Galileo Galilei (1564 – 1642)

Credit: Leoni

Galileo

Galileo was born in the same year as Shakespeare and on the day of Michelangelo’s death. Appointed to the Chair of Mathematics at the University of Pisa when he was 25 his studies of motion there and later at Padua provided the foundation of the study of dynamics. His contributions to the the development of gravitational theory and motion were to terminally undermine the tenets of Aristotelian motion and physics.

In 1604 a bright new star appeared in the constellation Serpentarius. Galileo’s observations detected no parallax, suggesting it was a star and not some atmospheric phenomenon. This result confirmed Brahe’s findings from the nova of 1572 that stars could change and again challenged the Aristotelian orthodoxy.

When Galileo heard about a new optical device, the telescope, in 1609 he quickly built his own version. He then used it and more refined telescopes to systematically observe the night sky. Details on Galileo’s use of the telescope can be found in the HSC Astrophysics section. His findings, published in 1610 in Sidereus nuncius (The starry messenger) had important implications.

The Moon:
According to Aristotelian principles the Moon was above the sub-lunary sphere and in the heavens, hence should be perfect. Galileo found the “surface of the moon to be not smooth, even and perfectly spherical,…,but on the contrary, to be uneven, rough, and crowded with depressions and bulges. And it is like the face of the earth itself, which is marked here and there with chains of mountains and depths of valleys.” He calculated the heights of the mountains by measuring the lengths of their shadows and applying geometry. He also detected earthshine on the lunar surface, that is the Moon was lit up by reflected light from the Earth just like we receive reflected light from the Moon.
Stars in the Milky Way:

Galileo’s drawing of the Pleiades shows many more stars than visible to the unaided eye.

Even through a telescope the stars still appeared as points of light. Galileo suggested that this was due to their immense distance from Earth. This then eased the problem posed by the failure of astronomers to detect stellar parallax that was a consequence of Copernicus’ model. On turning his telescope to the band of the Milky Way Galileo saw it resolved into thousands of hitherto unseen stars. This posed the question as to why there were invisible objects in the night sky?
The Moons of Jupiter:

The moons of Jupiter as drawn by Galileo over successive nights.

Observations of the planet Jupiter over successive night revealed four star-like objects in a line with it. The objects moved from night to night, sometimes disappearing behind or in front of the planet. Galileo correctly inferred that these objects were moons of Jupiter and orbited it just as our Moon orbits Earth. Today these four moons are known as the Galilean satellites; Io, Europa, Ganymede and Callisto.
For the first time, objects had been observed orbiting another planet, thus weakening the hold of the Ptolemaic model. The Earth was clearly seen to not be at the centre of all motions.

Two subsequent observations also undermined the Arisotelian-Ptolemaic Universe. Galileo found that Venus exhibits phases, just like the Moon. This of course could be accounted for in a Copernican system but not in a Ptolemaic one. He published a letter in 1613 announcing his discovery of sunspots in which he also proclaimed his belief in the Copernican model. Monitoring sunspots showed that the Sun rotated once every 27 days and that the spots themselves changed. The concept of a perfect, unchanging Sun thus also became untenable.

In presenting his views in his Dialogue concerning the two chief systems of the world, the Ptolemaic and the Copernican in 1632 Galileo reignited his earlier conflict with the authorities of the Catholic Church. Eventually forced into publicly recanting his belief in the Copernican system and being placed under comfortable house arrest his Dialogue, along with the works of Copernicus and Kepler was placed on the Index of Forbidden Books.

Galileo spent the last years of his life working once again on trying to understand motion. The resultant final book Dialogues concerning two new sciences had to be smuggled out of Italy before being published in Holland in 1638. It primarily dealt with describing motion, kinematics, but also revealed that acceleration resulted from the application of a force and that he was aware of the concept of inertia. He rejected Aristotle’s ideas of forced and natural motions after studying falling or rolling objects and projectiles and realised that gravity was some type of force acting in terrestrial situations though he does not seem to have extended this to heavenly motions.

Whilst Galileo did not propose his own model of the Universe, his observational, experimental and theoretical work provided the conclusive evidence need to overthrow the Aristotelian-Ptolemaic system. His work on forces was to help Newton develop his dynamics. Galileo died in 1642, the year that Newton was born.

