Fusion or Antimatter?

Getting around interplanetary space is a challenge for chemical rockets – it’s barely worth doing for probes, and pointless for people. Eventually something better is needed. Two high-tech approaches are to use nuclear fusion or antimatter annihilation. Another old paper (1996) makes a detailed comparison…

Comparison of Fusion/Antiproton Propulsion
Systems for Interplanetary Travel

…and fusion wins easily. Antimatter energising of propellent is incredibly tricky and difficult, producing a lot of pesky neutrinos and nasty gamma-rays. The gammas can be effectively thermalized by big tungsten shields, but with a weight and cooling system penalty.

Much easier to use some kind of fusion ignition system (Stanley Borowski, the author, examines several) and enjoy the higher performance allowed by much easier to handle fusion by-products. The best performance is, of course, pulsed fusion using fuel pellets detonated by lasers (Inertial Confinement Fusion), which is what sent the good ship “Daedalus” to the stars in the JBIS’s 1973-1978 study. For interplanetary flight a more modest performance is sufficient – roughly 1/10th the exhaust velocity gets a ship to Pluto and back on a single tank in less than 2 years.

Antimatter ain’t what it used to be…

Antimatter annihilation propels the most memorable SF starship of all – the USS Enterprise – between the stars at FTL speeds by powering the manipulation of “subspace”. But what can real matter annihilation do? A naive view would claim an antimatter drive is a photon rocket with an exhaust velocity of lightspeed. The problem with that is that antimatter and matter turn into some seriously nasty gamma-rays. And nothing known can reflect a mega-electron volt gamma-ray. Is antimatter annihilation a hopeless cause then?

Several analyses say otherwise. Robert Frisbee studied the concept a bit more closely for NASA…

Systems-Level Modeling of a Beam-Core Matter-Antimatter Annihilation Propulsion System (Robert H. Frisbee)

ADVANCED PROPULSION FOR THE XXISt CENTURY (Robert H. Frisbee)

HOW TO BUILD AN ANTIMATTER ROCKET FOR INTERSTELLAR MISSIONS

…basically concluding that building the thing was possible, but the performance somewhat poorer than first imagined. So much energy is lost as gamma rays that the effective exhaust velocity he computed was 0.33 c. That’s way, way above the piddling 0.01-0.05 c hoped for from fission or fusion reactions, but a long way from ideal. A major problem was storing the antimatter – very cold anti-hydrogen ice could be levitated due to the residual magnetic field of hydrogen (and anti-hydrogen) molecules, but the storage density had to be very low, maybe 1/10th the density of hydrogen ice (itself just 75 kg/cu.metre.) Another problem was cooling off systems exposed to the gamma-rays produced by the engine. Frisbee’s starship design is very narrow, and very, very long. But he believed a transit speed of 0.25 c was feasible, and allowed a return mission design.

A slightly poorer performance, just 0.2083 c, was derived by Ulrich Walter in his text Astronautics (from Google book preview.) Not bad, but no one is likely to be powering to near lightspeed with any reasonable amount of antimatter. At least with normal matter structures.

Hans Moravec speculated on an interesting material over 20 years ago… Higgsinium …which is composed of heavy, charged supersymmetric versions of the Higgs boson (still undiscovered.) If such could be made in sufficient amounts it could be used as a gamma-ray reflecting material and enable true antimatter/matter photon rockets. However such SUSY particles have yet to be observed – maybe they exist, maybe they don’t. Once the Large Hadron Collider has obliterated enough particles in the TeV range of energies we may well know. Watch this Space for an update in ~2 years.

Nuclear Power Forever

Bernard Cohen studied the renewability of uranium for breeder fast-reactors back in 1983…

Breeder Reactors: A renewable energy source

…concluding that uranium pulled from seawater at 6,500 tons per year was sustainable for 5 billion years. That’s 18.5 TW.yr of raw energy since uranium in a breeder can have virtually 100% of its energy potential liberated. As we humans use about 15 TW.yr from all sources annually that’s pretty good news. He argues that as crustal matter is continually dissolved and carried into the sea the total level in the ocean (over 4 billion tons) will remain much the same for aeons.

Deuterium – a fusion fuel – is available too. Some 44 trillion tons in the ocean. It produces about 300 TJ/kg energy, thus it too could last for aeons at modest power usage levels. Thorium is more plentiful than uranium in the crust, but its solubility is much lower, thus it has to be mined. In the long term, so the crust isn’t overturned for mining, uranium is the fission fuel of choice.

