Gravity Definition and 1000 Threads

On wikipedia, I found one of the objections to MWI. "We cannot be sure that the universe is a quantum multiverse until we have a theory of everything and, in particular, a successful theory of quantum gravity.[73] If the final theory of everything is non-linear with respect to wavefunctions then...

Homework Statement I learned how to deriving Maupertuis's principle from Hamilton's principle under conservative energy condition and feel this interesting. But when trying to derive a particle's behavior in gravitivity field,stuck... The direction of grativity is along ##x## axis,the particle...

https://arxiv.org/abs/1411.1935 This paper says LQG does not produce the Unruh effect effect, and therefore must be wrong. What do you guys think. You can click into the PDF to see the entire paper.

Ok - I have qualified this question with the word stupid so please go gently with me. Imagine you are talking to a rather stupid child when answering please. My question is about dark matter. Every time I do a search for "proof of dark matter", I get the same answer, which is that using the...

Homework Statement There are a big object and an astronaut in space. How do we calculate the gravitational force between them. I enclose a photo. I have given the mass of an astronaut, the dimensions of this giant ring and density of the ring. There is also a mistake in the photo. The astronaut...

Homework Statement Homework EquationsThe Attempt at a Solution For part a) I got t=0.571 seconds For part b) I got s=4.6m But part c) is what I'm stuck on. If his centre of gravity has shifted lower by 50cm, is his time spent in the air greater?

PeterDonis submitted a new PF Insights post The Schwarzschild Geometry: Part 4 Continue reading the Original PF Insights Post.

Hi All, Long time absentee from the forums hoping for some direction in my new research. I've been tasked to look into the stages of motion of objects (dropped from various heights), hitting a body of water (of various depths) and eventually hitting the seabed (of various sediment types). I am...

We see in our solar system the planets orbiting the sun, but why doesn't with all of the other forces in play the perpendicular velocity seemingly not decrease(or does it?). And if the perpendicular velocity of the planets slow down would that result in static planets not moving around the sun?

Thanks to all of you for your kind attention to my questions. May I ask another? If what we're calling dark matter interacts with visible matter via gravity, wouldn't it be concentrated along these arms if density waves are a feature of gravitational force?

PeterDonis submitted a new PF Insights post The Schwarzschild Geometry: Part 3 Continue reading the Original PF Insights Post.

PeterDonis submitted a new PF Insights post The Schwarzschild Geometry: Part 2 Continue reading the Original PF Insights Post.

I'm only an interested layman so please forgive my lack of knowledge but assuming the universe is flat, infinite and the cosmological principle holds then why wouldn't the attractive force between galaxies cancel out? If I think of a 2d analogy such as the surface of a sphere, and there were...

Thought experiment: Assume two galaxies in a galaxy group, initially at rest (with respect to one another). The distance between the centers of the galaxies is r = 1 Mpc. The total mass of each galaxy is mg = 6 ∙ 1042 kg (including dark matter). This is ≈ 3 ∙ 1012 solar masses. The...

I am totally confused. What is concerning the anisotropy the difference between re-entering gravity waves and re-entering density perturbations and more important for me what is then happening in the photon-baryon plasma between end of inflation and these re-enterings.

So there's a spaceship 5 light years away from Earth and they want to send a signal to Earth but a cloud of interestellar dust don't let them use light or radio signals so they decide to send a gravity signal. So they produce a violent huge thermonuclear explosion that will send some particles...

I was sitting at my bed when I suddenly have an idea. The unification of electromagnetism and gravity was made with General Relativity. For this to happen, one just need to write the energy due to electromagnetic field in "differential geometry" form, through the Electromagnetic tensor, in a...

PeterDonis submitted a new PF Insights post The Schwarzschild Geometry: Part 1 Continue reading the Original PF Insights Post.

If my understanding is correct, all particles are sources of gravitational fields (albeit minor ones), and the gravitational potential energy between two bodies is given by: U = -GMm/r So, if we have two Z bosons (or any other bosons with mass but no repulsion due to charge) which are traveling...

Dears, In the film the Arrival, of Denis Villeneuve, the alien ship apparently subjugates the terrestrial gravity. It's one thing to simulate gravity in space, another thing would be to simulate a gravitational system apart on Earth. Is there at least a theoretical possibility for this? How...

Assume the universe is flat and neither expanding nor contracting whatsoever, and the only matter in it are two marbles separated from each other by a distance of 10 million light years. Would the marbles eventually begin pulling toward each other (after 10 million years)?

I understand that General Relativity can make a difference between a spinning and non spinning mass thus can make better prediction for planetary orbits for example. The effect is frame dragging. However if we simulate a Newtonian gravitation and instead of representing a planet as a sphere...

1) If the idea that gravity is a curvature of space that causes objects to move towards each other then would it be appropriate to say that magnets do the same thing to space? 2) Does it seem a bit coincidental that both gravity and magnetism operate by the inverse square law or is it possible...

Suppose you have a perfect spring. By that I mean a spring that experiences no friction whatsoever, even internal, and that there is no elastic limit. Thus it obeys Hooke's law perfectly. Its own weight is negligible, and there is a point mass attached to the end of the spring. Now, the spring...

I ever read that there is no meaning for quantizing fluid mechanics. While left hand side of Einstein field equation used fluid-like approximation for matter (momentum-energy tensor). Therefore, if the first statement is correct then there is no meaning for quantizing gravity, right?

Homework Statement A baseball is thrown straight up with a speed of 25 m/s. Determine: (a) The maximum height reached by the ball (b) How long it takes the ball to reach its maximum height (c)The time it takes for the ball to reach a velocity of 5m/s (down) (d) The speed of the ball when it...

