Faster than a speeding bullet or how to measure the speed of a beam of light.

Do you remember how fast we are travelling? I did tell you back at the start of this journey.

“Well that was a while ago Albert, but as we’re travelling on a beam of light we must be going at the speed of light.”

If I told you we’d covered 18,382,965,104,070,000 miles in just over three thousand years could you work it out from that?

“I guess that means we must be going very fast.”

186,000 miles every second. Light moves so fast that it seems to arrive without any delay at all. Light could get from London to New York in just two hundredths of a second. That's faster than a human blink.

“So what’s our speed in miles per hour, I can’t really imagine a speed in miles per second.”

Oh, let’s see. Almost 671 million miles per hour.

“Wow, how could you measure something going that fast?”

It’s difficult but not impossible. How would you do it?

“I don’t suppose we have a speedometer with us do we? Would a stop watch work?”

Not a bad idea. You can measure the speed of anything if you can record how far it goes in a second. That idea works fine for checking the speed of people running or even cars but as light goes 186,000 miles in a second it gets a bit difficult. Can you think why?

“Because I couldn’t see that far? I’d have to be able to see the start and the finish line to see when the light beam started and finished.”

Of course you can see that far. The moon is further away than that. The furthest object in the sky you can see, the Andromeda galaxy, is 2.5 million light years away. The real problem is that the light coming from the starting line would take exactly the amount of time to reach you as the beam of light you were trying to measure. So by the time you saw it leaving it would have arrived at your position and appear to arrive with no delay at all.

“That’s weird.”

But true. Imagine a bullet being shot at you and travelling faster than sound. You would be hit by the bullet and dead even before you the sound of the gun being fired reached you.

“Kind of unfair, but I can picture that.”

So if the bullet was travelling at exactly the same speed of sound you would hear the bullet at the same moment it hit you. It would seem to have arrived without delay. It would be the same if the gun was two feet away. The sound and the bullet would always arrive at the same time.

“I could look for the flash from the gun. That would arrive faster and use that to measure the speed of the bullet.”

True but for measuring light you would need something much faster than light.

“How about radio signals?”

No, radio waves travel at the speed of light because they are part of the same electromagnetic spectrum as light. That is the one of the starting points of my theory of relativity, nothing travels faster than light.

“So just by knowing that nothing travels faster than light, I can understand a little bit of your theory of relativity?”

Not just a little bit, a very important part of relativity. Thinking about the speed of light and when things seem to happen is the heart of special relativity. I told you earlier that one of my scientific heroes, Galileo, was the first person to start thinking about relativity almost four hundred years ago. Galileo was also the first person to challenge the ancient Greek ideas about how light travelled by measuring the speed of light. It was clear to the ancient Greeks that light and sound don't travel at the same speed. The delay between lightning and thunder when a storm isn't overhead showed that light travels much faster than sound but they thought it just arrived without any delay.

“How did Galileo do it. I thought you just explained that you couldn’t measure the speed of light?”

Not at all. I was trying to show that it is difficult but not impossible. Galileo managed to solve the problem we were talking about by being at the starting line and finishing line at the same time.

“How can you be in two places at the same time?”

It happens all the time in a lap race. The start and finish are usually at the same place aren’t they?

“Yes, but light travels in straight lines”

Usually yes, but you can reflect it back or get someone to send another light beam back, like a relay race. That’s what Galileo tried. He sent an assistant to a hill a mile or so away and positioned himself on another hill so there was nothing blocking their view of each other. Galileo had sorted out lamps for himself and his helper and the plan was for Galileo to switch his lamp on and start a clock. As soon as the assistant saw the first lamp light go on, he would turn his lamp on. By measuring how long it took before the light from the second lamp to get back, Galileo hoped to work out how fast light was travelling. Unfortunately even a remarkable man like Galileo couldn't spot the 5 millionths of a second delay while the light travelled from one hill to another a mile away.

"So he just needed a better clock."

Well a better clock would have helped but that kind of accuracy was centuries away, but a much bigger distance for the light to travel would help. With a little thought a Danish astronomer by the name of Ole Rømer had a go at measuring the speed of light in 1676 and came up with a speed of 133,000 miles per second.

"But he got the wrong answer?"

OK, so he got the wrong answer by 50,000 miles a second or so, but back then that was a pretty impressive achievement. But he did show that light took time to get places, it didn’t just arrive.

"Better than a million miles off I suppose. So how did he measure the speed of light?"

He used a clock but didn't need to measure millionths of a second because the light he was measuring wasn’t coming from a nearby hill but from the planet Jupiter which is 400 million miles away at its nearest point to Earth. Rømer carefully measured the time it took for Jupiter's moons to circuit the planet. He noticed that the times when the planet Jupiter's moons disappeared from view behind the planet, like an eclipse, seemed to be different at different times of the year. Since the Earth's moon is regular as clockwork this seemed odd. It only made sense if light from Jupiter's moons took some time to reach the Earth. At those times of year when the Earth was nearer to Jupiter, the light had a shorter distance to travel so the eclipses of Jupiter's moon seemed to happen earlier. Later on when Jupiter was getting further away, the light had further to go so it arrived later. It looked as though the moon had disappeared behind Jupiter later than it should have done.

“So how did scientists come up with the answer of 186,000 miles per second?”

A Frenchman called Leon Foucault used an updated version of Galileo's idea in 1862 but rather than use an Italian servant to open the shutter on a lamp, he used a rotating mirror.

"How does that help?"

If the light beam is reflected off a spinning mirror before being sent off to another mirror some distance away, then by the time the light gets reflected back to the spinning mirror it will have rotated a fraction.

“So?”

So the reflected light beam doesn’t get back to where it started but ends up a small distance away because by the time it gets back the rotated mirror….

“…rotated.”



Exactly so the light beam is reflected off at an angle. It's easy to measure the distance the light beam has moved with a ruler and how fast the mirror is spinning than it is to measure a few millionths of a second. So much easier that Foucault only needed to bounce light from a mirror sixty feet away. He got an answer pretty close to the actual figure but was finally beaten by an American in 1879. Albert Michelson improved on the Frenchman's machine by borrowing $2,000 from his father-in-law to built a bigger better version. This was no mean feat as $2,000 was a huge sum in those days and would be the same as almost a million dollars today. Michelson’s final answer was a speed of light of 186,355 miles per second which is impressively close to the figure accepted today of 186,282 miles per second or 299,792,458 metres per second.

“Impressive but does measuring the speed of light that accurately really help anyone?”

It was an important step for physics. A few years later in 1887 Albert Michelson also helped finally to bury the idea that light needed a strange substance called ether to carry it through space.

"Who's Ether?"

Ether, or aether, is a non-existent substance invented because no one could imagine waves without water.

"I don't follow."

Remember I explained that light can act like a wave.

"Like a sound wave or a wave on the ocean?"

Precisely, but what is left of an ocean wave if all the water is taken away?

"The fish?"

Very funny, the real answer is absolutely nothing. When you take away air and make a vacuum, sound waves disappear like ocean waves without water. A Mexican wave in a football stadium is carried by the spectators, with an empty stadium there is no conceivable way of having a Mexican wave. So if space is empty then how can light waves cross it? Some people were so attached to light being a wave that they said – ‘Simple, space isn't empty, it is full of ether and that's the stuff that carries light waves’.

Proving ether didn't exist rested on the simple idea that swimming in a river with the current is faster than swimming against the current. As the Earth rotates around the sun each year then it must be sweeping through the ether if it exists, effectively producing a flow of ether in the same way as wind whistles past a moving car even on a windless day. Light travelling in the same direction as the movement of the Earth will be travelling into this ‘headwind’ and so get slowed down. Light travelling sideways compared to the Earth won't face any extra resistance. In 1887 Michelson and Morley measured light travelling in these two directions and showed there is no difference in the speed of light. Discounting the remote possibility that all the ether in the universe was rotating around the sun in exact unison with their laboratory on Earth, this showed that light can't be travelling in anything like ether.