Resources of the Solar System: Venus

Venus orbits the Sun at ~ 0.72 AU and receives roughly twice the insolation as Earth – with very low variation because its orbit is nearly circular. Its mass, radius and surface gravity are very close to our own – 0.815 Earth masses, 6052 km and 8.87 m/s^2 respectively. Its orbit around Sol lasts 243.1 Earth-days and its rotation on its axis is 224.7 Earth-days, but retrograde. This means the solar-day – time between sunrise and sunrise – is 116.8 Earth-days. Thus, without centrifugal force countering gravity, the Venusian globe is very nearly spherical, unlike all the other large planets, Earth and Mars included.

The most striking feature of Venus is its atmosphere – opaque, very reflective and very massive. By volume it’s 96.5% carbon dioxide and 3.5% nitrogen, and little dashes of everything else. The clouds are a practically unbroken haze of fuming sulfuric acid (H2SO4 + SO3 in solution with a bit of water) and opaque because they’re very deep, many kilometres. Surprisingly, if condensed, they would amount to only a few centimetres of acid. Beneath the haze banks the air is clear, though there are unidentified particles floating around that might be decomposed acid (i.e. grains of sulfur.) From the visible top-deck of the clouds to the surface is about 70 km and the surface pressure is a very high 92 bar. The surface temperature is ~ 735 K or 462 C/864 F, and would glow a dull red if the sunlight didn’t sufficiently penetrate the clouds to give a hellish eternal glow.

So what’s available on such a nasty planet? Let’s do an inventory

(1) Atmosphere – carbon dioxide is about a quarter carbon, which is the high-strength material of the future. Nitrogen is more abundant than on Earth – 2.7 times as much in fact. Combined the atmosphere could supply C,N,O for millions of space-cities, but that’s an as yet non-existent market.

(2) Surface – while hot there are materials that happily tolerate such conditions and retain strength, so teleoperated machines would work just fine, especially using high temperature electronics developed by the US DoD. Venus is similar in bulk density to Earth so its mineral resources will be akin, but differently distributed. Water has played a major role in concentrating ores on Earth, powered by plate tectonic processes. Venus doesn’t seem to have enough water in its mantle and upper crust for the same processes to occur. It might, however, do something completely different yet with similar results. We don’t yet know what so much water-free chemistry with a hot surface might do.

(3) Energy – deuterium is 150 times more prevalent than on Earth and might need removal from what hydrogen is available in the clouds to make potable water. Thus it’s a natural fusion fuel resource, though diffusely spread throughout the clouds. Above the cloud decks the energy flux from the Sun is almost twice Earth’s – and almost as much is reflected back up by the clouds as rains down from above. Thus solar energy is abundant.

(4) Gravity – Venus’s surface gravity is 90% Earth’s, so human health issues of ‘low gravity’ will be non-existent. Gravity also makes some industrial processes easier and that will be a boon.

(5) Space – at the 1 atmosphere level of the atmosphere Venus has 90% of Earth’s surface area, thus 3 times Earth’s dry-land area. Of course any colonies will need to float, but breathable N2/O2 mixtures are lighter than the ambient CO2/N2 mix. Thus vast inflated habitats will float naturally in the nicest part of the planet. The super-rotating atmosphere will mean the effective day-night cycle will be just 4 Earth days, not the 116.7 days of the rocks below. Thus a vast area for building habitats, if so desired.

(6) Any suggestions? Make a comment and let me know.

Dark Matter Stars and SETI

Any species which advances beyond “the Singularity” stage of development has essentially become immortal and is faced with the inconvenient fact that natural power-sources like stars aren’t. While they might persist for eons there will always come a day when the fusion flame goes out and most of a star’s mass is left to cool to ambient temperatures.

But are there other options for powering stars? Two new sources of power have emerged in recent arXiv.org submitted papers, Black Hole Quasi-Stars and Dark Matter Stars. Let’s have a look at those ideas.

Quasistars: Accreting black holes inside massive envelopes

…Begelman, Rossi and Armitage describe a scenario in which an accreting black-hole forms the core of a star-like object. A star is essentially an object in which the pressure created by inward pull of gravity is counteracted by the outward pressure of escaping electromagnetic energy – either indirectly as particle agitation (what we call ‘heat’) or directly as radiation pressure. In side a quasi-star the hot layers of gas above the black hole are bloated into a heat-pressure supported radiating surface, a luminous star, by the energy of infalling matter. As matter falls into a black hole it can lose up to ~5.7% of its mass energy as radiation – this is more efficient than a star’s piddling 0.7% energy production via nuclear fusion.

This might sound rosy as a long-lived star power-source but it has a down-side. It’s violently unstable because of opacity. Opacity is the tendency of a gas to get in the way of radiation passing through it – it’s what turns radiation into ‘heat’. Too much and the radiation gets pent up until something gives… in this case the gaseous envelope that looked so much like a star gets blown away and the naked raging x-ray source that is an accreting black-hole is revealed. Not healthy.

