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Yonatan Zunger
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Yonatan Zunger

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We have observed gravitational waves!

This morning, the LIGO observatory announced a historic event: for the very first time in history, we have observed a pair of black holes colliding, not by light (which they don't emit), but by the waves in spacetime itself that they form. This is a tremendously big deal, so let me try to explain why.

What's a gravitational wave?

The easiest way to understand General Relativity is to imagine that the universe is a big trampoline. Imagine a star as a bowling ball, sitting in the middle of it, and a spaceship as a small marble that you're shooting along the trampoline. As the marble approaches the bowling ball, it starts to fall along the stretched surface of the trampoline, and curve towards the ball; depending on how close it passes to the ball and how fast, it might fall and hit it. 

If you looked at this from above, you wouldn't see the stretching of the trampoline; it would just look black, and like the marble was "attracted" towards the bowling ball.

This is basically how gravity works: mass (or energy) stretches out space (and time), and as objects just move in what looks like a straight path to them, they curve towards heavy things, because spacetime itself is bent. That's Einstein's theory of Relativity, first published in 1916, and (prior to today) almost every aspect of it had been verified by experiment.

Now imagine that you pick up a bowling ball and drop it, or do something else similarly violent on the trampoline. Not only is the trampoline going to be stretched, but it's going to bounce -- and if you look at it in slow-motion, you'll see ripples flowing along the surface of the trampoline, just like you would if you dropped a bowling ball into a lake. Relativity predicts ripples like that as well, and these are gravitational waves. Until today, they had only been predicted, never seen.

(The real math of relativity is a bit more complicated than that of trampolines, and for example gravitational waves stretch space and time in very distinctive patterns: if you held a T-square up and a gravitational wave hit it head-on,  you would see first one leg compress and the other stretch, then the other way round)

The challenge with seeing gravitational waves is that gravity is very weak (after all, it takes the entire mass of the Earth to hold you down!) and so you need a really large event to emit enough gravity waves to see it. Say, two black holes colliding off-center with each other.

So how do we see them?

We use a trick called laser interferometry, which is basically a fancy T-square. What you do is you take a laser beam, split it in two, and let each beam fly down the length of a large L. At the end of the leg, it hits a mirror and bounces back, and you recombine the two beams.

The trick is this: lasers (unlike other forms of light) form very neat wave patterns, where the light is just a single, perfectly regular, wave. When the two beams recombine, you therefore have two overlapping waves -- and if you've ever watched two ripples collide, you'll notice that when waves overlap, they cancel in spots and reinforce each other in spots. As a result, if the relative length of the legs of the L changes, the amount of cancellation will change -- and so, by monitoring the brightness of the re-merged light, you can see if something changed the length of one leg and not the other.

LIGO (the Laser Interferometer Gravitational-Wave Observatory) consists of a pair of these, one in Livingston, Louisiana, and one in Hartford, Washington, three thousand kilometers apart. Each leg of each L is four kilometers long, and they are isolated from ambient ground motion and vibration by a truly impressive set of systems.

If a gravitational wave were to strike LIGO, it would create a very characteristic compression and expansion pattern first in one L, then the other. By comparing the difference between the two, and looking for that very distinctive pattern, you could spot gravity waves.

How sensitive is this? If you change the relative length of the legs of an L by a fraction of the wavelength of the light, you change the brightness of the merged light by a predictable amount. Since measuring the brightness of light is something we're really good at (think high-quality photo-sensors), we can spot very small fractions of a wavelength. In fact, the LIGO detector can currently spot changes of one attometer (10⁻¹⁸ of a meter), or about one-thousandth the size of an atomic nucleus. (Or one hundred-millionth the size of an atom!) It's expected that we'll be able to improve that by a factor of three in the next few years.

With a four-kilometer leg, this means that LIGO can spot changes in length of about one-quarter of a part in 10²¹. That's the resolution you need to spot events like this: despite the tremendous violence of the collision (as I'll explain in a second), it was so far away -- really, on the other end of the universe -- that it only created vibrations of about five parts in 10²¹ on Earth.

