Dark Stars

The media is full of stories about the impending release of the first ever images of a black hole – images that not only represent cutting edge science, but are the culmination of 200 years of speculation and theory moving ever closer to observation.

This is huge for astrophysics, and a stunning example of how the mundane rules of our tangible, everyday world give physicists the ability to make intellectual leaps into the unknown – and the long-thought unknowable.

Dark Stars

The idea of a black hole can be traced to a calculation by English “natural philosopher” John Michell in 1783.

Along with his contemporaries, he imagined light as a stream of particles. Like balls thrown with superhuman force, these corpuscles emitted by the sun had no trouble reaching escape velocity and sailing away into space.

Michell pulled on a loose intellectual thread, asking how big a star would need to be before its own light could not escape its clutches.That is, when would the gravitational field of a star become so strong that, rather than setting off into the distance, light would fall back to the stellar surface?

Using no more than today’s high-school physics, Michell worked out that you could reach that point of no return by scaling up the sun by a factor of 500. A star 500 times greater in radius than the sun would be 125,000,000 (500x500x500) bigger by volume, and thus would “weigh” 125 million solar masses.

At that size, such a giant would become a dark star – vanishing from view.

From Newton to Einstein

Michell was working with the Newtonian model of gravity. In 1915, Einstein wove space, time and gravity into a single fabric with his General Theory of Relativity. Working in Berlin, at the heart of an empire locked in a global conflict, he hammered out a set of equations that explained gravity as the stretching of spacetime by massive objects. In the Einsteinian framework. the gravitational field of single, stationary object is described by the Schwarzschild metric, named for artillery officer Karl Schwarzschild, who did the maths while being treated for a painful (and eventually fatal) disease in a hospital near the Russian front, in the months after Einstein published his original equations.

To derive the Schwarzschild metric, you first imagine the object as a single idealised point. In the vicinity of this point, the math tells us that space and time turn inside out. This makes it impossible for light – or anything else – to escape past the event horizon into the world beyond. Beyond the event horizon normality is restored, and to the outside observer, it marks the external boundary of the black hole.

Mathematically, the black hole itself consists solely of empty space — it is a pucker in the fabric of spacetime. It is not so much that a black hole has mass; rather, the black hole is a memory of where mass once was.

The event horizons you would have calculated for all known astronomical objects in 1915 were far smaller than their actual size, in contrast to the vast dark stars extrapolated by Michell. In other words, after Einstein, black holes came to be understood as the endpoint of a collapse.

Relativistic Astrophysics

By the 1930s the pieces of what we now call “modern physics” – relativity, quantum mechanics, nuclear physics, along with a nascent understanding of particle physics – had been assembled. Armed with these tools, astrophysicists set about exploring the inner workings of stars. In the 1950s and 1960s we first observed pulsars and quasars (powered, respectively, by neutron stars and black holes) whose very existence is contingent upon these newly discovered laws of physics and these discoveries inspired that new field of “relativistic astrophysics”.

Texas became the epicentre of this emerging field – and two New Zealanders had ringside seats in the early 1960s. One of them, Beatrice Tinsley, went on to do fundamental work on the evolving universe; the other was mathematician, Roy Kerr, who took the step beyond Schwarzschild by solving Einstein’s equations for a spinning black hole, giving us what is now known as the Kerr metric.

SEEING IS BELIEVING

The first black holes to be discovered were 10 or 20 times the mass of the sun; the husks of massive stars that burn fast and quickly die. But there’s another category of black hole in our universe — the supermassive black holes that live at the hearts of galaxies.

And it is these giants that we are about to see, as it were, in person.

The coming images have been captured by the Event Horizon Telescope, a network of radio telescopes stretching from the South Pole to Greenland that can operate as a single instrument. This composite telescope has been trained on the centre of our own Milky Way galaxy, and the much more distant heart of the giant elliptical galaxy M87.

The Milky Way’s black heart corresponds to an object weighing millions as much as our sun; and that of the M87 galaxy amounts to billions of times the weight of our sun. Perhaps poetically, they sit at either end of a similar scale to the dark stars originally imagined by Michell.

