Unexplainable astronomy? Part 2: Of pigeons and cosmic uniformity

By Sam Jarman

Ask just about any scientist, and they will tell you that the very best discoveries they can make are the ones which force us to change how we think about the world completely. That’s exactly what happened in 1923, when Edwin Hubble looked out at a small, hazy nebula from his mountaintop telescope in California.

Without Henrietta Leavitt’s work, we might still think of galaxies like Andromeda as funny-looking nebulae which don’t make any sense | Image: NASA

Based on previous research by Henrietta Leavitt, Hubble was looking for Cepheid variable stars in the nebula, which would tell him exactly how far away it was. He managed to find a variable star easily enough, but it revealed a pretty big problem. The star showed Hubble that the nebula he was looking at was not even a nebula after all, but a vast cluster of stars situated very, very far away.

The distance was greater than any we had thought possible for thousands of years. For scientists, that meant that we needed to completely rethink our model for the structure of the universe. With the discovery of the Andromeda galaxy, the fundamental idea of billions of galaxies occupying a vast, ever-expanding universe was born. It was a lot for the world of astronomy to take in.

In the decades following Hubble’s discovery, theoretical physicists struggled to discern the scientific rules which could possibly govern such a newly colossal, mysterious universe. Some of their theories seemed promising, but In the meantime, the imaginations of the media and the public ran wild. If our understanding of the universe could be so shaken by one discovery like Hubble’s, they reasoned, what other revelations could be hiding in plain sight? And, of course, the biggest question: could intelligent life be out there somewhere?

Apprehension had grown strong by the 60s. Theoretical physicists had some convincing ideas about the nature of the universe, but at the time, technology just wasn’t sophisticated enough to prove experimentally which were correct, and which were misguided. Their ideas so often disagreed with each other that it seemed like no-one had really got anywhere at all. But change was coming. Improved telescopes meant that by 1964, mysterious signals, the likes of which would make or break previously speculative cosmological theories, began to come in.

A dodgy radio receiver? 

Pigeons. It had to be. Since making their home in the cosy hovel that was the Holmdel Horn Antenna, the birds had been busy and painting the walls with the remains of their dinner, which was giving off who knows what kind of radiation. How else could radio astronomers Arno Penzias and Robert Woodrow Wilson explain the uniform, unwanted microwave signal, which wouldn’t go away no matter where in the sky they pointed the telescope?

Whenever you’re feeling down, just remember that a sad metal toilet in New Jersey once became one of the most important telescopes of the 20th century| Image: NASA

The pair cleared out the avian intruders, and scrubbed the inner surface of the antenna clean. They checked the equipment thoroughly yet again, making sure the receiver was still cooled to just above absolute zero, minimising any interference from within the telescope. But still, the low, intense noise hindering their experiment persisted. Clearly, the radiation was originating from something more interesting than pigeon droppings. Something, they realised, beyond our own galaxy.

Coinciding with Penzias and Wilson’s predicament, a group of astrophysicists at Princeton University were working on a theory they believed could explain what was happening in the earliest moments of the universe’s existence. Based on earlier work by cosmologist Ralph Alpher, their idea hinged on the concept that the universe had all started with a rapid, high-energy expansion from a single point: a Big Bang. They proposed that if the Big Bang had really happened, it must have released a colossal surge of radiation, still observable to this day with the right equipment. The radiation would have cooled significantly by now, but if its remnants were detected, it would be definitive proof that the scientists’ notion was correct.

The idea was in direct conflict with the earlier-proposed Steady State theory, which suggested that the universe had no beginning, meaning no Big Bang, at all. Instead, the universe had existed for an eternity beforehand, and had never changed significantly. The two theories were radically different, and both appeared to have equal weight. But without any hard evidence, neither could be validated over the other. That all changed in 1964, when Penzias and Wilson caught word of the Princeton scientists’ research.

The Colossal Clang and Tremendous Toot were also considered, but never really caught on| Image: NASA/WMAP Science Team

Through a friend, Penzias got access to a preview of the Princeton paper. What he read astounded him: using first principles, the astrophysicists had argued that a surge of electromagnetic radiation had permeated the entire universe following the Big Bang. As the universe expanded, so too did the wavelength of the radiation, until reaching its current wavelength of 7.35 cm. That was the very same radiation Penzias and Wilson had attributed to pigeon poop – almost exactly. The pair got in touch with the Princeton scientists as soon as they could.

The rest of the story is history. The two groups teamed up to publish not only a theory of how the Big Bang could have hypothetically played out, but indisputable experimental proof of the Big Bang itself on top of it. The Steady State theory had been disproved, and was consigned to history. In 1975, Penzias and Wilson earned a Nobel Prize for their ground-breaking discovery.