A nicer way to stoke the flames is via dark matter… except we still don’t really know what it is. One candidate is WIMPs – short for Weakly Interacting Massive Particles – and by “Weakly” they interact via the Weak nuclear force as well as gravity. With the least number of assumptions and actual prediction by minimal extensions of the Standard Model (i.e.how known particles and forces work), WIMPs are prime candidates for the super-abundant Dark Matter that out-masses “light matter” 5 to 1. Because they’re Weak force particles, they happily are their own antiparticles and thus a good energy source for a star.

A recent reference…

Dark Stars: Dark Matter in the First Stars leads to a New Phase of Stellar Evolution

…in which the effect of WIMP annihilation on the first stars is discussed. Basically a low mass newly minted star glows brightly from Dark Matter when it gets to about 17 AU across and 0.6 Solar Masses, then it gradually accretes more mass and becomes 800 Solar Mass behemoth radiating 1,000,000 times as bright as the Sun, lives a million years, then dies by collapsing directly into a Black Hole, perhaps forming the core of a Quasi-Star?

Such very early objects allow the super-massive Black Holes at the cores of Galaxies to form rapidly – some eventually massing as much as the Milky Way’s visible matter (~60 Billion Solar Masses.) That doesn’t sound like a very long-lived star candidate, but further work has revealed a light-weight version…

Dark Matter annihilations in Pop III stars

…with the right density of WIMPs (about a billion per millilitre) a 20 Solar Mass star can be ‘frozen’ and still be happily burning on the Main Sequence for as long as the current age of the Universe. That’s a life extension of about ~2,000 fold, so it’s definitely enticing to imagine ETIs shepherding Dark Matter into the Galactic Core and giving their stars a life-extension. With a mass-energy conversion efficiency of ~60% the Galaxy’s 1.2 trillion Solar Masses of Dark Matter could keep its stars burning at 30 billion Solar Luminosities (current output) for ~350 trillion years. Much of that luminosity is from over-active O, B and A stars, so the useful light level is more like ~3 billion, thus 3.5 quadrillion years of starlight is available for all to bask in. Not forever, but substantially better than the darkness awaiting the natural Galaxy in that epoch.

Eventually something better will be needed. But just what will replace starlight?

Another WIMP Star paper is Low mass stellar evolution with WIMP capture and annihilation …which discusses the effect of WIMP burning on low mass stars like the Sun. As already pointed out the stars’ lifetimes are enhanced greatly – after 30 billion years the model star looked as it did at age 0.1 billion. In other words so long as the WIMP supply lasts the structure is stable and nuclear fusion takes a back-seat. The star can last for trillions of years given the right WIMP halo. Gravitationally bound structures composed of more than two roughly equally massed objects are inherently chaotic and prone to eventually fall apart – even the Galaxy will disperse over about 10 million trllion years. Nothing lasts forever.

Solar Energy Storage breakthrough?

MIT researchers have developed a new way of storing solar energy – use the energy to breakdown water into hydrogen and oxygen, which can be used in a fuel cell later…

MIT solar breakthrough

…which may sound a bit like old news but they’ve developed a new high-efficiency way of doing it at low temperatures. Another advance announced a few months back involves a new thermoelectric generator potentially able to work at 85% of the Carnot Limit.

The real issue with God #3

Apocalyptic was a literature of protest. Between about 150 BC and 100 AD a style of prophecy developed which remained with us ever after – a way of talking about one’s oppressor without talking about them, thus making sedition hard to prove. The New Testament book, the Revelation of Jesus Christ, or the Apocalypse in scholar-speak, is but one of many, many similar “revelations” (apocalypsis in Greek) and it very cleverly disguises who it is targeting with theriomorphic (“beast shaped”) imagery – so cleverly that most people are shocked to discover that the original target was probably Nero Caesar.

But Apocalyptic turns up in the Gospels – specifically Matthew 24, Mark 13 and Luke 21 – as what’s called the Synoptic Mini-Apocalypses. Jesus gives lurid prophecies of the destruction of Jerusalem and “the End of the Age”. We also have numerous parables of Jesus which give us insight into what he thought about the End. One in particular is pertinent to the present question – the Parable of the Wheat and the Tares – which pictures the Devil as throwing weed-seeds into a field on God’s farm. The wheat and the weeds sprout, but the two can’t be separated else some of the wheat might be pulled up with the weeds. However at harvest the two are at different heights and can be cut separately, divided into two batches and kept or burnt up.

“Weeds” might sound innocuous, but the particular Greek word refers to a poisonous grass very similar to wheat, except it matures quicker and at a different height to the wheat. Even small amounts could spoil a whole harvest of wheat, sickening anyone who ate it. I’m not sure a modern person can really appreciate the life-or-death nature of ancient grain-based economies, but any loss could be disastrous for a small agricultural community in Judea, or anywhere in the old Roman Empire. An analogous situation would be the effects that drought in Australia is having on grain-markets around the world, where cheap grain is the difference between satiation and hunger in the Third World.

