Paper is publicly available
Physical Review Letters is behind a paywall
The authors list is 3 pages long... good luck to the Nobel committee to pick 3 people out of that for the prize.
A 15-year experiment using some of the most advanced technology known to Man has picked up the first detection of a gravitational wave, the first direct measurement of black holes, and the first direct evidence of binary black holes. It has also opened up an entirely new field of astronomy. The signal was picked up on …
I read about it at www.physicsworld.com and picked up the paper from journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102 with no paywall. (And that's my first use of links in El Reg.)
Recorded livestream from an unknown point in spacetime: here
The actual livestream sites now show only static. Hopefully a blackhole didn't decide to visit.
And a sad look at A. Zee's "Einstein Gravity in a Nutshell" in my library which I probably will never have the occasion to read.
Thanks for the link - the Ligo one worked for me. My highlight from a quick scan: peak luminosity (in gravitational energy) of 3.6×10⁴⁹ W (that's 10^49). If this had been in electromagnetic form, it would easily (though briefly) have outshone everything else in the visible universe put together. I wonder what the effect would be on anything reasonably 'close' - say a few light-years?
Edited to say, I guess that means a billion times closer, so a billion billion times stronger, which means a full metre displacement on a Ligo-style detector. It would give any nearby planets a substantial jolt (unless I've misplaced a dp somewhere).
Of course it's a tidal effect. That's what LIGO measures, right? But not in an N-body system, but in a self-sustaining wave.
Assume for simplicity the energy in the gravitational wave is fully deposited in a body blocking its path with a surface of 1m² orthogonal to the radiation vector at 1 ly distance. How much energy is that? Should be easy to compute...
...it's not due to the mirror moving - it's due to the compression / expansion of the fabric of the universe caused by the gravitational wave making one of the laser paths in the 'L' shape a different length to the other, causing the interference pattern to change.
So the detected change is a direct result of the compression/expansion of the fabric of the universe as the gravitation wave passes through the detector site.
Not nitpicking, just trying to wrap my mind around the whole thing: if a gravitational wave passes through the detector, in other words through the path of the laser beam, and that wave compresses spacetime (which I understand to be a reversable/temporary effect) between the laser and the mirror - wouldn't that mean that, from the perspective of the laser, the mirror did move?
Anyway, I'm utterly amazed at the engineering that must have went into the detector. Filtering out noise and environmental factors alone... I did some experimental stress analysis using laser interference around 1990 at university. You can detect really small dimensional changes with that, but setting up the test rig was always a pain.
I suppose (and could be very wrong) but is this not the same thing with the article "Australian astroboffins reveal hundreds of hidden galaxies" where they write that "our whole Milky Way is moving towards them at more than two million kilometres per hour". Is the Milky Way moving or is it not rather space that expands. "Newtonian" is easier to understand and explain I suppose.
Is the Milky Way moving or is it not rather space that expands.
The movement of the Milky Way is measured by checking relative velocities of all the crap around it out to a billion LY or so.
"Space expanding" is something else, it just means everything is moving away from everything else by sheer inertia (in imagery along the fourth dimension, like the coins on the expanding balloon moving along the third dimension which doesn't actually exist on the balloon surface). If the coins exhibit mutual attraction, there can be serious movement on the balloon surface (plus local crumbling)
I came to the comment section with this in my copy-and-paste buffer: "mirrors at the end of the tunnels move". In other words, I spotted the same possible wording issue. Minor of course. And debatable.
So... How come the laser beam itself isn't shrunk and stretched in precise lock-step with the 4-km long tunnel?
Must be back to the constancy of 'c'...?
"and the shift in phase can only occur if the actual space itself is contracted then expanded."
Yeah, but... if the fabric of spacetime itself is warping then the distance experienced by anything in that spacetime - including photons - shouldn't actually change because all the contents of spacetime are warped along with it. Otherwise you're saying that new extra spacetime appears out of nowhere then disappears again which I presume is not the case.
Otherwise you're saying that new extra spacetime appears out of nowhere then disappears again which I presume is not the case.
