View Full Version : time dilation and electronic charge.

2006-Sep-03, 04:40 AM
If you get a big piece of wood and spin it around then according to relativity the atoms at the far end of the wood are experiencing slower time than the atoms closer to the hub of the spinning wood. Does this differentiation of time have an effect of the charge of the electrons in the wood(ie perhaps lower charge) and if so how does this effect chemical bonding, and what other effects are there?

Ken G
2006-Sep-03, 04:46 AM
Rotation of charges induces magnetic fields, which is at the scale of relativistic effects, but over the scale of an atom this would be miniscule, and over larger scales you'll have cancellation between positive and negative charges. I can't say the effect is zero, but it's so close to zero that you might rather worry about whether or not you are breathing on the wood. Still, there are other scales on which this kind of thing does matter-- black holes can have charge, and if that is combined with a spin, this can affect things in ways that require general relativity and are way beyond anything I know about.

2006-Sep-03, 04:49 AM
but do high speeds reduce apparant charge relative to the rest of the Universe.

2006-Sep-03, 06:11 AM
Frog March,

Electric charge is invariant under relativistic transforms. All observers see the same total charge of a system. However, how the charge is distributed and how it appears to be moving does vary from frame to frame. IOW, the charge density and current density is frame dependent, but the total charge is not.

Now, what I'm wondering, which is related to your question, if some charge arrangement that appears completly neutral, *no external field of any sort* could ever have an external field in some other frame. That is could E and B both zero in some frame transform to non-zero E' and B' in some other frame. I don't think so.

For example, imagine a spherical capacitor of sort. A small sphere of positive charge surrounded by a sphere of equal negative charge. The external field will be zero -- internal field will be radial, the chunk of out a point charge field between the two spheres, but completely cancelled outside the sphere.

If that sucker gets to moving, the sphere will be squashed and become an ellipsoid, but the charge density will still cancel. The internal field will change shape, but the external field will remain zero. And that applies to the magnetic field as well as electric. I think. :)


Ken G
2006-Sep-03, 02:31 PM
Yes, fields transform in a way that depend on the fields in the previous frame, and if they are all zero, the transform gives zero also. The "dot product" of the fields is invariant, so if there are no co-aligned components in one frame, then there are none in any frame. But fields do transform into perpendicular components, which is essentially what is meant by the "Lorentz force".