So what did LIGO see?

About 1.5 billion light years away, two black holes -- one weighing about 29 times as much as the Sun, the other 36 -- collided with  each other. As they drew closer, their gravity caused them to start to spiral inwards towards each other, so that in the final moments before the collision they started spinning around each other more and more quickly, up to a peak speed of 250 orbits per second. This started to fling gravity waves in all directions with great vigor, and when they finally collided, they formed a single black hole, 62 times the mass of the Sun. The difference -- three solar masses -- was all released in the form of pure energy.

Within those final few milliseconds, the collision was 50 times brighter than the entire rest of the universe combined. All of that energy was emitted in the form of gravitational waves: something to which we were completely blind until today.

Are we sure about that?

High-energy physics has become known for extreme paranoia about the quality of its data. The confidence level required to declare a "discovery" in this field is technically known as 5σ, translating to a confidence level of 99.99994%. That takes into account statistical anomalies and so on, but you should take much more care when dealing with big-deal discoveries; LIGO does all sorts of things for that. For example, their computers are set up to routinely inject false signals into the data, and they don't "open up the box" to reveal whether a signal was real or faked until after the entire team has finished analyzing the data. (This lets you know that your system would detect a real signal, and it has the added benefit that the people doing the data analysis never know if it's the real thing or not when they're doing the analysis -- helping to counter any unconscious tendency to bias the data towards "yes, it's really real!")

There are all sorts of other tricks like that, and generally LIGO is known for the best practices of data analysis basically anywhere. From the analysis, they found a confidence level of 5.1σ -- enough to count as a confirmed discovery of a new physical phenomenon.

(That's equal to a p-value of 3.4*10⁻⁷, for those of you from fields that use those)

So why is this important?

Well, first of all, we just observed a new physical phenomenon for the first time, and confirmed the last major part of Einstein's theory. Which is pretty cool in its own right.

But as of today, LIGO is no longer just a physics experiment: it is now an astronomical observatory. This is the first gravity-wave telescope, and it's going to let us answer questions that we could only dream about before.

Consider that the collision we saw emitted a tremendous amount of energy, brighter than everything else in the sky combined, and yet we were blind to it. How many more such collisions are happening? How does the flow of energy via gravitational wave shape the structure of galaxies, of galactic clusters, of the universe as a whole? How often do black holes collide, and how do they do it? Are there ultramassive black holes which shape the movement of entire galactic clusters, the way that supermassive ones shape the movement of galaxies, but which we can't see using ordinary light at all, because they aren't closely surrounded by stars?

Today's discovery is more than just a milestone in physics: it's the opening act of a much bigger step forward.

What's next?

LIGO is going to keep observing! We may also revisit an old plan (scrapped when the politics broke down) for another observatory called LISA, which instead of using two four-kilometer L's on the Earth, consists of a big triangle of lasers, with their vertices on three satellites orbiting the Sun. The LISA observatory (and yes, this is actually possible with modern technology) would be able to observe motions of roughly the same size as LIGO -- one attometer -- but as a fraction of a leg five million kilometers long. That gives us, shall we say, one hell of a lot better resolution. And because it doesn't have to be shielded from things like the vibrations of passing trucks, in many ways it's actually simpler than LIGO.

(The LISA Pathfinder mission, a test satellite to debug many of these things, was launched on December 3rd)

The next twenty years are likely to lead to a steady stream of discoveries from these observatories: it's the first time we've had a fundamentally new kind of telescope in quite a while. (The last major shift in this was probably Hubble, our first optical telescope in space, above all the problems of the atmosphere)

The one catch is that LIGO and LISA don't produce pretty pictures; you can think of LIGO as a gravity-wave camera that has exactly two pixels. If the wave hits Louisiana first, it came from the south; if it hits Washington first, it came from the north. (This one came from the south, incidentally; it hit Louisiana seven milliseconds before Washington) It's the shift in the pixels over time that lets us see things, but it's not going to look very visually dramatic. We'll have to wait quite some time until we can figure out how to build a gravitational wave telescope that can show us a clear image of the sky in these waves; but even before that, we'll be able to tease out the details of distant events of a scale hard to imagine.

