Archive for the ‘Space Sciences’ Category

A Galactic Internet
September 26, 2008

Timothy Ferris is a popular science writer. In Interstellar Spaceflight: Can We Travel to Other Stars?, he starts writing about space travel but ends up speculating about an interstellar internet.

Living as we do in technologically triumphant times, we are inclined to view interstellar spaceflight as a technical challenge, like breaking the sound barrier or climbing Mount Everest – something that will no doubt be difficult but feasible, given the right resources and resourcefulness.

…[But] the technical problems involved in traveling to the stars need not be regarded solely as obstacles to be overcome but can instead be viewed as clues… that point through other ways to explore the universe.

The high cost of interstellar spaceflight suggests that the payloads carried between the stars… are most likely, as a rule, to be small. It is much more affordable to send a grapefruit-sized probe than the starship Enterprise. Consider spacecraft equipped with laser-light sails, which could be pushed through interstellar space by the beams of powerful lasers based in our solar system. To propel a manned spacecraft to Proxima Centauri, the nearest star, in 40 years, the laser system would need thousands of gigawatts of power, more than the output of all the electricity-generating plants on Earth. But sending a 10-kilogram unmanned payload on teh same voyage would require only about 50 gigawatts – still a tremendous amount of power but less than 15 percent of the total U.S. output.

What can be accomplished by a grapefruit-sized probe? Quite a lot, actually, especially if such probes have the capacity to replicate themselves, using materials garnered at their landing sites… The probe would mine [an] asteroid and use the ore to construct a base of operations, including a radio transmitter to relay its data back to Earth. The probe could also fashion other probes, which would in turn be sent to other stars. Such a strategy can eventually yield an enormous payoff from a relatively modest investment by providing eyes and ears on an ever increasing number of outposts.

[Another] clue – that radio can convey information much faster and more cheaply than starships can carry cargo – has become well known thanks to SETI, the search for extraterrestrial intelligence. SETI researches use radio telescopes to listen for signals broadcast by alien civilizations…

When SETI was first proposed… in 1959… the object most frequently raised to the idea of interstellar conversation was that it would take too long. A single exchange – “How are you?” “Fine.” – would consume 2,000 years if conducted between planets 1,000 light-years apart. But… conversation is not essential to communication; one can also learn from a monologue… We learn from Socrates and Herodotus, although we cannot speak with them…

In 1975, when I first proposed that long-term interstellar communications traffic among advanced civilizations would best be handled by an automated network, there was no model of such a system that was familiar to the public. But today the Internet provides a good example of what a monologue-dominated interstellar network might be like and helps us appreciate why extraterrestrials might prefer it to the arduous and expensive business of actually traveling to other stars.

The most profound gulf separating intelligent species on various star systems is not space but time, and the best way to bridge that gulf is not with starships but with networked interstellar communications.

The gulf of time is of two kinds. The first is the amount of time it takes a signal to travel between [civilizations]. Therefore, it makes sense of send long, fact-filled messages rather than “How are you?”

The other gulf arises if… communicative civilizations generally have lifetimes that are brief by comparison with the age of the universe… Even if we manage to survive for a robust 10 million years to come, that is still less than a tenth of 1 percent of the age of the galaxy.

Any other intelligent species that learns how to determine the age of stars and galaxies will come to the same sobering conclusion – that even if communicative civilizations typically stay on the air for fully 10 millions years, only one in 1,000 of all that have inhabited our galaxy is still in existence. The vast majority belong to the past. Is theirs a silent majority, or have they found a way to leave a record of themselves, their thoughts and their achievements?

That is where an interstellar Internet comes into play. Such a network could be deployed by small robotic probes like the ones described earlier, each of which would set up antennae that connect it to the civilizations of nearby stars and to other network nodes… one could get in touch with many civilizations, without the need to establish contact with each individually. More important, each node would keep and distribute a record of the data it handled. Those records would vastly enrich the network’s value to every civilization that uses it.

