23.11.2014 Views

Single-Particle Electrodynamics - Assassination Science

Single-Particle Electrodynamics - Assassination Science

Single-Particle Electrodynamics - Assassination Science

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

The reason for us naming it after FitzGerald can be seen by computing its<br />

three-magnitude:<br />

σ ′ 2 = 1 − (v·σ) 2 .<br />

The magnitude of σ ′ is (like that of σ) unity, if σ lies in a plane perpendicular<br />

to the three-velocity v; but it is contracted by a factor of<br />

√<br />

1 − v2 ≡ 1 γ<br />

if σ lies parallel or antiparallel to the direction of v; in other words, σ ′ acts<br />

like a FitzGerald–Lorentz contracted [87] version of σ.<br />

It is curious, but probably not fundamentally meaningful, that a concept<br />

from the pre-relativistic days should be a useful tool in a completely<br />

relativistic analysis.<br />

G.4.6<br />

Proper-time and lab-time derivatives<br />

The proper-time rate of change experienced by a particle moving with fourvelocity<br />

U, of an arbitrary quantity external to the particle (e.g., an external<br />

field), is computed by means of the relativistic convective derivative operator,<br />

d τ ≡ (U ·∂) ≡ γ∂ 0 + γ(v·∇).<br />

(G.7)<br />

The proper-time rate of change of a component of a kinematical property<br />

of a particle (i.e., the partial proper-time derivative), as seen in some given<br />

lab frame, is defined to be related to the lab-time rate of change of that<br />

component by means of the time-dilation formula:<br />

[d τ ] ≡ γd t . (G.8)<br />

G.4.7<br />

Partial kinematical derivatives<br />

We now take successive proper-time derivatives of the components of U and<br />

Σ (i.e., partial proper-time derivatives). To do so, we first take d t of (G.2)<br />

422

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!