# Tag Info

## Hot answers tagged general-relativity

63

No, gravitational waves cannot pass through a black hole. A gravitational wave follows a path through spacetime called a null geodesic. This is the same path that would be followed by a light ray travelling in the same direction, and gravitational waves are affected by black holes in the same way that light rays are. So for example gravitational waves can ...

50

Another way to answer this question is to apply the Equivalence Principle, which Einstein called his "happiest thought" (so you know it has to be good). The equivalence principle says that if you are in an enclosed box in the presence of what Newton would call a gravitational field, then everything that happens in that box must be the same as if the box was ...

48

No you can't and the behaviour of bodies with mass and of light is completely different near a compact, massive object if you use Newtonian physics rather than General Relativity. In no particular order; features that GR predicts (and which in some cases have now been observationally confirmed) but which Newtonian physics cannot: An event horizon. In ...

41

Well, yes, but we must be careful with the meaning of "predict". The Schwarzschild solution, developed by Karl Schwarzschild in 1916 [1], is the first closed-form, explicit solution of Einstein's field equations for gravitation. It describes a spherically symmetric, static, vacuum spacetime. The solution goes singular at a specific radius (the ...

31

There are a couple of ways one could approach your question: Black holes are regions of space that have been deformed by a sufficiently concentrated mass. Light waves/particles always travel in a straight line at a constant velocity ($c$). Although a photon approaching a black hole will continue traveling in a straight line through space, space itself has ...

29

Going from Newton's theory to Einstein's theory is not simple. It's not like you can just add a term to Newton's gravity, like $\textbf{F}=-{GmM \over r^3}\textbf{r} + \textbf{f}(\textbf{r})$ and obtain the right formula. To say it with Feynman's words, you can't make imperfections on a perfect thing, you have to make another perfect thing. And the new ...

27

No more than the observation of light waves disproves quantum mechanics. Light has properties of both a particle and a wave. At low energies, the particle nature of light is hard to detect: radio waves are made of photons, but individual radio wave photons are pretty hard to detect. I'm not sure that we have directly detected individual photons with ...

26

Since I like math, let's throw some math into this. I'll try to keep it as simple as possible though. Kerr Black Holes A rotating black hole is known as a Kerr Black Hole (named after Roy Kerr who found the numerical solution to GR equations for rotating black holes). In the case of a rotating black hole, there are two important parameters used to describe ...

22

Yes, observations of this kind are within the technical scope of amateur astronomers. Several groups succeeded in replicating the experiment during the 2017 eclipse that crossed the USA. For example Donald Bruns measured deflections of 2.8 arcseconds of multiple stars. Nasa published a "How To" page for anyone wanting to test GR themselves.

21

Birkhoff's theorem is very useful: in general relativity, if you are in vacuum and there is a spherically symmetric gravitational field, then it will be the Schwarzschild solution. This solution only depends on the mass, not on the size of the object. So the neutron star and the black hole will give rise to exactly the same orbits.

20

The impact of this measurement on the status of quantum gravitation is exactly zero. The proper statement of the incompatibility of general relativity and quantum mechanics is that the quantum field theory of general relativity is not renormalizable. Renormalizability essentially means that the theory is well-defined at all energy scales, which seems like a ...

20

If they were spinning they would be distinguishable (in principle), otherwise not. Astrophysical black holes and neutron stars are expected to spin. In the case of a neutron star that automatically means that the mass/energy distribution is not spherically symmetric and therefore that the detail of the potential outside the surface depends on the detail of ...

18

The answer is yes time dilation does affect how much time an observer experiences since the big bang until the present (cosmological) time. However there is a certain set of special observers called comoving observers, these are the observers to which the Universe appears isotropic to. For example we can tell the Earth is moving at about 350 km/s relative ...

18

Another question, how can we identify the ripple's origin (let's say that if it's the result from the big bang or another big event)? (I'm just answering this part of the question, as James has already answered the main part about GR vs QM.) LIGO have produced an image which shows their best estimate of where these two black holes were: All they can say ...

16

This is the Newtonian model of gravity. It is a very good model, it is used for accurate calculating the motion of objects in the solar system to a very high degree of accuracy. However, for very strong gravitational fields you need to use Einstein's model, which accounts for things like the constant speed of light for all observers. I'm not going to go into ...

