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The principle of least action/time, and geodesics of spacetime

  1. May 1, 2003 #1
    Hi all,
    I am trying to reformulate the axioms of Special Relativity. It seems intuitively true that all inertial frams should be equivalent (*), but there seems to be no philosophical justification that light should travel at constant velocity to all inertial frames (+).
    Could someone show me, without sacrificing too much detail, the proofs for the following:
    1. the principle of least time for light, which states that light always travels in paths of least time. Does the proof need the premise of condition (+)?
    2. given the priciple of least action/time, derive that light always travel in geodesics of the space to which it is confined.
    Thanks, all.

  2. jcsd
  3. May 2, 2003 #2


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    Light is an electromagnetic wave.
    The wave equation for electromagnetic waves indicates that they should propagate at speed 1/sqrt(&mu0&epsilon0)
    &mu0 and &epsilon0 are constants of the vacuum, and are the same for all observers.
    Therefore, the speed of light = c = 1/sqrt(&mu0&epsilon0) is constant for all observers.
  4. May 2, 2003 #3
    you can't refer to Maxwell equation(s) trying to prove constancy of speed of light, because Maxwell equations are CONSEQUENCE of relativity, because they are DERIVED from relativity (namely, from existence of electric charge, Lorents transformations of coordinates (which gives rize to magnetic component), and 3-dimensionality of space).

    Better approach is to postulate that ALL fundamental constants (G,h,c,e) are velocity-invariant (=independent of observer's motion). In essense this means that all physics is velocity-invariant as usually postulated to be separate from constancy of c postulate.
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  5. May 2, 2003 #4
    actually fundamental constants are not only velocity invariant, but also acceleration invariant, position invariant, time invariant (as far as we know). So they are invariant in any and all reference systems and for any and all observers no matter where/when he/they are or how weird they are moving.
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  6. May 2, 2003 #5


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    er... [?]


    Maxwell's theory was published in 1873. Special Relativity was published in 1905.

    Heck, Maxwell died in the year Einstein was born (1879), how could his equations possibly be derived from Special Relativity?

    The equations comprising Maxwell's Equations date back even further.


    The equations composing Maxwell's laws are:

    Gauss's Law. (Gauss died in 1855)
    Absence of magnetic monopoles
    Faraday's Law. (Faraday died in 1867)
    Ampere's Law. (Ampere died in 1836)

    And Maxwell's modification of Ampere's law to include displacement current.

    Even the first line of Einstein's paper On the Electrodynamics of Moving Bodies references Maxwell's electrodynamics:

    And Einstein even uses the equations in his paper. See the first sentence of section 6:

    And from there, Einstein goes on to derive the transformation law for the EM field based on the hypothesis that the Maxwell equations are valid in all inertial frames of reference.

    It's true that relativity added to Maxwell's theory by explaining what happens when one changes their frame of reference, and corrects the EM force law (which is independant of the Maxwell equations), but by no means imaginable were the Maxwell equations derived from Special Relativity.
    Last edited: May 2, 2003
  7. May 2, 2003 #6
    Do you care about historic way of who and in which sequence stubmles upon observed facts, or do you care how these facts are related to each other (which one is causal consequence of which other one)?

    Prime cause af it all (of e/dynamics) is an "electric charge" property of some particles. So far we don't know what it is (what causes it) - this is where present knowledge blurrs into unknown. Correct me if I am wrong and electric charge is finally derived from more fundamental entities.

    Electric charge + 3D space --> Electric field --> Coulomb inverse square electric field law (square only if to use 3D space, 1st power if to use 2D, etc) ---> Gauss law (First Maxwell equation).

    Electric field + motion of observer --> magnetic field. (Here Lorents transformations of coordinates are vital. There is no magnetic field if to use Galilean ones. Magnetic field is simply relativistic component of moving electric field. Absense of magnetic monopole as a consequence.) --> Magnetic Gauss law (second Maxwell equation).

    Magnetic field (better say, component) + motion of observer ---> Faraday's law of electric induction (3rd Maxwell equation) and equation for displacement currents (last Maxwell equation).