Hello all! Lately i have been looking at many rocket equations, and i was wondering about the gravity loss on a rocket launch. I found the formula for the gravity loss where g is of course the gravitational acceleration, and y is the flight path angle, but can somone explane why it looks like...

Going through several definitions, it appears that escape velocity is equal to the potential energy. That is:$$\frac{1}{2}m v^2=-\frac{G M m}{r}$$but if I solve for velocity, $v$, I get:$$v=\sqrt{-2\frac{G M}{r}}$$So how do I get an escape velocity that isn't imaginary?

I'm trying to understand potential energy, especially with regard to gravity. There's a double negative that's difficult to grasp: The direction of the force is negative because the vector points radially outward and the force of gravity is directed inward, and the work done to bring an object...

In Schutz says When we have weak gravitaional fields then the line element *ds* is $$ ds^{2}=-(1+2\phi)dt^{2}+(1-2\phi)(dx^{2}+dy^{2}+dz^{2}) $$ so the metric is $$ {g_{\alpha\beta}} =\eta_{\alpha\beta}+h_{\alpha\beta}= \left( \begin{array}{cccc} -(1+2\phi) & 0 & 0 & 0\\ 0 & (1-2\phi) & 0 &...

For my physics assignment we have to design and test a way to allow an egg to fall from a three storey building and not crack. The only requirments are the egg must be visible in atleast one place and the smallest design wins. I tried making a crumple area and then protecting the egg with...

Is swimming affected by gravity? How will swimming in micro gravity compare to high gravity and low gravity.

If a ship could cancel out the effect of gravity, wouldn't it be able to pass through the event horizon of a black hole, and take a tour of the singularity, and then just report back with its observations? Also, if a ship had the capability to cancel out the effect of gravity, wouldn't this...

Propose an experiment: Simultaneous test of matter warping space (Gravity) and quantum entanglement "spooky action" at a distance where the spin of the other particle instantly changes to clock or counter clock. Will the result be that both Symmetric Changes are instantaneous? Or will it...

Here's what my prof says: "Define F_{mean} to be the mean force, F_close to be the force on the side of the Earth closer to the moon, and F_far to be the force on the side of the Earth furthest away from the moon. On the closer side the net force is F_close - F_mean > 0 On the further side the...

Some time ago I tried to define classical inverse-square gravity on a 3-dimensional (cubical) torus T3: the quotient space obtained by identifying opposite faces of a unit cube. (Or more rigorously, the quotient space T3 = R3/Z3 of R3 by its subgroup Z3 of integer points.) I assumed there...

Is the theory that inflatons become dominate when gravity is strong (as in right after the big band) and when gravity is weak (as in driving the current expansion of the observable universe)?

Homework Statement A horizontal pole is made of 8 pieces, 1 meter apart each. Forces that do not act at the ends are shown in the picture. The pole does not rotate, and weighs 20N. a. What is the sum of forces applied on the pole? b. Where is it applied? c. What are the forces at the ends...

Homework Statement I searched around and found a manual, so this time I have the complete question in English (I'll use different numbers though, because mine is the 8th edition, and the manual is from the 14th, but it's not something that affects this exercise): 2.98) An alert hiker sees a...

I just read about how professor Erik Verlinde saying that 'gravity is not a fundamental force of nature, but an emergent phenomenon. In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in...

Here is the arxiv link. I'd be interested to hear comments from people more expert than I. The claim is that both dark energy and dark matter can be understood as consequences of gravity as an entropic force.

Does gravity affect waves such as gamma, xray, radio etc. and how does it interact with other wave forms considering gravity is a wave itself. Respectfully, Pat Hagar

Homework Statement Sputnik I was launched into orbit around Earth in 1957. It had a perigee (the closest approach to Earth, measured from Earth's center) of 6.81 × 106 m and an apogee (the furthest point from Earth's center) of 7.53 × 106 m. What was its speed when it was at its perigee? The...

I know that there is constant acceleration due to gravity. A regular force that is applied to a moving object causes the object to accelerate once and as long as the force is present. Meaning, if a ball that is rolling experiences a wind force of 5N the ball's movement will increase 2 mph for...

Pressure can exist in a container of gas even if there is no gravity. From kinetic theory of gases it is assumed that pressure exists due to momentum transfer between molecules and the wall. Suppose I have a closed container on Earth at sea level filled with air. Now I carried it to zero gravity...

Homework Statement A 10kg bucket of water is lifted vertically 3.0m at a constant speed. How much work did gravity do on the bucket during this process? Homework Equations Work=Force*Distance (what thought to use) Wext=Fextdcosθ (what the answer key says to use. The Attempt at a Solution My...

Does a static uniform E field in space appear to be a mix of E and B fields to an observer on Earth (ignoring the Earth's motion)? My GR is not so great, but would the rigorous way to calculate the new fields be to parallel transport the Maxwell 2-form ##F_{\mu\nu}## from free space, where it...

Howdy! So I wrote a novel a while back called Breaking Gravity (info at the bottom) and about two months ago, the book suddenly took off found a pretty big audience. I'm now considering adding a follow up, but I'm looking for ideas. Specifically, the reason I'm here is that I'm hoping you...

Is this line of research a reality? https://www.sciencedaily.com/releases/2016/01/160108083918.htm Produce and detect gravitational fields at will using magnetic fields, control them for studying them, work with them to produce new technologies -- it sounds daring, but Prof. André Füzfa of...

When International Space Station is at rest, it revolves around it's axis once per orbital revolution (92.65 minutes). In other words, is tidally locked. Does the spinning around it's axis do create non-zero artificial gravity? Does it have non zero angular momentum?