Sir Isaac Newton (1642 – 1727)

Sir Isaac Newton

Isaac Newton is the pivotal figure in the scientific revolution of the 16th and 17th centuries. He discovered the composition of white light, and laid the foundations of modern optics. In mathematics he invented infinitesimal calculus and the binomial theorem. His work on the laws of motion and of universal gravitation became the basis of modern physics. Whilst today remembered for his immense contributions to science the bulk of his writings were actually in the fields of theology and alchemy though as his views on both of these was contrary to the establishment he kept many of them secret.

During 1665-6 Newton returned to his home at Woolsthorpe from Cambridge when the University closed due to the Great Plague. This period allowed him time to develop his ideas on optics and light, planetary motions and the concept of gravitation. By 1670 he was Lucasian Chair of Mathematics at Cambridge, had developed his corpuscular theory of light and built the first successful reflecting telescope, thus avoiding the chromatic aberration problems inherent in the lenses of refracting telescopes. For this he was elected a Fellow of the Royal Society. He withheld publication of chief work on light, Optiks, until 1704, the year after his adversary Robert Hooke died.

Newton’s scientific legacy rests on his other work, the Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), generally known as Principia published due to Edmond Halley’s urging and funding, in 1687. His detailed exposition of the concepts of force and inertia is summarised eloquently in his three axioms or Laws of Motion (from the translation in On the Shoulders of Giants, ed. by Stephen Hawking, Running Press, 2002).

Newton’s Laws of Motion:

Law I: Every body preserves in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
This is more commonly stated as: An object remains at rest or in a state of uniform motion unless acted on by an unbalanced force.
Law II: The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
This is now commonly referred to as F = ma and emphasises the vector nature of force.
Law III: To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

The true genius of his work is that he then went on and applied them not just to motion on Earth but realised that they applied equally to the motions of other bodies such as planets in space. He applied his mathematical techniques to investigate the nature of the force between the Earth and the Moon, and the Earth and the Sun. His solution revealed the force to obey an inverse-square relationship and result in elliptical orbits as calculated by Kepler.

Newton’s Law of Universal Gravitation

(The formulae used in this section are not required for the NSW Stage 6 Preliminary Course. They are explicitly required for unit 9.2 Space in the HSC course)

As he showed in Book 3, System of the World of his Principia, Newton could apply his law of universal gravitation to accurately predict the motions of planets, the orbits of comets and even account for tides on Earth. His law can be mathematically expressed as follows:

F ∝ m₁m₂/ r²

where F is the force between any two objects of masses m₁ and m₂ respectively and separated by a distance r.
As there are no other variables involved the equation becomes:

F = Gm₁m₂/ r² (1.2)

where G is a constant known as the Universal Gravitational Constant.
(G = 6.673 × 10^-11 Nm²kg^-2)

This can also be expressed in words as:
The force of attraction between any two bodies in the Universe is proportional to the product of their masses and inversely proportional to the square of their distance apart.
Having shown that gravitationally all the mass of an object can be assumed to be at its centre of mass, the gravitational force therefore acts along a line joining the two bodies. It is always an attractive force. The gravitational mass of an object was shown to be identical to its inertial mass (that which hinders its change in motion).

For a two-body system such as the Sun-Earth an equilibrium exists such that the gravitational force = centripetal force. Using this relationship Newton was able to derive Kepler’s Third Law.

Since F_G = F_C
then: F = Gm_Sm_E / r² = m_Erω² (1.3)

where m_S and m_E are the masses of the Sun and Earth and ω is the angular velocity of the Earth around the Sun.
Simplifying (1.3) gives

Gm_S/ ω² = r³ (1.4)

Now the time taken for one complete revolution of the Earth around the Sun,is its orbital period, T such that:

T = 2π / ω (1.5)
so ω² = 4π²/ T² (1.6)

substituting this into (1.4) gives:

Gm_ST² / 4π² = r³
which can be rewritten as:

T² = 4π²r³/ Gm_S
Note this is of the form:

T² = kr³
which is Kepler’s Third Law, and the value of k is:

k = Gm_S/ 4π² (1.7)

This value of k is a constant for all bodies orbiting the Sun as it only depends upon the mass of the Sun and the constant, G.

