Inertia

Let's go back to thinking about masses and springs. Imagine a mass held between two springs as shown in the animation below. If the mass is moved away from the equilibrium position the restoring forces provided by the springs will make the mass oscillate back and forth in a similar manner to the playground swing example.

What happens if we change the size of the mass?

An increase in mass increases the inertia (reluctance to change velocity...i.e. reluctance to accelerate / decelerate) of the object. (In this example we have switched off gravity, so the heavy mass does not 'sag' on the springs. Note - massive objects have inertia even in outer space where they have no weight!). An increased inertia means that the springs will not be able to make the mass change direction as quickly. This increases the time period of the oscillation. The greater the inertia of an oscillating object the greater the time period; this lowers the frequency of its oscillations.
The oscillating objects we've looked at up to now all vibrate with a rather special 'shape' or waveform. Let's think some more about that...

Sine Waves

The animation below shows how an oscillating pattern (waveform) can be modelled by placing a marker on the edge of a rotating disc. If we just think about the VERTICAL location of this marker (i.e. view the disc edge-on) then we can see that it moves up and down with simple harmonic motion. It generates the sine-wave (sinusoidal) waveform we have been using for mass-spring systems and pendulums. Objects which oscillate sinusoidally (which = most things in practice!) are described as oscillating with simple harmonic motion.

(This 'disk' idea is very useful for thinking about phase of oscillation - we'll come back to this later).

Simple Harmonic Motion and Resonant Frequencies

Here's the maths which underlies what we've been talking about.
An oscillation follows simple harmonic motion if it fulfils the following two rules:

1. Acceleration is always in the opposite direction to the displacement from the equilibrium position
2. Acceleration is proportional to the displacement from the equilibrium position

The acceleration and displacement are linked by the following equation:
Acceleration = - ω² * x
Here ω is called the angular frequency of oscillation, and is given by 2π / T or 2π f
T is the period of oscillation (s), f=1/T = frequency of oscillation (Hz) and x is the displacement (m).
Using Newton's 2nd Law (F=ma) we can show that the Force on the object due to inertia will be:
F= -m ω² x
Using Hooke's Law for springs(F=-kx), we know that the force on the oscillating mass due to the springs is simply
F=-kx
These two forces are always in balance, so
m ω² x - kx=0
From this we can find the resonant frequency:
ω²=(kx) / (mx) = k / m,
ω = sq.root (k / m)
f = 1/ 2π * { sq.root (k / m) }
This might look a little 'tricky'...but it is a massively useful equation!

Damping

When we were talking about playground swings, we mentioned damping - a loss of energy from, in that example, movement (kinetic) energy to heat. There other examples of damping we could think about - here's one:
Imagine hitting a cymbal. This causes the cymbal to oscillate. These oscillations cause the air around the cymbal to vibrate, and these vibrations travel to your ear and you hear this as sound. The sound of the cymbal will eventually die away, as the air resistance and internal losses within the cymbal reduce the restoring forces, causing the oscillations to get smaller and smaller. Placing your hand on the cymbal after hitting it can greatly speed up the damping process, as your fingers absorb the kinetic energy (being soft and a bit 'pudgy'!) very effectively..
Normally it is hard to see a cymbal vibrating because it is moving too fast. Here we've slowed it down to 1/ 80th of normal speed. You can see that the oscillations take ages to die away, as damping is small. For many oscillations (including this one) the damping forces are roughly proportional to velocity, and this leads to an exponential decay of amplitude over time.

Wavelength frequency and velocity

Wave-Length, Velocity and Frequency

When you pass your Basic Qualification, one of the first ham bands you will probably use is the 2 metre band. Why are you allowed to use that band? Your Basic Qualification allows you to use any ham band provided that the frequency of the radio signals is 30 MHz or higher and athe 2 metre band falls into this category.
Now you are probably thinking "Wait a minute, first he was talking about metres, then he was talking about MHz. Can't he make up his mind?" The answer is that I can't and luckily it doesn't matter.
Metres and Megahertz are simply two ways to measure radio waves. Radio Amateurs switch back and forth, depending on which which way of measurement is most convenient. It's not hard to do. In fact, many people can do it in their heads once they know how.
Before you can convert back and forth you have to know the basic formula: "distance = velocity * time". You may know this formula already from travelling. For example, if you are in car going 100 km/hr. and you travel for 1 hour then you will have gone 100 km. In two hours you will go 200 km and so on. You take the speed the car is going and multiply it by the number of hours you have travelled to get the distance.