Decades later, the radiation observed for the first time by Penzias and Wilson – now known as Cosmic Background Radiation – has been studied in meticulous detail. It has been found to be incredibly uniform, but not perfectly so. In the very earliest moments of the universe, some regions emerged where matter was densely clumped together, whereas it was sparser in others. As the regions formed, the surge of radiation created by the Big Bang was slightly distorted by the clumps of matter.

Of all the scientific results which were first thought to be literally crap, the Cosmic Microwave Background has got to be the most impressive | Image: NASA

Billions of years later, the denser regions have become areas abundant in galaxies, while the sparser regions are now vast, empty voids in space. The Cosmic Microwave Background has retained the distortion it underwent at the very start. By picking up the subtle variations in the radiation, we can now create maps of the structure of the universe with detail which must have been unthinkable just a few decades ago.

So from a mysterious error first attributed to pigeons with poor housekeeping skills, came another fundamental theory in cosmology. But the 60s weren’t over, and there were yet to be more discoveries which would prove to shake up our understanding of how the universe works. In 1967 came another bizarre signal, this time with a somewhat more exciting misguided idea about where it came from.

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!


Unexplainable Astronomy? Part 1: The Wow! Signal

By Sam Jarman

The scribble that started it all | Image: Big Ear Radio Observatory, NAAPO

Monday was dragging for Jerry R. Ehman, in a way that only a volunteer astronomer at the SETI project could truly understand. The frequency data printed out by the IBM 1130 computer was, as always, infuriatingly ordinary. The stream of paper he needed to analyse by hand was seemingly endless. Coffee in a Styrofoam cup was slowly growing lukewarm. Then… something different.


On August 15 1977, Ohio State University’s Big Ear radio telescope picked up an intense and unexpected burst of radio waves, which lasted for a full 72 seconds. Stunned by the break in the monotony of his task, Ehman couldn’t help but briefly forget his deeply engrained skills in data analysis. Taking a bright red marker, he drew a circle around the numbers and letters indicating the burst, and wrote one word next to it: “Wow!

Ehman wasted no time in letting his fellow members of SETI know about his discovery. The implications of the burst were not lost on any of them. The SETI volunteers knew there and then that the burst could not be explained by any current science. On each of their minds was the possibility that the purpose of their project had at long last been fulfilled: evidence of intelligent life beyond the Solar System.

 It doesn’t sound much like ‘We come in peace’, or ‘As you no doubt will be aware, the plans for the development of the outlying regions of the western spiral arm of the galaxy require the building of a hyperspace express route through your star system and, regrettably, your planet is one of those scheduled for demolition’, but the Wow! Signal could be the closest thing we have to a message from aliens

Hey, I know that frequency!

To understand why Ehman and his colleagues were so excited by the burst, we need to look at the characteristics of the particular frequency their telescope was picking up. The Big Ear radio telescope had been programmed to detect signals with the very specific frequency of 1420.406 Megahertz. That number might not roll off the tongue, but it’s incredibly important in astrophysics, and even has its own name: the Hydrogen Line frequency.

The red colours shows where hydrogen gas has been detected within the Milky Way. As it turns out, there’s a lot of it | Image: NASA

Hydrogen gas is extremely common in the wide expanses of space which lie between the stars. Atoms of the gas, consisting of an electron orbiting a single proton, are normally very stable and pretty uninteresting. But occasionally, the electron will flip over spontaneously due to quantum processes – a phenomenon known as spin flip transition, making the system unstable. To return to normal, the electron needs to flip back around, releasing a flash of light with a specific energy – a radio photon – in the process.

This process is incredibly rare for individual hydrogen atoms, but when hydrogen gas is gathered in clouds which span many light years, enough radio photons are given off by spontaneously flipping electrons that they can be easily picked up by radio telescopes on Earth. These photons make up the radio signal of the Hydrogen Line, which is important both to radio astronomers and to those hopeful that other technologically advanced civilisations could be out there somewhere.

The Hydrogen Line frequency is useful to astrophysicists, as they can use it to detect the exact locations of clouds hydrogen gas within our galaxy, and in those beyond. The clouds will always be densest in galactic arms, allowing radio astronomers to map out the distinctive structures of galaxies. They can also use the Doppler Effect to measure how fast hydrogen gas at various distances from the centre of galaxies is moving. From this, they can then create rotation curves for different galaxies, which currently give the clearest evidence we have for the existence of Dark Matter (but that’s a whole other story!)

