H is for Holograms

By Siobhan Fairgreaves

After a little summer break, welcome to the eighth post in this series- and today we’re going to be finding out more about Holograms.

First things first- what even is a hologram, and where would we encounter them?

Holograms are popular optical illusions that seem to make a moving image appear from a still photograph. The moving images appear 3D and can be viewed from different angles. However, holograms aren’t just reserved as fairground gimmicks; we can find them all around us. From our money to our DVDs, holograms are everywhere and they have some very important uses.

Hello dove my old friend, I’ve forgotten the 3-digit security code on the back of my card again| Image: Dominic Alves

Why use a hologram though? Sure, the shiny bits on money and credit cards look nice but what is the point? Holograms on money are used to reduce forgery. When a financial system becomes inundated with forgeries the strength of the currency can be weakened. Incorporating a hologram printed onto a piece of metallic film into the design of bank notes and credit cards means the forgers will find it much harder to copy. Other goods, such as DVDs, games or software programmes may also feature holograms so you can be sure they are genuine products.

Now that holograms are starting to sound important it would be nice to know how they’re made. This is where it gets a bit more complicated, try and bear with me though. The exciting news is that we do get to talk about lasers, everybody loves lasers. We’ll be finding out more about how light works in a few posts time but for now, we need to know that holograms rely on the coherent, synchronised waves of light that make up a laser beam.

Tired of only seeing images of 3D objects from just one boring old angle? Now you don’t need to! | Image: Explain that stuff

The synchronised waves of light in a laser beam can be split using a half mirror which allows half of the laser beam through and reflects the other half. By carefully angling the half mirror and a normal mirror you can ensure that the reflected half of the laser beam hits the object you’re trying to turn into a hologram. The reflected half creates what is known as the reference beam. The other half of the laser beam, having passed straight through the half mirror, creates the object beam which can then also be directed onto the target object.

But how do we know this was the real Dennis Gabor? | Image: Keystone

When these two, usually synchronised, halves of the laser beam recombine on the photographic plate having reflected off the target object they now show the object from different angles. This is because each half of the laser (the object beam and the reference beam) took a different route to get to the same place. From whichever angle you view the hologram you can now see how the light would have hit the object if it had been real. This allows the image to change as you move your head around to capture different angles.

A post about this complex process would not be complete without reference to their creator, Dennis Gabor. Incredibly, Gabor knew how to make a hologram before lasers – which are needed to make holograms work – were even invented. His invention and development of holography (the art of making holograms) in the early 1950s eventually earned him the Nobel Prize in Physics in 1971.

That was definitely one of our tougher posts so far but it just goes to show how often we use complex physics and take it for granted. I know I’m definitely guilty of thinking physics was the least interesting of the sciences because it just didn’t feel as relevant. How wrong was I!

A fun fact to finish off now and something that is sure to inspire a few YouTube searches. If you break a hologram into tiny pieces the whole object can still be seen in each fragment.

G is for Gravity

By Siobhan Fairgreaves

“Gravity pulls everything down”


Now we know what gravity doesn’t do- let’s take a look at what it does.

If it helps, you can think of the force of gravity as ‘falling’ into this thing – a gravity potential. If you’re ever floating in the void of space and you’ve lost your keys again, simply having mass would be a good way to get them back | Image: AllenMcC

Before I get too stuck in I’m going to clarify- we’ve all heard of gravity right? Just in case you missed that class here is a quick reminder. Gravity is one of the many invisible forces acting upon us and our environment every minute of every day. We learn about it in the early days of school, but gravity is still hugely important to physicists. The famous Sir Isaac Newton had a ‘eureka!’ moment concerning the force – more on that later though.

Every object with mass has some gravitational pull (that includes you!) That means your own body is exerting its own force, however tiny, both on the clothes you are wearing, and on distant stars. But for gravity, a bigger mass means a bigger force. So you never need to worry about walking around with lots of small objects stuck to you simply because the gravitational pull created by the Earth is much, much stronger.

For humans, animals and absolutely everything we share our planet with, gravity pulls us towards the centre of Earth. It might seem pedantic but there is an important difference between pulling towards the centre of the Earth and pulling ‘down’. An important law in physics, called Newton’s Third Law, states that “for every action, there is an equal and opposite reaction”.