“So what difference did that make?”

With ether gone, the time was right for a complete rethink about how the universe works. That rethink was my theory of relativity. Now give your brain a rest for a while. Next time I’m going to throw some really strange ideas at you.

Quantum mechanics for cat lovers – Newton strikes back.

Let me you ask you a question. If you close your eyes does the world still exist?

“Of course it does. What a daft question.”

How do you know?

“Well, I can feel the chair I am sitting on. I can hear noise from the street outside.”

Yes but what if a tree falls in the middle of a forest and no-one sees or hears it falling?

“It still happens because the world exists, we are just part of it.”

You believe that the world is a physical reality?

“Of course, why are you asking these crazy questions?”

To show why I had such a hard time believing in Quantum mechanics. At the start of all this back in the 1920’s all of us theoretical physicists were excited by what we discovered about light and atoms. Then some people like my friend Niels Bohr took quantum mechanics to an extreme and claimed that nothing exists until it is measured. A tree wouldn’t really have fallen until someone went to see.

“So the big bang didn’t happen until someone came along and could measure it?”

Crazy idea, huh?

“Raving. If the universe couldn’t have been born until someone checked it had happened where did that person come from?”

Brilliant, now you’re thinking. Do you know what we call that? A paradox, where something contradicts itself or common sense. Quantum mechanics is full of them and I spent a lot of time tormenting Niels Bohr with paradoxes but he still believed in quantum mechanics. The crazier it got the more he believed in it. Niels Bohr once said if quantum mechanics hasn't profoundly shocked you, you haven't understood it yet.

“Well I’m shocked and I’m still not sure I understand it. How did they even start to believe this?”

In quantum mechanics any situation is a blend of every possible option of what might happen and this blend is called a wave function. This seems to work for light. Sometimes light can act as a particle and sometimes as a wave. Niels Bohr and his friends showed that atoms seem to follow the same rules. As the world is made of atoms, the world must follow the rules of quantum mechanics. Obviously in the real world doesn't spend its life sitting on the fence, things just happen. But in quantum mechanics things happen only when this wave function collapses and only one possibility is left.

"What on earth does that mean?"

Sorry that’s the sort of jargon quantum mechanics use all the time. It means that at some point a situation has to stop having every possible outcome. When an event is observed then all the other possibilities suddenly disappear.

"Hmmm. Still not sure I get this at all."

It's like saying that the universe is based on chance. One enormous casino. What happens next is based on chance not on an absolute certainty. Imagine the universe as a horse race with lots of evenly matched horses. Until the race is over you can't tell which horse is going to win. With quantum mechanics the idea is that the race isn't over until someone decides to check on the result. This is where the science fiction idea of ‘parallel universes’ comes from. If every possible outcome is waiting to happen perhaps it really does happen in another quantum universe. Every horse wins in some reality.

“Gamblers must love quantum mechanics, but it seems too weird to be true.”

That’s what I started to think. But it wasn’t just me. A friend of mine Erwin Schrödinger was the man who first discovered the equations that quantum mechanics relies on. Even he couldn’t believe the idea that nothing happens until someone looks to check it. He invented the most famous cat in science - Schrödinger's cat. If nothing happens until it is observed then imagine the following. A cat is put in a box with a small gadget that will release poison.

"A real cat?"

No this is just an imaginary cat, so whatever happens the cat doesn't really get harmed. Like this journey, it's what is called a ‘thought experiment’ as you have to imagine it happening.

“OK, I’m sure I want to even imagine poisoning a cat but let’s hear where this is going.”

This poison will be released by something that is controlled by the laws of quantum mechanics, for example radioactive decay. Radioactive atoms are ones that are unstable and spontaneously break down into smaller atoms. So there is a lump of radioactive material and a device to detect if an atom has broken down. This atomic break-up has a 50:50 chance of happening in one hour. According to quantum mechanics, until the box is opened an hour later both outcomes should co-exist. The cat should be both dead and alive at the same time until someone observes the result.

"Can't the cat tell if it's dead or not?"

Only if it's alive.

"Hmmm. That’s as daft as the ancient Greeks thinking that seeing involved feeling rays coming out of the eyes."

Well despite what some people think, this story was meant to show how Niels Bohr’s interpretation of quantum mechanics was wrong. It was just an interpretation. I think there is an easier way of thinking about this. Quantum mechanics does seem to explain a lot of things about atoms and light. This craziness of a cat that is both dead and alive only applies if you stick to the idea that everything happens until it is measured by a person. There is no paradox if you just change to the idea that a quantum event happens when the result interacts with anything. When the radioactive atom in the box decays, the cat will only die when the radioactivity detector in the box detects it. When a particle that follows quantum mechanics interacts with anything it has to commit to being one thing or another. So a quantum mechanic event can set up a sequence of events that end up with a cat that is dead or alive without needing it be both at the same time.

“I thought you didn’t believe in quantum mechanics?”

Well I didn’t believe the extreme version, but perhaps in my re-creation inside this computer I’ve mellowed a bit. All this cat really tells us about quantum mechanics is that trying to use quantum mechanics to explain normal day-to-day life doesn't work. Understanding atoms doesn't help you understand a whole cat, but then again understanding cats doesn't help you understand atoms, so it works both ways. At the end of the day quantum mechanics does make sense in its own realm and offers explanations for strange effects that have no other explanation. My problem with quantum mechanics was summed in the my idea that 'God doesn't play dice'. Everyone seems to remember that but do you know not what Niels Bohr said in reply?

“No.”

It is not the job of scientists to prescribe to God how he should run the world. Not a bad reply I think. My real problem with quantum mechanics was that I couldn’t see why the universe would have one set of rules for big objects and another set of rules for the particles inside atoms. I spent most of the second half of my life trying to join this all together into one beautiful theory of everything.

“Did you get there?”

No. Once or twice I thought I was close but it slipped away, like sand through my fingers. Someone out there will solve it I’m sure one day.

“The world needs another Albert Einstein or Isaac Newton to solve that.”

Well the world needs a lot of things more than another Einstein or Newton. Peace, kindness and fewer weapons would be a good start. Mind you, I don't suppose Isaac Newton would have been too happy with the Schrödinger's cat experiment either. One of Newton's less well known claims to fame is as the inventor of the cat flap. In the simple understandable universe that Newton described, the cat would have got bored and left out of the flap at the back, leaving the quantum mechanics scratching their heads and wondering where the cat had gone.


(No cats were harmed in the writing of this blog post. In fact one was fed, let out of the kitchen door, let back in and back out again. I don't have a cat flap.)


CHAT WITH ALBERT 2.0's CAT MIMI

Why you should never trust a Quantum mechanic

“Albert, that stuff you were saying last time. I’m still not sure how a light can be a wave and a particle at the same time.”

Well light can be different things at different times. William Bragg who won a Nobel prize for physics said ‘On Mondays, Wednesdays and Fridays light behaves like waves, on Tuesdays, Thursdays and Saturdays like particles, and like nothing at all on Sundays.’

“He got a Nobel prize for saying that?”

No. He got the prize for work that would end up revealing the secret of DNA but, smart as he was, he found light baffling too. A whole new science called Quantum Mechanics was invented a hundred years ago to explain how light can be two things at once. Before that nearly all scientists thought like was a wave because of the experiments I told you about last time. Then came along some pesky young man working in a government Patent office in Switzerland with proof that light came in little bits he called Quanta.

“Who was that?”

Oh some man who couldn’t get a job in university called…what was his name?...oh yes Albert Einstein.

“You?”

