Questions tagged [gravity]

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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68 views

How do time elapse for free falling clocks vs. clocks in flat space-time? [duplicate]

Complete re-edit. Trying to rephrase my question, based on some good comments. Made several questions in the hopes that they combined paint the picture and that atleast one of them is understood :) Q1:...
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How does the curvature of space-time explain a ball falling in reverse?

If we throw a perfect uniform ball up in the air without any rotation in a vacuum room in a perfect perpendicular direction of earths gravitational pull and mark the part of the ball that is pointing ...
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Gravito-magnetism and gravitational waves outside of General Relativity

An important implication of (linearised) Einstein equations is that you can write a gravito-magnetic field in addition to the gravito-electric field (the classic gravitational field). And from there ...
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Event horizon around branes

I'm wondering if a brane is a massive topological defect, does it have an event horizon? Is it related to the effect capuring matter fields on the brane? Thanks for the answers
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How Does a Rocket Even Go Up? [closed]

The earth pulls all bodies towards it at $9.8\ \mathrm{m/s^2}$. I would like to imagine that the ground is accelerating up at $9.8\ \mathrm{m/s^2}$ (just as an analogy). So this means that a rocket ...
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Extent of verification of household-name physical concepts? [closed]

I was listening to some news coverage of the IPCC's newest report, which included a statement that "the evidence that climate change is real is now provided by over 14,000 studies worldwide."...
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Can the Newton field law be applied to electrostatics, gravity as well as thermal radiation in the same way?

Can the Newton field law be applied to electrostatics, gravity as well as thermal radiation in the same way?It is well known that the net gravity inside a homogenous shell is zero.Also happens to the ...
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Why does the inverse square law make impossible for an object to levitate throught a combination of distance action forces?

From the book Thinking Physics: Suppose the moon had a negative charge. Then it would exert a repelling force on electrons near it. But the gravitational force of the moon exerts an attracting force ...
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Rotation of galaxies and gravitomagnetism [duplicate]

Can we explain the bigger speed of rotation of the galaxies from gravitomagnetic field acting from the "inner layers" of the galaxy to the "outer layers" of the galaxy?
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What are the possible effects of a change in the Earth's gravitational potential?

I have been trying to figure out the possible consequences of a small relative change (in the neighborhood of $10^{-4}$) in the Earth's gravitational potential, such as gravitational time dilation, ...
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Does movement of the earth bring difference in the distortion of fabric of spacetime?

The earth is constantly moving with huge velocity. Doesn't this tremendous velocity bring difference in distortion of spacetime at the opposite ends? I mean the end towards the direction of motion and ...
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Can an observer in a double tidal locked system figure out it is orbiting?

Tidal locking, when the spin rate of a body matches the orbital rate so that it always faces the other body with the same side, usually occurs for just one of the bodies in orbit. However, there are ...
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Combining Newtonian Gravity and length contraction from SRT?

The value of $\gamma$ that I will use is $$\gamma=\sqrt{1-\frac{v^2}{c^2}}$$ And length contraction is $$L_0=\frac{L}{\gamma}$$ Where all variables have usual meaning. Let the gravitational pull on ...
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Maximum altitude of a vertical launch taking into account the change in gravity

Say that a body with mass $m$ is launched vertically from the surface of a planet with mass $M$. What is the maximum altitutde the body will reach? My attempt: the equations $$\frac{d^2}{dt^2}d(t)=a(...
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Could you make a space elevator anywhere besides the equator?

I’ve read about and understand the basic concept of a space elevator. Currently there is no material strong enough to withstand the tension force applied to such a device. But what would happen if you ...
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String Theory, understading Einstein-Field Equations

Let me give some insight. I was researching about how Einstein-Field Equations arise from String Theory, and I came upon this post. It explains, mathematically, how you can arrive at the corrected ...
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What would be the challenges of my new telescope idea? [closed]

I have an idea of using a black hole as a new design of a telescope by placing lenses in orbit around it. Since light bends around a black hole, it should be possible for us to place lenses in a coil ...
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Time dilation in the center of the earth [duplicate]

The time dilation seem to slow time in stronger gravitational fields. Lets say we put a person with a clock in the center of the earth. It will feel no gravity since it's pulling from every direction ...
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How would the density of a sphere of uniform composition vary with depth due to gravity?