So when astronomers point their radio telescopes at hydrogen gas clouds, it’s hardly surprising for them to observe radio waves at the Hydrogen Line frequency. But what if we observe them in places in the sky where we aren’t expecting them, or at higher-than-expected intensities? Any astronomer will agree that if this happens, something unexplained is going on.

“Huh, these aliens know about hydrogen spin-flip transition and have an in-depth understanding of their place in the galaxy… so why haven’t they invented clothes yet?” | Image: NASA

Astronomers at SETI became interested in the Hydrogen Line frequency because it is such a fundamental figure in astronomy. Clouds of hydrogen gas are so abundant in the galaxy that their signal can be picked up no matter where you go in space. SETI figured that if there are any other enlightened civilisations elsewhere in the universe, then they must realise this too. No matter how different their scientific units are to ours, if we asked them to create a radio signal at the Hydrogen Line frequency, it would be exactly the same as the signal we would create. So what better way to announce your presence to the galaxy than to send out a distinct, high-intensity transmission of the frequency?


SETI has already sent signals like this out into space, in the hope that others out there may be listening. In fact, they have a strict control over the transmission of the Hydrogen Line signal; it is now illegal to get anyone’s hopes up by transmitting the frequency yourself. But that isn’t the only method we have used to broadcast our knowledge of the Hydrogen Line. In 1972, under the efforts of Carl Sagan, Frank Drake and other journalists and astrophysicists, the famous Pioneer plaque was attached to probes Pioneer 10 and Pioneer 11.

In the top-left of the plaque is etched a diagram representing the distinctive electron flipping process which causes the Hydrogen Line. Between two representations of hydrogen atoms with electrons in the two different states is a horizontal line 21.106 cm long – the exact wavelength of a radio wave at the Hydrogen Line frequency of 1420.406 MHz. If an intelligent civilization ever finds one of the probes then it’s a safe bet that they will understand exactly what the diagram is representing, no matter their language.

 An explanation, or an alien-killing pretender?

Not long after Ehman’s bizarre discovery in 1977, the newly dubbed Wow! Signal, named after Ehman’s hasty scrawl in red pen, had taken the worlds of both science and the media by storm. While astrophysicists rushed to discover the astronomical source of the signal, journalists began to enthusiastically speculate that without any existing scientific explanation, it could have been more purposefully created in origin. For hopefuls of the existence of extra-terrestrial intelligence, the evidence was now more tantalising than ever.

Sorry it was me all along… or was it? | Image: NASA, ESA, J.-Y. Li

Yet for all the attention the Wow! Signal gained, the search for its origin proved fruitless for scientists and alien hunters alike. For over 40 years, the result sat there unexplained; frustrating some, and instilling hope in others. But in April 2017, astronomer Antonio Paris from St Petersburg College, Florida claimed in a paper to have solved the mystery once and for all.

Paris argued that on August 15, 1977, two comets inside the Solar System – 266P/Christensen and 355P/Gibbs – passed directly in front of the Big Ear radio telescope. Surrounding one of the comets was a cloud of hydrogen gas, which was given off by one of the masses of ice and rock. Naturally, the telescope picked up the Hydrogen Line signal of the cloud, but only as it passed through Big Ear’s field of view. For the first time, it looked like the mystery had been solved. But not everyone was satisfied with the new explanation.

Within weeks, Paris was receiving backlash from scientists, who had some strong criticisms of his paper. In June, Robert S. Dixon, director of the SETI project himself, published a rebuttal to Paris’ paper, claiming that the two comets weren’t in fact within Big Ear’s field of view on the day of the Wow! Signal. Other criticisms included claims that the comets were a long way from the Sun and therefore inactive, meaning neither of the comets could possibly maintain a hydrogen cloud around themselves. Some scientists even had many harsh words to say about Paris’ scientific methods in general. Paris stands by his theory, but he’s open to debate.

So for now, many astronomers still regard the Wow! Signal as a mystery, and hope remains that the true explanation of the burst could be another advanced civilization broadcasting its existence. But this isn’t the only case where strange signals in astronomy have gone unanswered for long periods, or where far-fetched and alluring explanation theories have been thought up.

Over the last century, the world of astronomy has been become famous for detecting mysterious signals, making bizarre discoveries, and throwing up seemingly unanswerable questions. Some of these questions have had mundane explanations. Others have led to new research which has come to revolutionize our understanding of the universe. And, like the Wow! Signal, still others remind us just how much we have yet to learn.

In this series, we will find out more about the signals which have both answered and created some of the most enticing questions in modern astronomy. Next time, we will turn back the clock to before 1977, when astrophysics was reeling at the revelation that our place in the universe was far less significant than we realized.

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!