Yes me, well the real me that is, back when I was a person not just an idea in a computer.

“You didn’t have a job at a university?”

No-one would have me. I wasn’t the best of students back then and some of my professors did all they could to stop me getting a job in a university so I ended up working in the patent office in Bern. It wasn’t so bad as I had plenty of time to read and think because the work wasn’t too hard. A hundred years ago I was reading about a new discovery called the photoelectric effect.

“What’s that?”

When light shines onto a metal surface it's not just light that is reflected but also a stream of electrons, the small charged particles that make up atoms.

“Why is that so interesting?”

The odd thing is that the speed of these electrons depends on the colour of light but not on how bright the light is. There are more electrons given off in bright light but they all come off at the same speed. The first of my big ideas was that this would happen if light arrived in small packets of a fixed energy. The speed of the electrons depends on how much energy was in each packet, so the electrons all fly off at the same speed. The brighter the light the more packets there would be to knock electrons from the surface of the metal. I worked out that different coloured light contained packets of different amounts of energy, with blue being the highest energy packets and red the lowest. So in blue light the electrons would be knocked with more energy and so move faster.

“And this proved that light wasn’t a wave?”

It showed that light had to come in little packets. Just before that a great German scientist called Max Planck had seen a similar effect by looking at the light emitted by glowing metal. Light energy was only emitted in multiples of a certain value. These small packets of energy were called quanta. He thought it was something about the metal that made it happen, I said it must be that light comes in packets.

“Photons.”

Well, we discovered them before we called them that. The name ‘photon’ was invented 20 years later in 1926 by a scientist called Gilbert Lewis. Best of all was that this could be described by an equation so simple it is beautiful; E=hf, where E is the energy in each quantum of light, h is Planck's constant (a very small number) and f is the frequency of light or how fast it is vibrating.

“How can an equation be beautiful?”

It’s not the equation as much as the natural law behind it. If there is a God his handiwork should be visible in the natural laws that shape the universe. This is so beautifully simple it could only make God smile. It certainly made me smile.

“I’m glad to hear it made you happy but you promised me there'd only be one equation on this trip, your E=MC²?”

Did I? Well that was a very long time ago, but if you prefer it described in words this equation just means that as the frequency of light increases from red to blue, photons carry more energy.

"OK. I can understand that explanation, but how can these particles behave like waves?"

This is where things start to get a strange. A whole science called quantum mechanics has been invented to explain what simply doesn’t make sense. Even though I was in part responsible for starting quantum mechanics I could never really believe it could be true.

“You didn’t believe you own theories?”

No, I believed my theories but some of my friends and colleagues starting making discoveries that would make your brain implode. For a start it turns out that to be able to a particle and a wave means that a photon can be two places at the same time. Remember Young's experiment with the two slits?

"The one where light is shone through two narrow slits?"

That's the one and remember it is constructive and destructive interference that produces the fringes of light, where the peaks of two waves meet to make a bigger wave or the peak and trough of two waves meet and destroy each other. In 1909, almost a hundred years after Young's first experiments, that experiment was repeated by Geoffrey Taylor with very faint light and using photographic film to record the shadows and fringes. The light source was so dim that only a single photon was released at a time and so these individual photons could only go through one slit or the other but not both at the same time. It took 3 months to get enough photons through to produce a picture on the film. So what happens with two slits when a single photon can only pass through one or the other?

"There can't be any interference pattern because there is only one photon at a time going through the slits so they can't interfere with other photons because there aren't any others there at the same time. It can only go through one slit or the other."

That is the common sense answer. But it turns out that an interference pattern is still produced. So single photons can interfere with themselves or be in two places at once.

“I can’t believe that.”

Strange isn’t it. But it doesn’t stop there. This quantum mechanics seems to apply to all the particles that make up atoms. Quantum mechanics was developed to explain what happens inside atoms. Atoms behave more according to the laws of probability or chance and have more in common with a casino than a physics book. In the quantum world, the question of whether light is a particle or a wave doesn't really matter since everything can act as a particle or a wave. At an atomic scale objects stop being solid and dependable objects. Instead they become very slippery creatures. The more you try to work out where a particle is inside an atom, the less well you can tell how fast it is moving. The better you know how fast it is moving then the less sure you can be of its location. You can never tell where anything is, all you can know is the probability of it being somewhere. This is called Heisenberg's Uncertainty Principle.

“There’s a scientific theory called the Uncertainty Principle?”

Probably but then again perhaps there’s not.

“Is there anything certain about quantum mechanics?”

Certainly, never buy a used car from a quantum mechanic because you can't believe a word they say.

What am I? The confused life of a light particle

Remember I told you about Isaac Newton when we were talking about gravity?

“The apple man and his laws of motion. I remember him.”

Well he is just as famous for working out where the colours from a rainbow come from. He discovered that white light was made up of lots of different colours.

“How did he do that?”

Well the strange thing was that everyone in the local town market knew it already. They sold triangular bits of glass, as novelties. This is what Newton wrote in one of his books – ‘I procured me a triangular glass prism to try therewith the celebrated phenomena of colours’.

“Strange way of talking. What does that mean?”

This was how they spoke three hundred years ago. It means he bought a prism to look at the colours it makes. He worked out that raindrops must act in the same way to turn sunlight into a range of colours.

"So if everyone knew about these colours why is Newton famous rather than the person who first made these prisms?”

He did something much more important than discovering about prisms and colours, he examined it and worked out why a prism makes colours. By experimenting with these prisms Newton worked out a set of rules for how light could be split up into different colours and recombined again. He started by showing how light could be split up by a prism into different colours.

“But we knew that already”

Yes but people thought it was something magical about the glass not a feature of light. He also showed that these individual colours couldn't be split any more by passing the light through another prism. So the prism must separate light into its individual components like unravelling a rope made from lots of different coloured threads. What no-one suspected was you make white light from mixing all the colours back together again.

“Was that an important discovery? It doesn’t seem as big an issue as gravity.”

Not on a galactic scale but closer to home colour televisions use Newton’s discovery.

“Really?”

Oh yes. Go up close to an old fashioned television and you’ll see that all the colours including white are made up from coloured dots. You can’t see the individual dots on some modern televisions but every colour is still made up of a mix of three single colours. But what was much more important than television was that all this inspired Newton to produce an idea or theory about what light is.

"And the answer was?"

Newton concluded that light was made up of little particles, corpuscles as he called us, of different types. Each different type represents a different colour across the rainbow, or spectrum as scientists call it, from red to violet. White light is an equal mixture of all the different types. Every hue imaginable can be made by mixing the different types in varying proportions.

“But you said from the start that we were light particles or photons.”

I did because on this point I agreed with Newton. But for the next two hundred years, this was one thing that people thought Newton got wrong. Christiaan Huygens a Dutch man and French man Augustin Fresnel, argued that light was entirely wave-like spreading like ripples across water and most scientists started to believe that light was a wave.

“But you said near the start that we couldn’t be waves because if we were we couldn’t get through empty space.”

Very true but all that took a lot of working out and it just shows that scientists spend as much time getting things wrong as they do getting them right. The wave idea seems odd to most people because light casts shadows and so appears to always travel in straight lines. Waves on the sea spread a little around objects like boats, rocks and harbour walls. Sound waves carry sounds around corners.

“I can hear around corners but I can’t see around corners. So light and sound must be different.”

Yes, but in fact light does go around corners.

“No, it doesn’t.”

It does but only by a tiny amount and you have look carefully to see it. Just after Newton died another clever Englishman Thomas Young started investigating shadows. Although the edge of a shadow looks like a sharp line between dark and light, if you look closely you can detect faint bright lines running alongside the main light-dark line in the areas that should be dark.

"You're really telling me that light spreads a very tiny amount around corners?"