We're in the territory of metaphysics here. But yes, that's what happens. Spacetime is "created" and "destroyed".
Actually, relativity says nothing about what spacetime actually is; it just says the distances increase and decrease, albeit by the width of a proton's hair.
warping then the distance experienced by anything in that spacetime - including photons - shouldn't actually change
Light waves, lets avoid photons since this is classical physics, are special. Their velocity is constant. So if the distance changes, the frequency of the light must change to keep the speed constant. The most dramatic example of this is the cosmic microwave background. These electromagnetic waves were emitted as gamma rays but so much space has been "created" in the intervening years that they're now microwaves.
Another way to think about this is as conservation of Energy. The quantity of energy in the wave doesn't change. But it's spread over a greater or lesser volume; i.e. the energy per unit volume varies. And by E = hν that produces a change in frequency. That's why I say light waves have been "stretched", but space has been "created".
"Their velocity is constant. So if the distance changes, the frequency of the light must change to keep the speed constant"
Bit what is distance? If you had a ruler it would be warped along with any other matter so you wouldn't see a change in distance.
"so much space has been "created" in the intervening years that they're now microwaves."
But the expansion of the universe "creates" extra space like filling a bathtub with water. Whereas gravity waves surely are more like waves on the water already in the tub.
> So... How come the laser beam itself isn't shrunk and stretched in precise lock-step with the 4-km long tunnel?
Thank you! That's exactly what's been confusing me. Can anyone please point out the no doubt obvious explanation for this?
> > So... How come the laser beam itself isn't shrunk and stretched in precise lock-step with the 4-km long tunnel?
> Thank you! That's exactly what's been confusing me. Can anyone please point out the no doubt obvious explanation for this?
Isn't it simply because the beams are at 90 degrees? Imagine a gravitational wave approaching exactly along the line of one of the arms: that beam will be stretched along its whole length; the other beam is stretched across its width and - crucially - not along its length. It is this difference which is being detected.
No - it is the time-dilating blip that is indirectly observed. Changing the shape of the fabric of space also changes the shape of what is in it, including the photons, mirrors and tunnels, and the interesting additional note is that there is no change to the light frequency.
"Boffins' gravitational wave detection hat trick blows open astronomy"
I am not exactly 100% sure that a confirmation of relativity in some way "blows open astronomy". Quite the contrary, in fact.
The gravity wave won't have a redshift correct? I wonder how they knew the fairly specific mass sizes involved and the distance. Guess will have to wait for smarter people to explain it me as I am sure the math is going to be impressive. All around congrats to all the boffins for giving us a new tool. Also Congress get off your butts and fund the gravity wave detector in space NASA has mentioned before.
Pretty sure it does have a redshift. If it goes at c it must show all the phenomena exhibited by the electromagnetic field (the classical description of which has practically the same mathematical form as the "gravitational field" but doesn't involve varying distances along "time" IIRC).
The New York Times allows us to read a few articles without a subscription, but after a given limit -I think it was three articles the last time I bothered to check- they smash our faces with the paywall. So either you have read other NYT articles in the last month or so or some other user of your computer has.
I like to think I'm a pretty intelligent bloke, but reading articles like this and trying to comprehend the sort of minds who can conceive of science that can measure a 1,3 billion year old wave deflecting a laser beam by a ten-thousandth of the diameter of a proton makes me feel like a plankton, by comparison.
... in a good way.
Thank God the human race has at least a few people left who are dragging us in the right evolutionary direction.
Thank God the human race has at least a few people left who are dragging us in the right evolutionary direction.
Unfortunately, there is no "right evolutionary direction", there is just local optimization in a changing landscape. It can go downhill fast, so to say. The "Whig Theory of
HistoryEvolution" does not apply.
Lady luck smiled on us. Better be fast before she FROWNS again.
> Have we found aether :)
No, just another type of wave travelling through the same non-medium.
Michelson-Morley validated special relativity (before Einstein even published it).
LIGO validates general relativity.
