All Questions
5,503 questions
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Possible reasons concerning decreased suspension height of an copper object in an electric field
I am transcribing an article that discusses electrostatic levitation. A segment of this article pertains to the author describing a charged spherical object composed of copper with its suspending ...
1
vote
1
answer
56
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Is there any way in which orbits emitting gravitational waves could avoid coalescence?
In the very far future, all orbits will eventually coalesce as they lose orbital energy through gravitational waves emission.
However, can there be 3-body or N-body interactions in which the members ...
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12
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8
answers
3k
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Would weightlessness (i.e. in thrill rides, planes, skydiving, etc.) be different on a Flat Earth?
I was listening to a Scientific American podcast, dated March 27th, 2020, about what Flat Earthers believe and why they believe it. As I listened, Michael Marshall mentioned that FEers believe that ...
- 237
3
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0
answers
62
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Are relic neutrinos gravitationally bound to large scale structures?
I have been interested in how neutrinos arrange themselves around gravitational potentials. However, I found contradictory claims and I would like to clarify this
After reading a bit into this paper (...
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1
answer
58
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Assuming that proton decay occurs, will all types of particles be dispersed without exception?
Proton decay is a theoretical possibility, as it is predicted to happen by some GUTs, so the proton may not be stable and could decay into lighter particles considering enormous time scales.
If proton ...
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9
votes
4
answers
2k
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Can there be clouds of free electrons in space?
Electrons are repelled from each other by the electromagnetic force, which is stronger than gravity. However, if there is a sufficiently high amount of free electrons in some region of space, could ...
- 2,878
1
vote
2
answers
91
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How does an atom change in the presence of a large gravitational force?
Question / light thought experiment on atomic structure in the presence of a large gravitation force.
Background:
This is a general, conceptual question, not looking to solve any particular problem, ...
- 111
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3
answers
51
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Pressure of column of air and pressure of cinder block - what makes the pressure to apply from below in the former and not in the latter? [duplicate]
Pressure is force per unit of area.
I understand that the pressure at the ground is the force the column of air (above a surface of one square meter) exerts on that surface of one square meter. To be ...
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1
vote
1
answer
241
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If electron is in two places at same time, is its gravitational pull also at two positions at same time? [duplicate]
If electron is in two places at same time, is its gravitational pull also at two positions at same time?
Have experiments been done to investigate this question?
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0
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78
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Non-metric gravity calculations
According to "Gravity and Strings" by T. Ortin (2015), the non-metricity tensor is calculated as
$$
Q_{\rho\mu\nu}\equiv\nabla_\rho g_{\mu\nu}=\partial_\rho g_{\mu\nu}-\Gamma^\beta_{\rho\mu}...
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2
answers
163
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Quantum mechanics does not fit well with gravity (GR) - does this mean one of these two theories is wrong or incomplete? [closed]
Quantum mechanics does not fit with gravity (GR) - does this mean one of these two theories is wrong or incomplete?
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1
answer
135
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Can we have negative Ricci scalar?
Recently I faced a negative Ricci scalar in some calculations and looking for a physical interpretation for it. Is there any physical Energy-Momentum tensor that could produce a negatively signed ...
- 219
2
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1
answer
85
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Can dark matter in neutron stars cause the formation of naked singularities?
I've been studying the concept of dark core collapse, particularly as discussed in the paper "Low Mass Naked Singularities from Dark Core Collapse" on arXiv. The paper explores the idea that ...
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8
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2
answers
3k
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A study on the speed of gravity
I have just come across a paper which involves a study about the speed of gravity. This has rocked me a bit, since I thought it was well known gravitational fields travel at the speed of light, in ...
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4
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1
answer
120
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Can there be a progressive and slow vacuum transition event instead of a sudden one?
I have a question on transition events between vacua, and what do our models predict that would happen:
Could there be a transition between vacua without involving a sudden event like a bubble ...
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1
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0
answers
45
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How does string force us to include gravity & GR? [duplicate]
Some physicists (e.g. Witten) often claim that string theory does not only makes gravity possible, but actually force its existence.
I have studied an introduction to string theory (mainly from ...
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1
answer
62
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Basic understanding of Gravity Wells
In all the diagrams I have seen, a large astronomical body eg. a planet or star, shows its gravity well as one dimensional in spacetime. My basic question is: Is it truly one dimensional or multi-...
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1
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93
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Is curved spacetime related to entropy and/or quantum decoherence?
