LIGO-Australia Proposed

Bikeman (Heinz-Bernd Eggenstein)
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Mike Hewson
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RE: That is a shame. A real

Quote:
That is a shame. A real shame.


Well, as they say : timing is everything !
It was just a really bad time in the political cycle to be asking ... :-(
Obviously, spending is always easier with a surplus.

Cheers, Mike.

( edit ) More power to LIGO in India then ! :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

geonerd
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Guys, if two LIGOs haven't

Guys, if two LIGOs haven't observed anything yet, how will a third facility improve the overall system?

Bikeman (Heinz-Bernd Eggenstein)
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RE: Guys, if two LIGOs

Quote:
Guys, if two LIGOs haven't observed anything yet, how will a third facility improve the overall system?

I understand that this is not about a third detector, but moving one of the detectors.

Anyway, several detectors can help in many ways:

- not all detectors can be on-line all the time. Sometimes they are thrown out of operation mode because of local disturbances, or are undergoing repairs/maintenance/upgrades. It would be a shame if some some interesting astrophysical event would happen and not a single detector was on-line (galactic Super Nova, "close" gamma ray burst, whatever).

- the detectors are in general omnidirectional, but still sensitivity varies with orientation relative to a source.

- longer baselines between detectors could help to determine teh location of a transient event provided that adequate timing can be achieved.

CU
HBE

Mike Hewson
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RE: Guys, if two LIGOs

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Guys, if two LIGOs haven't observed anything yet, how will a third facility improve the overall system?


Hi Geonerd! :-)

I guess your question is : why haven't we heard anything yet? Due to the very low level of effect that we are after - 1 part in some 23 or so orders of magnitude - then the LIGO program was designed to be evolutionary. Meaning it gets better with time, hardware upgrades, technical experience and the like. A learning process across a whole spectrum of matters and disciplines. Currently we are awaiting upgrades to the Advanced configuration of the detectors. If the Enhanced configuration had been less noisy ( ie. S6 ) I think there was good odds of a detection, so it was close but no cigar. The current infrastructure was designed for component switching so we await with interest for things like more powerful lasers, heavier test masses, improved seismic isolation etc .....

As to the need for a widespread network : this depends on the particular character of gravitational waves, they are transverse & traceless with spin 2. This means that the alteration of the spacetime geometry is perpendicular to the direction of propagation of the wave, and there are two base modes that are at a 45 degree offset in that perpendicular plane. For a given mode a contraction along one direction implies an expansion along the one perpendicular to that, and vice-versa ( that's the traceless bit ). When that interacts with the IFO arrangement there is only a mild directional dependence to the detector response. So a single IFO can hear, but will not localise the direction to source particularly well. Indeed there are certain incoming directions for which there is no antenna response at all. So one can resort to good old 'triangulation' in that with sufficiently precise timing standards within a network of detectors, one can deduce the incoming wave vector well. And somewhat like the precision of a GPS fix ( the more satellites the better ) this scheme works best with widely separated IFO's on the planet, with what one misses ( for whatever reason ) redundancy can catch.

Cheers, Mike.

( edit ) So to be quite specific : even the original design assessments ( circa late 1990's ) put the chances of detection at around < 20% with our current detector layout, so lack of a detection so far is no real surprise ( for those who are aware of that prediction ). In any case that still misses a crucial point : even if we fail to detect a single GW with a well functioning Advanced stage ( yet to happen ) then that will be of enormous importance akin to the famous Michelson-Morley null result. After all, if we 'knew' the result then why bother looking? This is science. One doesn't assume a finding, one goes and asks reality. It's a heck of a challenge either way ( firm detection or firm absence of detection ) and indeed Einstein when first appraised of the wave solutions to his GR framework immediately despaired of any hope of confirmation. He sensed the difficulties ...

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Here you go : I've found the

Here you go : I've found the diagram from Peter Saulson's book Fundamentals of Interferometric Gravitational Wave Detectors which explains the LIGO design rationale very well. I recommend it [ but I'm not sure why Amazon is quoting $1,809.57 for a single used copy !!! ]

If you can imagine the interferometer with it's arms lying horizontally and aligned with the faces of the displayed boxes above, then :

- the left hand image represents the IFO response for a given 3D direction for one choice of polarisation ( called F+ or 'F plus' ).

- the middle image is for the other polarisation ( called Fx or 'F cross' ), also note that it's symmetries are rotated 45 degrees about a vertical axis c/w F+. Just like the '+' and 'x' symbols are !! :-)

- the right hand image is for a combined average, assuming at least equal mixes of both polarities.

The best response for any polarity ( or mix thereof ) is from directly above or below the plane of the IFO, this exposing the IFO arms to the full 'force' of the spacetime distortion and hence the greatest length differential b/w each arm. This translates to the greatest phase difference b/w photons traveling the two arms and hence the best IFO's output signal.

So they're funny looking peanuts, where the 'pucker' points are the nulls for which there is no response : depending upon the specific signal's modal composition. Certainly the right side image is to a first approximation a sphere, thus implying poor angular discrimination of the signal's incoming direction as a general overall statement. So these are the especial reasons why GW detection is quite a unique challenge, in addition to that 23 orders of magnitude, and indeed the proposed LIGO site in Western Australia was nicely situated in a null zone of hearing ( for some polarities ) from the existing US sites.

