November 14, 2010 § Leave a comment
There is no way back;
we go and never return.
Death lies up ahead
and birth behind;
further and further away
and more so.
No road, no friendly sign;
just the leaves falling on the patio,
the bitter weeds on the graves
turning bitterer still.
– Rua Breathnach –
November 9, 2010 § Leave a comment
Pulsars are a type of rapidly rotating neutron star; the remnants of supernova explosions. These “stellar corpses” represent, perhaps, the ultimate state of matter, as we know it, in the Universe. Akin to the regular pulses of lighthouses, pulsars emit beams of electromagnetic radiation that can be detected as radio waves here on Earth. The rate at which these beams are emitted ranges from a few pulses to over 1,800 pulses per second. Listening to recordings of the waves emitted by a neutron star spinning 8 times per second, one is reminded of the steady click of a stylus on a scratched record. In contrast, the individual pulses of the fastest spinning pulsars can be heard only by slowing them down considerably, so that they sound like skipping CDs.
The existence of neutron stars was predicted in the early nineteen thirties by two American scientists; Baade and Zwicky. The third component of the atom, the neutron, – the electron and the proton were already known – had only recently been discovered. Baade and Zwicky believed that neutron stars, if they really existed, would consist entirely of neutrons packed very tightly; the remnants of exploded stars. Thirty four years later, in 1968, the Italian physicist Franco Pacini went so far as to say that if these stars existed, they were likely to rotate very rapidly and to emit electromagnetic waves.
Convincing evidence of the reality of these strange celestial bodies was found in 1969. Jocelyn Bell, a Belfast-born PhD student working at Cambridge in England, detected what she thought at first to be signs of interstellar communication: pulses of radio waves travelling across the Universe at fairly regular intervals. She and her colleague, Anthony Hewish, slowly gathered evidence for these “pulsed radio sources” on mile after mile of magnetic tape. To their dismay, they discovered that these pulses were not alien civilizations trying to contact the Earth (or each other), but hard evidence for the existence of swiftly rotating star remnants, from then on known as pulsars (a coinage derived from ‘Pulsating source of radio emissions’). Nevertheless, once other pulsars had been detected, it was perceived that some of them emitted radio waves in irregular blips, and not in the regular patterns that had been predicted. This meant that their physics was much more complicated – i.e. difficult to measure accurately – than had previously been assumed.
The Crab pulsar, at the heart of the Crab Nebula in the constellation of Taurus, is one of the best known pulsars. This is mainly due to the fact that it emits a very regular pulse – 30.2 times per second – and is easily detectable by radio-astronomical instruments on Earth. Like all known pulsars, it is thought to be the end product of a supernova explosion. Incidentally, the Crab nebula (and, by extension, the Crab pulsar within it) was the first astronomical object identified with a historical supernova explosion. It corresponds to a bright explosion visible in the sky in July of the year 1054, recorded by, among others, Chinese, Japanese and Persian astronomers. It has also been claimed that an obscure entry in a number of Irish monastic annals originally referred to the same event.
A supernova explosion occurs during the gravitational collapse of a star that has burnt out the nuclear fuel – mainly hydrogen and helium – in its core. Approximately 4 billion years in the future, our own Sun will die such a death. All the matter in our solar system, including ourselves – from gold and oxygen, to the molecules that make up our DNA – is the product of at least two previous stellar explosions. Depending on the mass of the star, the end product of its collapse can be either a black hole, a pulsar or a white dwarf. A star with roughly 6 to 8 times the mass of the Sun, or above, will become a black hole; ordinary matter being unable to resist the force of the gravitational collapse of such a massive body. A smaller or medium star, at least 1.4 times the mass of our Sun, will most likely become a pulsar: on collapsing it will blow off much of its excess material, and stabilise again to become an extremely dense body of roughly 20 – 30 kilometres across. One teaspoon of the matter of such a pulsar would have a mass equivalent to all the water in Galway Bay.