Not to be pedantic, but the gravitational pull of Earth isn’t the same everywhere: this image from NASA’s GRACE mission shows the subtle differences across the Earth’s surface | Image: NASA

Applying the law to gravity; the force you exert on the Earth (pulling the Earth towards your body), and the force the Earth exerts on you (pulling your body to the Earth’s centre), are equal and opposite, keeping you firmly in place on the planet’s surface. Understanding this principle helps us understand how gravity works when we scale things up.

So how does this come into play for the rest of the Solar System? Well, the Earth has a bigger mass than our moon – just as all the other planets have bigger masses than their moons. But the circular orbits the bodies make around each other mean that gravity doesn’t simply cause the bodies to crash into each other. Instead, the force of gravity maintains the circular paths.

Another one of Newton’s laws – the second this time – states that the acceleration of an object is equal to the force acting on it, divided by its mass. For planets and the moons, the force acting on both of them will be the same, but the mass of the planet will always be much larger. This means that the acceleration of the moon is much larger, making the path its’ circular orbit much longer than that of the planet around it.

Fortunately there is one very important celestial body holding this all together (for our Solar System at least) and that is the Sun. The Sun has a by far the biggest gravitational pull which allows it to hold Earth and the other 7 planets (sorry, Pluto) in position. Due to their respective size and distance each of the planets will experience the gravitational pull from the Sun differently.

With so many laws with his name on them, it’s no wonder Isaac Newton was a little uptight. It must have been the apple which probably didn’t actually fall on his head | Image: Eden, Janine and Jim

Newton summed this all up nicely with yet another law: the law of gravitation. The equation calculates the gravitational force between two bodies using their two masses, the distance between them, and a gravitational constant Newton discovered himself. The law is still hugely important to astronomers today.

Hopefully this makes it clear why we shouldn’t say that gravity pulls everything ‘down’! I don’t know exactly what is ‘down’ from our position in the cosmos but I’m not so sure I want to find out. In a way, it would be more correct to say that gravity pulls everything ‘in’.

I’ve mentioned our old friend Sir Isaac Newton a lot now, and we heard about his famous apple at the very start. Legend has it that an apple fell on Newton’s head and just like that, he understood gravity- but did it really happen? Well it certainly seems to be true that a falling apple did inspire Newton’s work on gravity but there was no instant moment of understanding as the story seems to suggest.

The truth is that, whilst sheltering from the wrath of the bubonic plague at his family home in the countryside, a young Newton witnessed an apple fall from a tree. This sight led him to wonder, “Why should that apple always descend perpendicularly to the ground?” However it was not until around 20 years later that Newton first published this principle.

So that’s one myth busted, and another classic example of science, or scientists, being inspired by nature.

See you next time for a futuristic post all about holograms!


F is for Flying

By Siobhan Fairgreaves

Don’t listen to Jon Snow, summer is coming!

Whether you will spend your (hopefully) sunny days looking up at blue skies or jetting off on an adventure, one thing may cross your mind- how do those aeroplanes stay up there!? In this post we will learn a bit more about how aeroplanes get up, stay up and get back down again.

Plane wings: black magic sorcery, or a clever bit of engineering? (This isn’t actually discussed in the article, it’s obviously the second one)

This is one post I’ve been really looking forward to writing- I’ve been well and truly bitten by the travel bug and I can’t think of a better feeling than the rush of the engines as your plane prepares to take off. It’s certainly obvious from the noise of the engines and the speed of the objects flashing past outside that it takes a lot of force to get in the air but what changes between just going forwards really fast and flying?

In order to understand this (and everything else about flying) it’s important to consider another very important F in the physics world- Forces. There are four key forces acting upon an aeroplane.  Horizontally, there is thrust and air resistance. Thrust, generated by those powerful jet engines, propels the plane forwards. Air resistance, or drag, slows the plane down- like when you stick your head out of a car window and the air hits you hard in the face, that force increases the faster you go, and planes go very fast!

Vertically the plane is subject to gravity (more on that next time) and overcoming the weight of the plane which is a downwards force. The final force is lift. This is slightly less obvious but extremely important and keeps the plane in the air.

