Hot answers tagged

23

Yes it's falsifiable and indeed has been falsified. The geometry of spacetime is described by an equation called the metric that we get by solving Einstein's equation. We can get some information about the metric that describes the universe by studying the motion of objects like galaxies, galaxy clusters etc. When we do this we find the observations are ...


11

Two things about this. First of all, it is good to remember that LIGO is a bit different from experiments like the LHC, where reproducibility just means doing the experiment over and getting the same result. LIGO is a gravitational wave observatory (that's what the "O" stands for), and hence they can't really reproduce the experiment since the event they ...


8

There are a few big differences: Tidal effects change the rate at which the orbit decays The neutron stars touch before the black holes would have merged Ejected matter can contribute to the gravitational-wave signal The merged neutron star can have "mountains" that keep radiating Probably the most important effect: the matter in NSs can emit photons and ...


7

There's a few things going on here. Let's get this out of the way: yes, declaring we've discovered gravitational waves based on just LIGO's observation is overselling it. Respectable scientists know this. What LIGO found is the first direct evidence for gravity waves. Many more observations will be required before it can be said gravitational waves are ...


6

@JohnDuffield: I can give you both a correct answer in simple terms and the fairy tale, together with references to an explanation how the fairy tale is related to the real thing! The dry facts are that two real particles (e.g., an electron and a positron) are created from the energy in the very strong gravitational field near the horizon of the black hole ...


6

To answer this we need to talk a bit about how particles are described in quantum field theory. For every type of particle there is an associated quantum field. So for the electron there is an electron field, for the photon there is a photon field, and so on. These quantum fields occupy all of spacetime i.e. they exist everywhere in space and everywhere in ...


5

Is there another way to conclude the Schwarzschild solution has a mass M It's not so much a conclusion as a definition. From Schutz in "A first course in general relativity", section 8.4 "Newtonian gravitational fields", pages 207 - 208: Any small body, for example a planet, that falls freely in the relativistic source's gravitational field ...


5

Let's first have a look at the concept of reproducibility. This is a concept for application to experiments. The idea is that if we make an experiment and give a careful description of it, we should in principle be able to redo the experiments and obtain the same outcome. A very good example of an ideal experiment is the LHC. It's also one of the biggest ...


4

The material part of a black hole is (classically) compressed into a zero volume area, and almost all of the information of the matter that eventually became the black hole was dissipated away, so the original notion of your question is unanswerable. There IS another sense in which we can think of your question, though. Black holes are known to shine light ...


4

Conserved quantities in GR In GR, energy (or mass) is typically an ill-defined concept. In flat spacetime, we define energy as the conserved quantity corresponding to time translational symmetry. Extending this to GR is quite tricky mainly because, what one is calling time is already observer dependent (this is of course also true in flat spacetime, but at ...


4

For a given mass the gravitational attraction remains the same -- but only if you are far away. For example, the surface gravity of Sol, our sun, is $274$ $ m/s^2$, about 28 times the surface gravity of Terra, which is $9.8$ $ m/s^2$. But as the material is compacted, the surface gravity increases: this is because the effective mass can be treated as ...


3

As the comments have suggested, the problem is that your description of virtual particles appearing to vacuum fluctuations is wrong. Have a look at my answer to Black holes and positive/negative-energy particles for more on this. There isn't an explanation of what is really going on that is accessible to the non-quantum field theory nerd (though I have ...


3

Spacetime isn't an object, like some sort of elastic jelly, that galaxies move through churning it up as they go. Spacetime is a mathematical object that we use for calculating observables in relativity. So there isn't any sense in which a singularity twists up spacetime. The treatment of singularities is quite subtle in relativity. We describe spacetime as ...


3

Yes, an observer inside the (Schwarzschild) black hole region may receive signals from the external region if the BH is sufficiently large. The motion of an internal observer is arbitrary, it is pictured by a generic timelike curve. The only constraint is that, within a finite interval of proper time, the curve reaches the internal singularity (roughly ...


3

According to the cosmic censorship hypothesis all singularities are assumed to be behind horizons. This is not the same as saying "all horizons contain a singularity". The black hole solutions to Einstein's equations (which contain singularities) are all stationary, vacuum spacetimes. Stationary means that the spacetime does not explicitly depend on time. ...