You can read the full paper at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102 , including all of the technical details. Many congratulations to the entire LIGO team: you've really done it. Amazing.

Incidentally, Physical Review Letters normally has a strict four-page max; the fact that they were willing to give this article sixteen pages shows just how big a deal this is.
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Please explain it
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Disney movies seem to often be based on Idiot Plots, which rely on at least one major character being an idiot. Seanan McGuire here shares how most of them would run if the average RPG group were playing the stories instead. (Hint: generally much shorter)

h/t +David Priebe​​
daddyto2switches: “ curiosity-discoverer-of-worlds: “ zerofarad: “ curiosity-discoverer-of-worlds: “ deadcatwithaflamethrower: “ ““Who gives a shit about your gentials, you have a fucking dragon.” ” I...
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+Louis Wasserman The Sally/Jar Jar pages have me crying with laughter.
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+Andreas Schou reshared this, and his summary is pretty much bang on, so I'll quote him. All I can add is that Robert Smalls was, without a doubt, one of the great badasses of American history.

----

I just want to amplify what +Tshaka Armstrong  said about how awesome Robert Smalls was. Just to give you some details which fall between the big bullet points here:

* Born into slavery. 

* Started out with seriously unpleasant menial jobs. Taught himself to read. More importantly, taught himself navigational trigonometry. This is not simple to do yourself. This is especially not simple to do when you grew up speaking Gullah creole, and your first exposure to standard-dialect English was when you were ten. 

* In general, was seriously awful at being a slave. Ran away. Resold tobacco and candy to make money his master didn't have access to. Bailed out of slave lockup over and over again because, despite the fact he never took to the whip, he was too competent to punish.

* Stole a Confederate ship. Sailed off with it. Gave it to the Union.

* Pushed for Congress to pay him the bounty, and was paid about $37,000 for it. Which is to say, "more money than a slave would likely see in five lifetimes." 

* Joined the US Navy. Which is notable, because the US Navy was not admitting black sailors at this point. 

* Convinced the US Army to admit black soldiers. You know. Like you do.

* Oh, did I mention that all of this happened before he turned 23? Because it all did.

* Assigned to pilot an experimental ironclad steamship in an attack on Charleston harbor. This fails. The ship sinks. Smalls is nonetheless commended for bravery.

* Reassigned to the Planter, the ship he stole less than two years ago, with some of the black crew which originally stole it. The captain of the Planter, caught in crossfire between Confederate and Union ships, attempts to surrender to the Confederates.

* Decided he's going to have none of that, because black soldiers and sailors are killed on capture. Sails the ship back to the Union lines against his captain's orders, saving the lives of his black crew. 

* Commended for bravery again. And promoted. Which makes him the first black naval captain in US history. He's actually captain of the Planter, the ship he stole less than two years ago. 

* The war ends.

* Used the money he got from stealing Confederate ships to buy the house he lived in when he was a slave. Moves in. Runs for Congress.

* Won.

* Kept running for Congress. Kept winning. Became the longest-serving black Congressman until the late 20th century. 

* Reconstruction ended. Gerrymandering, poll violence, and the like keep him from running again.

* Stayed active in politics. Attempted to return the black vote to South Carolina. Fails, but consider that this is precisely the sort of thing which would get you lynched between the years of 1876 and 1920.

* Appointed to be customs inspector. Which, again: this is a math-heavy job, and Smalls had no formal education. 

Name an important thing which a human being could have done between the Civil War and World War I, and Smalls did it. He didn't just rise up from poverty: he rose up from the most abject position an American could be consigned to, and just ... kept rising. Even after the tragedy that ended Reconstruction, he somehow managed to keep his head above water. 