In this article (https://physics.iitm.ac.in/~dawood/resources/pedagogical-articles/GRFessay_Kothawala_2013.pdf) in the abstract, it is said that
Spacetime curvature will generically perturb the ...
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3
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0
answers
184
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Understanding Feynman: why gravity is not a spin-0 theory?
I'm struggling to understand a certain paragraph in Feynman's "lectures on gravitation". It's lecture 3, why gravity cannot be a spin-0 theory. Here's the text:
The rejection of spin-zero ...
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5
answers
100
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A theoretical experiment about gravity and propulsion
An observer travels in a spherical ship drifting through space. The observer cannot 'see' anything outside the ship.
At some time, the ship approaches a massive object P and describes an hyperbolic ...
- 213
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0
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30
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Dilaton and scalar hair
I see that there are many papers for dilaton black hole with the scalar hair. So, what is the difference between dilaton and scalar hair of the black hole?
1
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1
answer
52
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Heat transport in earth's gravitational field
Consider a Carnot machine that exploits the earth's atmospheric temperature gradient using air at lower altitude as a warm reservoir and air at higher altitude as a cold reservoir. Due to the ...
- 482
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2
answers
60
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Is it possible to detect a photon using gravity without information about its emission and absorption?
Is it possible to detect a photon using gravity without information about its emission and absorption?
Speculation. Body No.1 emits a photon and changes its momentum. The photon passes by body No.2 ...
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1
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1
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72
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Is rotation invariant under gravitational time dilation?
Imagine a giant ferris wheel with a ten-mile radius on a planet with a strong gravitational field. As it spins, different parts of the wheel experience varying degrees of time dilation due to the ...
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3
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1
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463
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Astronomy Without Fusion
Let us begin with diffuse clouds of atomic hydrogen as we believe was the case in the early universe, but assume that nuclear fusion is not an existing mechanism.
What would happen under the influence ...
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3
answers
127
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Can two particles ever be in equilibrium under their mutual gravitational forces alone?
Can two particles be in equilibrium under the influence of their mutual gravitational forces alone?
Obviously, if the two particles are kept at rest at a distance apart, one will exert an attractive ...
- 12.3k
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0
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30
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Do lightbeams redshift one another upon passing?
If it's true that anti-parallel light beams affect eachother gravitationally does this imply that they mutually redshift in addition to bending their paths towards one another? For example consider ...
- 630
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0
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44
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Tensor differential equation
How to solve the following differential equation with tensor indices?
$\epsilon_{\mu\nu}\partial^{\gamma}\partial_{\gamma}f-2i\epsilon_{\mu\nu}p.\partial f+ip_{\mu}x^{\gamma}\epsilon_{\nu\gamma}+ip_{\...
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0
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40
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Derivation of Equation of Motion of Graviton
In Feynman's Lectures on Graviton, Feynman tries to simplify the equation of motion of gravitation field,
$$h_{\alpha\beta,\sigma}^{,\sigma} - (h_{\alpha\sigma,\beta}^{,\sigma} + h_{\beta\sigma,\alpha}...
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3
answers
2k
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Can perfectly stable orbits exist in GR?
Defining "stable orbit" between two bodies as one where, in the absence of other bodies or non-gravitational forces, the distance stays between some value pair $r_{min}>0$ and $r_{max}$. ...
- 393
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answer
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Cause of Coordinate Acceleration in Free Fall [duplicate]
So I understand that objects in free fall are in an inertial frame, at rest in terms of relativity. However, from a person on the surface of earth, a falling apple is accelerating constantly until it ...
- 475
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42
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What is a simple example that contradicts the theory that light (photons) could have a super small mass? [duplicate]
The obvious example is that, to bring something that has mass to the speed $c$ requires infinite energy due to special relativity. But what if a kid asked me "What if light/photos had a super ...
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36
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Energy of the gravitational field within a sphere of radius $R$ in the Schwarzschild metric
The Landau-Lifshitz energy-momentum pseudotensor $t^{μν}$ is defined by
$$16πt^{μν}
= -2G^{μν} - g^{-1} \left[ -g \left( g^{μν}g^{αβ} - g^{μα}g^{νβ} \right) \right]_{,αβ}$$
where $g=\text{det}[g^{μν}]...
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5
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239
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What is the gravitational field of a hole in an infinite perfect crystal?