Pulsars are among the most extreme objects known in the Universe. Examples have been detected that have a surface gravitational force 1013 (ten with thirteen zeros after it) timesstronger than the Earth’s. As Einstein predicted in his theory of general relativity, space-time is extremely warped under such conditions. That is to say, light radiated from a star with a gravitational force of this magnitude bends such that parts of the normally invisible rear surface become visible. Time, on the other hand, as measured from a distant perspective (e.g. Earth’s orbit), will seem to pass more slowly in the star’s proximity. Pulsars have also been shown to have an atmosphere, and even a mantle and crust. The atmosphere of one of the roughly 2,000 known pulsars, for example, is thought to be composed of gaseous iron, about two inches thick, and extremely dense. The surface temperature of such a body is reckoned to be somewhere in the region of 1,000,000 Celsius, growing hotter as you move towards the core.
The most significant feature of pulsars, however, and the feature that distinguishes them from ordinary, slowly rotating neutron stars, is their massive magnetic fields. Millisecond pulsars – pulsars that rotate more than once every millisecond – have been shown to generate a magnetic field of over 1015 times greater than the Earth’s. As the pulsar spins, energy in the form of electrons and protons travels along the magnetic field at speeds sometimes approaching the speed of light. This energy is ejected from the magnetic poles producing an electromagnetic beam that we can detect on Earth as radio waves (provided the beams are pointing edge-on relative to our line of sight). The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. This misalignment causes the beam to be seen once for every rotation of the neutron star, which leads to the “pulsed” nature of its appearance: the so-called ‘lighthouse effect’.
At this point, two questions might spring to mind:
- Why does a pulsar spin so fast? And,
- What is a pulsar made of?
Basic physics teaches us that the larger the size of a body the slower it spins. When a figure skater extends her arms and legs, she turns more slowly than if she were to keep her limbs tightly tucked in towards her centre of gravity. In the same way, the velocity of a planet’s or a star’s spin depends on the distribution of its mass (how much space it occupies). Our Sun has a radius (the distance from its centre to its outermost surface) of approximately 700,000 kilometres, and a rotation period of 24.47 days (i.e. it takes around 24 and a half days for it to rotate once). Were you to retain the mass of the Sun but shrink it so that its diameter (2 × radius) was no more than 20 kilometres, it would spin much, much faster: about once every millisecond.
Three quarks for Muster Mark!
Sure he has not got much of a bark
And sure any he has it’s all beside the mark.
James Joyce, Finnegan’s Wake, p. 383.
The answer to the second question is: nobody knows for sure. There has been plenty of speculation, however. Some astronomers still believe that the cores of neutron stars are composed of quickly spinning neutrons (subatomic particles with a neutral electrical charge that are slightly heavier than protons). Others posit quarks (elementary particles that combine to form composite particles, the most stable of which are protons and neutrons, the building blocks of atomic nuclei).
Finally, millisecond pulsars might one day replace atomic clocks as the most accurate instruments we have to measure time. By comparing the rotational stabilities of these stars with one another and with terrestrial time scales, we might one day be able to narrow the margin of error in our time-keeping significantly. However, we would still need to adjust our timepieces by 1.3 seconds every million years: the current estimated rate at which pulsars slow down over time. We might take comfort in the fact that all civilizations have had to contend with the same problem; that nature does not produce regular astronomical patterns. Until we find a more accurate clock, we shall have to add 130,000 nanoseconds (a nanosecond is a billionth of a second) to our calendar once every century, or so, to keep the accounts balanced.
– Rua Breathnach
November 1, 2010 § Leave a comment
Urashima ran up among them and grabbing hold of one of the children’s sticks, held it above his head and shouted, “This turtle has done you no harm! Why do you torment it?”. The children were quiet and stared at their feet. After a few moments, their mothers could be heard calling them inside for their evening meal of fish and rice. They left silently in groups of twos and threes, trailing their sticks behind them in the sand. Urashima watched them go and then stooped down to see what condition the turtle was in. The poor creature was not physically damaged but was quite obviously exhausted and afraid. It had tucked its head and feet in under its black and green patterned shell.