He died the owner of the house in which he had been a slave, serving the country which had both rewarded and betrayed him.
 
#BAMF for #BlackHistoryMonth  Now THIS is a cat I'd love to see a biopic about!

From Wikipedia: As a politician, Smalls authored state legislation providing for South Carolina to have the first free and compulsory public school system in the United States, and founded the Republican Party of South Carolina.
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Superb
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To this day, this movie is the first thing I think of when I see Bill Maher.

"This is war! The battle between the sexes! Anything less than cannibalism is just beating around the bush."
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+Craig Froehle  Kentucky Fried Movie! I completely forgot about that one. 
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Question: Is there anyone out there who is really concerned with rapper B.O.B.'s Twitter rants about how the Earth is really flat? (No, seriously)

Answer: No. However, Neil Tyson's reply to B.O.B.'s diss track (seriously; rap battles of theoretical physics) is pretty epic. And ample demonstration of why he is awesome.
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+Mark E. I think I like You, Sir.... Not in a bad way of course.... All I have to say to all of the Atheist in the group, is; Prove to Me that God in Heaven, does not exist????.... And the rest will follow.... ☺.... 
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The headline is just what it says on the label: the NSF (which is one of the key mechanisms by which science gets funded in the US) has made a public statement on the subject of harassment, and specifically gender-related harassment, to the effect of this will not be acceptable at any place which is receiving our money.

This is in no small part in response to a recent slew of stories in which pervasive sexual harassment in astrophysics has been brought to light. Nor is that field particularly bad in this context; if anything, it's slightly above average, which is not a thing to be proud of. The NSF's statement on this is likely to have a very marked impact.
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Good think
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A short silent film involving Nazis, spies, and a failure to tip one's bartender. It's great fun.
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That was fun.
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The UK is known for rather careful attention to public safety, whether it be the Tube's perpetual admonitions to "Mind the Gap" or the tendency of pharmacists to make sure that you are taking exactly the right medicine. But one of the least-known yet most important public safety measures can be heard every Sunday, as church bells continue to ring not just once or twice, but for a good fifteen minutes or so. This gives ample warning to the citizenry whenever the gods have returned to feed, calling on the faithful elect to attend the sacrifice while warning heathens of all stripes to flee, flee, for the gods are upon us and shall not depart until their hunger is sated. Ia! Ia!

Chalk up a point for ordinary British thoughtfulness, in not just ringing the combination dinner gong and warning bell once and hoping everyone hears it. 
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+John Riddell or indeed apps   https://disciplr.com/world-map-christian-apps/  Anything but parochial
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I don't have time to write an explainer for this, so you'll just have to settle for the raw news. Short version: In the past few months, a large number of street kids, mostly refugees from Morocco, have been living near Stockholm's main train station. The police have been wanting more powers to get rid of them, but haven't gotten much. Last week, they announced that the main train station was now "unsafe" and had been "taken over" by said street children. 

Yesterday, a group of about 200 masked men stormed the station, distributing leaflets and beating anyone who "didn't look Swedish." This action was coordinated by the NMR (a local Nazi group), and a follow-up rally was held this morning in Stockholm by the SDP (a far-right party, think the Swedish version of Donald Trump). 

Police made no attempt to stop the attack. Three people were arrested, one for punching a plainclothes cop, another for possession of brass knuckles; all have been released. The official police statement is that they "could not confirm that violent attacks took place.”

For a little context, the presence of far-right groups and their readiness for actions like these is not even remotely secret, and the police statement that the area was now unsafe and there was nothing they can do was widely read as a coded message that vigilantes were welcome. (There's a whole history of complicated interplays between Swedish police, the local and national government, and these groups, with "if you don't let us do something, I guess it'll just have to be them" being a classic negotiating tactic)

Important notes here are the extreme coordination of the group (200 people showing up on schedule, dressed in black with balaclavas, literature ready to distribute, and ready for an organized attack), and the tacit cooperation of the police.

