## Fundamental particles and forces

The most common force of nature is
**gravitational force**!

This law basically states that two objects in universe attract each other at a force defined by this universal law of gravitation equation:

F = G*m1*m2/ d^2

Where, G = 6.673 x 10^{-11} Nm^{2 }kg^{-2}

m_{1}
= mass of first object (kg)

m_{2}
= mass of second object (kg)

d = distance between two objects (m)

F = gravitational attraction force (N)

This equation is applicable to objects which are small (few mm) to infinite distance apart.

It pulls matter together, causes you to have a weight, apples to fall from trees, keeps the Moon in its orbit around the Earth, the planets confined in their orbits around the Sun, and binds together galaxies in clusters.

Classical mechanics (or Newtonian physics) laws are applicable to cases where size of particles are larger than atomic level and speed of travel is much lower than that of light.

If you are looking at physics of
particles at atomic level, then you need quantum theory. If you are
approaching near speed of light, you
need Einstein’s **theory of relativity**.

Example, two cars are coming towards each other at a speed of 30 km/h each their relative speed is 30 + 30 = 60 km/h.

However, if two spaceships are travelling at 270,000 km/s each their relative speed will be found using following formula

u = (v + w)/(1 + v*w/c^2)

Where c is velocity of light, v and w are speeds of two spaceships measured by
a 3^{rd} observer. Note that for low values of v & w, the above
equation becomes u = v + w.

Atom = nucleus ( = protons + neutrons) + electrons

Electrons have -ve charge while protons have +ve charge.

Since particles with same charge must repulse from each other, there must be a
force which holds same +ve charged protons together at nucleus. This force is
known as **strong nuclear** force. This force acts only in a atomic
range (10^{-15} m).

Because the strong force binds nuclear particles so tightly together, huge amounts of energy are released when lightweight nuclei are fused together (fusion reaction) or heavy nuclei are broken apart (fission reaction).. The strong nuclear force interaction is the underlying source of the vast quantities of energy that are liberated by the nuclear reactions that power the stars.

The force for which opposite charged particles (eg. protons & electrons)
repels from each other, is known as **electro magnetic force**.

Which is defined by Coulomb’s equation

F = k1 * k2 / d^2

Electrons are attached to nucleus by this force – thus it holds atoms and molecules together! Like gravitational force, electromagnetic force is applicable to an infinite range.

The electromagnetic force binds [negatively charged] electrons into their orbital shells, around the positively charged nucleus of an atom. This force holds the atoms together.

The electromagnetic force controls the behavior of charged particles and
plasmas (a plasma is a mixture of equal numbers of positive ions and negative
electrons) as, for example, in solar prominences, coronal loops, flares, and
other kinds of solar activity.

The electromagnetic force also governs the emission and absorption of light
and other forms of electromagnetic radiation.
Light is emitted when a charged particle is accelerated (for example, when an
electron passes close to an ion, or interacts with a magnetic field) or when
an electron drops down from a higher to a lower energy level of an atom (from
an outer to an inner 'orbit' around the atomic nucleus).

The **weak nuclear force** causes the radioactive decay of certain
particular atomic nuclei. In particular, this force governs the process called
*beta decay* whereby a neutron breaks up spontaneously into a proton, and
electron and an antineutrino. If a neutron within an atomic nucleus decays in
this way, the nucleus emits an electron (otherwise known as a *beta particle*)
and the neutron transforms into a proton. This increases (by one) the number
of protons in that nucleus, thereby changing its atomic number and
transforming it into the nucleus of a different chemical element. This force
is applicable to atomic (10^{-17} m) range only.

The weak force is responsible for synthesizing different chemical elements in stars and in supernova explosions, through processes involving the capture and decay of neutrons.

When confined within a stable (non-radioactive) atomic nucleus, a neutron is a
stable, long-lived particle. Once removed from an atomic nucleus, a free
neutron will undergo beta decay, typically in about twenty minutes. The
reverse process of beta decay occurs in the collapsing cores of supernovae,
where protons and neutrons are fused together to create the vast numbers of
neutrons that populate the end product of the collapse - a *neutron star*.

It has been proved recently that electromagnetic force and weak nuclear force
are basically same force – called **electro weak force**.

Before we move on, there are some other theories we need to learn. One among that is

**quantum mechanics**. The key idea here is that the smaller the
scale at which you look at the world, the more random things become. **
Heisenberg's uncertainty principle** is a famous example of this. The
principle states that when you consider a moving particle, for example an
electron orbiting the nucleus of an atom, you can never ever measure both its
position and its momentum as accurately as you like. Looking at space at a
minuscule scale may allow you to measure position with a lot of accuracy, but
there won't be much you can say about momentum. This isn't because your
measuring instruments are imprecise. There simply isn't a "true" value of
momentum, but a whole range of values that the momentum can take, each with a
certain probability. In short, there is randomness. This randomness appears
when we look at particles at a small enough scale. The smaller one looks, the
more random things become!

A major difference between quantum mechanics and classical mechanics is that law of physics depends on sizes of particles in very small scale dimensions. At very small scale (atomic level) the classical laws of physics no longer work. That is the place where quantum mechanics laws will apply.

## The theory of everything

We now aim to define earlier fundamental forces by a single theory! That’s
where **string theory** comes.

Theory of everything = to combine, quantum mechanics + relativity + classical mechanics

Classical physics assumes that a particle is the smallest existence. String theory asserts that the fundamental building blocks of nature are not like points, but like strings: they have extension, in other words they have length. And that length dictates the smallest scale at which we can see the world. The estimated size of these strings is 10-34 m.

The mathematics behind string theory is very complex! Our conventional physics so far assumed only 4 dimensions – namely length, width, height and time. But string theory, in its various forms, assume dimension of 10, 11 or even 26. However, it is stated that most dimensions actually exists only in atomic range distance.

Strings can vibrate. In fact they can vibrate in an infinite number of different ways. This suggests that the different particles and forces are just the fundamental strings vibrating in a multitude of different ways.

In a nutshell, the string theory gives us an exciting vision of nature as miniscule bits of vibrating strings in a space with hidden curled-up dimensions.