Gravity is an attractive force that affects and is effected by all mass and - in general relativity - energy, pressure and stress. Prefer newtonian-gravity or general-relativity if sensible.

learn more… | top users | synonyms

12
votes
3answers
18k views

How do we determine the mass of a black hole?

Since by definition we cannot observe black holes directly, how do astronomers determine the mass of a black hole? What observational techniques are there that would allow us to determine a black ...
9
votes
3answers
382 views

Field created by varying Gravitational field

Changing Electric Field causes Magnetic filed and changing Magnetic Field causes Electric Field. Is there anything similar in relation to Gravitational Field? What sort of field is created by varying ...
8
votes
3answers
227 views

How would two equally massed stars orbit?

In an empty universe, except for two equally massed stars, how would they orbit? Or, for another example, if the earth suddenly grew to be the mass of the sun, how would they orbit, or interact? Would ...
11
votes
1answer
433 views

Does gravitation of a sphere equal gravitation of a point?

Under Newtonian model of gravity, a perfect sphere creates the same gravitation field as a point mass in its center. General Relativity describes gravitation differently. How much this difference ...
3
votes
3answers
411 views

Can a planet cover the whole sky as seen from its satellite

Sorry if this is a trivial question, but I do not know enough physics to answer this on my own. Can there exist a satellite that revolves around a planet so large such that the planet covers the ...
2
votes
4answers
1k views

Can the effects of gravity be broken by jumping?

I was having a debate the other day with a work colleague where I explained that gravity is a weak force because it is easily broken. Then I remembered a lecture by someone, I forget who, that ...
3
votes
2answers
374 views

Astronaut on a rotating asteroid

This might sound like the silliest question ever, but can someone shed some light on what is actually happening in the following situation: There is an asteroid in outer space what is rotating with ...
6
votes
2answers
253 views

Possibility of “graviballs”?

Looking at the relevant wikipedia page, one can read that the graviton should be massless. Is it 100 % certain that it is massless or is there room in any "nonstandard" models for a tiny non-zero mass ...
22
votes
6answers
13k views

How can a black hole emit X-rays?

Considering that 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 ...
1
vote
1answer
222 views

Sun-Earth Virtual Gravitons?

How many virtual gravitons do the sun and earth exchange in one year? What are their wavelengths?
8
votes
2answers
1k views

The energy of a Graviton

Maybe another stupid question, but what's the energy of a graviton? Is it $\hbar \omega$? Does it emit gravitons when an apple falls onto the ground, like photons be emitted when an electron transits ...
-2
votes
1answer
330 views

future light cones and light paths

I understand that an event, in a four dimensional space-time, produces a light cone. As time increases the cones gets larger on either side of the event (past and future). For example the if the sun ...
1
vote
2answers
164 views

A question on a system of particles governed by laws of gravity and electromagnetic field

Consider a system of many point particles each having a certain mass and electric charge and certain initial velocity. This system is completely governed by the laws of gravitation and electromagnetic ...
1
vote
2answers
218 views

Question about gravity probe B

I have a question about the gravity probe B experiment. According to this site: http://science.nasa.gov/science-news/science-at-nasa/2011/04may_epic/ The measurements they made confirm Einsteins ...
7
votes
2answers
308 views

Would dark matter absorb gravitational waves?

Would the vast and seemingly diffuse clouds of dark matter floating around our galaxy (and most others) absorb gravitational waves? Is this perhaps why we haven't detected any yet?
3
votes
0answers
218 views

Calculation of a Gravity Resonance Keyhole

Can anyone describe the mathematics behind the calculation of a resonance keyhole (for a two-body model)? It seems like the size and position of the keyhole should be a function only of mass and ...
4
votes
3answers
4k views

Gravitational time dilation at the earth's center

I would like to know what happens with time dilation (relative to surface) at earth's center . There is a way to calculate it? Is time going faster at center of earth? I've made other questions ...
11
votes
1answer
2k views

Why does gravity need to be quantised?