In order for the plane to change take off it needs to move forwards and upwards (we’re talking passenger planes here- some military aircraft, like the Harrier, are capable of extremely short take offs.

We’ve got moving forwards covered thanks to the powerful jet engines providing enough thrust to overcome the force of drag. Moving upwards requires the cleverly designed wings to come into play. As air rushes over the wings it is thrown down towards the ground creating lift. In order to get off the ground, there must be enough lift created to overcome the weight of the plane due to gravity.

So when thrust is bigger than drag we go forwards and when lift overcomes the weight we go upwards. Very useful for take-off but we don’t want to keep going up forever.

Enough is enough! I’ve had it with these stably-balanced forces on this stably-balanced plane! | Image: Amada44

The wings are shaped and angled to ensure that we stay up, without constantly rising, throughout the flight. The curved and thicker front of the wing is designed to ensure air is at a lower pressure above the wing and a higher pressure underneath. The angle of the wing effectively allows the plane to push down on the air which also helps it stay up- think about pushing down on the water in a swimming pool.

Once the pilot has reached the right altitude they can level out the aircraft and just keep moving forwards. As long as the vertical forces of lift and weight, and horizontal forces of thrust and drag are all balanced, the plane will cruise at a steady altitude at a constant speed.

Getting down again simply (easy for me to say!) requires the forces to be reversed. We now want to go slower so the thrust force can be reduced and drag is allowed to help with the braking. As we also want to get closer to earth we allow the weight of the plane to help. By changing the shape of the wing using flaps, you may have seen these during take-off and landing, the pilot can reduce the strength of the lift force and transfer the weight of the plane onto the wheels which are lowered in time for landing.

Of course, numerous other environmental factors make the business of flying a plane very difficult indeed, but with a little more knowledge of the physics pilots use to help them out you can now sit back, relax and enjoy the journey.

Until next time!

E is for Electricity

By Siobhan Fairgreaves

Last time, in D for Detectors, we looked at some of the applications of physics that you might encounter during your day. In this post, we’re going to find out more about something I can guarantee you use almost every day- electricity!

Far from simply being a piece of tack on your ‘cool’ uncle’s shelf, the plasma lamp is one of the best ways to see electron flow  in action

A reliable source of electricity is something we could easily take for granted, but how does it actually work? Well, surprise surprise, it all comes back to our friend- the humble atom. More specifically, electricity is the flow of electrons – remember them?

Electrons are the tiny negatively charged subatomic particles which whizz around the outside of the nucleus. In some materials, such as copper, the outer electrons break off quite easily and their movement through the material (a copper wire for example) creates an electric current.

Of course, not all materials can conduct electricity. I’m sure you remember the experiment in school where you tried to complete a circuit with different materials- they didn’t all work. Rubber, for example, holds onto its electrons pretty tightly so they can’t easily flow. This means rubber can’t conduct electricity.

Now back to that circuit experiment you may have tried. Your copper wires alone aren’t enough to light the bulb- you also need a source of power, like a battery.  The battery is a source of “pushing power” to move the electrons along. The official name for this is the electromotive force (EMF) but it’s more commonly known as voltage.

Ah, the good old Mexican current… at least that’s what it’s called among scientists who advocate Mexican wave-particle duality | Image: Martin Thomas

One way of thinking about electricity is a bit like a Mexican wave in a stadium. It usually takes a few people acting together to get it started- that’s the battery, and then the wave (or energy) transfers through each person and onto the next. The moving wave is a bit like the moving electrons.

Time for a fun fact now- did you know that the band AC/DC can actually teach us more about electricity? Their name came from seeing a symbol on their sisters’ electric sewing machine. This symbol stood for Alternating Current/Direct Current and meant that the machine could work with either type or electrical current.

“I’m on the hiiiiiiighwaay to… oh yeah, I’m periodically changing direction, so I guess I’m not really going anywhere” – an electron in an AC power line

Direct Current is best explained by the Mexican wave analogy used above. All the electrons move in the same direction. This type of current is used in toys and small gadgets. Larger machines tend to require Alternating Current.

