Prepared By

A.K.M. Moinul Haque Meaze

Center for High Energy Physics

Kyungpook National University

Daegu 702-701 Korea

2003-09-30

For the class of

Respected Professor Kihyeon Cho

INTERACTIONS

(Gravity & Electromagnetic)

Introduction

Interaction between particles and the large-scale behavior of matter throughout the universe, are determined by four fundamental forces---

The fundamental forces of nature determine how the particles are arranged and how they interact with one another

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Gravitational Force

• It is the weakest of all forces

• Only plays a significant role in large mass structure

• Force is postulated to transmitted by the graviton boson

• Is least understood of the fundamental forces

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2r

MmGF

For spherical symmetric body

rGF )3/4(

2

3

)3/4(r

RGF

r<R

r>R

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Graviton

The proposed quantum of gravitational radiation is called graviton.

A theoretical particle having no mass and no charge that mediates the gravitational force. The graviton is boson. The existence of a graviton has not yet been confirmed experimentally, although string theory predicts the existence of gravitons as closed strings with the minimum possible energy. It is also theorized that gravitons interact with lepton and quarks.

Detecting of graviton, if it exist, would probe rather problematic. The particle carry very little energy so detecting them would be very difficult.

Problems of Gravity

•It assumes that changes in the gravitational forces are transmitted instantaneously when positions of gravitational bodies change. However, this contradicts the fact that there exists a maximum velocity at which signals can be transmitted.

• Assumption of absolute space and time contradicts Einstein’s special theory of relativity

• It does not explain gravitational waves or black whole

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Basic Properties of Gravitational Force

•Gravitational force keeps things on the ground and stops them floating away

•The pull of gravitational force is directed towards the center of the earth

•The size of this force is the weight of the object

•The size of this gravitational force is not only dependent on their masses but also the distance between these objects

•The size of gravitational force is determined by the mass of the object and the mass of the earth and moon.

•There is gravitational force between any two objects

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The basic electromagnetic events•It’s range is infinity

•The photon is the boson that carries the force between particles

•This is responsible for all electric and magnetic phenomena we observe

•Force between stationary charges describe by Coulomb’s law

• Magnetic force arises from relative motion of charges

•Mutual repulsion of an electron/electron or positron/positron pair

•Mutual attraction of electron and positron

•The strength of the interaction is given by the Fine Structure Constant

•The quantum approach to the electromagnetic force is called QED

Examples: Photoelectric effect, Rutherford scattering, Pair production, Compton scattering

137

1~

4

2

c

e

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Photon

* Photon was first postulated by Plank

*The photon is a massless, chargeless, spin 1 boson that interact with charged particle

*The energy of the photon is

•Low energy photon-radiowaves or microwaves

•Medium energy photon-lightwaves or visible waves

•High energy photon-Xray

•Above higher energy-Gamma rays

*Photon have non zero momentum

*Photons can mediate interaction between quarks and charged leptons

kh

c

h

c

Ep

hE

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Feynman Diagrams

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Summary on Electromagnetic Interaction

•If the electromagnetic field is defined in terms of the force on a charged particle, then it is tempting to say that the field itself consists of photons which causes a force on a charged particle by being absorbed by it or simply colliding with it

•Mutual repulsion of an electron/electron or positron/positron pair

•Mutual attraction of electron and a positron

•Annihilation of an electron and positron to form a photon

•Creation of an electron positron pair from a photon

•Photon can convert into pairs of oppositely-charged, like-type leptons or quarks.

__ ,, eettbbccssdduu ,,,,, ,, bdcu (NO)

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FIFTH FORCE?!

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rmGmV /21

}1){/( /21

nrermGmV

Proposed by

Fischbach & collaborators-1986

Strength~0.01

Range~100m