Throat singing: a storm of sound on the steppes

By Sam Jarman

The day dawns cold, clear, and still. A nomadic Mongolian herder knows that sound will carry well in these conditions, so he takes his chance. Climbing onto his horse, he rides into the wilderness, intent on finding a place where he can honour one of his people’s most sacred traditions. Using only his voice, he will imitate the song of birds, the hum of wind streaming over a rugged mountainside, and the hooves of wild galloping horses.

Finding the right spot is crucial. The herder needs to probe the nearby river valleys and mountaintops, to discover the place where his song will sound the most clearly.  If he chooses well, his voice could be heard for miles across the surrounding plains. He has learnt from his ancestors to use the Mongolian steppe as a sound studio; its open landscape providing the perfect acoustics to carry his voice. When he finds the right place, he begins his song.

His mesmerising chant often doesn’t sound like anything a human could create with their voice. The song could vary in pitch between being hauntingly deep, and as high as a flute. But astonishingly, he can sing both of these pitches, and many tones in between, at the same time.

For generations, people in Mongolia and surrounding areas in Siberia and northern China have learnt to use their voices to mimic the sounds of nature with a hypnotic accuracy. Even in the face of modernisation, the tradition remains as strong as ever today. It’s hardly surprising that the extraordinary talent of throat singing has become one of the most famous and iconic traditions in Mongolian culture.

Many Mongolians learn to throat sing from a very young age, but the techniques they use to produce such a bewildering array of sounds are anything but simple. To truly understand how they do it, we need to explore the anatomy of our own voices, and the physics of the acoustic waves we create with our vocal apparatus. Throat singers have learnt to intricately manipulate these acoustic waves to produce some astonishing sounds.

How it’s made: the human voice

Like any breath, or a normal speaking or singing voice, the song of a throat singer begins life as the lungs contract, forcing out a column of oxygenated air. The air travels upwards through the tube connecting our lungs to our throats, known as the trachea. But to distinguish the breath from a noiseless exhale, it first needs to pass through the larynx, sitting just above the trachea. The larynx contains a set of two vocal chords, which contract to a thin slit when we decide we have something to say, or rather sing.

Looking top-down: this little slit is responsible for every word you’ve ever said, even the embarrassing ones | Image: Henry Vandyke Carter

As air passes through the contracted vocal chords, they are forced to vibrate. These vibrations chop the air column up, forcing it to vibrate itself. At regular intervals, the air will be blocked entirely, and then be let through in a sudden rush. So, the air coming through the vocal chords vibrates in a pattern we are probably very familiar with: a sound wave.

The pitch, or frequency of these sound waves is ultimately decided by the size of our vocal chords. The tension in the chords, and the size of the gap between them, determine how much the sound waves they produce will vibrate over time. This gives them an important property, called the fundamental frequency – a value which entirely defines the unique pitch of each of our voices.

The fundamental frequency is heavily dependent on gender. Women’s vocal chords are usually smaller, meaning the air column in the trachea vibrates more over time, giving the sound waves a higher frequency. For men, larger chords will cause the air column to vibrate at a lower frequency. But the fact that no two voices sound the same can’t be explained by variations in pitch alone.

A sound wave in it’s purest form – you can see how air is compressed and spaced out at regular intervals. This type of wave is easy to understand, but lacks emotional depth… | Image: Pluke

On top of a fundamental primary wave in our voices, there are many smaller, higher-frequency waves also created by the vocal chords. Called overtones, each of these waves has a distinctive frequency; the first will have twice the frequency of the fundamental wave, the second will have three times the frequency, and so on. When stacked on top of each other, all of these waves create one intricate and complicated sound wave. This is the sound of our voices, as unique to each of us as our fingerprints.


…The sound waves of our speech are much more complicated

Already, these sound waves can accurately express our emotions. When listening to other people, our minds can tell if their voice indicates happiness, sadness, anger, or surprise, simply by picking up subtle patterns in its loudness and variations in frequency. But to turn the sound waves into a coherent language, the air finally passes through our mouths. The continuous movements of our throats, tongues, and lips sculpt sound waves into the words we use to communicate with each other (here’s a pretty hilarious demonstration of how it works).

We use our vocal apparatus so often that we don’t need to consciously think about how we need to manipulate it to produce the particular sounds we want. But for Mongolian throat singers, the anatomy we normally use to talk and sing has untapped potential. The secret of their captivating sound lies in an important phenomenon in acoustic physics, known as resonance.