As strange as that sounds it's true. Young wasn't the first person to see strange things in shadows but he was the first to be able to explain it. In 1803 Thomas Young looked at the shadows made by shining light through two narrow slits that were very close together. In the middle of the shadow cast by the solid bar between the two slits was a faint line of light. Some of the light must have been bent around the solid edge in the same way that waves in the sea can bend around the edges of boats or harbour walls. There were also fainter lines of light, fringes, each side of this central line. All this only makes sense if light are like waves.

“It doesn’t quite make sense to me I’m afraid Albert.”

It’s a bit complicated but here is how it works. Young worked out that these fringes depend on the light from one slit ‘interfering’ with light from the other.

"What does ‘interfering’ mean?"

Interference is science-speak for where two waves collide. It doesn't matter what type of wave you are thinking about; sound waves, light waves and even waves on the ocean can all produce interference. Thomas Young worked out that the lines of light appeared where the peak of the two light waves from the two slits combined and added together. Imagine the peak of a wave hitting the two slits at the same time. At the two slits only a small amount of the wave gets through, just enough to make two smaller waves which are in exact synchrony because they are starting off at the same time. If these little waves travel the same distance then they will remain in sync with each other. So at the centre of the bar's shadow they will have travelled exactly the same distance and all the peaks of the little waves will all line up. The peak of one wave will meet the peak of the other making a wave that is twice as big. This is called constructive interference and that’s how you can get a light of light in the middle of a shadow. At all the places where this interference happens you get a fringe of light. Here is a website that shows what happens (CLICK HERE).

“So light is a wave after all.”

Well it certainly can certainly act like a wave. Young also found that different coloured lights had fringes that were different distances apart, fringes from red light being further apart than fringes from blue light. From this he worked out the light of different colours had a different wavelength.

"What's a wavelength?"

It's how long a single wave is, from the peak of one wave to the peak of the wave following immediately behind it. In the sea there might be 10 metres between waves so a sea-wave has a wavelength of 10 metres. A sound wave will have a wavelength of a bit less than a metre depending on the pitch. Young worked out from the spacing of the fringes that light had a wavelength too but it was very very small. He calculated that red light must have a wavelength of 700 billionths of a metre and blue light 400 billionths of a metre.

"How big is my wavelength?"

Tiny, about 500 billionths of a metre. You also have a frequency; if you could stay at one point and let another light wave pass you by, then the number of peaks that pass you in a second is the frequency of the wave. It also means how many times a second you are vibrating. Frequency and wavelength are related. The smaller the wavelength, the more bunched together the waves and the more waves that will go past in a second and so the higher the frequency.

"So if our wavelength is so small then our frequency must be very large."

It certainly is, our frequency is 600 trillion. In other words in our beam of light we are vibrating 600 trillion times each second. Sound waves from a note somewhere in the middle of a piano keyboard will only vibrate a few hundred times a second. Light of different frequencies and wavelengths have different colours but the whole range of colours is squeezed into a surprisingly small range of wavelengths. If you think of a piano keyboard, which only covers a fraction of the range of possible pitches of musical notes, then you could fit the whole range of visible light into just one octave or a scale of eight notes.

“But I can see millions of different colours, not just eight.”

The high frequency or long wavelength light on this keyboard would look red and be like the low pitched notes in sound terms. The short wavelength light would be blue and be like the high pitched sounds with the whole of the rainbow in between. There appears to be so many colours because the eyes is much better at detecting fine variations in light than ears are at detecting differences in the pitch of a sound, unless you are a bat of course.

"So if I made my wavelength shorter and shorter, would I get bluer and bluer then disappear completely?"

No you'd still be a photon; it's just that humans couldn't see you.

"What would I be?"

An ultraviolet photon. If you were a really short wavelength you might be an X-ray like the ones that black holes make.

"And if I got stretched into a longer and longer wavelength, what would I be then?"

An infrared photon, that's the sort of photon that carries the heat and warmth across space. And if you kept on being stretched you would change into a microwave photon, then a radio wave.

"Could I be stretched or squeezed into a sound wave?"

Never. Sound waves are just vibrations in air. Light is a completely separate breed altogether. After all no matter how much you stretched or compressed a dog, it would never become a cat.

“And all this could be worked out from looking at shadows?”

Amazing isn't it. Most importantly none of this made sense if light was just like small billiard balls travelling in straight lines.

“So are we travelling on a beam of light particles or on a beam of light waves?”

For most of the rest of our journey scientists on earth will be arguing about that with each side convinced that they are correct and the others are wrong.

"Will they have made up their minds by the time we arrive?"

Well sort of, but it won't be a clear victory for either side of the particle-wave argument.

"Why not?"

By then both sides will have conclusive proof that they were right all along. So by saying that light must be a particle and a wave at the same time they reach some sort of agreement.

“That sounds a bit weird.”

Oh, there are weirder things than that ahead of us. I’m only just getting you prepared for the really strange stuff like quantum mechanics and my theory of relativity.

The end of the Dark Ages and Invention of the Frozen Chicken

“I’ve told you a lot about science, now let me ask you a question.”

Okay, Albert fire away.

What is science?

“Science is the all the facts about how things work, space and all that kind of stuff.”

No it’s not.

“Yes it is.”

Science is not just about the facts it’s about the discovery of things. It’s a journey not a place.

“So how do you know when you’ve arrived?”

You never quite do, that’s the beauty of science. A lot of people think that all the important things have been discovered already.

“They haven’t?”

Not at all. A wise man knows how little he knows, only a fool thinks he knows everything. Science is like a travel across a vast ocean. You are happy for a while living on one island comfortable in how well you understand that island. Then someone comes along and decides to explore a little further and finds a bigger and better island just over the horizon. Facts and even reality are only what we currently think of as true.

“So you mean your theories are wrong too?”

Newton’s theories lasted over two hundred years and mine are only a little over a hundred years, so it’s too early to say. You can still fly a spaceship around the solar system using Newton’s theories or work out how the planets move pretty accurately. I really just extended his theories. If you start flying your space ship very fast or start circling a black hole then you’ll need to start thinking about my theories of gravity and general relativity but we’re getting ahead of ourselves Europe hasn’t even heard of me yet. They have barely started thinking about science properly at all.

“What date is it on Earth now then?”

We’re a little over 500 light years away so it’s the early 1500’s still on Earth. For a thousand years nothing much has happened in science in Europe. This is the end of the dark ages. Monks have been making beautiful copies of manuscripts from the ancient civilizations, keeping alive the knowledge that is over a thousand years old while most people couldn’t care less. As I told you last time, Arabic scholars were making advances in astronomy and mathematics but nothing much new was happening in Europe. The dark ages ended when Europe started thinking for itself.

“You mean no-one had any thoughts in Europe for a thousand years?”

Oh they were thinking about all sorts of things I’m sure, and fighting of course this was the age of the crusades, but not much science. Most of the ancient knowledge had been lost or forgotten in Europe. The first thing they had to do was find it all. The monasteries had some of the books from the ancient Greeks, but a lot were brought back with the Crusaders. Others were stolen back from the Arabic libraries in Spain when it was won back from the Moors.

“Why were there Arabic libraries in Spain?”

The Moors had taken over Spain for centuries and set up an Islamic culture and had brought thousands of ancient books with them. The library in Cordoba was the best in world back then, full of the best writings and ancient Greece and Arabic science.

“I thought you said the ancient Greeks got it all wrong?”

They didn’t get everything right and were certainly wrong about seeing but they had invented the art of thinking. Do you know what the word philosophy means?

“Thinking?”

