The Story of Gravity: The weakling, the plague and the apple.

“Albert, is the gravity that makes stars the same as the gravity on Earth?”

It’s the very same force that keeps your feet on the ground and makes apples fall out of trees.

“Was that story true about the man who invented gravity being hit on the head with an apple really true?”

Well the man who was supposed to be under the tree was Isaac Newton and he certainly existed. Many people think he was the most brilliant scientist who ever existed.

“I thought you were the most brilliant scientist that ever existed Albert.”

Like Newton said, if I have seen farther it is by standing on the shoulders of giants. Newton was one my giants.

“So tell me about Isaac Newton then.”

Well he was born on Christmas Day in the year 1642 in the small village in Lincolnshire in England and he almost died the same day.

"What happened?"

He was tiny and feeble when he was born. He was so small his mother, Hannah Newton, said he'd fit in a two pint pot. Luckily for science young Isaac proved them wrong. He proved them so wrong he lived for 84 years which was almost a miracle back then. He also survived an outbreak of a deadly plague in 1665 that closed down Cambridge University. That was when he was sent back to deepest Lincolnshire for some uninterrupted thinking time. This was when he was supposed to sitting under the apple tree.

“Surely people had always known apples fell of trees?”

Of course they did. The difference was that Newton asked the question ‘Why?’
He was also smart enough to answer the question by developing his theory of gravity.

“So what was his theory?”

Newton said that everything big and small has a gravitational pull. The bigger the thing the stronger the gravity. He also worked out that the pull of gravity from an object gets less and less as you get further away.

“So even an apple creates gravity? I thought it was just the earth pulling the apple down.”

Exactly. The Earth pulls the apple and the apple pulls the Earth so they move together. It’s just that the Earth is so huge it has a much greater gravitational pull than the apple does. In mathematical terms gravity increases with the sum of the masses of two objects and reduces with the square of their distance apart.

“Whoa, hold on try that again in English.”

OK, for a start if you double in weight (or mass as we scientists call it) your gravity doubles.

“Got that.”

Right, the next thing is that if there are two objects then each pulls the other.

“Sounds weird, but OK.”

And lastly if you move away from a big object gravity reduces so that at twice the distance it is reduced by four times, two times two.

“I know what two times two is Albert.”

OK, so what happens to gravity at fives times the distance?

“Oh, er…”

It’s just five times five.

“Twenty five. I get it, so at three times the distance gravity is reduced by 9, three times three.”

Exactly, so now you know Newton’s theory of gravity.

“Can you really explain how a whole planet moves with this law of gravity?"

You certainly can, But there is one thing that gravity doesn’t do.

“What’s that?”

Gravity doesn’t make people fall in love.

“Thanks Albert, I’ll try and remember that.”

Now Newton didn’t stop there he worked out the rules for how things move in the universe, his laws of motion. How many rules or laws do you think that would take?

“Oh, millions I suppose. It’s a complicated place the universe.”

Newton only needed three extra laws but.these three laws explain how objects, from footballs to planets, move and what happens when you kick a football. Now pay attention.
“I’m listening.”

Newton's first law of motion is that objects keep doing whatever they are doing unless a force is applied to them. This works if the object is still or moving. Leave a football on the ground and its stays there. Kick it and it moves.

"But it doesn't keep moving for ever."

No because two other forces act on it. Gravity pulls it down to earth and friction between the ball and the air and ground stops it. In space there is no friction so a football could keep going forever as long as it didn't hit anything.

Newton's second law is a bit more mathematical but explains the link between how much an object accelerates, how big it is (it's mass or in simple terms weight) and the force applied. Simply put it says that for a given force, the smaller the object the faster it will accelerate. If you hit a golf ball and a football with a golf club, the golf ball goes much further because it is smaller and lighter.

“That makes sense.”

Newton's last law sounds a bit odd but is just as important as the others. It says that for every action there is an equal and opposite reaction. Most of the time you don't see this in action but it's still there. Step out of a small rowing boat at a jetty and it becomes a little more apparent. As your front foot moves forward, the equal and opposite reaction pushes the boat away from the jetty. Ignore Newton's third law and you'll get wet.

Together with his theory of gravity, these laws create what scientists call Newtonian mechanics. Four laws to explain most of the movement in the universe.

“So if Newton got it right, what was left for you to discover Albert?”

Well these four rules seemed to explain everything about how the universe worked and for most situations they still do. It's only when things are going spectacularly fast or gravity becomes very strong that Newton's law start to stop working but I'll explain all about that one day. Even though I ending up disagreeing with Newton on some things, when they sent a man to the moon they used Newton’s equations and not mine to make sure he got there.

The Universe’s Darkest Hour

"These stars we're travelling past, were they all made in the Big Bang?"

Not at all, the stars you can see now were born well after that. Even though all the matter in the universe was made in the Big Bang, it's been rearranged a few times since them. New stars are being created all the time in space from clouds of gas and space dust. Some of this dust has been here since the Big Bang and the rest is the leftovers from stars that have long since exploded.

"How can a star form out of just gas and dust in the middle of space?"

