Gr Definition and 952 Threads

What is the difference between both theories? From what I've heard, GR replaces SR as the more correct theory, but if that is so, then why do we still refer to SR and GR separately, rather than just refer to GR? (Don't have much of the physics background, so I would appreciate it if people...

Hi, The explanation I have been given for why GR breaks down at the quantum level is that GR requires spacetime to be mathematically smooth, and quantum foam is 'un-smooth'. If this is correct, then is a reworking of GR in a mathematics that does not require 'smoothness' one possible...

Consider the variational principle used to obtain that in the vacuum the Einstein tensor vanish. So we set the lagrangian density as L(g,\partial g)=R and asks for the condition 0 = \delta S =\delta\int{d^4 x \sqrt{-g}L} proceeding with the calculus I finally have to vary R such that...

I don't really know much about GR, but I was just wondering about this theoretical question. I am asking this, because I think Special Relativity is a theory of four dimensions, and wasn't sure how many dimensions GR involved.

In a popular book on quantum gravity "Physics meets Philo on the Planck Scale", it is mentioned there are 4 roads to quantum gravity: "1. quantising General Relativity 2. quantising a different classical theory, while still having general relativity emerge as a low- energy (large-distance)...

My physics background is sparse--I only know classical mechanics and E&M, and I have a self-taught (and therefore foggy) quantitative understanding of special relativity to go with my (I think) strong qualitative understanding. My math background is a bit better but not much: multivariable...

1. A soccer ball is kicked at 25m/s with a 25° angle to the ground. What is a) time it takes the ball to reach max height b) the max height of the ball c) time it takes to land on the ground again d) range the ball travels e) final velocity of the ball (with angle)2. v2= v2 +aΔt Δd= .5(v2 +...

If we assume: E = mc^{2} and for photons: E = hv Then we can derive an effective mass: m = \frac{hv}{c^{2}} And using simple classical gravity obtain: hv - \frac{GMm}{r} = hv - \frac{GMhv}{c^{2}r} = Constant You can derive the constant by evaluating the equation above at...

I know that GR deals exclusively with tensors (at least, in every book I have), but how does the same concepts of affine connection etc extend to non-tensor entities? Example would be a spinor, or even a mix of spinor / tensor. Are there different affine connections? Something different yet...

Can anyone please explain the difference between Newtonian and GR when used to describe gravity especially with reference to gravitational attraction inside an event horizon. Or how do we get over the problem of an object in freefall exceeding the speed of light as it approaches the singularity.

Hello guys, so I have a problem. I am familiar with a fair bit of physics but I cannot seem to cross the gap between classical to relativistic physics. If someone could give me a road map on how to get to GR that would be helpful. For those of you wondering this is where I am at. Mathematics...

Space time Geometry and GR, taken from lecture notes http://www.mth.kcl.ac.uk/courses/cm334.html http://www.mth.kcl.ac.uk/courses/cm334/Lectures0910.pdf from pages 3 and 4 Newton's universal law of gravity is getting modified. it says using the result, from where this result...

In another thread here, https://www.physicsforums.com/showthr...=548148&page=3 , post #36, Peter Donis posted this: "...But we *can* describe a generic spacetime in GR using a curved geometry (*which* curved geometry depends on the specific spacetime), and we *can* describe any given...

HI all There is smth I don't understand in building initial data for BH The equations everybody uses uses spacetime or timelike hypersurfaces but for example horizon is a null surface!

Let's say you have two entangled particles. Is it possible for one of them to fall into the event horizon of a black hole and the particles still be entangled? Or say the two particles are far apart and the universe undergoes extreme expansion (just after the Big Bang, say) such that they are...

For instance you have time dilation in special relativity which is said to be due to something moving faster than something else relative to it. You also have the time dilation in GR where time speeds up the further away you go from a body such as the Sun, Earth, etc. Now could it really...

If you are traveling very very fast in a space ship, are there any visual effects produced by general relativity that you wouldn't see from just special relativity? Assuming we don't get close to large gravity wells like black holes. Does the acceleration of the ship produce any visual...

The relativistic relativistic energy–momentum relationship is: E^2 = \left(pc\right)^2 + \left(m_0c^2\right)^2 The relativistic energy (for a mass) is: E = \gamma m_0c^2 It has been said that the curvature of space is contributed to by photons, not just mass. However: The curvature of...

I emailed a prof about sitting in on his graduate-level gravitational theory class next semester. He said he would like his students to have understanding of electromagnetism, special relativity and quantum mechanics at the advanced undergraduate level. This is the course description...

This is really a continuation from another thread but will start here from scratch. Consider the case of a static thin spherical mass shell - outer radius rb, inner radius ra, and (rb-ra)/ra<< 1, and with gravitational radius rs<< r(shell). According to majority opinion at least, in GR the...

I am interested to know what Newton's second law (F=ma) looks like in general relativity. Looking at the geodesic equation, it appears to have some similarity. Multiply the mass of a particle by both sides and it looks like this...

Neutrinos are thought to be particles with mass. So they aren't even supposed to be traveling at the speed of light to begin with, let alone travel faster. They would have to have some type of imaginary mass in order to do so. So what do you guys think will have to be modified in SR and GR if...

We know that in GR it is not possible for arbitrary spacetimes to define a conserved energy by using a 3-integral. There are some obstacles like the covariant conservation law DT = 0 (D = covariant derivative; T = energy-momentum-tensor) does not allow for the usual dV integration (like dj...

I'll be taking a first course in quantum field theory and general relativity this fall at the masters level and I have a few weeks until I start. I was wondering if anybody had some advice on what to brush up or even self-teach before I'm neck deep in them (on top of other classes) so I can be a...

Basically since we use QM for the very small, and GR for anything bigger than an Atom, can we assume GR has no place in the universe if there is no matter?

