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Gravitational wave doubt

acey

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acey

acey

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The recent high-profile announcement of gravitational wave detection is in doubt, with claims that the observations may really have been due to galactic dust.

The controversy was sparked by this paper:

http://arxiv.org/abs/1405.7351

leading to this article in Nature:

http://www.nature.com/news/big-bang-blunder-bursts-the-multiverse-bubble-1.15346

and lots of  news articles, e.g:

http://news.yahoo.com/experts-cast-doubt-big-bang-bolstering-discovery-060400200.html

http://www.dailygalaxy.com/my_weblog/2014/06/astrophysicists-see-serious-flaws-in-big-bang-gravity-wave-observation-1.html

http://www.scientificamerican.com/article/backlash-to-big-bang-discovery-gathers-steam/

http://time.com/118525/doubts-over-big-bang-discovery/

http://edition.cnn.com/2014/06/05/opinion/urry-big-bang/

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Tiki

Tiki

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If it's 'too good to be true' it often is. I'm sure that gravitional waves will be detected one day but I do wonder how as gravity is such a weak force.

From the Scientific American link in Acey's original post:

"In the ensuing two months, the doubts have only grown stronger, as physicists have attempted, and failed, to reproduce the BICEP2 team’s calculations—admittedly, without the original data, which the team hasn’t yet provided, and without the “systematics” paper, laying out the possible sources of error, which the team has promised but not yet completed. The paper describing the results themselves has not yet been published by a peer-reviewed journal, although that process is underway."

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Ganymede12

Ganymede12

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Pardon my ignorance but what exactly are gravitational waves? I feel I have read articles about them wothout understanding their exact nature.I've pictured them as a distortion in space-time moving in a ripple motion but I don't understand how they are created or what someone would experience if one past over them. Can anyone offer an explanation?

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D4N

D4N

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Well you wouldn't notice it go past you unless you were very close to the source, but since they are generated (in theory) by high energy events like supernova you wouldn't really be worried about the gravitational wave!

The sensors set up to detect these measure tiny changes over many kilometres.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

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Paul M

Paul M

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Pardon my ignorance but what exactly are gravitational waves? I feel I have read articles about them wothout understanding their exact nature.I've pictured them as a distortion in space-time moving in a ripple motion but I don't understand how they are created or what someone would experience if one past over them. Can anyone offer an explanation?

I visualize them in the same way as seismic waves (P waves in particular) travel through the earth from the focus of an earthquake. 

A kind of compression wave that distends the medium it passes through. In this case space is distended causing bodies it passes through to be stretched and compressed.

I find it easy to imagine how gravitational waves are created by a cataclysmic shock just as P Waves are created by an earthquake.

Hey, this might all be academic unless someone eventually finds the elusive creatures. As already stated we don't want to be on the front row seats at an event that generates a big one!!

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acey

acey

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Pardon my ignorance but what exactly are gravitational waves? I feel I have read articles about them wothout understanding their exact nature.I've pictured them as a distortion in space-time moving in a ripple motion but I don't understand how they are created or what someone would experience if one past over them. Can anyone offer an explanation?

Imagine an elevator with its cable cut, so that everything inside is in free fall. Imagine a cloud of dust particles inside the elevator. All the particles are accelerating downwards, so all are effectively weightless. But particles near the bottom of the compartment are closer to Earth than those at the top, so feel a bit more gravity, so accelerate a bit faster. The cloud is therefore stretched in the vertical direction. Also, however, since all the particles are accelerating towards the centre of the Earth, they must gradually be moving closer together in the horizontal direction. So the cloud is being squeezed as well as stretched: if it started out as a sphere then it becomes an ellipsoid. This is called a tidal effect (it's what makes tides on Earth).

In general relativity this tidal effect is mathematically encoded by the "Weyl tensor". The consequence is that it becomes possible to find solutions for Einstein's equations in which the tidal effect propagates through space as a wave. The effect of the wave is to stretch and squeeze whatever it passes through. The effect would be tiny, but in principle you could have a quadrupole aerial which would record its own changes in length and width and thus detect a passing wave. People have been trying to do this for decades.

Alternatively you could model the effect of gravitational waves in the very early universe, and look for a signature of this in the cosmic microwave background radiation. That's what the scientists in this case claimed to have done.

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Tiki

Tiki

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Pardon my ignorance but what exactly are gravitational waves? I feel I have read articles about them wothout understanding their exact nature.I've pictured them as a distortion in space-time moving in a ripple motion but I don't understand how they are created or what someone would experience if one past over them. Can anyone offer an explanation?

It is worth pointing out that gravitational waves are very subtle in their nature. Einstein himself was not always convinced of their existence, so if you don't understand gravitational waves you are in very good company to say the least. It was not until the 50's and 60's that much of the details were worked out.

The tidal effect described by acey above is Newtonian and has nothing to do with gravitational waves (assuming that the elevator is somewhere on the earth). The Earth's gravitational field, being spherically symmetric is independent of time. Time varying gravitational fields give rise to gravitational waves eg. Binary pulsars/black holes, the collapse of stellar cores, black hole mergers. In the unlikely event of a perfectly symmetric collapse of a star then no gravitational waves would be emitted.

As Paul M points out above, gravitational waves are transverse. This is key to their understanding. Suppose we knew that the sun was about to act completely out of character and emit some gravitational waves. In order to best detect them we would configure our apparatus in the tangent plane. If we aligned our apparatus radially then we would detect nothing. Our apparatus in this case could consist of a bar and we would be looking for deformations of the order of 1 part in 10^21. Early detectors were commonly of the 'bar' type.