Or equivalently and more interestingly: In the early universe when there was uniform H/He gas everywhere, gravitational field was close to 0 everywhere. Every test particle was pulled from all sides ...
4
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2
answers
661
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Areas with anti-parallel gravity in classical physics
I cannot indicate an error in the following reasoning if it is done in the framework of classical physics.
Let's make the imaginary setup with two wedges and the ball, when the gravity in the left ...
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2
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67
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Confused about Weinberg's result of gravitational time dilation
I am reading Weinberg's Gravitation and Cosmology. In section 3.5, the author got a result$$\frac{dt}{\Delta t}=(-g_{00})^{-1/2}\tag{3.5.2}.$$Here $dt$ is the time interval of a stationary observer in ...
- 613
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0
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54
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Does gravity accelerate you towards the geodesic of light between you and the mass?
If there's a planet far away, you will accelerate straight towards it due to gravity. If you place a Schwarzschild black hole right in the middle between you and the planet (the distance between the ...
- 171
3
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0
answers
62
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Negative (absolute - not potential) energy of the gravitational field; how to generalize to GR?
Alan Guth gives a thought experiment to show that a gravitational field has negative energy. (See the picture below.) Consider a thin spherical shell of elastic, compressible matter, of radius $R_o$. ...
- 467
4
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0
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98
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Lorentz force error in the present 2024 version of the gravitoelectromagnetism Wikipedia page? [closed]
I noticed that the Gravitoelectromagnetism (GEM) Wikipedia page has been edited recently. The factor of 4 in the GEM Lorentz force equation is now missing. But the GEM field equations are identical to ...
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2
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0
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97
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Gravitational halos made of neutrinos...?
I have been recently interested in how halos made of standard model particles could be formed and behave.
After asking some questions in this site, I was told about how neutrinos could form such halos....
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2
votes
1
answer
93
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How to properly combine kinetic and gravitational time dilation effect?
I developed a time dilation calculator that includes both kinetic (Lorentz Factor) and gravitational (Schwarzschild Metric Formula) factors to assess the time difference between Earth and satellites. ...
8
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5
answers
1k
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Do you always experience the gravitational influence of other mass as you see them in your frame?
You see a galaxy far away. That galaxy is attracting you with a certain amount of gravity. I'm wondering if the gravity influence of the galaxy on you, as measured by you, always ends up being what ...
- 171
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3
answers
82
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How much time does it take for an object to fall from space? [closed]
Let's say there's an object of mass $m$ in space, $h$ meters away from the surface of the Earth. $h$ is large enough that $g$ cannot be assumed to be constant. The acceleration varies according to ...
- 127
3
votes
2
answers
89
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Tug of war between observers in frames with different rate of time
You have a very dense hollow sphere of matter. Observer A is inside the sphere inside a rocket. Observer B is in an identical rocket outside the sphere where the ring's gravity is negligible. They are ...
- 171
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3
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137
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Do clocks tick faster when gravitational forces are weaker?
A professor last year taught us that "gravity slows clocks," when teaching about the relationship between gravity and time. This led me to think about places, such as intergalactic space, ...
-3
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3
answers
81
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Does Matter Cause Curvature or Vice-Versa [closed]
From the way explanations about gravity-acceleration-curvature equivalence are usually phrased here or elsewhere, it would appear many or most think that matter causes space-time curvature.
I cannot ...
- 115
4
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1
answer
206
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Binary black hole merging condition
Assuming two black holes with the same rest mass $m$ collid coming from infinity with velocity $v$ and impact parameter $b$. Lets ignore spin at first. For which values of $v$ and $b$ would these ...
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0
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19
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How significant is the parabolic arch effected with an incline versus a decline in angle versus a decline in angle . For example throwing a dart
A dart players height can vary as can the height in which they raise their arm to aim.
Is there less parabolic arch when throwing at the same strength from above the target compared to below, due to ...
5
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1
answer
472
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Bound states between neutrinos using Schrödinger's equation?
I would like to see if it's possible that neutrinos (with sufficiently slow velocities) could form bound states in a universe with matter (such as ours)
There is a cosmic neutrino background in the ...
- 2,878
2
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0
answers
51
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Second-order equations of motion for higher derivative gravity?
We know that Lovelock gravity is the most general theory of gravity possible for Lagrangians which depend only on the metric tensor and the Riemann tensor
\begin{equation*}
L = L \left(g_{\alpha\beta},...
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