisfactory: "Physical insight into the ...
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1answer
3k views

How large can a ball of water be without fusion starting?

How large can a ball of water be without fusion starting? Peculiar question: some explanation might be necessary. My young son is into ‘space’ and astronomy. One of his posters says that Saturn could ...
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1answer
293 views

How can artificial gravity hold the water in swimming pool? [closed]

I was watching Passengers (2016). In this movie, a swimming pool exists in the space station. In middle of the movie, Artificial Gravity is turned off and the water rises, with the swimmer suffocating ...
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1answer
174 views

Why is the moon's motion more complicated than that of those planets?

I am of course not an expert on astronomy. But from the literature, I have the impression that the lunar motion is very complicated. It seems that Kepler can make sense of the motion of the planets ...
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4answers
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How does gravity have an effect from the inside the event horizon of a black hole with the rest of the universe?

According to general relativity, as I understand it, the space around a black hole's event horizon is distorted by gravity, such that the distance to a point approaching the event horizon from an ...
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2answers
403 views

Why black holes have positive energy?

How could Hawking get the conclusion "[...] and black holes have positive energy. That’s why empty space is stable. Bodies such as stars or black holes cannot just appear out of nothing. But a whole ...
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1answer
384 views

Requirements for a satellite/planet to be tidally locked to a planet/star

The Moon is tidally locked to the Earth, the Four Galilean are tidally locked as well, and the recently found planetary sistem TRAPPIST-1 has seven tidally locked planets, but Venus or Mercury are not....
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Is the Roche limit used this way?

Has the Roche limit affect been observed having an effect on an constructed object gravitational wise to cause damage? Could the Roche limit be used to slow an object?
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1answer
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Can a bullet be fired on the moon and sent it into orbit?

I know you need oxygen for the ignition, but presumably, if the bullet is impervious (or water-tight) and if there is a little air enclosed in the bullet case, wouldn't that be sufficient to fire it ? ...
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1answer
211 views

What does Gauss' divergence theorem say about compression of a body under self-gravitation?

Below this question there is a comment that caught my interest: First, there's a misconception. Compressing a body does not increase the force of gravity on outer-lying masses. That's Gauss's ...
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3answers
219 views

Where do rocks come from?

Consider a loose pile of rocks in an early solar system, say a tiny asteroid. As it accretes more and more material, the pressure at its center eventually becomes great enough to collapse the gaps ...
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2answers
108 views

Gravitational waves

We know that gravitational waves stretch space-time so light travelling through that space should be stretched as well. So my question is how do we know anything is stretching if the effect is ...
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1answer
274 views

In zero gravity - I'm floating inside a torus as they spin it up, do I float do the “ground”?

Imagine this scenario - I'm in a space station with one of those huge torus shaped constructions, that you spin to simulate gravity through centripetal force. But it's turned off right now, so I'm ...
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1answer
164 views

Is energy required for gravity to have effect, or is it an energy itself?

If magnetism requires some energy to exert change of motion to attract ferrous metals, then, does gravity require energy to exert attraction, or is gravity energy itself, creating attraction through ...
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1answer
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If dark matter bends light, how do we know the stuff in the sky is where we think it is?

We measure movement, position, and many other things of an object in space because of its light and what we can measure with it. But as far as I know there's supposed to be a HUGE amount of dark ...
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1answer
339 views

Can Earth have a ring?

Why doesn't the earth have a ring? If solid material such as dust and moonlets, and common components of satellite systems are brought nearer to the Earth, then what will happen? Will it take the ...
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1answer
617 views

If earth changed its orbit, what would happen to the moon?

Let there be an accelerating force acting on earth over an arbitrarily long time to change its orbit around the sun (e.g. by asteroids repeatedly passing by close to earth (intentional or not) or ...
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2answers
794 views

How can gravitational waves escape from the gravity of black hole?

I know that even light can't escape from a black hole's gravity and the velocity of light and gravitational waves are the same. How can only gravitational waves escape from its gravity?
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0answers
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How can X-rays be emitted from a black hole? [duplicate]

A black hole's gravity prevents light from escaping, how can a black hole emit X-rays? Visible light and X-rays are both electromagnetic radiation, shouldn't the black hole's gravity prevent X-rays ...
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2answers
411 views

Why the comets can cross the orbits of planets?

Why are comets able to cross planetary orbits without colliding with the planets - Why is the comets' orbits elliptical, intersecting all the other planets' orbit. Why don't they get attracted by the ...
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1answer
220 views

Why is the eccentricity of Triton so low?

Triton is very special, since its orbit is retrograde respect Neptune, its obliquity is near 0° and its orbit is almost circular (according to Harris (1984) is $5 \times 10^{-4}$). I'm familiar with ...
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0answers
387 views

Horseshoe orbits and integration in C

I'm studying a particular case of the restricted three-body problem. It's been found that some objects follow a horseshoe orbit pattern, and I'm trying to sort out something through an integration ...
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1answer
281 views

How can a gas planet become tidally locked with a star?