I hope this helps.

Electro magnetic waves are the closest analogy to gravitational waves but the manner in which they interact with matter is somewhat different.

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acey

acey

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The tidal effect described by acey above is Newtonian and has nothing to do with gravitational waves (assuming that the elevator is somewhere on the earth).

What I said was correct: in general relativity the tidal effect is controlled by the Weyl tensor; the rest is described by the Ricci tensor. Spacetime can be Ricci-flat yet have non-zero Weyl tensor. Picturing these tensors by considering co-moving particles (e.g. dust in an elevator) is a standard way of getting the idea without resorting to equations. It is true that this picture does not explain gravitational waves per se, but those waves are controlled by the Weyl tensor, so their effect is the same, i.e. stretching and squeezing.

Tiki is incorrect in his description of these waves. The key is that they are quadrupole, unlike electromagnetic (EM) waves which are dipole. To detect EM waves it is enough to use a dipole aerial, but for gravitational waves you need a quadrupole (a cross) to detect the stretching and squeezing in orthogonal directions.

What neither of us has mentioned is excitation of the corresponding quantum field. For EM waves this is the photon, which is spin 1 (corresponding to dipole) while the quadrupole nature of gravitational waves (and the infinite range of the force) imply a spin 2 massless particle dubbed the graviton. Its coupling would be incredibly weak (because gravity is very weak in comparison with the other fundamental interactions), and its predicted properties lead to horrible problems in quantum field theory (non-renormalisability) which have yet to be resolved.

Einstein (and others) doubted the existence of gravitational waves because it seemed weird that you could have a spacetime with no gravitational sources (Ricci flat) yet a non-zero Weyl tensor (tidal effects or gravitational waves). It was thought that this was just a trick of co-ordinate systems, rather like the "singularity" at Earth's north pole (where lines of longitude all meet). The supposed stretching and squeezing implied by gravitational radiation would really be a coordinate artefact with no physical meaning. However someone (can't right now remember who) argued that if there was a bead free to slide on a gravitational wave receiver then its motion would generate heat, i.e. the waves transmit energy and must be real.

I just googled "gravitational wave bead heat" and discovered it was actually Feynman (though usually attributed to Hermann Bondi)

http://en.wikipedia.org/wiki/Sticky_bead_argument

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zyxwv99

zyxwv99

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Yesterday I was reading an article about rapidly spinning torlillas with mountains on them. Or Middle-Eastern flat bread, whichever you prefer. If the bread has a crust of solid iron, and it's been worked in a heavy forge so that it's a million times as strong as steel, what you have is a very crusty tortila, one that can spin around at more than a hundred revolutions per second, even though it's several kilometers around and has a mass of 0.12 solar masses, and has a mountain on it. The mountain doesn't fall through the crust because of the stength of the crust, and stays in place because of mysterious magnetic forces.

If you were anywhere nearby, the gravity waves would make your belly fat jiggle up and down.

Gravitational Waves From Low Mass Neutron Stars by C. J. Horowitz

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Ptarmigan

Ptarmigan

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The recent high-profile announcement of gravitational wave detection is in doubt, with claims that the observations may really have been due to galactic dust.

The controversy was sparked by this paper:

http://arxiv.org/abs/1405.7351

And the update that we have all been waiting for :-

"Inside Science" Radio4 has just said a joint study/paper by the Planck and BICEP  teams agree that the signal was due to dust.

A brief search online gives me

http://sci.esa.int/planck/55362-planck-gravitational-waves-remain-elusive/

"Despite earlier reports of a possible detection, a joint analysis of data from ESA's Planck satellite and the ground-based BICEP2 and Keck Array experiments has found no conclusive evidence of primordial gravitational waves. "

and a hyperactively feevered article in S&T 30Jan

"Yes, it was too good to be true. The cosmic "discovery of the century" last March has officially blown up. Or will blow up next week"

at

http://www.skyandtelescope.com/astronomy-news/biceps-big-bang-finding-reduced-dust/

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acey

acey

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Abstract

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10−21. It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410+160−180  Mpc corresponding to a redshift z=0.09+0.03−0.04. In the source frame, the initial black hole masses are 36+5−4M⊙ and 29+4−4M⊙, and the final black hole mass is 62+4−4M⊙, with 3.0+0.5−0.5M⊙c2 radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

Full paper:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102

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Tiki

Tiki

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Exciting times indeed. I have always believed in gravitational waves I just thought that they might prove elusive.

Cut and pasted the last paragraph of this tutorial:  http://www.astro.cardiff.ac.uk/research/gravity/tutorial/?page=4blackholecollisions

'The detailed knowledge of the detected signal will allow us to infer the masses and spins of the binary components and its orientation in space. The knowledge so gained will enable strong field tests of general relativity such as the no-hair theorem which states that the geometry of a black hole is entirely determined by its mass and spin. Additionally, it will be a new observational tool to map and measure the Universe. For instance, being standard candles binary black holes will make it possible to measure the luminosity distance to sources at cosmological distances thereby providing a new tool for cosmology. In particular, Cardiff relativists have shown that by observing many such binaries at cosmological distances, it will be possible to measure the mean density of the Universe and its expansion rate very accurately. LISA observations of binary black holes will also help in measuring the dark energy equation of state (see the Section on super-massive black holes).'

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