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Re: Kaluza-Klein help needed

  1. Oct 11, 2006 #1
    [SOLVED] Re: Kaluza-Klein help needed

    To John et al...
    I agree with everyone else who posted.
    For a few comments, I'll ref to P.G. Bergmann's
    "Intro to...Relativity", he details KK's 5D stuff.

    John Baez wrote:
    > For a course on a classical mechanics I decided to have my
    > students work out the geodesics on a 5-dimensional manifold
    > M x U(1) with the metric h given by


    > h_{ij} = g_{ij}
    > h_{i5} = A_i
    > h_{55} = 1

    See ref, Eq.(17.61). John's statement looks accurate summed
    over 4D.

    > where i,j = 1,2,3,4, g is a metric on M and A is a 1-form
    > on M describing the electromagnetic vector potential.
    > I was hoping to get the equation for the motion of
    > a charged particle in a electromagnetic field, namely
    > m (D^2q/Dt^2)^i = e F^i_j (dq/dt)^j

    Yes, see Eq.(12.57).
    The last term on the LHS, can also be zero and
    yield information, that corresponds to John's
    RHS above. If john's LHS above vanishes then it
    is a constraint on motion.

    > where:
    > dq/dt is the derivative of the path q(t),
    > D^2q/Dt^2 is its covariant 2nd derivative,
    > F_{ij} = d_i A_j - d_j A_i is the electromagnetic field,

    > m is the particle's mass

    Stop, same ref, see (6.14..19). A particles mass has
    3 possible components, covariant energy-momentum "p_0",
    invariant "p", and contravariant "p^0".
    (Mixing g-fields with EM-fields, IMO does requires
    care to define the meaning of mass).
    Presuming John mean's a conserved "p" as defined
    by (6.19), let's good ahead.

    > e is its charge.
    > And indeed, I get this assuming that the particle moves
    > around the circle U(1) at velocity e/m:
    > (dq/dt)^5 = e/m

    Above John pointed out h_{55}=1, that's a constant!
    How can variables exist in the direction of the 5th
    dimension if it's metric is constant?
    There cannot be a basis for acceleration in the
    direction of 5th dimension, i.e.

    (dq/dt)^5 = 0 , (Tucker suggests)

    IMO, that's a constraint in 5D. Look at it physically,
    if you have charges going off into the 5th dimension
    won't that rather make the other 4 lonely?

    > HOWEVER, I don't find that the velocity of the particle around
    > the circle is constant!
    > (dq/dt)^5 seems to have nonzero derivative, which is annoying:
    > the particle's effective charge-to-mass ratio changes with time!

    I think if John employs the quantum theory, (meaning energy changes
    discontinuously, applied to the "m", then the classical
    type predictions, made on the basis of Maxwell, of particles,
    like electrons spiralling into the nucleus will not happen.

    It looks like dm =0 but is not a constant.

    > Am I making a mistake or what?

    Well, by experiment we have found, "power" is
    not continuously radiated, hence a spirally orbiting
    electron descending into a nucleus obeys the vector
    dot product,

    q*E.V = 0

    (q=charge, E=E-field, V-Velocity),

    IOW's it can't spiral. Otherwise a spiral type orbit
    would have

    q*E.V =/=0,

    as the charge sinks into the nucleus, continuously
    radianting power, that is classically predicted but
    physically not apparent and gave birth to Quantum T.

    In the case of the Lorentz force John introduce above,
    the component,

    f_0 = q*F_i0 U^i == q*E.V =0.

    I think that means there is no force in the direction of

    > It's all the more annoying because the spacetime M x U(1) has
    > rotational symmetry in the U(1) coordinate, so by Noether's theorem,
    > the momentum in the 5 direction is conserved.
    > However, the velocity in the 5 direction appears not to be
    > conserved, basically because we obtain the velocity from the
    > momentum by raising an index:
    > (dq/dt)^5 = h^{5a} (dq/dt)_5
    > and h^{5a} is time-dependent.
    > I wish I were making some mistake here - am I?

    Don't know, but IMHO, one should give due consideration
    to the quantization of energy exchange. AE's GR gives
    a good foundation for that following his GR1916 Eq.(65a),
    where he suggests the Lorentz force vanishes. I provided
    one sample "f_0=0" above, but many disagree.
    If you ref to Dover's PoR GR1916, pg 156, and read past
    Eq.(66) you'll see, "if kappa_sigma vanishes", (that's
    Lorentz force in Eq(65)), and find that's how EM-forces
    are included into the T_uv side of G_uv=T_uv, and I think
    ok with quantum theory.

    Ken S. Tucker
  2. jcsd
  3. May 25, 2007 #2
    Using the wrong g(5) would only introduce 'error' like A_a A^a = (V/m)^2 =.01%. So why the 'error' like 10^10% by getting Plank mass instead of electron mass?

    I think the 'error' was more due to using 'wrong' simple L(5) rather than wrong g(5). To get the Plank mass out to the 'KK-miracle' need a geometric L with R curvatrure etc. => Brans-Dicke gravity etc. as you would expect from a grand geom theory.

    But keep it simple - go for the simple L(5) & g(5) and we get
    1. x^..M = - Gamma5^M_BC x^.B x^.C
    = - Gamma4^m_bc x^.b x^.c + F^m_c x^.c
    via another sort of '5d miracle'.
    But this one introduces a factor of charge/mass (of electron) and not Plank mass (what I referred to as the 10^10% 'error'.

    2. for simple QM using corresponding g(5)^BC
    L(5density) = 0.5 (d_B psi)* g(5)^BC (d_C psi)
    = 0.5 (D_b psi)* g(4)^bc (D_c psi) + 0.5 mm (psi* psi) = KG equation.
    again we get the charge/matter for electron, not Plank mass.

    3. By the way, within the omitted definitions & calculations, A_5 ==1 is used to simplify
    x^.B g(5)_BC x^.C = x^.b g(4)_bc x^.c + x^.B A_B A_C x^.C
    But always A = A(4) ensures g(5)^BC g(5)_CD = KronDelta^B_D
    leads to:
    x^.B A_C = 1 => x^.5 = (1 - A_c x^.c) approx= 1
    So x^5 approx= tau >>>>>>>>>>>> the x^5 associated with Plank mass.

    4. Back in 1978 Franchi and in 1984 Kubo developed 5d QFT using s = x^5
    id_s <=> m of electron etc.

    5. My background: I am new to PF, spotted this question months ago, saved it in bookmarks till I had time to follow it up. I dont know if there is any more correspondence or whether this is going down a black hole. I have been struggling with path integral representation, QFT etc. So have been dabbeling in QM, CM, 5d as a pre-retirement hobby. Apologies for omitting the working, but it is such hard work producing the symbols - especially if this is not going anywhere.
    Does this clarify anything, or am I on the wrong track? Thanks.
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