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.
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1answer
610 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 ...
6
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3answers
2k views
Why is Higgs Boson given the name “The God Particle”?
Higgs Boson (messenger particle of Higgs field) accounts for inertial mass, not gravitational mass.
So, how could it account for formation of universe as we know it today? I think, gravity accounts ...
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5answers
3k views
How does gravity work underground?
Would the effect of gravity on me change if I were to dig a very deep hole and stand in it? If so, how would it change? Am I more likely to be pulled downwards, or pulled towards the edges of the ...
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8answers
455 views
Gravity theories with the equivalence principle but different from GR
Einstein's general relativity assumes the equivalence of acceleration and gravitation. Is there a general class of gravity theories that have this property but disagree with general relativity? Will ...
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7answers
1k views
How does Newtonian gravitation conflict with special relativity?
In the Wikipedia article Classical Field Theory (Gravitation), it says
After Newtonian gravitation was found to be inconsistent with special relativity, . . .
I don't see how Newtonian ...
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4answers
504 views
Where's earths death bulge, destroying everything in it's path?
I was watching a BBC documentary on space last night. It was talking about gravity, and it said that the reason we only ever see one side of the moon, is because the earths gravity is strong enough ...
6
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1answer
476 views
Grain of sand attracting the sun?
My friend keeps telling me that according to physics...
"The sun attracts a grain of sand on the earth with the same force that the grain of sand attracts the sun"
or
"A grain of sand on the earth ...
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5answers
2k views
How come gas molecules don't settle down?
If the earth's gravity exerts a net downward gravitational force on all air molecules, how come the molecules don't eventually lose their momentum and all settle down? How is the atmosphere is still ...
6
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2answers
554 views
What physical forces pull/press water upwards in vegetation?
Each spring enormous amounts of water rise up in trees and other vegetation. What causes this stream upwards?
Edit: I was under the impression that capillary action is a key factor: the original ...
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4answers
1k 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?
6
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1answer
262 views
In what limit does string theory reproduce general relativity?
In quantum mechanical systems which have classical counterparts, we can typically recover classical mechanics by letting $\hbar \rightarrow 0$. Is recovering Einstein's field equations (conceptually) ...
6
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2answers
263 views
What is the weight equation through general relativity?
The gravitational force on your body, called your weight, pushes you down onto the floor.
$$W=mg$$
So, what is the weight equation through general relativity?
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3answers
240 views
Tidal force on far side
I have a question about tidal forces on the far side of a body experiencing gravitational attraction from another body.
Let's assume we have two spherical bodies $A$ and $B$ whose centers are $D$ ...
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5answers
384 views
Event Horizon violability?
Is the "event horizon" of a black hole potentially violable? Black holes are commonly described as being unidirectional (matter / energy goes in, but doesn't come out), but is the event horizon of a ...
6
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2answers
140 views
What are the limitations of performing music in space?
Canadian astronaut Chris Hadfield's song Space Oddity is making news around the world today. It makes me wonder: What are the limitations of performing music in space? Clearly, there is no point to ...
6
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3answers
228 views
Can low-gravity planets sustain a breathable atmosphere?
If astronauts could deliver a large quantity of breathable air to somewhere with lower gravity, such as Earth's moon, would the air form an atmosphere, or would it float away and disappear? Is there a ...
6
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3answers
344 views
Charging a black hole?
What would happen if we have a black hole and we start shooting at it a single electron at a time, and go on doing it forever? Would the electrons start to bounce off eventually?
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3answers
493 views
Without gravity, is there still up and down?
I'll try to be clear: example: If you send the ISS far enough for it not to undergo the Earth's gravity anymore, then you turn it and the—sleeping—astronauts in it upside down, when they wake up, will ...
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3answers
98 views
What happens with the force of gravity when the distance between two objects is 0?
so I had my first approximation to the gravity equation
\begin{equation}
F=GmM/r^2
\end{equation}
and some questions arose that my teacher couldnt respond:
if r approximates to 0 with mM being ...
6
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1answer
146 views
Why doesn't my particle simulation end in a flat disc?
I've made a 3d particle simulator where particles are attracted to each other by the inverse of the square radius. The purpose of my experiment is to see if this alone would create a flat disk (like ...
6
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1answer
82 views
Quantum mechanical gravitational bound states
The quantum mechanics of Coloumb-force bound states of atomic nuclei and electrons lead to the extremely rich theory of molecules. In particular, I think the richness of the theory is related to the ...
6
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1answer
276 views
Why does Jupiter emit more energy than it receives?
I hear that Jupiter and Saturn emit more energy than they receive from the Sun.
This excess energy is supposedly due to contraction.
Is this accepted as fact (or is it controversial)?
Does this ...
6
votes
4answers
640 views
Can gravity be shielded, like electromagnetism?
If I remember well, they said that it can't, but I do not know why.
Yes, I meant if gravity can be shielded using something like a Faraday cage
(or something else?).
Thank you.
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1answer
113 views
Why is $R^2$ gravity not unitary?
I have often heard that $R^2$ gravity (as studied by Stelle) is renormalisable but not unitary. My question is: what is it that causes the theory to suffer from problems with unitarity?
My naive ...
6
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1answer
187 views
Gravity duals to Navier Stokes and interpretation of non linear contributions
I have been reading the paper The Incompressible Non-Relativistic Navier-Stokes Equation from Gravity. In it they state,
"An instability, if it occurs, must necessarily break a symmetry ...
...
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2answers
142 views
What exactly is the microgravity field in orbit?
