mmeent
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Why did the neutron star merger signal last for so much longer than the black hole merger signals?
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51 votes

It is not that the merger of two neutron stars takes longer, the inspiral and merger of a pair of neutron stars just spends a longer time in the frequency range where LIGO is most sensitive. Let me ...

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Why do gravitational waves circularize a binary?
28 votes

Note that the emission of gravitational waves does not necessarily makes an orbit more circular. This happens to be the case in the weak field (as detailed in G. Smith's answer). However, for ...

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Can a Black Hole be moved by lasers?
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25 votes

A laser carries linear momentum, so when a laser is fired into a black hole, this linear momentum is transferred to the black hole, causing it to accelerate. Of course, for a realistic astrophysical ...

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Do wormholes really exist?
20 votes

No, as far as we know there are no wormholes anywhere in the universe. In fact, we know of no realistic physical mechanism that would lead to the creation of a wormhole in nature.

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Gravitational waves on rigid bodies
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13 votes

The mirrors at the ends of LIGO's arms have been mounted in such away that their motion along the direction of arms is (almost) completely decoupled from their physical surroundings. The motion of ...

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How intense are gravitational fields, where we have been able to test General Relativity?
13 votes

Quantifying how strong a gravitational field is, is notoriously tricky. The two main quantities that people use to qualify the strength of the gravitational field probed in an expertiment are: The ...

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Why can photons be used as qubits?
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11 votes

A photon is a massless spin-1 particle. This means that a photon has exactly two spin states, just like an electron.

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What is a layman explanation for why the wavelength of light increases when space itself expands?
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10 votes

Here is simple explanation: Suppose a light source and a observer are in an expanding space. Now think of two subsequent crests of wave emitted by the lightsource. The second crest is emitted ...

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In which direction would gravitational waves be emitted when two black holes colide?
9 votes

Much of this question can be answered qualitatively based on basic symmetry arguments without doing any calculation or resorting to any approximation. The key observation here is a system consisting ...

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What causes a black hole ringdown and why can it prove the no-hair theorem?
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9 votes

The ringdown is simply the black hole formed in the merger settling down to a final stationary form. According to the no-hair theorem, this stationary form has to be described by the Kerr family of ...

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Could a trajectory around a large mass ever deflect by more than 180 degrees due to general relativistic effects?
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9 votes

For a test particle there is no limit on how many "laps" a hyperbolic orbit can make before it returns to infinity. However, once you start accounting for the objects own mass, there will be a ...

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Where is the wiggle room in current gravity theories?
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8 votes

To understand where the "wiggle room" in general relativity is it is useful to look at one of the main theorems that constrains GR, Lovelock's theorem. This says that if we start from an action that ...

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How can a black hole rotate if time dilation stops time at the event horizon?
6 votes

The statement "time is dilated to infinity at the horizon" is a (very imprecise) way of saying that the event horizon is a null/lightlike surface. However, as is clear from light-rays, being ...

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Andromeda & Milky Way Merger: Gravitational Waves
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6 votes

The peak strain of GW150914 was about $10^{-21}$. Strain scales linearly with the total mass of the system, and inversely proportionate to the distance. A merger of the of two supermassive black holes ...

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What would Happen if a Primordial Black Hole, with 5-10x time the mass of Earth, were to fall into our Sun?
6 votes

Anders Sandberg's answer assumed a black hole that somehow is already embedded in the Sun's core and stays there (despite travelling at an ultra-relativistic speed). In reality, a ten Earth mass black ...

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Do black holes resonate?
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5 votes

Yes, black holes resonate when perturbed by gravitational waves (or otherwise). The characteristic frequencies with which a black hole does this are called the quasinormal modes (QNMs) of the black ...

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Horizon of extremal black hole
5 votes

It means exactly what it says. For any point outside the horizon, the distance to the horizon (as measured along a slice of constant $t$) is infinite. Note that this statement depends on the choice of ...

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Can you detect gravitational waves emitted by quark stars?
5 votes

Quark stars, if they exist, would emit gravitational waves in a similar manner as neutron stars. In fact, their gravitational wave signals would be almost identical to that of a pair of merging ...

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What is the expected size of primordial black holes?
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5 votes

The answer is pretty much anything you want. Models for their formation are flexible enough to allow pretty much any reasonable size. There are some practical limits. If they would too small, we would ...

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Hawking Radiation from a Physical Black Hole
5 votes

Saying that "a physical black hole takes an infinite amount of time collapse, because time is redshifted near the Schwarzchild radius" is just a wordy way of saying that an event horizon will not ...

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How does a neutron star collision turn mass into gravitational waves?
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5 votes

This not too hard to understand. In fact, applying even Newtonian intuition will give an answer that is a reasonable (qualitative) approximation of the (always complicated) relativistic reality. ...

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What would happen if you dumped negative mass into an extremal black hole?
4 votes

Ignoring the plethora of theoretical problems that negative mass would cause (see examples on the Wikipedia page's introduction), the naive answer is yes a negative mass should be able to "...

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Crossing a Kerr wormhole
4 votes

The short answer is: you do nothing. It is surprisingly hard to hit the ring singularity in Kerr. The vast majority of geodesics (test particle trajectories) plunging into a Kerr black hole miss the ...

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Gravitational radiation from a perturbed metric other than Minkowski
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4 votes

You can linearize general relativity on any background. With an appropriate choice of gauge the resulting of equations of motion for the perturbation closely resemble a wave equation on a curved ...

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What is the energy integration constant from time symmetry in general relativity?
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4 votes

A priori, the interpretation of this energy is not obvious. It is the covariant t-component of the momentum 4-vector, but by itself it does not say much, other than that in flat Minkowski space it ...

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No-hair theorem for several black holes in presence of matter fields
4 votes

There is a Living Reviews in Relativity article on the status of stationary black hole solutions. The upshot is that for non abelian gauge theories (and some other nonlinear theories) you can find “...

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Is a Schwarzschild solution possible in 3 space-time dimensions?
4 votes

As mentioned in ApolloRa's answer, in 2+1 dimensions there exist no asympototically flat black hole solutions. However, you can still solve the Einstein Field Equations to find the metric of a non-...

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The Information Paradox and the Copenhagen Interpretation
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4 votes

Collapse of the wave-function, as required by the Copenhagen interpretation indeed requires a non-unitary evolution of the system. This, however, is also widely viewed as problem. It is one of the ...

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Do stellar black hole precess like Earth do?
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4 votes

Yes, black holes can precess. In binary consisting of two black holes, both spins and the orbital angular momentum will precess about the total angular momentum. In April, LIGO and Virgo announced the ...

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What is the Critical impact parameter for photons of a black hole?
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4 votes

The impact parameter $b$ of a scattering orbit is given by (in units with $G=c=1$) $$ b= \frac{L}{E}$$ A critical photon trajectory will have the same ratio $L/E$ as the photon orbit. This we can ...

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