By Paul Sen
Series Producer, Science You Can't See: The Atom
A new BBC Four series examines humanity's quest to understand the atom.
Here, series producer Paul Sen explains why the realisation that everything we see in the Universe is made of atoms could be the most important discovery ever.
An atom isn't just tiny, it's over 99.9% empty space
No-one ever expected the atom to be as bizarre as it turned out to be. Since 1905 when a young Albert Einstein demonstrated for the first time that atoms must exist, they have consistently flummoxed scientists by their weird, almost contradictory behaviour.
Here's a quick flavour of just how strange an atom is. And remember atoms aren't obscure objects: everything in the world around us is made of atoms; we are made of atoms.
First of all, atoms are ridiculously small - they're about one tenth of a millionth of a millimetre across. That means that a human hair, one of the narrowest things visible to the eye is around a million atoms across.
Put another way, there are more atoms in a glass of water than glasses of water in all the oceans in the world.
In a new BBC Four series, Professor Jim Al-Khalili, a nuclear physicist, claims that the hunt for the secrets of the atom - the basic building block of matter - is the most exciting and unexpected of scientific detective stories.
And the story gets really strange. An atom isn't just tiny, it's over 99.9% empty space. All the weight of an atom is concentrated in a mind-numbingly tiny object at its centre. It's a trillionth of a centimetre across and is called the nucleus.
The rest of the atom is entirely empty apart from a few ghostly objects called electrons that skim about at a great distance from the nucleus.
To give you a sense of how empty an atom is - if the nucleus was the size of a football, the nearest electron would be half a mile (0.8km) away.
The programme is presented by Prof Jim Al-Khalili
That means even the most solid-looking objects we see are predominantly nothingness. Put another way, if you were to remove all the empty space in the atoms that make up a human being, he or she would be a lot smaller than a grain of salt.
If you removed all the empty space from the atoms that make up all the humans on the planet, then you could fit all 6 billion of us inside a single apple.
This astonishing discovery that atoms are mainly empty was made in 1909 at Manchester University by the indefatigable Ernest Rutherford. Rutherford had great courage as a scientist and was prepared to fly in the face of convention.
So when he announced that the atom was mainly empty, he did so knowing his claim flatly contradicted the then known laws of physics. These demanded that all atoms collapse instantly. It was a seismic moment in the history of science.
Forced to explain the atom's mysterious emptiness, scientists had to jettison everything they had believed to be true for the previous two centuries. Their response was to invent an entirely new science, which we now call quantum mechanics.
The strangest and most disturbing fact that scientists uncovered while investigating the atom was a law called "Heisenberg's Uncertainty Principle".
In a nutshell, this states that atoms are in more than one place at the same time until a conscious observer looks at them.
Think about this for a moment - if no-one's looking at the atoms that, say, make up your hand, they're effectively spread out across the entire Universe.
Then when someone, maybe even you, looks at your hand, the atoms instantly coalesce into the hand-like shape you're familiar with.
Of course, this is an extreme example. We know that once trillions of atoms bind together to make up everyday objects, like your hand, they stop behaving in a weird spread-out way but no one really knows when and how they switch from one state to the other.
The point is, it seems arbitrary where we draw the line between the strangeness of the atomic realm and the commonsense world of our senses.
If ideas like this make your head hurt, don't worry. Even Albert Einstein, who as a young man pioneered atomic physics, was horrified by the idea that we somehow "invent" the Universe every time we look at it.
He said: "I like to think that the Moon is there even if I am not looking at it."
Exploring the atom has tested humanity's imagination and intellect more than any other scientific endeavour. Even now, as we peer deeper and deeper into the atom, it throws back as many questions at us as answers.
Science You Can't See: The Atom will be broadcast on Thursday 26 July, Thursday 2 August and Thursday 9 August 2007 at 2100 BST on BBC Four.