Let's say we have a planet-sized sphere of some uniform material floating isolated in space, not rotating, with the only significant gravity acting on it coming from its own mass. How would the ...
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How do I calculate evection and variation for the moon in my simple solar system model?

I am building a model of a simple sun-planet-moon solar system (not the real Sol-Terra-Luna system). Purpose: to calculate the positions of the sun and moon in the sky of the planet at a given time, ...
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Curvature in Minkowski space? [closed]

Lately am thinking of how the equivalence principle has made up GR and how can I represent it as elementary as possible in my head. So the idea that had Einstein back then was that given an inertial ...
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Regarding distance laws of gravity from “A Brief History of Time” and why the Earth doesn't fall into the Sun

In “A Brief History of Time”, Hawking explains Newtonian gravity in Chapter 2, Space and Time. The further apart the bodies are, the smaller the force. The gravitational attraction of a star is ...
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The Force of Gravitational Waves

How could one calculate and derive the "force" or displacement and stretching of an object due to gravitational waves?
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Conformal blocks in AdS/CFT

In a CFT one can expand correlation functions in conformal blocks, for example the four-point function can be written (schematically) as: $$\langle \mathcal{O}_1 (x_1) \mathcal{O}_2 (x_2) \mathcal{O}...
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Why can't the speed of gravitational waves be greater than the speed of light if the universe can expand faster than the speed of light?

Since the expansion speed of the universe can be greater than the speed of light, why can't gravitational waves, which also uses space as the medium, travel faster than the speed of light?
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Schwarzschild energy-momentum tensor

How can I figure out the components of the stress-energy tensor for an object i.e. a planet, star, black hole, etc. And especially what is the stress-energy tensor of a Schwarzschild-geometric ...
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Spacetime diagram for static weak field metric

Can anyone draw/provide links to the spacetime diagram for the static weak field metric (where Newton's law applies)?
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A closed laboratory free falls and approaches the event horizon of a Black Hole. Are measurements of physical constants affected?

A closed laboratory is in perfect free fall and hence is an inertial frame. This lab is falling toward the event horizon of a Black Hole. The lab is sufficiently small and the event horizon is ...
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Is there a term that describes the (immediate) loss of mass at the center of a gravitationally bound system?

I'm curious if there is a general term in physics that describes the (immediate) loss of mass at the center of a gravitationally bound system? For instance, if (as contrived as it may be) the Sun were ...
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The velocity of gravitational waves or the velocity of the effect of gravity [duplicate]

If the sun disappears from the solar system, the earth will remain revolving for 8 minutes and will not be freed from the sun's gravity until after 8 minutes because the speed of the effect of gravity ...
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Two bodies A and B fell from rest Compare the time of arrival of objects A and B to Earth [closed]

Two bodies A and B fell from rest from a height of 100 m, and body A reached the ground Directly while body B hits an inclined surface making its velocity horizontal as shown in the figure, Compare ...
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What would a theoretical non-rotational artificial gravity generator look like? [closed]

Assuming we invented a device that can create gravitational waves or bend spacetime (or through any non-motion related phenomenon) and produce artificial gravity, would it pull everything towards a ...
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Non-factorization of partition functions in AdS/CFT

I have seen this comment a lot in recent papers. This has to do with the AdS/CFT correspondence and Euclidean asymptotically AdS wormholes. So when we calculate the partition function of two $D-1$ ...
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Geodesic: Why should an apple fall? [duplicate]

A geodesic is the path an object follows to extremise (maximise) its proper time. I am still confused why the final position of the apple should be near the earth. My argument: an apple falls so that ...
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What is the difference between AdS and BTZ conditions in (2+1)-D gravity?