    Finally, motion of charge with acceleration ----> both electric and magnetic components changing in such mathematical manner which we label "e/m waves (or light, depending on rate of acceleration change)".

    All arrows here stand fot "mathematically results in".
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  8. May 2, 2003 #7


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    It's somewhat imprecise to talk about causality when the objects involved don't occur at points in time. One can talk about logical implication, but the arrows of implication generally do not have a unique direction.

    The "historic way" demonstrates that Maxwell's laws can be derived without the assumption of a constant speed of light.
  9. May 2, 2003 #8
    If you assume that speed of light is not constant, then there is no Lorents transformations. Then magnetic field vanishes (as a mathematical result - there is NO magnetic field if you transform coordinates using Galilean transformations instead of Lorents). As a consequence of absense of magnetic field last 3 Maxwell equations vanish. Only Gauss law survives.

    Instead others you'll have, for example, just ordinary wave equation for ELECTRIC wave (notice here: NOT for e/m wave, because there is NO magnetic field). Speed of this wave would depend on observer and on source motion - like sound wave for instance.
  10. May 2, 2003 #9


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    Who cares about galilean transformations?

    The magnetic field is the force field that deflects currents. The inverse square magnetostatic force between stationary current patterns is

    F = (&mu0/4&pi) I2 (I1 r) / d2

    from which we conclude an inverse square magnetic field

    B = (&mu0/4&pi) (I r) / d2

    which is in direct analogy with electrostatics. Stationary charge patterns generate inverse square electric fields, stationary current patterns generate inverse square magnetic fields.

    Varying magnetic field introduces an emf via Faraday's law. Varying electric fields emulate a current. Combine with Gauss's law to yield a wave equation. Assume the vacuum looks the same to all observers and you have constant speed of light.

    Last edited: May 2, 2003
  11. May 3, 2003 #10


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    Unless I misunderstand what you are asking... why would there be a philosophical basis for the measurement of c yielding a constant value? This is a consequence of the theory of relativity, which was put forth to explain the observed facts. You cannot derive special relativity from other theory, ergo it cannot be proven.

    I think it is interesting that the least action principle for light applies equally well when light moves from one medium to another. I am not sure if this could be called a geodesic. However, a photon changes its path in response to a change in medium such that its path is still the optimal fastest path.
  12. May 5, 2003 #11
    Looks like you got it all backward. Magnetic field is NOT some kind of new field. It simply is relativistic part of electric field. Move by electric field - and you'll get magnetic. All 3 Maxwell equations mathematically follow from the definition of magnetic field B=[v x E]gamma/c2, which in turn follows from Lorents transformations of electric field into moving reference system of observer.
  13. May 5, 2003 #12
    Formulation of space-time mechanics


    Space-time mechanics are derived from relativity and parodoxia of the manner relative to the fundamental forces of nature. Using {x,y,z,t},
    in association of manifold energies from one part of the universe to the other to balance motion of subspace entities as superpartner
    particles that support subspace and hyperspace. Subspace: being a manifold of gravity space-times associated to protect us from hyperspace radiation.Hyperspace: being bosonic and fermion dynamics
    that are the equivalancy principal of the universe. The parodoxia:
    involves repeating associations to relativity and the basic structure
    of the four fundamental forces of nature. Using this information toward that end involves membranes and p-branes where {t}does not equal zero but is a step toward dimensional exploration. A step up from gravity. The exo-gravitational universe, we cannot see it because it to heavy to be supported by non-dimensional space. It is what causes space to expand.
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  14. May 5, 2003 #13