Now examine my animation of a radio wave. It is a pictorial representation of the electrical and magnetic waves as a radio wave moves through space. The electric wave is in blue and the magnetic wave is in green. This wave is said to be vertically polarized because the electric wave is up-and-down.
How fast does a radio wave travel? That depends on what it is going through. To make things simple will will look at a radio wave doing through the vacuum of outer space. Radio waves actually travel a bit slower in air but the difference in speed is fairly small.
How fast is a radio wave moving in a vacuum? It moves the same speed as light. Light is just another type of radio wave. Physicists like to use the term c to denote the speed of light. This is also the speed of radio waves.
So how big is c? That is, how fast does a radio wave move? The answer in general terms is fast. In exact terms the answers is 300,000,000 metres per second.
Is that fast? You bet! Suppose you could send a radio wave all the way around the earth; how far would it go in one second? It would go seven times around the earth and be almost half way around again by the time that one second was gone. I don't know about you, but I can't even get out of my chair and turn on a light in one second, let along go more than seven times around the earth!
Ok, once again, the speed of radio waves in a vacuum is 300,000,000 metres per second. This is the same as 300,000 km per second or 300 Mm per second (ie. 300 mega or million metres per second). We will use these speeds interchangeably depending on what is convienient.
Ok, so how does this relate to wavelength and frequency. By definition, the wavelength of a wave is the distance travelled while the wave goes from zero to its maximum, then to its minimum and finally back to zero. This is one complete cycle. (Some people say one cycle is when a wave goes from one maximum to the next. This gives the same wavelength and frequency, but may be a littler easier to describe and imagine.) The frequency is how many cycles there are in one second.
Since we know the speed of a radio wave, if we know how long it takes to complete one cycle we can figure out the distance travelled, and this gives us the length of the wave or as we Radio Amateurs like to call it, the wavelength.
Now as an example, the frequency of the electricity in your house is 60Hz. Since there are 60 cycles in a second, each cycle can only take 1/60th of a second. As a general rule you can get the elapsed time by "fippping-over" the frequency. The mathematical purists call this "getting the reciprocal", but we won't mention that--whoops, we just did...
Remember "distance = velocity * time". Wavelength is a type of distance, time is the frequency flipped over and the velocity is c. Put this all together and we get: lambda = c/f
So where does lambda come from? It's just a fancy way of writing "wavelength". Don't worry, you'll get used to it!
Now, remember all the different ways we wrote c? All right, here's the deal. If the frequency is measured in Mhz it's easiest to use the speed of light measured in Mm/s. That way the "mega's" cancel and we can work with small numbers.
For example, what is the wavelength of a 30Mhz radio wave. One way to calculate this is to convert to hz and calculate 300,000,000/30,000,000 to get 10 metres. Or we can leave the whole thing in Mm and Mhz and calculate 300/30 to get 10 metres. I don't know about you but I don't like writing zeros when I don't need to.
With a Basic Qualification you are allowed to use any ham band where the frequcncy is higher than 30Mhz. It follows from the above paragraph that you can use any ham band where the wave length is shorter than 10 metres.
So what about the 2 metre band? This band goes from 144Mhz to 149Mhz. In terms of wave length this works out as follows:
300/144 = 2.08 metres
300/149 = 2.01 metres
So now you see why this is called the two meter band.
Another band you can use with a Basic Qualification is the 6 metre band. This band goes from 50Mhz to 54 Mhz. Let's calculate the wavelength of the two ends of the band.
300/50 = 6 metres
300/54 = 5.55 metres
So the wavelengths in this band vary from 5.55 metres to 6 metres.
What about going the other way, from wavelength to frequency? You can use the same technique. Since \lambda= c / f it is also true that f = c / \lambda.
Suppose you have a friend who builds a type of antenna called a half-wave dipole measuring 1.5 metre from end to end. What is the frequency of radio waves this antenna will be best at receiving.
Ok, so you may be saying "Whoa! Hold on here just a cotton-picking minute! What's a half-wave dipole?" Well, I don't know how picking cotton got involved with this but I can explain the other things. A dipole is two wires, typically of equal length which are physically hooked up to form one long line, but the two wires are insulated from each other. A half-wave dipole is one half a wavelength long.
So if your friend's antenna is 1.5 metres long then the wavelength is twice that or 3 metres. Now according to the formula, the frequency is given by 300/3 = 100 Mhz. Chances are your friend wants to listen to FM radio.
One word of caution is in order if you want to use the above formula to calculate the size of antennas. Electricity actually travels a bit slower in wires than radio waves travel in a vacuum. Is the difference much? No, but once you get your Morse code endorcement and start using bands below 30Mhz the difference in antenna lengths is noticeable enough to warrant using a separate formula. A good approximation is to use \lambda = 286 / f.
For a more thorough discussion of antennas, please refer to the section on antennas.
In summary, as an electromagnetic wave (ie. a radio wave or a light wave) goes through a cycle it moves through space. The number of cycles it goes through in one second is called the frequency of the wave. The distance it travels as it goes through one cycle is called the wavelength. The speed of radio- wave is basically the same as the speed of light, ie. VERY FAST. The speed of light is 300 Mm/s or 300,000 km/s or 300,000,000 m/s. The formula to convert between wavelength in metres and frequency in MHz is as follows: f =300/ \lambda or \lambda=300/f Of course if ht frequency is in kHz use 300,000 instead of 300. And if the frequency is in Hz then use 300,000,000 m/s for the speed of light.