It all comes back to gravity. After the Big Bang, all the matter started to spread out in all directions. If all the matter spread too evenly then the universe would have just got thinner and thinner and fizzled into nothing; no stars, no galaxies. On Earth they talk about the dark ages lasting from the end of the Roman empire to the time when Europe became civilised again. That was only about 500 years. After the Big Bang, the Universe was dark for 150 million years. Not a single star and no light. Total darkness.

“No stars for 150 million years? Are you serious?”

Absolutely. But remember those microwave ripples formed by the Big Bang? Well, luckily for us, they showed that the universe just after the Big Bang was lumpy. From these microwave ripples scientists at NASA have worked out the lumpiness of the universe 300,000 years after the Big Bang. Look here is a picture of the lumpiness of the universe in terms of temperature back then. The yellow and red bits are hot and the blue bits are cold.


"Why are lumps lucky for us?"

Because without the lumps, there wouldn't be any stars and without our star we wouldn't be here. For gravity to be able to collect matter together into stars there needs to be a few irregularities or lumps. The bigger you are the more gravitational pull you have. So it takes one lump of matter in the centre, or anywhere, to start slowly pulling in other smaller pieces of matter. The central lump then starts getting bigger and bigger and so its gravity gets stronger and stronger. The whole process speeds up with more and more atoms being pulled into a big ball.

“So why did it take 150 million years to make a star?”

Because the ripples were very small so the first lumps didn’t contain much and it took ages for gravity to pull in enough gas to make a star.

"And what exactly is in these balls?"

Three quarters of it would have been hydrogen, remember that's the simplest and smallest type of atom. Nearly all the rest was Helium which is the next smallest. Between them, hydrogen and helium make up most of the universe. As these balls of gas got pulled together by gravity, they got hotter and hotter like I explained before. Now hot things glow, but gravity alone will not make hydrogen glow as brightly as a star. When they get really hot, the hydrogen inside starts combining with itself by nuclear fusion reactions and ....carrummmph..... there you have it.

"Have what exactly?"

A fully fledged star using nuclear fusion energy to shine for millions or billions of years.

“How can we know how stars were made billions of years ago?”

Because we can see it happening today.

“Where”

The easiest place to see where stars are being born is in the constellation Orion, the hunter. That’s one of the easiest constellations to see. It has a belt of three stars, just below the belt is what looks like a hazy area. That is a massive cloud of dust and gas where stars are being made as we speak. It looks like tiny fuzzy blob when you look up at Orion at night but it is huge, 30 light years across. With the Hubble telescope, scientists have even seen new stars with discs of gas around them. Look this picture taken about ten years ago. Those fuzzy beige blobs are stars being born surrounded by swirling discs of dust that one day may become planets.


“Albert, how can you know about all this stuff that happened after you died, like the Hubble telescope?”

Well, when I was rebuilt in this computer system I found I could roam about cyberspace catching up on all the science I missed out on by dying. What do you think I do when I’m not talking to you?

“Be careful there is a lot of rubbish out there on the Internet too.”

Sparks of brilliance and generosity in a sea of dross and unpleasantness.

“Is that one of your famous quotes?”

No, I just made that one up. But it sums up the Internet pretty well don’t you think?

Dark Matter and how WIMPs may save the universe

“Albert, you said earlier that the Belgian priest came up with the Big Bang idea by thinking backwards.”

Father Lemaître. That’s right.

“Well if I start thinking forwards about an expanding universe all I can imagine is a universe that gets bigger and bigger until all the stars would be so far apart you couldn’t see any.”

Excellent. That brain of yours is now starting to imagine things. Now what you say might happen but it isn’t the only way the universe could end. It might just keep on expanding, getting thinner and thinner, older and older. But then there is my great friend gravity. If there is enough stuff in the universe to generate enough gravitational pull it will start to slow down and then start contracting. Eventually everything will end up in same place. The Big Crunch.

"Then what happens?"

Well it might start all over again with Big Bang II: The Sequel. Perhaps it already has. There is one thing that science can’t fathom - what happened before our Big Bang. Gravity is trying to pull everything in the universe back into the centre. The more mass, the more gravity and the more likely it is that gravity will be able to put the brakes on the present expansion and start pulling things back together again. Looking around the universe there isn't enough matter to overcome the present expansion. But there is a lot more to the universe than meets the eye. The fate of the universe rests on how much dark matter there is.

"Dark matter? That sounds like something from Star Wars."

Dark matter is very real and very important. It's all the bits of the universe that can't be seen. Left over hydrogen, dust, interstellar debris, pollution from advanced but ecologically unsound civilisations - that sort of thing. What sounds even more like a Star Wars invention is the idea of the 'Dark Energy.' In the last few years scientists have started to see signs that the expansion of the universe is getting faster, as if there is something pushing galaxies away.

"Something like the opposite of gravity."

That sort of thing. But no-one knows what sort of mysterious force could do this so they've called it 'Dark Energy'. Remember I put something extra into my theory to stop the Universe expanding when I didn’t want to believe it could, the Cosomological Constant.

“Your biggest mistake?”

That one. Well it turns out that today that same extra part of the equations is exactly how this Dark Energy could work.

“So you were right?”

