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Questions tagged [gravity]

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

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If a photon were 1 Planck length away from the event horizon of a blackhole, would it be able to escape the blackhole's gravitational pull?

I understand that one could theoretically replace the sun with a blackhole without affecting the orbits of the planets. This leads me to believe that a blackhole isn't a "cosmic vacuum cleaner" so ...
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1answer
32 views

Mass of Photon and Black holes [duplicate]

The gravity caused by a black hole is said to be so strong such that even light(photons) can not escape from it. since gravity exist only in between objects with mass, How does a black hole attract a ...
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1answer
771 views

How can Halley's Comet be predictable?

How can we know that Halley's Comet is going to come again every 76 years? I understand that the comet has an orbit around the sun but how can scientists know that it won't be thrown off course for ...
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1answer
76 views

How to determine the mass of a galaxy?

I am having a slight disagreement with my professor. We can measure the mass of a distant star cluster from: A.Its color B.Its radius C.Its metallicity D.Smearing of lines in ...
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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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3answers
118 views

Can't pulsars and stars be used for gravitational wave measurement?

Suppose that we collected photons from a distant star, and they arrive regularly at 15 photons every microsecond onto a CCD, when a gravity wave bends space time, wouldn't the regular 15 photons graph ...
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4answers
540 views

Why do Earth and moon move apart but binary black holes move closer?

According to the accepted answer on Is the moon moving further away from Earth and closer to the Sun? Why?, the moon is receding from Earth because tidal forces and friction cause energy to be lost. ...
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3answers
4k views

Why do astronauts move so slowly in zero gravity? [closed]

When astronauts are floating about in a spaceship or space station, they nearly always move very slowly. After doing a bit of research I can't see why being in zero gravity would restrict movement to ...
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1answer
82 views

Looking at the Expanding Universe through the lens of relativity [closed]

The basis and most observable metric for how the universe is understood to be expanding at an accelerating rate is the red shift in observable light. I am assuming it has been thought about or maybe ...
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1answer
81 views

Why does gravity make time go slower and warp light?

I know that objects with lots of gravity are capable of warping light or slowing time (an example being a black hole). I also know this is because of the theory of relativity, but I need someone to ...
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3answers
249 views

Why do sun spots appear near the equator of the Sun?

I noticed on many examples sun spots are near the equator of the Sun. Why are there no sun spots closer to the poles of the Sun?
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2answers
139 views

A neutron star without a flat surface and an atmosphere?

In the Kurzgesagt video Atoms As Big As Mountains — Neutron Stars Explained they state that neutron stars have an atmosphere that reaches about 10 centimeters above the surface. How is this possible?...
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3answers
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Can two planets in an empty universe meet/be pulled together?

For this question assume that the entire universe is completely empty. The universe is not expanding or contracting, it is completely motionless since time immemorial and has never moved before. Only ...
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1answer
40 views

Mini disk of a blackhole meaning

What is meant by the "mini disk" of a blackhole? Is it the same as accretion disk? If not, whats the difference? Eg: In the abstract of this paper https://arxiv.org/abs/1806.05697 it is mentioned "...
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1answer
178 views

Is the moon inside earth’s atmosphere? If so, what are the consequences?

Recently, a paper was published claiming that the Earth’s atmosphere extends far beyond the Moon. This has been reported on by a number of news sources and websites, including the ESA, Business ...
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2answers
135 views

Is gravitational lensing a good way to search for negative mass?

Is it possible to verify negative mass with gravitational lensing? The following video proposes an idea that galaxies are surrounded by negative mass: https://youtu.be/MZtS7cBMIc4 Could this be ...
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1answer
86 views

First observation that the movement of a planet or asteroid in its orbit was affected by another planet?

Answers to the question How did Kepler determine the orbital period of Mars? describing careful observations centuries ago got me thinking. Question: What was the first analysis of observations that ...
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4answers
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What is the minimum mass required so that objects become spherical due to its own gravity?

Is this minimum mass known? or maybe, is it given in terms of density? If so, how much density is the minimum to have an spherical object due to its own gravity?
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1answer
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Can the moon's gravity influence be observed on the space station's orbit?

In browsing through Astronomy Stack Exchange, I came across this question about orbiting the moon (Is it possible to achieve a stable "lunarstationary" orbit around the moon?). In further ...
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1answer
31 views

How to separate binary and optical stellar pairs?

I have list of stellar pairs from The Washington Double Star Catalog (WDS), i.e. I have next parameters: angular separations position angles stellar magnitudes of both components Additionally I have ...
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2answers
1k views

If an asteroid twice the size of Earth passed super close would half of the Earth be pulled towards it?

So let's say that an asteroid the twice the size of Earth passed by going relatively slow say in between the Moon and Earth. Among so many other things (and perhaps you can touch on some of the ...
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1answer
759 views

Could dark energy be negative gravity?

Main question: Could dark energy (the mysterious accelerating expansion of the universe) be explained by "negative gravity"? "Spin off" questions: Does antimatter have negative gravity? If ...
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0answers
190 views

How does the size of a naked black hole effect its photon sphere?

What effect does the size of a black hole have on the size or density of the photon sphere and it's proximity to the horizon? For example: A person is in space looking at 2 black holes, one of them ...
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1answer
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Wouldn't the rings of Saturn experience tidal effect?