The ISS and other objects in orbit still experience small acceleration outside from the perfect line of orbit (of the system CM). For instance, two objects in the ISS that are let to be at rest will ...
6
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0answers
92 views
Equation of state of cosmic strings and branes
I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ...
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4answers
668 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 noninertial frame accelerating in the sense opposite to the gravitational field - is ...
6
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4answers
282 views
Why do 3d spheres and gravity tend to rotating discs on one plane?
Whether is it our solar system or a whole galaxy, there is usually a massive object (star or black hole) at the centre with gas and objects rotating around it.
The gravitational effect of the ...
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votes
3answers
376 views
Is it possible to have a floating bullet in the air?
I've been asking myself a question for quite some time :
say that a bullet gets out of a gun at 900 km/h (I'm european, hence the metric system).
say a train could go in a straight line at 900 km/h
...
5
votes
7answers
981 views
What is the proof that the universal constants ($G$, $\hbar$, $\ldots$) are really constant in time and space?
Cavendish measured the gravitation constant $G$, but actually he measured that constant on the Earth. What’s the proof that the value of the gravitation constant if measured on Neptune would remain ...
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4answers
5k views
Electrical force vs gravitational force
Given that the electrical force is so much stronger than gravitational force at atomic levels, why is it that it's the gravitational force between you and the earth that keeps you on the ground rather ...
5
votes
6answers
430 views
Why do liquids separate in space?
I've seen videos of people in space (on ISS) who squeeze a bottle or something and liquid comes out, it then separates into smaller balls.
Why is this surely it should stay pretty much together ...
5
votes
4answers
473 views
Why do we weigh less when falling?
I don't want to go to science world to find out because it would be a long round-trip.
I understand that acceleration/deceleration would effect the weight and I can also imagine that someone at ...
5
votes
1answer
441 views
Why can you remove the gravitational constant from a computer game simulation?
I've seen in a few gravity simulation games (ie. bouncing balls) the equation:
force = G * m1 * m2 / distance^2
shortened to this by removing the gravitational ...
5
votes
2answers
332 views
What is the likelihood of ever discovering the graviton?
How would one look for and confirm existence of a graviton?
Someone was speaking to me about perhaps one day discovering the graviton, but to me it seems unlikely, although I'm young and essentially ...
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4answers
702 views
A Basic Question about Gravity, Inertia or Momentum or something along those lines
Why is it that if I'm sitting on a seat on a bus or train and its moving quite fast, I am able to throw something in the air and easily catch it? Why is it that I haven't moved 'past' the thing during ...
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3answers
333 views
Black holes in a head-on collision
Assume two uncharged non-rotating black holes traveling straight at each other with no outside forces acting on the system. What is thought to happen to the kinetic energy of these two masses when ...
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4answers
497 views
Please clarify how entropy increases when matter gravitationally coalesces
On John Baez's website, http://math.ucr.edu/home/baez/entropy.html, he discusses the problem of how entropy increases when a cloud of ideal gas collapses gravitationally (no black holes - keeping it ...
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2answers
2k views
Why do helium filled balloons move away from the Earth?
From my understanding objects do not fall but are pulled to the earth from gravity.
With this in mind, I can't understand why if helium filled balloons are not pulled by gravity then shouldn't they ...
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2answers
227 views
What is the largest sphere of liquid water that could exist in space held together under it's own gravity? [duplicate]
What is the largest theoretical sphere of liquid water that could exist in space held together under it's own gravity? I've always wondered if a planet the size of earth could exist as a single volume ...
5
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2answers
672 views
Calculating gravity when taking into account the change of gravitational force
This is a problem that has bothered me for a couple of weeks now, and I can't seem to wrap my head around it and understand it.
Let's say we have a planet with a mass of m. We also have an object of ...
5
votes
2answers
302 views
Does rotational energy have effect on gravity/metric?
Intuitively, if energy can be stored in rotational motion, it has to obey $E=mc^2$.
Does rotation of typical stellar-sized objects - BHs, pulsars, binaries - have measurable effect on their overall ...
5
votes
2answers
833 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 ...
5
votes
3answers
265 views
Do rotating bodies emit gravitational waves?
Suppose we have a cylinder of mass $m$, radius $R$ and height $h$ in rotation with speed $\omega$ around its symmetry axis with no friction (ideal situation).
I'd expect this cylinder to emit ...
5
votes
2answers
185 views
Do objects with mass “suck in” spacetime?
I don't really understand the general theory of relativity (GTR) really deeply, but according to my understanding, the GTR say that gravitation is caused by the curvature of spacetime by objects with ...
5
votes
5answers
402 views
Why can't airplanes just keep going up? [duplicate]
Possible Duplicate:
Why do space crafts take off with rockets instead of just ascending like an aircraft until they reach space?
Ignoring that the engines only work in air, so say its a ...
5
votes
3answers
304 views
Does MOND make good predictions?
Well,it does according to this preprint: http://arxiv.org/PS_cache/arxiv/pdf/1102/1102.3913v1.pdf
for certain scales
How would be a simple way to explain MOND to a layman?
Does it ignore mainstream ...
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3answers
84 views
Could dark energy be the effect of gravity at great distances?
This may be a silly question, but is it possible that dark energy and gravity are related to each other?
Space-time is deformed everywhere in space by objects with mass. The more massive the object, ...
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2answers
551 views
What does it mean for objects to follow the curvature of space?
In science documentaries that touch on general relativity, it is often said that gravitational pull isn't an actual a pull (as described by classical physics), but rather one body travelling in a ...