NB that violence against immigrants, Muslims, and Jews has been on a sharp rise across all of Europe, and far-right parties have been making significant political inroads, but this is the first highly coordinated attack. Earlier attacks have primarily been by small groups (3-5 people) on individuals, and haven't shown signs of organization, uniforms, etc.

There's lots more that could (and should) be written here, to give context about just why there were so many refugees living there, the six-month ramp-up of the conflict, and the political shifts within Sweden and more broadly within Europe as a result, but I'll have to leave those for someone else to write.

Useful additional coverage: http://www.telegraph.co.uk/news/worldnews/europe/sweden/12131460/swedish-far-right-migrant-attack-stockholm.html

(And yes, I realize that I just linked the Daily Mail as the primary source below. Going through English-language coverage of the story, theirs was actually the most thoroughly researched and informative, and didn't have any egregious biases that I could spot. Damned if I know why their coverage was the best, but it was. Normally the Mail is mostly suitable for wrapping fish in.)
A mob of black-clad masked men went on a rampage in and around Stockholm's main train station last night targeting refugees. At least three people were beaten up, according to witnesses.
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+John Hanley lmao
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This article is surprised and upset that police aren't investigated for damaging police property, namely the cameras which are supposed to be recording them. But this is inevitable, given the incentives: police get little protection from the cameras, and are placed at greater risk of complaints sticking. (Let's not say legal action, since actual prosecution for misconduct of even the most egregiously felonious sort is very rare) Police chiefs have no interest in alienating their own officers. And nobody seems to have clear authority outside of that.

But there's an alternate legal solution, part of which would require a simple change to the laws mandating the cameras in the first place, and part of which is already in the hands of judges today. It's to use the legal notion of "adverse inference."

In court cases (both civil and criminal), there's a phase of discovery, where the sides can demand that the other side produce relevant documents and so on under court order. If you try to cheat at discovery, concealing or destroying evidence, there are a range of exciting penalties available to the judge. (And given that trying to ignore or defy a court order is one of the best ways to end up with a very unhappy judge very quickly, this is not something you want to happen.)

"Adverse inference" is the mildest of these, and it's something a judge can order when a piece of evidence is "mysteriously" unavailable: that whatever that evidence was meant to prove, it will be assumed that what the other side claimed about it was true. (So if e.g. your financial records were subpoenaed in a breach of contract case to prove that you had earned more than you claimed on a deal, and you tried to fake their nonexistence, that would count as evidence that you had earned exactly what the other side had claimed you had)

If a body or dashboard camera failed due to the actions of the police officers in question, then the resulting lack of evidence isn't and shouldn't be neutral, as far as the law is concerned. It's a deliberate attempt by a party to a case to thwart the collection of evidence, and courts should treat it as such.

The relevant change in the law would be to create a presumption that these camera failures are not accidental, so as to put the burden of proof on the officers in question instead of on whoever is up against them in court.

(For those who are legally minded: a failure to produce evidence from body or dashboard cameras due to damage to or nonoperation of the camera, or loss of evidence outside of the retention processes defined for said footage by statute, would create a rebuttable presumption of spoliation, which the judge would then have leave to remedy according to the relevant rules of procedure in that court)

I'm not certain whether this would be a good or bad idea. It's a powerful enough change to the evidence system that it's honestly dangerous, and one would have to think through the potential consequences, pro and con, very carefully. Such a law would essentially place the burden of proof on the police, whenever evidence was unavailable for a suspicious reason, that they had not deliberately sabotaged it.

If cameras actually do fail fairly regularly without sabotage, for example, this would be a terrible law. On the other hand, if it is in fact the case that "camera failure" is almost always a sign of monkey business, this would be a quick way to straighten it out. Likewise, if standard operating procedures were worsening the situation – e.g., cameras which break naturally aren't getting fixed for a long time because nobody cares, or the handling of camera evidence is sloppy and frequently loses things – then a law like this would change priorities very effectively.