It means the love of wisdom and that is what the ancient Greeks had and the Europeans had forgotten. Back then science was called natural philosophy, the love of wisdom about the natural world. When all these ancient books were recovered and read it opened their eyes to wisdom and the pursuit of knowledge. The advances made by Arabic scholars also showed them that new things could still be discovered. If you don’t believe that you’ll never discover anything new. So a few brave souls started to challenge the ancient ideas, people you’ve probably heard of like Leonardo da Vinci.

“He was just a painter. He did the Mona Lisa and the Last Supper.”

Oh he was more than that he was a brilliant architect, scientist and inventor, thinking of things that wouldn’t or couldn't be built for hundreds of years like helicopters and parachutes. This was the start of an explosion of thinking called the Renaissance or literally rebirth. Another of my heroes is Galileo, he came a little after Leonardo, and was the worlds first great scientist. He made huge discoveries in astronomy and physics. He was even the first person to talk about relativity in very simple terms.

“He invented relativity?”

No, but he introduced the concept to science. He compared the situation of someone standing on deck of a moving boat or inside a cabin on the same boat with no windows. On deck the person could tell they were moving forward, but inside the cabin apart from a little side to side sway they’d feel they weren’t moving forward at all. If you dropped a ball it would seem to fall straight down even though really it was also moving forward at the same speed as the ship as it falls. That is the simplest form of relativity, what you experience is only relative to what is around you. It just took a few hundred years more to work out the details.

“What took you so long?”

I had to be born first and that took hundreds of years. While I was waiting to be born Europeans were still busy working out what science was. Science was really given a kick start in the year 1605, when an Englishman called Francis Bacon published a book called The Proficience and Advancement of Learning. This gave birth to what would now think of science. Rather than just studying the writings of the great Greek philosophers, Bacon urged people to think for themselves and come up with new theories for how the universe worked. Bacon is now thought of as the father of modern science.

“What did Francis Bacon discover?”

Well he invented the scientific process, or at least a version of it. He thought that a theory would naturally come from examining the world. In his writings he didn’t talk much about experiments and sparks of creativity but in his last week alive he managed to invent the frozen chicken and die as a result.

“Francis Bacon died trying to create the first frozen chicken?”

In a snowy March in 1626 Bacon was visiting London with the King’s doctor when he had the spark of an idea that cold could stop meat from going off. They didn’t have refrigerators back then of course. So he got out of his warm carriage in Highgate to buy a chicken and have it stuffed with snow. Sadly the chicken probably stayed fresh as long as he did. He caught pneumonia and died a few days later.

A brief history of Light, from the daft Greeks to the brilliant Iraqi Al-Haytham

“Albert, what about us?”

What about us?

“Well, you’ve told me a lot about stars and the universe but not much about light. Aren’t we meant to be imagining this trip of yours across the universe on a beam of light?”

Of course, light is after all one of my favourite topics. A lot has happened in terms of light during the journey so far. When we started out starlight arriving on Earth was greeted by a truly appreciative audience that placed great importance on the twinkling lights in the sky. Mesmerised young faces would gaze up into the stars and ask their parents what stars were.

"So what did they tell them?"

They told them stars were little holes in the floorboards of heaven, where light shone through.

"Lovely idea."

A beautiful notion, but a little wide of the mark scientifically. Those ideas lasted for over a thousand years from the time of the ancient Greeks until the 1600's. That was when natural philosophers, which is what scientists were called back in the year 1605, started to realise that they could abandon the old ideas of the ancient Greeks and think for themselves.

"So what did these ancient Greeks think about light?"

The Greek philosophers had a good think about light and then most of them came up with all the wrong answers. Plato, born in 427 BC, was most famous for his writings on politics, but also dabbled in science and his idea was that light works by sending out 'feeling rays' from the eyes to whatever you happen to be looking. This sounds a bit odd today but as an idea it lasted for almost two thousand years.

"So if all humans went blind overnight, light would cease to exist."

True, but I suppose they thought cats and dogs saw things the same way. Another problem with this great theory is that if light comes from the eyes why can't people see in the dark?

"Of course, why didn’t I think of that?”

Don’t worry, neither did most of the Greek philosophers. More worrying is what this says about progress in politics, no-one takes any notice of Plato now on science but his writings on politics are still highly regarded – most famously Plato’s Republic. The other great greek philosophers weren't much better on light. Have you heard of Aristotle?

“I’ve heard the name but I’m not sure what he is famous for.”

Aristotle was perhaps the best philosopher of ancient Greece and was brilliant in developing logic and studying animals and biology. He wasn’t so good at light and thought that all colours were a mixture of black and white. Another ancient Greek, Epicurus, got the right idea in 300 BC with light coming from objects to the eye, but since he was a small second division philosopher no-one listened to him.

“Why didn’t anyone listen to him? It seems so obviously true.”

Not back then. The notion of light travelling from objects to the eye seemed nonsense to most people- how could the light coming from something big like an elephant fit through the tiny pupil of an eye?

"So how does it all fit?"

Very easily. Light isn't made up of atoms so it doesn't take up any space. You can fit more light than the eye could cope with through even the tiniest pupil.

"So did no-one have a clue about light back then?"

After the Greek and Roman empires collapsed Europe was stuck in the dark ages for a thousand years.

"I guess they weren't likely to work out much about light in the dark ages."

Very true, but it wasn't so bad everywhere. To find someone talking sense about light 1500 years ago, you would have to skip Europe and look towards the Middle East at a time when Arabic culture was leading the world in mathematics, astronomy and science. Iraq was the major science centre back then.

“Iraq?”

It may come as a surprise to many people but Baghdad and Basra a thousand years ago were some of the most cultured places on the planet rather than just another war-zone. Did you know the numbers 0 1 2 3 4 5 6 7 8 9 are called arabic numerals?

"I thought they were invented in Europe.”

Well you’re wrong. The numbers we now use around the world can be traced back even further to Hindu and Indian mathematicians. Imagine if we were still lumbered with Roman numbers, all those X's and V's. And it's not just the numbers, the foundation of modern mathematics was created by arabic mathematicians a thousand years ago. Think of the word algebra.

“That’s a type of mathematics. Equations and stuff.”

It certainly is, but the ‘al’ part also reveals its Arabic origins. The father of this type of mathematics, Muhammad bin Mūsā al-Khwārizimi, was from Persia which would now be part of Iran but he spent most of his life working in Baghdad. Algebra is work that comes his famous book on the subject that contained the word al-Jabr in the title. But the most important Arabic scientist for our story was Abu Ali Muhammad bin al-Hasan bin al-Haytham.

“Wow, long name.”

He’s often just called Ibn al-Haytham or even just Alhazen. He was born in the year 965 A.D. in Basra, in what is now Iraq. He knew all about the writings of the Greek philosophers and thought this ‘feeling ray from the eye idea’ just didn’t make sense. More than that he worked out things that we know take for granted like how light travels in straight lines and how light is reflected by mirrors.

“How did he get his inspiration?”

Al-Haytham had plenty of time to think about light since he spent years under house arrest pretending to be insane after giving up his attempts to stop the Nile flooding. At that time in Egypt the flooding of the river Nile was the most important event in the year. He was invited to Egypt after claiming he could stop this annual flood. The plan involved a dam on the Nile at Aswan. Overwhelmed at the scale of the task he admitted defeat and pretended to go mad as a result. The ruler of Egypt at the time, Fatimid caliph al-Hakim, was pretty upset at this. Since al-Haytham appeared mad it seemed a little unfair to kill him for failing in his mission, so he let him off and simply placed him under house arrest for life. Unfortunately the Caliph had a long memory so al-Haytham had to pretend to be mad until the Caliph finally died years later. Al-Haytham has the consolation that his plan would have worked if he had completed it. A dam was built at Aswan almost a thousand years later and it did stop the Nile flooding.

“So when did he have his great ideas on light?”

