Where Grey Matter meets Dark Matter

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Episode 29 - 9 November 2010

Bushfires tend to get a bad rap, and with good reason. Houses get burned down, and people and animals die - lots of them. It's scary. But it's not all bad - fire can be good for the environment, and there are some species that turn these infernos to their advantage.

A photo of the Black Saturday bushfires, February 2009.
Source: The Weather Channel

In Australia, there are two main types of bushfires - grass and forest. Grass fires are common in the savannah parts of the country, where there are wet seasons and dry seasons. During the wet season, the grass grows tall, and during the dry season they provide ample fuel for any fires that start. Forest fires happen mainly because of trees, like Eucalyptus types, whose leaves are filled with oil - flammable oil that burns very hot. Once this fire gets going it's hard to stop.

New growth on a Eucalypt tree.

There are a few ways these fires can start. Humans are the most obvious cause, but we can't take full responsibility. Lightning can also be to blame, as well as 'spontaneous' combustion – when some chemical process provides the ignition for nearby fuel to start burning, like a hay stack in the sun whose decomposition causes it to catch fire.

Of course, considering the flammability of most plants under the burning light of evolution tells us that maybe there's more to the story than "Fire bad, water good". Some plants are incredibly flammable, but why would that be so if catching fire was so bad for them?

It turns out that a lot of species use fire as part of their life cycle. As a rule of thumb, the more flammable a plant is, the more likely it is to rely on fire to propagate. As a result, an area with frequent fires will tend to grow more flammable plants, and infrequent fires promote the growth of less flammable plants (think rainforest-type vegetation). You wouldn't want fires every year though, because then plants don't get a chance to grow. A healthy flammable ecosystem would have fires every 3-10 years.

There are two questions that immediately come to mind: How do the plants survive the fire, and how do they use it to propagate?

Life tends to finds some way to survive (ask Dr. Ian Malcolm) - some plants have dense bark, so the fire doesn't burn all the way through; some have a high water content, so they're not as affected; others shed lower branches so the fire doesn't reach their leaves, or have 'reserve shoots' that only grow when there are no higher branches in the way (throughout the area north of Melbourne where the Black Saturday fires happened just under two years ago, trees that look like green furry poles are a common sight).

Hakea Pods - Thick Skins.

Of course, some don't need to try, because their offspring are safe. Many plants drop seeds which hide underground until the fire's gone. Some plant species do a similar thing but with 'propagules', which includes not only seeds, but any plant material that can germinate and grow into a full grown plant. Some propagules can lie dormant for years and only begin to grow when a fire has cleared the surrounding area of competitors. Some plants even use smoke to promote germination, by inducing production of butenolide.

There is the wonderful example of giant sequoia trees, which grow in the Western USA and have a larger diameter than any other tree species in the world. The US used to have a policy of trying to put out all forest fires, until it became clear in the 1960s that there were no new giant sequoia trees growing. The warm air from a fire cracks open the cones which then drop seeds, and the fire clears the undergrowth to allow the new trees to start growing. The new policy is to allow controlled fires.

Giant Sequoia tree 'The Sentinel', at Sequoia National Park in California.
Source: Travel plan idea blog

Good for plants means good for the ecosystem. But the fact that some plants gain direct benefit from fires isn't the only reason fires can be good for the ecosystem. They break down dead and dying matter and return the nutrients to the soil. They clear the undergrowth, allowing the sun to reach the forest floor and encouraging new growth. They also can act as a moderator - if there's a locust swarm a good fire should tame it. Fire also removes diseased plants and stops the diseases from spreading, in much the same way as the Great Fire of London in 1666 put a lid on the Bubonic Plague.

There is a research program that's called "Burning for Biodiversity", which is dedicated to studying the effects (both positive and negative) of regular fires on the ecosystem of the tropical woodlands in the Northern Territory. A similar program is in place in South Africa. The results of this research will shed light on many of the unknowns about fire's place in the ecosystem.



Disney and fantasy = yes. Disney and sci.fi = dubious (notably excepted, of course, is Tron). The Black Hole was a Disney film with science fiction at its core, including real fringe scientific theories, heavily marinated in gobbledygook, and a crazed German physicist who had delusions of grandeur.

Geologists and mathematicians hypothesised about a cosmic object so dense that light couldn't escape, but it took a physicist to come up with the real deal - and it turned out way more rad than anybody ever thought. 1915 = Einstein's general relativity, which is the theoretical framework for how the large scale structure of the universe operates. (By 'large scale' we mean from about human-sized up to universe-sized.) The guts of the theory are the Einstein field equations which have the geometry of space-time on one side, and the density of matter on the other. Ever heard the saying 'Space tells mass how to move, and mass tells space how to curve'? That's a decent rendering of Einstein's equations into English.

A black hole with the path of a particle (red) and plasma jets.
Source: Sky and Telescope, J. Bergeron

The first solution of the equations came but a mere few months after they were published, by a German soldier on the Russian front called Karl Schwarzschild. He showed that black holes can exist, and that if you ever got inside its event horizon (located at the Schwarzschild radius) you never come out again, nor does any of your email. There is the slimmest of possibilities that there also exist white holes, which are the opposite of black holes - things that you can't ever get close to. And if we shave another sliver of slimness off the probabilities, then we might even be able to enter a black hole, avoid destruction, and emerge from a white hole. (Enter the science fiction writers.)

It might be helpful to think of the 'rubber sheet' diagram. Everything that you normally think of in space has some type of mass (or energy) and you show its gravity by depressing the rubber sheet downwards. The bigger the depression the more mass the object has. Now a black hole would be, in the words of Bill S. Preston Esq, 'a totally deep hole' in the sheet. And, it actually has a hole in the bottom, meaning that if you ever got there, then you're never getting out. A white hole would be a hole, but stetched waaay upwards, meaning that you would need a totally big run up to get to the top. (Actually you would never get there).

A rubber sheet diagram of a body orbiting a star.
Source: Einstein for Everyone

The 'no hair' theorem tells us that black holes have only three (3) defining features: mass, electric charge, and angular momentum (rotation, that is). So we can get different black holes by varying these three parameters. A charged black hole gives you the possibility of finding naked singularities (a point in space of inifinite density) without being inside an event horizon, but this probably doesn't happen without some funky physics inventing negative mass.

The coolest black holes are the rotating ones. In addition to the event horizon, they also have an ergosphere outside them. In this region you are forced to orbit in the same direction that the black hole is spinning. You can still leave and go home (since you haven't crossed the even horizon yet), but if you miss seeing something down below, you need to go the long way around before you can see it again. The reason for this is called 'frame dragging'; the mighty black hole is dragging space with it as it rotates. On the rubber sheet, you can think of this hole being twisted. Some people have figured that you can use this effect to get the black hole to do work for you - some people try so hard to be so lazy.

A rotating black hole affecting spacetime.
Credit: A. Broderick & E. Mer

These rotating black holes have other far out effects on space-time as well. If you enter at just the right angle (as our German Disney friend was attempting) you can time and space travel, you might enter a wormhole and emerge at a white hole, you might travel back in time to meet yourself, or you might just die. Probably just die.

You should all study general relativity. In the meantime, you should definitely see the film: the robots were ace.

An animation of stars orbiting some massive dark spot in the centre of our galaxy.


Oops. Mistakes we shouldn't have made but did:

Anthony said that a charged black hole would be like dropping a battery into a black hole. This isn't true since a battery has, in total, no net charge, otherwise you would get zapped every time you picked it up. Instead you could brush your hair with a comb and then throw that in.


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