[sdiy] MRI time dilation?

[Martin Czech]

:::Perhaps it has something to do with aluminum being a slightly magnetic
:::material?
:::
::: Aluminum? Yes, aluminum. Remember, the first two letters of the phrase
:::"Alnico Magnet" represents the element aluminum. It is slightly magnetic,
:::but most people don't realize that. I should add that by "magnetic" I mean
:::it is attracted to magnetic fields, just as a block of iron would be, only
:::the effect is *much* weaker.


Any matter might interact with magnetic fields.  Therefore the theorists
split it up in a purely geometric law, and a law for matter properties.

INT(H)ds = Phi  means the sum of the H-field along a closed loop
is equal to the total current in that loop (be it real current or
"virtual" displacement current due to dE/dt).

This is a purely geometric law, it doesn't need any infromation about matter
in that loop. H is only a auxilary field, it can not be observed.

Next we have:

B = mu*H

which means that the real observed magnetic field B (which leads to force and
induction of voltage) depends on H field times some matter constant mu
(greek letter mu).  For practical reasons we set mu:= mu0*mur (0 and r beeing
subscript!)

mu0 was set for historical reasons to 4*PI*1.0E-7 H/m, it has a unit,
mur is the relative "difference" between different types of matter, it
is a simple number, vacuum is defined as mur=1.0

Now we have:

I) diamagnetism: the electrons on their orbits work like currents which
   weaken the field, thus mur < 1.0 (less then vacuum), eg. bismuth
   in principle all matter is diamagnetic, but there is also
   
II) paramagnetism: the atoms don't compensate their fields, thus act like
    little magnets, thus mur > 1.0 (more then vacuum).
    
III) "very strong paramagnetism", ferromagnetism because wellknown in iron:
    mur >>> 1.0, the ferromagnetic matter mur is not constant (B saturates
    whereas H gets higher
    and higher, and also it does depend on crystal direction. Therefore mu
    becomes a function of H, and it may be a tensor, not a simple number.

           mur  
Wi         1.0 - 160e-6 (strongest dia)
Cu         1.0 - 10e-6 (dia)
Ag         1.0 - 25e-6 (dia)
H2O        1.0 - 9e-6 (dia)
air        1.0 + 0.4e-6 (slighly para)
Al         1.0 + 22e-6 (para)
Pt         1.0 + 300e-6 (para)
pure iron  >250000      (ferro)


So, yes , there are little magnets in Al, but very weak, maybe you could
attract an Al plate with a magnet in space. Eddy currents make much stronger
effects, however. The rotating disk found in domestic eletricity meters
where Al, they where moved by AC field and eddy currents.


m.c.