Newton’s contributions profoundly influenced subsequent generations. His view of the Universe was a mechanistic one that ran like clockwork and had a designer. The success of his law of gravitation was confirmed in 1758 when a bright comet returned as predicted earlier by Edmond Halley. He realised that it would be the same comet that had previously been seen in 1531, 1608 and 1682. This comet was subsequently named in his honour and we now know it was the same comet shown on the Bayeaux Tapestry commerating the Norman invasion of England in 1066.

String Theory and Newton’s Law of Gravity

By Andrew Zimmerman Jones and Daniel Robbins from String Theory For Dummies

String theory is based upon our understanding of matter and the other forces of nature, in terms of quantum mechanics, including Newton’s law of gravity.

Sir Isaac Newton developed his theory of gravity in the late 1600s. This amazing theory involved bringing together an understanding of astronomy and the principles of motion (known as mechanics or kinematics) into one comprehensive framework that also required the invention of a new form of mathematics: calculus. In Newton’s gravitational theory, objects are drawn together by a physical force that spans vast distances of space.

The key is that gravity binds all objects together (much like the Force in Star Wars). The apple falling from a tree and the moon’s motion around Earth are two manifestations of the exact same fundamental force.

The relationship that Sir Isaac Newton discovered was a mathematical relationship (he did, after all, have to invent calculus to get it all to work out), just like relativity, quantum mechanics, and string theory.

In Newton’s gravitational theory, the force between two objects is based on the product of their masses, divided by the square of the distance between them. In other words, the heavier the two objects are, the more force there is between them, assuming the distance between them stays the same.

The fact that the force is divided by distance squared means that if the same two objects are closer to each other, the power of gravity increases. If the distance gets wider, the force drops. The inverse square relationship means that if the distance doubles, the force drops to one-fourth of its original intensity. If the distance is halved, the force increases by four times.

If the objects are very far away, the effect of gravity becomes very small. The reason gravity has any impact on the universe is because there’s a lot of it. Gravity itself is very weak, as forces go.

The opposite is true, as well, and if two objects get extremely close to each other — and I’m talking extremely close here — then gravity can become incredibly powerful, even among objects that don’t have much mass, like the fundamental particles of physics.

This isn’t the only reason gravity is observed so much. Gravity’s strength in the universe also comes from the fact that it’s always attracting objects together. The electromagnetic force sometimes attracts objects and sometimes repulses them, so on the scale of the universe at large, it tends to counteract itself.

Finally, gravity interacts at very large distances, as opposed to some other forces (the nuclear forces) that only work at distances smaller than an atom.

Despite the success of Newton’s theory, he had a few nagging problems in the back of his mind. First and foremost among those was the fact that though he had a model for gravity, he didn’t know why gravity worked.

The gravity that he described was an almost mystical force (like the Force!), acting across great distances with no real physical connection required. It would take two centuries and Albert Einstein to resolve this problem.

Boning Up (Er, Down) on Gravity

Gravity is a force of nature that you experience every day. It’s produced by all matter in the universe and attracts all pieces of matter, regardless of type. The Earth produces gravity and so do the sun, other planets, your car, your house, and your body.

Gravity basics

Sir Isaac Newton invented gravity in 1687 when he failed to pay attention while sitting under a tree and got bonked on the noggin by an apple. Before that, gravity didn’t exist, and everyone just floated around. Okay, Isaac Newton didn’t invent gravity. But the famous mathematician was the first to study gravity seriously, and he came up with the theory of how gravity works.

Newton’s law of universal gravitation states that every object in the universe attracts every other object in the universe. The amount (force) of the attraction depends on the mass of the object. If you’re sitting in front of your television, you may be surprised to know that the television set is attracting you. However, because the mass of the TV is so small compared to the mass of the Earth, you don’t notice the physical “pull” toward the television set.

On Earth, gravity pulls objects downward toward the center of the Earth. The force of gravity acting on an object is equal to the weight of the object. Of course, other planets have lesser or greater masses than the Earth, so the weight of objects on those planets, and thus the amount of gravitational pull, will be different.

Newton’s law also says that the greater the distance between two objects, the less the objects will attract each other. In other words, the farther away an object is from the Earth (or any large body), the less it will weigh. If you stand at the top of a high mountain, you will weigh less than you do at sea level. (Don’t get too excited about this weight-loss technique; gravitational pull isn’t the next big diet craze. The difference is incredibly small. Sorry!)

