Having looked at this question online, multiple sources overwhelmingly support the statement that 'Gravity results from the curvature of space-time due to mass-energy'. However, discussions on the H-Bar (See ) have led to me doubting this.   So, is the above statement correct, or not? If not, then can it be corrected, or should it be completely disregarded? I cannot see how the curvature of space-time and gravity cannot be related, as gravity and curvature are both linked to the mass of an object. For example, a black hole results in massive curvature of space-time, and the gravitational forces are also massive.
6794
They say that gravity is technically not a real force and that it's caused by objects traveling a straight path through curved space, and that space becomes curved by mass, giving the illusion of a force of gravity.  That makes perfect sense for planetary orbits, but a lot less sense for the expression of gravity that we are the most familiar with in our day-to-day lives: "what goes up must come down."  Imagine that I hold a ball in my hand, several feet off the ground, with my fingers curled around it, and my hand is above the ball.  Then I open my fingers, releasing my grip on it, being very careful to not impart any momentum to the ball from my hand as I do so.  An object at rest remains at rest unless acted upon by an outside force.  If the ball is not moving (relative to my inertial reference frame), it has no path to travel that's any different from the Earth's path through space.  It should remain at rest, hanging there in the air.  And yet it falls, demonstrating that an outside force (gravity) did indeed act upon it.  How does curved space explain this?
90598
Greetings StackExchange!  We were having a conversation with a peer about stupid ways of interpreting theories. We would go to and fro with an interpretations, but we would always find a way to disprove them. There was one, however, that we couldn't refute:  Gravity on a plane in a bidimensional space can be interpreted as the acceleration of spacetime towards an object.   We tried to invalidate in saying that the inertial mass and gravitational mass would be different, thus violating GR, but after some thought this couldn't be. The gravitational mass would accelerate spacetime towards it. If that object we were to be pushed, the pulling of spacetime would make it difficult for it to move.   We know for a fact this can't be true, and we would greatly appreciate any refutation of this thought. I can clarify if needed, or do a sketch. I apologise in advance as I'm not a native speaker.
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First, I'll state some background that lead me to the question.  I was thinking about quantization of space-time on and off for a long time but I never really looked into it any deeper (mainly because I am not yet quite fluent in string theory). But the recent discussion about  got me thinking about the topic again. Combined with the fact that it has been more or less decided that , I decided I should give it a go.    So, first some of my (certainly very naive) thoughts.  It's no problem to quantize gravitational waves on a curved background. They don't differ much from any other particles we know. But what if we want the background itself to change in response to movement of the matter and quantize these processes? Then I would imagine that space-time itself is built from tiny particles (call them space-timeons) that interact by means of exchanging gravitons. I drawed this conclusion from an analogy with how solid matter is built from atoms in a lattice and interact by means of exchanging phonons.  Now, I am aware that the above picture is completely naive but I think it must in some sense also be correct. That is, if there exists any reasonable notion of quantum gravity, it has to look something like that (at least on the level of QFT).  So, having come this far I decided I should not stop. So let's move one step further and assume string theory is correct description of the nature. Then all the particles above are actually strings. So I decided that space-time must probably arise as condensation of huge number of strings. Now does this make any sense at all? To make it more precise (and also to ask something in case it doesn't make a sense), I have two questions:        In what way does the classical space-time arise as a limit in the string theory? Try to give a clear, conceptual description of this process. I don't mind some equations, but I want mainly ideas.   Is there a good introduction into this subject? If so, what is it? If not, where else can I learn about this stuff?      Regarding 2., I would prefer something not too advanced. But to give you an idea of what kind of literature might be appropriate for my level: I already now something about classical strings, how to quantize them (in different quantization schemes) and some bits about the role of CFT on the worldsheets. Also I have a qualitative overview of different types of string theories and also a little quantitative knowledge about moduli spaces of Calabi-Yau manifolds and their properties.
60549
Is gravitational time dilation caused by gravity, or is it an effect of the inertial force caused by gravity?    Is gravitational time dilation fundamentally different from time dilation due to acceleration, are they the same but examples of different configurations?    Could you recreate the same kind of time dilation without gravity using centrifugal force?