The electrons forming an Alternating Current reverse direction 50 to 60 times a second. It’s a bit difficult to imagine how that could create a current but remember it’s all about transfer of energy. The battery still provides the initial push but these electrons don’t run in a straight line, they run on the spot instead. It doesn’t work in quite the same way but, just like Direct Current, still requires loose electrons and they still need energy.

See you next time for a convenient start of summer post- F for Flying!

D is for Detectors

By Siobhan Fairgreaves

So far in the series, we’ve looked at some big physics concepts, but now, we’re trying something a little different. In this post, we are going to have a look at some of the practical ways we use physics in our daily lives. Specifically, we’re looking at detectors.

Smoke detectors

Air molecules ionized by the radiation given off by americium-251 can detect smoke particles. Unfortunately, they can’t tell the difference between smoke particles caused when your house is burning down, and when you’ve put your crumpet down one too many times in the toaster | Image: Whitepaw

Hopefully, you are pretty comfortable with what a smoke detector is, and why it is useful- but have you ever wondered how they work? Surely that’s not physics!?

Well, surprisingly- yes it is.

Inside your humble smoke alarm is a small amount of americium, a radioactive material. The presence of this radioactive material means the particles of air inside the detector are ionized, allowing an electric current (more on that next week!) to flow across the chamber.

When smoke particles go into the detector some of the ionized particles attach to them. The current drops, setting off the alarm.

There are actually two different types of smoke detectors. Most common is the type that uses an electric current generated by radioactive material but there is another type.

Photoelectric detectors work in a totally different way but are still a pretty good example of how physics can help us out every day. Under normal conditions, in a photoelectric detector light travels from one end of the chamber to another unobstructed. When smoke enters the chamber the light hits the smoke particles, this causes the light to scatter and hit a sensor which then triggers the alarm to sound.

And now you know what is going on when you burn your toast!

Metal detectors

Heheh… those fools will never know about my emergency supply of plastic knitting needles! | Image: Hunter Desportes

I don’t know about you but when somebody says metal detectors I still think of somebody trawling beaches in search of buried treasure. Of course, there are also much more serious applications of this technology. Those beeping things at airports? They are metal detectors too, and they work in more or less the same way.

Metal detectors rely on the fact that an electric current will always generate a magnetic field in a direction perpendicular to its movement. And likewise, a magnetic field across a wire will always induce a current in the wire.

When forming wires into coils of many loops, strong magnetic fields can form through the loop of the coil, simply by firing a current through the coil. Metal detectors can cleverly use these physical laws to their advantage.

In an airport, metal detectors work by firing currents which rapidly alternate in direction through coils of wire – producing a magnetic field which itself alternates in direction. The presence of metal in your pockets, belt or jewellery as you pass through can be enough to disrupt this magnetic field.

Side-view of a magnetic field as it compellingly comes through a current-carrying coil in characteristic curves | Image: Geek3

Due to an important affect in physics known as Lenz’s law, small currents are induced in the metal on your body, which produce their own magnetic fields to resist the field of the detector.

Another coil of wire in the detector is there to measure the overall magnetic field. This time, instead of having an alternating current fired through it directly, a current will be induced by the magnetic field – the strength of the field can be measured by measuring the strength of the induced current.

If the overall magnetic field is any different to that created by the initial coils, the detector will know something is up. An opposing magnetic field was generated as you passed through, which can only mean that there is metal on your body, and the detector sounds the alarm.

Radiation detectors

Geiger and Muller had some trouble persuading everyone that their invention wasn’t just a silly looking hammer, and could actually save thousands of lives | Image: Mrjohncummings

Well, this one might seem a little more abstract but you’d be surprised to realise how common radiation detectors are.

Radiation detectors can be given a number of names, but a Geiger counter or Geiger-Muller tube is a pretty common choice. “Geiger” and “Muller” refers to the scientists Johannes “Hans” Wilhelm Geiger and Walther Muller who discovered an improved method of detecting radiation way back in 1928.

The Geiger-Muller tube detects ionizing radiation such as alpha particles, beta particles, and gamma rays. Once radiation has been detected an electric pulse is produced which shows on the Geiger counter display allowing you to get out sharpish if things start to reach scary levels.