It would have been much more spectacular if they hadn't detected gravity waves, with the sensitivity that LIGO has. That would have been like not finding any Higgs boson.
But what most people forget is that electromagnetism is a fully relativistic theory.* It says the speed of light is constant and that the strength of the electrical and magnetic field depend on your velocity. This was understood at the time and was a reason for thinking Maxwell must be wrong. IIRC it part motivated the search for the aether.
Einstein's moment of genius was to accept Maxwell's prediction that the speed of light is constant, add in one other eminently reasonable postulate (the laws of physics are the same in all inertial frames), and figure out the maths to make it work. But the ideas were all there. He just tied the bow.
You falsify Maxwell, you've falsify relativity.
* Electromagnetism is so relativistic it can be merged it with General Relativity to produce a theory of five dimensional spacetime that explains gravity and electromagnetism.
The phrase "Michelson-Morely" is used in the paper. Repeatedly I believe. In the same context you're referring to.
The detectors wouldn't work if the 'aether' was noisily drifting past. They'd never get 'lock' under such conditions.
.: They've conclusively proven there's no aether, as if it was still an open question.
DL the paper. It's lovely.
As the direction of the source is not known all that can be calculated is a maximum speed (direction of propagation is aligned along the speration of the two detectors. Four detectors are really needed unless each Ligo itself can give some information on the diretcion of the wave.
"Four detectors are really needed unless each Ligo itself can give some information on the diretcion of the wave."
They may just be able to give an insight on direction because each LIGO structure is two-dimensional (the L-shape mentioned). Plus we don't know how each LIGO structure is oriented relative to each other, which could help in terms of orientation of the detected wave.
"But...no one has yet established they travel at the speed of light"
Yes, as far as I'm aware this was a pretty contentious argument throughout the 20th century, with no-one finding any concrete evidence either for or against the idea. However IIRC an experiment in the early noughties involving the observations of a quasar as it passed behind Jupiter found strong supporting evidence that gravitational effects propagate at approximately the speed of light, though I think it was also argued that what was being measured was actually the speed of light, and not gravitational waves. Hopefully these latest experiments will help clarify exactly what the hell gravity is and how it works.
On a less serious note, I wonder if the boffins involved have been the very first people in history to hear the whalesong of the universe, and if so, should I start investing heavily in really, really big joss sticks...
Hopefully these latest experiments will help clarify exactly what the hell gravity is and how it works.
Unfortunately the article at Quanta:
invites you to also read
which indicates in no uncertain terms that we are very unsure what spacetime actually is. Looks like some kind of patch cabling. Or a database. Or maybe God is just using a symbolic processor and only gives out numeric results if HE absolutely must. Or this universe is filled with ghosts. Who knows!!
Which is why physicists prefer not to think about it and stick to systems of coordinates. "In what" is a metaphysical question.
Well, systems of coordinates are evidently artificial though practical, so the quest to find the (most compressed?) model of spacetime covering all empiricial evidence (and beyond), in particular, naturally giving birth to the Standard Model, goes on. This isn't a "metaphysical" quest.
Saying that "physicists prefer not to think about it" is inherently wrong. It's engineers who prefer not to think about it.
See also this writeup: Space is Meaningless
The postmodern unobservable and extravagant "multiverse" sure is metaphysical, or worse: It sounds like an alibi for being impatient and unwilling to do the hard work while still staying in the limelight.
From how I understand it, the US based detectors wouldn't detect a gravity wave approaching from directly above the United States because it would affect all four arms of the two detector equally; or at least with a much lesser difference than a gravity wave approaching from the horizontal direction.
About a quarter of the way around the Earth would be the ideal place to have another detector and India seems to fit the bill for that.
"About a quarter of the way around the Earth would be the ideal place to have another detector and India seems to fit the bill for that."
Ummm India is more like half way around the earth from the US. They don't call it the antipodies for nuthin'. The antipodies of the US is actually somewhere in the Indian ocean, but you get the idea.
I heard the recording of this event replayed on the Beeb 5 o/c news.