And, since astrophysical black holes are likely born spinning (since they inherit the rotation of whatever produced them), the images we are about to see may reveal not just a black hole, but a black hole for which the Kerr metric is a key part of our ability to understand it.

Of course, “seeing” a black hole is a paradox. What we see is the matter circling its event horizon, caught in the grip of the black hole’s gravitational field, a spinning vortex around a cosmic plughole. Detailed images of this accretion disk alone would be spectacular enough, but on top of that, we will it apparently twisted into fantastical shapes as the light is pulled and twisted along complex paths in the vicinity of the event horizon.

These images will test Einstein’s understanding of gravity, and provide unprecedented proof that black holes truly exist in our universe. Because seeing really is believing. Even for scientists.


Header image: Oliver James et al 2015 Class. Quantum Grav. 32 065001

We Don’t Need No Education?

The news in New Zealand is that the kids are revolting, along with kids in Sweden, Denmark, Germany, the United Kingdom and dozens of other countries. Just as previous generations of teenagers challenged their parents to widen their perspective on what’s possible, today’s students are protesting the world’s lack of progress in addressing climate change.

At this point, the implications of climate change are widely known: rising temperatures, rising sea levels, and more chaotic weather events – which will be both more frequent and more extreme than their present-day counterparts. Climate change will damage global ecosystems, disrupt farming and agriculture, accelerate extinction events, and create instability in countries unable to mitigate the impact of a changing climate on the lives of their citizens. For today’s teenagers, climate change holds a similar position to nuclear war in their parents and grandparents’ generations. The difference is that climate change — unlike nuclear war — has actually begun.

What has not yet begun is a response with the scale and scope needed to make a genuine difference. The good news is that we know what needs to be done. We need to minimise greenhouse gas emissions (carbon dioxide and methane), and we can do this by developing and introducing new transport technologies, decarbonising energy production, changing industrial and agricultural practices, and modifying some of our own habits and choices.

My mother and father – whose own childhoods took place against the backdrop of the Pacific War and the Blitz, respectively – hoped that their children’s lives would be safer and more comfortable than their own. But parents today should have no such illusions about what our children could face in the anthropocene if climate action does not begin right now. We remind our children to do their chores and homework, chivvying them through breakfast and off to school when they don’t particularly want to go. But on March 15, striking students are reminding the grownups of the jobs we need to do and the messes we should be cleaning up. It won’t be a small job; it may in fact be the biggest challenge we have ever tackled as a species, but it is both describable and doable. But just as we say to our kids about doing their homework, the sooner we start, the easier it will be to make progress.

This is an uneasy moment for educators; schools have programmes and plans that are carefully fitted to the teaching term. And indeed, the Chair of the Secondary Principals’ Council, James Morris was less than enthusiastic when interviewed in the Herald. Mr Morris helpfully added that “It would be a real sign of the students’ commitment to the cause if they chose to give up after-school jobs or weekend activities to ‘strike’ rather than school time.”  This may be well-meaning, but “kids protest climate on a weekend” would be a quick newsblip, but “school strike for climate” certainly has our full attention.

In short, these young people know what they are doing. And, if my memory serves you can actually wag school for a day without going to the media about it – if all these kids wanted was a day off, they don’t need to organise a march.

Principals are charged with ensuring that the schools they lead run smoothly and deliver the results their communities expect; it is easy to see why they might be cautious. But some principals fully understand what is at stake. At Western Springs College a classroom was made available for students to hold meetings to organise their strike and their principal clearly knows that leadership is sometimes about more than just sticking to the timetable. We expect that schools will nurture our children toward informed and engaged citizenship, producing society members equipped to innovate, lead, and respond to the challenges of a changing world. From this perspective, a climate strike at your school is a marker of success as well as an administrative challenge.

Better yet, engaged students are teachable students. Climate touches almost any NCEA subject you care to name: chemistry, physics, biology, mathematics, statistics, art, technology, economics, social studies, media studies and no doubt many more. Clever teachers will find ways to tie climate to content, using the interest and passion of their students to facilitate their learning.

