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We live on a lump of rock...

I am starting this page on the 13 June 2013. To give me some focus, I am aiming it at our grandchildren - in descending order of age, Anastasia, Barney, Ewan, Josie and Tom. I want to draw together what I’ve learnt and what I’ve decided to believe through researching and writing other pages in The diary of a wandering mind - most recently and most importantly Life, the Universe and Everything - and make it as understandable as possible for young and non-scientific readers.

Our home

You and I live on a lump of rock which we call ’Earth’ or ’the Earth’, or just ‘the world’.

Our lump of rock is hurtling through space. The natural thing for lumps of anything moving in space to do is to carry on in a straight line, but our world isn't doing that. It is moving round and round our nearest star - the one we call ’the Sun’. Its path around the Sun is roughly a circle, and is called the Earth's orbit. It keeps us about 150 million kilometres, or 93 million miles, from the sun.

This makes our Earth a planet, which is what astronomers call the larger lumps of rock and other stuff that orbit around stars

There are eight planets in orbits around our Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn and Uranus - all except the Earth re called after ancient Roman gods and goddesses. With the Sun and various smaller chunks of rock and ice, they make up our Solar System (the word 'solar' comes from 'sol', which is the official name of our Sun).

The inner planets - Mercury, Venus, Earth and Mars - are all lumps of rock, but Jupiter, Saturn, Uranus and Neptune are called gas giants. There used to be a ninth planet called Pluto, but in 2006 it was re-classified as a 'dwarf planet'.

The Sun is very hot, and if the Earth was much closer to it, like our neighbour Venus, or even very close, like Mercury, it would be much too hot for us - or anything else - to live on. And if it was much further away, like Mars and the other planets, it would be much too cold.

Luckily, we are just far enough away for the sun's heat to keep most of the Earth's surface safe and comfortable for us and many other forms of life - animals, plants and all sorts of other creatures like bacteria, which are neither animals nor plants. There are animals, but not many plants, which can survive in the hottest places like the Saraha desert. There are also a very few animals, but no plants at all, that can survive in the icy wastes of the Antarctic (have you seen those wonderful films about Emperor Penguins?).

The reason why there are forms of life that can survive in all these hostile places in, of course, that over many millions of years they have evolved to live in them. We will come back to evolution later on this page...

Why don't we fall off the Earth?

I said the Earth is hurtling through space - and I meant it. We are travelling round the sun at about 30 kilometres every second. A spaceship travelling that fast could get to the Moon in just four hours. So why don't we fall off?

The Earth is also spinning like a top - and it's doing that very fast, too. At the equator, where the planet is widest, the surface is moving at over 465 metres every second - 1675 kilometres or over 1000 miles an hour. Which makes it seem even more surprising that we don't fall off!

So why don't we and everything else that isn't  firmly fixed to the Earth, including the water in the sea, the sand on the beaches and the air we breathe, all fall - or even fly - off into space?

The short answer is one word: gravity. And that will have to do for now, because I need to tell you more about our beautiful planet.

More about our Earth

Actually, it's a bit unfair to call the Earth 'a lump of rock'. To start with, it isn't a lump: it's shaped almost - but not quite - like a perfect ball, and it's very beautiful when seen from space, as this magnificent photograph shows.

The Earth photographed from

I say 'not quite' because it actually looks as if the ball has been slighty squashed, as if someone very big has balanced the Earth on its south pole and sat on the north pole. Mathematicians call this shape an oblate spheroid, but ’a squashed ball’ will do for us - except that it isn't actually squashed: it's stretched. More about that soon...

And, calling the Earth 'a lump of rock' isn’t the whole story, either. Certainly, if you dig down into the crust from anywhere on the Earth, you will go a long way down before you find anything but rock – as little as 5 kilometres in some places but as much as 70 in others. Then you will find more rock, but now it will be a thick liquid, because as you go down the earth gets hotter and hotter, and at these depths it is hot enough to melt stone. You have left the crust and are now in the mantle.

Of course, if you really did this your journey would end very quickly as you would have made your own volcano - with yourself in the middle of it! You would be blasted back to the surface on top of a column of red-hot lava.