Two black holes swallow each other ...
If they'd had the patience to wait and leave the machines on for a few more seconds they might have heard >Burp!<.
2 black holes cosy up to each other in deep spaace.
1st black hole: "Sighh!"
2nd black hole: "Did the universe move for you too?"
Seriously - Incredible bit of engineering to get a result from that kit through all the noise here on earth. Bet there was some serious signal processing there to pull the signal out of the noise floor. I hope they get some more results just to prove that this wasn't some anomaly caused by something here on earth. We really need the wider scientific community to tear this experiment apart (in a good way) to help improve it and the harvest of knowledge it will hopefully lead to.
Now if the devices can be re engineered to be built an space as I believe they hope to do ....
Optical lasers wavelengths are in the few thousands of angstroms (angstrom = 10^-10 meters). But a proton is 10^-15 meters in diameter, and 1/10,000 of that is 10^-19 meters, or one billionth of a photon of visible light.
So, how can you use such large photons to see such a small change in length?
Also, how do you know the size of the black holes involved, at such a distance?
The paper itself is an easy read. Anyone with even a passing interest should review it.
The 'strain' (warp) is 10^-21. The signal is certainly in the noise. The paper explains how they did it.
I thought that the signal was on the order of the diameter of a proton (10^-18), not "1/10,000" that.
km-scale (10^3 m). 10^-21 strain (warp ratio). Proton (10^-18).
+3 scale, subtract 21 orders of magnitude = -18 scale
Did I miss a memo?
"Also, how do you know the size of the black holes involved, at such a distance?"
Computer modeling to match the waveform of the signal.
In fact, they needed to predict the waveform in advance to help find it.
But this signal was big enough that they found it as an arbitrary transient too.
There's a lot about this that seems a little too precise. Two black holes collided 1.3 billion light years away, and we know how much energy it put into a wave we have never measured before?
Seems more likely that some kid was banging mud off of his shoes in Fish Kill Montana. Meanwhile we are on a planet with a thin rocky crust floating on a soft flexible core, the orbit of our moon is causing very strong gravity waves moving millions of tons of water every second, pulling and pushing on the tectonic plates, causing them to flex constantly grinding the boundaries of said plates.
If the gravity of our moon can cause such distortion visible to the naked eye, how come that is not what we measured?
"the orbit of our moon is causing very strong gravity waves "
The moon is having a measurable effect by gravity on our planet but any gravitation waves produced by the moon are miniscule.
gravity != gravitational waves
What you propose to measure is a water wave, not a gravity wave. Gravity wave is a "a change" in gravity force, or even better, a change (dynamic, propagating) in the the "shape" of the time-space. Like shrinking and stretching of the peek traffic on a highway. Yes, your moon does cause gravity waves hitting earth, but with a frequency of 2 per day. Does not make for good study. Kind of like measuring the speed of light with your torch and a handheld stopwatch.
A kid in Montana could explain things, if the identical signal was not detected at the other end of the continent.
Results are too precise? They pre-calculated several thousands of possible wave patterns the black holes could cause, in variety of scenarios. Among those patterns they found the one best matching what the detector, well, detected. Then they tweaked the inputs (masses, distances and rotation frequencies, and other stuff I don't understand) into their pattern producing model till they got the near perfect match. And voila. And it's not really that precise. Some things (like spin of one of the black holes) they can't say match about, many different values can match the produced pattern.
Of course the model they are using might be flawed. Of course you can dispute this, but you better know your stuff (and there's crapload of it) if you want to do that.
Little comment has been made (so far) about the fact that this event showed up fairly soon after switching the device on. (I think I'm right in saying they were still commissioning it and weren't officially in "observing" mode yet.) That would suggest that detectable events like this one are fairly common, which is encouraging news for those trying to get funding in this area.
"Little comment has been made (so far) about the fact that this event showed up fairly soon after switching the device on."
This got mentioned in the press conference. They're pretty optimistic that the thing will pick things up quite regularly. It's a big universe with a lot of possible black hole binaries.
