Equivariance, Variational Principles, and the Feynman Integral
We argue that the variational calculus leading to Euler's equations and Noether's theorem can be replaced by equivariance and invariance conditions avoiding the action integral. We also speculate about the origin of Lagrangian theories in physics and their connection to Feynman's inte...
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| citation_txt | Equivariance, Variational Principles, and the Feynman Integral / G. Svetlichny // Symmetry, Integrability and Geometry: Methods and Applications. — 2008. — Т. 4. — Бібліогр.: 8 назв. — англ. |
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| description | We argue that the variational calculus leading to Euler's equations and Noether's theorem can be replaced by equivariance and invariance conditions avoiding the action integral. We also speculate about the origin of Lagrangian theories in physics and their connection to Feynman's integral.
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Symmetry, Integrability and Geometry: Methods and Applications SIGMA 4 (2008), 032, 13 pages
Equivariance, Variational Principles,
and the Feynman Integral?
George SVETLICHNY
Departamento de Matemática, Pontif́ıcia Unversidade Católica, Rio de Janeiro, Brazil
E-mail: svetlich@mat.puc-rio.br
URL: http://www.mat.puc-rio.br/∼svetlich/
Received November 02, 2007, in final form March 13, 2008; Published online March 19, 2008
Original article is available at http://www.emis.de/journals/SIGMA/2008/032/
Abstract. We argue that the variational calculus leading to Euler’s equations and Noether’s
theorem can be replaced by equivariance and invariance conditions avoiding the action in-
tegral. We also speculate about the origin of Lagrangian theories in physics and their
connection to Feynman’s integral.
Key words: Lagrangians; calculus of variations; Euler’s equations; Noether’s theorem;
equivariance; Feynman’s integral
2000 Mathematics Subject Classification: 49N99; 49Q99; 58D30; 58K70; 70S05; 70S10
1 Introduction
This paper is part of an on-going investigation into Lagrangian theories in an attempt to under-
stand why they are so prevalent in physics. Part of the mystery is that though one uses variation
of the action integral to get the equations of motion or conservation laws (via Noether’s theo-
rem), the actual convergence of the integral is generally not relevant, unless one is really trying
to extremize the value, such as finding the shortest path or smallest area. Such manipulations
are formal yet very useful. When one comes to quantization, in the Feynman integral approach
the integrand is a phase with the action integral in the exponent. The Feynman integral again is
used as a formal object whose very definition is unclear. Why are these procedures so successful?
We won’t be able to answer this question, only hope to give some hints of an answer. A conve-
nient starting hypothesis which would explain the ubiquity of Lagrangian theories is to consider
all successful Lagrangian theories as effective theories arising from some truly fundamental theo-
ry by relegating some degrees of freedom to the background. The fundamental theory is such
that its successful effective theories are of Lagrangian type. The success of Lagrangian theo-
ries is thus due to a specific nature of the fundamental theory. Lagrangian theories have the
flexibility of hiding and revealing degrees of freedom thus marking a necessary property of any
fundamental theory: it must be possible to extract effective theories from it. It must be possible
to effectively deal only with small and well chosen combinations of variables in the whole set of
degrees of freedom that make up the world.
This is also part of an attempt to replace the calculus of variation by geometric constructs
and reinterpret the Feynman integral as something other than integration. The hope is that this
may shed light on the structure of fundamental physical theories and what makes the successful
ones succeed.
In Section 2 we present the geometric structure of the calculus of variation in bundle-theoretic
terms. This is fairly standard and well known material, much of which can be found in Saun-
?This paper is a contribution to the Proceedings of the Seventh International Conference “Symmetry in
Nonlinear Mathematical Physics” (June 24–30, 2007, Kyiv, Ukraine). The full collection is available at
http://www.emis.de/journals/SIGMA/symmetry2007.html
mailto:svetlich@mat.puc-rio.br
http://www.mat.puc-rio.br/~svetlich/
http://www.emis.de/journals/SIGMA/2008/032/
http://www.emis.de/journals/SIGMA/symmetry2007.html
2 G. Svetlichny
ders [1]. See also Olver [2] for related topics. In Section 3 we re-examine Euler’s equations
from the point of view of equivariance of certain bundle maps, deducing, as a new result, what
all such are. Section 4 addresses Noether’s theorem under a new perspective, that by a “de-
effectivation”, that is, the introduction of equivalent Lagrangians with extra degrees of freedom,
variational invariance can be re-expressed as ordinary diffeomorphism-induced invariance of the
Lagrangian function. This frees Noether’s results from a reliance on the action integral. Sec-
tion 5, the last, offers some remarks on the whole program and its relation to the Feynman
integral. Further discussion of the Feynman integral is to be found in a separate article [3].