So what does this mean about God? Basically Jesus is saying the unjust and evil in the World can’t be removed until the End of Time, not without negative effects on the good and just. But surely God can do anything? Why not that?

More next time…

The real issue with God #2

Can God be defended from the charge of the atheists that “he” is evil and stupid? That’s the challenge that the plight of Job, the afflicted righteous man, posed to faithful scribes some millennia ago.

Not long after the later Redactor (c.400 BC) the people of Israel faced new challenges when they were forcibly included in the Greek Empire of Alexander the Great – then traded back and forth between “the King of the North” and “the King of the South” – the Greek rulers of Anatolia and Egypt respectively. According to the 1st century historian Josephus the Jews cleverly convinced Alexander that his coming had been predicted in the prophecies of “Daniel” – a Babylonian exile of the 6th century who succeeded even as a repressed minority in a foreign land, much to the inspiration of Jews ever since.

But Daniel wasn’t the only culture hero from that era. A slightly earlier figure, and an originally pagan one, is Ahiqar, who lived under the 7th century Assyrians, and also triumphed as a righteous man against scheming adversaries. We have some earlier texts of non-Jewish origin that feature his tale from the Assyrians. He was rewritten as a Jewish exile in Assyria in the Apocryphal book “Tobit” where he features as a minor character. What was the appeal to Jewish scribes of both these figures, Daniel and Ahiqar? Individual heroics, but also a theological answer – God will rescue the righteous from their enemies – to the charge against God.

By the 2nd Century BC religious freedoms under the Greeks, originally negotiated with Alexander, had deteriorated badly, so much so that the last native High Priest, Onias, had been assassinated and Greek puppets, of a modernising bent, had been put in his place. Jewish culture – a life and death matter in those days – had been under pressure to become even more Greek and even less Jewish, while much of the territory of the Jews had been settled by Greeks. The pressure to assimilate was intense, and exploded into armed insurrection when the Greek King, Antiochus, proceeded to replace the Jewish altar worship to Yahweh with a sacrifice of a pig to Zeus.

Many, many righteous Jews died in the conflict… posing an even tougher charge against God’s role as “Judge of the World”, the absolute standard of Justice. So what was the answer? Move the balancing of the scales into the Next World and/or the Next Age. God would settle the accounts, rewarding the righteous and punishing the wicked in the Next World and, perhaps, inaugurate a new Dawn, a new Age of Righteousness. But this wasn’t exactly new… a similar vision underpins prophecies from the 6th century onwards.

So how did the prophecies of Deutero-Isaiah, Malachi, Ezekiel and Zechariah, of a new Jerusalem under a new Deal with God become morphed into a New Dawn and a New World?

The real issue with God #1

George Williams, a great evolutionary biologist, made a point about God in several of his books which deserves exploration. He had no time for arguing over the existence or otherwise of God, merely defining “God” as the creative Something which caused the world we observe. He then proceeded to demonstrate from the apparently stupid and cruel features of the biosphere that God was both unintelligent or blind to consequences, and evil, not good. Fortunately this apparent evil was off-set by an huge measure of stupidity.

I think, for many atheists and agnostics – including myself for many years – the problem with the idea of God was the character of such a being, as presented to us by the Creation, is not worthy of worship. The Cosmic Creativity seems inhuman and inhumane, and it seems better to believe that God is not a loving Father, but it merely a mindless cosmic process that doesn’t deserve the name “God”. In fact the character of God, as ostensibly believed by many of the Faithful, is antithetically opposed to the values of modern liberal democracy and humanistic society, that such a being can only be a diabolical delusion to be opposed by right-thinking persons.

But, a big “but” admittedly, the faithful have pondered the inconsistencies of believing in Divine Justice and have tried to answer the problems in many different ways. One way is the “Answer to Job” – the Theophany in the tale of Job, which answers the claim, by a righteous and afflicted man, that God is unjust by pointing out the wonders of Nature and God’s visible power in all those mighty works… which doesn’t really make a direct answer. Indirectly God is saying that God’s reasons for Job’s plight are beyond Job’s understanding just like the “big wide world” is beyond Job’s understanding.

Does that work? Many of the wonders that God shuts Job up with aren’t as beyond human understanding as they once were, and so the whole exercise – to me – rings a bit hollow. I think it rang a bit hollow to the Redactor of “Job” because the original dialogue has a start and conclusion which seem to be later accretions to the core text, and the Redactor’s conclusion has a happy ending for Job with him receiving double what he had lost. But that doesn’t really work for me, especially since Job’s children were killed.

So what did the faithful do next?…