The electroweak and strong forces seem to be completely different types of forces to gravity. The latter is geometric while the former are not (as far as I'm aware!). So why should they all be ...
3
votes
1answer
314 views

Olympic games and the local g

As you probably know, the two next Olympic games will take place in London and Rio. Randall, author of xkcd, did an interesting observation (that I had never thought of) in http://xkcd.com/852/ , ...
3
votes
1answer
1k views

Voyager local time dilation (caused by gravity)

Voyager I, as an example, taking account gravity and setting aside effects of speed as cause of time dilation. If it is very far away from earth and sun, so then there must be a difference in the ...
0
votes
1answer
202 views

Is relativistic motion equivalent to fluctuating gravitational fields?

The theory of relativity makes very precise predictions about how an object's motion through space-time affects the passage of time for both the object and observers in other frames of reference. I ...
1
vote
3answers
2k views

Terminal Velocity

Let Suppose a bullet of 2 gram is falling with the speed of 200km/h. Now how can we know the time after which it gains it terminal velocity. Also when it gain its terminal velocity the force of ...
-1
votes
1answer
1k views

Modeling and Simulating Pendulum Motion

I've been having difficulty creating a mathematical model of a pendulum in my code. While there is plenty of info on general equations describing a the motion of a pendulum, I seem to be having ...
2
votes
2answers
664 views

Three-Dimensional Gravity

Does anyone have any references that discuss gravity in three-dimensions? I'm trying to make my way through some papers by Witten relating $SL(2,\mathbb{C})$ Chern-Simons theory and gravity in three ...
-1
votes
2answers
448 views

Space-time hospitality

Are there any reasons to assume that organic processes cannot arise in relatively flat spacetime environments? An argument for the existence of a critial value or window.
1
vote
1answer
205 views

How should I REALLY have done this continuous moment formula derivation?

One of my more recent hobbies is making "swords" out of PVC conduit, foam, and duct tape. As all the weight is forward of the cross guards, I needed to put some amount of some substance inside the ...
4
votes
2answers
3k views

Theories that Relate Gravity, Electricity, and Magnetism [duplicate]

There are some people who (without having a stated theory that I know of) insist that Gravity, Electricity, and Magnetism are related. Some point to symmetry in Maxwell's Equations as a potential ...
1
vote
1answer
284 views

Why is it that the splash pattern for every drop is never the same?

This was just a random thought that crossed my mind, and I'm sure there is a simple physics explanation. Say we have a leaky faucet 20 feet above the ground that lets out the exact same amount of ...
7
votes
5answers
590 views

Why aren't all rocks in space orbiting bigger rocks?

Why do only big rocks (planets) have satellites, and not small ones? Why doesn't cosmic dust orbit rocks that are many times heavier than the dust grains? If dust is still too heavy then what about ...
0
votes
2answers
365 views

Distance increase between Sun and Venus

Is the distance between the Sun and Venus increasing? What is the measurement? May I request for a formula?
-3
votes
1answer
741 views

Gravity anomaly in Lacnov, Slovakia: Gravity ashamed

http://www.youtube.com/watch?v=4qxwqNqL-Nc http://tvnoviny.sk/sekcia/spravy/technologie/v-lacnove-sa-predmety-kotulaju-do-kopca.html (Sorry: above links in Slovak.) Can someone explain this to me? ...
1
vote
3answers
806 views

Anti-matter repelled by gravity - is it a serious hypothesis? [duplicate]

Possible Duplicate: Why would Antimatter behave differently via Gravity? Regarding the following statement in this article: Most important of these is whether ordinary gravity attracts or ...
7
votes
3answers
1k views

Why would Antimatter behave differently via Gravity?

Confinement of antihydrogen might help provide a future answer. http://arxiv.org/abs/1104.4982
12
votes
4answers
2k views

Is there a fundamental reason why gravitational mass is the same as inertial mass?