Only in part and for all the wrong reasons. But whatever this dark force is, if it exists, the only thing that will stop this force is gravity. So it comes back to seeing how much dark matter there is. If there is enough dark matter then one day the universe will stop expanding. If not the universe will just keep going and eventually fade away.

"So the fate of the universe depends on how much dust there is lying around?"

Well, basically yes. The problem is that as it’s dark we don’t how much is out there because you can’t see it with a normal telescope.

“So we’ll never know until we start travelling around the galaxy like in Star Trek?”

No, astronomers have already found a way to find invisible things but strangely it wasn’t invented by astronomers but by an American telephone company, Bell Labs, in 1931. Karl Jansky was looking for all the different types of interference or noise that could affect radio telephone signals and he noticed a certain type of interference that appeared at round about the same time each day. This interference was found to come from certain positions in the sky, from outer space. This was the birth of radio astronomy, mapping and understanding radio signals that come from stars and galaxies.

"So are these like alien radio stations?"

No, radio waves can be created deliberately to send music and news, but the radio waves picked up radio telescopes from space seem to be naturally occurring coming from stars, galaxies and other strange objects that live out there in space. And guess what you can detect with a radio telescope?

"Stars?"

Well some stars but the most important things are the things that you can't see with a regular telescope like dust and interstellar gas.

"So astronomers finally have a way of looking for the missing dark matter."

Exactly. Unfortunately, despite all this, they still don't think they have found all of it. Another thing that might make up dark matter are WIMPs that some scientists think will determine the fate of the universe.

“The fate of the universe lies in the hands of wimps? Albert you really have lost it now.”

WIMPs are weakly interacting massive particles. They are called sub-atomic particles because they are smaller than atoms but they don’t really interact with normal matter. A WIMP could fly straight through the whole earth, which makes them even harder to detect than dust. Wimps may make up 80% of the dark matter but no-one is quite sure.

"So most of this dark matter is still missing?"

Either they can't find it or it isn't there.

"Are there clouds of it lying around in this galaxy?"

Oh, yes lots of it. There are also things called MACHO’s which stands for Massive Compact Halo Objects, these are large lumps of dark matter like old dead stars or dense clouds.

"So which are more important for the universe wimps or machos?"

Oh, the wimps I'd say. They could be responsible for the most of the dark matter in the universe.

"And we couldn't tell if we were about to fly into a cloud of this dark matter?"

No. That's why it is called dark matter.

"And we could be absorbed, just like that, before we had reached anywhere?"

True, but we have another two thousand years of this journey to go, so I think we can reasonably assume we are safe for a few years. Anyway I never think of the future. I always find it comes soon enough. But don’t forget this is a thought experiment. So don’t imagine it happening or it just might.

The Big Bang. What really happened?

"Ok, Albert. So I’m convinced about this Big Bang idea, now tell me what actually happened at the start of the Universe."

Well, here’s what we think happened, because of course no-one was there to watch it. At the start there was no such thing as space, no atoms. All the energy and matter in the entire universe started in one super-concentrated blob. Then for reasons we will probably never know it exploded with a massive release of energy and particles.


Remember the quarks, gluons, protons and neutrons I told you about before? (If you never read that bit click here).

“Well, I remember the names.”

That will do for now. Well the quarks and gluons were the first matter to be made and all these particles rushed out in the first moments of creation. At the start that was all there was, there weren’t any atoms to begin with. It took a millionth of a second for the quarks to stick together to make protons and neutrons. It took a few minutes more for things to cool down to a few billion degrees, cool enough to allow protons and neutrons to sick together into simple atomic nuclei. Even space itself was popped into existence out of nowhere. Imagine the fabric of space scrunched into a tiny ball then suddenly bursting out and expanding – a bit like one of those air-bags in cars.

“How can you know about air-bags? They weren’t invented when you were alive.”

You have forgotten that I once worked in the patent office and I was in fact alive when the airbag was invented. John W. Hetrick got a patent on that idea in 1953 but just like the big bang it took people decades to catch onto the idea.

“So was the start of the universe like an exploding egg as you said last time or an exploding airbag?”

Well, the Big Bang was perhaps a little closer to an exploding airbag than an exploding egg, but scaled up a trillion fold.

“So that was it, one mega explosion and all the matter in the universe just burst into existence?”

It wasn't just that lots of matter was created. There was lots of anti-matter created too.

"What's anti-matter?"

Science fiction writers love anti-matter but it is real stuff. Anti-matter and matter are complete opposites in some ways and exact copies in another. They are sort of mirror images of each other with one crucial difference. Anti-matter and matter can't mix like normal materials. In fact when they mix they annihilate each other and release energy. One big question about the start of the Universe is where did all the antimatter go, since most theories need it there as the universe was born. There was the slight possibility that all the matter in the universe might have been destroyed in the first few seconds by colliding with all the anti-matter flying around at the same time. So it would have been a very short lived universe. If that had happened we wouldn’t be here to worry about it.

“So what did happen?”