But, ignoring that, if you carried on down you would eventually reach the upper core, which is liquid metal – mostly iron.

Keep going even further, and in spite of the rising temperature the metal would become solid. This is the lower core, the actual centre of the Earth, where the metal is very hot and under incredible pressure.

Conditions like these do strange things to some materials. Carbon – crumbly charcoal or the soft graphite that makes pencils write – turns into crystals of diamond, the hardest (and one of the most beautiful and valuable) of the natural materials found on Earth. The effect on the iron in the Earth’s lower core is also to make crystals, but these are very big. They are also long and thin, and lie parallel to one another, which turns the inner core into a huge magnet. That is what makes compass needles point north.

So that’s what our world is made of. What about how it behaves?

The Earth in space

As I’ve already mentioned, the Earth spins like a top. It turns once every 24 hours, making the Sun look to us, stuck to the surface, as if it’s going round the Earth. That is what gives us day and night.

In fact, most people believed that that the Sun was going round the Earth, and that the Earth was the centre of the Universe, until around the time Christopher Columbus discovered the Americas in 1492. Most also believed that the Earth was flat and that if you went too far in a straight line you would fall off the edge. (Columbus was quite sure the Earth was round, which convinced him that he could go west instead of east to get to the Far East. He was right, but he didn't know the huge continents of North and South America were in the way!)

Before that, the ideas that the Earth was a ball rather than flat, and that the sun instead of the Earth was the centre of the Solar System, were considered so heretical that the early church would burn you at the stake for even suggesting them. As late as 1632, the great scientist Galileo Galilei published a book aboiut these theories and was arrested,  tried and put under house-arrest by the Church.

The Earth spins round a line (which is called its axis) from the north pole to the south pole, so the ground is moving fastest at the equator where the distance around the Earth is the greatest. And it does move really fast: 465 metres a second. That’s nearly 1700 kilometres or over 1000 miles an hour - faster than almost any aircraft can fly.

The Earth is also hurtling through space, because it goes round the Sun, which is around 93 million miles away, once a year. It’s a long way round a circle 93 million miles in diameter, so the Earth’s average speed is 30 kilometres or around 18 miles a second. Or 108,000 miles an hour.

So how is it that we – and the sea and the sand and all the other loose stuff – can cling to this spinning, speeding squashed ball? More amazing: how is it that we don't feel as if we're moving at all?

And what about people on the other side of the world? They are actually upside-down compared to us! Why don’t they fall off?

The short answer is still one word: gravity.

Gravity (and Isaac Newton)

Gravity is one of three forces of nature which you’ll meet if you go on reading this, and it’s The Big One in the sense that it holds all the big stuff in the Universe together. The other two hold the much smaller stuff – atoms, the bits inside atoms, and even the bits inside the bits inside atoms! – together.

The slightly longer answer to the last question is: the Earth’s gravity. And - although we are obviously affected by the Sun's gravity, which keeps our world in its orbit, and even by the gravity of the Moon, which drags our oceans around the Earth to produce tides - the Earth's gravity has by far the biggest effect on us, so we experience everything in relation to the Earth and only modern science stops us believing that the Sun goes round the Earth!

So what is gravity?

The short - and disappointing - answer to to this is that we don't really know. It is a force - something that can affect or prevent the movement of stuff. It's the force we live with throughout our lives, keeping our feet on the ground, our bums on our seats and us in our beds. Yet science still can't explain how gravity works (though scientists talk about some pretty weird stuff as if it's absolute fact).

But what we do know is that every object has mass, which is the measure of how much stuff there is in the object. We experience mass as weight, but you and I would weight a lot less if we were on the Moon because the Moon's gravity is far less powerful than the Earth's. But our mass would stay the same.