2 Geometry of the variational calculus
Let M be a differentiable manifold, which we shall take to be the configuration space of some
classical physical system with a finite number of degrees of freedom. For simplicity’s sake we
shall not deal with field theories, the arguments here should be extensible to the field theoretic
case also. The cotangent bundle T ∗M is then what is known as phase space. We shall designate
a generic point of phase space by (q, p). The tangent bundle TM is the space of “positions and
velocities” whose generic point we shall designate by (q, v) or (q, q̇). We shall deal with the
iterated bundles T 2M = T (TM), T ∗(TM), T (T ∗M), and T ∗2M = T ∗(T ∗M).
It is useful to see how various objects look in bundle trivializations, in particular those
induced by a choice of local coordinates q1, . . . , qn in an open set U ⊂ M . In relation to such
local coordinates a typical vector and co-vector in coordinate basis are
v =
∑
i
vi
∂
∂qi
, α =
∑
j
pjdq
j .
In what follows we shall adopt a summation rule by which repeated indices, one lower and one
upper, are to be summed over from 1 to n where n is the dimension of the manifold M . Note
that the index i in the expression ∂/∂qi is to be considered as lower.
In the four doubly iterated bundles, using coordinate bases again, generic points will be
denoted as follows
Bundle Generic Point Abbreviation
T (TM)
(
qi, vj , Ua ∂
∂qa + V b ∂
∂vb
)
(q, v;U, V )
T ∗(TM)
(
qi, vj , Aa dq
a +Bb dv
b
)
(q, v;A,B)
T (T ∗M)
(
qi, pj , S
a ∂
∂qa + Tb
∂
∂pb
)
(q, p;S, T )
T ∗(T ∗M)
(
qi, pj , Qa dq
a +Rb dpb
)
(q, p;Q,R)
One has to be careful in dealing with these expressions. The eight quantities U i, V j , Ai, Bj ,
Si, Tj , Qi, and Rj don’t necessarily transform under coordinate change in the way the indices
suggest. U is a vector and B a co-vector, but in general the transformation properties are more
complicated, a fact that will be important.
There is a rather remarkable canonical isomorphisms as bundles over M
Λ : T ∗(TM) → T (T ∗M),
which plays a central role in the variational calculus. In local coordinates this is
Λ : (q, v;A,B) 7→ (q,B; v,A).
Equivariance, Variational Principles, and the Feynman Integral 3
To check that this is coordinate independent is a tedious and unenlightening exercise. There
seems to be no way to define it without recourse to local coordinates and we suspect that it
may in fact be impossible to define it any other way.
There are two projections T (TM) → TM given by π : (q, v;U, V ) 7→ (q, v) and π∗ :
(q, v;U, V ) 7→ (q, U). There is a subbundle E(TM) ⊂ T (TM) of elements satisfying π(X) =
π∗(X), that is U = v. Sections X of E(TM) over M are vector fields whose flow is the equiv-
alent first order system of second order ordinary differential equations. The flow defined by
(v, v, V ) would be q̇ = v, v̇ = V , that is q̈ = V . So if we want to construct a second order
ordinary differential equation in M we have to give a section of E(TM). From now on we shall
indicate an element of E(TM) by (v, v, a) using the lower case letter “a” to signify acceleration.
One has a canonical map π̃ : T ∗(TM) → T ∗M , given in local coordinates by
π̃ : (q, v;A,B) 7→ (q,B).
One has a subbundle Z∗(TM) ⊂ T ∗(TM) consisting of forms A such that π̃(A) = 0. This
subbundle allows for another map π[ : Z∗(TM) → T ∗M which in local coordinates is given by
(q, v;A, 0) 7→ (q, A),
which can be seen to be consistent with coordinate changes by another tedious and unenlight-
ening exercise.