And of course, for deliberate destruction of evidence to conceal felonies, I have no sympathy whatsoever. A crime committed under color of law is worse, not better, than one committed in the usual fashion.

h/t +Alex Scrivener.
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How many fail to check their vehicle has fuel and run out of gas during their shift ? 

How many fail to check their guns have ammo before they start their shift ?  


May be an occasional failure of audio&video gear during a shift,  such should be treated same as vehicle engine failing... replace ASAP. 
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Thirty years ago today, 74.130 seconds after liftoff, the Challenger fell. 

I want to write a long story about this – to tell the story of the people aboard and the people on the ground, of the crew and of the people who studied what happened, of the things that we learned and changed, of all the things we should remember about that day. But thirty years later, it still hurts too much for me to write it.

I can still see the two SRB's spinning out of control after the airframe disassembled, forming two horns coming out of an oddly round cloud, if I close my eyes. I can still remember the utter confusion and disbelief that followed, as nobody could even tell, at first, if they had actually seen what they thought they saw. I remember being so upset, that night, not being able to fully wrap my heart around it. And nightmares still sometimes wake me, of rockets falling out of a perfectly blue sky.

To the seven who gave their lives that day – CDR Francis R. Scobee, pilot Michael J. Smith, mission specialists Ellison S. Onizuka, Judith A. Resnik, and Ronald E. McNair, and payload specialists Gregory B. Jarvis and S. Christa McAuliffe – you are not forgotten.


Photo: STS-51L as it cleared the tower, approximately T+2.7 seconds, 16:38:02 UTC, January 28th, 1986, seventy seconds before it began to disintegrate. The fatal failure of the O-ring on the right booster had already happened. Image KSC-86PC-0081, from NASA.
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I remember it well, the disbelief,of what I had just seen while I was getting ready for work.
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This is possibly the most pointless thing I have ever seen. It's all about a trivia question whose answer you would think is obvious: Does the ACT – the Australian Capital Territory, which is to Australia what Washington, DC is to the US – have a coastline or not?

You would think that questions like this would have fairly obvious answers, and people would be able to say "why yes, my capital does have a coast," or "no, my capital is nearly sixty miles inland." Or that at least the government of Australia would be able to say this fairly easily. But no, that is not how this works.

Madness lays ahead.

Via +Andreas Schou.
 
Fun project for +Anthony Baxter and I today: trying to work out, once and for all, if the ACT has a coastline.

For you non-Australians: the ACT is the Australian Capital Territory. The place where Canberra is. Think DC in the USA; there are many similarities, including the fact that states donated land to carve out a neutral HQ for the national capital. (Side note: you may see references to the FCT, or Federal Capital Territory, in some of the stuff linked below. It's the same place; it was renamed along the way.)

You'd think "does a federal subdivision have a coastline" would be an easy question to answer. You'd be so, so, so wrong. This is a pub trivia kind of question in Australia; the problem is, most people get it wrong. At best, they get it right, but for the wrong reasons. Like, maybe it does have a coastline, but not the one they think.

At least 3 Wikipedia pages cover the topic. Each of them give different answers to the question.

Regardless, this is a fascinating geopolitical quirk. So here's what we know:

Easy answer: no, it's inland

This is the answer you get when you look up 'Australian Capital Territory' in your favourite online map site, or (heaven forfend) a paper atlas. The ACT is landlocked, as any fule no (cf. http://www.cmd.act.gov.au/__data/assets/image/0004/117526/Australia_map_downsized.jpg). Obviously it doesn't have a coastline, some will say.

These people are wrong.