On the BCFT, at lower Temperatures, the AdS condition is favored while at higher ones, BTZ conditions are. But, I do not understand how the conditions for either one change the gravitational theory. ...
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Minimum mass of asteroid that could knock moon away [closed]

What is theoretically the minimum mass of asteroid, coming from deep space, that could knock the Moon away from Earth? The asteroid is not hitting the Moon, just interacting with it by gravity. I was ...
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How AdS/CFT solves the non-renormalizability of gravity?

According to AdS/CFT, all gravity calculation in AdS can be mapped to CFT calculations on the boundary. Then what are the loop divergences in AdS gravity be mapped to the CFT side? Or put it ...
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How the wavelength of a photon is affected by an uniform gravitational field

In the presence of a uniform gravitational field two observers at fixed positions obtain different measurements of frequency of the same photon. One observer at the origin of some coordinate system ...
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Why is weak equivalence principle reformulation of the equivalence of gravitational and inertial mass?

The weak equivalence principle is (from the Weinberg book) At every space-time point in an arbitrary gravitational field it is possible to chose a 'locally inertial coordinate system' such that, ...
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Mass hanging from vertical spring

Why does a mass hanging from an ideal spring, under gravity, undergo periodic oscillations? Isn't gravity a damping force here? Equation for mass spring system is given by $m\ddot{y}+ky=0$ where ...
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Curvature of space without mass or energy [duplicate]

Are there solutions of Einstein's equation $$ R_{i,k}=0 $$ which imply there are 'curvature' without matter or energy? For example, space is curved but there are no matter or any energy there so an ...
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Warped space shouldn't cause movement in Gravity so what does? [duplicate]

I'm trying to get an intuitive understanding for how gravity works and have been reading various explanations from answers here to Sean Carroll and others' youtube videos. I have a set of university ...
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Is the Hubble constant actually constant? [closed]

For this question I completely understand the the constant is different over time, but just having one of those daydream thoughts and thought I'd ask. As a layman, I've been made to believe the ...
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Heisenberg uncertainty principle: Have unusual methods of measuring position (other than photons) been proven equally imprecise?

I'm reading Hawking's A Briefer History of Time and it's explained that Heisenberg demonstrated his uncertainty principle based on fundamental limitations of positional meaurememt by basically ...
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Topologically, is a curvature singularity just a hole?

Topologically speaking, a hole can be introduced into a manifold and it will still be a manifold, e.g. remove points within a 2-sphere of some radius from the cartesian plane and you'll still have a ...
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Can I just add a cosmological constant to an on-shell supergravity action?

Suppose one has some supergravity Lagrangian $\mathcal{L}_\text{sugra}$ in flat (Minkowski) space. Moreover, assume that this Lagrangian is on-shell, in the sense that the auxiliary fields have been ...
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Would an inertia electric motor propel a spacecraft in zero gravity?

There's an interesting mechanic that is used by self balancing proof of concept robots where they have a wheel on two axis and when the stock robot starts to pivot and fall over, it spins the wheel ...
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Newton's first law - elevator lift upward and downward force [duplicate]

Why does $F_c$ is equal to $F_g$ and why $F_c$ isn't greater than $F_g$? $F_c$ is moving it upward right, so the force must be greater than gravity to pull it upwards I think.
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A perhaps naïve question, but will help with my understanding [duplicate]

Now, I get that gravity according to GR is merely an illusion caused by acceleration, and therefore the force I feel on Earth is not gravity at all, but the Earth accelerating ahead at $9.8 \;\text{m/...
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Is there anything peculiar happening with General Relativity in $D=1+1$ dimensions?

I have noticed the following with Einstein's equations in empty space (with zero cosmological constant): when $T_{\mu\nu}=0$, the Einstein tensor is zero, $G_{\mu\nu}:=R_{\mu\nu}-\frac{1}{2}Rg_{\mu\nu}...

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