No, it’s never a good idea to play a game of ‘Who can make the Geiger counter beep the most?’| Image: Cygaretka

Sounds interesting but not exactly relevant? Think again.


Radiation detectors can be found all over the UK in schools, hospitals, airports and of course power plants. Any workplace that handles, or could handle, radioactive material must use one of these clever little detectors to ensure that radiation stays at a safe level.


Well, radiation can be pretty dangerous stuff if you’re exposed to it for too long. With a range of effects ultimately all stemming from corrupted cell functions it’s really important to stay safe. You may remember the suspicious poisoning of Alexander Litvinenko? Exposure to radiation can be deadly.

Until next time!

C is for Collider

By Siobhan Fairgreaves

In this post, we will learn a little more about the world’s largest particle accelerator, the Large Hadron Collider (LHC).

Do you remember a few years ago when there was a lot of fuss about a black hole being created? I know there were a few nervous questions fired at our unfortunate science teacher that day.

Proton beams in the LHC can at least enjoy a few thousand trips through the scenic Alpine countryside before being annihilated | Image: CERN

The reason for all that fuss is situated 175 metres underground, on the border between France and Switzerland. The LHC is a circular tunnel with a diameter of 27 kilometres, where scientists and engineers are working to solve some of the big unanswered questions in physics.

But how does this machine work, and how is it going to help?

Well, as the name suggests, colliding is a pretty big part of the whole idea. This machine usually uses protons (the positive subatomic particle) but can sometimes use the whole nucleus of the material lead. Remember what they are? If not, we’ve already covered a bit about what protons are.

Naturally, it’s not as easy as just throwing them at each other and observing what happens. The particles are so small that the chance of getting a successful collision is described by the creators themselves as about as likely as “firing two needles 10 kilometres apart with such precision that they meet halfway.”

Hmmmm, not easy stuff then.

Rumour has it that to make sure the LHC’s superconducting electromagnets were cold enough, CERN hired a group of Canadians to touch them, and were only satisfied when they admitted “they’re pretty chilly, eh?” On a side note, you shouldn’t believe everything you read on the internet | Image: MaGIc2laNTern

To add to the complications, the particles must be going at almost the speed of light to have enough impact when (or if) they do collide for anything to happen. The Large Hadron Collider is designed to help speed up the particles using thousands of seriously strong magnets. These magnets actually have a pretty impressive name, officially they are superconducting electromagnets. There’s your dinner party lingo for the day!

With all that whizzing around and accelerating you’d imagine things are getting pretty hot down there, right? Well, no. Another complication arises. In order for the electromagnets to work they must be kept at -271.3 °C. That’s colder than outer space! In order to achieve this, a complicated cooling system is in place which uses liquid helium to keep things chilly.

I’m beginning to understand why this project is such a big deal.

But what is it all for?

Collision data for the Higgs event. When your raw data looks this cool, it’s not too hard to persuade anyone that your research is justified | Image: Lucas Taylor/ CERN

Well, sometimes science for science’s sake is a good enough reason to conduct an experiment. However when setting up that experiment cost an estimated ÂŁ6.2 billion and involves over 10,000 scientists and engineers in an international collaboration you need a slightly better excuse.

The team at the European Organisation for Nuclear Research (CERN) have certainly got more than one decent reason for this mammoth undertaking. They hope to answer some fundamental questions about the structure of space and time, to better understand forces which are part of our lives every day and even to discover brand new particles. In July 2012 the team announced the discovery of the Higgs boson, a particle which will now be studied intensively to help answer some of these big questions.

The Large Hadron Collider is at the forefront of some of the most profound scientific discoveries of our time and we should certainly stay tuned for more exciting discoveries. If you’re interested in finding out more visit the CERN website which even includes a virtual tour of the tunnel itself.

Until next time!

B is for Black Holes

By Siobhan Fairgreaves

In our last post we looked at atoms and their subatomic particles. That’s the tiny end of physics, this time we look at something at the other end of the scale- black holes.