Urashima spoke to it kindly, saying: “Now little turtle. Your ordeal is over. You can go on your way. I won’t hurt you.” But the turtle wouldn’t budge an inch. It lay there helplessly, no doubt wishing to be left alone. Urashima cupped his hands, and scooping up some sea water, poured it over the turtle’s back. He did this four or five times before the turtle dared to poke out its curious head and open its beady eyes. Urashima sat with it and spoke to it soothingly. After a while, the turtle extended its legs, and looking around warily began slowly, very slowly, to move towards the breaking surf. Urashima watched it as it tumbled around in the first breakers and finally disappeared beneath the waves. “Goodbye, my friend!”, he whispered, and turned for home.
Five years or more passed before Urashima fulfilled his wish to sail into the horizon. His father had grown old and seldom used the boat for fishing. In the natural course of things, Urashima had taken over as the family’s breadwinner, and it was now he who plied the waters and brought home the catch. And so, it happened that one evening Urashima was fishing a few miles out to sea. He had been fishing all day, but had caught nothing. He was loath to return home empty-handed. After all, his ageing father and mother depended on him for food. He made a wish and cast his net one last time into the darkening waves. Immediately he felt a small tug. “At last,”, he thought to himself, “something to show for my troubles!”. When he pulled the net in, he did not see the familiar silvery struggling body of a fish. Instead, a round body thumped down on the planks in the bow of the boat, entangled in the nets. “Turtle meat!”, he said aloud, and laughed. But when he had disentangled the creature, he saw that it was too small to feed even one person; let alone three hungry people. He was about to throw it overboard when a small voice said, “Don’t throw me back! I’ve come to find you. You who saved me all those years ago”. Urashima stood holding the turtle for a moment. He looked around him to see who was speaking. He was afraid. It was getting dark and he feared that it might be an evil spirit that was summoning him from the deep. He kept calm, however, and asked aloud, “Who speaks?”. Again the little turtle piped, “Here I am. Do you not remember me?”. Urashima had no doubt this time who had spoken and, in his fright, dropped the creature overboard. At this, the turtle grew to an enormous size and swam alongside the boat. Lifting its head out of the brine, it said, “Don’t be frightened. I was sent by the daughter of Ryujin, the Dragon of the Sea. I have come to fetch you and bring you to Ryugu-jo, the Dragon’s Palace in the deep. The turtle paddled around to the stern of the boat and pushed its hind quarters up against it. “Sit up on my back and I will take you there.”
As though in a trance, Urashima, forgetting all danger, climbed over the stern and onto the turtle’s back. As he did so, he felt a great internal change come over him. He gasped like a fish on dry land and suddenly wished only to plunge into the sea’s murky depths. Soon, he was gliding through the deep waters, breathing naturally as though it were his native element. He held tightly to the turtle’s neck and thought not of his vessel far above nor of his distant home and parents. How long the journey lasted, he could not say. For who can tell the time when all is unchanging darkness? He must have slept, however, for when he awoke he was on dry land, lying on a bed of lush, green sea-grass.
When his eyes had re-adjusted to the light, he saw that a beautiful maiden was seated next to him, weaving a garland of anemones and singing a strange and enchanting song:
“Come to my father’s palace in the deep,
where time does pass but none do weep.
I’ll be your companion and flower-wife,
for this day you spared my life.”
She smiled down at him and made him drink a thimbleful of water. To his parched lips and tongue it tasted sweeter than honey. She kissed his forehead and bade him sleep. But he was not tired. He gazed around him in wonder. There was no sky above; only a vault made of red coral, resting on slender pillars of solid crystal. He looked and listened for what seemed like a long time and then fell into a deep slumber.
Urashima awoke to the same sight every morning – if morning it really was; the smiling maiden, the garlanding of flowers, the gorgeous palace above his head. He felt no need or desire to move. Words were rarely exchanged between him and the maiden. An understanding and appreciation of beauty and love seemed to pervade all things. He lived in a blissful, awakened dream, and wanted for nothing. He neither longed for the past nor hoped for the future. All was an eternal, unchanging present, where the hours were undivided and toil and hardship unknown. On awakening, he would drink a thimbleful of water and listen to the song of the beautiful maiden. The song never varied, and he never got tired of hearing it sung. The maiden, indeed, was a flower-like creature, and her kisses were as sweet as nectar. But, unlike an earthly flower, her splendour did not fade.