3

Let us for simplicity work in units where the speed of light $c=1$ is equal to one, and assume that there is no cosmological constant $\Lambda=0$. A spherically symmetric vacuum solution to the EFE of the form $$\tag{1} ds^2~=~g_{tt}(r)dt^2 + g_{rr}(r)dr^2 +r^2 d\Omega^2,$$ such that it asymtotically becomes Minkowski space $$\tag{2} ...


2

A theory that describe the black hole interior is intended to give some predictions that is testable even outside the horizon. For instance the internal "dynamics" could be relevant to tell what is the final state of the hole after the evaporation and what are, if there are, the correlation in the Hawking radiation. If the putative theory gives the right ...


2

The field of quantum black holes is an hot topic of research right now, and the firewall proposal is still being debated. I have the feeling that no one really take the proposal seriously. By saying this I don't mean that it was a bad paper, on the contrary it's a nice thought experiment that forced us to think even more about the black hole information ...


2

Technically/mathematically correct answer Here's an example of a black hole that is technically inside another black hole: the maximally extended Reissner-Nordstrom solution. In this case, what we mean by being "inside" the black hole is that the event horizon for one black hole is completely to the future of the other (see below for why this isn't ...


2

Time-dilation is defined independent of retardation. That is, you are assumed to use some kind of range sensitive detector (perhaps a really big phased array) to collect the light of distant events and the reconstruct their "real" time and position in your frame of reference before computing the dilation. A result of this is that red- or blue-shift of a ...


2

Once you are inside the event horizon every form of energy just increases the gravity. For example when discussing the weak force you suggest: By being random over enough time has an expected value of having "lucky spurts" of weak force over large enough time may play a part in tearing apart a black hole. by which I assume you're thinking of a decay in ...


2

The statement that at the beginning of the universe energy/mass was concentrated in a single region under conditions of extreme temperature and density is usually extrapolated from experimental data on the energy/mass content of the universe and its expansion, which is then analyzed through the classical (i.e. non-quantum) theories of general relativity and ...


2

Great question. Black holes are some of the brightest objects in the universe. While we think they require the Blandford-Znajek (BZ) mechanisms to produce things like Relativistic Jets, the bulk of the light (emission) they produce is just the efficient thermalization of gravitational energy when material falls into (`accretes' onto) them. The simplest way ...


2

The conservation of $\vec{k}\cdot\vec{u}$ only holds in the test particle limit. That is, it considers the metric to be unaffected by the motion of the particle. In this limit, there are no gravitational waves, since the metric has no time-varying quadrupole. If you want to see gravitational waves, you need to allow the metric to evolve dynamically, ...


1

BHs can carry charge - so you squirt some electrons into it to charge it and then use an electrical charge on the parabolic reflector to couple it to the ship.


1

Suppose you have two systems $S_1,S_2$ with Hilbert spaces $H_1,H_2$ with a density matrix $\rho$ on $H_1\otimes H_2$. The partial trace of $\rho$ over the Hilbert space of one of the systems, $H_1$ say gives you a reduced density matrix $\rho_2$. The reduced density matrix predicts the expectation values of all the measurements you can conduct on $S_2$ ...


1

That's fairly small for an object. It wouldn't have any significant gravitational effect on the moon or the earth. Tidal effects go as the cube of the distance. So the sun has about half the tidal effects of the moon. If this object were in low earth orbit (400km altitude), then the relative tidal effects on the surface when it is overhead would be about ...


1

It is not a coincidence. It has to work like that. The deficit angle has to be zero. It's most convenient to see it in the Feynman's path integral approach to quantum mechanics. One works in the Euclideanized spacetime to calculate the temperature $T=1/\beta$ partition sum. Let us consider the full finite-size black hole; the Rindler geometry is a local ...


1

For the simple case of a spherically symmetric, uniform, pressureless ball we have an analytic solution to describe the collapse called the Oppenheimer-Snyder metric. In this case the event horizon starts at the centre before the singularity is formed and grows outwards. The matter remains as a uniform sphere that eventually shrinks to a point. At no point ...


1

You have to consider the singularity and event horizon. By observing the orbits of nearby planets and such, we can calculate the approximate mass of the black hole, and thus where the singularity is. This then further gives us the Schwarzschild radius; $R_{Schwarzschild}=\frac{2GM}{c^{2}}$ This defines the event horizon, thus allowing us to then calculate ...



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