Cheesy sci-fi films have been trying to warn us for decades… but we never listen! As it turns out, that’s probably for the best | Image: Martin

Ooooo, now this is the good stuff, right? The fuel of sci-fi movies and something that you think you should be mildly concerned about. Remember when scientists were apparently going to make another one in Switzerland? That was a bit of an anti-climax.

I was at school when the Large Hadron Collider in Switzerland was switched on and I remember a lot of panicky talk about black holes and us all being sucked into space. To find out more about what on earth a Large Hadron Collider is you will have to read the next post- C is for Collider, cheeky I know.

For now, though, what is a black hole?

I know you’re hiding in there somewhere – astronomers can only find black holes by mapping out the paths of stars orbiting around them | Image: ESO

Well let’s start off with a quote from the brilliant Stephen Hawking, “It is said that fact is sometimes stranger than fiction, and nowhere is this more true than in the case of black holes.”

He certainly got that right. Black holes are pretty mysterious and teams of scientists are still figuring out what exactly goes on up there. This is made all the more difficult by the fact it’s not even possible to see black holes, only the effect they have on other objects. In a nutshell, though, black holes are an area of space where gravity has become so strong that nothing can get out- not even light.

Born from the death of a star: the Orion Nebula, made up of the explosive remnants of a star which could once be seen just under Orion’s belt, is thought to have a black hole at its centre| Image: Ljubinko Jovanovic

Because this gravity is so strong it’s easy to go along with the assumption that one day we will be sucked into one and disappear. You’ll be pleased to hear that NASA thinks this is very unlikely- there simply isn’t one close enough for us to worry about.

For a long time, I imagined black holes as a sort of Pacman travelling around in space munching on stars, planets and everything else in their way but this is not the case. Black holes are actually caused by a star collapsing and as the star collapses in on itself the gravity gets so strong that a black hole is created.

A horrific yet tasty way to go: the difference in gravity between an astronaut’s head and their feet as they fall into a black hole would lead to the process of ‘spaghettification’ | Image: Cosmocurio

That’s all well and good but there are billions of stars out there, does this mean that every star will one day make a black hole? And, hang on a minute, our Sun is a star- are we orbiting a wannabe black hole? You’ll be pleased to hear that our Sun simply isn’t big enough to become a black hole- it takes a pretty big star to cause such a powerful pull of gravity when it collapses.

Instead of Pacman, you could think of a black hole like water going down a drain. When the bath is full, the water rushes down the drain and sucks everything with it but when there is only a dribble left it isn’t even strong enough to take down any leftover bubbles.

So yes, black holes are crazy giant plugholes travelling through space which, as the European Space Agency says, would see an astronaut “pulled apart by the overpowering gravity” if they were to get too close- but should we be worried about them right now? No.

Until next time!

This probably won’t ever happen to you, but if you do ever find yourself in the devastating clutches of an inescapable gravity source, it might be nice to know what to expect

A is for Atom

By Siobhan Fairgreaves

Never trust an atom- they make up everything!

Scanning Tunnelling Microscopy can give us incredible images like this one of a piece of graphite, showing how its atoms fit together like Lego. Luckily, treading on atoms isn’t like treading on actual Lego, or walking would be incredibly painful | Image: Frank Trixler

Terrible jokes are finished, for now…


In this post, we will look at the basic structure of the atom. But first, what are atoms?

Atoms are the building blocks of the world- think of Lego. If the whole world was made of Lego an atom is that tiny single square block. Imagine how many of those tiny blocks would be needed to build the whole world and all the people, animals and stuff inside it… That’s a lot of Lego, and there are a lot of atoms. A single grain of sand contains millions of these tiny particles.

For a long time atoms were thought to be the smallest piece of the puzzle. Then in 1897 a scientist named J.J. Thomson identified an even smaller particle which helps to make an atom.

Cheer up J.J., you’ve revolutionised modern physics! | Image: Benjamin Crowell

Thomson made the discovery when he was experimenting with mysterious beams of particles called cathode rays. When firing cathode rays at hydrogen atoms, he measured how the path of the beams changed as they interacted with the atoms. Thomson realised that the cathode rays were made of tiny, negatively charged particles – around 1/2000th the size of a hydrogen atom. He named the particles ‘corpuscles’, but we know them today as electrons. But Thomson’s discovery doesn’t tell the whole story about what we came to know about the atomic structure.

