Detector Watch

Thanks for the encouragement! :-)
Yes it certainly looks like hours to recover. I don't think it's the persistent shaking of the ground, but the nature of the gadget. I guess it takes a while for the energy of the induced wobbling to subside. There are lots of possible disturbances, say this stuff at Livingstone ( 'AS' is a seismic sensor, 'Yend' is the end of the 'Y' arm of the interferometer - the other is the 'X', 'UTC' is Co-ordinated Universal Time - a high precision standard for time worldwide ):

Speaking of disturbances, the North Korean nuke test may have upset the LIGO's:

This will behave like an earthquake, the delay refers to the time taken for the quake wave to travel across the Pacific ( under it actually ). The USGS refers to the United States Geological Survey which tracks seismic events, you can actually hook up to an RSS feed if you like!
Cheers, Mike.

( edit ) 'Earthquake' depth at 0.0km is the give-away for a nuke test BTW. Standard causes are at kilometre depths.

( edit ) Correction AS is 'anti-symmetric' ie. the science signal output of the interferometer.

OK, detector fiends, I thought I'd give an explanation of what I've derived from my study of some of the screens available on the logs. Forgive me if it seems too simple or over-explained.

( Bear in mind I have no official status here, this is a private reverse engineering effort, based upon the following:

)

So let's start with an easier one - the 'State Vector' plot. I don't think it's actually a vector in the usual sense, but like 'DEFCON' ( defense condition ) used to indicate the status of the project at a glance. It's not science data, and they are replicated at both Hanford:

and Livingstone:

In both, the bottom left indicates T0=date time. T0 pronounced 'time zero' is when the screenshot was taken. Date is in DD/MM/YYYY ( D = day, M = Month, Y = year ) format and time in HH:MM:SS ( H = hour, M = minute, S = second ) format. A 24 hour clock format ie. 00:00:01 ( 1 second past midnite ) to 23:59:59 ( 1 second prior to midnite ). It is in UTC or 'Universal Co-ordinated Time', pretty well the replacement for the old 'Greenwich Meridian Time' for those that recall that.... :-) Thus the data is logged in this time reference and not with regard to any particular local time.

As it's name suggests it treats the entire world as a single time zone. That way events in different places can be compared easily if brought back to that clock. ( Recall that light can travel about seven times around the world in a second. That's fast, but not instantaneous - so strictly speaking events aren't exactly simultaneous in the usual sense - but we'll leave that be for now ).

Anyhows that sets the time that the plot is marked at '0' on the right hand edge of the lower axis. This axis is then marked along to the left in hours before the time in question ( smaller divisions 1/4 or 1/2 hour, larger divisions 1 or 2 hours ). So it is marked in Time(h). I belabour this explanation as many of the other plots have this setup.

Now the vertical axis on the left is the State (*) where the asterisk '*' indicates no physical units apply. So it is simply a number. Values clearly go from 0 thru to 4. I haven't yet seen used the '-1' that appears on the Hanford plot. The LIGO has three interferometers ( IFO's ) in the continental US. ( Though I believe GEO in Germany is contributing I can't find any publicly access route to that operational data ). H1 ( in red ) at Hanford has 4km long arms, H2 ( in blue ) also at Hanford has 2km long arms, with both contained in the same building - indeed the same vacuum space. L1 ( in green ) at Livingstone has the same design as H1. Their substantial physical separation is important for a host of good reasons. Note the plots from both sites have the same features but are offset horizontally simply because they were snapped at different times ( ie. different T0's ).

The horizontal bars used to plot each detector are slightly offset vertically and of slightly different widths, so that you can see them distinctly when overlaid. But the values are exactly integral.

So what does the scale mean? Well here's my shot/guess:
0 - not working
1 - mode cleaner ( MC ) operating
2 - case 1 plus arms powered up
3 - case 2 plus control system powered up
4 - case 3 plus IFO 'locked'

I envision it like the flow of photons from the laser through 'gates' into the machine proper.

The mode cleaner, as it's name suggests, 'purifies' the light by removing unwanted photon types ( my guess is wavelength, polarisation, maybe unwanted resonances/sidebands ).

The 'Both arms locked 12h Export' refers to the optimal ( ie. science ) operation of the IFO's. This produces the good data for us crunchers to churn and burn! :-)

So with the recent plots shown above, clearly it was a fiddly time for H2 and L1, but rather well locked for H1.

Cheers, Mike.

Yes, I've been mulling over that one too... there are a few quantities/signals expressed in that general form. I think I understand 'per Hertz', but I'll get back to you when I've conquered 'per root Hertz'..... :-)

Yep, I read that too. Have you found the LIGO document server here? It helps to have the key to their coding/naming as well. I've been steadily trawling the ( thousands of... ) papers there and downloading some of the more interesting/pertinent pdf's. I'm currently in the 'T' or technical group..... :-)

I suppose you're referring to one like this at Livingstone :

If I am correctly reading the upper left legend, then it includes the end stations ( ETM ) the corner apparatus ( LVEA ) for both arms ( X and Y ) in the frequency ranges 0.1 to 0.2 Hz and 0.2 to 0.35 Hz ( where 'p' represents the decimal point ). Thus covering that 0.1 to 0.35 Hz seismic range on that plot at least. Anyhow, all those plots presumably cover the gamut of seismic disturbances - to aid understanding of loss of interferometer lock from shaking, and perhaps any overlap in that range with our desired astronomical detections.

Yes it's a terrific instrument isn't it?
H1 - Hanford 4km arms
H2 - Hanford 2km arms
L1 - Livingstone 4km arms
The difference this year is that the interferometers are 'up' at science mode more of the time - they are simultaneously locked for the best data. Each increment in range ( of detection ) increases the volume enclosed somewhat proportionally more - as a sphere's volume goes by it's radius cubed - thus potentially snagging more targets.

Thanks for that link, downloaded in a jiffy :-)

Cheers, Mike.

So let's next look at the NS/NS Inspiral Range.

This has the horizontal axis showing time as defined before. The vertical axis represents distance, but in a standard astronomical unit. A parsec is short for 'a parallax of one arc second', a fairly bizarre name for everyday use. It's a lot further away than the shops... :-)

Historically parallax was/is a way of measuring distances to relatively nearby stars or other objects. If you make two measurements of the direction of an object, when the Earth is on opposite sides of the Sun in it's orbit ( ie. taken six months apart ) then you can form a triangle. The 'base' is the diameter of the Earth's orbit, and the angles between that line and each of the two directions of observation, to the same object of interest, give the other sides.

The angle between the long sides ( or strictly speaking half of it ) is referred to as the parallax angle. It is very similiar to standard surveying 'triangulation'. It only works if the said object is not too far distant, as the angle gets smaller and is harder to measure the further out the object is. For things outside the range of parallax deduction then you use some other measure(s) which overlap in some range - so one can calibrate one type of yardstick to the other. Hence you can still speak of the parsec measure for really long distances even if no parallax measurement actually applies. Now in a full circle you have 360 degrees, each of which can be finely divided into 60 parts - called arc minutes - which in turn can be divided in to another 60 parts - called arc seconds. Hence 1 arc second = 1/(60 * 60 * 360) = 1/1296000 of a complete circle...... pretty small huh?

For the Earth's orbit ( average about 150 million kilometres in radius ) then an object would have to be placed at a distance of 3.26 light years to have an evident parallax of one arc second. Thus 'one parsec' is a shorthand way of saying that.

[ Mind you this gives a linear scale, not an inverse one, so that 'two parsecs' is twice the distance of one parsec and NOT a parallax of two arc seconds, which would in fact give a distance of half a parsec .... ]

So now I'm talking 'light years'! A light year is the distance that light, at 300,000 kilometres per second, would travel in one year. This is 300,000 ( km ) * 60 ( seconds per minute ) * 60 ( minutes per hour ) * 24 ( hours per day ) * 365.25 ( days per year ) = 9,460,730,472,580,800 metres. I have used the International Astronomical Union's ( IAU ) definition of year length, see here.

The second nearest star ( our Sun being closest ) is one of the Alpha Centauri group, Proxima Centauri, and is around 4 and 1/3 light years away or about 1.4 parsecs. The 'thickness' of our Galaxy is approx. 1000 light years and say 100,000 light years in diameter. Andromeda ( next nearest ) Galaxy is 2 and a bit million light years away.

So back to the vertical scale is marked in Mpc - Mega-parsecs ie. millions of parsecs. Altogether the 'design' range of the LIGO's is roughly 15 Mpc = 45 million light years!

Phew! Nearly there! :-)

Now because big things make big waves, and smaller things make smaller waves then the 'detection range' refers to a standard type of event. This is a bit arbitrary in a sense, but has been chosen to be representative of expected detections. The exact definition needn't worry us too much, but see here and here if you are curious. So if the NS/NS Inspiral Range is at 15 Mpc then we expect to at least detect an event ( as defined ) out to that distance.

I do hope that is clear..... :-)

Also if the images are to large ( I am trying to keep well under 100Kb ) for dial-up users, let me know. I'm using mostly gif's which compress pretty well for diagrams and aren't overly lossy.

Cheers, Mike.

( edit ) Again, red is H1 ( Hanford 4km ), blue is H2 ( Hanford 2km ) and green is L1 ( Livingstone 4km ).