A Crash Course In Particle Physics (1 of 2)

the world seems almost infinitely complicated made up of thousands if not millions of different materials throughout history people have tried to collect categorize and analyze them to find some underlying pattern that would help simplify this seemingly incredibly complicated world now at the dawn of the 21st century we've made some progress to achieving that long yearned for simplification with the use of particle accelerators we are starting to understand the nature of the world around us these machines have revealed a whole array of particles which we believe may be the fundamental building blocks of matter but back in the 19th century scientists thought that everything on earth was made of just over 80 elements these elements were famously arranged in a periodic table by Dmitri Mendeleev at the time it was thought the elements were made of indivisible spheres called atoms but each of the elements behaved in a different way did that mean that there were 80 different kinds of atom and if so what made them different were they different shapes or sizes or maybe the atoms were divisible maybe they were built of even smaller objects it was here in Cambridge that the first clear evidence for smaller objects inside the atom was found many of the great scientists of history of walked these streets and one of the greatest with JJ Thompson who became the director of this the old Cavendish laboratory in 1896 Thomson had just got his hands on this new piece of kit it's essentially a particle accelerator when this plates heated particles are emitted they're accelerated by these electrodes they pass through these two plates across which you can apply a voltage and they hit the end of the bulb here on a screen which glows so you can see the beam now this is a modern version of Thomson's apparatus again we've got the particle accelerator and there's a screen in there so you can see the beam glow what Thomson did was he varied the voltage across the plates and he measured the amount of bending as the voltage changed that allows you to deduce the mass of the particles in the beams now the lightest known particle in Thomson's day was the hydrogen atom but Thomson found from these measurements that the particles in this beam are almost 2,000 times lighter than hydrogen atoms Thomson had discovered the first subatomic particle the electron the electron hose is practical agility utility to its smallest ignite apparently shakespeare say my youth is great because i am so small the electron was the first discovery of a fundamental particle and it is interesting to realize that more than a hundred years later the electron is still to the best measurements we can do today a fundamental letter of nature's alphabet we can use electrons as ways to probe materials and look at the structure in electron microscopes or in big machines like this accelerator behind me pretty much all of everything we do in the in the 21st century depends on understanding the properties of electrons thomson had discovered that the atom is not the fundamental building block of matter there are smaller objects inside so atoms could no longer be thought of as hard indivisible spheres but how did the electrons fit inside the atom Thomson suggested that the atom was something like this muffin with a negatively charged electrons embedded in a positive body it will be student of thompson's that proved him wrong the mystery of how the electrons fitted inside the atom was eventually sold here in Manchester in this building in 1911 by Ernest Rutherford Rutherford was in my opinion one of their first proper particle physicists because he used beams of particles as projectiles to explore the structure of matter now of course in Rutherford's day there was no such thing as a particle accelerator so he used the decay of radioactive elements to produce his beams of particles this is Rutherford's original desk and in fact if you hum around a little bit you can detect traces of radioactivity 100 years later Rutherford has two of his students hands Geiger and Ernest Marsden to fire some alpha particles at a piece of thin gold foil see what happened so imagine these tennis balls are the alpha particles now if the atom were as Thompson had suggested kind of amorphous blob then you expect the output particles pass right through and that's indeed what happened to most of them but to their surprise they found that around 1 in 8,000 bounced right back after two years of puzzling go with a meaning of these results Rutherford realized that in order for the alpha particles to bounce back they must hit something small and dense so his new model of the atom was a bit like the solar system with all the mass concentrated at the center and the electrons orbiting like planets around the Sun today we know that this picture isn't quite correct quantum mechanics tells us that we can't know precisely where the electrons are but we can predict that they reside in distinct shells around the nucleus Rutherford's alpha particle scattering experiment was remarkably direct and simple and it showed nature of what the atomic structure is by the way the alpha particles bounced off the atom he worked out with a positive charge of the atom Louvre's Rutherford had come to the astonishing conclusion the most of the atom and therefore most of what we think of as ordinary matter is in fact empty space so this Apple with the atomic nucleus the electrons would be a kilometer away after discovering the nucleus Rutherford continued doing experiments firing particles at different targets to delve into the structure of the nucleus itself by 1932 Rutherford and his colleague James Chadwick had found that the nucleus is made of two kinds of particles positively charged protons and electrically neutral neutrons the discovery in these experiments of neutrons and charged atoms of mass 1 has food of great significance and importance and has given us a much clearer understanding of the actual structure of nuclei less than a century after Mendeleev published his periodic table scientists had arrived at a seemingly beautiful simplification all this is made of just three fundamental particles proton the neutron and the electron this was a giant step forward in our understanding of matter but there was still phenomena that couldn't be explained in terms of just these three particles in the early 20th century scientists recorded mysterious new particles bombarding the earth from outer space they had discovered cosmic rays and they rushed to study them by the late 1930s they came to the conclusion that the experimental results could not be explained using the then known fundamental particles protons neutrons and electrons some other more mysterious particles were responsible using cosmic rays to detect new particles isn't particularly efficient however because you never know when or where they're going to turn up it'll make much more sense to make your own thus entered into physics this the particle accelerator a way of making cosmic rays in the laboratory particle accelerators built in the 1940s and 50s led to the discovery of many new particles given exotic names like pi ons Sigma's lambdas and deltas by the mid-1960s over 80 apparently fundamental particles have been discovered so many in fact that particle physicists began to refer to them as a zoo this was no better than Mendeleev's periodic table eventually order and elegans were restored by American physicist Murray Gelman there was a comparatively simple underlying structure to all this and the classification say of the strongly interacting particles depended a great deal on symmetries and brokens in particular broken symmetries approximate symmetry that were violated Galman had noticed patterns which physicists can explain in terms of symmetries and by identifying the underlying symmetries he found he could explain the properties of the particles according to him protons neutrons and the whole zoo of apparently fundamental particles were made up of just three types at basic building blocks which he named quarks just the simple inspection of the particle chart would suggest immediately the quarks scheme so the difficult thing was not noticing the quarks scheme that was essentially trivial what was difficult was believing that it had any relevance there anything for anything for anything at all to happen in the universe for some stacked for anything at all last night we usually think of forces as moving things around pulling this Apple towards the ground or pushing a car up a hill but forces also cause the Sun to shine they make the ice melt in your drink and they cause a plant to emerge from a seed forces are the agents of change in the universe to help us understand forces thousands of scientists around the world have spent billions of pounds to build this machine I'm standing a hundred metres below the ground CERN in Geneva and this is the CMS detector pass of the largest and most complicated scientific experiment ever attempted this experiment will give us deeper insight into the forces of nature than ever before it's a long way from when Isaac Newton pondered the laws of gravity but all part of the same story

Channel: powerphyzix

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Keywords: Particle Physics, Brian, Cox, Brian Cox (physicist), Large Hadron Collider, CERN

Id: HVxBdMxgVX0

Length: 13min 1sec (781 seconds)

Published: Sun Jan 01 2012