For an object to really lose weight, it must be far away from the Earth (or any other large body). When an object is far enough away from these bodies that it experiences practically no gravitational pull from them, it is said to experience weightlessness — just like the astronauts you see on TV.

False gravity of a spinning object: Centrifugal force

An object traveling in a circle appears to experience a gravitational force. This isn’t really gravity, but instead it’s a concept known as centrifugal force. The amount of force depends on the mass of the object, the speed of rotation, and the distance from the center of the rotation:

The more massive the object, the greater the force.

The greater the speed of the object, the greater the force.

The greater the distance from the center of rotation, the greater the force.

Centrifugal force doesn’t really exist, so many scientists refer to it as a false force. It’s not a force at all, but rather a product of Newton’s (remember him?) laws of motion. This characterization seems wrong because when your car goes off the road and crashes or when your bicycle skids out from under you when cornering a slippery curve, you feel like this force had something to do with it. Because it feels real, it’s often useful to treat it as if it’s a real force.

If you’re riding a merry-go-round on the playground, you have to exert a constant force to keep from flying off. This force isn’t due to something actually pushing you in that direction, but by your body’s inertia trying to keep you moving in a straight line. Because one of Newton’s laws states that accelerating objects tend to want to travel in one direction, as the merry-go-round turns, your accelerating body wants to keep traveling in one direction, so you feel you’re being “pushed” outward.

