Standard Model - The Four Interactions

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/