A Lagrangian, conventionally expressed as L(t, q, q̇) is to be though of as a time-dependent
function on TM , that is L : R× TM → R. There is a formal device by which a time-dependent
Lagrangian can be replaced by an equivalent time-independent Lagrangian in TR × TM ′ with
another manifold M ′, which we explain later. For what we do now, it’s more convenient to treat
the time-independent case and so we assume, until further notice and without loss of generality,
that L is time-independent. Given L there is now the map pL : TM → T ∗M which in local
coordinates is given by
pL(q, v) =
(
q,
∂L
∂vj
dqj
)
=
(
q, π(L)i dqi
)
but is a coordinate independent construct. This is the familiar “conjugate momentum”. Familiar
but not quite innocent, its coordinate independent definition is
pL = π ◦ Λ ◦ dL,
where π : T (T ∗M) → T ∗M is the canonical projection. Thus without Λ one would not have
conjugate momenta, nor the much traveled bridge between Lagrangian and Hamiltonian systems.
Euler’s equations whose coordinate version is conventionally written as
∂L
∂qi
− d
dt
∂L
∂q̇i
= 0,
in coordinate independent notation can be shown to express a condition on a vector X = (v, v, a)
over a point q in E(TM), namely
Λ ◦ dL− (pL)∗(X) = 0.
Here pL∗ is the differential of pL, this is where the second derivatives of L appear. Note again the
role of Λ. Under a certain condition of regularity, obeyed by most systems without constraints,
this condition defines a unique section of E(TM) and thus a system of second-order differential
equations on M .
4 G. Svetlichny
Equivalently, Euler’s equation can be seen as the condition EL(X) = 0 where EL is the
bundle map E(TM) → T ∗(M) given by
EL(X) = π[(dL− Λ−1(pL∗X)),
where an easy exercise in local coordinates shows that dL− Λ−1(pL∗X) ∈ Z∗(TM).
We can thus identify EL as an element of a bundle of bundle morphism
EL ∈ Hom(E(TM), T ∗M).
In local coordinates the one-form EL(X) is given by
EL(X) = ELi(X) dqi =
(
∂L
∂qk
− ∂2L
∂qi∂vk
vi − ∂2L
∂vj∂vk
aj
)
dqk.
We see from this that EL in a coordinate basis is built up from the first and second partial
derivatives (in the same coordinate basis) of L. These derivatives constitute coordinates of the
second jet of L and so the construction of the Euler equations is a bundle map
E : J2(TM) → Hom(E(TM), T ∗M). (1)
Concerning this map, there are two things to point out: (1) The bundles on both sides of (1) are
natural bundles [4], that is, diffeomorphism of the base manifold M lift to bundle maps (which
are also diffeomorphisms), and (2) The map E is equivariant , that is, Φ] ◦ E = E ◦Φ] where Φ is
a diffeomorphism of M and Φ] is its lifting to Hom(E(TM), T ∗M) on the left-hand side of the
equation and to J2(TM) on the right-hand side.
One can now ask the natural question: what are all the equivariant maps between the two
natural bundles that appear in (1)? We show below that these are very few, defined by three
constructs of one-forms, among which are the canonical momentum and the Euler construct.
We will supply a “physicist’s proof” of this result, meaning a discussion about the possible ways
of contracting indices among quantities that transform under the diffeomorphism group. A true
mathematical proof using invariance theory is still being worked out and so we state our results
as conjectures, though the “physicist’s proof” is generally a reliable method of quickly reaching
the same result, providing thus strong evidence for the truth of the conjectures.
3 Equivariance
We work in a local coordinate system. The second jet of L has the following coordinates induced
via the local coordinates we are using:
1) the function L;
2) the first partial derivatives of L,
∂L
∂qi
,
∂L
∂vi
;
3) the second partial derivatives of L
∂2L
∂qi∂qj
,
∂2L
∂qi∂vj
,
∂2L
∂vi∂vj
.
Also X = (v, v, a) ∈ E(TM) has components:
Equivariance, Variational Principles, and the Feynman Integral 5
1) the velocities vi which in Euler’s differential equation become d
dtq
i;
2) the accelerations ai which in Euler’s differential equation become d
dtv
i = d2
dt2
qi.