Motion Characteristics for Circular Motion

Acceleration

An object moving in uniform circular motion is moving in a circle with a uniform or constant speed. The velocity vector is constant in magnitude but changing in direction. Because the speed is constant for such a motion, many students have the misconception that there is no acceleration. "After all," they might say, "if I were driving a car in a circle at a constant speed of 20 mi/hr, then the speed is neither decreasing nor increasing; therefore there must not be an acceleration." At the center of this common student misconception is the wrong belief that acceleration has to do with speed and not with velocity. But the fact is that an accelerating object is an object that is changing its velocity. And since velocity is a vector that has both magnitude and direction, a change in either the magnitude or the direction constitutes a change in the velocity. For this reason, it can be safely concluded that an object moving in a circle at constant speed is indeed accelerating. It is accelerating because the direction of the velocity vector is changing.

To understand this at a deeper level, we will have to combine the definition of acceleration with a review of some basic vector principles. Acceleration as a quantity was defined as the rate at which the velocity of an object changes. As such, it is calculated using the following equation:

where v_i represents the initial velocity and v_f represents the final velocity after some time oft. The numerator of the equation is found by subtracting one vector (v_i) from a second vector (v_f). But the addition and subtraction of vectors from each other is done in a manner much different than the addition and subtraction of scalar quantities. Consider the case of an object moving in a circle about point C as shown in the diagram below. In a time of t seconds, the object has moved from point A to point B. In this time, the velocity has changed from v_i to v_f. The process of subtracting v_i from v_f is shown in the vector diagram; this process yields the change in velocity.

Direction of the Acceleration Vector

Note in the diagram above that there is a velocity change for an object moving in a circle with a constant speed. A careful inspection of the velocity change vector in the above diagram shows that it points down and to the left. At the midpoint along the arc connecting points A and B, the velocity change is directed towards point C - the center of the circle. The acceleration of the object is dependent upon this velocity change and is in the same direction as this velocity change. The acceleration of the object is in the same direction as the velocity change vector; the acceleration is directed towards point C as well - the center of the circle. Objects moving in circles at a constant speed accelerate towards the center of the circle.