Speaking of which - this detection was also confirmation that such things actually exist (or "did" until these two merged) and that's another first. Up to now they've been purely theoretical.
The LIGO result does put an upper bound on the speed of gravitational waves - the two interferometers (one in Washington State, the other in Louisiana) are far enough apart that light would take about 10 milliseconds to travel between them (The great circle distance is roughly 3000 km, and light travels at very close to 300,000 km per second - I'm not bothering to work out the straight line distance rather than great circle [if you want to work it our yourself, the Earth's circumference is 40,000 km, near enough])
The signal, seen by both detectors, was offset in time by about 7 milliseconds. If the event occured exactly on a line extended between the two interferometers and out into space, this means the maximum possible 'speed of gravity' is roughly 1.4c as the distance between the detectors is the maximum possible difference in path length for the signal (to a first order approximation). If the event occurs at any point on a plane bisecting the distance between the two interferometers, the difference in path length would have been zero, and we would have no information on the maximum possible speed of gravity.
There are very good theoretical reasons for expecting the 'speed of gravity' to be the same as the speed of light, but I believe this is the first direct measurement/test. (There are indirect methods, see https://en.wikipedia.org/wiki/Speed_of_gravity#Possible_experimental_measurements).
When more detectors come on line, we will be able to get better experimental bounds (using this direct measurement method) on the value. Very few people would expect it to to differ from c, but it is always worth measuring to check.
Note, the detector being built in India is being built there because the Indians agreed to pay.
I will quite happily concede that my understanding is limited in this area, but as far as I know an Alcubierre drive would work by causing massive distortions on spacetime to construct the warp field needed to propel the craft. Does anyone here know enough to hazard a guess at what kind of gravity waves would be created by such a distortion moving at superluminal speeds? Just out of curiosity, of course :)
So we could re-configure a LIGO-array as a speed trap for alien ships and levy fines? Neat, further research would pay for itself.
(Hey, I've just had a great idea for planetary defense: when alien ships appear a la Independence Day - just give them a parking ticket, they'll be gone in no time. No one defeats a meter maid.)
Everyone else here has summed up my feeling on the discovery - and the preliminary thinking that went into this pre-discovery - far more eloquently than I'm capable of on a Friday morning. However, I've got to say my favourite part of the article was this:
As part of their jobs, four members of the team have to try and introduce faults into the signal, and all four said they weren't responsible for the signal.
That must either be the most fun job in the world, or the most annoying...
Scientist 1: "I've done it! I've found a gravitational wave to a 5-sigma certainty! Now, just to check to make sure it was the 'Red Team' interference..."
'Red Team': "Ah, no, sorry. That was us again. Sorry about that."
Scientist 1: "Well just fuck you guys."
What's *really* extremely cool about this discovery is that gravitational waves will allow us to observe the opaque early universe (pre recombination). Until now we can only observe back to about 400,000 years after the big bang (post recombination). We will, in theory, be able to observe the singularity itself.
Doesn't WMAP already kinda do this by mapping out anisotropies in the microwave background radiation?
It will take some time to have a gravitational eye with any sort of resolution able to do the same for low-frequency / low-energy gravitational waves. Maybe the galactic empire can look into that..
The BBC "Expert" told the news presenter, condescendingly, that science was wonderful because now we had discovered this it could affect her life, but before we discovered it, it couldn't - that - he said - was the power of science.
Tell that to James Watt or Richard Trevitick, who invented the steam engine without knowing the underlying science, or Darwin, who worked out evolution without knowing about DNA.
It makes me wonder what you have to do to be a BBC Expert - it certainly isn't understand your subject.
When gravity waves, it's just waving goodbye.
I'm afraid we must part with particles,
Einstein's beach has very minor ripples.
(wasn't that a cheap booze we used to imbibe?)
If we catch that wave, can we hang ten by dint?
(or just find a toehold, too small to mount)
I've seen things that some of you people'd astound, like
Gravity's Rainbow off Orion (if you really really squint)
I worked with the Glasgow team as a Postgrad <mumble> years ago and so I am thrilled that this has finally come to fruition. I realised during my studies that I didn't have the patience required to make a career of it (along with other limitations).