In fact, we have another scientist to thank for that. In 1909, New Zealander physicist Ernest Rutherford fired some positively charged radioactive particles through a sheet of gold atoms, and measured the different paths they took. He was testing out J.J. Thomson’s ‘plum pudding’ model, which proposed that atoms were made up of electrons sitting happily inside a positive sphere, holding them together.

Thanks to Ernest Rutherford, scientists can now study atoms without constantly thinking about delicious desserts | Image: Library of Congress

Rutherford noticed that most of the particles passed straight through the atoms, but a tiny proportion were deflected back. That meant that instead of being plum puddings, each atom was made up of a small positive nucleus, surrounded by orbiting electrons, with lots of empty space between them. And so, the basic model for  an atom was born!

A typical illustration of an atom will show a ball in the middle surrounded by orbits- but what is going on in there? The ball in the middle is the nucleus which Rutherford discovered, and inside the nucleus are protons and neutrons. The things whizzing round the outside are the electrons. Protons, neutrons and electrons are known as sub-atomic particles, now that’s an impressive dinner party phrase.

Electrons in different energy levels form a cloud of negative charge around the nucleus| Image: Mets501

It might look like electrons are in a messy, complicated cloud but they are actually very precisely arranged. Around each nucleus are different shells, or energy levels, which have space for a different number of electrons.


The very first energy level around the nucleus can only hold 2 electrons. In an atom of the element Helium both of the spaces in the first energy level are filled by an electron. So, using Helium as an example, what else is in there? To work that out you should know that it is really important for an atom to be balanced. Each electron carries a negative charge so to balance Helium we now need two positive charges. Fortunately, protons have a positive charge each. So we’ve got two negative electrons whizzing around the outside, two positive protons snug in the nucleus, the charges are balanced. So, are we finished?

It might seem fun to try and chop a uranium nucleus in half, but it’s not actually a very good idea… | Image: Scienities

Almost, don’t forget the third component, neutrons. Luckily, they are- you guessed it- neutral, so it’s okay for the number of them to vary between different forms of the same element.  Most of the time, Helium has two neutrons and with two of everything it is nicely balanced and known as stable.

Helium is a nice example with small numbers but not all elements are quite so compact. Take Uranium, for example, there are a lot more protons (92!) and electrons involved to try and get Uranium to balance.

That’s a very basic introduction to atoms- and for sticking with it, you’ve earned yourself another terrible joke! What a treat…

A neutron walked into a bar and asked for a drink.

“How much?” asked the neutron.

The bartender replied, “For you, no charge!”

Hopefully, that joke will make a bit more sense now you know your stuff about subatomic particles and their charges.

Until next time!

Z is for Zeno

By Jonathan Farrow from the Thoughtful Pharaoh

It’s early in the morning.  The caffeine from your morning cup of coffee has yet to fully kick in, but as you turn the corner, you see your bus.  It’s just pulling in to the stop and is only 50m away.  You know you can make it, so you break into a sprint.

It takes you 3.5s to travel 25m and get halfway to the bus.  In that time, an old lady has gotten off.  You’re halfway there and there’s still a few people who need to get off.  You’ll definitely make it.

In only 1.75s you’re already halfway to the bus again (12.5m). There’s only one person left to get off.

Another 0.875s and you’ve travelled the 6.25m that gets you halfway to the bus again.  There is nobody left to disembark.

In less than half a second, you’re halfway again, just over 3m from the bus.  The driver must see you.  He’ll wait, right?

In less time than it takes you to blink (0.22s), you’re 1.5m away, almost close enough to touch the bus.  So close, and yet, somehow, you’re not quite there yet.

In order to catch the bus, you need to get halfway to the bus first.  Getting to the halfway point, no matter how short a journey, will take you some finite amount of time.  Unfortunately, there are an infinite number of halfway points between you and the bus.  According to a grumpy Greek philosopher from the 5th century BCE named Zeno of Elea, you will never get to the bus.  In fact, he argued that all motion is impossible.  It is merely an illusion.  This paradox, also called the Dichotomy, is one of four paradoxes that Zeno used to demonstrate this idea and it has been notoriously hard to refute.