The acceleration of an object is often measured using a device known as an accelerometer. A simple accelerometer consists of an object immersed in a fluid such as water. Consider a sealed jar that is filled with water. A cork attached to the lid by a string can serve as an accelerometer. To test the direction of acceleration for an object moving in a circle, the jar can be inverted and attached to the end of a short section of a wooden 2x4. A second accelerometer constructed in the same manner can be attached to the opposite end of the 2x4. If the 2x4 and accelerometers are clamped to a rotating platform and spun in a circle, the direction of the acceleration can be clearly seen by the direction of lean of the corks. As the cork-water combination spins in a circle, the cork leans towards the center of the circle. The least massive of the two objects always leans in the direction of the acceleration. In the case of the cork and the water, the cork is less massive (on a per mL basis) and thus it experiences the greater acceleration. Having less inertia (owing to its smaller mass on a per mL basis), the cork resists the acceleration the least and thus leans to the inside of the jar towards the center of the circle. This is observable evidence that an object moving in circular motion at constant speed experiences an acceleration that is directed towards the center of the circle.

Another simple homemade accelerometer involves a lit candle centered vertically in the middle of an open-air glass. If the glass is held level and at rest (such that there is no acceleration), then the candle flame extends in an upward direction. However, if you hold the glass-candle system with an outstretched arm and spin in a circle at a constant rate (such that the flame experiences an acceleration), then the candle flame will no longer extend vertically upwards. Instead the flame deflects from its upright position. This signifies that there is an acceleration when the flame moves in a circular path at constant speed. The deflection of the flame will be in the direction of the acceleration. This can be explained by asserting that the hot gases of the flame are less massive (on a per mL basis) and thus have less inertia than the cooler gases that surround it. Subsequently, the hotter and lighter gases of the flame experience the greater acceleration and will lean in the direction of the acceleration. A careful examination of the flame reveals that the flame will point towards the center of the circle, thus indicating that not only is there an acceleration; but that there is an inward acceleration. This is one more piece of observable evidence that indicates that objects moving in a circle at a constant speed experience an acceleration that is directed towards the center of the circle.

So thus far, we have seen a geometric proof and two real-world demonstrations of this inward acceleration. At this point it becomes the decision of the student to believe or to not believe. Is it sensible that an object moving in a circle experiences an acceleration that is directed towards the center of the circle? Can you think of a logical reason to believe in say no acceleration or even an outward acceleration experienced by an object moving in uniform circular motion? Additional logical evidence will be presented to support the notion of an inward force for an object moving in circular motion.

Meaning and Definitions in Particle Physics

Types of particles and forces

Quark

A fundamental particle. Six types (or flavours) ofquarks are known. Up and down flavours are constituents ofprotonsand neutrons.The other, heavier, quarks are called strange, charm,bottom, and top.

Electron

A negatively charged particle (lepton) making up the outer shell ofthe atom.

Positron

An Anti-electon.The positively-charged antiparticle of anelectron.

Muon

A heavier flavour of leptonthan the electron.

Tau

The heaviest known lepton.

Neutrino

An uncharged, massless (or at least extremely light), lepton.Like the charged leptons, they can come in three types (orflavours):electron neutrinos, muon neutrinos,or tau neutrinos.

Anti-neutrino

The antiparticle of aneutrino.

Baryons

Particles consisting of three quarks.Neutrons,protons, and the lambdas areall baryons.

Mesons

A family of particles consisting of a quark and an anti-quark.pions, kaons,and B-mesons are all mesons.

Hadrons

Particles made up of quarks. There are two types of hadrons:baryons and mesons.

Leptons

A family of particles consisting of the electron,the muon and the tau, along with theirneutrinos.

Proton

A positively-charged particle (baryon) consisting of two up and onedownquarks which is found in and makes up the atomicnucleus.

Neutron

A neutral particle (baryon) consisting of two down and oneup quarks which is found in and makesup the atomicnucleus.

Lambda

The lightest strange baryon, consisting of one up, one down,and one strange quark.

Pion

Pions are the lightest mesons. They consist of up and down quarks (eg. the pi+ consists of an up quark and a down anti-quark).

kaon

The lightest strange meson, consisting of an up or down quark, with a strange anti-quark.