These have various transformation properties in relation to a diffeomorphism ofM , which locally
we can take as a change of coordinates qi → q̃i. Obviously L is a scalar and v is a vector. The
transformation law of the other quantities in j2L can be obtained from the relation
L̃(q̃i, ṽj) = L
(
qi,
∂qj
∂q̃b
ṽb
)
,
where on the right-hand side q is considered a function of q̃. We find
∂L̃
∂q̃i
=
∂L
∂qk
∂qk
∂q̃i
+
∂L
∂vj
∂2qj
∂q̃i∂q̃k
ṽk, (2)
∂L̃
∂ṽi
=
∂L
∂vk
∂qk
∂q̃i
, (3)
∂2L̃
∂q̃i∂q̃j
=
∂L
∂qa
∂2qa
∂q̃i∂q̃j
+
∂L
∂va
∂3qa
∂q̃i∂q̃j∂q̃b
ṽb +
∂2L
∂qa∂qb
∂qa
∂q̃i
∂qb
∂q̃j
+
∂2L
∂qa∂vb
[
∂qa
∂q̃i
∂2qb
∂q̃j∂q̃c
+
∂qa
∂q̃j
∂2qb
∂q̃i∂q̃c
]
ṽc +
∂2L
∂va∂vb
∂2qa
∂q̃i∂q̃c
∂2qb
∂q̃j∂q̃d
ṽcṽd,
∂2L̃
∂q̃i∂ṽj
=
∂L
∂va
∂2qa
∂q̃i∂q̃j
+
∂2L
∂qa∂vb
∂qa
∂q̃i
∂qb
∂q̃j
+
∂2L
∂va∂vb
∂2qa
∂q̃i∂q̃c
ṽc
∂qb
∂q̃j
, (4)
∂2L̃
∂ṽi∂ṽj
=
∂2L
∂va∂vb
∂qa
∂q̃i
∂qb
∂q̃j
. (5)
The components of X transform as
ṽi =
∂q̃i
∂qa
va,
ãi =
∂q̃i
∂qj
aj +
∂2q̃i
∂qa∂qb
vavb.
We shall also need an expression for the second derivatives of q̃i with respect to qi in terms
of the other order of coordinates
∂2q̃i
∂qk∂ql
= − ∂2qa
∂q̃b∂q̃c
∂q̃a
∂qk
∂q̃b
∂ql
∂q̃i
∂qa
.
So the problem now becomes: how do we put all the above ingredients together to get a one-
form?
Some of the transformation pattern above are quite familiar: L is a scalar, ∂L
∂vi is a co-vector
and ∂2L
∂vi∂vj is a symmetric contravariant tensor of rank 2. The one form ∂L
∂vi dq
i is, as was already
mentioned, the canonical momentum. Since vi is a vector, one can form another scalar ∂L
∂vi v
i
which along with L constructs the Hamiltonian H = ∂L
∂vi v
i−L (defined in TM). The symmetric
tensor ∂2L
∂vi∂vj dq
i⊗dqj , if not degenerate, assures the regularity of Euler’s equations by providing
a unique section of E(TM). There is a third scalar given by ∂2L
∂vi∂vj v
ivj which is not as widely
used as the other three. A fourth scalar is given by 〈EL(X), v〉 = ELi(X)vi, the contraction of
the Euler one-form with v. We conjecture that all other scalars are functions of these four.
The above transformations express an action of the diffeomorphism group which we now take
to be on J2(TM)×E(TM) and we are asking for an equivariant bundle map J2(TM)×E(TM) →
6 G. Svetlichny
T ∗(M). This is equivalent to our previous request. Let Ω = ωi dq
i be the putative one-form
constructed in an equivariant way from the above data. Now because ∂2L
∂q̃i∂q̃j involves the third
derivatives of qi and no other term does, Ω cannot depend on this element of j2L. None of
the other components in the q̃i coordinates receive contribution from ∂2L
∂qi∂qj so they form an
invariant set of components and it is consistent to assume Ω is built only from this set. This
set is still reducible as both the ∂L
∂vi and the ∂2L
∂vi∂vj form invariant sets. The first of these gives
rise to the canonical momentum one-form and the second to the one-form η(L) = ∂2L
∂vi∂vj v
i dqj .
