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Four-vectors and conservation laws in relativity

This lecture was delivered to the final year honors class of 3 year science degree students on 21 November 2017 as part of the Classical Dynamics paper.

In this lecture we will discuss some of the important tools of relativistic mechanics. We will discuss the idea of proper-time, 4-velocity, 4-acceleration, 4-momentum, 4-force and related conservation law of the 4-momentum.

A. Proper-time. 
The proper time is the time interval in the rest-frame of any event. The proper time is related to time-interval in other inertial frame by: tau = (1/gamma)t where gamma  > 1 always.

Gamma is the Lorentz factor or Lorentz boost factor directly related to the speed of an object in speed-of-light units, i.e. beta.

gamma = 1/sqrt{1-v^2/c^2}

Hence proper-time is the smallest possible time interval for an object in motion in among all possible inertial frames of reference and it occurs in the rest frame.

d(tau) < dt

Proper-time is necessary to define other basic quantities in theory of relativity if we are to preserve their basic meaning in terms of the non-relativistic mechanics definitions.

B. Four velocity. 
Four velocity of a particle is the rate of change of 4-displacement …

So, …  is the position vector — or space-time interval in the Minkowski  space — akin to the difference of two 3-dimensional vector in coordinate space, this time with 4 coordinates rather than 3.

The proper-time interval d(tau) is a Lorentz invariant i.e. when we move between arbitrary inertial frames of references given by the Lorentz factor beta or  gamma this interval retains its value — because it retains its form. Any variable which would retain its form under such transformation are said to be Lorentz invariant quantities.

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Relativistic Doppler effect: source is at rest in unprimed frame S and observer is at rest in the primed frame that is S' frame. Photo Credit:

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Relativistic Doppler effect

Relativistic Doppler effect. 

There is an apparent shift in the observed frequency of any electromagnetic wave (light) when there is any relative motion between the source of light and the observer. This can be easily determined by using the 4-vector formulation of theory of relativity.

Lets discuss the details of this phenomena under two situations.

A. Source is at rest and observer is in motion. 
Lets us consider two inertial frames S and S’. S’ is moving wrt S, along the x-axis with speed v = (beta) c where the observer is at rest in S’ frame but the source is at rest in the  S frame.

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Feynman Diagram of Higgs Boson Production.

This is a copy of the diagram from Wikipedia. I produced this using codes developed by me as previously instructed here with other examples. — What to do when its 2 am around here, you are fresh but nowhere to go.

Here is the code:

% Feynman diagram

% Requires PGF >= 2.0








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Creating Feynman diagrams with Latex.

I have successfully installed Cygwin, GV, GS, MikTex, Lyx etc on my win-07 Vaio. So I want to check to see my Cygwin functionality on some physics exercises.

The following link is broken now. Feyntex at Colorado.

— Somehow I could not produce this and another simple example for Feynman’s diagram using the codes here and at another place. Fortunately though I have the following which works excellent so far. So I am just learning.

My TikZ (PGF) codes are working.

So I will try to see if the codes written in Feynmf package in above links, how I can convert into Tikz ==>> and I tried and it all works nice: the diagrams are given above, compare with the same with the ones given in the above links, Feyntex at Colorado. 

I have posted the Feynman amplitude diagram of the electrons scattering off the hadrons elastically.

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