B-meson

One of the heaviest mesons, containing a bottom quark.

Strong Force

A force which binds quarks together.Its range is limited to the distances between quarks in hadrons, but an indirect effect of the strong force is to bind protons and neutronstogether to form nuclei.The strong force is carried by gluons.

Electromagnetic force

A force with infinite range which acts between objectsaccording to their charge. Specific cases are the electric and magnetic forces.The electromagnetic force is carried by photons.

Weak Force

Interactions that change the flavour of particles; for example thedecay of a neutron into a proton,electron, and anti-neutrino; are governed bythe weak force. The weak force is the only one that affects neutrinos.

Electroweak Force

A force resulting from the combination of theelectromagnetic force and theweak force.

Gravity

A force with infinite range which acts between objects, such as planets,according to their mass.

Gluons

The carrier particle of thestrong force.

Photon

The carrier particle of the electromagnetic force.Electromagnetic radiation, such as light, can be thought of as beingcomposed of photons.

Boson

A force-carrier particle. Photons, gluons, W, and Z particles are allbosons. Another type of boson, the Higgs, is proposed as the mechanism by whichparticles acquire mass.

Fermion

A matter particle. Leptons (such as the electron and neutrinos) and quarksare fermions.

Definitions

Antiparticle

A particle and its antiparticle have a number of opposite properties.If they have charge, then these will be opposite. For example themuon is negatively charged and the anti-muon is positively charged.However the neutrino and antineutrino are both uncharged, but are neverthelessare different (they have a property called lepton number, which isopposite).

Antimatter

Matter composed of the antiparticles ofnormal matter.

Flavour

A characteristic that distinguishes different types ofhadrons and leptons withdifferent masses (apart from theneutrinos,which as far as we know, are massless).

Colour

Property of quarks associated with their binding with gluons.

Sub-atomic

Smaller than the atom. The structure of the atom.

Cosmic rays

High energy particles from outer space.

The quark theory of the structure of matter

The theory that all hadrons, such as the protons andneutrons in the nucleus, are made ofquarks.

Electron Volt (eV)

One electron Volt (eV)is the energy gained by an electron when it is accelerated through apotential of 1 Volt.The binding energy on an electron in an atom is of the order of 1 eV.

Giga electron Volt (GeV)

One billion electron Volts, or 1,000,000,000 eV.The energy-equivalent (E=mc^2) of the mass of a proton is about 1 GeV.

Tera electron Volt (TeV)

One trillion electron volts, or 1,000 GeV.So far the most powerful accelerators in the world (eg. the Tevatron atFermilab) can produce beams of protons with an energy of about 1 TeV.

CP violation

In almost all circumstances antimatter seen in a mirror behaveslike normal matter. Very occasionally, in the decay ofkaons (andperhaps in the decay of B-mesons, though this has not yet been seen), thisrule is violated. This is known as CP violation.

Experimental Particle Physics

Particle Physics

Particle Physics is the study of thebasic elements of matter and the forces acting among them. It aims to determinethe fundamental laws that control the make up of matter and the physicaluniverse.

RAL

The Rutherford Appleton Laboratorysituated at Chilton, near Oxford.

CERN

The European Laboratory for Particle Physics, located near Geneva on theSwiss-French border. The LEP accelerator is located at CERN.

LEP

The Large Electron Positron collider, the world's largest particleaccelerator, which is 26.7 km in circumference and some 100 metres underground,situated at CERN. LEP collides electrons and positrons atenergies sufficent to produce the Z and (soon) W particles, carriers of theweak force.

Fermilab

The Fermi National AcceleratorLaboratory which is situated 30 miles west of Chicago.The Tevatron accelerator is located at Fermilab.

Tevatron

A proton accelerator at Fermilab which can accelerate protons tonearly one trillion electron volts (1 TeV). Two detectors, CDF and D0,detect the results when these protons collide. Recently these detectors havedetected the heaviest-known quark, the top.

DESY

The Deutsches Elektronen-Synchrotron (DESY) in Hamburg.The HERA accelerator is located at DESY.