One theory goes that at the start there wasn't a perfect balance between matter and anti-matter. The division was about 50.0000001% matter and 49.9999999% anti-matter. So that there was lots of energy, lots of annihilation but at the final whistle matter came through by a whisker. Of course perhaps anti-matter won, and the universe is full of anti-matter. How could you tell? Physicists would still call the other stuff anti-matter because it was different from the stuff that made up the universe. Perhaps in a parallel universe somewhere there are anti-physicists who are sighing with relief that antimatter won out at the start of it all. One more thing to worry about I suppose.

“Albert?”

Yes?

“You’re rambling a bit there.”

Oh, sorry. I got a bit carried away.

Fire Engines, Microwaves and Eggs

"Albert, how can you tell the universe is expanding? It’s far too big to see the edges."

Because wherever you look, everything is moving away from each other.

"Yes, but how do they know that?"

In the same way that a blind man can tell whether a fire engine is coming towards him or travelling away from him. The siren from a fire engine that's travelling towards you is slightly higher pitched than the sound of the same siren when it's travelling away from you. An effect that Christian Doppler will discover in Vienna in 1842 towards the end of our journey and is still named after him as the Doppler Effect. It was easy to demonstrate the effect with sound in 1842, but Doppler also predicted that the same principle could apply to light coming from a stars. Rather than becoming lower pitched when moving away, the light is shifted very slightly in colour becoming a little more red. The faster the movement, the bigger the shift. The effect that Doppler predicted in stars is called the Red Shift and was discovered by Edwin Hubble in 1929. From looking at light from distant galaxies it seems that all the galaxies are moving apart. The further away a galaxy is, the faster it seems to be moving.

"So why don't fire engines look redder when they are going away from you."

The Doppler Effect only works with the sirens of fire engines because fire engines can travel at a reasonable fraction of the speed of sound. The speed of sound is only 760 miles per hour. Light travels almost a million times faster at 670 million miles per hour and so a fire engine would have to be going a million times faster than normal to be 'red shifted' in the same way that the siren changes pitch. Galaxies are a lot faster than fire engines and can travel at millions of miles per hour and so that's why their light appears slightly redder than it is.

“So what made scientists believe in the Big Bang?”

One by one, scientists came around to the idea as more clues were found. You know you asked about hearing the Big Bang? Well there are still echoes of the Big Bang out there in the universe, not in the form of sound waves but as faint ripples of microwave signals coming from space.

“Hold on Albert, you’re losing me here. Microwave ovens in space? Poets and priests? Are you sure they put you back together right when they put in this computer?”

I’ve never felt better and trust me this is all true. Microwaves are more than just white boxes to heat food. Microwave ovens are named after the wave, and microwaves are like a small radio wave. Luckily these microwaves from space are so faint that there is no risk of you being cooked, but the pattern of these faint signals gives clues about the start of the universe. This is how science works, you first have theory and idea and then you have to find clues or evidence that support it or disprove it. The Big Bang theory predicted three things. That the universe is expanding. That is should mostly be made of hydrogen and the heat of the explosion should be still detectable as faint traces of microwaves. The first two are true, so if you can find the microwaves and there is no other good explanation for them then your theory is probably correct.

"How long ago was this?"

Like I said before, about 14 billion years ago.

"And there are still microwaves echoing around? I can't believe that."

OK, I admit it's like trying to work out the shape of a stone thrown into a pond fourteen billion years ago by looking at the ripples left. But the ripples are very small and the Big Bang was the biggest splash of all time.

“Is that why you said you were so impressed with the microwave oven back at the start of the journey?”

Of course, isn’t it amazing that the same principle can tell you about how the universe begain and cook popcorn without any obvious source of heat?

“So how does a microwave oven work?”

A microwave is part-way between light and a radio wave. Light can't get through a piece of chicken at all. But a radio wave can pass straight through a chicken and carry on half way around the world. Microwaves are in the middle, they can get most of the way through a chicken but a lot of them will be absorbed. When absorbed they give up their energy and so heat up the food from inside. But there’s another reason why I like a microwaves. The fact they can heat things up was accidentally discovered by Percy Spencer in 1946 when he was experimenting with something called a magnetron. Here's the picture from his patent on the microwave. The first odd thing he noticed was that a chocolate bar in his pocket melted when he was standing next his magnetron. The next thing he tried was popcorn and that started popping too. The day he showed his discovery to someone else in the laboratory using an egg. As they watched the egg it exploded on their faces. It was like recreating the birth of the universe.

“Exploding an egg in a microwave is like the start of the Universe?”

Don’t you see? Lemaitre’s cosmic egg, explosions, egg on my face for not believing in it.

“Hmmm.”

Well next time you cook popcorn you should think about the start of the universe but just don’t try the egg trick. Trust me it explodes and trust me you’ll get in trouble.

Albert, the Priest and the Pope

“What did the priest say that was so important?”

Georges-Henri Lemaître did the most important things of all, he asked the right question. Just accept for the moment that the universe is expanding, what happens if you think backwards in time?

“I’m not sure I get what you mean?”

Imagine the universe in reverse, shrinking over time. Where would that end up?

“As some really tiny universe?”

Go back a bit further and you don’t even have a universe, just everything squeezed into a dot. This was Georges-Henri Lemaître’s big idea. Working backwards in time he showed that the universe must have started at a place and time when everything in the universe was in a single point. He called this the Cosmic Egg, the moment of the creation. Then on the first day of the universe, 'a day without yesterday', it exploded in the biggest explosion that’s ever been. Georges-Henri Lemaître said it has been expanding ever since.