Normally, I would agree with Mr Morris but right now, things are not “normal” – and will become less and less normal the longer we wait. Just like the principals, striking students are seeking control and order. But what our children are hoping to control is the environment they inherit, and it is easy to see why they are pushing their communities to start real work on climate now,. And, let’s face it, if you can take a day out of lessons for the cross-country, and the athletic sports you can probably take time out to call for climate action across the country – and beyond.

Its Dark Materials

Each galaxy lives within its own three-dimensional halo of dark matter whose gravitational field corrals the stars within it. Without the stars, the halo would still be there, albeit invisible to our eyes; but if the halo vanished, the stars would scatter into the depths of the universe – just as a Christmas tree remains a tree with or without the pretty lights. Whereas without the tree, the lights would merely be a puddle of colour on the snowy ground.

Growing up in the Southern Hemisphere, the traditional trappings of Christmas were always out of step with the onset of summer around us: Santa in his cozy suit, imagery of roaring fireplaces, snowy scenes on Christmas cards, a heavy meal we consumed on a hot day before a swim.

But after I’d experienced my first Northern Hemisphere Christmas in Ithaca, New York — which, with several feet of snow on the ground, was something close to Narnia — the childhood strangeness of that transplanted holiday melted away and the seasonal symbolism at last made sense. A festival of light in the darkness of midwinter; gathering around a warm hearth while it snowed outside.

Fast forward a few years and I was living in New York City, where the Rockefeller Christmas tree (and the ice skaters beneath it) stands as a marker of the turning seasons. Another few years further on, my family and I were living in Connecticut, where the town of New Haven marks the season by selecting an enormous local evergreen to make the ultimate sacrifice in exchange for the chance to stand, dramatically lit, in the New Haven Green through the Yuletide season. We lived close by, and it became an annual ritual to walk to see the tree with the kids, always in winter jackets, often with snow on the ground.

New Haven Green. Image: Richard Easther

New Haven Green. Image: Richard Easther

Once, as we approached the illuminated tree, my wife Jolisa – a literature person, never not searching for metaphors to help make sense of science – asked: “You know this dark matter stuff that you talk about, is it something like a Christmas tree at night — we can see the bright twinkling lights, but we can only make sense of why they’re hanging in the air in that shape if we know about the tree that holds them up?”

And she was exactly right.

Our sun is one of roughly 100 billion stars in the Milky Way Galaxy, and the Milky Way is itself one of roughly a trillion galaxies in the visible universe. For over 100 years, astronomers and physicists have been trying to understand how galaxies, the giant islands of stars that are the large-scale buildings blocks of the universe, hold themselves together. If galaxies are made entirely of stars – in other words, if what we see is all we’ve got – the stars would be moving too fast for their mutual gravitational attraction to hold a galaxy together.


A selection of galaxies; each image contains over a hundred billion stars.


To make sense of this, the vast majority of astrophysicists and astronomers have come to believe that the cosmos is now contains far more than our eyes can see. As we now see the universe, each galaxy lives within its own three-dimensional halo of dark matter, whose gravitational field corrals the stars within it. Without the stars, the halo would still be there, albeit invisible to our eyes; but if the halo vanished, the stars would scatter into the depths of the universe – just as a Christmas tree remains a tree with or without the pretty lights. Whereas without the tree, the lights would merely be a puddle of colour on the snowy ground.

So if you are seeking a secular interpretation of the iconography of Christmas, you could do worse than seeing a well-trimmed Christmas tree, illuminated with lights and bedecked with tinsel, as a metaphor for the cosmos.

Dark matter – by definition – neither emits nor absorbs light, and cannot thus be made of atoms, or indeed any of the fundamental particles known to physicists; it must be something entirely novel. So likewise, let the spectacle of a galaxy serve as a reminder that there is literally more to the physical world than meets the eye, and that there are deep mysteries for us to solve in the years and decades to come.

Happy holidays and compliments of the season.

View fullsize

The Milky Way over Portobello, Dunedin. Image: Ian Griffin.

The Milky Way over Portobello, Dunedin. Image: Ian Griffin.


Footnote: Actually this sort of was my TED talk. And, full disclosure, we can’t (and shouldn’t) be sure about dark matter until we have a better idea of its properties and composition.

What’s In A Name?

