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
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
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
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
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
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
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
So that’s what our world is made of. What about how it
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 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
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
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
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
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
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
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
This same centrifugal force is what stops the moon
crashing down onto the Earth and the Earth diving into the
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
So now we know why our just-warm-enough World has day and
night and the seasons.
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
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
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
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
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
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
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
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
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
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
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
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
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