If there were any truth in this fancy, what would our galaxy look like? Well, we would find that interstellar voyages by starships of the Enterprise class would be rare, because most intelligent beings would prefer to explore the galaxy and to plumb its long history through the more efficient method of cruising the Net. When interstellar travel did occur, it would usually take the form of small, inconspicuous probes, designed to expand the network, quietly conduct research and seed infertile planets. Radio traffic on the Net would be difficult for technologically emerging worlds to intercept, because nearly all of it would be locked into high-bandwidth, pencil-thin beams linking established planets with automated nodes. Our hopes for SETI would rest principally on the extent to which the Net bothers to maintain omnidirectional broadcast antennae, which are economically draining but could from time to time bring in a fresh, naive species – perhaps even one way out here beyond the Milky Way’s Sagittarius Arm. The galaxy would look quiet and serene, although in fact it would be alive with thought.

In short, it would look just as it does.

Here is a PDF of the full essay.

Bob Evans, Supernova Hunter
September 1, 2008

Bill Bryson knows how to tell a good story. He always loved science, and he wanted to know “Wait, how did we figure that out?” His A Short History of Nearly Everything is the story of how we figured things out. Here Bryson recounts his meeting with Bob Evans, amateur scientist extraordinaire.

By day, Evans is a kindly and now semiretired minister in the Uniting Church in Australia… But by night he is, in his unassuming way, a titan of the skies. He hunts supernovae.

Supernovae occur when a giant star, one much bigger than our own Sun, collapses and then spectacularly explodes, releasing in an instant the energy of a hundred billion suns, burning for a time brighter than all the stars in its galaxy. “It’s like a trillion hydrogen bombs going off at once,” says Evans.  [But] most are so unimaginably distant that their light reaches us as no more than the faintest twinkle.. It is these anomalous, very occasional pricks in the crowded dome of the night sky that the Reverend Evans finds.

To understand what a feat this is, imagine a standard dining room table covered in a black tablecloth and someone throwing a handful of salt across it. The scattered grains can be thought of as a galaxy. Now imagine fifteen hundred more tables like the first one — enough to fill a Wal-Mart parking lot, say, or to make a single line two miles long — each with a random array of salt across it. Now add one grain of salt to any table and let Bob Evans walk among them. At a glance he will spot it. That grain of salt is the supernova.

[When Bob asked the astronomical community] if they had any usable field charts for hunting supernovae, [they] thought he was out of his mind. At the time Evans had a ten-inch telescope — a very respectable size for amateur stargazing but hardly the sort of thing with which to do serious cosmology — and he was proposing to find one of the universe’s rarer phenomena. In the whole of astronomical history before Evans started looking in 1980, fewer than sixty supernovae had been found. (At the time I visited him, in August of 2001, he had just recorded his thirty-fourth visual discovery; a thirty-fifth followed three months later and a thirty-sixth in early 2003.)

Evans, however, had certain advantages. Most observers, like most people generally, are in the northern hemisphere, so he had a lot of sky largely to himself, especially at first. He also had speed and his uncanny memory. Large telescopes are cumbersome things, and much of their operational time is consumed with being maneuvered into position. Evans could swing his little sixteen-inch telescope around like a tail gunner in a dogfight, spending no more than a couple of seconds on any particular point in the sky. In consequence, he could observe perhaps four hundred galaxies in an evening while a large professional telescope would be lucky to do fifty or sixty.

Looking for supernovae is mostly a matter of not finding them. From 1980 to 1996 he averaged two discoveries a year—not a huge payoff for hundreds of nights of peering and peering. Once he found three in fifteen days, but another time he went three years without finding any at all.

“There’s  something  satisfying,  I  think,” Evans said, “about the idea of light traveling for millions of years through space and just at the right moment as it reaches Earth someone looks at the right bit of sky and sees it. It just seems right that an event of that magnitude should be witnessed.”

Buy A Short History of Nearly Everything.

What If Gravity Was Weaker or Stronger?
August 26, 2008

Martin Rees is an astrophysicist famous for his work on Big Bang theory and galaxy formation. In Just Six Numbers, he explains what the universe would be like if gravity was a bit stronger or weaker compared to other forces.