It used to be thought that the world was flat. That beyond the horizon lurked a bottomless void, measureless to humans. As our ships got better and our navigators more confident, vessels that disappeared over one horizon began returning triumphant from the other. The Earth was curved, a sphere with no edges. We didn’t fall off because a mysterious force called gravity kept everyone and everything stuck to the planet’s surface. Then Albert Einstein came along and told us that space itself was curved. Now we realise that if we sail off into the sea of curved space we call the Universe, there are indeed bottomless voids that the unwary traveler can fall into – and they are measureless to humans. They lurk on the other side of their own dark horizons, and we call them Black Holes. FATAL ATTRACTION A black hole is an astronomical contradiction – a dark star, an invisible nothing, a prison of light. Its boundary is marked by the so-called Event Horizon, a sphere of darkness that shrouds the inside and defines the point of no return. There is no solid surface beyond, just a bottomless gravitational whirlpool so strong that it sucks everything – even light – relentlessly inward. Oblivion waits at the centre in the form of the Singularity, Gravity’s fatal attractor. Hidden eternally from view, the Singularity marks the spot where an immense gravitational force has been concentrated. All the mass, light and energy that has ever fallen into the black hole is compressed by its own overwhelming gravity into a point that is infinitely small and infinitely dense. The more a black hole swallows, the heavier it gets, yet the singularity never changes. Space has been squelched out of existence and Time has been squeezed to a stop. Step over the event horizon and for all intents and purposes you’ve fallen off the edge of the universe. A BRIEF HISTORY OF GRAVITY The concept of Gravity, this unseen force that dominates our lives and pulls us eternally towards the ground, has long challenged the greatest human minds. Even in Galileo’s day, the tower at Pisa had a good lean to it – perfect for dropping things off. Galileo wondered why no matter how heavy or light objects were, they all took the same amount of time to fall to Earth. He puzzled too about why the planets moved they way they did. His conviction that they orbited around the sun led to house arrest for heresy. He was still trying to put the gravity puzzle together when he died in Florence in 1642. On Christmas Day that same year, the gravity baton was handed to Isaac Newton, born weak and premature in a Lincolnshire farmhouse. Twenty three years later, Newton returned to Woolsthorpe Manor to sit out the plague sweeping southern England. With 18 months quiet thought in the countryside he discovered calculus, unravelled the nature of light, and began formulating laws for the motion of the planets: discoveries that still underpin most of modern physics. One day when he was having a short break with a cup of tea, a falling apple interrupted his thoughts and led him to ponder gravity itself. The reason the apple fell straight down was that it was trying to fall to the centre of the earth where gravitational attraction was focused. And the Earth wasn’t the only object that had gravity, so did the moon, the sun and the planets. In fact, Newton reasoned, every object in the universe – including ourselves – has gravity. The bigger and heavier, the greater it’s gravitational force. We are glued to the surface of the Earth – and not the other way round – only because it is has so much more mass. The Earth orbits around the sun for the same reason. Finally, Newton had found a reason for the heavens to move the way they do. DARK STARS In 1784, John Michell, Rector of Thornhill Church in Yorkshire and great forgotten 18th Century scientist, became intrigued with the idea of escape velocity – the minimum speed with which you need to travel upwards from a star or planet to escape its gravitational clutches. He knew that gravity depended upon mass and he knew the speed of light was fast but finite. How heavy, he wondered, would the sun have to become before its gravity would become so great that even light (which travels at 299,792 km/second) would be held back at its surface? The answer, Michell reasoned, was that if the sun was the same size but weighed 500 times more, the light from the Sun would not escape the Suns own gravity. The Sun would simply disappear from view. A few years later the great French mathematician Laplace came to the same conclusion independently. The concept of the Dark Star was born. A UNIVERSE OF HOLES Black holes remained an ignored theoretical curio until a young clerk in a Swiss patent office published his General Theory of Relativity in 1915. Albert Einstein realised that the universe was a fundamentally different place to the clockwork universe of Newton and commonsense. The three dimensions of space could not be separated from the fourth dimension of time. Together they form the ‘Spacetime’ continuum, a kind of invisible scaffolding that defines existence. Spacetime, though, is not an absolute, fixed thing. It can be warped, bent and curved. Spacetime is ‘straight’ only when it doesn’t have anything in it. Wherever there is mass, there is gravity. Wherever there is gravity, space is curved. The curvature of space dictates how an object will move through it. The object will dictate how space bends around it. Gravity, according to Einstein, is the curvature of space. Einstein’s thought experiment was to imagine space and time being squashed flat like a 2D rubber sheet. Put a massive object like the sun on the sheet and it will bend. The more dense an object is, the deeper the depression it makes in Spacetime and the stronger the gravity. Eventually a point is reached when the walls of the depression are stretched so steeply that nothing can climb out of it. It is, quite literally, a hole in the universe. A STAR IS BORN To understand the very large in the universe, you need to start with the very small. With the unlocking of the secrets of nuclear energy, scientists finally got a clue to how black holes might form in nature. Stars are born when enormous clouds of cosmic dust and hydrogen begin to clump and condense under their own gravitational weight. Gravity grows stronger by the hour as the increasing density of the protostar curves space more strongly. Faster and faster, the hydrogen gas falls in upon itself in the condensing core. The more it collides the hotter it grows. When the core reaches 10 million degrees, the hydrogen protons begin to fuse into helium. Some of the mass disappears, having being turned into energy and light. Like a giant cosmic light bulb, the star has switched itself on. Every star we see in the heavens has a giant nuclear reaction raging at its core. It’s what makes a star like our sun shine so hot and bright. Gravity is still trying to pull the star’s gas tighter and tighter but is matched now by the energy pouring outwards from the nuclear reaction in the core. The star settles into a precarious balance that gravity will always win in the end. THE BIG SQUEEZE The ultimate fate of a star depends upon its mass. Our sun is middle-aged. It switched on 5 billion years ago and has enough fuel to burn for 5 billion more. But when, in that far distant future, the spent heart of the sun sheds its outer layers and shuts down, gravity will squeeze the core so tight it cannot be squeezed any more. It will become a ‘white dwarf’, a feeble ember the size of the earth but a hundred thousand times more dense. The more massive the star is, the faster it burns its fuel and the shorter its life expectancy. A star 10 times as massive as the sun may survive only millions, not billions, of years. As it starts to collapse, the crush of in-falling matter slamming into the iron core sends the temperature rocketing to 50 billion degrees. The core has only seconds to respond – and it does so Supernova-style. A supernova is a massive explosion. Huge quantities of material are blown into space, but only from the outer regions of the star. Most of the star has actually imploded, with the core being given a gravity bear hug so extreme that the protons and electrons have been squeezed into a ball of superdense subatomic particles called neutrons. The resulting ‘neutron star’ would weigh about one and a half times as much as the sun but would measure only about 20 kilometres across – about the size of Brisbane. Astronomers can prove that neutron stars exist, because they give off a unique distress signal. Like a lighthouse warning of a