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Questions tagged [gravitational-waves]

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Classical mechanics gravitational waves

Would there be gravitational waves even if general relativity was wrong? For example imagine there was a theory of gravity that was consistent with special relativity. How different could ...
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Hulse-Taylor binary data gap in the nineties

I was wondering what is the reason there are no data points in the famous Hulse-Taylor plot of the period decay in the 1990s. Does anyone know why no one collected data during this period?
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1answer
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Why is the Ricci scalar non-zero in this case?

The Einstein equations can be written as (1): $$R_{ab}-\frac{1}{2}Rg_{ab} = -8\pi GT_{ab}$$ or by contracting the above equation with the metric tensor and resubstituting: (2) $$R_{ab}=8\pi G(\frac{1}...
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Could a powerful gravitational wave cause electrons to emit light?

I imagine electrons being accelerated by passing gravitational waves, say from a nearby kilonova, so I would expect the electrons to emit light. Am I right?
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What is the experimental evidence for the gravitational field having positive energy density?

Recent direct observation of gravitational perturbations attributed to merging black holes and merging neutron stars has reliably confirmed the existence of gravitational waves. The observed fact that ...
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Redshift z=40 and gravitational waves

Why z=40 is the critical value of the redshift (more or less) to decide find out primordial gravitational waves around it? I have read that is related to the following issue: black hole mergers ...
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Attributing Gravitational waves to a source

At a given time, there would be n number of binary neutron stars or Black Holes or even Super Novae, all of which would be leaving a gravitational wave imprint ..So how do LIGO scientists know which ...
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1answer
321 views

Lowest 'Order' of Radiation

I've noticed an interesting phenomenon (admittedly from only two data points). In electromagnetism, $A^\mu$ obeys Maxwell's equations: $$ \square A^{\mu} = j^\mu . $$ where I've chosen $\mu_0 = \...
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Do gravitational waves carry energy?

We now know that gravitational waves are real. Do they carry energy? I mean thy are waves and every wave carries energy. If they carry energy, how do they do it? Is there any equation to describe ...
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How to use Ashtekar's variables in classical gravitational physics?

I have often heard of Ashtekar's variables in General Relativity, because of the naturalness with which they would allow a canonical formulation of gravity, useful for a hypothetical quantum gravity ...
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2answers
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How does a neutron star collision turn mass into gravitational waves?

I heard on the TED Radio hour that in the neutron star collision that LIGO recently detected, some of the mass of the neutron stars was turned into energy in the form of gravitational waves. How does ...
3
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1answer
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Determine expansion rate of universe from gravitational waves LIGO?

I heard on the TED Radio Hour that the LIGO team was able to make an independent measurement of the Hubble constant and the increasing expansion rate of the universe when they detected the ...
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How to show that the polarization tensor of the gravitational plane wave in vacuum is transverse

This is Ta-Pei Cheng's book p.254. I am trying to understand (13.28). The Lorentz gauge condition (13.18) is like below. How can I use the (13.18) condition to show (13.28)? I am trying to calculate ...
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477 views

What is the wavelength of gravitational waves?

What is the wavelength of gravitational waves? I have looked for an answer but so far not identified one.
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Gravitational waves & cosmological redshift

Are gravitational waves streched by the expansion of the universe in the same way as EM radiation is? In that case how does one differentiate between a gravitational wave from a given event (say ...
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Stochastic versus primordial gravitational waves

I have read differences terms but I doubt if they are exactly the same in Cosmology: Primordial gravitational waves (and their background). Stochastic gravitational waves (and their background). ...
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Gravitational waves' path and interaction with black holes

As a gravitational wave propagates away from its source, how does its path get affected if a black hole is in its way? I was trying to picture it visually and here was my 'before' (with the line being ...
3
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1answer
137 views

Could LIGO have detected R'lyeh if it existed?

In Possible Bubbles of Spacetime Curvature in the South Pacific, that R'lyeh might be exist within a bubble of spacetime. (It appears to be a joke paper since some of the references are fictional, but ...
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1answer
160 views

Observation of Hawking radiation

Is it conceivable that Hawking radiation could be observed using e.g. gravitational waves or imprints in the CMB?
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76 views

Gravitational waves detector orientation

For a wave of the form, say, $$ ds^2 = -dt^2 + [1 - A\sin(\omega(t-z))]dx^2 + [1 + A\sin(\omega(t-z))]dy^2 + dz^2$$ how should one orient the detector (that works by monitoring the distance between ...
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Difference in Gravitational Waveforms for different objects?

This is an extension of one of my older questions: How would the gravitational strain waveform look like for a planet in orbit with a star? Let's say at some distance D there are two objects which ...
3
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1answer
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Why is it correct to conceptualize LIGO's mirrors as “test masses?”

In the LIGO paper on the first detection of gravitational waves, they have a diagram of one of the interferometers in which they label the mirrors with the conceptual label of "test masses." The ...
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1answer
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Primordial Black Holes mass diference with isomass stellar black holes

How can we distinguish, for a given mass (measured from gravitational waves experiments and or other experiments) of a black hole or black hole binary, if they are PRIMORDIAL or they are stellar black ...
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On gravitational waves of dark matter collisions

It is known that dark matter interacts gravitationally (and weakly in other ways as predicted by the WIMP model), and dark matter is present in the same space-time fabric as that of matter. When two ...
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Black hole hair, gravitational wave emission, and information, from outside observer

I have a few conceptual puzzles that I cannot see my way through. Time and again, I've read that from the point of view of a static outside observer, infalling matter just reaches a black hole's event ...
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1answer
255 views

What is the relationship between a gravitational wave and a graviton? [duplicate]

Gravitational waves were theorized a century ago and recently discovered, leading to the awarding of the 2017 Nobel Prize in Physics. According to Wikipedia: Gravitational waves transport energy as ...
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2answers
444 views

Why does gravity travel at the speed of light?