The principle of equivalence - that, locally, you can't distinguish between a uniform gravitational field and a non-inertial frame accelerating in the sense opposite to the gravitational field - is ...
5
votes
3answers
2k views

Formula for implementing a simulated weighing scale

Background: I'm on the pledge drive committee for a non-profit group, and there is an issue that is always contentious within this group; thankfully the group is amicable, although competitive, about ...
12
votes
2answers
2k views

Is dark matter repulsive to dark matter? Why?

I think I saw in a video that if dark matter wasn't repulsive to dark matter, it would have formed dense massive objects or even black holes which we should have detected. So, could dark matter be ...
2
votes
1answer
640 views

What is the mathematical nature of space time quantization in string theory/super string theory?

I don't know much about string theory, apart from it being a theory of everything which brings QM, QED and nuclear forces and gravity under one single roof. I am curious to know from a mathematical ...
3
votes
3answers
1k views

How would you move without gravity?

I'm wondering how would you move without gravity? You would still have air pressure at 1Atm. Would you "swim" in the air or would you have do something else?
15
votes
5answers
68k views

jumping into water

Two questions: Assuming you dive head first or fall straight with your legs first, what is the maximal height you can jump into water from and not get hurt? In other words, an H meter fall into ...
3
votes
2answers
397 views

General Relativity Paradox: Holding a string across a gravitational gradient

The paradox I envision involves two objects that exist in very different gravitational potentials -- one very high (eg: close to a black hole) and one very low (eg: far away from a black hole). Now, ...
6
votes
1answer
679 views

String theory and trace anomaly in semiclassical gravity?

what does string theory have to say about the trace anomaly in the expectation value of the stress energy tensor of massless quantum fields on a curved background and its interpretation as the ...
9
votes
4answers
22k views

How can gravitational forces influence time?

How does it work that gravitational forces can affect time and what usable applications could arise from this?
5
votes
1answer
365 views

Determining Average Tidal Effects

Maximum tidal heights vary widely across the globe, from 16 m in the Bay of Fundy to mere centimeters elsewhere. These variations are due to coastline and shoreline differences. This makes it ...
3
votes
1answer
509 views

Do all atoms in the universe gravitate each other?

I understand that matter will gravitate toward matter. (ex: Earth gravitates a satellite toward it, and the satellite toward Earth.) Does this always apply, regardless of distance?  Take two atoms, ...
3
votes
3answers
215 views

Making a “heavier-than-air” craft float

How big would a hollow rigid object need to be to float, (not in water but in air) if all of the air was vacuumed out and the container sealed?
4
votes
4answers
484 views

Can a disk like object (like UFO's) really fly?

UFOs as shown in movies are shown as disk like objects with raised centers that emit some sort of light from bottom. Can such a thing fly? My very limited knowledge in physics tell me that a disk ...
6
votes
2answers
2k views

Escape Velocity of Asteriod 243 Ida

I was reading about this asteroid (apparently, it has a moon, isn't that awesome?) and I started thinking about if I was on this asteroid, and I jumped, would I fall off? It's been a while since I ...
8
votes
5answers
1k views

Two orbiting planets in perpendicular planes

Inspired by this question. Can a 3 body problem, starting with two planets orbiting a larger one (so massive it may be taken to stand still) in perpendicular planes, be stable? Is there known an ...
13
votes
4answers
1k views

Why do all the planets of the solar system orbit in roughly the same 2D plane?

Most images you see of the solar system are 2D and all planets orbit in the same plane. In a 3D view, are really all planets orbiting in similar planes? Is there a reason for this? I'd expect that the ...
1
vote
2answers
4k views

Forces in a Pendulum

Help me visualise the forces found in a pendulum. I know weight can be decomposed into $F_x$ and $F_y$. Centripetal force, acceleration and velocity are three words that I'm confused. Centripetal ...