dangerous shore, a neutron star sweeps space with a blinding beam of radiation, generated by a magnetic field more than a trillion times greater than the Earth’s. Such a neutron star is called a pulsar. To astronomers, the pulsing beam sweeping the darkness of space is an unmistakable warning that extreme gravity lurks nearby. GRAVITY’S FINAL TRIUMPH A neutron star resist the ongoing crush of gravity, only with its neutrons packed in like sardines in a tin. But if the remnants of the star after supernova weigh more than three times the mass of the sun, even neutrons cannot hold back the inexorable force of gravity. The neutrons are squashed into oblivion. The star’s core becomes so dense that gravity overwhelms space itself, distorting it so horribly that it, and time with it, is wrenched off from the outside universe. A darkness forms at the star’s heart and moves relentlessly outwards as the stars brilliance is sucked inwards. This is the hungry, growing maw of a black hole: gravity’s final triumph. There is no escape, no turning back, until the entire mass of the star has been swallowed and its brilliance completely extinguished. A BLACKNESS BEYOND BLACK Visible only by its invisibility, the margin of the black hole is marked by the event horizon, so-called because all events beyond are hidden from view. For a black hole like this the event horizon may be only a few kilometres in diameter but the void beyond impossibly deep to measure. The entire mass of the star has been reduced to a singularity – a point of infinite smallness and infinite density at the very centre of this black malevolence. The singularity is where science ends and speculation begins. Space and time have ceased to exist, replaced by a seething chaotic mass we call quantum foam. This bizarre conjecture is where Einstein’s laws fail. This is where the laws of quantum mechanics fail. This is the realm of something called Quantum Gravity – one of the hottest areas of advanced mathematical research. It is from a singularity that the Universe is believed to have begun. In many ways the collapse of a star to form a black hole singularity is the reverse of the Big Bang. Is this the way the Universe is going to end? Wilder speculation is that our entire universe might lurk inside someone else’s singularity. or even that universes can bud off from each other like this, like some sort of heavenly breeding organism. It wasn’t until 1967 that John Archibald Wheeler slipped the term “Black Hole” into his paper at a scientific conference, and into the lexicon of the late 20th Century. They may have become a household name – but are they real? BLACK HOLE OR WHITE ELEPHANT Einstein himself couldn’t believe that such an invisible impossibility as a black hole could exist in the real universe beyond his theories. Today, his successors have no such problems. Astronomers not only think they have identified nearly 30 black hole candidates in our own Milky Way galaxy, they are now getting the proof that the holes behave in the relativistic way that Einstein’s theories predict. A black hole is an elusive quarry with perfect camouflage: total blackness in the blackness of space. Searching for a black hole no bigger than Sydney’s CBD across hundreds or thousands of light years of space demands a sneaky approach. First you have to find a visible star that a black hole has trapped in orbit. Then you have to study how the star wobbles. John Wheeler described it as like looking for a pair of dancers on a dark dance floor. The heavy man dressed in black is invisible, but the bright white dress of a light women is an easy target as she is whirled around. Astronomers look for the bright stars that ‘orbit’ dark partners in the same way. One of the best candidates is the star called V404 Cygni. Calculations shows V404’s dark partner is twelve times more massive than our Sun, yet totally invisible. But for every black hole orbiting another star, there must be many more solitary ones yet unseen. One of these could lurk quietly much closer to home. THE COSMIC HOOVER Although black holes have the power to hoover up anything and everything that strays too close, they can’t hunt. Contrary to popular belief, if you replaced our Sun with a black hole of the same mass the Earth wouldn’t get sucked in, there just wouldn’t be any sunlight. You could even orbit a black hole in a spacecraft just so long as you kept a safe distance. Get too close though and strange things start happening. Space gets stretched longer and skinnier. You would find your feet being pulled miles away in front of you while your body is squeezed sideways. You will have become a piece of space spaghetti long before you reach the event horizon. Then you’d be ruptured into your own fundamental particles and disappear behind the veil of darkness. It’d be a spectacular way to go but no-one would see it because time is being stretched as well. The photons carrying the image would struggle harder to leave your body the closer you fell. Even with a few million years to spare, an outside observer will see you slow to a halt above the event horizon, before slowly fading from view. HEARTS OF DARKNESS Most astronomers now concede that a black hole heavyweight lurks in the centre of our own Milky Way galaxy. Latest estimates are that it weighs in at a whopping 2 million times the mass of the sun – a dwarf in comparison to some of the truly supermassive black holes that may lurk in the cosmos. By the 1950s, astronomers began turning optical telescopes towards some of the strongest signals that the new radio telescopes were picking up. Source number 3C 273 was found to be a bright star-like object with a ‘jet’ of intense radiation sticking out of it. It was the first of a number of similar objects given the name of ‘quasar’ or ‘quasi-stellar radio source’, but their real identity remained hidden for decades. Quasars have now been revealed to be the energetic hearts of very active galaxies: brilliant discs of superheated gas and ruptured stars swirling at nearly the speed of light. Great jets of charged particles are blasted thousands of light years into space from above & below – like an axle through a wheel. The central engine that is driving all this activity, though, is hidden deep inside. It has to be small and it must be extraordinarily dense. The mathematics demand that the only beast that can drive such a display of raw power is a supermassive black hole. The heavier the hole, the faster the gases whirl in orbit. Astronomers have observed speeds which tally with black holes weighing up to five thousand million suns. The theory goes like this: a galaxy evolves from a vast rotating cloud of gas that begins to clump and condense under its own weight into billions of stars arranged like an enormous Catherine wheel, a Mexican hat or a bee swarm. In the centre, where the gas is concentrated, enough matter to make millions or even billions stars has undergone titanic gravitational collapse to make a supermassive black hole. While the hole is still actively feeding on the inner part of the new galaxy it manifests itself as a quasar. Later, when all nearby food has been consumed, the black hole becomes quiescent, leaving a relatively quiet galactic core like the one in the Milky Way. If this theory is correct, then supermassive black holes are present in all but the smallest galaxies. TOWARDS THE WITHIN For all their ferocity, these supermassive black holes are surprisingly gentle giants up close. You can fall into one without turning to spaghetti. Suppose you or I were an astronaut about to step into such an abyss at the edge of the universe. As I approach the event horizon, blackness spreads upwards around me. The Universe shrinks to a bright point directly overhead. As I meet and cross the horizon the universe above disappears in a blinding flash of photons trapped in orbit around the hole. I am now inside the black hole and falling towards the Singularity. It’s not dark like I expected. I see a ring of dancing light where the singularity should be. It must be spinning so fast that the centrifugal force has balanced out gravity. Now it’s a naked glowing hula-hoop of indeterminate size. Around it I see glimpses of heavens unimaginable to humans, universes within universes, time within time… But hey! No matter what I might see or experience inside the black hole, I could never send a message out. The secrets I discover will die with me as I achieve oneness with the Universe, at the central Singularity.
	“A black hole is an astronomical contradiction – a dark star, an invisible nothing, a prison of light.”