“Cosmic egg?”

Well I’m glad to say that later on he called it the primaeval atom which sounds a bit more scientific.

“Did you believe him?”

Not even then. I told him 'your calculations are correct, but your grasp of physics is abominable.'

“Ouch!”

The problem with being famous is that people always write down what you say, the smart things and sadly the stupid things too. The next stupid thing I did, what I call my biggest mistake, was to change my own equations with an extra little bit I called the “cosmological constant” to keep the Universe just as it was.

“Did it work?”

Of course not, the Universe didn’t care what I wrote down. Most importantly of all I was wrong.

“So what made you change your mind about the Big Bang?”

Over time as everyone starting talking about the idea and I thought about it more I began to realise he was probably right. I was listening to Lemaître give a talk in California a few years later, 1933 I think it was. At the end I stood up, clapped and told him and the whole audience, this is the most beautiful and satisfactory explanation of creation to which I have ever listened.

“So a priest discovered how the Universe began?”

Well that Russian mathematician, Friedmann, I told you about started the ball rolling but Lemaître made the world sit up and listen. Still I don’t think Georges Lemaître got the credit he deserved. People seemed to be uncomfortable about mixing up science and religion, even if Georges himself was clear that he was just thinking about the science. The Catholic church seemed to like his ideas as Pope Pius XI promoted him to the Pontifical Academy of Science. The next Pope, Pius XII, went further and embraced the big bang theory as agreeing with the Book of Genesis. The Big Bang became the moment that God said 'Let there be light.'

Even when I was convinced not everyone was. Even when they discovered that the universe was expanding, with all the galaxies flying away from each other, a lot of scientists still thought the idea of a big bang was crazy. The name 'Big Bang' even started as a joke by another astronomer Fred Hoyle, to poke fun at the idea. Fred Hoyle also asked ‘What kind of scientific theory is conceived by a priest and endorsed by a pope?’ If Fred Hoyle had known that a poet had come up with the same idea almost a hundred years earlier he might have lost his reason completely.

“You are trying to tell me that a poet discovered how the universe began?”

Edgar Allen Poe, a poet and writer, put his ideas about the start of the universe in a piece he called Eureka which he wrote in 1848. It’s all in there, the universe starting from one point, the primordial particle. Then all the atoms and everything spreading out in all directions. ‘The Priest, the Pope and the Poet’ now that would a great name for a book on the start of the universe. But the difference between poets and scientists is that poets can write down whatever they imagine and their job is done. A scientist can imagine new ideas but that is only the start of it. The hard part is trying to find out if it is true or at least possible.

The Biggest Day in History: The Big Bang

“Albert, How long have we been travelling?”

Oh a thousand years or so.

“That’s impossible?”

Why?

“Well, it seems like no time at all.”

Time, like most things in life, is relative. Talk to a pretty girl for an hour and it seems like a minute. Sit on a hot stove for a minutes, it seems like an eternity.

“So what year is it now on Earth?”

Let me check my watch…it’s the year 4BC.

“Four years to go until Jesus Christ is born.”

You’d think so wouldn’t you, but it seems that with all the messing around with calendars over the years and history being a bit woolly, things got a bit out of sync. It seems that Jesus was probably born four years earlier than he should. Around now in fact.

“How can you mess with time, and lose four years?”

Oh, it’s not so hard. History is what happens while people are just getting on with their lives. By the time history has become important time has moved on, and people’s memories aren’t as good as they think. As for messing with time, how about leap years? Every four years an extra day appears out of nowhere?

“Well it seems easier to get an extra day than loose one.”

In England in 1752 they lost eleven days. Wednesday September 2^nd was followed by Thursday September14^th when they changed the calendar to get back in step with the rest of Europe. Before that the English and the French couldn't even agree on what date it was. So time and dates can’t really be relied on, but sometime around this date back on Earth, three old men are on a journey too, trekking across Asia following a star.

“The star of Bethlehem! Was that a real star or just a story?”

Well that’s a big question that we sadly don’t know. There are lot’s of theories. It could have been a real star that suddenly exploded, or another bright object like some of the planets lining up or a comet. When we get near to Earth we’ll have a closer look at comets and planets, but for now no-one really knows.

“But Jesus’s birthday, that’s a big day in history even if they did get the date wrong.”

Not as big as the biggest day.

“What day was bigger than that?”

Well, the day the universe was born.

“You mean all this hasn’t always been here?”

It’s been here a very long time but not forever. We now think the Universe had its own birthday; the Big Bang.

"The Big Bang? We've been travelling through space for a thousand years and I haven’t heard a thing, apart from you talking in my head of course.”

Well, as we are imaging ourselves to be photons we don't have ears anymore. Also the Big Bang was around fourteen billion years ago and sound waves don't travel in space.

"Why not?"

Because sound waves are just vibrations in air or water or whatever they are travelling through. Without water there can’t be waves in the ocean and without air can’t be sound waves in space. There is no such thing as a sound particle.

“All those crashes and explosions in the Star Wars films wouldn’t really happen?”

