# Questions tagged [gravitational-waves]

For questions about the propagation of waves carried by space-time, for instance as described by general relativity. Not to be confused with gravity waves, such as ocean surface waves.

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### What happens to the momentum when 2 black holes collide and no matter is present?

Imagine 2 non-spinning black holes with identical mass collides with each other at the same speed in the absence of other matter, I think the gravitational waves can only carry a little of the ...
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### Can a charged (likely) binary black hole ( BBH) system be in a stable circular orbit (SCO)?

A BBH is a system consisting of two black holes in close orbit around each other. The last stable orbit or innermost stable circular orbit (ISCO) is the innermost complete orbit before the transition ...
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### What is meant by “gravity waves we see at sea surface”?

I've read in an answer to this question, of which I think that it was a very good, but somewhat long one, (I include the context in which it was written): Let me expand this point. When this ...
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### Is space time interval formula $ds^2=\eta_{\mu\nu}dx^\mu dx^\nu$ changed in presence of gravitational wave perturbation?

In relativity theory, in absence of any perturbation of the space time, we have conservation of spacetime interval : $$ds^2=\eta_{\mu\nu}dx^\mu dx^\nu$$ where $\eta_{\mu\nu}$ is the Minkowski metric....
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### Free fall and gravitational waves

If you take the Earth-Moon system, from what I understand, the Moon for instance is in free fall towards the Earth-Moon center of mass. However Einstein's equivalence principle says that a body in ...
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### Can gravity be conceived of as 'repelling' regions of lower energy density?

*I'm a writer, not a physicist, so please bear with me in my 'ham'-handling of physics and its terminology! My question is about whether repulsive forces over large distances could exist. My ...
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### How do we see an object fall into a Black hole in gravitational waves? [duplicate]

If an object emitting light falls into a black hole, the conventional wisdom is that an observer (at infinity) will never actually see it fall in - they will see it moving slower and slower as it ...
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### SXS gravitational wave data question

I need to do some analysis for a project with the SXS gravitational wave data: https://data.black-holes.org/waveforms/catalog.html but I am a bit confused about the initial conditions of their ...
58 views

### Dirac Delta Function and Schwarzschild Singularity [duplicate]

Pardon my naive question. I recently found out about Dirac Delta function. It is interesting to note that Schwarzschild singularity gives the infinity values of the General relativity field equations ...
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### An equation for derivation of gravitational waves polarization forms

When I was reading Spacetime and Geometry An Introduction to General Relativity by Sean Carroll(Page 297,Equation 7.110), I couldn't solve this problem in the proper way to get an approximation. ...
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### Extracting energy from the universe's expansion using gravitational waves

Would it be possible to extract energy from the universe's expansion by having two massive bodies orbit each other in such a way that their rate of infall due to emitting gravitational waves is ...
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### Gravitational waves detection using lunar retroreflectors

According to Wikipedia, the wavelength of a gravitational wave is about 600,000 km. The Earth moon distance is about 384,400 km. Considering that the latter is roughly half the former, it seems to ...
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### What are gravitational waves from the early Universe/how could they be produced?

Gravitational waves can be produced by the merger of two black holes or two neutron stars and those have been detected. Now people also thinking about GWs produced in the early universe. By what ...
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### The variation of the flux of the linkage for the asymptotically flat spacetime

The concept of the linkage for the asymptotically flat spacetime is defined and dicussed in this paper by Geroch and Winicour. This is a nice paper, but I came across one problem. The linkage is an ...
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### Kinnersley’s “photon rocket” and gravitational radiation

In this paper by Carlip, a comparison is made between electromagnetic and gravitational aberration. For the latter case, he takes as a study subject the Kinnersley’s “photon rocket”, an exact ...
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### Mathematical relations of Gravitational waves and the Metric Tensor $T$

Ok so as we all know that Spacetime Curvature has Geometric Disturbances which are mathematically called Gravitational Waves. But the question I am asking is that why the Coordinative value of the ...
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### Sidebands, cavity tuning and detection in interferometric gravitational wave detectors

I know the basics of gravitational wave detection but struggle to put things together and view the whole picture of the signal path. Most of my knowledge refers to second generation gravitational wave ...
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### Momentum in Gravitational Waves - Maggiore's explanation

In Maggiore's book on Gravitational Waves, he derives the energy-momentum tensor $t_{\mu\nu}$ associated with a gravitational wave. From this, he gives in equation (1.161) that the momentum (in the $k$...
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### Gravitational wave thought experiment: changing circumference?

I just started learning about gravitational waves and how they can cause the distance between 2 neighbouring particles to increase and decrease in an oscillatory manner. I made up a thought experiment ...
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### Derivation of Teukolsky equation

I have been trying to derive the Teukolsky equation via the Newman-Penrose formalism. I have derived the following formula (see Eq. (2.14) in Teukolsky's paper) \begin{equation} (*) \left[(\Delta +3\...
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### Do photons generate gravitational waves?

I guess I’m trying to understand the difference between a rock orbiting earth, that would radiate gravitational waves. And say a photon orbiting a black hole that is just following a straight line ...
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### Gravitational wave energy fluxes and bracket averaging notation

Typically the rate of energy loss due to gravitational waves is given in the form (e.g. MTW or Eq 33 of these notes), $$\dot{E} = - \frac{1}{5} \langle \dddot{I}_{jk} \dddot{I}_{jk} \rangle$$ where ...
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### Moving objects and gravitational radiation

I have read that GR predicts that moving, massive objects emit some of their energy as gravitational waves. In reality, the energy loss is negligible and undetectable, but in some systems, like PSR ...
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### In a gravitational wave, why is the effect on path length due to the change in path negligible?

