Quantum chromodynamics - David Gross

this year actually last year is a hundred year anniversary of the birth of the nuclear force it was a hundred years ago in 1911 that Ernest Rutherford discovered that atoms had a core a center called which he called the nucleus before that people were very confused as to what atoms were they knew there were electrons little points of electric charge inside atoms but they didn't know what the structure was rather furred bombarded atoms with alpha rays actually the nucleus of helium atoms and measured the deflection of the atoms of these heavy nuclei and realized that the only way he could explain those experiments was to assume that in the center of the atom there was a little place where all the masked 99% of the mass of the atom and it's positive charge was located which he called the nucleus and over the last hundred years we have been probing and trying to understand what goes on inside the nucleus of atoms and it it was very difficult because one can't just look inside the atom one has to follow Rutherford and hit the atom with all sorts of projectiles other particles and see what happens in the collision that journey has been a long journey but we think we have arrived in fact over now over 30 years ago at the answer we now understand the forces that act within the nucleus and the culmination of that hundred year journey maybe a thousand year journey is to have a very detailed picture of all the forces of nature that act around us they are of course the force of gravity which we all feel the force of electricity and magnetism that pulls electrical particles together and governs radio waves and light and then there do these two mysterious forces a hundred years ago that act within the nucleus of atoms and they are harder to understand because the distances involved are a million times smaller than the size of an atom so it was not easy and only after World War two did we start building these big particle accelerators to probe the structure of the nucleus what was discovered in the latter part of the 20th century was that it looked very much like nucleons the things that make up the nuclei of atoms protons and neutrons were actually made out of particles that were even smaller and more fundamental called quarks that were named quarks but it was very strange because no one has ever seen a quark in the laboratory and in fact we now understand that you can never see a cork in the laboratory but still they believed that the proton and the nuclei Neutron were made out of quarks the proton was actually three quarks somehow held together in a way that you could never pull them out that was the situation when I entered physics as a student people did not understand what was going on at all there was no true understanding the situation became even more confusing when people shot electrons which are don't feel the strong force but feel the electricity at protons and they could probe how these electrons interacted with whatever it is that makes up the proton and discovered or their experiments could be interpreted as saying that the proton actually did consist of three quarks moving around as if there was no force between them that seemed very strange in of itself but also how could that be so if you couldn't pull the quirks out of a proton see if you hit an atom an electron comes out and you can run it in wires and do all sorts of things with electricity but you can't get the cork out of the atom no matter how hard you hit it people went to higher and higher energies hit it harder and harder no quarks and yet in these experiments it seemed that the quarks were moving around freely inside the proton and that confusion was explained finally by our discovery of us at what we call a called asymptotic freedom asymptotic freedom means that there are forces in nature in fact there's only one force of nature that does this which is what we discovered that has the very strange and unusual property that the force between two particles becomes weaker and weaker the closer they are together and the farther and farther apart it becomes stronger and stronger it's very counterintuitive usually force gets weaker when you pull things apart they interact less strongly but in this case it was exactly the opposite which was surprising and led us to a very unique specific theory of the nuclear force which is called now quantum chromodynamics it's not easy to explain how that works but we understand in great detail how that comes about it has to do with the properties of the quantum vacuum which are is fluctuating and boiling with virtual particles and quantum fluctuations but we understand that now and in this theory we can say an awful lot about what happens at these very short distances where the four becomes weak we also can understand why the quarks can't get out because when you try to pull them apart the force and effect get stronger and stronger so strong that you can never pull the quarks out this theory which has been developed for the last many decades has become extraordinarily powerful and useful and explore in physics and in understanding what we're made out of for example you probably don't know what what is the origin of your weight when you step on the scale every morning and you say oh my god I've gained another kilogram what makes up that mass it's not the mass it's the mass of all the protons and neutrons that the nuclei of one carbon and all the atoms that exist in your body are made out of but where does their mass come from those protons and neutrons are made out of quarks so does it come from the mass of the quarks in your body quarks are almost massless they don't have any mass that's not what makes up your weight what makes up your weight is energy so your pure energy energy according to Einstein is just the measure of mass our mass is the energy of a body at rest so what makes up the mass of the proton is the energy of all the quarks inside the proton which are zooming around very fast almost the speed of light inside a little region of sphere and that sphere is created by the fact that when the protons get too far apart say the radius of the sphere then the force between them gets very strong so you are just confined kinetic energy of quarks and so-called gluons that quanta of the force inside this region of we call confinement where there those particles that are racing around are held together by this force that's getting stronger and stronger the farther apart they are so it's a nice point of view and you step on the scale in the morning you see gained 1 kilogram you've gained 1 kilogram of energy so quantum chromodynamics is undoubtedly now we believe the correct theory of the nuclear force it's a rich theory much richer than the theory of electricity and magnetism the equations of QCD of quantum chromodynamics can be written down on a half of 1 t-shirt but to solve them is not easy there are many many questions about nuclear matter that are very interesting that we are still working on and we'll be working on undoubtedly for hundreds of years people still are working on the equations that govern the dynamics of fluids for maybe 200 years and there are many important unanswered questions turbulence we can't still predict the weather with great accuracy the same is true in quantum chromodynamics the equations in in the case of QCD are more beautiful and richer but an enormous progress is being made on solving them but this will take hundreds of years and will go on because in the realm of physics we do we believe understand these basic laws basic equations in some areas in the area of what happens when you when quarks interact at very high temperatures or very high energies very high densities the force gets weak and when things are weak it's easy to understand them their enormous progress has been made it allows us for example it's one of the elements that allows us to to understand the history of the universe back to the first microseconds where the temperatures of the universe were so high and so dense that nuclear matter were underwent a change of phase we believe though what happened in the early universe is that there was a different phase of nuclear matter much like ice when you raise the temperature if you raise the temperature in this room to a hundred million degrees the nucleons in your body would melt and the quarks would come out and you would have a kind of liquid of quarks and gluons and we can observe that in experiments now by bombarding nuclei together at very high energies we form a fireball which is very hot and for a very short time quarks escape from the proton and there is a quark-gluon liquid that whose properties we are trying to understand in more detail so there are many questions that will be continued to be explained like in all fields of physics for probably centuries you

Channel: Serious Science

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Keywords: science, lecture, Serious Science, Quantum Chromodynamics (Field Of Study), David Gross (Academic), physics, Quantum Mechanics (Field Of Study)

Id: eLhmUYiDjrU

Length: 11min 45sec (705 seconds)

Published: Thu Jul 24 2014