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Comment by lpage | Hacker News Reader

For those curious about the future of LIGO...

At present there are two LIGO facilities - one in Hanford, Washington and another in Livingston, Louisiana. This is necessary for both denoising (it's unlikely that a seismic event or random perturbation would effect both simultaneously) and triangulation via parallax.

Right now having just two facilities (that are relatively close together) limits localization to broad regions of the sky. Additional facilities are under way/in discussion for Europe, Japan, and India. This would significantly improve both the sensitivity of the array and its ability to localize events in a smaller region of the sky. Hopefully these projects get funded. LIGO stands to resolve some of the biggest open questions we have in cosmology.

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There is also an effort to get these detectors in space with eLISA: http://physics.aps.org/articles/v9/63 This will be able to detect wave frequencies that would normally be drowned out by terrestrial sources.

terrestrial sources of gravitational waves?

Terrestrial sources of run-of-the-mill vibrations. The detector can pick up both.

I recommend the two animations on the wiki page of eLISA, they are worth a thousands words to understand the (crazy!) orbit of this contraption:

https://en.wikipedia.org/wiki/Evolved_Laser_Interferometer_S...

Wow, zero-drag satellites are a really elegant idea!

https://en.wikipedia.org/wiki/Zero-drag_satellite

You must see umbrellas and parasols as elegant as well.

A precursor concept of a zero-drag satellite can be seen in Stargate SG1: "The Serpent's Venom", where an armed mine from an orbital minefield is brought aboard a shuttle, and the pilot must match the mine's trajectory changes.

"The Drag-free Satellite", the paper referenced in the Wikpedia article is from 1964, slightly older than SG1.

Also, your comment would be much nicer without the first sentence. And if you think about it, umbrellas are pretty neat. The folding mechanism is nifty, if you ask me.

look at this idiot, he can find beauty in elegant mechanisms!

It's so beautifully elegant to hold it aloft with your outstretched arm.

Wow, I didn't think it was possible to have three orbits that maintain an equilateral triangle like that. (I'm assuming the satellites only need to thrust against smaller influences like solar wind.)

If you like that orbit, look into the hopf fibration!

They should build one on the Moon.

Moonquakes exist too... I'd naively expect a lot less noise in space, and you don't have to worry about moon dust gumming up the works. And you can just lob three satellites up one at a time and arrange them more or less as you like, rather than having to deal with intervening geography.

One of the opening scenes from the transhuman adventure novel Diaspora by Greg Egan involves a robot on the moon tending a large laser interferometer which observes two inspiraling neutron stars. Perhaps my favourite book.

Egan's Quarantine had me checking out dozens of books about Quantum Mechanics while I was in High School. Love Egan's work, even if I had quite a bit of trouble with Diaspora (I probably just need to give the audiobook a second listen).

Love Diaspora, but it is really more of a philosophical thought experiment that drops you into two worlds rather than a novel with a plot. Half the book is geometry!

One of my favorite books as well. Every time I've gotten to the ending (especially the part where they discover the massive sculpture through trillions of universes) my mind is blown and I end up sitting and thinking about it for hours.

I have a similar reaction to Egan's Permutation City when the couple living in the simulation adjust the clock rate so the computer running them only executes one clock tick every second, every day, every year, every million years, every billion years, every trillion years, etc. Puts me in a contemplative mood.

But bad things happen on Earth after they coalesce. But that was in our galaxy. I guess it's just that these detected events are much farther away. Right?

The Moon also has seismic waves, albeit less than Earth. seismic waves have similar waveforms and frquencies as gravitational events. However the delays between two detectors is much larger for seismic since they propagate 50,000 times slower than gravitational waves.

There is a current space experiment to measure the behavior of two elisa nodes. They are only a foot apart instead of a planned million kilometers. The purpose is a low cost in situ test of instruments. And also to survey the types of in situ instrumental and evironmental noise. It took decades to reduce LIGO noise to acceptable levels.

One thing I don't understand in your post: given how ridiculously far away these black holes are, how could you possibly even dream of triangulating them with two facilities on Earth via parallax...?

You don't definitely don't, you only get angular information for exactly the reason you say. The distance information comes (I believe) by first determining the masses, and then computing how fast the energy falls off with distance.

The localization isn't performed through parallax, it's done through triangulation and time-of-arrival. For a pair of observatories, a difference in the arrival time of a signal (moving at the speed of light) defines a circle on the sky of possible source locations. With three observatories, there are three circles on the sky, which intersect at a single point.