I was interviewed by Ron Drever for the post (and worked with him briefly before he moved to Caltech) and I'm desperately sorry to hear he he may no longer be capable of realising his work was so successful.
This laser interferometer array can detect the very, very, very long wavelength ripples in space-time from massive events like the collision of two, or more, black holes. If the array is extended will it be able to detect ripples from even earlier events, such as the moment of inflation of the known universe? Further, will scientists be able to detect ripples returning from a journey to the edge of the universe and back again - echoes as it were, like ripples bouncing back from the edge of a pond/puddle?
Lastly, and with a pinch of humor, given enough notice of the immanent arrival of the next wave, should I prepare a long board with lashings of sex wax and persuade the Falcon X team to provide a lift, as it were, to where I can perform a zero-gravity cutback on this big old gnarly?
Can someone educate me please?
If I dropped a pebble into a swimming pool, I would generate ripples that head outwards from a central point and eventually they 'disappear' once the energy has 'run out'..
How the buzz are we able to detect 'waves' generated over a billion years ago?
I'm well out of date here (and the memory is foggy) but this analogy may be helpful (or completely wrong).
If you drop something into a pool the ripples spread out until they hit a boundary, then they reflect, rinse and repeat until the energy has been evenly distributed around the pool, the boundary, etc. No more ripples.
These waves haven't hit any boundary, they haven't reflected. They suffer from the dissipation of energy as they move from a 'point' source into a larger and larger volume but they are not absorbed, reflected or otherwise homogenated (word?) before reaching us.
1. The energy doesn't run out it just gets spread progressively thinner (at least for light in a vacuum and gravity waves that's were your model falls short).
2. The same analogy applies to light and yet we manage to see distant galaxies.
3. It helps to have a very energetic source and a very sensitive detector.
Gravity waves spread out in all directions following a standard inverse-square law for power dissipation. By the time it reaches us after 1.3 billion years traveling at the speed of light it has spread to such a vast sphere that it's a very tiny signal. Working backwards they can determine that the wave was created with a burst of energy 50 times greater than the entire rest of the visible universe was producing at that instant. It basically consumed 3 x our sun's mass in an instant, converted to energy using E=mc2.
It basically consumed 3 x our sun's mass in an instant,
Various reports seem to have muddled this aspect.
I'm not sure that the process 'consumed' mass, certainly not in the form of a nuclear reaction or suchlike as some reports seem to suggest. Wasn't it simply that the kinetic energy which the two black holes had accumulated as they attracted each other to travel at something like half the speed of light went into rotational kinetic energy when they got close. Much of this was then radiated away as gravitational waves as the pair of them did their final twirls in a closer and closer embrace before they coalesced.
The radiated energy was equivalent to three sun's worth, but the mass that was lost was the relativistic increase in the black holes' masses due to their high speeds, which they gave up when they stopped each other, rather than conversion of the matter that comprised them.
The radiated energy was equivalent to three sun's worth, but the mass that was lost was the relativistic increase in the black holes' masses due to their high speeds, which they gave up when they stopped each other, rather than conversion of the matter that comprised them.
Yes, because the black hole has no matter. It's just a horizon. (If spacetime were a block of wood with constant time (only locally valid) 2-D slices, the black hole is the edge of the tunnel the carpenter made with a bore, right?)
So, we have:
Energy of outgoing wave generated during merger and (mainly?) merged black hole settling
mass/enery of "settled" merged black hole (... comprising rest mass, energy in angular momentum ... don't know how much of it in the surrounding 'dragged spacetime' ... and energy in linear momentum + error terms for non-zero charges)
Mass/energy of the incoming black holes and overall two-blackhole bound system (see above)
3 solar masses of m*c² (indicating how hard it is to give spacetime the wobbles)
some error terms to cover for "but doctor, I can't compute that" extremely messy gravitational radiation emitted while merging
@ John Mangan - Would it be correct to assume therefore that the older the waves, the longer the wavelength, due to the fact that space-time (the universe) continues to expand from origin and therefore stretches everything out - the oldest waves having been stretched the most?
