In the last post we discussed about the fundamental particles which constitutes matter.
To summarize again, the whole universe comprises of just
1) 6 sets of quarks ( up, down, charm, strange, top, bottom)
2) 6 sets of Leptons( electron and its neutrino, muon and its neutrino, tau and its neutrino)
Along with this matter particles, there are also antimatter particles like the anti quarks and anti leptons.
Now the time has come to speak about their interactions.
Physics has a holy grail and that is to have a unified theory of interactions.
The Standard Model is the best theory of a unified model that we have about matter and its interactions. But it is still not a complete model.
What are the interactions in nature?.
They are in decreasing order of strength
1) Strong Interaction
2) Electromagnetic Interaction
3) Weak Interaction
4) Gravity
In order to understand about what you mean by interaction, you need to understand a bit about motion of matter.
Let us go back to our school days and reread our old physics textbooks.
Newton's first law of motion is a good starting point .
It says that " Unless compelled by an external agency a body will continue its state of uniform motion".
What it says is rather subtle. It is actually defining an Inertial frame of reference.
A frame of reference is just a vantage point .
If you are traveling in a Bus that is your vantage point and that becomes your frame of reference with respect to which you describe everything.
Now what Newton's first law is saying is that there are Frames of References in which a body will continue its state of motion( state of rest or state of motion) unless some external agency interacts with it.
So essentially what this means is that, when you see a body changing its state of motion then you can be sure that it interacted with its surroundings. Now the quantity of this interaction is what is known as force.
Note that this is true only in an inertial frame. In simple terms an Inertial frame is simply a frame which is non accelerated. Consider a spacecraft floating in space moving with uniform velocity . Consider a body inside it like a ball for e.g it will move with uniform velocity unless something interacts with it.
On the other hand consider a vehicle accelerating(change its velocity) while its moving . It is not an inertial frame. So even when nothing is interacting with the body inside it there may be change of state of motion because of the acceleration of the frame of reference itself. Newton's law is not strictly applicable here. We have to bring in the acceleration of the frame of reference also in order to describe the motion and it could be complicated. But in an inertial frame, no such problems exist. All you need to consider is that if there is change in state of motion then somebody touched it. ( that is applied a force)
What Newton's first law is defining is that " There are places or circumstances in nature where a change of motion implies an interaction ".
Newton's second law defines how much this force is . It says that
F = mass m multiplied by acceleration a
or
F = ma
acceleration is nothing but change of velocity in unit time. Say a car is changing its speed from 10 miles/hr to 20 miles/hr in one second , its acceleration is 20 -10 = 10 mil/hr/ sec.
The last law of Newton says that " For very reaction there is an equal and opposite reaction".
What this means is that a Force is nothing but an interaction.
When a body interacts with its surroundings , the surroundings interact with it also as a consequence. The quantity of the interaction is what we call the force.
So in a nutshell what the Newton's laws are defining is that in an inertial frame, a change of state of motion implies an interaction of the body with its surroundings.( and vice verse).
The first law states that there are such frames, the second law states how much this interaction would be and the third law states that the nature of the interaction.
The digression was just to give you an idea about interactions in general.
Physicists often use the term Force and Interaction interchangeably.
So you may see lot of references in which they speak about the Strong force , the electromagnetic force etc . Both descriptions are identical.
But the modern view point tend towards interaction since it more accurately describes its nature.
Newton's laws of motion was found to be inaccurate in the world of small particles and a new theory called Quantum Mechanics was formulated for small elementary particles.
Now what are these interactions that we spoke about.
Let us take Gravity first since we know it from our everyday experience. The law of gravity as given in Newton's theory states that everybody interacts with every other body with a force proportional to masses of the interacting bodies.
Thus the earth and the sun interact through gravity.
Mass is nothing but the capability of the object to take part in a gravitational interaction.
The more the mass , the stronger the gravitational interaction.
You can see that every interaction has a parameter that determines how strongly they interact.
For gravity this parameter is mass.
Let us consider electromagnetic interaction next.
For electromagnetic interaction the parameter is charge. So when you see 2 like charges repelling or unlike charges attracting, you are seeing electromagnetic interactions.
If you take gravity and electromagnetic interactions it will cover up all the usual physical phenomenon that you experience in your everyday lives.
However there are things like radioactivity, the binding of protons inside a nucleus etc which need to be explained.