The "Lord of the Rings" has more than 60 moons, some of which are larger than Mercury. My question is, wouldn't the ring, which is mostly comprised of rocks, experience tidal effect whenever a moon is ...
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1answer
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How can a galaxy in-between our view and the galaxy behind it have a 'lensing effect'?

In this article, the author writes: This galaxy is so distant that it would have been invisible without the gravitational lensing effects of a large galaxy cluster called Abell 1689, located nearly ...
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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
60 views

What would happen to the solar system if Earth's mass was removed?

This question is inspired by the "Stargate SG-1" episode "The Road Not Taken". In that story a device is used to shift the entire Earth to another dimension to defend against an attack. I'm not ...
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1answer
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Is an elliptical orbit around a black hole, or an orbit consisting of three bodies (including the black hole) possible?

If this is possible, would it then lead to an object undergoing varying time dilation (predictable variation with elliptical and random variation with three body)?
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1answer
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Can light be curved due to heavy gravitational force?

Lets suppose there is a huge planet having large amount of gravitational pull, then will the light passing through the planet get attracted towards it.
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1answer
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Do falling bodies ever slow down?

I've had some discussions recently about black holes, and the issue of infalling bodies taking forever to reach the event horizon. That's essentially what Einstein said in his 1939 paper on a ...
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0answers
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Change in axis of the Earth if the Moon would dissappear [closed]

I read that, if the moon would not be there or if it would dissapear or be destroyed it could cause the earth to rotate into a possible axis of 45° degrees. Because it would be missing the ...
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3answers
464 views

Techniques for locating origin of gravitational waves

The gravity wave detections (GW150914 and GW151226) have both been ambiguous in their exact position, the LIGO papers (GW150914 and GW151226) give no precise information about the location of the ...
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3answers
250 views

Why doesn't the vertical light beam get out of a black hole?

I'm asking this question as a follow-up to if light has no mass why is it affected by gravity? Imagine you’re standing on a gedanken planet, shining a laser beam straight up into space. The light ...
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4answers
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Is a black hole a perfect sphere?

I know all about how black holes form and why their gravity is so strong. However, is the gravity equally powerful in all directions? Will the event horizon appear as a perfect sphere?
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1answer
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Are other planets' gravity measurable on Earth?

I understand that we can calculate the gravitational force other planets exert on Earth, but can it be measured?
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Could light be dark matter?

Is it possible that light itself is dark matter? I am speaking of photons (e.g. visible light, infrared, ultraviolet, etc...). I realize light is understood to be massless, but it is obvious it at ...
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4answers
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Why do nebulae turn into accretion disks but planets do not?

What are the factors that prevent planets from also turning into disks, like the stellar dust in a nebula does? The Earth is not a perfect sphere, but rather it's squished at the poles and it bulges ...
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3answers
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Is there any orbit at which the Roche limit can be “felt”?

Do any of the planets have a Roche limit that is strong enough to be felt by an astronaut whilst in orbit?
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3answers
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Gravitational acceleration inside a planet

The gravitational acceleration, g, inside the Earth generally decreases with decreasing distance to the center: However, apparently for Jupiter, the gravitational acceleration only increases with ...
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1answer
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Largest non-hot gas giants

Excluding 'fluffy' or 'puffy' gas giants that are 'inflated by heat from their stars, what is the maximum radius of a gas giant planet. I keep reading things like "...Jupiter is 'about' as big as ...
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8answers
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Explain to a 4-year old why people below the equator are not tilted or suspended upside down

So I just bought a globe for my extremely inquisitive soon to be 4-year old son. He knows the very basics of the solar system and knows that we live on planet Earth and also knows the concept of ...
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0answers
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Can eternal inflation and gravity explain dark energy?

First of all, I have absolutely no background in astronomy and physics. I just watch a lot of "i-hope-scientific" youtube channels talking about physics and astrophysics and I can kind of follow the ...
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0answers
164 views

Gravitational eddies across the galaxy?

I read about natural gravitational eddies that travel in a wave that black holes have. They also have a strong magnetic field. Does those eddies follow magnetic field lines of the rotating black hole? ...
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1answer
83 views

How would humans with appropriate equipment navigate the surface of Saturn's moon Titan on foot?

Taking into account the physical characteristics of Titan (e.g. surface gravity, atmospheric pressure), what would be the most efficient method of motion for an astronaut to navigate the surface of ...
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3answers
3k views

Why don’t we feel the Earth rotating beneath us?

I’m not sure all of the details in the experiment but basically it went as follows. Someone marked their location of take off in a hot air balloon and went straight up in the air. They stayed ...
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1answer
598 views

How does gravity affect pressure under water?

How different would being under water in Europa differ from being under water on Earth? Would it be possible to dive deeper on Europa with a submarine than on Earth and by how much?
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2answers
1k views

What is the orbit of Earth around the Sun?

According to wikipedia and other sources, a planet and a star always move in a circular orbit around the common center of mass of the both bodies, in case of Earth and sun, this center of mass lies ...
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1answer
114 views

Black hole, escape velocity, going up?

I do know nothing can escape a black hole, and I understand popsci explanation of geodesics of space-time around and inside the event horizon. I however have some trouble with following thought ...
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2answers
95 views

Could a moon be passed from one planet's orbit to another?

Like if you had two planets that had a close orbital path could the moon be pulled away from the first planet?
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2answers
127 views

Dark matter virtual objects hypothesis [closed]

I've hypothesized for many years in my head that dark matter are the gravitational after effects of objects that have exploded. My understanding of physics is that when an object explodes, its center ...