@ Alan Brown - Thank you for that.
TIL: Space time is gigantically wibbly-wobbly, but sadly not raspberry flavoured (yet).
Scott, bear in mind that I am working from near thirty year old memories here but broadly yes.
I remember seeing graphs with a range of postulated sources; Big Bang, supernovae, merging black holes and others thatI've forgotten. Each had predicted ranges for strain and frequency. I'm pretty sure that the Big Bang was low frequency and amplitude because of the elapsed time/intervening expansion of the universe. The longest wavelength/lowest frequency waves will require a space-based detector with 'arms' thousand of kilometres long, see LISA.
I'll have to dig out my thesis to see compare how the predictions from back then compare with the current thinking on the subject (if I can still understand any significant fraction of it).
Other stuff I remember is that the gravitational waves were quadrupoles with two polarisations usually represented as + and x (similar to photons with l and - ). The 'best' signal would be with the wave travelling perpendicular to the detector (up/down into the ground) with the polarisation aligned with the two arms. This gives maximum 'stretch' to one arm with maximum 'squeeze' to the other alternating as the wave propagates.
I understand that the wide physical separation eliminates local sources when they see the same signal at almost the same time in both detectors, but how do they eliminate the possibility that the vibration originated deep in the earth at a point nearly equidistant from both detectors? After all, we're talking about very, very tiny signal. Surely the earth produces its own wide range of grumbles at depths all the way down to the core. Maybe one of those just happened to look like a merging-black-holes "chirp". Probably the reason they used the word "chirp" to describe the signal is that it's a familiar pattern produced by many phenomena.
While we can’t stop the world and its inhabitants from causing vibrations, what we can control is LIGO’s responses to these environmental disturbances. We do this through the use of hundreds of levels of feedback and control systems, which maintain all of LIGO’s parts in near-perfect stillness in the quietest man-made environment on Earth.
Yes, I understand that they did a lot of work to reduce and nearly eliminate local sources of vibration in what must be one of the most sensitive vibration detectors on earth. But the fact that they spent so much time and effort working on it shows that it's not easy, and there's no single clean solution to it. The argument that vibration transmitted through the earth from a site equidistant from both detectors wouldn't be strong enough to travel that far without being detected by other sensors is plausible, but I'd like to see data supporting that. However, I've just finished reading the paper and I see that in section IV they wrote specifically about this topic and state that the environmental sensors should be sensitive enough to detect any vibration in the same magnitude range as their gravity wave signal, and in fact the level of noise detected by the environmental sensors at that time amounted to no more than 6% of their signal magnitude.
The mirrors are suspended to avoid vibrations of similar frequencies. So if it came from the Earth crust, it must have been pretty strong to shake two facilities 2000 km apart. So why it was not recorded by seismometers? And with a frequency of up to 75 Hz, we would actually hear this continent-wide chirp. Finally, seismic waves of acoustic frequencies don't travel far. That's why we do not actually hear them.
As per above. If local physical interaction is ruled out, then gravitational is suspected.
I guess it's like looking at a pool. If you see a boat causing a wake, then that is obviously not the tide. The wind can be seen separate from the ripples from a stone. But of cause a lot of checks need to be done to make sure it's not someone diving under and playing tricks. ;)
nano- earthquake that moved the mirrors half a wave length from each other?
This is not how LIGO works (well, kinda, but there is more).
Plus, two "nano earthquakes" at the same time in different locations (whatever "nano-earthquakes" are, stop making stuff up) giving an expected signal and not flagged as error sources? I don't think so.
It's quite readable. Even a interested child could understand most of it.
Opens as a pdf.
It's only half as long as it looks, due to the vast number of 'authors', their organizations, and endless citations.
