# Tag Info

46

The actual paper (pdf) is very heavy in error quantification - and rightly so. They presented an experiment result that is statistically extremely difficult to obtain. But for the rest of us, conclusion is the most important part. The abstract says: The observed B-mode power spectrum is well fit by a lensed-$\lambda$CDM + tensor theoretical model with ...

19

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 ...

18

They announced that through observation of the Cosmic Microwave Background, via the BICEP2 experiment in Antarctica, particularly the polarization on a 2-4 degree angular scale, gravitational waves from inflation during the early universe are being indirectly observed. Link to FAQs about the release: http://bicepkeck.org/faq.html Link to pre-print: ...

16

Yes, gravitational waves will undergo the same red-shift as any wave that propagates at $c$. There were probably very violent gravitational waves in the very early universe. If those waves hadn't been red-shifted, they'd be ripping us apart right now. If so, could observations of them be used like red-shifted electromagnetic waves from distant sources ...

15

A common procedure to determine the spin of the excitations of a quantum field is to first determine the conserved currents arising from quasi-symmetries via Noether's theorem. For example, in the case of the Dirac field, described by the Lagrangian, $$\mathcal{L}=\bar{\psi}(i\gamma^\mu \partial_\mu -m)\psi$$ the associated conserved currents under a ...

12

The rubber-sheet analogy is often used to "explain" the basics of GR to beginners, but actually it has nothing to do with real gravity. It acts much more like a scalar field (the up/down freedom degree) - and there were several attempts to build a scalar gravity. But the correct description turned out to be tensorial and purely geometrical. GR has 10 ...

9

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 ...

8

Dr. Matt Strassler has some great info on his site, see here: http://profmattstrassler.com/2014/03/17/a-primer-on-todays-events/ http://profmattstrassler.com/2014/03/17/bicep2-new-evidence-of-cosmic-inflation/ http://profmattstrassler.com/2014/03/18/if-its-holds-up-what-might-bicep2s-discovery-mean/ Here's a summary of some key points in my own words (any ...

8

Yes, most likely, unless there is something fundamentally wrong with our understanding of gravity. The most promising candidate for detection is Advanced LIGO, which is currently in the process of being designed and built. The website has some really interesting information listed, including the construction schedule (PDF), and the upgrades, such as ...

8

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 ...

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

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 ...

7

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 ...

7

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 ...

7

Photons or cosmic rays don't (normally) emit gravity waves. Consider the comparison with radio waves. A moving electron doesn't emit radio waves. It has to be accelerating to emit EM radiation. Specifically radio waves are only emitted when there is a changing dipole moment. So you wouldn't expect a particle moving at constant velocity (photon or ...

6

As mentioned, this is not the first evidence for gravitational waves. The data from BICEP2 shows that there is a much higher amount of B-mode polarization than what is predicted by gravitational lensing alone. According to theory, this could only be due to higher amplitude tensor modes in the CMB than previously observed (or rather, lack of observed). These ...

6

In order to do perturbation the expansion parameter needs to be small. Otherwise the the system will be strongly coupled and you're in the non-perturbative regime. It's the same as for instance in QM: for perturbative calculations the pertubation must be small.

6

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 ...

6

If gravitational waves exist are they technically just another form of light/electromagnetic wave? No. Electromagnetic waves are (classically) disturbances in the electromagnetic field that propagate with speed $c$. Gravitational waves are disturbances in the geometry of spacetime that propagate with speed $c$. I would imagine a gravitational ...

6

Gravitational waves have never been directly detected. Gravitational waves are predicted by general relativity and have been inferred from other observations. Strong evidence of gravitational waves is the change in period of the Hulse-Talyor binary star system. Energy is being lost from the system at a rate consistent with radiation of gravitational ...

5

The gravitational wave (graviton) emission by atoms is completely analogous to the electromagnetic case except that the relevant observables of the atoms are not dipoles but quadrupoles etc. The ground state doesn't emit gravitons because of energy conservation: there is no lower-energy state that the atom could fall into after it emits a positive-energy ...

5

To add to Hindsight's great answer: one of the reasons that the analogy fails is the same reason why Nordström's Scalar Theory of Gravitation fails: Waves on rubber sheets are described by linear wave equations; at least in the small amplitude limit. However, by analogy with Maxwell's equations, waves in gravitation should bear energy. But we are also ...

5

In a way you are right because LIGO hasn't observed anything. But the theory for it working is sound, so you're wrong on that aspect. The light path itself is also affected by the gravitational wave. The Wikipedia article on LIGO says, Note that the effective length change and the resulting phase change are a subtle tidal effect that must be carefully ...

5

A theorized object called a geon, "an electromagnetic or gravitational wave which is held together in a confined region by the gravitational attraction of its own field energy", would seem like a match for what you're talking about. The wiki article mentions that exact solutions involving geons have been found (one is discussed in this paper), though it's ...

5

If not, what are the other applications? Calculating the relativistic precession of Mercury, for one. This post-diction was one of the key things that helped with the rapid acceptance of general relativity. Modeling GPS, and calculating the orbits of LAGEOS and Gravity Probe B, for another. A full-blown general relativistic formulation works quite ...

5

Typically solving the full Einstein equations is rather difficult, so to calculate stuff about gravitational waves people typically use the following approximation $$g_{\mu\nu} = \eta_{\mu\nu} + h_{\mu\nu}$$ That is, they approximate the full metric $g_{\mu\nu}$ as some perturbation of flat Minkowski spacetime. This approximation is called 'linearized ...

5

This is not possible at long distances because of special relativity. If gravity is a long-range force, the effects must be transmitted at the speed of light, so that there must be gravitational waves. The reason is that if you shake a mass at one point, the different position of the mass must lead other masses far away to shake later, at the speed of light. ...

5

Yes, they expect to see a signal when advanced LIGO is up and running. Unless there is something supressing the Bh-bh, bh-ns and ns-ns merger rate, if gravitational waves exist (and the hulse-taylor binary makes this almost undeniable), then they should have a positive detection by 2020, unless there is something unknown modifying the way that gravitational ...

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.

5

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 ...

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