Showing visitors around the University of Auckland can be a quick lesson in our colonial history. The University is named for the city, which is named for a spectacularly unsuccessful Governor-General of India. Our oldest streets are centred on Queen Street — which is crossed by Victoria Street: the settlers took a belt and braces approach to honouring their young monarch when the city was laid out in 1841. And my office overlooks Albert Park, named in the 1880s in memory of the Prince Consort at about the time the University itself was founded.

Of all New Zealand’s universities, though, the name of Victoria University of Wellington ties it the most tightly to our colonial past. Their founding coincided with that long-lived monarch’s 60th year on the throne, and the name commemorates the anniversary. Unsurprisingly, a number of outposts of Empire had similar ideas as they built their own replica Englands. Consequently, our “Vic” is one of several dotted round the world and there is very little about the name that says “New Zealand”. So it is not surprising that Victoria’s leadership felt that “University of Wellington” would be a better name and would assist with the near-sacred tasks of international student recruitment and gaining a notch or two in the rankings.

What was possibly more surprising was that those leaders actually initiated the long and laborious process involved with a name change. The arguments in favour are mainly technocratic, but the arguments against have roots in the visceral connections many students and alumni feel for their alma mater, so the latter were advanced with more passion and intensity. And what was truly surprising is that while the University was happy to shoulder aside this opposition from within the community, the Minister overruled the University Council and decreed that Victoria it would stay.

image.png

That said, Victoria is not the only oddly named institution of higher learning in New Zealand. To most people, Cornell University is an Ivy League school with all the trimmings: a handful of resident Nobelists, cut-throat competition for admission, a glorious campus, and shops that are entirely devoted to selling clothing, mugs and other paraphenalia adorned with the university name, along with bumper stickers reading “My kid and my money go to Cornell.” But there is also a Cornell in Auckland, the Cornell Institute of Business and Technology.

Our “Cornell” is most definitely not an offshoot of Cornell University, even if its logo does rather remind you of a place whose sports teams are affectionately known as Big Red. And while Cornell University offers advanced degrees in everything from archaeology to zoology, the Auckland Cornell is more practical, with non-degree qualifications in cooking, business administration and computing. There is even short course in Barista Skills. Like the real Cornell, our Cornell does make the news on a regular basis, just not in particularly positive ways. Here’s one on Filipinos angry over an alleged immigration scam, or last year’s coverage of courses with rampant plagiarism and incomprehensible English in student work, and an older tale of students being admitted who couldn’t speak English.

The government doesn’t approve the names of private providers, but it does approve their courses, and it beats me how a place engaging in this sort of blatant misrepresentation gets a license to operate. If the Minister is taking an interest in names, the fake Cornell would be my nomination for the next spot on his to-do list.


Image: The Cornell campus and Lake Cayuga, in Ithaca, New York. Auckland’s “Cornell” is located in a scruffy looking building in the central city – on Hobson Street, named for the young colony’s first governor.

We Have The Technology

Last weekend. former astronaut, fighter pilot and David Bowie fanCommander Chris Hadfield posted some stats about the safety of flying, showing that per mile traveled, planes are about 10 times safer than trains, and 100s of times safer than cars. 

https://platform.twitter.com/widgets.js

Planes travel further and faster than cars and motorcycles,  and that got me to thinking about the safety of all kinds of travel.

Space travel certainly looks pretty risky, and astronauts are selected for their ability to stay calm in the face of danger. But just how risky is spaceflight, by Hadfield’s measure? It turns out that human beings have spent a total of 144.1 years in space.* An astronaut orbits the earth at around 18,000 miles or 28,000 kilometres / hour, so the total distance covered by human beings in space is about 35 billion kilometres.** On the other side of the ledger, 18 astronauts have died in flight (excluding those deaths that happen in training and testing, such as the Apollo 1 fire), making for roughly one death per 2 billion kilometres traveled. In Hadfield’s units that’s about 0.8 deaths per billion passenger-miles, putting space travel in much the same category as riding the subway.*** 

The second thing we learn from Hadfield’s numbers is that it is not just planes that are safe. By this measure, commercial or public operators (trains, buses, planes) are all safer than cars by at least a factor of 10, and are vastly safer than motorcycles. Hadfield shared his numbers to reassure nervous fliers. But because we are nervous about flying, we have – as a society – insisted that commercial flying be as safe as it can possibly be. People launching rockets likewise go to enormous lengths to make safety a paramount priority.