And we know that everything with mass has gravity - the greater its mass, the greater its gravity. We know very exactly how gravity affects the way things move - so exactly that we were able to put astronauts on the Moon and bring them home again safely half a century ago. Incredibly we have known this much about gravity since 1687, when Isaac Newton published his Law of Universal Gravitation. Put simply, this says that the gravitational force pushing two bodies together (we might think of it as pulling) is equal to the mass of one multiplied (or 'timesed', as you seem to say in school maths these days) by the mass of the other, all divided (shared) by the distance apart multiplied (timesed) by itself. There's a thing called the Gravitational Constant thrown in as well, to make the maths work, but as it never changes it isn't terribly important.

There is a  story that Newton had a sudden flash of inspiration when an apple fell off the tree he was sitting under and hit him on the head. A good story, but probably not true! Stories like this are quite common: there's another about the ancient Greek philosopher Archimedes suddenly understanding why things float in water while he was sitting in his bath!

But what Newton's Law actually says is that it isn't just the heavier body that attracts the lighter one: two bodies (like, for instance, the Earth and the Moon) attract each other.

It also tells us that the heavier (actually the more massive) a body is the more gravity it has, and that the attraction gets much stronger as the bodies get closer together. There's something called the Inverse Square Law, which shows that when the distance between two bodies is halved, the gravitational attraction between them increases to four times as much. And when the distance between them is doubled, the attraction is shared by four.

But, over 300 years after Newton published all this, we really don't know how gravity actually works. Maybe we never will!

This is weird, because we do know an awful lot about how other stuff works. Just wait 'till we get to what happens inside atoms...

Anyway, we need to look a bit more closely at one or two things I've already mentioned.

Hearly but not quite a ball

First, why are the Sun, the Earth and the Moon - not to mention all the other planets and all the stars in the universe - almost perfectly spherical (ball-shaped)?

The answer is that they were all formed from clouds of gas or dust - or both - which gradually came together under its own gravity. Yes - even atoms of gas and grains of dust are attracted to, and attract, one another. Without anything else interfering, they naturally fall into the tidiest arrangement, which is a sphere.

Ah - but why is the Earth a squashed sphere?

Or, as I said earlier, a stretched sphere. The answer to that is that, like the Earth, all what we call 'heavenly bodies' spin and therefore experience centrifugal force. I also mentioned earlier that the natural direction for anything moving is a straight line. If nothing interferes, it will go on forever in a straight line. Centrifugal force is the rotating surface of the Earth trying to shoot off in a straight line but failing. It has to keep going round, but it pulls strongly outwards from the spin. When a sphere spins, the surface is moving slowly near to the ends of the axis - in  the Earth's case, these ends are the North and South Poles - and much faster around the middle (on Earth, the equator). So centrifugal force is far more powerful at the equator than at the poles and pulls the planet into its stretched-ball shape.

This same centrifugal force is what stops the moon crashing down onto the Earth and the Earth diving into the Sun.

Now a couple more things about the Earth's behaviour.

Day and night, summer and winter

First, it's easy to see that as the planet spins different areas face towards and away from the Sun. In the areas facing towards the Sun it is day, and in those facing away it is night. Half way in between are morning and evening. Pretty straightforward so far.

But then there is also the fact that the Earth's axis - the line it spins around - is tilted: it isn't quite upright relative to the planet's orbit. This means that during one part of the orbit the 'top' (North) part of the planet is tilted towards the sun, and during the opposite part it is tilted away from the Sun. When light and heat hit a surface at a shallow angle, the rays are spread out over a bigger area so they are not so bright or warm. When they hit at a steeper angle they are more concentrated - brighter and hotter. So in the first case it is winter and in the second it is summer.

For the same reason - the tilting of the axis - the sun shines longer in summer and not so long in winter. And half way between summer and winter we have spring and autumn.

So now we know why our just-warm-enough World has day and night and the seasons.

More gravity

Okay - back to gravity...

The Earth and everything movable on it, plus the Moon and an awful lot of man-made stuff in orbit round the Earth, form a stable system. Everything movable is firmly anchored to the surface by gravity - and that includes the air we breathe. And everything else - the Moon and all the artificial satellites - is equally firmly anchored in orbit.

Most of the other planets have one or more moons, with which they form stable systems too.

And the whole lot orbits round the Sun, forming one much bigger stable system.

