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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13answers
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How does gravity escape a black hole?
My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to ...
43
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5answers
2k views
Is there a small enough planet or asteroid you can orbit by jumping?
I just had this idea of orbiting a planet just by jumping and then flying upon it on its orbit kind of like superman. So,
Would it be theoretically possible or is there a chance of that small body to ...
43
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10answers
3k views
How can I stand on the ground? EM or/and Pauli?
There is this famous example about the order difference between gravitational force and EM force. All the gravitational force of Earth is just countered by the electromagnetic force between the ...
28
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1answer
501 views
Why are our planets in the solar system all on the same disc/plane/layer?
I always see pictures of the solar system where our sun is in the middle and the planets surround the sun. All these planets move on orbits on the same layer. Why?
26
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6answers
1k views
Can I survive a free fall using a ramp and a rope?
Can I survive a free fall by carrying a very light and resistant ramp using a rope?
Note: lets assume the ramp is a little bit heavier at the bottom and I am very skilled at making it always land ...
22
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2answers
2k views
Why can't we feel the Earth turning?
The Earth turns with a very high velocity, around its own axis and around the Sun.
So why can't we feel that it's turning, but we can still feel earthquake.
21
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3answers
679 views
Why is gravity such a unique force?
My knowledge on this particular field of physics is very sketchy, but I frequently hear of a theoretical "graviton", the quantum of the gravitational field. So I guess most physicists' assumption is ...
21
votes
1answer
194 views
Vassiliev Higher Spin Theory and Supersymmetry
Recently there is renewed interest in the ideas of Vassiliev, Fradkin and others on generalizing gravity theories on deSitter or Anti-deSitter spaces to include higher spin fields (utilizing known ...
20
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2answers
87 views
experimental bounds on spacetime torsion
Did Gravity Probe B provide any bounds on Einstein-Cartan torsion? is a non-zero torsion value at odds with the results regarding frame-dragging and geodetic effects?
19
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2answers
66 views
Kerr Geometry, Separability and Twistors
One of the remarkable properties of the Kerr black hole geometry is that scalar field equations separate and are exactly solvable (reducible to quadrature), even though naively it does not have enough ...
19
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5answers
1k views
Could gravity be an emergent property of nature?
Sorry if this question is naive. It is just a curiosity that I have.
Are there theoretical or experimental reasons why gravity should not be an emergent property of nature?
Assume a standard model ...
18
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4answers
2k views
Would you be weightless at the center of the Earth?
If you could travel to the center of the Earth (or any planet), would you be weightless there?
18
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6answers
1k views
The speed of gravity?
Sorry for the layman question, but it's not my field.
Suppose this thought experiment is performed. Light takes 8 minutes to go from the surface of the Sun to Earth. Imagine the Sun is suddenly ...
17
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6answers
2k 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 ...
17
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8answers
1k views
Why are L4 and L5 lagrangian points stable?
This diagram from wikipedia shows the gravitational potential energy of the sun-earth two body system, and demonstrates clearly the semi-stability of the L1, L2, and L3 lagrangian points. The blue ...
16
votes
3answers
613 views
Are modified theories of gravity credible?
I'm a statistician with a little training in physics and would just like to know the general consensus on a few things.
I'm reading a book by John Moffat which basically tries to state how GR makes ...
16
votes
1answer
268 views
Flames with no gravity?
I was watching "Solaris" (Tarkovsky) today, and noticed this: in some moment the space station changed orbit and the people inside experienced zero-gravity. At that moment, a candlestick passed ...
15
votes
2answers
285 views
Does gravity travel at the speed of light?
Whenever I did calculations in high school physics involving gravity, it was either "a ball falling to the earth" type scenario, or a basic measurement of the gravitational attraction between two ...
15
votes
8answers
5k views
How exactly does curved space-time describe the force of gravity?
I understand that people explain (in layman's terms at least) that the presence of mass "warps" space-time geometry, and this causes gravity. I have also of course heard the analogy of a blanket or ...
15
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8answers
2k views
How can anything ever fall into a black hole as seen from an outside observer?
The event horizon of a black hole is where gravity is such that not even light can escape. This is also the point I understand that according to Einstein time dilation will be infinite for a ...
14
votes
3answers
614 views
Why doesn't a fly fall off the wall?
Pretty simple question, but not an obvious answer at least not to me. I mean you can't just place a dead fly on the wall and expect it to stay there, he will fall off due to gravity. At first I ...
14
votes
7answers
612 views
Is the quantization of gravity necessary for a quantum theory of gravity?
The other day in my string theory class, I asked the professor why we wanted to quantize gravity, in the sense that we want to treat the metric on space-time as a quantum field, as opposed to, for ...
14
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1answer
63 views
Instantons and Non Perturbative Amplitudes in Gravity
In perturbative QFT in flat spacetime the perturbation expansion typically does not converge, and estimates of the large order behaviour of perturbative amplitudes reveals ambiguity of the ...
13
votes
8answers
997 views
What are the reasons to expect that gravity should be quantized?
What I am interested to see are specific examples/reasons why gravity should be quantized. Something more than "well, everything else is, so why not gravity too". For example, isn't it possible that a ...
13
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7answers
2k views
What does it mean to say “Gravity is the weakest of the forces”?
I can understand that on small scales (within an atom/molecule), the other forces are much stronger, but on larger scales, it seems that gravity is a far stronger force; e.g. planets are held to the ...
13
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7answers
2k views
Is it theoretically possible to shield gravitational waves?
Electromagnetic waves can be shielded by a perfect conductor. What about gravitational waves?
