The usage of four-vectors (vectors in Minkowski space) can simplify a number of problems where special relativity must be considered. For an example, we will consider the phenomenon of Compton scattering. Note, for the rest of this I will use the (+,-,-,-) signature; namely that the scalar product of four vectors is .

The energy-momentum 4-vector, also known as the 4-momentum, or sometimes by the (IMO atrocious) portmanteau “momenergy”), is the four-vector extension of momentum, the time component corresponding to energy. Specifically, I will use the convention

(as seen here). In this form, we see

;

thus the (invariant) length of the four-vector is simply the rest energy of the object.

Now, in Compton scattering, a photon of wavelenth *λ* collides with an electron at rest in our laboratory frame, and is scattered by an angle *θ* from its original direction, with new wavelength *λ’*. We desire to derive a formula for *λ’* in terms of *λ* and *θ*.

The key is to apply the conservation of energy and momentum, which the use of 4-vectors makes simple: the combined energy-momentum 4-vector must be the same both before and after the collision. We choose our spatial coordinates so that the photon is initially moving in the *x* direction, and scattering is in the *xy* plane, with *θ* in the usual direction in that plane.

As discussed here, the energy and momentum of a photon are given by , and , giving us an energy-momentum 4-vector for our photon pre-collision of .

And letting *m* be the rest mass of an electron, then our electron has pre-collision energy-momentum 4-vector .

After collision, we have new energy-momentum 4-vectors and . From our energy and momentum relations for photons, along with the scattering angle *θ*, we have post-collision that .

Now, our conservation of energy and momentum is

.

Solving for the post-collision 4-momentum of the electron, we have

.

Taking the norm square of both sides, we see

.

But since the norm squared of a four-vector is its scalar product with itself, and the scalar product is a bilinear form, the right hand side of the above is

, and so

. But we recall that the length of an energy-momentum 4-vector is the rest energy; thus , as photons have no rest mass, and

. Thus we need only find the scalar products , , and . As for the first two, since all the spatial components of are zero, and . For the last scalar product,

.

Plugging in these, we see

.

Multiplying both sides by *λλ’*, we get

,

and then dividing both sides by 2*hmc*^{3} gives

,

where the quantity is the Compton wavelength of the electron.

Note that our use of the 4-momentum meant not only could we combine energy and momentum conservation into a single equation, by taking the norm square as we did, we eliminated the need to consider the individual components of the post-collision 4-momentum of the electron.

Tags: Conservation of Energy, Conservation of Momentum, Four-Vector, Friday Physics, physics, Relativity

February 12, 2010 at 12:06 am |

[…] Friday 108 By twistedone151 Last week, I illustrated an example of momentum and energy conservation under special relativity, with the […]