Pub trivia answer: yes, on Jervis Bay

Some background. When the various states federated into the Commonwealth of Australia (1901), Australia didn't have a capital per se. Melbourne acted as capital, with the promise that they'd sort a real one out later. In 1908,  the Seat of Government Act was passed, which basically said "we're going to build something in the Yass-Canberra area, the New South Wales government will give us some land once we've worked out somewhere mutually agreeable". The interesting part is the quote "The territory to be granted to or acquired by the Commonwealth for the Seat of Government shall contain an area not less than nine hundred square miles, and have access to the sea." (emphasis mine). The astute amongst you will note, from your maps, that the "district of Yass-Canberra" is nowhere near the sea. No problem, New South Wales will carve out another bit, on the sea, and pony that over too. The land they chose was at Jervis Bay (https://en.wikipedia.org/wiki/Jervis_Bay), a bay due more-or-less east of Canberra.

So, people say, this land they carved out (you can see it on a map!) is actually part of the ACT. It does have a coast!

These people are wrong.

Advanced double-bluff pub trivia answer: no, Jervis Bay isn't part of the ACT

The next (correct) argument is that the thing at Jervis Bay is not part of the ACT; it's part of the Jervis Bay Territory (JBT), a completely separate part of Australia. This is fairly startling to many Australians; we are all taught that Australia has 6 states (NSW, Victoria, Queensland, Western Australia, South Australia, Tasmania) and 2 mainland territories (Northen Territory and the ACT). But this isn't true; there are three mainland territories. Jervis Bay Territory is, legally, exactly like the other two: an independent top-level division of Australia. Finding out there's a third territory is startling for many Australians: it would be like if the US actually had 51 states, but no-one ever bothered to mention, say, a South Rhode Island. Anyway, it's true. Legally, in Australia, JBT is just like the ACT. The difference is: it's smaller, almost no-one lives there, and lots of people have never heard of it. But that's irrelevant.

Really quite advanced pub trivia answer: the Jervis Bay Territory is PART of the ACT, so yes

This is wrong, as stated above. But people believe it, because of one key fact: the JBT doesn't have a government. Because almost no-one lives there, giving it a government is kind of wasteful. So the ACT administers it. That is, the laws of the ACT apply; commit a crime there, you're tried in the ACT courts. Live there, you vote for the ACT government. But the law is clear; it's as if it's part of the ACT, but it's not. This is an administrative convenience.

Exhausted and confused person answer: so it's no then?

Ahahaha. No.

Epic map nerd smart arse answer: yes, but not the one you're thinking of.

Ahh. Here's where we get really tricky. All that stuff above? You know where I said the "pub trivia answer" people who said "yes" were wrong? Well, they're very possibly right. But for the wrong reasons. There's a completely separate parcel of land, also on Jervis Bay, which may well be part of the ACT.

Look at Bing Maps (no, really): http://binged.it/1nngW39. The Jervis Bay Territory (NOT part of the ACT, as established above) is the thing outlined in green. But that's irrelevant to us. Look north-east of there. See the land at the north headland of Jervis Bay? That's the Beecroft Peninsula. This is in fact the bit of land that may be part of the ACT.

Cadastral surveying nerd answer: a-ha! That's not part of Commonwealth land; Beecroft peninsula is merely leased to to Commonwealth by NSW! So no!

Oh-ho, cadastral surveying nerd, hold up. I'm not talking about all of Beecroft peninsula. In the majority, you're right. But there's one part where I'm not sure you are. See http://i.imgur.com/giylEo1.jpg - I'm not saying A or B are part of the ACT. All I'm talking about is C: the land given to the ACT under the Seat of Government Acts of 1908 and 1922.

That land is part of the Jervis Bay Territory too! So no!

No, it's not. This is actually really quite clear. The Jervis Bay Territory Acceptance Act 1915 (https://www.comlaw.gov.au/Details/C2004C00038; hereafter JBTA) makes it clear what's part of the JBT. See "The Schedule". Following the descriptions is complicated, but this describes the parcel of land on the south headland. It mentions nothing about the North one. 

If your argument is based around the JBTA: nope, it's not in there.