HTH :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

geonerd
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Thanks, Mike. Give me some

Thanks, Mike. Give me some free time to digest this - I feel some questions nucleating. :)

Mike Hewson
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Actually the 'traceless'

Actually the 'traceless' feature is worthy of some elaboration. When expressed in a particular form the transformation that is experienced by the test masses' separations has a sequence of numbers along the leading diagonal of a certain matrix. The 'trace' of a matrix is the arithmetic sum of those entries along that leading diagonal. Traceless means that the sum is zero. For the matrix in question those entries are each an eigenvalue : meaning a 'typical amplification' factor that 'characterises' the transform. The German word 'eigen' translates as 'typical' or 'characteristic'. One entry is +1 and the other is -1, hence the sum being zero. Now the plus one and minus one refer to orthogonal directions ( x and y for the sake of discussion ) so when a z-axis aligned ( direction of propagation ) wave comes along : the plus one refers to the rate of change along x ( say ) with a simultaneous minus one rate of change along y ( say ). So in natural language we say the x direction expands while the y direction is squeezed. And vice versa in a cyclic fashion for the total interval of wave passage.

However there is a deeper aspect or interpretation. The wave neither 'creates' nor 'destroys' volumes of spacetime. If viewed as a 4D bag of jelly then the local distortions induced by wave passage don't change that volume, but merely the 'instantaneous' shape ( one struggles with time descriptive language when talking of spacetime, but I hope you get the gist ). With gravity, there being only one type of 'gravitational charge' ( positive mass ), then a mode where it acts like a balloon inflating and deflating can't occur. That would be tantamount to a breach of conservation of mass/energy. Likewise, to avoid the breaching of conservation of momentum, then an 'isolated' system ( whatever that is! ) cannot just wholly shift to one side and return. There must be something else to react against, whether that be something nearby or the rest of the universe or whatever.

An inflation/deflation mode is referred to as 'monopole', while a shift to side and back is called 'dipole'. What is available for gravitational field change is 'quadrupole', and is what we have been discussing. This wave behaviour is generated when 'non-axi-symmetric' oscillation happens. Examples of which are :

- a 'mountain' on a rotating neutron star. Sort of like an out of kilter centrifuge, or tyre on a car wheel not properly balanced.

- two masses orbiting around each other. Thus all manner of binary star system variants.

- a sufficiently asymmetric relativistic fireball that is a supernova explosion. Mind you they probably all are asymmetric to some degree. This will yield all manner of transient waves ( not studied by E@H ) and generally punts off the residuum ( eg. a neutron star ) at speed in some direction away from it's system of origin.

Hence gravitational waves have energy which propagate away from source, this one can equate to an effective mass and hence explain other objects ( more readily identifiable as 'massive' ) as going the other way. But this is a deep rabbit hole, so I'd best pull up here. :-)

[ This topic is quite separate to Big Bangs, Dark Energy, Dark Matter etc. ]

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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More on the Amazon price :

More on the Amazon price : that's gotta be either an error ( shift decimal point one place to left methinks ) or hopeless optimism ( unless it's signed by someone really famous etc ). But maybe there has been an intense shift in interest in that expose of this topic. Who can say ? :-) :-)

I have a copy of the exact same hardcover edition, which I did buy from Amazon about five years ago. From memory it was certainly under $200 AUD including P&H to DownUnda.

Here's another place to buy which supports the Decimal Place Shift Hypothesis. :-) :-)

Seriously, go and buy it. It's about as close to everyday language that you'll get without the introduction of silly simplicities that weaken the truth of it. It says the target audience is 'physics undergraduates' and above ( which is true ) but even if you skim the messy stuff you'll still get a good feel for the broad nature of the challenges of GW detection.

Cheers, Mike.

( edit ) @Geonerd : nucleate away! I'll be here to help, if I can. :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

astro-marwil
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Hallo ! RE: *** if two

Hallo !

Quote:
*** if two LIGOs haven't observed anything yet, ***

There aren´t only 2 detectors but thereof 4 (Geo660 and Virgo too) and the japanese one coming soon in operation as the 5th. All they are in a belt on the north half of the globe.
My point of view is: As long as there is no proven evidence of GW by more than one of these detectors, it is irresponsible to install a 6th detector, as they realy can´t be paid just from our pocket money. To make plans, also detailed plans, for the days just after the advent of that proven evidence is fine, so one can start off immediately.

We are searching now for 6 years for this waves without success, with detectors that have the neccessary proven sensitivity to find something valuable. The only result now, that has been concluded is, that neutronstars are obviosly more precisely ballshaped than allowed by theoretical calculations. At minimum 2 detectors become upgraded to 10 fold sensitivity just now. By schedule they will start the testphase in the next year and come to regular operation one year later (2014). To check, whether there are GW or no and to take basic measurements, the existig 5 detectors are more than sufficient.

Of course, if there would be found no GWs or theire strenght is engraving lower than expected, that would be very spectacular and a strong indicator for some sort of new physics.

Kind regards
Martin

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