60376
Firstly I think shades of this question have appeared elsewhere (like , or ).  Hopefully mine is a slightly different take on it.  If I'm just being thick please correct me.  We always hear about the force of gravity being the odd-one-out of the four forces.  And this argument, whenever it's presented in popular science at least, always hinges on the relative strength of the forces.  Or for a more in depth picture .  But, having had a single, brief semester studying general relativity, I'm struggling to see how it is viewed as a force at all.  A force, as I understand it, involves the interaction of matter particles with each other via a field.  An energy quantisation of the field is the force carrying particle of the field.  In the case of gravity though, particles don't interact with one another in this way.  General relativity describes how space-time is distorted by energy.  So what looked to everyone before Einstein like two orbiting celestial bodies, bound by some long distance force was actually two lumps of energy distorting space-time enough to make their paths through 3D space elliptical.   Yet theorists are still very concerned with "uniting the 4 forces".  Even though that pesky 4th force has been well described by distortions in space time.  Is there a reason for this that is understandable to a recent physics graduate like myself?     My main points of confusion:   Why is gravity still viewed as a force? Is the interaction of particles with space time the force-like interaction? Is space-time the force field? If particles not experiencing EM/weak/strong forces merely follow straight lines in higher-dimensional space (what I understand geodesics to be) then how can there be a 4th force acting on them?     Thanks to anyone who can help shed some light on this for me!
63416
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 that gravity can be described by a QFT?  But I find this weird, because gravity seems so incredibly different from the other forces (yes, I know "weirdness" isn't any sort of scientific deduction principle).  For relative strengths:   Strong force: $10^{38}$ Electromagnetic force: $10^{36}$ Weak force: $10^{25}$ Gravity: $1$   Not only does gravity have a vastly weaker magnitude, it also has a very strange interaction with everything else.  Consider the Standard Model interactions:    No particle (or field) interacts directly with all other fields.  Heck, gluons only barely interact with the rest of them.  So why is it then that anything that has energy (e.g. everything that exists) also has a gravitational interaction?  Gravity seems unique in that all particles interact through it.  Then there's the whole issue of affecting spacetime.  As far as I'm aware, properties such as charge, spin, color, etc. don't affect spacetime (only the energy related to these properties).
11921
So I understand the concept of gravity, in that it's not actually a force, but more of a displacement in the spacetime grid. An object with a big enough mass will bend the spacetime, causing smaller objects to "attract" to it (not really, but good enough for what I am trying to figure out)   Ok so that makes sense, but what I am not understanding is, how come planets don't just get engulfed by the Sun? There must be something acting against the gravity for the planet to be a certain distance away from the sun. Am I not understanding this concept correctly?
18173
Does such a set of equations exist or does our universe have an intrinsic curvature that can't be explained by an embedding in a flat Minkowski space of 1 higher dimension? Even if general relativity can be explained by such equations, it still doesn't prove our universe is embedded in a flat Minkowski space of higher dimension.
1399
General relativity tells us that the universe is bent by gravity, but this curvature is intrinsic to the universe (the universe bends, but not in a fourth spatial dimension, the universe having only three bent spatial dimensions).  How could we know the difference between the intrinsic curvature of a 3D spatial universe, and a the extrinsic curvature of a 3D shape in a 4D universe?    Obviously, there is a similar question about our curved 4D spacetime inside a 5D spacetime.
70220
Is Einsteins Equivalence theorem in General Relativity correct? It seems to me that it neglects the fact that gravitational acceleration depends upon separation distance squared, thus neglecting the effect of tidal forces.   For example, as I sit on earth, I experience the affect of earth's gravity; Although the acceleration on my head is slightly less than the acceleration acting on my feet. If I make the claim that my frame is equivalent to me being in a space ship traveling at g, doesn't that mean my whole body is accelerating at g uniformly? This is contrary to the previous statement though.
40590
We know through  (GR) that matter curves spacetime (ST) like a "ball curves a trampoline" but then how energy curves spacetime? Is it just like matter curvature of ST?