In all those years of house arrest he wrote 209 books on science, philosophy, mathematics and astronomy. The most famous and influential of these was his book on light Kitab Al Manazir or the Book of Optics. This amazing book started the science of light. Just about everything he said about light made more sense than the ideas of the ancient Greeks and so he overturned what was thought to be true for over a thousand years and no-one improved on his ideas for another 500 years. When Europe started to escape from the dark ages it was Alhazen’s book that helped to get people thinking sensibly about light.

"So Alhazen was the first ray of light to illuminate the gloom of the dark ages."

You're getting quite poetic in your old age.

"Why thank you, that's your first compliment in over two thousand years."

If I have seen further....the nose, the moose and the telescope.

“So it wasn't really until Galileo got his hands on a telescope that anyone really understood anything about space?”

The telescope was a huge advance but one of the most famous astronomers of all time, Tycho Brahe, died in 1601, just seven years before the telescope was invented in Holland in 1608.

“What was he famous for?”

As well being the worlds best astronomer four hundred years ago, he is also famous for having a magnificent moustache and loosing part of his nose.

“That's a bit careless, how did he do that?”

He had a duel in on 29th December 1566 with a Danish nobleman by the name of Manderup Parsberg and lost the end of his nose.

“What his fighting over, some woman I suppose?”

No, would you believe it, they were fighting to settle and argument over who was the best mathematician. Tycho Brahe was probably right about being the best mathematician but his sword skills were clearly not quite so sharp.

“So he spent the rest of his life without a nose?”

No he had a new one made of silver and wax.

“That's a bit weird isn't it?”

I presume that was what you did back then when someone sliced off your nose, but Tycho was certainly an unusual character. At one stage he had pet moose that he brought along to a party at the castle of Landskrona in Sweden. The moose apparently drank so much beer it fell down the stairs and broke its leg and died shortly after. Despite these series of unfortunate events, Tycho became famous across Europe for his talent in making extraordinarily accurate astronomical records of the positions stars and planets. Remember Johannes Kepler I told you about who worked out how the planets move? Well, Kepler was Tycho's student and he used all these measurements to produce his laws of planetary motion.

“How did he make all those measurements without a telescope?”


He used his eyes along with a variety of instruments like the sextant and quadrant to help measure the position of the stars. He didn't believe in the ancient Greek view of the universe with the Earth in the centre or the new outrageous Copernican notion of the Earth moving around the Sun. It may not surprise you to know that this slightly larger than life character believed most strongly in his own theories. His theory had the Earth at the centre but the rest of the planets moving around the sun which in turn moved around the Earth. This was as close to Copernican thinking as you could get without getting excommunicated by the Church but it was also wrong. He had great faith that his young student Kepler, with his mathematical abilities, would be able to use Tycho’s own huge volumes of measurements of planetary positions to prove his own theories right. He fortunately died before Kepler showed something completely different.

“But the telescope must have changed things completely.”

It certainly did, it also helped to finally separate astronomy from astrology.

"I thought you said that astrology was all rubbish."

I did and it is, but hundreds of years ago people wanted their fortunes told just like they do today. The astronomers of the day like Tycho Brahe had to make predictions to get paid. They had become so good at predicting the movements of the planets and eclipses that ordinary people and a few kings as well, assumed they could predict anything which of course they couldn't. But just like today, as long as they were fairly vague in their predictions no-one seemed to mind if they didn't come true.

"So how did the telescope change things?"

By making astronomy into a science. A lot of famous names we've already met designed new types of telescopes, like Johannes Kepler and Isaac Newton. Newton developed a totally new type of telescope based on mirrors rather than lenses and the biggest light telescopes today use mirrors to collect and focus light. As bigger and better telescopes helped astronomers see fainter and more distant objects, the size of the known universe steadily grew.

“So how far away can we see now with the best telescopes.”

With telescopes like the Hubble space telescope, galaxies have been seen that are up to 13 billion light years away. The light from these galaxies has been travelling for so long that some of these light beams started their journey just 700 million years after the universe was formed in the big bang, . From these distant galaxies we can look into the past and tell what the universe was like when it was very young.

“So you can see into the past with telescopes, but not into the future.”

Exactly and deeper into space. In just four hundred years mankind's understanding of the universe has stretched from the solar system to galaxies billions of light years away. Isaac Newton once said, 'If I have seen further, it is by standing on the shoulders of giants', but you could change that to say 'If man has seen further, it is because Newton developed the reflecting telescope.'

Little green men and Black holes

"Is a star completely blasted into dust in a supernova?"

No there is usually a core left behind and this is where the story gets interesting. Last time you asked a very intelligent question.

“Did I?”

You certainly did, you asked what stops gravity collapsing a star into nothing.

“..and you said it was the charged particles in atoms pushing away from each other like the poles of two magnets.”

Very good, you were listening. Well, that pushing effect which scientists call electron degeneracy is true up to a point.

“Electron degeneracy? What a great name for a rock band.”

Rock band?

“Rock, a new type of music, invented 50 years ago. You definitely need to get your entertainment circuits updated Albert, anyway you were telling me about gravity.”

Ah yes, gravity well if it gets too strong even the forces inside atoms aren’t strong enough and the electrons and protons in atoms get crushed together to end up as neutrons. Neutrons are not charged so can be squeezed together very tightly. A spoonful of pure neutrons would contain billions of tons. These dense neutron remnants are called neutron stars.

“So neutron stars must be tiny?”

They are. A neutron star with twice the mass of the sun might be only 15 miles across.

“So how can scientists see something that small thousands of light years away?”

When they first discovered them they thought they were alien radio stations. I told you earlier about radio telescopes. Well an astronomer Anthony Hewish from Cambridge University became interested in radio waves coming from stars. In 1967 one of his students, Jocelyn Bell, discovered a celestial radio source giving out regular radio pulses every 1.3373011 seconds. It was like radio-lighthouse in space.

“It wasn’t really a radio-lighthouse was it?”

No, but it was initially called LGM-1, short for Little Green Men because it seemed so fast and regular that an alien radio station seemed as likely as anything other explanation. Then they started finding more and more of them all over the galaxy, LGM-2, LGM-3, LGM-4. If you convert the radio signal into sound this is what a pulsar sounds like.

“So the universe does have a ticking clock then, like Newton said.”

No, it just has lots of spinning neutron stars. Like most things in space, the sun, the earth and the rest of the planets, neutron stars spin. And like ice skaters that spin faster as they pull their arms inwards, these solid lumps of matter spin faster and faster as they contract. You end up with a very small, very dense, star spinning around so fast that it can do a complete turn in a couple of seconds or even a fraction of a second. With each spin they give out a flash of radio-waves with every rotation which is why they are now called pulsars.

The Hubble telescope is so good that it has taken pictures that show the spinning of these stars.

“Are you serious?”

Oh yes take a look at this video. These are real pictures of the the supernova seen in the year 1054. The explosion left a cloud of gas and there is a pulsar in the middle of it. You can see the rotation and the ripples coming away from it. That’s the beauty of science, if you get it right everything starts to link together and make sense.

“I’m getting to see why you like science so much Albert.”

If you thought pulsars were interesting I have another surprise for you. Even stranger, and the only thing in the universe that we photons have to fear, is a black hole. They also can be created from exploding stars. If a collapsing star is a bit bigger than a neutron star, then the gravity will crush it to an even small size and create black hole. Like so many things in space, understanding black holes is all about gravity.

"Our friend Newton again?”

I don’t think Isaac would ever have imagined something like a black hole. In fact neither could I. A colleague of mine, Karl Schwarzschild, showed the my theories showed they could exist but I thought it was just a quirk of the equations. He even managed to do these calculations while he was serving in World War I calculating trajectories for artillery shells.

“Black holes and artillery shells?”

