Physics Made Easy

Special Relativity

Einstein’s postulates are where it all began.

  1. The laws of physics are the same in all inertial frames.
  2. Light propagates in a vacuum rectilinearly with the same speed at all times, in all directions and in all inertial frames.

Time dilation: proper time (τ) is the time measured in the rest frame of an object. The time taken in any other frame is given by
where \gamma=\frac{1}{\sqrt{1-\frac{v^{2}}{c^{2}}}}
as always.

Length contraction: the proper length (L) between two events (x1, t1) and (x2, t2) is the distance between them in the frame where t1=t2; i.e., the frame where the two events occur simultaneously. The length in any other frame is given by l=\frac{l}{\gamma}

Lorentz transformations for going from one frame (S) to another frame (S’) travelling at speed v relative to S.





or, in matrix form

\left[  \begin{array}{ c } ct' \\x' \\y'  \\z' \end{array} \right] = \left[  \begin{array}{ cccc} \gamma&-\beta\gamma&0&0 \\-\beta\gamma&\gamma&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{array} \right] \left[  \begin{array}{ c } ct \\x \\y\\z \end{array} \right]

where \beta=\frac{v}{c}

Lorentz transformations of energy and momentum





or, in matrix form

\left[ \begin{array}{ c } E' \\p_{x'} \\p_{y'} \\p_{z'} \end{array} \right] = \left[ \begin{array}{ cccc} \gamma&-\beta\gamma&0&0 \\-\beta\gamma&\gamma&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{array} \right] \left[ \begin{array}{ c } E \\p_x \\p_y\\p_z \end{array} \right]

Natural units operate on the principle that c=\hbar=1. The reason for doing this is basically because it involves less writing and makes equations look tidier- right up to the point where you need to get a meaningful number out. Then you need to look at the dimensions of the quantities you are working with and multiply by the appropriate powers of c and \hbar to get an answer that makes physical sense.

Don’t worry if natural units sound pointless and difficult to use at first- the more you work with this field of physics, the more you get used to them.

4-vectors are just vectors with four components. Below are some common 4-vectors in natural units:

Position: \underline{X} = (t, \underline{x})

Momentum: \underline{P} = (E, \underline{p})

Wavevector: \underline{K} = (\omega, \underline{k})

Velocity: \underline{V} = (1, \underline{v})

Force: \underline{F} = (\frac{\partial E}{\partial t}, \underline{f})

Magnitude of 4-vectors: when taking the dot product of a 4-vector, the fourth component has the opposite sign to the other three, e.g. |\underline{P}|^2 \equiv E^2-|\underline{p}|^2c^2. We could represent this with a factor of i in the 4-vector, but that’s just a convention that is not universally used.

Compton Scattering with 4-vectors: scattering of high energy photons off electrons.

Photon: initial 4-momentum \underline{P}=\frac{h\nu}{c}(1, \underline{\hat{n}}) where \underline{\hat{n}} is the unit vector in the direction of motion.

final 4-momentum \underline{P}'=\frac{h\nu'}{c}(1, \underline{\hat{n}'})


Electron: initial 4-momentum \underline{Q}=(m_ec,0) as it starts at rest.

final 4-momentum \underline{Q}'– we don’t know or care about this, so it will be eliminated later.


Conservation of 4-momentum



\underline{P}^2+2. \underline{P}.\underline{Q}-2\underline{P}.\underline{P}'+\underline{Q}^2-\underline{Q}.\underline{P}'+\underline{P}'^2=\underline{Q}'^2


Now let’s work out what these dot products actually are

\underline{P}.\underline{P}'=\frac{h^2\nu\nu'}{c}(1-\cos\theta) where θ is the scattering angle.



Substitute back into conservation of momentum equation




But of course, \nu\lambda=c



Where \lambda_c=\frac{h}{m_ec} is the Compton wavelength.

Invariance: the key to solving most problems in this subject is remembering that for a system of particles, |P_{tot}|^2=E_{tot}^2-p_{tot}^2c^2 is invariant.

Remember, for a single particle E^2-p^2c^2=m^2c^4, the rest mass (invariant).

For a system of particles, it just means that there is always some frame we can find where the sum of their momenta is zero (i.e. there is always a centre-of-mass frame).

TrackBack URI

Create a free website or blog at

%d bloggers like this: