Detector Watch S6 V3

It's OK. First anniversary of our firestorm, so a busy time for a local doc ....

Thanks for asking! :-)

Cheers, Mike.

Now it looks like Livingston has had very good duty cycles and decent inspiral range. They've also had some trouble with data collection, and nicely managed by redundancy :

Now I've been reading somewhat avidly on the topic of General Relativity in the hope of understanding what's what. I'll share a minor epiphany with you, if you'll suffer it! :-)

I've been pondering on what is actually meant, in practical or theoretical terms, by spacetime "curvature"? What entity is it that is "bending" or "warping"? For that matter, what is meant by "straight"? Here's my run at it :

Start by throwing out, if you can, any ideas/preconceptions about either space and/or time. Set your mind to a blank canvas of nothing and then see how we can 'construct' a universe that is consistent with the principles of GR. You see we tend to be, naturally, fixated upon the everyday world. Thus we need to adjust to the notion that our usual experiences are in but one corner of the universe, and that 'foreign' notions may reign elsewhere but which none-the-less approximate quite well here. For this discussion ignore non-gravitational forces, lets see what gravity does alone.

Think of a particular point in space, at a particular time, but by itself and not described with regard to any greater framework of description ( co-ordinate system ). For definiteness say this point is when & where a particle collides with another, a so-called 'event'. It is thus a 'real' thing, not abstract. How do I get to a nearby point? ( there must be a nearby point, else the universe has no extension ). Refer locally to a set of vectors that exist for this point alone ( technically called the "tangent space" at the point ). These are pointers to nearby points. A bit like road signs at the main intersection of a village - there aren't lines along the road to follow, but a set of choices to make about 'where to next?' for reaching another close-by village. Trot along the chosen road to the next village. What now? Well there is another set of vectors again ( another tangent space ) to guide any further travel. Et cetera ..... the image in my mind is of the French bocage country where you can't see over hedges and whatnot to gain a 'distant bearing'.

We are used to vectors going between known points you see, according to some co-ordinate system. So if I have a point A and a point B, I draw 'a line' from A to B and stick an arrow or some such marker to specify which end of the line ( segment ) is the 'beginning' and which is the 'end'. This is the standard vector idea, which implies all sorts of things - like being able to pick the vector up and drag it somewhere else ( retaining it's qualities like length and orientation ) and having it represent the general idea of displacement upon a known/predetermined background.

Not so with these local ( point dependent sets of tangent ) vectors, which aren't displacements, aren't draggable, don't really have a length in the commonly meant sense, but effectively are road signs to navigate through space and time. For the mathematically inclined they are derivatives implying the notion of 'most direct route'. However there isn't a standard set of 'direct routes' that work for all places, in the sense that the use of a compass would offer. You have to use one and one set only for each and every point in spacetime.

This no doubt seems are rather obtuse way of going around things. Why bother with such a baroque approach?

Firstly this does work for our 'everyday' life. It implies all the things we enjoy about our intuitive Euclidean understanding of the world. I replace all traditional 'point to point' vectors with a trail of tiny little steps, and I iterate the same technique at each step. I'm at a point, I look up the tangent vectors for this point, select one and go that way to a nearby point .... at this new point I examine the tangent vector set, etc. Are we there yet? :-)

Secondly, I don't need to make any global statements. I'm only ever referring to local decisions.

Thirdly, and this is the true majesty of this approach, I can describe any 'reasonable' universe. The 'curvature' concept becomes a statement that I can have more than one 'direct route' between well separated events. So a transit between a 'Berlin' event and a 'Paris' event ( just names for this analogy ) can have several routes each of which are 'minimal'. I can set off in different directions from Berlin, taking a village-to-village navigation technique with each initial choice, and still wind up in Paris with no reason to 'prefer' one total route over another. In a classical sense I would say "parallel lines have met somewhere in the distance". Is this not curvature or warping or bending?

Fourthly, I can make global statements if I want. I examine various paths which differ slightly from one another - two 'close' paths near a black hole horizon, say - and see the result of such variation. This is a way of saying that I can construct a co-ordinate system, rather than assume one, by looking at the totality of spacetime paths. Reference frames then emerge as a result of applying local rules ( the tangent vectors especially ), not local rules deriving from a larger framework. The 'metric' of spacetime is the characterising of the variation of tangent vector sets as you move from point to point between events.

Just ignore this post if it doesn't make sense ..... :-) :-)

Cheers, Mike.

( edit ) As applied to a LIGO detector the two key events are :

(A) two identical photons separating at the beam splitter in the corner station, and

(B) the same two photons returning to the beam splitter having each experienced paths in a different interferometer arm

with the phase difference ( as evidenced by the probability of photo detector capture ) mapping the relative topology of the arm paths.

I've been having a look the 'glitch' business. The logs show a shift by shift glitch report, like this :

With a glitch being defined ( see here ) as :

The glitch reporting and study is overseen by the glitch group, a subgroup of the Detector Characterization Committee, with members plucked from various other groups that design, commission and operate the detectors or those that deal with the output. Glitches matter because some of the astronomical signals happen over a similar time span and we don't want any terrestrial causes mimicking that. Obviously if glitches can be reduced - because their origin is understood and correctable - then the IFO's may work better. But even if not that, then those data segments produced at times when glitches occur can be identified. Which helps in selecting better quality data for any analysis. There would be more than a few non-correctable causes, though. Power supply problems, earthquakes, someone stubbing their toe .... :-)

My feeling is that glitches aren't anywhere near as important for the continuous wave sources that we are trying to detect, compared to other efforts. We integrate over rather longer periods than a few seconds so glitches would be somewhat less likely to fool our searches - particularly if they have no real periodicity.

I wonder if there may be some analogy with chlorophyll here? No single photon can/will trigger a chemical reaction, however the electrons run up and down a band within the molecule absorbing successive photon energies. The reactive focus is where that is released. Think of a kettle on a slow heat, it will eventually boil on account of accumulated input. Or the boiler with a safety valve on a steam train. So ( recalling an earlier discussion of resonance ) an interferometer may simply have mechanisms of sudden energy transport, at certain thresholds, exhibiting as glitches. Resonant bar detectors are known to integrate effects in that way. Which is not only bloody annoying, but would make the glitch events slaved to history ..... that is, the response to a signal depends very sensitively on the state of the detector prior to it's arrival.

Now while the surface of the Earth is evidently an especially noisy place, LISA could still suffer rather too. Far more high energy particle interactions for starters, not having the benefit of any of Earth's magnetosphere shielding it.

As for GR : while we may well have this 'nearby village signposts' business, and the Einstein equation ( well a group of 16 actually, but only 10 are independent ) to specify how they ought behave one-to-another across our spacetime landscape, we are barely into the problem at all. GR can describe entire universes other than this one, and not simply different parts of this one. So that becomes more than a matter of expertise at solving or approximating solutions, as clever as one can be, but a more wider concern of initial conditions. What you start with certainly determines what happens later on. GR can't tell you what those initial conditions are, only how stuff evolves. In any case GR is classical so really has nought to say about extremely high density/energy/quantum scenarios - except to label them as singularities. Like the Big Bang or the center of a black hole. Ironically Einstein loathed the quantum concept, and notably probabilistic interpretations, that he helped deliver to us. So no surprise GR has no quantum component, not-withstanding having done spectacularly well at those non-quantum scales it has been tested upon.

Cheers, Mike.