In electromagnetism, Maxwell's equations predict that electromagnetic disturbances travel at the speed $$c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}.$$ Does general relativity predict that gravitational ...
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0answers
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Does special relativity apply to gravity wells?

If the gravity well of a non-rotating heavy object is in roughly the same inertial frame as an observer, its spacetime geometry (as measured e.g. by photon lensing) will present as spherically ...
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2answers
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GW170817 optical and infrared light

I'm reading about GW170817 in various research articles. One of them, The X-ray counterpart to the gravitational-wave event GW170817. E. Troja et al. Nature 551, 71 (2017), arXiv:1710.05433 ...
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1answer
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Bondi news in BMS supertranslation

What does "news" mean in "Bondi news"? Is it information? What about memory in gravitational memory, is it information as well?
2
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1answer
145 views

What ways are there for a black hole to lose its mass?

How can a black hole lose its mass? Is Hawking radiation the only way?
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Neutron Stars of unequal mass merging

Is the difference in masses between merging Neutron Stars the cause of the massive gravitational disturbances due to the unbalanced decaying orbit right before the merger?
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2answers
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Kerr BH effects on inspiral binaries

In the gravitational wave calculation for binary systems: what is the effect of rotation of two BH (or neutron stars, BH-NS,...) on the usual calculations? Is there any EXACT result known?
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1answer
74 views

What information can we know by LIGO's detection of gravitational waves?

LIGO's detection of gravitational waves can help estimate Hubble's constant in a new way. Another paper might be informative. Apart from that what information can we get from the observed ...
8
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1answer
156 views

Can gravitational waves orbit a black hole?

Assume (for the sake of simplicity) a Schwarzschild black hole (non-rotating, non charged). This black hole has a photon sphere in $r=1.5r_s$, where photons may travel in a circular orbit. Will a ...
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2answers
332 views

Gravitons with negative mass?

I have been reading several papers on massive gravity. All of them have equations that involve the square of the graviton mass, rather than graviton mass itself. See for example, equations 43 and 44 ...
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Help finding references on WKB approxiamtion of gravitational waves in a curved background

In MTW Gravitation, the shortwave approximation on gravitational waves in a curved background is given in Section 35.13. This treatment is similar to the WKB approximation in quantum mechanics, or the ...
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1answer
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Why do interferometers only split a wave into two beams?

I was recently reading about interferometers and was wondering why it isn't common practice to split a wave into three orthogonal beams? I know very little about the topic and this may seem daft, but ...
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0answers
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Is the speed of gravitational waves lower in a medium? [duplicate]

The speed of light in a medium is often lower than in the vacuum but does the same apply for gravitational waves. Is the speed of gravity in all media the same as light in that medium? This question ...
2
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1answer
152 views

Gravitational wave or “pure gauge”?

Given the following linearized metric tensor ($c=1$) $$h_{\mu\nu}=\begin{bmatrix} 0 & f(t-z) & 0 & 0 \\ f(t-z) & 0 & 0 & -f(t-z) \\ 0 & 0 & 0 & 0 \\ ...
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1answer
69 views

Atom interferometry,gravity and inertia: What can it measure that light interferometry can't? [closed]

What previously unexplored effects in gravity and inertia can be examined with atom interferometry in ways that hasn't already been done through light interferometry? Can atom interferometry be used ...
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1answer
206 views

Would photons “riding” a gravitational wave appear different to an observer?

Gravitational waves travel at the speed of light. A photon travelling in exactly the same direction as a gravitational wave will therefore remain in exactly the same position relative to the wave - at ...
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Gravitational waves formula

Gravitational waves are transmitted in environment in form of waves. Is there any formula for their energy of these waves?
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If gravitational waves are ripples in space-time, then electromagnetic waves are ripples in what?

If the answer is the electromagnetic field, then is it also ubiquitously present as space-time?
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1answer
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If we could only see gravitational waves, how “bright” would the sky be?

I feel like we actually are not sure about the answer (as there have not been many detections so far, so our knowledge about gravitational waves is probably not very deep), so this might be too ...
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3answers
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Detection of gravitational waves and the expansion of the universe

Very recently, there was the fascinating news of new measurements being done by LIGO, in this case detecting the gravitational waves of two Neutron stars colliding. These detections prompted the ...
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1answer
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If light was 2 parts per trillion slower than gravitational waves, would we know this from Gauss' law? [duplicate]

In the recent neutron star merger. The gravitational waves started 100 seconds before the collision, which is when they reached a max and STOPPED. The mass quadrupole had stopped changing, which means ...
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2answers
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Is there a model relating GW signal and luminosity of the merger / burst?

To understand fully the importance of the recent observation I would like to know if it is possible to infer the distance or luminosity (in whatever wavelength, or say, the overall power) of the ...
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1answer
250 views

Why is there blindspot for LIGO interferometer?

I am thinking maybe when the gravitational wave hits the 2 mirrors at the same time but the two mirrors are very far apart, so what could be the cause for the blindspot?
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Effects of a neutron star merger

After the announcement of a neutron star merger on 16 October, I have been wondering how close such a merger would have to be in order to generate effects that could affect life on Earth. For example,...