In space, no one can hear you scream

New Zealand is suddenly and unexpectedly a “spacefaring nation”, with locally built rockets regularly launched to orbit and even the Moon. This is a shock to many – it certainly surprised me, and I live and breathe this stuff. But now that we find ourselves with an unexpected lead in a couple of key events in the Space Olympics, how do we make the most of it?

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Arm The Disruptors

Last week, Science Twitter was roiled by claims that “disruptive science” was on the wane and that this might be reversed by “reading widely”, taking “year long sabbaticals” and “focussing less on quantity … and more on …quality”. It blew up, which is probably not surprising given that it first pandered to our collective angst and then suggested some highly congenial remedies.

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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 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. 

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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. 

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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.