The Strong interaction explains how the protons which are like charged particles are held together inside a nucleus. On a smaller level it is the interaction between quarks .
Since we know that the protons and neutrons are made of quarks.
The weak interaction explains a radioactive phenomenon called beta decay.
So we see that gravity and electromagnetic phenomenon are controlled by mass and charge.
Similarly the strong force is controlled by a parameter called color. This color charge has nothing to do with the visual color. It is just a convenient term.
Thus in order to take part in a strong interaction you need color .
Since quarks are the only entities with color , they are the only particles which can undergo strong interaction.
Now in order to further understand the interaction Physicists formulated that there must be a mediating particle for each interaction.
Thus the electromagnetic interaction is mediated by the photon, the strong force by the gluons and the weak force by the W+ , W- and Z0 bosons.
Thus what we see as an interaction is actually an interchange of the force carrier particles.
Gravity has defied a formulation like this is the major tumbling block for the standard model.
In order to understand the interaction let us look at an example
You see 2 electrons coming together and repelling each other.
In the modern Quantum mechanical picture what has happened is that the electrons have exchanged a virtual photon ( light particle) and like the skaters who seem to repel each other when they exchange a ball , the electrons seem to repel each other. We have seen an electromagnetic interaction.
But where did this virtual photon come from?. The answer lies in the strange world of Quantum Electro dynamics. (QED). The vacuum is actually a see of particles which exist for a fraction of a time and go out of existence. The net result is that nothing seem to exist. But particle are constantly getting created and destroyed before you blink your eye. They can exist only a fraction of time allowed by what is known as the Uncertainty principle.
Such particles are what are called virtual particles.
But an electron can excite a virtual photon into existence by virtue of its charge so to speak.
But even this can exist only during the time of interaction. Before it vanishes it interacts with the other electron and goes into thin smoke.
We don't see the photon we see only the repulsion.
This is how quarks too interact inside a nucleus they exchange gluons instead of photons. The theory is called Quantum Chromo Dynamics(QCD).
This year's Nobel prize for Physics goes to the pioneers of this theory see reference(2) below.
Next we will discuss more about how the interactions are unified in the standard model.
References
1) Particle physics General - http://particleadventure.org
2) Nobel Prize in Physics 2008 -- http://physicsandphysicists.blogspot.com/
Tuesday, October 7, 2008
Thursday, September 25, 2008
Standard Model -Elementary Particles
Every day we hear about the LHC experiment in the news.
Physicists talk strange language when it comes to describing their work which sounds like greek for most people. But Steven Weinberg one of the most important particle physicist of the 20th century had often proclaimed that most physics concepts can be explained in simple terms.
In order to completely comprehend what is going on at the LHC(Large Hadron Collider), the biggest scientific experiment of mankind ever, you need to understand many concepts in physics.
From the wildest ideas like SUSY (Supersymmetry) to the darkest matter in the universe, from the higgs bosons to black holes, stranglets to antimatter and so on.
What are these people talking about?.
Let us discuss Higgs boson first. Since we have lot of ground to cover we will take it in steps.
Physicists build models of the universe. A model is nothing but a possible blueprint of reality. From a model comes predictions. Predictions need to be verified. If a model cannot produce predictions that can be verified, it is not worth its money. It would be promptly thrown away.
But when a model survives the acid test of experimental verifications, it becomes accepted and the model leads to further refinements and so on.
What science believes is that , the closer it is to reality better is its chance for survival from experimental verifications . Science is an asymptotic progression towards reality .
Basic questions about the universe are no longer a passtime but are part of serious investigations.
As far as the Higgs boson is concerned, it is one of the predictions of the Standard model.
Now what is the Standard Model?.
It is the Model that Physicists have built over a period of time for understanding what matter is made up of and how different things interact.
If you look at ancient civilizations, you can observe that they all had a concept about some fundamental entities out of which all matter is made up. For e.g. in the greek and indian philosophies it was postulated that the whole world is made up of primary elements such as earth, air, fire, and water.
From there on science advanced, but it was only in the 1900s that physicists came up with the idea of the atom as the indivisable unit that constitues matter. It was speculated the entire universe could be thought of as made up of only 92 different kinds of atoms.
This can be called the Atomic Model .
Based on this model came predictions which were verified again and again.