One morning Urashima awoke and found that he was alone. He was greatly astonished and afraid. He called out but only the echo of his own voice replied. He stood up, feebly, and walked slowly towards a source of light at one end of the palace. As he passed through a high arched doorway, he could hear long-forgotten sounds: the chirping of birds, the hum of insects and bees, and the great surging movement of life itself. He wandered, leaving the glittering palace behind. He walked through flower-carpeted meadows until he reached the sea. He walked along the beach blindly, drinking in the tastes and sounds and smells that reached his senses. He walked and walked following the curve of the shore. The landscape became barren and rough. The sea roared and thumped against the cliffs. Sea-birds were swept inland by high winds, shrieking. The sea-spray bit into his skin. It was cold and harsh. Still he continued onwards, the faint sun at his back. Again his surroundings changed. Thick drifts of snow blanketed the fields. Life seemed scarce in the air and on the ground. Small birds clung to the black-barked trees and did not sing. His feet were red and frost-bitten; his hair and eyebrows were encrusted with ice. Urashima realised that he must be on an island, or that he must have wandered a long time, for presently the sun shone on his face again. It was springtime and the grass grew underfoot. The Dragon’s Palace stood out against a clear blue sky.
At the doorway of the palace stood the maiden, his wife. She looked at him oddly. “Where have you been?”, she asked. “Have you seen the four seasons?”, she added with a smile. Urashima replied, “Yes, my love, and my heart is filled with sadness and longing.” The maiden looked puzzled as he uttered these words. “How can you be sad when you have me and all else you desire?”, she asked. “I must travel back to see my home and family again. I will never be happy unless you grant me this wish”, Urashima replied bitterly. The maiden looked at him strangely, and said, “You will be taking a great risk in parting from these shores. No mortal has ever done so and lived to tell the tale. But if it is your heart’s wish, I cannot stop you.” There were tears glistening in her beautiful eyes. She took him gently by the hand and said, “Before you go, take this box, where the weight of all the time you have passed here is stored. It will protect you. But I warn you: under no circumstances must you open it. For then surely we shall never meet again. ” Urashima accepted the box and kissed his wife on the forehead. A swirling mist enveloped the palace and, in the twinkling of an eye, the giant turtle appeared by his side and bade him sit up on its back. He took one last look around, but the maiden had vanished, and the Dragon’s palace grew faint like an image distant in time and space.
When Urashima reached the shores of Suminoye, he was surprised to find it familiar but somehow different to how he remembered it. No longer were there colourful fishing boats bobbing on the waves, and the wooden huts among the dunes were rotting, half-buried in the sand. No children played on the shore and, apart from an old woman sitting on the beach cooking a herring on a stick over a fire, not a soul could be seen. The old woman had spotted him as soon as he emerged from the waves. She hid the charred fish under her shawl and watched him suspiciously as he approached. He bowed to her and asked her if he might dry himself by the fire. He stood in silence for a long time. The old woman, perceiving that there was little threat to her fish, began toasting it over the embers again. Eventually, he asked her did she know his parents and if some disaster had overtaken the fishing folk thereabouts. The old woman looked at him curiously and said that there had been no fishing folk here for as long as she could remember, and that the names he had mentioned belonged to a time long past, and all but forgotten.
He was puzzled, and leaving the old woman he walked again among the dunes searching for a sign of life. He found nothing; only scraps of wood and iron. Desolately, he came down to the shore and looked out to the west. The evening sun projected a tongue of flame over the calm waters. He sat down on the sand and, handling the box the maiden had given him, thought of his father and mother and how they must be buried somewhere in the dunes. Perhaps some planks from his little fishing vessel had floated in with the tides and confirmed their worst fears and imaginings? Perhaps they had died of hunger, or of grief? Too much time had passed to know the truth; and yet, for Urashima it had hardly passed at all. For a long time he looked out to sea. At last, the sky grew dark and slivers of moonlight were reflected on the surface of the inky black waters. The chariot of stars rode across the heavens from east to west. His thin cloak ruffled and flapped in the breeze. “Goodbye, eternity”, he whispered and, lifting the lid of the box, he was immediately turned to dust.
– Freely adapted from the Japanese folktale, Urashima Tarō, by Rua Breathnach.