If you argument is that a subsequent piece of legislation post-JBTA has changed it: [citation needed], as I'm not aware of any.

THE LAW

So, let's look at the law. There are a few relevant parts here, beyond the ones we've already discussed.

There's the Seat of Government Acceptance Act 1909 (https://www.comlaw.gov.au/Details/C2004C00608). This was actually two acts: this one, and a corresponding one from the NSW Government, the Seat Of Government Surrender Act 1909. That is, NSW passed an act surrendering the land; the Commonwealth passed one accepting it. Each was conditional on the other; both were passed and both came into effect. This does cover the north headland; for example, "Eastern Division, Land District of Nowra, County of St. Vincent, Parish of Beecroft, area five hundred and thirty‑one acres. The Crown lands within the following boundaries: Commencing on the High Water Mark of Jervis Bay at Longnose Point, and bounded thence on the east by that High Water Mark and the right bank of Duck Creek generally northerly to the road leading to Point Perpendicular Light House, thence by that road, generally westerly and north‑westerly to the High Water Mark of Jervis Bay at a wharf, and thence generally on the west and south by that High Water Mark southerly and easterly to the point of commencement. Plan Misc. 1393 Sy." (Yes, it's ALL like this. Gripping). I chose this example deliberately: the lighthouse is recognisably on the north headland, so you know that's where they're talking about. If you follow up on the others, they all seem to be on the north too (with one exception, but let's not go there).

There's the Seat of Government Acceptance Act 1922 (https://www.comlaw.gov.au/Details/C2004C00609; again, there's a corresponding NSW act). This complicates things, but then again… it doesn't. It does because it defines a whole new set of land parcels; it doesn't, because it's the same set. This exists only because _"certain errors and misdescriptions exist in the descriptions of lands set forth in [SoGA 1909]". That is, it's covering the same stuff, but more precisely. Nothing (really) to see here.

There's the Jervis Bay Territory Acceptance Act 1915; the one I cited above. I've already said this is irrelevant; what complicates it a tiny bit is that the corresponding NSW state act was called Seat of Government Surrender Act 1915. Ignore that, it's nothing to do with the Seat of Government. It's totally seperate. They just, like… copied and pasted the name of the 1909 state act, or something. Ignore it.

There's the Australian Capital Territory (Self‑Government) Act 1988 (https://www.comlaw.gov.au/Details/C2014C00617): this is the act that gave the ACT the right to make its own laws. This should be useful, but… it's not. Its entire definition of the actual boundaries of the ACT is "Territory: (a)  when used in a geographical sense, means the Australian Capital Territory". That's really helpful, you bastards.

As far as I can tell, that's all the legislation that's relevant. 

So, my answer: as far as I can tell, it's unambiguously part of the ACT. It was ceded in 1909 (and clarified in 1922). These acts, as far as I can tell, are the best source we have for defining the boundary of the ACT. If there are other sources, I don't know them.

To address some likely objections:

"The Jervis Bay Territory Act says…" I'll stop you right there. Irrelevant; these acts don't cover the north headland. Ignore JBT, it's a red herring.

"This map says…" Maps don't actually define boundaries. This is an obscure point of geopolitics: it's obvious that many maps don't bother to get it right. Even government maps: we know some of them get it wrong, because many of them disagree. They can't all be right. So which ones are?

"The boundaries have changed since the 1909 Act" [citation needed]. Where? Give me a source dammit.

"NSW ceded the land, and the Commonwealth accepted it. But they didn't make it part of the ACT; it's now just regular Crown [commonwealth-owned] land" Great, good argument. But where is it defined which bits are part of the ACT? Again, [citation needed]. If not the act, find me a source.

In conclusion: damn, I need a stiff drink.

No, wait.

In conclusion: I'm pretty sure it is part of the ACT. But it's deeply murky, and not only do the three goverments seem to disagree on the exact state of this land, but individual sources from the same government do.

Geopolitics is fun!
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