HERA

HERA is an electron-protoncollider at DESY, which uses theelectrons as a probe to understand the structure of the proton.The results of these collisions are detected by two detectors, ZEUSand H1.

Particle beams

A stream of particles guided into a defined direction by anparticle accelerator.

Solenoid (magnet)

An electromagnet produced by current flowing through a single coil of wire.Many particle detectors are surrounded by a solenoidal magnet, since thisproduces a fairly uniform magnetic field within.

The experiments CDF (Collider Detector Facility) andD0 (named after its location on the accelerator ring) at Fermilab have searched for the top quark by looking for evidence of its decay to the next heaviest quark, the bottomquark. This decay bears a direct relationship to the more familiar beta-decayof the neutron, in the following way.
The neutron decays into a proton, an electron and an electron-antineutrino. Interms of quarks, a neutron consists of one up quark and two down quarks(udd), while the proton consists of two up quarks and one downquark (uud). So at the quark level, a d changes into a u. This decay occurs through the weak force, which in this case is mediated by thenegative carrier particle of weak force, the W-. Thus the dchanges into a u by emitting a W- which almostimmediately materialises as an electron (e-) and anelectron-antineutrino.

The top quark, t, decays similarly via the weak force, but to the bottomquark, b. In this case a positive carrier particle, W+, isemitted, which can materialise in various ways since the mass difference betweenthe t and the b quarks is sufficient to create a variety ofparticles. However, the easiest decay modes to detect are those when theW+ decays into a positron (e+) and an electron-neutrino, or apositive muon and a muon-neutrino.

In the collisions at Fermilab we expect the top quark to be createdsimultaneously with its antiquark. The antitop will decay in similar ways tothe top, but with particles replaced by antiparticles and vice versa. So theantitop can decay to a bottom antiquark, together with an electron and anelectron-antineutrino, or a negative muon and a muon-antineutrino.
The bottom or anti-bottom cannot emerge on their own into the detectors, butinstead create a characteristic "jet" of particles. The electron or muon (ortheir antiparticles) do emerge, and can be identified through theirinteractions in the detector. The neutrinos (or antineutrinos) likewise emergebut leave the detectors unseen as they are so weakly interacting; they can beidentified only by the "missing" energy with which they escape.
The picture circulated by Fermilab shows an identified muon, an identifiedelectron and two jets of particles - in other words, one of the "signatures"expected for the decay of a top-antitop pair.

What is matter?

All matter consists of basic substances, or elements, with well definedphysical and chemical properties, ranging from hydrogen, the lightest, touranium and beyond.
Each element consists of building blocks - atoms - uniqueto the element, but the different atoms can combine to form anenormous variety of compounds from simple water to complexproteins. Yet, as scientists first discovered towards the end of the19th century atoms are not the simplest building bricks of matter, so what isthe most basic of our building blocks?

What is the atom made up of?

To find out what the atom was made up of theories had to be worked out,explained and proved. The structure of the atom was discovered in anexperiment where protons were fired at a piece of gold foil. Approximately onein eightthousend was bounced back, and when it was bounced back it returnedwith greater speed. This suggested a small, positively charged nucleus, withnegatively charged particles orbiting the nucleus, since like forces repel, andso the positive particle colliding with the nucleus would be powerfullyrepelled.

Splitting the atom

We now know that most of the mass of an atom is concentratedin a small, dense, positively-charged nucleus. A cloud of tinynegatively-charged electrons envelopes the nucleus, but at arelatively large distance, so that much of the volume of an atomis empty space.
In most atoms the nucleus contains two types of particle ofalmost equal mass: positively-charged protons and electrically-neutral neutrons. To make the atom neutral overall the number ofprotons exactly balances the number of electrons.
This picture of the atom stems largely from pioneering work atCambridge and Manchester Universities. At Cambridge in the1890's, two physicists began unwittingly to probe the worldwithin the atom. One, Joseph ('J.J.') Thomson, discovered thefirst known subatomic particle, the electron, while one of hisstudents, Ernest Rutherford, started to explore the newphenomenon of radioactivity, in which atoms change from onekind to another. This was to lead Rutherford eventually to thediscovery of the nucleus, in work with Hans Geiger (of Geigercounter fame) and Ernest Marsden at Manchester University in1909-10.
Later back at Cambridge, Rutherford found that atoms containpositively-charged particles, identical to the nucleus of hydrogen.He called the particles protons. And at Cambridge in 1932, JamesChadwick showed that the nucleus must also contain neutrons.By this time Rutherford and his colleagues had established much ofthe modern picture of the atom.