What are the causes that led einstein formulate curvature of space-time? what it has to do, if any, with the discrepancy in mercury orbit.

I'm going to attempt to start a beginner GR book since we're covering it in Modern Physics and I've been covering the material up a notch so far (I did the SR by reading Spacetime Physics by Taylor and Wheeler & I've already read Griffith's for the Qmech section etc.) I don't intend to finish an...

This is a very basic question in understanding General Relativity, but the answer still eludes me. The simples way I can state it is: "what exactly represent the 4 basis vector at a certain point on the manifold?" But let me explain myself. Let's take a 4 dimensional Minkowsky space. In this...

The attached PDF shows an analysis sequence of books on the left. If I followed the sequence all the way to differential geometry, would I be prepared to study the subject of general relativity? Also, would it be feasible to skip the real analysis books or would they be required as well...

I may have the option my senior year to take a GR class at the graduate level. I was wondering if taking this class would be sufficient enough for me to say I "know" general relativity? Do you have to be an active researcher in the field to have a firm grasp of GR, or can one simply take a...

MTW has a cute argument on p. 980 that the maximum power radiated from any system is a fixed number, about 10^53 W. I came across this and filed it away as a piece of cool trivia. But it turns out that apparently you can derive the GR field equations from nothing more than the assumption that...

can you construct (or if yes, is it regularly done) a Hamiltonian in curved spacetime? If you took a system and moved it into a strong gravitational field or accelerated it to relativistic speeds can you still do Hamiltonian mechanics...

I was reading something about the Cartan (vierbein) formalism in GR, in which the connection is allowed to have torsion, and it got me interested in the Einstein-Cartan theory. Apparently both GR and Cartan theory with torsion should give the same experimental results in vacuum, what I would...

I've just read the FAQ about this and IMO it is "not even wrong" to say that energy conservation doesn't apply to Cosmology. The fact is energy conservation is clearly stated in the EFE by the vanishing divergence of the stress-energy tensor and since GR is the theory we currently use in...

forces (Ricci tensor etc.) which prevent universe from collapsing and the tensorial forces of surface tensions which prevent a soap bubble from collapsing? Are these two phenomena absolutely incomparable? Could someone explain in simple words why? I'm new, and a student in quest for...

http://www.esa.int/esaCP/SEM5B34TBPG_index_0.html "ESA’s Integral gamma-ray observatory has provided results that will dramatically affect the search for physics beyond Einstein. It has shown that any underlying quantum ‘graininess’ of space must be at much smaller scales than previously...

From an online book where the author tries to discredit GR, there's a part where he talks about forgetting about the rubber sheet analogy and just remembering: "A body moves along the path that makes time dilation a minimum." Then he presents a diagram and a little argument that is really...

It has come up a few times in recent threads here that the energy conditions on the stress-energy tensor (weak, null, dominant, etc) traditionally used to prove global results (e.g. the singularity theorem, the positive energy theorem, geodesic motion theorems*) are problematic: they allow more...

This is a diversion from the "Interesting energy problem" thread. That thread lost its direction a bit lost due to implications of Perpetual motion machines or over unity systems, but in this thread it is my intention to avoid those complications so that maybe we can have an sensible discussion...

GR "slightly" Wrong? http://www.wired.com/wiredscience/2011/06/clumpy-universe/ The universe appears to be clumpier than astronomers expected, according to the largest galaxy survey to date. The extra clumps could call for a redesign of the standard model of cosmology, and maybe a new...

Here's an idea I came up with recently, that I wanted to share: No true black holes exist. Every "apparent" event horizon is really a separation of two universes, where the outside universe is entangled geometrically with the inside universe. The Hubble volume is sitting inside of an...

The FAQ on rotation of the universe contains the following remark: A question that emerges from this is whether or not GR is actually contradictory to Mach's principle, or if it simply isn't built to accommodate it without additional modification. This is a very technical issue, as GR...

I watched The Elegant Universe, and it was somewhat disappointing. At the current moment, I only know some of the math of SR, no math of GR, and no math of QM. The special kept giving vague overviews of why GR and QM don't mix. I do know some of the concepts for GR and QM, and I do not see why...

we know that on Earth, an object falling with a rate of g horizontally, and the formula for it is a=g*cos x. So according to GR, an object falls into the warp of space time that a huge object causes. So should there be a new "g" applied to all the warp in space time? Am i right or not?

A friend of mine had the following funny question: Imagine I have a metric ansatz with two unknown functions. The Einstein equations give both real and complex solutions for the unknown functions. Question: Is there a decent interpretation of these complex solutions in GR? We know about...

In this thread I would like to discuss aspects of this separate from some recent related threads. In particular, I prefer that proposing that mathematical results of differential geometry and well known results in GR are wrong please not occur. I have a disussion question at the end of this...

A local velocity-dependence of local curvature of space would violate the notion that space is given a curvature only by surrounding matter. Any arbitrary test particle or object moving at some velocity isn't going to experience a different gravitational "force" under the same curvature...

Mostly I have little interest in this topic, but the discussion came up in a recent thread about what is the axiomatic basis of GR? I threw out what I would consider a 'physicist view': the action or field equation plus rules for relating mathematical object to natural objects are all you need...

I am orbiting a neutron star on an edge of stability. If I drop a rock on it, it collapses and I get a Black Hole. If I don’t do it, I can fly away, leaving it as is. It is my choice, and both options are compatible with GR, both are valid GR solutions. I wonder, where exactly...

Ok I feel like there is a really simple answer to this but I've been trying to get this to work for days and I just can't. Here is the basic problem: The "free-field" lagrangian for weak perturbations in the metric on a flat space is: L = \frac{1}{4}(\partial^\sigma h_{\mu\nu}\partial_\sigma...