New answers tagged

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Most orthodox theory is that black hole has singularity at it's center. A gravitational singularity or spacetime singularity is a location where the quantities that are used to measure the gravitational field of a celestial body become infinite in a way that does not depend on the coordinate system. These quantities are the scalar invariant ...


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So, just to clarify your approach: You take events A and C occurring at the same location $x'_a = x'_c$ but different times $t'_a$ and $t'_c$ in the primed frame. You also take event B occurring at the same time as A in the primed frame, $t'_b = t'_a$, but at the same location as C in the unprimed frame, $x_b = x_c$. For events B and C you then apply the ...


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He meant that You may think an orange is present in space when it actually doesn't exist. You imagine coordinates of space which don't exist. Can you see a dream? Similarly, here he meant space and time don't exist if you don't imagine them. But once you think about it, it exist. Have you seen space and time? You just imagine it out of nothingness and ...


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In physics, frames moving at the speed of light are not valid. What "list verse" means, more accurately, is that as you approach the speed of light, your time, as seen by a "stationary" observer, ticks slower. This is a well-documented effect that needs to be accounted for in all manner of applications, ranging from particle accelerators to GPS satellites ...


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I'll try to boil down several of your questions and answer what I think is most fundamental, and hopefully clarify things in the process: Gravity is completely synonymous with the shape of spacetime across all 4 dimensions (3 space, 1 of time). The reason we speak of spacetime is thus: When you (having negligent mass) stand in a "gravity field" such as ...


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Good question, Rovelli in his book Quantum Gravity writes: 1.1.3 GR is the discovery that the gravitational field and spacetime are the same entity. What we call 'spacetime' is itself a physical object, in many respects the same as the Electromagnetic Field. Hence gravitational waves, as the EM field has waves; he adds: We can say that GR is the ...


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It does not matter whether you are moving relative to anything else that you can observe or just in a fixed reference frame in a completely empty space. Special relativity treats both scenarios as the same because you experience no acceleration/gravity. No matter what state of motion you are in your time will pass with the same speed as always so in a ...


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Both vertical lines in this diagram, i.e. one "after" the evaporation as well as the vertical line in the left lower corner (and all vertical lines without "teeth" in all Penrose diagrams in the world) describe the vicinity of the point $r=0$ in polar coordinates. One can't "cross" these lines because there are no points with the radial coordinate $r\lt 0$. ...


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While the question about the twin paradox in a closed universe is a duplicate, I think it's worth a few comments to clarify what is meant by a closed universe. There are two different meanings for the word closed when used in connection with the universe. Observations suggests the universe is at least approximately described by the FLRW metric, and this ...


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We do not have an absolute rate for time flow. So, slow or fast depend in what reference frame you are fixing to define what is fast and what is slow. What make sense is asking if we can make the rate of time flow slowly as possible when compared with the proper time for example. And the answer to this question is yes. Any referential frame with constant ...


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In short: The particle horizon is the extent of our current past light cone, and the event horizon is the extent of our future light cone at $t=\infty$. It is important to be clear that these horizons are horizons only to an observer at this place in Space and Time. They do not mark physical boundaries between different regions in Space; rather, they ...


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The answer to your first question can be answered regardless of your misunderstanding of Big Bang. The time passing before atoms begin to form is 379,000 years, which is the time it took for the temperature to drop sufficiently for atoms not to be constantly ionized. – pela, in a comment.


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the cutting by universes is a way : to introduce possible new physics for each of these universes without leaving the homogeneity and isotropy cosmological principles, the known constants and the known physics of "our" universe to defer the infinity issue from our universe to a parent structure : the multiverse Homogeneity and isotropy are the main ...


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If it's inside the event horizon, can this light escape it? no , it cannot escape. Anything falling cannot see the ground, nor send message back, meaning in the sphere surface direction. can you escape it by the speed of light itself? no, it not sufficient the light is red-shifted means that an object near the BH may emit or reflect this light ...


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The time taken to travel to the planet, as seen by the bystander at point B, is the distance from A to B, is simply the distance divided by your speed. The time experienced by you doing the travelling is the time seen by B divided by your relativistic factor. It's just like moving from A to B in non-relativistic physics. So the faster you go, the shorter the ...