15

You are labouring under the misapprehension that how far we can see directly gives the age of the universe. Whilst it is true that the oldest light we can see was emitted some 13.7 billion years ago, the stuff that emitted that light is now roughly 46 billion light years away, thanks to expansion of the universe. The universe itself probably extends ...

14

Gravity doesn't affect the speed of light. It affects the space-time geometry and hence the paths of light. However, this can have a similar effect. Light emitted at source $S$ to pass a massive object $M$ that is very close on the otherwise (if M weren't there) straight path to an observer $O$ has to "go around" $M$, which takes longer than following the ...

14

The scenario you describe may occur. On the other hand it may actually be that neutronisation in a white dwarf is the trigger for a thermonuclear type Ia supernova. You may be misunderstanding the Pauli Exclusion Principle (PEP).The PEP states that no two fermions can occupy the same quantum state, not that they cannot occupy the same space or be compressed ...

14

I am not an expert in physics and the explanation of the others is excellent. However, I noticed a flaw in your reasoning which they did not address. You have written: Considering the Newton's Law of Gravity equation $F = GM/r^2$, if the radius of an object becomes super small, then it can technically have immense gravity. Hence I deduce that you read the $... 13 As Walter says, gravity doesn't bend light. Light travels along null geodesics, a particular type of straight path. Since (affine) geodesics don't change direction by definition, geometrically light trajectories are straight. Moreover, the speed of light in vacuum is$c$in every inertial frame, regardless of whether or not spacetime is curved, although a ... 13 There is evidence that both black holes and their event horizons exist. The primary evidence for stellar-mass black holes arises from observations of the dynamics of binary systems. What has been found, for at least 20 binary systems, is that the optically visible star has a dark companion, that is usually more massive, and more massive than can possibly be ... 13 There is no "up" direction within the event horizon. Most people get fixated on the speed of light, or energy or whatever. They're like, if light was faster, could it escape the black hole? If my rocket had bigger engines, could I escape? The problem is, all these questions make no sense. You can't get out because there is no way out. A black hole is ... 13 The trajectory of a ballistic body, whether in Newtonian or Relativistic physics depends on the initial energy and angular momentum. The difference is that in Newtonian physics, if the mass is compact enough, the infalling object (if given an initial kinetic energy) will never hit the central object, unless it is a direct hit, and will scatter off to ... 12 Cosmological parameters are measured in a variety of ways, and their values will depend on which measurements you trust the most. The paper you link to (Planck Collaboration et al. 2016) with the 2015 results from the Planck observations of the cosmic microwave background is probably the one that most people will accept, but even in that paper you will find ... 12 We can currently only detect gravitational radiation when it is extremely intense: in the last fraction of a second. For example the first gravitational wave detection lasted less 0.15 seconds. The black holes are releasing gravitational radiation with every orbit, but that radiation is too weak for us to detect. It takes a colossal amount of energy being ... 11 This has been considered long ago (Here's a paper talking about this). Wormholes are not forbidden by physics, but the creation of wormholes is iffy ground. THere are two possible paths one can take to create a wormhole: Choose a pre-existing wormhole in the quantum foam and "expand" it by feeding it exotic matter. "Tear and sew up" space — we're ... 11 The waves pass by at the speed of light. So you you would'nt see ripples, they would pass too fast, and remember the waves would be passing through you too. The wavelength was (relativly) long about 3000km. The wave doesn't pass you, you are inside the wave. The amplitude of the waves detected by LIGO was small, one part in$10^{21}\$, Now while the intensity ...

11

Technically, there would be minute differences due to the gravitational field generated by the mass of the spacecraft tidally deforming the neutron star leading a small response in the gravitational field of the neutron star, which in turn effects the orbit of the spacecraft. This tidal response is governed by the so called tidal Love numbers of the Neutron ...

10

It is correct that the Kerr black hole solution of GTR allows travel between universes. However, that does not mean that if you actually jump into any kind of black hole that you could go to another universe. To motivate the resolution to this conundrum, let's start off very easy: suppose you stand on the ground with a ball in your hand, and you throw it ...

10

The rubber sheet only is not meant to be a qualitative model, it gives one concept and one concept only: Mass causes curvature of spacetime. You can't get any more than that from the rubber sheet. If you have that idea in your head already then you are ready to drop the image because: The sheet is 2d but spacetime is 4d The 2d sheet is embedded in 3d ...

Only top voted, non community-wiki answers of a minimum length are eligible