A Pageant of Particles

This was only the beginning. The electron, proton and neutronproved to be the first members of a rich pageant of subatomicparticles. During the 1930's and 40's, many physicists studiedcosmic rays, the steady rain of high energy subatomic particlesthat originate in outer space.
The collisions of high-energy cosmic rays with atoms in theatmosphere prised open the nucleus to reveal new kinds ofshortlived particles that could be seen only through the tracksleft behind in sensitive detectors. There were particles such asthe muon, which behaves like an electron, but is 210 timesheavier; thepion, which is just a little heavier than the muon; thekaon at little more than half the protons mass; and the lambda,which is about 20 per cent heavier than the proton.

Enter the Positron

One particularly important particle, discovered in 1932 by CarlAnderson at the California Institute of Technology is the positron- as light as an electron, but with positive charge. Its existence, atfirst a puzzle, was soon explained by Paul Dirac, a theoreticalphysicist at Cambridge University.
According to Dirac's theory the positron is a particle withexactly opposite properties to an electron - an anti-electron.The theory showed how an electron and a positron can emergetogether from pure energy provided the energy is sufficient tosupply the total mass of the two particles in accordance withEinstein's equation, E=mc^2.
If they collide, the particle and antiparticle disappear to leaveonly energy - an act of mutual destruction called annihilation.Experiments have since demonstrated that most other particles-protons, neutrons, muons and so on-have antiparticles too.


The result of matter and antimatter colliding: theyannihilate each other, creating conditions like those that might have existedin the first fractions of a second after the Big Bang.

Cosmic Mimics

By the early 1950s, the study of these particles had become abranch of physics in its own right- particle physics had come ofage. To aid them, the physicists had machines that couldaccelerate protons and electrons to high energies, mimicking thecosmic rays but in more controlled conditions.
The 3.8 metre wide LEP tunnel, 100 metres below theFrench and Swiss countryside, has taken eight years to complete. The tunnelcarries bunches of accelerated particles in a 27km long aluminium 'beampipe'.
Work in the early 1930s by John Cockcroft and Ernest Walton atCambridge, and by Ernest Lawrence and Stanley Livingstone atBerkley in California, had provided the first artificially acceleratedprotons. Their pioneering ideas gave birth in the 195O's and 60's tolarge machines capable of producing millions of protons,electrons, pions or kaons each second. With the invention ofmore sophisticated detectors to complement the accelerators,physicists now had the tools to study the many varieties ofparticle in detail.

Into inner space

The results of this onslaught on the realm of 'innerspace' have beenspectacular. We know that matter has a deeper layer revealed only as we probe it more energetically, inaccelerators and in studies of cosmic rays.
The proton, neutron, pion, kaon, lambda and many other subatomic particles arethemselves complex structures, based on only a few more basic particles - the quarksand their corresponding antiquarks.There are probably at least six types quark in three pairs, which have beennamed: up, down; charm, strange; top andbottom - although the top quark has yet tobe positively identified. These quarks combine in groups of threeto form the proton, neutron, lambda and related particles calledbaryons. The quarks can also bind with antiquarks to makeparticles such as pions and kaons, which are collectively known asmesons.


The fundamental particles of Nature appear to fallinto two categories: leptons and quarks.
The electron and muon, on the other hand, are not made fromquarks but appear as far as we can tell, to be indivisible. Theybelong to a separate family of particles called leptons, which alsoinclude a third still heavier charged particle, thetau, as well asneutrinos - particles that are almost massless, neutral and difficult todetect.

Inside the Nucleus