Not at all. War in space would be totally silent, though I do hope by the time humans are flying around the galaxy for real they have come to realise that war is pointless.

"What about us, how can we… well how can light travel through space?"

We exist in our own right, we're photons, light particles. That's why we can travel through a vacuum like space. Don’t worry I’ll explain all about what light is and how it travels later.

"So what exactly was this silent Big Bang all about?"

The Big Bang was the start of everything when all the matter in the universe and space itself suddenly came into existence in one place which was very, very hot; about a trillion degrees or so. From this first huge explosion of creation, everything started expanding in all directions and the universe has been expanding ever since. The Big Bang is the cosmological version of the creation story in the Bible. In the bible God had six days to create heaven and earth and one to rest. In the science version the pattern of the next few billion years was sorted out in the first thousandth of a second, everything after that is supposed to be following physical rather than divine laws. Talking of God, I have to confess something about the Big Bang.

“Confess?”

When I first heard ideas like this I thought it was bunkum, complete nonsense.

“How did you think the universe started?”

Well, I thought like you that it had always been there and always would be. I had never asked myself the question of where it came from. Then a young Russian mathematician, Alexander Alexandrovich Friedman, tried to show that my own equations allowed the universe to be expanding. That was back in 1922.

“What did you do?”

To my shame I assumed he must be wrong and wrote a letter saying as much. Then a year later I re-checked and found out his calculations were right and it was possible that the universe could expand, but I still didn’t believe that it really was expanding. No-one else seemed to believe it either until a young Jesuit priest, Georges-Henri Lemaître, from Belgium who was studying physics went one step further.

Albert and the Atom Bomb

"Weren't you trying to explain how stars shine?"

Oh yes, back to nuclear energy. The nuclear energy industry, nuclear weapons and stars have one thing in common: mess around with atomic nuclei and big things happen. There are two types of nuclear reactions; nuclear fusion and nuclear fission. Nuclear fission is the splitting of the nucleus of big atoms like uranium or plutonium into smaller atoms. This is what happens in a typical nuclear reactor and a typical nuclear bomb. Nuclear fusion is knocking two small atoms together to make a bigger atom and happens in stars and hydrogen bombs. This also produces a totally new element.

"So big atoms split and little ones stick."

Precisely. In stars four hydrogen atoms start sticking together to make one atom of helium. When four hydrogen atoms fuse to make one atom of helium, something is lost in the process. Helium is a little less heavy (or has less mass as a scientist would say) than four hydrogen atoms. That lost bit of matter is converted in the collision into energy like us.

"So why are nuclear explosions different from normal explosions?"

A normal bomb or explosion is just a chemical reaction. At the end there are the same number and same type of atoms as there were at the start, they're just rearranged. In the process lots of heat and gas are released that makes things blow apart. All the atoms stay the same but just end up joined up in different ways to make a new set of molecules. In an atomic bomb or the Sun it's the atoms themselves that are changed or destroyed by nuclear reactions. So you end up with different types of atoms.

"So where does the energy come from in nuclear bombs?"

Ever heard of 'E = Mc²’?

"That was Einstein. Wait a minute Albert that’s you!”

Everyone seems to remember that equation. Anyway this is the key to understanding what happens when matter is converted into energy in a star.

"I'm not that good with equations I’m afraid."

Don't worry, this is the only one I'll throw at you during our journey and it's easy enough to explain how it works as an equation. Remember, equations are only a type of short code that captures an idea, so every equation has an English description. In 'E = Mc²' M is mass (the amount of material or matter), c is the speed of light (300,000,000 metres per second) and c² is a sort of short hand to mean the speed of light multiplied by itself (a really big number).

"So that's the same as 'E = M multiplied by c multiplied by c' "

Exactly, so you see, it's not as complicated as it looks.

"Hold on, what about 'E', what's that?"

Sorry, 'E' stands for energy. If you want to know how much energy, 'E', is made when you loose that little bit of mass in a fusion reaction to make helium you multiply that amount of mass that is lost, 'M', by the speed of light, 'c', and multiply the answer by the speed of light again. The final answer is the amount of energy. Because 'c' or the speed of light is such a large number, even a tiny amount of mass lost creates a huge amount of energy. This wonderfully simple equation says that a little matter turned into pure energy releases a lot of energy. If all the mass in a normal 60 Watt light bulb could be converted into electrical energy, there'd be enough energy to power an identical light bulb for 80 million years. The reason you still have to pay for electricity to keep your electric lights working is that all this energy is trapped. Even in fusion reactions only a tiny percentage of the mass of the starting materials ends up as energy. Luckily stars are so huge that they can get energy from nuclear fusion for billions of years. Sadly that little equation can also measure the misery humans can inflict on each other.

“I don’t understand?”

