Fundamental Forces
In the previous post I talked about matter. The next question is, how does matter interact with other matter? Everywhere we see around us, things are in motion and are changing. As I type this sentence, my fingers are moving on the keyboard, the keys are getting pressed, in turn they are sending signals to the CPU which in turn is processing them and sending them to the OS running on this laptop and so on. The movement of the fingers themselves is happening because the muscles in my hands are making them move. The muscles in turn are responding to nervous signals being sent by my brain. Even the simplest interaction in the world can be broken down into smaller and smaller interactions. So just as we dissected matter and arrived at the elementary particles constituting matter, we can keep breaking up all interactions till we arrive at an interaction which cannot be further broken up.
It turns out that all matter and anti-matter interactions in the Universe can be reduced to a combination of four fundamental interactions (or forces) of nature:
1) Gravity: The force that holds the stars together, holds the earth to the sun, holds the moon to the earth, keeps our feet on the ground and pulls us down when we jump and makes an apple fall to the ground from a tree. This is the weakest force, but has infinite range. Since all mass exerts gravity, the net effect of gravity is cumulative and it becomes the most important force over large scale. The force of gravity falls in an "inverse square" relationship with the distance of two masses. This means that if at distance 1 m, the force of gravity is x, then at a distance of 3 m, the force of gravity would be x /9.
2) Electromagnetic Force: The force that drives all electronic equipments, the force in the magnet, the force which sends signals throughout our body and brain. Light is a form of electromagnetic force. Like Gravity, Electromagnetism has infinite range, but is much stronger - 72 orders of magnitude stronger - than Gravity. This means that if we take the strength of the force of Gravity to be 1, the strength of Electromagnetism would be 1072 or 1 followed by 72 zeroes. To give you an idea of the scale, a million has 6 zeroes, a billion has 9 zeroes and a trillion has 12 zeroes. So Electromagnetism is a trillion trillion trillion trillion trillion trillion times stronger than Gravity. Like Gravity Electromagnetic force also follows an inverse square relationship with distance.
However, despite being so strong and despite having an infinite range, electromagnetism does not have an impact on large scales. The reason is two-fold: One, unlike gravity which is a property of all matter, it is only charged particles that have an electromagnetic force. Second, while gravity is always cumulative, charges cancel each other out and therefore electromagnetic force also cancels out. At very large scales it is fair to assume that half the particles are positive and half are negative and hence the net force is zero.
3) Strong Nuclear Force: The force that holds the nucleus of an atom together. Without this force, the positively charged protons will repel each other and nuclei would rip apart. This force is the strongest in nature - it is a hundred times stronger than Electromagnetic force and 1074 times stronger than Gravity. However, it has a very small range - it operates only within a range of 10-15 meters (1015 th part of a meter, or a millionth of a billionth of a meter).
4) Weak Nuclear Force: This force also operates at nuclear levels and is responsible for radioactivity. This force is quite weak as compared to Electromagnetic Force and Strong Force, but much stronger than Gravity - it is actually 1061 times as strong as Gravity. However its range is the smallest - it operates only within a range of 10-18 meters.
Force Carrier Particles
Now that we know about Fundamental Forces, we can define the fundamental particles of matter in terms of fundamental forces. Quarks interact using all four of the Fundamental forces. Leptons do not interact using the Strong Nuclear forces. The question is how do the Quarks and Leptons interact using these Fundamental Forces? The answer is that there are other kind of particles which are called Force Carrier Particles. Matter particles exchange these particles as carriers of forces - you can think of these particles as messengers of matter particles. Each Fundamental Force is carried by a unique particle:
1) Electromagnetic Force is carried by a particle called Photon - a massless particle. Since light is electromagnetic, you can think of light as being made of massless particles called Photons. In fact that is where Photons get their name from.
2) Strong Nuclear Force is carried by particles called Gluons - there are eight types of gluons, all massless.
3) Weak Nuclear Force is carried by particles called W and Z Bosons - they are massively heavy, a hundred times as heavy as a proton.
4) Gravity is hypothesized to be carried by a particle called Graviton. This particle has not been detected yet, but scientists think it should be massless.
Notice that both Electromagnetism and Gravity are carried by massless particles and have infinite range. Weak Nuclear force uses massive particles and hence is very short ranged. Strong Nuclear force can also work with a massless particle since it has a small range, but then the force is very strong.
All massless particles move at the speed of light in vacuum - the fastest speed possible in nature - around 3 x 106 kilometers / second. (That means 3 followed by 6 zeroes or 3 million kilometers / second) Hence, Electromagnetism, Gravity and the Strong Nuclear Force are also carried at the speed of light.
Particle Types
As if the number of particles you have already read of are not enough, I am going to introduce a few more :) Also we will now classify them into a system so that we have a complete picture.
1) Fermions and Bosons: All particles are either of of type Fermion or of type Boson. No two Fermions can occupy the same space at the same time. All matter falls in this category. Hence all Quarks and Leptons are Fermions. Bosons are different - multiple Bosons with the same amount of energy can occupy the same point in space. All Force carrier particles are Bosons.
2) Hadrons: Hadrons are composite particles which can interact thru the Strong Nuclear Force. All hadrons are composed of Quarks. All Hadrons are therefore matter particles.
3) Baryons: Baryons are Hadrons made of three quarks - neutrons and protons consist of three quarks each and hence are called Baryons.
4) Mesons: Mesons are Hadrons made of a Quark and an Anti-Quark
There are more particles types but we are going to ignore those right now.
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