Class 11 Recent Trends Notes

Unit 5- Morden Physics

Chapter 26 – Recent Trends

What is the world made of? What are the most fundamental constituents of matter? Philosophers and scientists have been asking these questions for at least 2500 years. We still don’t have anything that could be called a final answer, but we’ve come a long way. This final chapter is a progress report on where we are now and where we hope to go.

Particle physics is the study of fundamental particles and their interactions in nature. Those who study elementary particle physics—the particle physicist—differ from other physicists in the
scale of the systems that they study. A particle physicist is not content to study the microscopic world of cells, molecules, atoms, or even atomic nuclei. They are interested in physical processes that occur at scales even smaller than atomic nuclei.

Fundamental Particles—A History

The idea that the world is made of fundamental particles has a long history. In about 400 B . C . the Greek philosophers Democritus and Leucippus suggested that matter is made of indivisible particles that they called atoms, a word derived from a- (not) and tomos (cut or divided). This idea lay dormant until about 1804, when the English scientist John Dalton (1766–1844), often called the father of modern chemistry, discovered that many chemical phenomena could be
explained if atoms of each element are the basic, indivisible building blocks of matter.

Particle Physics

Electron:

It is first fundamental particle discovered by J.J thompson in 1897. It has spin $\frac{1}{2}$.Electron plays direct role in all physical phenomenon. It is denoted by $-1e^{–}.$
$Charge of electron=–1.6\times {10}^{–19}$
$Mass of electron =9.1 \times {10}^{–31}kg$
$Specific charge\left(\frac{ e}{m}\right)=1.76 \times {10}^{11}Ck{g}^{–1}$

Positron(anti-electron):

It was discovered by the American physicist Carl D. Anderson in 1932, during an investigation of particles bombarding the earth from space. It is also known as positive electron. Its mass is same as electron but chage is opposite.It is denoted by +1^e+ or simply e⁺.

Proton:

It was discovered by Rutherford in 1911. It has a positive charge $+1.6\times {10}^{–19}C$equal to the electronic charge and its mass $(1.67\times {10}^{–27}Kg)$is 1837 times the electronic mass. In free state, the proton is stable particle. Its symbol is P. it is also written as ${}_1{}^{1}H$.

Anti-proton:

The antiparticle of proton is known as antiproton. Its charge and mass are same as those of proton, the only difference is that it is negatively charged. It was discovered in 1955. Its symbol is $\stackrel{\_}{P}$.

The Photon:
Einstein explained the photoelectric effect in 1905 by assuming that the energy of electromagnetic waves is quantised; that is, it comes in little bundles called photons with energy E = hf . Atoms and nuclei can emit (create) and absorb (destroy) photons. Considered as
particles, photons have zero charge and zero rest mass. (Note that any discussions of a particle’s mass in this chapter will refer to its rest mass.) In particle physics, a photon is denoted by the symbol γ (the Greek letter gamma).

Neutron:

It was discovered by Chadwick in 1932. It carries no charge. Its mass is same as that of proton. In free state, Neutron is unstable its mean life is about 17 minutes.
Its symbol is n or ${}_0{}^{1}n$

Anti-neutron:

It was discovered in 1956. It has no charge and its mass is equal to the mass of neutron. The only difference between neutron and antineutron is that if they spin in the same direction, their magnetic moment will be in opposite directions.
Its symbol is $\stackrel{\_}{n }$

Neutrino and anti-neutrino:
The existence of these particles was predicted in 1930 by Pauli while explaining the emission of beta particles from radioactive nuclei, but they were observed experimentally in 1956. Their rest mass and charges are both zero but they have energy and momentum. Both neutrino and antineutrino are stable particles. The only difference between them is that their spins are in opposite directions. Their symbols are ν and ̄ν respectively.

Fermions and Bosons:
Particles of matter can be divided into fermions and bosons. Fermions have half-integral spin and bosons have integral spin. Familiar examples of fermions are electrons, protons, and neutrons.
A familiar example of a boson is a photon. Fermions and bosons behave very differently in groups. For example, when electrons are confined to a small region of space, Pauli’s exclusion principle states that no two electrons can occupy the same quantum-mechanical
state. However, when photons are confined to a small region of space, there is no such limitation.

Particles and Antiparticles:

There are lots of pair of particles which have same mass but have some other properties exactly opposite. Such pair of particles are called as antiparticle of each other. For example, the electron and positron are said to be antiparticles as they have same mass and spin but opposite charge. When they come in contact, they combine with each other and emit two photons sharing equal energy.

The table of particle and antiparticle is given below

Figure: Particles and antiparticles

Classification of Elementary Particles:

Elementary particles can be classified on the basis of different properties of particles. They can be
classified on the basis of mass (massless, light, intermediate, and heavy), charge (positive, negative, and neutral) spin or statistics (Bosons and fermions), Interaction (Gravitational, strong, weak and electromagnetic).

