6 ms·

Maybe this helps: http://physics.stackexchange.com/questions/190259/%CE%97ow-does-a-current-density-becomes-a-charge-density http://physics.stackexchange.com/q

• reagency 11y ago
  Is there an analog to that in mass/gravity? Or no because mass doesn't have sign like charge does?
  • Maro 11y ago
    Yes. You can combine charge density and current to form a 4-vector in SR like (charge density, current vector). Other 4-vectors are (energy, momentum vector). There are also a bunch of others, the most famous/basic one being (time, position vector): https://en.wikipedia.org/wiki/Four-vector#Fundamental_four-vectors https://en.wikipedia.org/wiki/Four-vector#Fundamental_four-v... The reason it's a good idea to group these into 4-vectors is that the components of the vectors transform into each other under coordinate transformations in SR (see Lorentz-transformation).