Then there are stars in our area of the Universe which are all anchored together by gravity for form a really big stable system: our galaxy, the Milky Way.

The Milky Way is believed to contain between 100 billion and 400 billion stars, and to be just one of 170 billion galaxies in what we can see of the Universe. And there may be much more of the Universe that we can't see because its light still hasn't had time to reach us. For all we know, there may even be lots of completely separate universes drifting around in space, so far away that there's no way we could ever see them.

So is the Universe (or our Universe) a stable system? Not in the way that planets with their satellites, solar systems and galaxies are, because our Universe is growing bigger all the time. The galaxies (all 170 billion of them) are all moving away from each other very fast.

We seem to have come a long way from 'You and I live on a lump of rock' in a fairly short space, don't we?

Why all this stuff is the way it is

But where did it all come from?

You've probably heard of the Big Bang theory. This is agreed among scientists to be the most likely story of how our Universe got to be the way it is, but it doesn't explain everything.

The latest estimate of the age of the Universe is 13.8 billion years, but until quite recently (when an even better telescope took an even better picture of something called the Cosmic Microwave Background) it was believed to be only 13.7 billion years.

Let's just get some of these crazy numbers sorted out before we go any further.

Until fairly recently you only ever heard the word 'billion' used by astronomers, but now it's used about money too. It seems that there are billions and billions of pounds washing around in the economy, and journalists talking about the American economy even have to use the word 'trillion'.

So here goes...

I've already mentioned millions when talking about how far the Earth is from the Sun - 93 million miles. That is written as 93,000,000. Each zero after a number timeses the number by ten, so this means 93 x 10 x 10 x 10 x10 x10 x 10. A billion is one thousand million - 1,000,000,000 - 1 x 10 x 10 x 10 x 10 x10 x 10 x 10 x10 x 10. And a trillion is a thousand billion or a million million - 1 x 10 x 10 x 10 x 10 x10 x 10 x 10 x10 x 10 x 10 x 10 x10.

So the Universe has been in existence for roughly 13,800,000,000 years. A very long time indeed.

The obvious question to ask at this stage is 'What was there before the Universe came into existence?'. Scientists say that this is a meaningless question because there was no 'before'. But this is because scientists have to work with the evidence they have got, and any evidence there might have been of anything that might have existed before the Big Bang would have been destroyed by it. So physicists can say, quite cheerfully, 'time and space began with the Big Bang.'

Not much help, is it. The next obvious question is 'What caused the Big Bang, if there was nothing there before it?' And that hits the nail on the head for me, because things don't happen without something causing them. And events huge enough to create a whole Universe don't happen without something pretty huge causing them.

I'm not a scientist, so I'm allowed to play guessing games when the evidence runs out, and I'll say more about my guesses later. Meanwhile, I'll stick with the most popular theory.

This says that at the moment of the Big Bang everything that makes up our Universe - all the mass and energy - was squeezed together in a tiny space. Some even say an infinitely small space - which is much smaller than a full-stop on this page. It's as small as small can get! They call this a singularity.

Then a process called inflation began. When you blow up a toy balloon you inflate it, so you can imagine the early Universe as a balloon growing very fast.

But what was the early Universe - evreything that existed in the first millionth of a second after inflation began - made of? The theory says it was a quark plasma.

A small world

This is where we leave our ordinary, comfortable world behind and start to explore the world of the very very smallest things. Because without understanding that world, it's impossible to understand how the Universe developed.

A plasma is like a gas, but gases consist of atoms. Normal atoms don't have an electric charge so they don't bounce away from each other or get violently attracted to one another - they just drift around, quite gently if they are cool and more rapidly if they are hotter.

Remember when I said 'Gravity is one of three forces of nature you’ll meet if you go on reading this'? Well you are about to meet the other two, because they are what holds atoms together. When I say that, I mean that they hold the bits of atoms together so that the atoms don't fall apart and they also hold the atoms that make up stuff together so that the stuff - matter - doesn't fall apart.

I'll start with the biggest bits and work my way down to the smaller ones - in fact, the very smallest...

Every atom consists of a nucleus - the tiny, heavy bit in the middle - and one or more electrons.