On the basis of their masses they can be classified as follows

Leptons:
These are the light elementary particles having low mass. These
particles are electron (e^–), electro-neutrino (ν_e ), muon (μ^–), muon neutrino (ν_μ), tau (τ^–), tau-neutrino (ν_τ ). All the leptons have their corresponding antiparticles.

Hadrons:
Hadrons are strongly interacting particles. This family includes heavy particles.

There are two subclasses of hadrons:

1. Mesons
2. Baryons.

Mesons:
these are the elementary particles of intermediate masses. They are heavier than leptons and lighter than baryons. Mesons include pions
(π⁺, π⁰ , π^–), kaons (k⁺, k⁰ , k ^–) and Eta(η).

Baryons:
The elementary particles having mass equal to proton or heavier than proton are called baryons. They are also classified into two groups: nucleons and hyperons. The proton and neutron are called nucleons.
The baryons of mass greater than neutron and less than deuteron are called hyperons. Lambda (λ), sigma(σ),Xi(Ξ) and omega (Ω) are example of hyperons.

Four Fundamental Forces
An important step to answering these questions is to understand particles and their interactions. Particle interactions are expressed in terms of four fundamental forces. In order of decreasing strength, these forces are the strong nuclear force, the electromagnetic force the weak nuclear force, and the gravitational force.

1. Strong nuclear force :. The strong nuclear force is a very strong attractive force that acts only over very short distances (about 10^-15m). The strong nuclear force is responsible for binding protons and neutrons together in atomic nuclei. Not all particles participate in the strong nuclear force; for instance, electrons and neutrinos are not affected by it. As the name suggests, this force is much stronger than the other forces. The exchange particle in this interaction is mesons or glueon.
2. Electromagnetic force: The electromagnetic force can act over very large distances (it has an infinite range) but is only 1/100 the strength of the strong nuclear force. Particles that interact through this force are said to have “charge.” In the classical theory of static electricity (Coulomb’s law), the electric force varies as the product of the charges of the interacting particles, and as the inverse square of the distances between them. The exchangeparticle in this interaction is photon.
3. Weak nuclear force: The weak nuclear force acts over very short distances (10^-17m )and, as its name suggest, is very weak. It is roughly 10^-9 the strength of the strong nuclear force. This force is manifested most notably in decays of elementary particles and neutrino interactions. For example, the neutron can decay to a proton, electron,and electron neutrino through the weak force. The weak is responsible for β decay. The exchange particle in this interaction is W-boson or z- bosons.
4. Gravitational force: Like the electromagnetic force, the gravitational force can act over
   infinitely large distances; however, it is only 10^-38 as strong as the strong nuclear force. In Newton’s classical theory of gravity, the force of gravity varies as the product of the masses of the interacting particles and as the inverse square of the distance between them.
   This force is an attractive force that acts between all particles with mass. The exchange particle in this interaction is graviton.

Quark model

In the 1960s, particle physicists began to realize that hadrons are not elementary particles but are made of particles called quarks. (The name ‘quark’ was coined by the physicist Murray Gell-Mann) Initially, it was believed there were only three types of quarks, called up (u), down (d), and strange (s). However, this number soon grew to six—interestingly, the same as the number of leptons to include charmed (c), bottom (b), and top (t).

All quarks are spin-half fermions (s = 1/2 ), have a fractional charge( 1/3 or 2 /3 e) , and have baryon number B = 1/3 . Each quark has an antiquark with the same mass but opposite charge and baryon number.

Composition of Hadrons by quark

Hadrons, may be baryon or mesons are composed of quark. Baryons are formed from three quarks and mesons are formed by one quark and one antiquark. For example, Proton is formed by 2 up quark and one down quark.

Mesons are formed by two quarks—a quark-antiquark pair. Sample mesons, including quark content and properties, are given in Table below. Consider the formation of the pion $({\Pi }^{+}=u\overline{d} )$ Based on its quark content, the charge of the pion is $\frac{2}{3}e+\frac{1}{3}e=e.$

Standard model

The particles and interactions that we’ve discussed in this chapter provide a reasonably comprehensive picture of the fundamental building blocks of nature. There is enough confidence in the basic correctness of this picture that it is called the standard model.

Cosmology

Cosmology is a branch of physics that involves the origin and evolution of the universe, from the Big Bang to today and on into the future.

Cosmic rays

Cosmic rays are extremely high-energy subatomic particles – mostly protons and atomic nuclei accompanied by electromagnetic emissions – that move through space, eventually bombarding the Earth’s surface. They travel at nearly the speed of light, which is approximately 300 000 kilometres per second.