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  15. May 5, 2003 #14
    A magnetic field is not a constant, it is in transition to the next energy level of the infinite parodoxia determined by quark plasma densities of bosonic-fermion dynamics, change to form an equivalancy
    principal across the board energy level of the four fundamental
    forces seeing gravity is time, therefore, subspace is space-time
    membrane in gravity and p-branes are condensed forms of dimensional space-times in gravity at {t}, the promotion of lensing potiential
    gravity produces manifolds to transfer energy and motion at a different level, this can be done through every fundamental force.
    This creates the interaction chromodynamics and fermion dynamics in the same way.
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  16. May 5, 2003 #15
    OK. Then where is it derived from, and what is the definition of B ?
  17. May 5, 2003 #16
    It is derived from dt^2 and Einsteins equivalancy principal,
    and the parodoxia, as for B, I think your talking about hyperspace: the equivalant of a space based on pure bosonic-fermion space which exchanges and absorbs energies creating hyperluminal particles called
    a subuniverse. These items are applied to the above statements.
    All of the events mentioned herein are above present human mathematical abilities. This is why I sent the information to the
    jet propulsion lab, a manifold is the first part of a wormhole as
    I understand it, dualaity takes care of the second half.
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  18. May 5, 2003 #17


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    The definition of B is the force field associated with magnetic force, which is defined as the force which deflects currents. I.E. B is the vector field such that:

    F = I x B

    Where I is a current element and F is the force acting on that current element.

    The definition of a force field is always the field associated with a particular force.

    The field of electrostatics showed that the electric field at point x generated by a point charge q at y is (neglecting the subscripts and boldface):

    E = (1 / 4 &pi &epsilon) q (y - x) / |y - x|3

    The field of magnetostatics showed that the magnetic field at point x generated by a point current I at y is:

    B = (&mu / 4 &pi) I * (y - x) / |y - x|3

    Mix in the relation I = q v, and your "definition" falls out of the mathematics as being a relationship between the electric field generated by a point charge and the magnetic field generated by a point charge.

    Another way is to take the Maxwell equations as the fundamental law as opposed to the laws of electrostatics and magnetostatics (but the definition of the terms involved is still the same!!) Generate the wave equation and find the integral describing the solution, and your relationship pops out, again not as a true in general equation (it's obviously not true in general), but as a relationship between the fields generated by a point charge at a retarted time.

    Another way is to start with your relationship (where it makes sense) as the fundamental law. Since we're assuming it, it doesn't need to be derived, but it's still not the definition; magnetic field is always defined as the force field for the magnetic force, which is always defined as the force that acts on currents.
  19. May 6, 2003 #18
    magnetic field scattering

    See, the problem is your not looking a the ability for mass/energy
    to be linked, where energy, force and matter are all the samething
    just at different levels associated by electron and chromodynamics scattering due to space-time fields regulated by gravity waves
    at one level or the other fundamental forces placed in levels according to distribution of the enhancement of fields transitioning
    magnetic field of the electron so a state of transition takes place
    that create interia or microgravity. Normal space is potential gravity, knowing this gravity controls the forces abilities or inabilities.
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  20. May 6, 2003 #19

    No, this is not the DEFINITION of magnetic field. This is the DESCRIPTION of magnetic field of a current (=moving charge) which usually is given in introductory physics classes. In these classes when students did not take relativity yet, magnetic field is NOT defined, but only described. Yet there they mention that magnetic field is not field of its own, but a mere relativistic part of electric field of moving charge.

    In electrodynamics magnetic field is defined as a term [vxE] arizing from Lorents coordinate transformation of components of electric force F (thus, of the components of electric field E=F/q) from stationary coordinate system of electric charge into the moving with velocity v reference system of observer.

    Basicly, if you are STATIONARY versus electric charge, your force has only ONE direction (away or toward electric charge), but when you move, then Lorents transformations give you TWO vectors: one still directed radially toward (or away) charge, and another (delayed component) directed as if there is a new field in PERPENDICULAR to radial direction (and perpendicular to system's velocity vector direction). Namely this vector product [vxE] (with gamma/c2 factor) is what we nick name as "magnetic field" and label as B to simplify relativcistic math in calculations.
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  21. May 6, 2003 #20


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    Definition of force field = field associated with a given force. Nothing more, nothing less.

    And when we want to simplify relativistic equations, we use the electromagnetic potential 4-vector, or the electromagnetic field strength tensor.
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