I once applied to work at the LIGO in Washington. It is quite near where I live. The position I applied for was, for all intents and purposes, a very high tech dishwasher. All the parts that go into the vacuum area have to be extremely-high-temperature and low pressure washed to get them to shed as much material as they can so that they don't release that stuff into the vacuum and ruin results. I didn't get the job, but the interview and tour that came with it was very interesting stuff.
Bullialdus observatory, Moon
24 046 104 526 757 CST
2 April 2996, 16:42:03.911 UT
... Bullialdus was a gravitational wave detector, part of a solar-system-wide observatory known as TERAGO. A single laser beam was split, sent along perpendicular journeys, then recombined; as the space around the crater was stretched and squeezed by as little as one part in ten-to-the-twenty-fourth, the crests and troughs of the two streams of light were shifted in and out of alignment, causing fluctuations in their combined intensity which tracked the subtle changes of geometry. One detector, alone, could no more pinpoint the source of the distortions it measured than a thermometer lying on the regolith could gauge the exact position of the sun, but by combining the timing of events at Bullialdus with data from the nineteen other TERAGO sites, it was possible to reconstruct each wavefront's passage through the solar system, revealing its direction with enough precision, usually, to match it to a known object in the sky, or at least make an educated guess.
... As well as recording any sudden catastrophes, TERAGO was constantly monitoring a few hundred periodic sources. It took an event of rare violence to produce a burst of gravitational radiation sufficiently intense to be picked up halfway across the universe, but even routine orbital motion created a weak but dependable stream of gravitational waves.
... Lacerta G-1 was a pair of neutron stars, a mere hundred light years away. ... G-1a and G-1b were separated by just half a million kilometers, and over the next seven million years gravitational waves would carry away all the angular momentum that kept them apart. When they finally collided, most of their kinetic energy would be converted into an intense flash of neutrinos, faintly tinged with gamma rays, before they merged to form a black hole. At a distance, the neutrinos would be relatively harmless and the "tinge" would carry a far greater sting; even a hundred light years would he uncomfortably close, for organic life. Whether or not the fleshers were still around when it happened, Karpal liked to think that someone would take on the daunting engineering challenge of protecting the Earth's biosphere, by placing a sufficiently large and opaque shield in the path of the gamma ray burst. Now there was a good use for Jupiter. It wouldn't he an easy task, though; Lac G-1 was too far above the ecliptic to be masked by merely nudging either planet into a convenient point on its current orbit.
Lac G-1's fate seemed unavoidable, and the signal reaching TERAGO certainly confirmed the orbit's gradual decay. One small puzzle remained, though: from the first observations, G-la and G-1b had intermittently spiraled together slightly faster than they should have. The discrepancies had never exceeded one part in a thousand—the waves quickening by an extra nanosecond over a couple of days, every now and then—but when most binary pulsars had orbital decay curves perfect down to the limits of measurement, even nanosecond glitches couldn't be written off as experimental error or meaningless noise.
With a martyr's sigh, Karpal touched the highlighted name on the screen, and a plot of the waves from Lacerta for the preceding month appeared.
It was clear at a glance that something was wrong with TERAGO. The hundreds of waves on the screen should have been identical, their peaks at exactly the same height, the signal returning like clockwork to the same maximum strength at the same point on the orbit. Instead, there was a smooth increase in the height of the peaks over the second half of the month—which meant that TERAGO's calibration must have started drifting. Karpal groaned, and flipped to another periodic source, a binary pulsar in Aquila. There were alternating weak and strong peaks here, since the orbit was highly elliptical, but each set of peaks remained perfectly level. He checked the data for five other sources. There was no sign of calibration drift for any of them.
Baffled, Karpal returned to the Lac G-1 data. He examined the summary above the plot, and sputtered with disbelief. In his absence, the summary claimed, the period of the waves had fallen by almost three minutes. That was ludicrous. Over 28 days, Lac G-1 should have shaved 14.498 microseconds off its hour-long orbit, give or take a few unexplained nanoseconds. There had to he an error in the analysis software; it must have become corrupted, radiation-damaged, a few random bits scrambled by cosmic rays somehow avoiding detection and repair.
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