Cosmic rays are of two kinds:

1. Galactic
2. Solar

White dwarf

White dwarfs are the hot, dense remnants of long-dead stars. They are the stellar cores left behind after a star has exhausted its fuel supply and blown its bulk of gas and dust into space. These exotic objects mark the final stage of evolution for most stars in the universe – including our sun – and light the way to a deeper understanding of cosmic history..

A single white dwarf contains roughly the mass of our sun in a volume no bigger than our planet. Their small size makes white dwarfs difficult to find. No white dwarfs can be seen with the unaided eye. The light they generate comes from the slow, steady release of prodigious amounts of energy stored up after billions of years spent as a star’s nuclear powerhouse.

Neutron star

Neutron stars are formed when a massive star runs out of fuel and collapses. The very central region of the star – the core – collapses, crushing together every proton and electron into a neutron. If the core of the collapsing star is between about 1 and 3 solar masses, these newly-created neutrons can stop the collapse, leaving behind a neutron star. (Stars with higher masses will continue to collapse into stellar-mass black holes.)

Black Hole

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.
Because no light can get out, people can’t see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

Schwarzschild radius

The Schwarzschild radius is the radius of the event horizonv surrounding a non-rotating black hole. Any object with a physical radius smaller than its Schwarzschild radius will be a black hole. This quantity was first derived by Karl Schwarzschild in 1916:
$R_s =\frac{2GM}{C^2}$

Red shifts

Redshift, displacement of the spectrum of an astronomical object toward longer (red) wavelengths. It is attributed to the Doppler effect, a change in wavelength that results when a given source of waves (e.g., light or radio waves) and an observer are in motion with respect to each other
.${\lambda }_0= {\lambda }_s\sqrt{\frac{c+v}{c–v}}$

Hubble’s Law

Analysis of redshifts from many distant galaxies led Edwin Hubble to a remarkable conclusion: The speed of recession v of galaxy is proportional to the distance from the Earth. This relation is called Hubble’s law; expressed as an equation.

v = HR …………………………(1)

Where H is an experimental quantity commonly called the Hubble’s constant. The current best value is 2.3 × 10⁻¹⁸s⁻¹.

The Big Bang

The Hubble law suggests that at some time in the past, all the matter in the universe was more concentrated than it is today. It was then blown apart in an immense explosion called Big Bang. When did this happen? According to the Hubble’s law, matter at a distance r away from us is travelling with speed v = HR
The time t needed to travel a distance R is
$t=\frac{R}{v}=\frac{R}{HR}=\frac{1}{H}=4.3 \times {10}^{17}s$
By this hypothesis the Big Bang occured about 14 billion years ago.

Critical Density

The total energy E(kinetic plus potential) for our galaxy is
$E=\frac{1}{2}m{v}^2-\frac{GMm}{R}$
At critical density E=0, so that $\frac{1}{2}m{v}^2=\frac{GMm}{R}$
The total mass M inside the sphere is $\frac{4\pi R^3}{3} times density {\rho }_c$
We know from Hubble’s law:
v = HR
Substituting these expression for m and v in equation 2, we get
$\frac{1}{2}\left(HR\right){}^2=\frac{G}{R}\frac{4}{3}\pi R{}^3{\rho }_c$
${\rho }_c =\frac{3H^2}{8\pi G}$
This is critical density.If the average density is less than ρc , the universe should continue to expand indefinetely; if it is greather, the universe should eventually stop expanding and began to contract. The minimum density required to prevent expansion of the universe is called critical density.

Dark matter

The visible universe—including Earth, the sun, other stars, and galaxies—is made of protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, or baryonic, matter makes up less than 5 percent of the mass of the universe.
The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent).

Dark Energy

Dark Energy is a hypothetical form of energy that exerts a negative, repulsive pressure, behaving like the opposite of gravity. It has been hypothesised to account for the observational properties of distant type Ia supernovae, which show the universe going through an accelerated period of expansion. Like Dark Matter, Dark Energy is not directly observed, but rather inferred from observations of gravitational interactions between astronomical object.

Gravitational waves

Gravitational waves are ’ripples’ in space-time caused by some of the most violent and energetic processes in the Universe. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. Einstein’s mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that ’waves’ of undulating space-time would propagate in all directions away from
the source. These cosmic ripples would travel at the speed of light, carrying with them information about their origins, as well as clues to the nature of gravity itself.

The strongest gravitational waves are produced by cataclysmic events such as colliding black holes, supernovae (massive stars exploding at the end of their lifetimes), and colliding neutron stars. Other waves are predicted to be caused by the rotation of neutron stars that are not perfect spheres, and possibly even the remnants of gravitational.