One attempt at refutation was made early on by Diogenes the cynic, who was said to have silently stood up and walked across the room. [Incidentally, Diogenes was a hilariously stubborn man who was prone to philosophical stunts like intentionally distracting Plato’s students by obnoxiously eating food in lectures; walking around the market in daylight with a lamp in search of an “honest man”; and sleeping in a big ceramic jar in the market to prove that wealth was a corrupting influence.]  While this does contradict Zeno’s conclusion that motion is impossible, it doesn’t address the argument itself.  Zeno’s response would simply be that Diogenes crossing the room, just like you trying to catch your bus, is your senses tricking you into seeing motion where there was none.

Aristotle tried to refute the Dichotomy by distinguishing “actual” from “perceptual” infinities.  The 50m line between you and the bus at the start of the scenario can be divided into an infinity of half-runs (therefore it is a perceptual infinity), but that is a geometrically different phenomenon than the single, undivided 50m line (the actual infinity).  Aristotle conceded that Zeno found something that is impossible (running infinite half-runs), but maintained that this was not what actually happens when somebody moves (running a single finite line).

This doesn’t seem satisfactory to me.  Aristotle’s distinction is an artificial one and misses the point that Zeno was trying to make.  The world would need to wait for the 19th and 20th centuries for mathematicians to start talking about infinite series and to resolve Zeno’s Dichotomy paradox.

In mathematics, a series is what you get when you add up all of the numbers in a given sequence.  Consider the sequence of numbers 1, 2, 3, 4….  The pattern here is that you add one to the previous number.  The first three terms add to 6, the first four add to 10.  Every number you count up adds to the total and as long as you keep going, the total sum will get higher and higher.  This is an example of a divergent series because there is no number that the series settles on.

Now consider the sequence of numbers 25, 12.5, 6.125, 3.0625…  The pattern here is that each number is half of the previous one.  Unlike the sequence above, if you continue the sequence, the numbers get smaller and smaller.  You will get closer and closer to 50 until you run out of space to put the 9s after 49.99999…  For all intents and purposes, you will have reached 50.  This solves the problem practically and is analogous to the way that we understand derivatives and integrals.  Understanding how and when infinite numbers of parts can add up to finite (and known) quantities has been incredibly helpful for us.  It’s the principle behind the dampening of oscillations in springs and sound waves, it lets engineers understand how wind will affect their bridges, and it lets Usain Bolt get to the finish line.

Somehow, though, this resolution still leaves me dissatisfied.  It’s just a more useful and mathematically rigorous version of Diogenes’ walk across the room.  In some ways, Zeno’s Dichotomy paradox still haunts modern mathematics.  Kevin Brown (possibly a pseudonym for a mysterious math/physics writer), in his 2015 book “Reflections of Relativity” writes somewhat ironically of the paradox’s resolution, “it’s probably foolhardy to think we’ve reached the end. It may be that Zeno’s arguments on motion, because of their simplicity and universality, will always serve as a kind of “Rorschach image” onto which people can project their most fundamental phenomenological concerns…”

And with that, we’ve reached the end of the ABCs of interesting things.  Thanks for joining me on this wonderful journey.  That being said, it’s probably foolhardy to think we’ve reached the end of the Thoughtful Pharaoh.

[Featured image: Grandjean, Martin (2014); License: http://creativecommons.org/licenses/by-sa/4.0/]

Y is for You!

It’s been 25 weeks since we started this epic journey through the alphabet together, and sadly we are nearing the end.  At this critical juncture, just one letter away from the finality of zed, I thought I would bestow my Pharaoh powers on to you, dear readers.

Comment below with your burning science questions, and I will answer them all next week in my final ABCs of Interesting Things post.

Thank you for reading.  I leave this quest in your very capable hands.

I want YOU to ask me questions

For those still aching for some interesting science facts, how about these “you” facts:

There are more bacterial cells in and around you than human cells.

All of the atoms in your body were made inside stars, as the great Carl Sagan said the 1980 TV Series Cosmos: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies, were made in the interiors of collapsing stars.  We are made of starstuff.”