Supposing a planet is made entirely of gas, is it possible for the planet to become tidally locked with the star it orbits? I know when a terrestrial planet orbits a star in a different amount of ...
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2answers
615 views

How can gravity lead to the Big Crunch scenario?

According to modern cosmology, space is expanding, causing proper distances (but not comoving distances) to increase between galaxies. In the Big Crunch hypothesis, gravity halts and reverses the ...
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775 views

What causes the Earth to have gravity?

I've always wondered how our planet Earth or any other large spheroid-shaped thing in our universe has gravity? Does it have a small black hole on its core causing it to have gravitational force? ...
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1answer
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Revolution of Planets [duplicate]

Why do planets revolve around the sun? I understand that gravity plays a huge role in allowing it to move around. But what caused it to revolve in the first place? One possible solution is explained ...
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1answer
4k views

Which of the following statements about gravitational waves are true? [closed]

a) The emission of gravitational waves from merging black holes is predicted by Einstein's general theory of relativity. b) The first direct detection of gravitational waves, announced in 2016, came ...
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1answer
137 views

Gravity of other objects in black holes?

If something falls into a black hole, but has not yet hit the singularity in the center, can we (outside of the black hole) feel its gravity? On the one hand, I've learned that there's no distance ...
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2answers
592 views

Are there upper or lower mass limits for black holes?

Natarajan & Treister (2008) describes a practical upper limit for black hole masses at $\sim 10^{10} M_\odot$. This is all due to the black hole's interactions with nearby matter. However, is ...
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1answer
524 views

The deduction of the Hill Sphere formula

This question may be a little lazy, but can anybody give me a proof of the Hill sphere formula? Acording to wikipedia, the formula for the radius, $r$, is $$r\approx a(1-e)\left(\frac{m}{3M}\right)^{...
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0answers
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Modifying Heisenberg's unified field theory with gravity [closed]

How would Hisenberg's unified field equations be represented if gravity is incorporated. Would it result in a case similar to the Heisenberg-Ivanenkov equation, or am I just confused here? I would ...
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1answer
93 views

Examples of astrophysical objects NOT in virial equilibrium?

Astronomers always talk about astrophysical objects being in virial equilibrium (e.g., a stellar system, or a disk of gas within a galaxy, etc.). But I never hear about thermodynamical equilibrium. ...
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1answer
186 views

Why are galaxies distributed as walls and voids, or spider webs?

I have wondered why the "walls and voids" of the cosmological galaxy distribution seem to resemble a "foam", like rising bread that's ready to fall, with "voids" corresponding to empty bubbles in the ...
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1answer
53 views

How can we map masses orbiting the outer Solar System by determining the orbits of planets and smaller objects?

Neptune was discovered 170 years ago with the help of observations of Uranus' orbit. A great demonstration of the scientific principle for its time. Recently, Earth and cis-Lunar based radio ...
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3answers
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Energy needed to create a black hole vs energy needed to run the Large Hadron Collider?

I was disappointed to hear Brian Cox's flippant reply to whether or not the LHC could create a black hole big enough to swallow the earth in this video: https://www.youtube.com/watch?v=QFy87tFTZwY I'...
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3answers
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How did the Milky Way and Andromeda Galaxy separate before they began their collision course?

How did the Milky Way and Andromeda Galaxy separate before they began their collision course? I understand that gravity is pulling together the two galaxies, but what separated them in the first ...
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0answers
179 views

Is there mass i.e a physical bottom to a black hole? [duplicate]

All matter has a gravitational pull, a ball, a rock, a planet and a star - the bigger the matter the more it affects the space fabric around it thus having a greater gravitational pull. I see black ...
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1answer
185 views

How to correctly create planetary orbits from kepler elements

I would like to build a gravity model (with 3D graphics) that would build the orbit of the planets based on orbital elements input and the gravity calculation for each element. My initial code is ...
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1answer
244 views

Are gas giants supported by thermal pressure?

I've heard gas giants are supported because there is an equilibrium between thermal pressure and gravity. That is, if Jupiter were to suddenly begin compressing, temperatures would increase to the ...
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3answers
3k views

How can a supermassive black hole cause so much energy to enlighten its matter when its massive gravity prevents light to escape?

To cite the German newpaper article Astronomen beobachten erwachendes Schwarzes Loch: Das Materie-Monster sitzt den Angaben zufolge im Herzen der 42 Millionen Lichtjahre entfernten Polarring-...
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Do differentials in the Earth's density cause the flyby anomaly?

The flyby anomaly is an occasional, unpredictable, and as yet unexplained discrepancy in acceleration of satellites when they pass close by the Earth. It causes an increase of up to 13 mm/s during ...