	Hear Stephen Hawking describe the formation of a Black Hole (You will need the Real Audio 3 plug-in to hear the sound)
	“Step over the event horizon and for all intents and purposes you’ve fallen off the edge of the universe.”
	Hear CalTech professor Kip Thorne explain the Event Horizon.
	“Black holes remained an ignored theoretical curio until a young clerk in a Swiss patent office published his General Theory of Relativity in 1915. Albert Einstein realised that the universe was a fundamentally different place to the clockwork universe of Newton and commonsense.”

	Hear Sir Martin Rees, Britains Astronomer Royal, describe what would happen to you if you fell into a Black Hole.
	“To understand the very large in the universe, you need to start with the very small. With the unlocking of the secrets of nuclear energy, scientists finally got a clue to how black holes might form in nature. “
	Are Black Holes a way of jumping from one Universe to another? Hear what CalTech Professor Kip Thorne has to say.
	“Visible only by its invisibility, the margin of the black hole is marked by the event horizon, so-called because all events beyond are hidden from view.”

	Hear Professor Stephen Hawkings thoughts on using Black Holes as a way to time travel.
	“Einstein himself couldn’t believe that such an invisible impossibility as a black hole could exist in the real universe beyond his theories. Today, his successors have no such problems.”
	Does every Galaxy have a Black Hole at its centre? Hear what the British Astronomer Royal, Sir Martin Rees thinks.

	“Although black holes have the power to hoover up anything and everything that strays too close, they can’t hunt. Contrary to popular belief, if you replaced our Sun with a black hole of the same mass the Earth wouldn’t get sucked in, there just wouldn’t be any sunlight.”

	Hear what the British Astronomer Royal, Sir Martin Rees describe what it might be like to approach a Black Hole.
	“Quasars have now been revealed to be the energetic hearts of very active galaxies: brilliant discs of superheated gas and ruptured stars swirling at nearly the speed of light.”