Resonance: a perfect storm of sound

Every object in nature has a natural frequency it vibrates at. Normally, any vibrations the object interacts with won’t have the same frequency as its natural frequency. But if the two frequencies match up (with a few physical constraints), the object’s vibrations can become much larger. One of the most famous examples of resonance is the bizarre collapse of the Tacoma Narrows bridge in 1940:

The reasons why the Tacoma Narrows bridge acted so strangely before collapsing have been strongly debated, and still aren’t fully understood. But the most agreed-upon theory is that patterns in the strong winds blowing across the bridge that day had just the right frequency to induce resonance in the bridge, causing a huge standing wave to form in the it’s road. Eventually, the bridge’s suspension cables gave out, but not before giving an outlandish display of the properties of physics.

In the case of our vocal apparatus, the natural frequency is determined by the sizes of the gaps which the sound waves of our voices need to pass through. If the wavelength of the sound is equal to the size of the gap, the gap itself will be forced to vibrate, creating its own waves with the same frequency as the original sound. All of the waves add together in a ‘perfect storm’ to amplify the sound, making it much louder than it was originally – a resonant sound wave is formed.

Throat singers have learnt to manipulate parts of their vocal anatomy to produce resonant sound waves artificially. Different types of throat singing can cause resonance in different parts of the singer’s vocal apparatus. This creates a wide array of different types of throat singing, which each have their own name in the Mongolian language. The different styles are too numerous to cover in one article, but a smaller number of fundamental styles give rise to many of them.


In this style of throat singing, one or more of the smaller, higher-frequency overtone waves are caused to resonate as they pass through small, specifically-sized gaps in the singer’s throat and mouth. These waves are normally subtly engrained into our voices as individual overtones, but here, they can be heard as distinct, mesmerizingly clear sounds.

Astonishingly, the sound can be heard over the singer’s primary singing tone, meaning more than one note can be sung at a time by a single person. By the nature of our natural vocal sound waves, if we isolated all of the overtones which make up our voices, they would all be in harmony with one another. That means that the singer’s overtones create a one-person symphony without any further effort.

Typically in Khoomei, the singer will chant one continuous primary note, and then vary the overtones which are resonated. This creates a strong base sound, with a higher -pitched tune being sung above it. Mongolians believe the multiple tones of Khoomei give the impression of wind as it swirls around rocks and boulders, creating an enchanting natural chorus.

Sound: Alash Ensemble


The highest of our natural overtones have far higher frequencies than anything a human could create with their natural voice. In this style, singers create a tiny gap between their tongues and teeth, to match the wavelengths of their highest-frequency overtones. The sound is shrill and piercing – as close as the human voice can come to sounding like a flute.

Like Khoomei, this style involves a steady fundamental tone being sung, with the resonant overtone above it. Yet in Sygt, the high-pitched overtone dominates the song; the gap which sound waves need to pass through is so small that the primary wave is greatly diminished. To Mongolians, Sygt is intended to mimic the sound of birdsong, and the warm, gentle summer breezes over the steppes.

Sound: Alash Ensemble


Situated right above the vocal chords are two folds of membrane which look similar to the vocal chords, but normally serve an entirely different purpose. The ventricular folds are there to prevent food and drink from entering our airways, but throat singers have learnt to manipulate them to produce one of their most iconic sounds of all.

By manually contracting their larynx to exactly the right shape, throat singers can bring their ventricular folds and their vocal chords together. So when air passes through the vocal chords, the ventricular folds will resonate themselves, producing their own sound. However, this resonance is unlike the effects seen in other types of throat singing; here, the ventricular folds vibrate at exactly half the fundamental frequency, creating an artificial undertone.

In musical terms, halving the frequency of a sound will bring it down an entire octave. This has astonishing implications for throat singers. Kargyraa singers can reach a wide range of notes far deeper than anything they could sing using their vocal chords alone. The effect is a haunting, low-pitched sound, reminiscent of rolling thunder, or the mournful cries of a camel after losing her calf.

Sound: Alash Ensemble

Kargyraa in particular can partly explain why throat singing still remains so popular in Mongolia today, and is even facing a resurgence. In the past, social taboos meant that women weren’t allowed to practice the tradition, but now, these barriers are breaking down. Female vocal chords may be smaller, but women can still contract their ventricular folds to create a fantastically deep sound:

Now that men and women have equal opportunities to practice the art, throat singing is being taught to boys and girls across Mongolia and the surrounding regions; all eager to pass on the traditions of their ancestors. The iconic sound has made its way into concert halls and recording studios, and in the West, throat singing has gone from a mysterious, alien practice, to one which is beginning to influence our own culture.

Anatomically, there is no special adaptation among the Mongolian people that enables them to throat sing better than anyone else. It’s no easy talent to learn, but we are all capable of recreating the effects of resonance on our voices, if we practice for long enough.

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 £6.2 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