Wars and bombs. Remember I told you that stars are like continuous nuclear explosions? Well that same equation explains why atomic bombs are so destructive. In sensible hands atomic energy will be a great benefit for humanity, but powerful hands are rarely sensible in my experience. Remember my friend Leo Szilard that I invented those new types of refrigerator with? Well he turned into a brilliant physicist who worked out how an atom bomb might work. Leo helped me write a letter I sent to the US president Franklin D Roosevelt at the start of World War II in 1939 telling him that an atomic bomb could be made and would be immensely powerful. It also seemed Germany was starting to take an interest in an atomic bomb, so we felt forced to do it only to prevent a greater evil if Germany had developed the bomb first. Two hundred thousand innocent people paid with their lives in proof of that harmless little equation in Hiroshima and Nagasaki. I just hope the world has learnt that lesson, though I’m sure the point could have been made without so many innocent people having to die. One demonstration on an uninhabited island near Japan would have worked as well. What I really don’t understand is why Truman needed to drop the second bomb on Nagasaki after he knew the first had been so destructive. It can only have been on account of two of the worst human sentiments; revenge and a desire to humiliate the enemy.

“Do you wish you’d never discovered that equation then?”

The equation? No. I regretted writing the letter to Roosevelt but who knows what would have happened if I hadn’t. I think Roosevelt understood the significance of the atomic bomb. When Roosevelt died, President Truman took over and seemed to see the atom bomb as just another bomb, bigger and better than normal bombs. He was told America had the bomb so that was that, he wanted to bomb Japan in revenge for Pearl harbour. As to the equation, mathematics never hurt anyone, people do that. I only wrote it down. It has existed since the universe began and without it there would be no stars or starlight and no humans to make mistakes. Because it exists stars can shine for billions of years and that finally explains how humans had enough time to evolve on the planet called Earth. I just hope that humans evolve enough sense to look after our planet for the next billion years.

"So are all those scientists that worried about the age of the Earth happy now?"

Well they have enough time for all the theories of geology and evolution to make sense, but they're just worrying about different things now. Global warming, something no-one worried about in my first lifetime, seems to be worrying a lot of people now. As soon as scientists solve one problem there is always another one to solve and we’ve barely scratched the surface of science yet. We'll get to global warming much later in our trip but there's a lot of science to talk about before we get there.

"So we're only starting the journey now?"

This is still just the beginning. It will take us three thousand years to reach Earth but it would have taken us just as long to get from the centre of Deneb to here. There is so much going on in the centre of a star that light particles can't travel more than about a centimetre without colliding with some atom or other. A tiny fraction of a second later the collision produces another light particle or photon which makes a dash for it until colliding with something else. Finally photons get released from the surface and can make it into open space. But with all the random bouncing backwards and forwards it takes thousands of years for the energy of a single photon to reach the surface and escape. Even in our super giant star Deneb, light could travel from the centre to the surface in only a few minutes if there was nothing in the way. Like in a long car journey, getting out of the city is often the slowest part.

"So there’s no point in asking are we there yet?"

Not for a few quadrillion miles.

"Where are we actually going?"

Our destination? A rather pretty, bluey-green circle surrounding a pool of blackness that is situated in the back garden of a house on a planet called Earth. But there is a lot of space and time between here and there.

Stars and Atoms

Our journey will almost be over before the human race arrives at a reasonably accurate answer to the question of how old the sun is and where it gets its energy from. As stars are pretty much the most important things out here in space, it makes sense to explain how stars really work now rather than wait for humanity to work it out over the next few thousand years. The source of energy that makes the sun shine and the stars shine is a continuous nuclear bomb, an H-bomb or Hydrogen bomb. The surface of a star like the sun is around 6,000 ^oC but the core is nearer 15 million ^oC which for those of you more familiar with gas ovens is about gas mark 79,000.

"So long can this H-bomb burn for?"

Stars, in fact the whole universe, are made up of mostly of this material called hydrogen, and stars are huge so they can shine for billions of years. The sun has been shining for 4.5 billion years and can keep going for another 4 billion years. This energy comes from the process of nuclear fusion, where atoms are combined to create completely different atoms and in the process release colossal amounts of energy.

"I’m not sure I really understand what an atom is"

An atom? Atoms are what everything is made from. They are the building blocks of the universe, sort of like LEGO. Atoms, like LEGO, come in different shapes but atoms are really minute, so small you can't see them even with a microscope. This atom idea dates back almost to the start of our journey. Democritus of Abdera was an ancient Greek philosopher from around 400 BC and developed the first well thought out concept of the universe being made up of atoms. As a young man he travelled to Athens to meet Anaxagoras, the hot rock man, but was very upset when Anaxagoras refused to meet him. Democritus has the consolation that his own ideas have lasted far longer than the famous older philosopher who snubbed him. At one time everyone thought that atoms were unchangeable. You could break things down into the individual atoms but they thought that you couldn't turn one atom into another or break up an atom. There aren’t even that many types of atoms, only 92 different types exist naturally.

"You're trying to tell me that everything is made up from just 92 types of atoms?"

I am indeed and each of these different types of atom is called an element. They vary from the very smallest which is hydrogen to the biggest which is uranium. Even more surprising is that despite all these different types of atoms or elements, nearly three quarters of the whole universe is made up of hydrogen atoms. So most of the universe is made out of a single type of atomic LEGO brick.

There's a diagram called the Periodic Table, first developed by the Russian Dmitri Ivanovich Mendeleev in 1869 that lists out all the atoms/elements in terms of size. Hydrogen is the smallest, followed by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon…

“OK, I believe you. I don’t think I’ll be able to remember all 92 anyway.”