13
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2answers
695 views
Does a photon exert a gravitational pull?
I know a photon has zero rest mass, but it does have plenty of energy. Since energy and mass are equivalent does this mean that a photon (or more practically, a light beam) exerts a gravitational pull ...
13
votes
3answers
355 views
Why don't I feel pressure on my body when swimming under water?
If I put a couple of lead bricks on my foot, there would be a definite sensation of a heavy, perhaps even painful, force. Calculating the pressure for $20 kg$ of lead over a $100 cm^2$ area of my ...
13
votes
6answers
666 views
Why do we still need to think of gravity as a force?
Firstly I think shades of this question have appeared elsewhere (like here, or here). Hopefully mine is a slightly different take on it. If I'm just being thick please correct me.
We always hear ...
13
votes
2answers
276 views
Why aren't gas planets and stars fuzzy?
The edge of Jupiter looks very sharp.
Even more bothersome, the edge of the sun looks sharp, aside from kind of a soup of particles floating above it.
The sun's surface has an incredibly low ...
13
votes
2answers
547 views
How do horseshoe orbits work?
An asteroid was recently discovered that is in a horseshoe orbit with respect to the earth. Is there an intuitive explanation for these orbits? It seems that the earth acts as a repulsive force where ...
13
votes
5answers
2k views
Is Gravity an entropic force after all?
Recently, there was a rapid communication published in Phys.Rev.D (PRD 83, 021502), titled "Gravity is not an entropic force", that claimed that an experiment performed in 2002 with ultra cold ...
13
votes
1answer
160 views
Is period of rotation relative?
My question is inspired by the following answer by voix to another problem:
"There is a real object with relativistic speed of surface - millisecond pulsar. The swiftest spinning pulsar currently ...
12
votes
3answers
609 views
Why does each celestial object spin on its own axis?
AFAIK all the celestial objects have a spin motion around its axis. What is the reason for this? If it must rotate by some theory, what decides it's direction and speed of rotation?
Is there any ...
12
votes
5answers
2k views
Why do rockets have multiple stages?
What is the advantage for rockets to have multiple stages?
Wouldn't a single stage with the same amount of fuel weigh less?
Note I would like a quantitative answer, if possible :-)
12
votes
6answers
618 views
Particle wavefunction and gravity
Suppose a particle has 50% probability of being at location $A$, and 50% probability being at location $B$ (see double slit experiment). According to QM the particle is at both $A$ and $B$ at the same ...
12
votes
1answer
725 views
Can one black hole suck in another black hole?
In the recent news, scientists at NASA have found “unprecedented” black hole cluster near Andromeda’s central bulge. I wonder why doesn't all these black holes merge and such each other in until just ...
12
votes
4answers
512 views
Why does no physical energy-momentum tensor exist for the gravitational field?
Starting with the Einstein-Hilbert Lagrangian
$$ L_{EH} = -\frac{1}{2}(R + 2\Lambda)$$
one can formally calculate a gravitational energy-momentum tensor
$$ T_{EH}^{\mu\nu} = -2 \frac{\delta ...
12
votes
5answers
731 views
What is a good mathematical description of the Non-renormalizability of gravity?
By now everybody knows that gravity is non-renormalizable, what is often lacking is a simplified mathematical description of what that means. Can anybody provide such a description?
12
votes
3answers
3k 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 ...
11
votes
5answers
6k views
How exactly does time slow down near a black hole?
How exactly does time slow down near a black hole? I have heard this as a possible way of time traveling, and I do understand that it is due in some way to the massive gravity around a black hole, but ...
11
votes
3answers
846 views
Significance of the second focus in elliptical orbits
1.In classical mechanics, using Newton's laws, the ellipticity of orbits is derived. It is also said that the center of mass is at one of the foci.
2.Each body will orbit the center of the mass of ...
11
votes
1answer
382 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 ...
11
votes
1answer
48 views
Relationship between Weak Cosmic Censorship and Topological Censorship
The weak cosmic censorship states that any singularity cannot be in the causual past of null infinity (reference).
The topological censorship hypothesis states that in a globally hyperbolic, ...
11
votes
2answers
468 views
What is the terminal velocity for a mobile phone
You may have seen the story of the iPhone which was dropped from perhaps 13,500 feet by a skydiver - it survived.
This made me wonder how to work out the terminal velocity for something like that. ...
10
votes
5answers
973 views
The square in the Newton's law of universal gravitation is really a square?
When I was in the university (in the late 90s, circa 1995) I was told there had been research investigating the $2$ (the square of distance) in the Newton's law of universal gravitation.
...
10
votes
6answers
1k views
Is Newton's Law of Gravity consistent with General Relativity?
By 'Newton's Law of Gravity', I am referring to
The magnitude of the force of gravity is proportional to the product of the mass of the two objects and inversely proportional to their distance ...
10
votes
1answer
1k views
Does a guitar sound different in zero (or micro) gravity?
Seeing a video of astronaut Chris Hadfield playing a guitar on the International Space Station made me wonder if a guitar or other stringed instrument played in zero-G would sound any different than ...
10
votes
2answers
86 views
Why is Jupiter so sharply defined?
In photographs of Jupiter, the limb seems extremely definite. Being a gas giant, my naive self thinks that the atmosphere should have a more gradual cut off, creating a hazy effect similar to that on ...
10
votes
3answers
863 views
“Speed” of Gravity and Speed of Light
Some threads here touching speed of gravity made me think about that. This lead to some questions.
The speed of gravity was not measured until today (at least there are no undebated papers to that ...