Despite its importance for us… gravity is actually amazingly feeble compared with the other forces that affect atoms… The gravitational attraction between [two] protons is thirty-six powers of ten feebler than the electrical forces, and quite unmeasurable. Gravity can safely be ignored by chemists when they study how groups of atoms bond together to form molecules.

How, then, can gravity nonetheless be dominant, pinning us to the ground and holding the moon and planets in their courses? It’s because gravity is always an attraction… On the other hand, electric charges can repel each other as well as attract… But any everyday object is made up of huge numbers of atoms (each made up of a positively charged nucleus surrounded by negative electrons), and the positive and negative charges almost exactly cancel out. Even when we are ‘charged up’ so that our hair stands on end, the imbalance is less than one charge in a billion billion. But everything has the same sign of gravitational ‘charge’, and so gravity ‘gains’ relative to electrical forces in larger objects… An apple falls only when the combined gravity of all the atoms in the Earth can defeat the electrical stresses in the stalk holding it to the tree. Gravity is important to us because we live on the heavy Earth.

Sand grains and sugar lumps are, like us, affected by the gravity of the massive Earth. But their self-gravity — the gravitational pull that their constituent atoms exert on each other, rather than on the entire Earth — is negligible. Self-gravity is not important in asteroids, nor in Mars’s two small potato-shaped moons, Phobos and Deimos. But bodies as large as planets (and even our own large Moon) are not rigid enough to maintain an irregular shape: gravity makes them nearly round. And masses above that of Jupiter get crushed by their own gravity to extraordinary densities (unless the centre gets hot enough to supply a balancing pressure, which is what happens in the Sun and other stars like it). It is because gravity is so weak that a typical star like the Sun is so massive. In any lesser aggregate, gravity could not compete with the pressure, nor squeeze the material hot and dense enough to make it shine.

Gravitation is feebler than the forces governing the microworld by the number N, about 1036. What would happen if it weren’t quite so weak? Imagine, for instance, a universe where gravity was ‘only’ 1030 rather than 1036 feebler than electric forces. Atoms and molecules would behave just as in our actual universe, but objects would not need to be so large before gravity became competitive with the other forces. The number of atoms needed to make a star (a gravitationally bound fusion reactor) would be a billion times less in this imagined universe. Planet masses would also be scaled down by a billion. Irrespective of whether these planets could retain steady orbits, the strength of gravity would stunt the evolutionary potential on them. In an imaginary strong-gravity world, even insects would need thick legs to support them, and no animals could get much larger. Gravity would crush anything as large as ourselves.

Galaxies would form much more quickly in such a universe, and would be miniaturized. Instead of the stars being widely dispersed, they would be so densely packed that close encounters would be frequent. This would in itself preclude stable planetary systems, because the orbits would be disturbed by passing stars — something that (fortunately for our Earth) is unlikely to happen in our own Solar System.

But what would preclude a complex ecosystem even more would be the limited time available for development. Heat would leak more quickly from these ‘mini-stars’: in this hypothetical strong-gravity world, stellar lifetimes would be a million times shorter. Instead of living for ten billion years, a typical star would live for about 10,000 years. A mini-Sun would burn faster, and would have exhausted its energy before even the first steps in organic evolution had got under way. Conditions for complex evolution would undoubtedly be less favourable if (leaving everything else unchanged) gravity were stronger. There wouldn’t be such a huge gulf as there is in our actual universe between the immense timespans of astronomical processes and the basic microphysical timescales for physical or chemical reactions. The converse, however, is that an even weaker gravity could allow even more elaborate and longer-lived structures to develop.

Gravity is the organizing force for the cosmos… [It] is crucial in allowing structure to unfold from a Big Bang that was initially almost featureless. But it is only because it is weak compared with other forces that large and long-lived structures can exist. Paradoxically, the weaker gravity is (provided that it isn’t actually zero), the grander and more complex can be its consequences. We have no theory that tells us the value of N. All we know is that nothing as complex as humankind could have emerged if N were much less than 1,000,000,000,000,000,000,000,000,000,000,000,000.

Buy Just Six Numbers.