Well like I always say, imagination is more important than knowledge. As long as you can imagine a universe made from 92 different elements that is more important than remembering all their names. That’s what reference books are for, to hold all the boring facts you can’t remember or don’t really need to remember.

You don’t even need lots of different types of atoms to make complicated things. Human bodies are amongst the most complicated things in the universe, but incredibly humans and most living things are basically made up of just six elements; carbon, nitrogen, oxygen, hydrogen, calcium and phosphorus. These six types make up 99% of your atoms with the last 1% a range of other rarer atoms.

"That's not many types of building blocks; LEGO would be dull with only six types of blocks"Well when you start combining atoms or LEGO blocks there’s an almost infinite range of things that can be built. Take just six LEGO blocks of the same size with 2 by 4 little studs, all the same shape and colour, and you could make 102,981,500 different shapes. How fast could you click together 6 LEGO blocks?“Oh, two or three seconds I suppose.”

Well at that speed it would take you almost ten years to try all possible ways of clicking together just six LEGO blocks. So it’s no wonder that you can build an entire universe out of 92 different types of atoms.

“It must take a while to make a universe then?”

Oh, it does. This one is almost fourteen billion years old after all. .

"So is light made of atoms?"

No, although all physical things in the universe from stars to humans are made from atoms we’re not. We photons are different; we're a form of energy and exist just as we are. Like heat, we can affect atoms but aren't made of atoms. Take a fire. The smoke coming out of a fire contains atoms but the light and heat you feel from a fire is energy and, in fact, is a type of photon like us. But let’s finish off atoms first. Atoms were initially thought to be the smallest division of matter but, in fact, atoms themselves are made of particles. An atom is made up of a nucleus which defines what sort of atom it is and a cloud of particles called electrons that spin around the nucleus. The electrons have a negative electrical charge and are the particles that help to carry an electrical current through a wire. The electrons are also the parts of atoms that can link atoms together to make different combinations of atoms that are called molecules. Your body is held together by these interactions of electrons which are called, reasonably enough, chemical bonds.

"Hold on Albert, let me get this staight. Atoms join together to make molecules."

That's right.

"Are humans just big molecules then?"

Well humans are made up of thousands of different types of molecules and each of those molecules is made up of atoms. By combining different atoms in different shapes the same six atoms (with a few extras thrown in here or there) can make up all the important molecules in a human like DNA and proteins. Have you heard of DNA?

"Like in Jurassic park?"

Fascinating thought recreating dinosaurs. I just read about it while I was waiting for you to join me. I'll explain all that a bit later but let's sort out the basics first. Now, where was I? Oh, yes, the glue between atoms is made up of electrons which make up chemical bonds. These bonds are where two atoms both think an electron belongs to them and hold onto them tightly.

"So if both atoms are holding onto the same electrons then the two atoms stick together, like a pair of kids 'sharing' the same toy."

Well sort of, but it does get a little more complicated from here. There are a few more names to remember about how atoms are put together, but it's worth the effort because you need these facts to know how stars shine. The nucleus is made up of two types of particles called protons and neutrons. The number of protons in a nucleus defines what element it is and neutrons somehow help to hold everything together. Remember the periodic table I just told you about?

"Hydrogen, helium, lithium and all that?"

Precisely. Well one proton means that atom is hydrogen, two protons means helium, three makes lithium and so on.

"And it doesn't matter how many electrons or neutrons, two protons in an atom is always helium?"

That's right. The protons determine the essence of what an atom is. The neutrons and electrons are important but not as critical. Take a dog. If it looses a leg and some of its fur it's still a dog inside and will still look like a dog even if it is three legged and a bit mangy looking. Legs and fur are like neutrons and electrons, important pieces of whole picture but the essential doginess is set by the protons.

As well as defining the type of atom, protons also carry a positive charge that helps to hold the fast moving electrons in their orbits. Remember the electrons have a negative charge and like the opposite ends of two magnets, positive and negative attract each other. So protons help the atom to hold onto its electrons. The simplest element hydrogen has a nucleus of one proton and one electron whizzing around it. Oxygen has eight protons, usually eight neutrons and usually eight electrons. A really heavy atom like Uranium-238 has 92 protons and 146 neutrons. Still with me?

"I think so."

There are a few more names worth hearing about, mainly because the names are so weird. You'll also impress people if you know this last piece of the atomic jigsaw puzzle. Even protons, neutrons and electrons are not the end of the story. They are made up of smaller particles called quarks. Scientists have made up some curious names for these quarks like ‘up’, ‘down, ‘strange’, ‘charmed’, ‘truth’ and ‘beauty’. In an attempt to be taken more seriously some scientists have renamed truth and beauty to top and bottom, but I rather like the original names. A proton contains two up quarks and a down quark whereas a neutron has one up and two down quarks. Naturally they are all held together with particles called gluons.

"You're making this up as you go along."

No Honestly that's what they are called. But don't worry about quarks, unless you just like the names, we can still explain most of the universe without thinking about quarks. All the same it's nice to know that everyone of you has inner beauty, truth and charm no matter how well it's hidden at times.