The electron is called a fundamental particle because it is one of the basic building blocks of matter (stuff). It isn't made of anything else - it just is. The electron has a negative electric charge. In fact, it can be thought of as just a bit of electricity, because it is the electrons that move from atom to atom in a wire when an electric current flows in a circuit.

The nucleus has a positive electric charge - the opposite of the electron's. And in electricity, opposite charges attract one another just as opposite poles of magnets attract one another.

So the electrons are held captive round the nucleus - a bit like the way planets are held captive round their parent stars. An atom is a bit like the solar system, but held together by electricity instead of gravity.

An experiment with electricity

You may have been shown, or even tried yourself, a simple experiment that demonstrates the power of opposite electric charges. If you haven't I'd really like you to try it now.

All you need is some thin paper and a ballpoint pen - the old type with a hard, shiny barrel rather than some gimmicky modern one made of squashy rubbery stuff. Tear up the paper into really tiny pieces - as tiny as you can manage - and scatter these on a table. Now take the pen and rub it quite hard on your clothes - a sleeve, a trouser leg or a skirt. Keep rubbing for ten or fifteen seconds, then lower the pen slowly towards your bits of paper - but don't let it touch anything else first. If all is well, when the pen is a few millimetres above the paper the tiniest bits will jump up and stick to it. If not, you may need to try a different pen or rub it on something different.

I hope you manage to do this. I've just done it for the first time since I was a child and it worked first time.

The kind of electricity that does this is called static electricity. It is quite different from current electricity, the kind that flows around circuits to make light-bulbs bright and electric fires hot. What is lifting the scraps of paper up is the electromagnetic force - another of my three forces of nature.

Scientists will explain that what is happening is that, by rubbing the pen on some cloth, you are knocking some electrons off the outer atoms of the pen's barrel. This leaves those atoms with a positive electric charge, and when they get close enough to the electrons on the outside of the atoms in the scraps of paper they want to join up. The attraction is quite powerful - enough to work across several millimetres and lift the scraps of paper off the table.

The name electromagnetic force suggests that this is the same force that makes magnets attract metals. That is true - the electrostatic and magnetic attractions are different aspects of the same force. And, just as positively charged and negatively charged objects attract one another, but two positives or two negatives push one another away, the north pole of one magnet will attract the south pole of another one, but two norths or two souths repel one another, as you may have discovered when playing with magnets. The strange sensation you get if you try to push the north poles of two magnets together still fascinates me! How one object can make another one moved from as much as a centimetre away seems really mysterious.

I could go on about electricity and magnetism, but I must leave them for now and go back into the world of very very small things.

Inside the atom

It was the idea of a quark plasma that got us into the electricity bit. Just for now, I'll tell you that quarks are some of the other fundamental particles, like the electron. In fact, the electron and two kinds of quark are the building blocks of everything in the universe.

But I want to get to quarks from the outside, working down from the bigger very-tiny-bits to the very-tiniest. Some of the stuff I'm going to tell you now will surprise you. It certainly surprised me when I first learned about it - and it still does!

In the 'A small world' bit back up the page I said 'the other two [forces of nature]...hold the bits of atoms together'.

So we have gravity holding our local stable system of the Earth, the Moon and us - everything that is made out of atoms -  together, and then the electromagnetic force holding the bits of every atom together.

Until three years before the start of the 20th century (before even I was born), the atom was believed to be the funamental particle of matter, which meant that there had to be almost a hundred different atoms to explain all the different chemical elements (n element is something consisting of just one kind of atom). Then the electron was discovered and found to be a part of the atom. The early theory was quite wrong about the other part, but in 1909 the idea of a very heavy nucleus surrounded by a cloud of very light electrons - a bit like the Sun and planets in our Solar System - was suggested. Amazingly, over a hundred years later, this theory in still accepted, but the details of what goes on inside atoms are now far better understood.

It is believed that 98 different elements occur naturally on the Earth and in the Universe, so it follows that there are 98 different atoms. We will ignore the fact that scientists have managed to create other elements, most of which exist for a very short time and then turn into something else, with or without an explosion!