All the physical and chemical properties of substances were the result of atomic configuration of that substance.
Each atom has an atomic number. For e.g. Hydrogen is the first atom and it has an atomic number 1 and Uranium is 92nd atom.
Atoms were arranged nicely in a table called the Periodic table. Why the name Periodic table?.
This is because chemical properties of atoms exhibit a regualarity.
Now this lead many people think that may be the atom is not fundamental after all. Maybe there are more fundamental elements out of which the atom is made up and different combinations of these elements lead to different atoms. So instead of 92 different kinds of atoms, physicsts started dealing with only 3 different entities
1) Electrons
2) Protons
3) Neutrons
The protons and neutrons make up the central part of the atom called the Nucleus and electrons move around the nucleus and are arranged in fuzzy orbits.
All the different atoms are made up of different set and configuration of these entities.
Thus the Hydrogen atom has only a single electron and a proton while
the uranium has 92 electrons , 92 protons and 146 neutrons
Isnt it amazing to think that the whole world is just made up of 3 basic elements?.
However this cozy picture of the universe was shattered when cosmic rays were discovered.
Cosmic rays are showers of different particles bombarding the earth's atmosphere.
Cosmic rays are the Natural Particle accelerators!!!.
From a simplistic picture there was a profusion of elementary particles.
there were pi mesons, K particles, omega hperons and so on.
If there is a multitude of entities, the first thing scientists do is to classify them into nice groups.
Classification can be done in many ways in the case of particles.
If you classify them in terms of mass. There are heavy particles
called Hadrons, medium wieght particles called Mesons and light weight particles called Leptons.
A simple example of Hadron is the proton and that of lepton is the electron.
It was soon found that there were lot of hadrons and mesons while the Leptons were few in number. So maybe hadrons and mesons are not fundamental after all?.
It was postulated that the Hardons and mesons were thought of as consisting of simpler entities called Quarks.
Thus for example a Proton has a structure
p -- uud ( 2 up quarks and a down quark)
while the neutron ( 2 down quarks and a down quarks)
n -- udd
So the only difference between a proton and a neutron is that in a neutron a down quark is there for an up quark.
So in the 1960s physicists discovered that the whole zoo of particles( hundreds of them) could be thought of as consisting of simply 2 sets of particles
1)Quarks
2)Leptons
Quarks are of six types ( up, down, charm, strange, top and bottom)
Leptons are also of six types ( electron and its neutrino , muon and its neutrino , tau and its neutrino).
Since most atoms are made up of neutrons, protons and electrons we can see that they are made up of the up quark , down quark and the electron .
The other zoo of particles require different combinations of the other quarks but they occur very infrequently in the universe.
So most of the universe can be constructed out of
u quark
d quark and
electron.
In addition to this there is also the conept of antimatter. Antimatter is the mirror image of matter. This is also something that came out of a model and it was experimentally observed.
Thus the antimatter counterpart of the electron is called the anti electron or positron , the up quark has an anti up qaurk and so on.
From what we have said so far you can easily find out that the anti proton would be made up of
anti u , anti u and anti d.
When matter and anti matter meet they annihilate into pure energy.
It has been repeatedly shown that this picture is accurate to a large extent and can explain many phenomena in the Universe. You cannot break these particles to smaller entities even at very high energies found in Ultra big Particle Accelerators like the LHC.
Having understood about the fundamental particles , Let us talk about their interactions in the next dose.
Physicists talk strange language when it comes to describing their work which sounds like greek for most people. But Steven Weinberg one of the most important particle physicist of the 20th century had often proclaimed that most physics concepts can be explained in simple terms.
In order to completely comprehend what is going on at the LHC(Large Hadron Collider), the biggest scientific experiment of mankind ever, you need to understand many concepts in physics.
From the wildest ideas like SUSY (Supersymmetry) to the darkest matter in the universe, from the higgs bosons to black holes, stranglets to antimatter and so on.
What are these people talking about?.
Let us discuss Higgs boson first. Since we have lot of ground to cover we will take it in steps.
Physicists build models of the universe. A model is nothing but a possible blueprint of reality. From a model comes predictions. Predictions need to be verified. If a model cannot produce predictions that can be verified, it is not worth its money. It would be promptly thrown away.
But when a model survives the acid test of experimental verifications, it becomes accepted and the model leads to further refinements and so on.
