# Tag Info

13

Gravitational waves are transverse waves but they are not dipole transverse waves like most electromagnetic waves, they are quadrupole waves. They simultaneously squeeze and stretch matter in two perpendicular directions. Gravitational waves definitely propagate in a given direction but the effect that they have on matter is completely perpendicular to the ...

8

A gravitational wave will distort space-time and the light that is on a path affected by such a wave will be similarly affected, but it will still take longer (or shorter) to travel that path. Imagine a car travelling along the surface of a trampoline, a wave on the trampoline could cause its path to become longer, but it won't be impossible to detect ...

7

Nope. Gravitational radiation is a kind of radiation and it has a completely different equation of state than the cosmological constant. The cosmological constant has pressure equal to the energy density with a minus sign, $p=-\rho$: the stress-energy tensor is proportional to the metric tensor so the spatial and temporal diagonal components only differ by ...

7

The amplitudes do become arbitrarily small, and there's nothing at all wrong with this. In fact the exact same thing happens with electromagnetic waves. Sure we have a quantum theory with photons that places limits on how small a packet of energy can be detected, but light can travel across the universe just fine and become as dim as it wants. The intensity ...

6

There is no gravitational waves for a uniformly rotating axially symmetric body, because the metric doesn't depend on time. First of all, let me cite Landau, Lifshitz, The classical theory of fields, §88 The constant gravitational field: However, for the field produced by a body to be a constant, it is not necessary for the body to be at rest. Thus the ...

5

yes, the waves fall off in intensity as they get farther from the source. This does not violate conservation of energy, because you'll just be spreading the same amount of energy out over an ever larger volume, but the (energy density)*volume will be constant, minus energy transferred from the waves to matter.

4

An addendum to the answers of Daniel Grumiller and sb1: The major difference of the gravitational field and other fields is that according to general relativity the gravitational field defines space and time and therefore defines the relation of events. It is true that it is possible to do an "arbitrary" split of a certain linear approximation of the ...

4

First it's important to note that gravitational waves do require energy to produce. A good example of this is a binary pulsar, where the emission of gravity waves carries energy away so the two pulsars spiral in towards each other and will eventually merge. Having said this, it is theoretically possible to modulate a gravitational wave and use it to ...

4

in the linearized limit of General relativity, as FrankH said, all propagating perturbations of the metric are transverse. However, it must be noted that the full theory does allow for nonlinear longitudinal modes of propagation. According to the Petrov classification, such regions of longitudinal propagation are region III. There are usually not taken as ...

4

In short, if there is nothing to interact with the wave, it can't lose energy. EM and gravity waves do not experience friction with vacuum, so they just keep going. Of course, as they spread out, their energy becomes spread out as well. The power per unit area, or flux, is (somewhat trivially) inversely proportional to the area of the wavefront, so as long ...

3

You probably should look here first for some relatively up-to-date predictions of signal-to-noise from cosmological sources. Astrophysicists are very confident that advanced LIGO will see signals from merging compact objects like neutron star binaries and black hole binaries, on the order of a few tens per year (see first link). It is far less certain ...

3

It was not showed experimentally - the only experimental evidence of gravitational waves is that some binary pulsars change their orbiting frequency exactly as expected from their losing energy by gravitational waves of the GR-predicted intensity. However, it is surely established theoretically. The answer is Yes, gravitational waves that carry enough ...

3

There are experimental projects (LIGO, and friends) to detect large gravitational waves from the collisions of neutron stars and black holes with other dense massive objects, but the mechanism of such waves in no way resembles waves breaking on the beach: neither in mechanism nor in mathematics. If you have some space cycles, Einstein@Home is a BOINC ...

3

Given boundary conditions allow for a unique solution of the wave equation. If you do it your way and immediately guess the retarded Green's function you are fine, but in principle there is an infinite amount of solutions which have to be fixed by boundary conditions. By imposing Sommerfeld conditions, you make sure that only the retarded solution survives, ...

3

I, also a layman in physics, after watching one of Steinhardt's lectures, became curious as to the results he mentioned as well. I found this article in my search, which seems to state that the data which he is referring to has not yet been released, and is slated for a 2014/2015 release. In particular he refers to the polarized view of the cosmic microwave ...

3

Read A.Zee, Quantum Field Theory In a Nutshell, Princeton, Chapter I.5, p 30 (first edition) In Quantum Field Theory, "forces" between 2 "charged" particles correspond to an exchange of "virtual gauge bosons". For instance, the repulsive force between 2 electrons, corresponds to an "exchange" of a "virtual photon" (a perturbation of the photon field). Here ...

3

In the nonrelativistic limit the energy lost by the system due to gravitational radiation is defined by the third time derivative of quadrupole moment: $$- \frac{d E}{dt} = \frac{G}{45 c^5}\dddot{D}^2_{ij}.$$ Where indices $i$, $j$ correspond to (flat) 3D space, and dot denotes time derivative. This equation is taken from Landau & Lifshitz' 'Classical ...

3

The Weyl tensor is the trace-free part of the Riemann tensor. The latter describes the curvature of spacetime. In the absence of sources, the trace part of the Riemann tensor will vanish due to the Einstein equations, but the Weyl tensor can still be non-zero. This is the case for gravitational waves propagating in vacuum. The physical reason is that even ...

2

Firstly I would like to question whether anyone is really experimentally searching for a graviton. The effect would be too weak to detect with current technology. There is the closely related concept of "gravitational wave" which is a "curvature wave" in General Relativity and that is being searched for. There are probably two main reasons for expecting the ...

2

General relativity, currently our best model of gravity, not only predicts the gravitational force as a geometric effect, it also predicts the existence of gravitons, the "exchange particle for gravity". A theory without gravitons or gravitational waves (I'll use them interchangably) could exist, but as we have already indirect evidence for gravitational ...

2

A couple of links, they could be useful: On critical collapse of gravitational waves. Evgeny Sorkin. http://arxiv.org/abs/arXiv:1008.3319 Ultra Relativistic Particle Collisions Matthew W. Choptuik, Frans Pretorius http://arxiv.org/abs/arXiv:0908.1780

2

Unfortunately gravitational waves have not been detected yet. There is a number of Earth-bound detectors planned and already in operation (e.g. LIGO, Geo 600, Virgo, Nanograv and others). As for space-borne detectors, ESA works on Next Gravitational-Wave Observatory after NASA pulled out of LISA project in April 2011 due to funding problems. Joint NASA/ESA ...

2

It's important to stress, once again, that nothing is actually traveling faster than light in a literal sense. It's just that the universe expands, so that object that sent the signal is, measuring with today's ruler, farther away than the speed of light times the age of the universe. You can't receive signals from objects too far away, though - we can only ...

2

No, because the kinetic energy of an electron can be increased without limit by accelerating it close to the speed of light. It will always be possible to increase the kinetic energy of the electron to the point where it matches the potential energy due to the positive charge, so the electron can always escape to infinity. Similarly you can accelerate an ...

2

Gravitation waves require a system with an oscillating quadrupole moment. There's a good description of what a quadrupole moment is here. The simplest example of a quadrupole is a rotating dumbell, and two stars rotating around each other have this geometry. In fact gravity waves have been inferred for binary pulsars. In general galaxies are close to ...

2

It is difficult to design empirical tests that specifically check propagation at c, independently of the other features of general relativity. The trouble is that although there are other theories of gravity (e.g., Brans-Dicke gravity) that are consistent with all the currently available experimental data, none of them predict that gravitational disturbances ...

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