The nucleus of every atom has a positive electric charge, and it is this which attracts and holds the electrons in their orbit-like positions. That's the second force in the sequence - the electromagnetic force. The charge ranges from the tiny one of the hydrogen nucleus, which can hold only one electron in 'orbit', to the huge (relatively) charge of the heaviest atom: californium, whose atom contains no less than 98 electrons. In between, there are 96 other elements, using every possible number of electrons from 2 to 97.

So how can the nuclei of different atoms have different electric charges?

Well, the nuclei are all different, containing different numbers of a particle called the proton. Each proton has exactly the same amount of electric charge as the electron, but it is positive instead of negative, That means that the proton can hold the electron close to it. So a hydrogen nucleus has just one proton. In fact it is just one proton and nothing else. And a californium nucleus has 98 protons. Unlike the hydrogen nucleus, it also contains many of another particle - the neutron - which has no electric charge at all. We may get back to this at some point, but for the moment it's the electrons and protons that matter most.

Before we go any further, I want to tell you something really amazing about the atoms which make up all matter, including our own bodies. On another page in The diary of a wandering mind I wrote this:

The...radius of an atom’s electron cloud is more than ten thousand times that of its nucleus... A few (or, in heavy elements, many) massless electrons whizz about in the cloud, but the rest is just space. Despite the lack of ’stuff’ in the cloud, its is only the electrons in the outermost ’orbit’ that can form bonds with other atoms, to form the molecules of compounds, so it is the size of the cloud that defines the real, practical size of the atom

Just to get this into perspective, Wikipedia tells us that the atomic radii of different elements range between 30 and 300 picometres. A picometre is one trillionth (1/1,000,000,000,000) of a metre or one billionth (1/1,000,000,000) of a millmetre. It follows that all the mass of one atom of the lightest element is concentrated in something with a radius of three ten-billionths of a millimetre

Accepting that the volume of a solid is proportional to the cube of its radius, the volume of an atom’s electron cloud must be of the order of ten-thousand-cubed times that of the nucleus. If I’ve got all this right, that is one trillion times!

So our bodies, like everything else in our macrosocopic world, are made up of an infinitesimally small amount of very dense solid stuff and a vast amount of nothing - not even fresh air! That’s one of the weirder notions, because it implies that most of the volume of any form of matter (including us) is actually a hard vacuum. I still struggle with the idea that I’m mostly made of water - never mind a couple of cubic feet of absolutely nothing

We macro-people naturally think that this [vacuum] would suck in air - but, of course, atoms of oxygen and nitrogen can’t possibly be sucked inside other atoms. This again reinforces the vast difference between the macro world and the micro one. The suggestion that all matter is composed of dense nuclei scattered very thinly in a vacuum otherwise populated only by electrons in their orbits, the whole held together by electromagnetic forces, takes a lot of believing.

So, although we see and feel our world to be made of solid stuff, as we work our way down through the structure of matter we find that it consists almost entirely of absolutely nothing.

So we have the nuclei of atoms which consist of anything from just one proton (hydrogen) to 98, but all the nuclei other than that of hydrogen also contain another particle called the neutron. This has no electric charge, but it does contribute to the mass of the nucleus and therefore of the atom. To make life more complicated, many of the chemical elements (the kinds of stuff with unique atoms) exist in two or more forms called isotopes. The number of protons (and therefore electrons) tell us which element (hydrogen, oxygen, iron, carbon and so on) it is but the number of neutrons tell us which isotope of the element it is. The more neutrons, the heavier the isotope is.

The Story of Science: Power, Proof and Passion, came up with a spectacular illustration: if we could take the entire six-billion population of our planet and ’get rid’ of the empty space, what was left would fit into a space smaller than a sugar cube! Now that isPersonal site for Paul Marsden: frustrated writer; experimental cook and all-round foodie; amateur wine-importer; former copywriter and press-officer; former teacher, teacher-trainer, educational software developer and documenter; still a professional web-developer but mostly retired.

This site was transferred in June 2005 to the Sites4Doctors Site Management System, and has been developed and maintained there ever since.