Conversely, cars actually come out looking pretty dangerous for those inside them. On top of that, Hadfield’s numbers make no mention of deaths among bike riders and pedestrians, most of which result from collisions with motor vehicles. On a per-mile basis, pedestrians and cyclists are killed about ten times as often as people in cars, although this varies vastly by location. 

How can this happen? Cars are designed and built using the latest technologies, with all manner of safety features as their selling points. But unlike trains, planes, buses – and spacecraft – cars are vehicles that are typically operated by their owners, and thus mostly driven by amateurs like you and me, on roads that are too often designed to facilitate speedy travel.

As a result we kill ourselves, our passengers, and other people on the road at a rate that would be unacceptable for commercial operators, or even for astronauts flying in space.

That “we” is important, because human beings are terrible at statistics and great at fooling ourselves.  “We” don’t get behind the wheel expecting to kill ourselves or someone else, so we don’t necessarily see solving this problem as a priority. 

However, many cities and countries around the world are now talking about Vision Zero; the position that traffic fatalities of all kinds are not an unavoidable nuisance, but a menace that can be eliminated through good design, policy choices and decent engineering. People aren’t perfect, but the idea is that – as in the workplace – nobody should pay for a mistake with their own life or somebody else’s.

The key first step is to SLOW DOWN; a collision between a car and a person at 50 km/hr is far more likely to be life-changing or lethal than one at 30 km/hr. That’s just the laws of physics. And safer road design helps make safer speeds intuitive. Also, making it easier for people to use “active modes” (bikes and walking), and improved access to public transport can all make a huge difference to our collective safety on the roads and our personal and social wellbeing in general – by helping us shift from less safe modes to ones that are not only safer, but better for us and incidentally, the planet.

It is easy to make rules for pilots and airlines. The challenge of Vision Zero is that it asks us to make rules for ourselves. But just imagine living in a world where walking and biking was as safe – by Commander Hadfield’s measure – as flying in space. 

As the Six Million Dollar Man put it: we have the technology. All we we need is the will to use it. 

<div class="sqs-video-wrapper" data-provider-name="YouTube" data-html="[youtube=://www.youtube.com/watch?v=HoLs0V8T5AA&w=640&h=480]”>

CODA: There is a lot to say about the coming era of commercial suborbital spaceflight, which will make for a lot of new “astronauts”. And then there is the whole separate topic of self-driving cars, which are often touted as a boon for safety – but an urban environment with people walking and biking on the streets is far more challenging for autonomous vehicles than the highway. The question may be whether these cars can adapt themselves to human cities, or whether their designers will attempt to reshape cities to better accomodate autonomous vehicles – just as the first carmakers did a century ago. 

IMAGE: The header image shows the failure of the Antares 3 launch vehicle; the image is from Wikimedia and was taken by NASA/Joel Kowsky. 

 

* That link refers to “man years”. Sigh. 

** This does not account for the small number of astronauts who traveled to the moon; they move faster than orbiting astronauts at the beginning and end of their trips, but more slowly in the lunar environment. 

*** The total number of “astronauts” includes people who achieved an altitude of 100km or more during suborbital flights – the so-called Karman line at which aerodynamic control becomes impossible and space effectively begins. It’s also worth noting that Hadfield’s statistics are per mile, not per trip.  Plane trips are much longer than car trips so this statistic makes each individual flight look safer than a typical journey by car; if the average car trip is a few kilometres and the average plane ride is more like a thousand kilometres, the risk per trip is actually roughly similar for cars and planes. Those 18 astronaut deaths occurred among the roughly 560 people who have been into space, putting the overall death rate per participant at about 3% and at maybe 1% per trip (since some people have made multiple flights) which of course puts the safety of space travel in a very, very different light. Lastly, the deaths all occurred during launch or reentry so it’s not being in space that is dangerous, its getting there and coming home.