This is about Example 16.1 of Hartle's Gravity book, which considers a wave traveling along the $z$-direction given by $$ds^2=-dt^2+(1+f(t-z))dx^2+(1-f(t-z))dy^2+dz^2,$$ where $f(t-z)\ll1$. Of this ...
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### How to get this metric tensor perturbation equation? (Gravitational wave）

(I need some time to come back to re-edit this post) LHS of equation 7.37 is gauge invariant The Twisted H symbol is comoving hubble parameter, the h_ij is metric perturbation, I do not know how to ...
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### Teukolsky (Bardeen-Press) equation ingoing coordinates

EDIT: I can't seem to delete this question, so I've posted the solution below (I must have made an algebra error-someone checking all this would still be appreciated!). I've left the question as is. ...
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### Should LIGO have been impacted by Lorentz contraction?

What bearing would or could Lorentz-Fitzgerald length contraction have on Ligo detections? Was this accounted for?
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### Polarization of gravitational waves VS polarization of photons

This might be a stupid question, but I think i am mixing important concepts. It is well-known that gravitational waves (GWs) come in two polarizations or two states of helicity: the so called "+" and ...
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### How come this is a gravitational plane wave?

In Wikipedia's Gravitational plane wave article a metric for a gravitational plane wave is given by \begin{equation} ds^{2}=\left[a\left(u\right)\left(x^{2}-y^{2}\right)+2b\left(u\right)xy\right]du^{2}...
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### Does the photon decay due to loss of energy in a gravitational shock wave that it creates? [duplicate]

If photon has energy, it has gravity. If photon has gravity and move through space at light speed, it creates gravitational shock wave (like sonic boom because gravity move at the same speed as ...
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### Can dark matter actually be gravitational radiation?

It seems to exist like only space distortion, and it seems to move just like radiation (specially when galaxy clusters collide, dark matter goes trough unaffected just like some type of radiation). ...
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### Does frequency of light change in the LIGO interferometer arms?

There are a number of questions on the internet and on this site asking about how the LIGO interferometer measurement works given that the gravitation wave stretches both the length of the ...
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### Detectability of gravitational waves emitted by a free-falling object, by different observers

Does an object falling radially in a gravitational field of a planet emit gravitational waves as seen by an observer on the surface of the planet? What about an observer falling along with the object?...
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### If gravity is a wave, what medium does it pass through? Is there a medium that it can not pass through?

I listened to a podcast discussing gravity and had this question. My apologies if it has been aptly answered somewhere else.
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### What's the shortest “safe” distance from a neutron star merger?

Take GW170817 for example, the first neutron star collision picked up by LIGO. Given how much data we got from that event, can anyone figure out what the "blast radius" is, and how far away from the ...
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### Why does everyone seem to assume that gravitational waves will always be very small? [duplicate]

The size or effective distortion field of a gravitational-wave perturbation at any given location in space-time is determined by the distance separating the source and that location. This describes a ...
83 views

### Gravitons from linearized gravity

Linearizing gravity as follows: $$g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}$$ Up to this point, everything is a 4x4 matrice. How does one eventually recover a spin-2 particle which, according to https:/...
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### Newtonian Gravitational Waves

I was thinking about the classical equations for gravity. I got stuck on two equations: $$\vec{\nabla}.\vec{g}= 0$$ and $$\vec{\nabla} \times \vec{g}= 0$$ The first equation is Gauss law of ...
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### Can gravitational waves be visualized in the elastic rubber sheet model of gravity?

To visualize the space curvature related to gravity, there is a model or analogy using an elastic sheet and a weight. A rubber sheet is spanned like a very soft trampoline, and a heavy ball on it ...
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### Why gravitational waves can only be generated by a time-varying quadrupole moment of the mass distribution?

The (rather old) source I dispose of "Sexl, Urbantke : Gravitation and Cosmology" describes the radiation of gravitational waves only rather sketchy. So why gravitational waves are only generated by ...
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### Can GWs move objects (excerpt pressure on matter) like EM waves?

Gravitational waves are real, they have been observed. Gravitational waves are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from ...
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### Can gravitational waves be used to investigate the inside of black holes?

I do understand that nothing, no particles and no information can escape a BH. Gravitational waves are real, they have been observed. Gravitational waves are disturbances in the curvature of ...
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### Do gravitational waves disperse/refract (like EM waves in a prism)?

I have read this question: What is the relationship between a gravitational wave and a graviton? where kingledion says: Gravitational waves were theorized a century ago and recently ...
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### Confused about the gauge transformation of the amplitude tensor for gravitational waves

Far away from the field sources, where the energy-momentum tensor $$T_{mn}=0 \tag{m,n=0,1,2,3}$$ The linearized EFE becomes $$\Box \bar h_{mn}=0 \tag{1}$$ where $\bar h_{mn}$ is the trace-reverse ...
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### What really is GW quanta?

I have read this question: What is the difference between gravitons and gravitational waves? where annav says: So photons are the building blocks of light, and gravitons are (hopefully) the ...
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### Tianqin project vs LISA

How does Tianqin space-based gravitational wave detector compare to LISA? Will it be able to detect gravitational waves from e.g. inflation (considering possible upgrades)?
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### Gravitons (real) passing through a black hole

This is not a duplicate. I do not ask about the gravitational field of the black hole, or why it extends farther out then the event horizon. I do understand that the gravitational field is described ...
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### Superposition of gravitational waves and the dark energy

Is it theoretically possible to create a superposition of gravitational waves that form a locally static negative curvature, something like the dark energy?