The Virgo detector, outside of Pisa, is in the final stages of installing its advanced instrumentation. With LIGO+Virgo a source like the first detection, from September (a very loud event), could be localized to a patch of sky about 10 square degrees in area.

> 10 square degrees in area

Wow, that's much bigger than I expected. So basically we can only determine the general direction?

Not so bad as that, but it is a large area, and it's a challenge for optical astronomers to detect a faint source in such a large patch of sky. It's about the size of your two palms held at arms length. (Or, 40 times the size of the full moon, but that sounds less optimistic.)

> So basically we can only determine the general direction?

With current-generation detectors installed at three stations which is all we can hope to achieve at the current level of funding in the very near future.

More money, more detections at lower energies, better idea where they come from. There are plenty of instruments in cosmology, astronomy, and physics that have already been designed and planned out, but which do not have the funding to be built; Wait 20 years and five percent of them might come to fruition. You could pour trillions of dollars on these problems without running out of novel questions that we've already proposed ways of answering, and novel results from exploratory instruments we've already proposed building. We spend about 30 billion a year on basic science research according to the NSF, spread over all fields. For comparison, the military gets upwards of 600 billion.

We have good ideas about how to make gravitational wave detectors much, much better, but not how to make them much, much cheaper.

> > 10 square degrees in area

> So basically we can only determine the general direction?

10 square degrees is about 0.3% of the full spherical sky.

It looks 45 times as big as the sun.

Well, the goal of LIGO is to detect if gravity is a wave. So we had to build sensitive sensors, over large distances to be able to pull off this feet. It was (to my knowledge) only build to do this. The fact you can use it for triangulation is a bonus. If they wanted triangulation, I suspect they would have made it with 3 sensors instead of 2.

I believe you use timing differences, not parallax (it takes ~40 ms for gravitational waves to travel through Earth).

You don't use parallax, rather you calculate the direction of the source by using the arrival times of the signals. The nearer detector will receive the signal slightly earlier by an amount that will typically be on the order of a few hundredths of a second. This is essentially the same method that geologists use to determine the epicenter of an earthquake.

This seems horribly prone to manipulation by cosmic radiation, gravity, and a million slower alterations like the rotation of the earth which when combined produce a noticeable effect.

The assumption here is that no force has acted on the energy arriving enough to distort two arrays on earth.

I think the assumption is that the paths taken are almost identical because the distance to the source is vastly longer than the distance between the detectors.

> seems horribly

No physicists here but you don't seem to be either. Irc from what had been said with the first event gravitational waves pass through matter and aren't disturbed like em waves, that's why they supposedly open new doors. Secondly, just take it at face value, if they say so it is so, it might seem improbable and "horribly prone to manipulation" but they probably thought of that and have machines precise enough to compensate for whatever effects you or me imagined e.g. "LIGO is designed to detect a change in distance between its mirrors 1/10,000th the width of a proton"[1]. It's like your grandma, never having touched computers, expressing opinions on the next release of some developer tool.

[1]https://www.ligo.caltech.edu/page/facts

You do realise that gravitational waves are incredibly weakly interacting?

Also they travel at the speed of light, the rotation of the earth is pretty much constant (if not negligible).

Correction to my post above:

I used the word "parallax" poorly to refer to the offset between the detectors - localization is most definitely not done via parallax. It's arrival time + some signal processing, so with two detectors, resolution is limited.

Additional details and an image depicting the improvements made by adding a third, properly positioned detector are available at: https://www.ligo.caltech.edu/news/ligo20160404

What are some of those big open questions in cosmology that this could answer?

Got one answer to my own question. Waves from the Big Bang would be a big deal in cosmology.

Ditto. We cant see past the photon soup of the cosmic microwave background of the first 380,000 years of creation using EM detectors.

This is the EU Virgo collaboration based in Tuscany, Italy. It is in the process of being upgraded:

https://en.wikipedia.org/wiki/Virgo_interferometer

how much does each facility cost (ballpark)?

>At this point, NSF has invested approximately $1.1 billion in construction and upgrades, in operational costs, and in research awards to individual scientists, who study LIGO data to learn more about our universe.

http://www.ligo.caltech.edu/system/media_files/binaries/300/...

But it's important to point out that

1) That $1.1 billion was for two facilities

2) LIGO underwent a huge upgrade over the last decade, making the instruments tremendously more sensitive, and the cost of this upgrade is included in there as well.

So I suspect that building a third facility with the sensitivity of the two existing facilities would be a much more affordable endeavor. The first time around is always the most expensive.

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