Well they both have to do with gravity but the gravity in a black hole is so strong even light can’t escape.

“Which is why they are black.”

Precisely. Here is how to make a black hole. Take the Earth and squeeze it into a ball less than an inch across and run as fast as you can. Most black holes are much bigger than that of course. There is probably a massive black hole in the centre of this galaxy, the milky way, which is about 10 million miles across. As long as the stuff inside is dense enough, gravity will do the rest. For all their mystery, the properties of black holes are still understandable in an apple falling way.

"Hmm, can't quite see how."

It's not as difficult to understand as you might think. You know that the gravitational force of the Earth stops you from throwing an apple into space.

"Of course, that's how Newton got the idea in the first place."

Exactly, if you throw an apple into the sky it falls back down again. Earth is therefore an apple-hole; apples cannot be thrown fast enough to escape the gravity of the Earth. Earth is also a bird-hole; birds cannot fly fast enough to fly into space. It is also an aeroplane-hole. To escape from the Earth you need a space rocket travelling at 25,500 miles an hour, any slower and the rocket will not be able to overcome gravity. Now imagine gravity getting stronger and stronger. The rocket will have to go faster and faster. The fastest thing in the universe is light and I’ll explain in a little bit why nothing can travel faster than us. So a black hole is a photon hole because photons cannot go fast enough to escape from the force of the gravity, in just the same way the Earth is an apple hole, bird hole and aeroplane hole.

“That makes sense.”

A long time before I invented relativity someone had already imagined that black holes could exist.

“Who?”

A Frenchman Pierre Simon de Laplace used this same argument in the year 1798, the year we've just reached, to predict that if a star had enough gravity it would stop light from escaping so become what he called a black star. It seemed such a crazy idea everyone ignored until I started coming up with just as crazy ideas.

"If you can't see black holes, how does anyone know they exist?"

Even though you can't see the holes themselves, you can see the effects of these strange things on neighbouring stars. As matter gets pulled in, it accelerates faster and faster and gives off x-rays in the process, the same sort of x-rays that are used in hospitals to take pictures of bones. These x-rays are a very high energy photon but are invisible and can go straight through your body and out the other side.

"So how do they escape?"

They escape because they are released just before the point of no return as matter is being pulled at huge speeds into the black hole. A special telescope that can detect x-rays from space can tell you where black holes might be. The constellation Cygnus, where we are coming from, contains the first detected object that might be a black hole.

"So what's it called? Vlad the absorber?"

You've been reading too many comics. No it ended up with a very boring name, X-1, because it was the first x-ray source detected in the constellation Cygnus.

“That’s the constellation we came from!

It certainly was, but luckily for us it was 5000 light years in opposite direction to the one we took towards Earth. So aren't you glad you ended up on this journey with me rather than being eaten up by a black hole.

"Oh, yes, but there is a lot space out there ahead of us and it all looks pretty black to me."

Exploding stars and why everyone has a little star quality

“Albert, quick look at that amazing star. I can't believe it's shining so brightly.”

That star isn’t shining, it’s exploding with style. That’s how some stars die, in a blaze of glory where they can outshine a whole galaxy for a few days or weeks That’s called a Supernova. You are very lucky to see one as they don’t happen everyday, you are also lucky it was a long way away.

"How often do these supernovas happen?”

In our galaxy, the milky way, only a few stars have exploded that violently while we’ve been travelling for the last couple of thousand years.

“Couple of thousand years? Rubbish we’ve been gone hardly anytime.”

Strange isn’t it, that time doesn’t do what we expect. That’s where I disagree with Newton, I don’t think the universe has a ticking clock that tells the actual time. Time only exists to stop everything happening at once.

“What a strange way of looking at things.”

One day I’ll explain my theory of relativity to you. Then you’ll really know how strange a place the universe is. Anyway do you want to know about exploding stars or not?

“Oh, yeah, sorry.”

Well the earliest one the humans saw and wrote about was in the year 185 AD when Chinese astronomers discovered a new star that slowly faded. The brightest one ever seen was seen on Earth in May 1006. After that there others in 1054, 1181, 1572 and the last one seen from earth from this galaxy was 1604 which is probably the one you are looking at now.

“There hasn’t been one since?”

Oh there probably has been quite a few, but the light just hasn’t reached us. Don’t forget the Milky Way is almost 100,000 light years across. The light from a supernova happening right now half way across the galaxy won’t reach here for 50,000 years.

“Will the Sun blow up like that?”

The Sun is too small to explode. Only very big stars explode like that.

“So what do stars like the Sun do?”

Smaller stars first swell up as they start to run out of hydrogen and then start cooling and shrinking. To know why that happens you need to understand how a star normally stays the same size. When a star is shining normally, the nuclear fusion reactions in the centre make a huge amount of energy and particles that are continually rushing out of the centre of the star. This balances the force of gravity so the star stops shrinking until it finds a balance and just shines away and stays the same size for most of its life. Think of a balloon, the stretchy rubber of the balloon is trying to shrink the star like gravity and the pressure of the air in side is pushing against the balloon.

“Until the air in the balloon escapes.”

That’s right the air escaping would be like nuclear fusion stopping so the star would star to contract. In a star the nuclear fusion reactions happen not just in the centre but also in layers further out. So it’s like having lots of balloons inside each other.

“Or like an onion?”

Yes you could think of it like the layers of an onion, either way when the centre runs out of hydrogen and the nuclear reactions start to slow down, this upsets the balance with gravity and the star starts to contract again. This heats up the next layer up which is still full of hydrogen. The outer parts get hotter as the fusion reactions move further from the centre and so they expand. The bits in the centre start combining the helium made by nuclear fusion into bigger and bigger atoms. This keeps going until the star gets bigger and bigger and ends up as a something called a red giant. By the time this happens to the Sun in about 5 billion years humans, or whatever the dominant species on the planet is by then, will have to find somewhere new to live.

"So what happens to stars after they swell up and become red giants?"

That’s where the size of the star comes in. Once all the atoms that can be used in nuclear reactions are used up, a normal sort of star starts to contract. There is no more nuclear fusion energy from the centre counteracting the pull of gravity. So they get smaller and smaller because of gravity. These star remnants are called white dwarfs, because even though they are still shining they may be no larger than the Earth which is tiny compared to your Sun. A piece of a white dwarf star the size of a sugar lump could weigh more than a ton.

“So what stops them shrinking to nothing?”

Despite all their gravity they can't keep on shrinking forever because all the charged particles in the atoms like electrons and protons start pushing away from each other if they get too close, a bit like two north poles of a magnet. This helps to balance the force of gravity but doesn't help the star to shine so it slowly cools down. White dwarfs cool down, shine less brightly and end up first as red dwarfs and then finally even colder brown dwarfs and finally cold lumps of dead stars.

“You said that large stars explode, what makes them so different?”

The biggest stars have a very different way of living and dying. A star ten times the size of the sun can be born from a cloud of gas and dust in only a few hundred thousand years because the greater the mass the greater the gravity pulling everything together. So it all happens faster. At ten times the size it will shine 10,000 times brighter than the Sun but last only 2 million years until they do what that Supernova just did. They live fast and die young.

“But why do they explode when smaller stars don’t?”

The short answer is that big stars have so much gravity that when the balance between gravity and nuclear fusion goes wrong the star can suddenly collapse crushing the centre which gets massively hot and explodes. When really large stars run out of hydrogen fuel for fusion reactions they can move on to fusion reactions with bigger and bigger atoms because they have much more gravity which makes the centre much hotter than a normal star. All these fusion reactions build up bigger and bigger atoms ending up with the formation of iron.

“Iron doesn’t explode does it?”