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Very often we don't. We just stick to $k=0$, because for most practical purposes, this is fine. But all measurements contain uncertainties. The latest Planck results (Parade et al. 2015) combined with observations of the baryonic acoustic oscillations yield$^\dagger$ $\Omega_k = 0.000\pm0.005$. We don't know whether the curvature on much larger scales than ...


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The space between galaxies isn't that much more empty than what we have here. There is some curvature, enough so that it causes gravitational lensing, even enough to make us suspect there's some extra matter (dark matter) we can't detect by other means. What you're asking is directly measurable: it's the gravitational blueshift from whatever light sources ...


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There is nothing in physics that describes the sort of folding shown in your picture. I'm afraid it is an invention of the Science-Fiction community. The best tool we currently have for describing spacetime is general relativity, but GR does not and cannot tell us anything about the global topological properties of spacetime. The sort of wormhole you show ...


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The proportionality between angular momentum and mass of the Kerr black hole can be shown directly by performing the Komar integral for angular momentum. In fact, you find that $J=Ma$. The parameter $a$ in the Kerr metric is thus the angular momentum per unit mass. I am not familiar with the relation between $M$ and $R_S$


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Without being able to manipulate gravity. We are manipulating gravity all the time, except on earth, labs and constructions do not allow timing gravitational effects, which is why newtonian gravitational theory which has instantaneous effects is so successful. How do we know that gravity is restricted to the speed of light? or gravitational ...


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Such a spacetime is called an ultrahyperbolic spacetime, so called because it produces ultrahyperbolic equations (equations with more than one negative eigenvalue). Those spacetimes are not overly nice to work with. They pretty trivially include closed timelike curves, since a closed curve in any plane of two time directions will be timelike. They permit ...


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Mass balances some momentum and some energy into a location via $E^2-(|\vec p|c)^2=(mc^2)^2.$ So basically, every time you created some mass $m$ you had to put some energy $E$ into that location and some momentum $\vec p$ into that location and then it proceeds to take off with velocity $\vec v=\vec p c^2/E.$ So its the energy and the momentum that affect ...


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The derivative with respect to proper time is the derivative with respect to time of the instantaneously comoving inertial frame. This does not mean the particle is at rest. That's why I had to have the word instantaneously in there. As for four velocity, that's the unit tangent vector to the world line. It tells you what direction in spacetime something ...


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In short, the answer to your question is no. Time dilation can't ever run backwards. Consider a simpler case: Let there be a person A on a massive planet, and a person B floating in space far away. If both had huge clocks with them, then B would see A's clock running slower than his own. And although it seems contradictory, A will also see B's clock ...


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The main idea is that it follows the path of least resistance. The light "wants" to travel the shortest path, which is (almost) always a straight line. However, since spacetime is curved, this straight line happens to be curved as well! Shown in the image are two hypothetical light paths, one that follows the curvature of spacetime and one that doesn't. ...


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Space-time is not simply a manifold, but a differentiable and pseudo-Riemannian manifold. The topology of the manifold is already fixed by the (topological) manifold structure (that is: local homeomorphism to $\mathbb R^n$ without assuming any inner product). In this sense sufficiently small open sets from the manifold carry the topology of a subset of the ...


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As the person B takes off in a rocket, both of them would see the other clock move at a slower rate, assuming the rocket to be moving at a constant speed, both are in an inertial frames of reference, but when B wants to return to A, B should make a turn somewhere, so a turn, is an acceleration, and accelerating frame of reference is non-inertial, and this ...


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@CuriousOne posted this answer in the comments: The premise of relativity is that the speed of light is the same for all observers. This has consequences, but it doesn't change time. All clocks still behave exactly the same for all observers traveling with their own clocks. It is only between observers that clocks are running at different rate. This ...


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Is Bekenstein entropy limit inconsistent with universal continuity? Yes, but that doesn't mean the Bekenstein bound is correct and everything is fine. Entropy can be considered as "sameness", related to available energy. And the expression $S \leq \frac{2 \pi k R E}{\hbar c}$ has a c in it, but the coordinate speed of light at the event horizon is zero. ...



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