80295
I'm a total physics n00b (i.e. I only know the physics as taught in IT grades, and don't remember much of it), and was talking about entropy (initially, not with the physical implications). My friend talked about entropy and gravity and was not sure about the fact that entropy decreases with gravitationally bound systems.  , but "apparently proving" that gravity decreases it. Finally, in the final link where he will show the prove that gravitational systems do not decrease entropy, it shows this text:     So, you're wondering why gravity doesn't violate the 2nd law of thermodynamics, even though the entropy of a gas cloud decreases as it shrinks under its own gravitational pull? The answer is simple, but I'll just give you a hint. We've already seen that as it shrinks, it loses energy. The energy has to go somewhere. Where does it go? If you figure that out, you'll see that the total entropy is not actually decreasing - it's just leaving the gas cloud and going somehere else!   My question is: No, actually I cannot figure if out. Can someone help me to figure it out? (yes, perhaps it's a very simple answer but no, still don't get it).
18693
In General Relativity Theory, mass can warp spacetime. However, in my view interaction only occurs between pieces of matter. Spacetime is not matter; how can it be affected by matter?
32462
I'm trying to understand the conservation of momentum in general relativity.  Due to the curvature of space-time by matters and energy, the path of a linear motion appears to be distorted.   Therefore from the frame of reference of an observer who is at a not-so-curved region of space-time, it appears that the velocity vector of an object that is moving along a straight line in a very-curved space-time is changing?   Does that imply that from the observer's frame of reference (in a not-so-curved space-time) momentum of that object (in the very-curved space-time) is not conserved?   In this sense is momentum not always conversed in general relativity or did I misinterpret something?
2577
With only a limited knowledge of , I usually explain space-time curvature (to myself and others) thus:  "If you throw a ball, it will move along a parabola. Initially its vertical speed will be high, then it will slow down, and then speed up again as it approaches the ground.  "In reality, the ball in moving in a straight line at constant velocity, but the space-time curvature created by the Earth's gravitation makes it appear as if the ball is moving in a curved line at varying velocity. Thus the curvature of space-time is very much visible."  Is this an accurate description, or is it complete     nonsense?
115608
In most popular explanations of General Relativity, both in print and film/television, gravity is demonstrated using an example of a 2 dimensional plane being flat, then when putting a heavy object in it it curves the plane into a funnel shape and you can visualize how other objects roll down the funnel into the gravity well simulating something like the orbits of the planets around the sun as an example.  However this demonstration relies on the notion of "down", or even the actual gravitational attraction on Earth, so that the objects in orbit roll or fall down the well.  This kind of explanation seems to rely on a force in order to explain that the force of gravity is just the curving of spacetime. At the very least I find this to not really explain how the theory of General Relativity actually works.
86645
OK...before everyone blasts this with references to the relativistic invariance of the physical laws, time dilation, etc let me add some context. Also, I am an amateur with an interest in physics, so I don't know the details of the physics. However, I have read enough about the cosmological principle and cosmic time to develop this "confusion" .  Based on evidence gathered to-date, it appears that the universe is both , as evidenced by observations and the . This principle, when combined with the FLRW metric, allows space-time to be divided into non-intersecting slices and therefore establish a "cosmic time."   Now, the existance of such a time in no way suggests that it is the "right" time or a true "now", hence violating relativity -- although some presentist philosophers (e.g., W.L. Craig) have tried to make this argument, but it is not generally accepted in philosophical circles.  However, this leaves me confused about an apparent disconnect between relativity and the cosmological principle:  Isotropy only holds if we are at rest relative to the cosmic microwave background radiation (universal rest frame), o/w anisotropy is present - yet each reference frame is supposed to allow equally valid observations. How can these two be reconciled when most reference frames would lead us to conclude that the universe is NOT isotropic?  It seems that only by appealing to the idea of being at rest relative to "universal rest frame" can we explain away any discrepancies from isotropy as due to "peculiar motion". However, doesn't this give this "universal rest frame" and its associated time an empirically privileged status, even though physical laws work just fine in every reference frame?   For example (pardon any abuse of astronomy): if look off in some part of space and see only quasars, and then in another part of space and see only brown dwarfs, but I measure both as apparently the same distance from me, then can I conclude that we are in motion, since otherwise we would have an empirical contradiction (kind of like finding dinosaur and human remains in the same strata)?  Any help on where I am going wrong would be helpful. Intuitively, I don't think there should be a way to establish absolute ordering between space-like separated events, but the above reasoning suggests otherwise.