Not by itself. It’s when these big stars try to use iron atoms in fusion reactions that everything starts going very wrong. Rather than release energy, nuclear fusion reactions with iron, absorb energy. So rather than making the centre of the star expands to counterbalance the effects of gravity the centre of the star suddenly starts to collapse. The matter in the centre then gets squashed into a super-concentrated form and the gas around the centre gets blasted out into space. This cosmic explosion is a supernova, and for a brief time a single supernova can outshine an entire galaxy. In the process of exploding a huge variety of nuclear reactions happen which result in the formation of all the complicated heavy atoms that are even bigger than Iron.

Remember I told you that humans are mostly made up of just six types of atoms; carbon, nitrogen, oxygen, hydrogen, calcium and phosphorus. Well all these atoms, except the hydrogen, were made inside a star before it died. There are some even bigger atoms in humans and these would have made in a supernova explosion millions or billions of years before the sun formed

"So this huge explosion blasts into space all the new types of atoms the star's been making from nuclear reactions, but how does it get into humans?"

Well the remnants of these explosions become the gas and dust that makes up the next generation of stars and planets.

"So the planet Earth and all the people on it are recycled space junk?"

Every one of them is made up of little bits of recycled stars. It may be well hidden but there’s star quality in every one of them.

"Do they know that?"

Well the smart ones, like you, do. But remember exactly the same star quality is found in mosquitoes, earthworms and the ink in a Bic biro. So don’t be getting any grand notions about yourself.

The curious connection of Isaac Newton and Harry Potter (and how to weigh a whole planet with an apple and two metal balls).

“Albert who was smarter, you or Isaac Newton?”

I think we both did pretty well at unravelling how the universe works.

“That’s not an answer.”

It’s an answer of sorts. So let me ask you a question, which of us would you rather be?

“Me? Well, I quite like being myself but I suppose I wouldn’t mind being you Albert. As for Newton, I can’t really imagine what that would be like.”

I think I’d rather be me too. I not sure being Isaac Newton would be that much fun. He was certainly brilliant but I’m not sure how happy he was. Certainly my childhood was simpler. True we had to move around a bit as my father’s various businesses struggled, but we were happy enough. Isaac Newton wasn’t quite so lucky. He was given away as a baby to his grandmother. His real father died just before Newton was born and his mother abandoned the young Isaac to marry a vicar from the next village, Rev. Barnabas Smith.

“That’s a tough start.”

Certainly was. His stepfather only lived another eight years before he died, which was just long enough for Isaac to learn to hate him and resent his mother for abandoning him.

“But he turned out good in the end?”

Oh he did very well, a professor of Mathematics at Cambridge University by the time he was only 27. His theory of gravity would have been enough for most people but he had the laws of motion and all his work on light too. All brilliant work, and most of done as a young man he didn’t publish most of it until he was much older. He was as grumpy as he was brilliant and a terrible man for getting into arguments with people.

“What kinds of people?”

Oh, almost everyone including his friends, but mostly people who disagreed with him.

“So he would have hated you?”

Oh I don’t know, I think meeting Newton would be fun. At least meeting him when he was still interested in physics. He left Cambridge University after 20 years or so and literally made money working for the Royal Mint in London. He’d been fascinated with the strange art of alchemy all his life and later in life he spent more of his time on that, than he did on physics.

"Alchemy?"

A strange mix of witch-craft and science that tried to convert metals into gold and discover the secret of eternal life. Newton became fascinated with the story of Nicholas Flamel.

"Nicholas Flamel? He was just a character in the first Harry Potter book, ‘Harry Potter and the Philospher’s Stone’ What’s he got to do with Newton?"

There really was a man called Nicholas Flamel that some people claimed did discover the secret of immortality. Legend has it that Flamel discovered the secrets of making gold out of ordinary metals and the secret of immortality from a book he saw in a dream. One day someone came into his bookshop in Paris and offered to sell him the very book he'd seen in his dreams. He immediately bought it and spent years trying to unravel its secrets. Some people claim he succeeded, but no-one knows for sure.

"Gold and other metals are elements or different types of atoms. How can you change one type of atom into another?"

They are indeed. We now know that you can't change one atom into another type of atom without changing the atomic nucleus in a nuclear reaction. So no amount of mixing of mercury and magic potion ingredients is going to create gold atoms. That's why there are no more alchemists, and alchemy changed from a mystical magic to the science of chemistry. Sadly Newton seemed to become quite crazy for a while from playing around with the metal mercury in his alchemy experiments. The tested some of his hair they had in a museum in Cambridge a few years ago. It was still full of mercury.

"Why was messing with mercury in the first place?”

Mercury is an amazing metal as it is actually a liquid and you can dissolve other metals in it which is why the idea of making gold seemed possible. The problem is mercury is poisonous and can send you mad. Remember the Mad Hatter in Alice in Wonderland?

“The Mad Hatter’s tea party?”

Exactly, well Hatter’s or people who made hats were often crazy because they used mercury in the making of hats.

"What did hatters do with mercury?"

They used it to harden hats to help them keep their shape until it was eventually banned. Anyway Newton eventually recovered from his mercury madness, but it is hard to tell if he was happy. He was only recorded to have laughed once in his entire life when someone asked him what he thought was a stupid question.

What might have made him smile was an experiment that showed that Newton was right in that a kilogram of anything has a gravitational pull.

“You said ‘might have’, he didn’t even smile when his ideas were shown to be right?”

No, because he had died 71 years before Henry Cavendish did his famous experiment with two large metal balls. Cavendish used a very sensitive machine built by a churchman, the Reverend John Michell who had died before finishing the experiment, to measure how much gravitational attraction there was between two heavy pound lead balls. Each weighed 350 pounds but there was a tiny but definite pull between the two balls. Cavendish measured the tiny gravitational force between these balls. From that he could calculate the mass of the Earth, though in fact he was trying to work out how much denser the earth was than water.

“You can stand on the Earth, do an experiment with two metal balls and an apple and measure how heavy the Earth is?”

Absolutely, but it is really the Earth’s mass you are measuring not its weight.

“Weight and mass are the same thing aren’t they?”

Not at all. Weight is just an effect of gravity on mass. Out in space an astronaut is weightless, but not mass-less, he still exists and has just as many atoms in his body. When people say they want to loose weight, what they really mean is they want to loose mass.

“OK, Albert we can talk about the Einstein mass-loss clinic later, first tell me how you work out the weight, sorry mass, of the Earth from a falling apple.”

Well, if you drop an apple it starts accelerating, going faster and faster.

“Yeah?”

So Newton’s second law of motion that I told you about last time can tell us how much force is required to accelerate an apple at a particular rate. We can measure how fast an apple accelerates easily enough. His experiment with the metal balls told him how much gravitational pull each kilogram of Earth could have, so all he needed to work out was how much mass was needed to create enough gravity to accelerate the apple in the way that it does according to Newton’s laws of motion and gravity.

“I couldn’t work out the mass of the Earth from that.”

Well if you tried hard you might, but luckily you don’t have to, that is why God invented physicists. The important thing is to appreciate how with a little thought and imagination you can do what sounds impossible like working out the mass of the Earth.

“Which is?”

Oh, almost six thousand billion billion or 6,000,000,000,000,000,000,000 tons. One last thing. Remember I said that gravity doesn’t make people fall in love?

“Yes, one of your stranger statements, Albert.”

Well, it doesn’t make people fall in love but it does make them attractive, especially if they are a bit fat.

“What!?”

Well every body floating in space has a gravitational pull and so does everybody on Earth. Every kilogram of bone, heart muscle or fat has exactly the same gravitational pull as a lump of rock. It doesn’t matter if you’re pretty or not and a few extra pounds just makes you a little bit more gravitationally attractive. So everyone is in reality attractive to everyone else. A little fact of physics that the crueller aspects of human nature have hidden from view for far too long.