What science believes is that , the closer it is to reality better is its chance for survival from experimental verifications . Science is an asymptotic progression towards reality .
Basic questions about the universe are no longer a passtime but are part of serious investigations.
As far as the Higgs boson is concerned, it is one of the predictions of the Standard model.
Now what is the Standard Model?.
It is the Model that Physicists have built over a period of time for understanding what matter is made up of and how different things interact.
If you look at ancient civilizations, you can observe that they all had a concept about some fundamental entities out of which all matter is made up. For e.g. in the greek and indian philosophies it was postulated that the whole world is made up of primary elements such as earth, air, fire, and water.
From there on science advanced, but it was only in the 1900s that physicists came up with the idea of the atom as the indivisable unit that constitues matter. It was speculated the entire universe could be thought of as made up of only 92 different kinds of atoms.
This can be called the Atomic Model .
Based on this model came predictions which were verified again and again.
All the physical and chemical properties of substances were the result of atomic configuration of that substance.
Each atom has an atomic number. For e.g. Hydrogen is the first atom and it has an atomic number 1 and Uranium is 92nd atom.
Atoms were arranged nicely in a table called the Periodic table. Why the name Periodic table?.
This is because chemical properties of atoms exhibit a regualarity.
Now this lead many people think that may be the atom is not fundamental after all. Maybe there are more fundamental elements out of which the atom is made up and different combinations of these elements lead to different atoms. So instead of 92 different kinds of atoms, physicsts started dealing with only 3 different entities
1) Electrons
2) Protons
3) Neutrons
The protons and neutrons make up the central part of the atom called the Nucleus and electrons move around the nucleus and are arranged in fuzzy orbits.
All the different atoms are made up of different set and configuration of these entities.
Thus the Hydrogen atom has only a single electron and a proton while
the uranium has 92 electrons , 92 protons and 146 neutrons
Isnt it amazing to think that the whole world is just made up of 3 basic elements?.
However this cozy picture of the universe was shattered when cosmic rays were discovered.
Cosmic rays are showers of different particles bombarding the earth's atmosphere.
Cosmic rays are the Natural Particle accelerators!!!.
From a simplistic picture there was a profusion of elementary particles.
there were pi mesons, K particles, omega hperons and so on.
If there is a multitude of entities, the first thing scientists do is to classify them into nice groups.
Classification can be done in many ways in the case of particles.
If you classify them in terms of mass. There are heavy particles
called Hadrons, medium wieght particles called Mesons and light weight particles called Leptons.
A simple example of Hadron is the proton and that of lepton is the electron.
It was soon found that there were lot of hadrons and mesons while the Leptons were few in number. So maybe hadrons and mesons are not fundamental after all?.
It was postulated that the Hardons and mesons were thought of as consisting of simpler entities called Quarks.
Thus for example a Proton has a structure
p -- uud ( 2 up quarks and a down quark)
while the neutron ( 2 down quarks and a down quarks)
n -- udd
So the only difference between a proton and a neutron is that in a neutron a down quark is there for an up quark.
So in the 1960s physicists discovered that the whole zoo of particles( hundreds of them) could be thought of as consisting of simply 2 sets of particles
1)Quarks
2)Leptons
Quarks are of six types ( up, down, charm, strange, top and bottom)
Leptons are also of six types ( electron and its neutrino , muon and its neutrino , tau and its neutrino).
Since most atoms are made up of neutrons, protons and electrons we can see that they are made up of the up quark , down quark and the electron .
The other zoo of particles require different combinations of the other quarks but they occur very infrequently in the universe.
So most of the universe can be constructed out of
u quark
d quark and
electron.
In addition to this there is also the conept of antimatter. Antimatter is the mirror image of matter. This is also something that came out of a model and it was experimentally observed.
Thus the antimatter counterpart of the electron is called the anti electron or positron , the up quark has an anti up qaurk and so on.
From what we have said so far you can easily find out that the anti proton would be made up of
anti u , anti u and anti d.
When matter and anti matter meet they annihilate into pure energy.
It has been repeatedly shown that this picture is accurate to a large extent and can explain many phenomena in the Universe. You cannot break these particles to smaller entities even at very high energies found in Ultra big Particle Accelerators like the LHC.
Having understood about the fundamental particles , Let us talk about their interactions in the next dose.
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