If Ω is not to be a combination of the canonical momentum and η(L), then it must depend on
either ∂L
∂qi or ∂2L
∂qi∂vj or both.
The second derivatives of the qi in the transformations of these components can only be
compensated by a contraction of some of these with ãi. To facilitate this analysis choose
a diffeomorphism which fixes a point q0 in M and at which ∂qi
∂q̃j is the identity matrix. By
Borel’s lemma Aijk = ∂2qi
∂q̃j∂q̃k is an arbitrary set of components provided it is symmetric under
interchange of j and k. We have ṽi = vi, Ãijk = ∂2q̃i
∂qj∂qk = −Aijk, and ãi = ai −Aiabv
avb.
With this the transformation equations (2)–(5) become
∂L̃
∂q̃i
=
∂L
∂qi
+
∂L
∂vj
Ajikv
k,
∂L̃
∂ṽi
=
∂L
∂vi
,
∂2L̃
∂q̃i∂ṽj
=
∂L
∂va
Aaij +
∂2L
∂qi∂vj
+
∂2L
∂va∂vj
Aaicṽ
c,
∂2L̃
∂ṽi∂ṽj
=
∂2L
∂vi∂vj
.
Now the first two transformed jet elements above have only one free index and can suffer no
contraction while the other two can each suffer a contraction either with ṽi or ãi. It is now an
easy exercise that the only combination of all these possible terms in which A cancels out and
which contains no terms proportional to π(L) or η(L) is precisely, up to a multiple, the Euler
one-form which we know is an equivariant construct. From what was shown above we can now
state:
Conjecture. The equivariant bundle maps in (1) are of the form
E(J2L)(X) = απ(L)(X) + βη(L)(X) + γEL(X), (6)
where α, β and γ are functions of the four scalars mentioned above. In this expression the first
two terms depend only on the component v of X.
4 Noether’s theorem
We consider all Lagrangian theories as effective theories arising from a fundamental theory by
relegating degrees of freedom to the background. The set of effective theories form something
like a partially ordered set by which one theory is related to another if the former is an effective
version of the latter. In the quantum version, Feynman’s integral provides a mechanism for
forming effective theories by integrating over the degrees of freedom one wishes to suppress and
rewriting the rest in term of those one wishes to promote (we discuss this in Section 5). With
luck one passes from one Lagrangian theory to another with fewer degrees of freedom. The
inverse process of “de-effectivation” of a theory has not received mathematical attention though
Equivariance, Variational Principles, and the Feynman Integral 7
it has historic precedence. Passing from the Fermi theory of weak interactions to the Weinberg–
Salam electroweak theory is a prime example. The common practice of introducing new degrees
of freedom to either simplify the treatment or make a given Lagrangian theory conform better to
one’s designs calls attention to the importance of this process. A good mathematical treatment
of “de-effectivation” is long overdue.
In our search for a replacement for the variational calculus we shall take the attitude that
if the introduction of new degrees of freedom in such “de-effectivations” leads to a simplified
perspective, then this perspective should be the one adopted. We justify this by noting that
the suppression of degrees of freedom can lead to a theory in which certain simple relations in
the original can assume less transparent form in the new. One is thus not trying to replace
all Lagrangian theories and the concomitant variational calculi with something else, only those
that exhibit certain simplicity in relation to those that arise from them by passing to effective
or equivalent versions, with fewer degrees of freedom. This will become clearer with explicit
examples below.
We admit temporarily that L could depend on t. A variation is conventionally written as
t 7→ t+ δt = t+ ωτ, qi 7→ qi + δqi = qi + ωηi
with ω infinitesimal and τ and ηi functions on R ×M , depending on time and position, but
not on velocities. This lifts to a vector field on R × T (M) (see Olver [2, 5] for this and other
constructs we do below)
X = τD + εi
∂
∂qi
+ (Dεi)
∂
∂vi
, (7)
where εi = ηi − viτ and D is the total derivative
D =
∂
∂t
+ vi
∂
∂qi
+ ai
∂
∂vi
.
The vector field X decomposes conveniently as XH +XV where XH = τD is known as the hori-
zontal component and the rest as the vertical component. Note that ai refers to the acceleration
as a component of an element of E(TM). This means that D is not a field on R × TM and
so at first sight neither would be X , but the ai contributions from the two terms containing D
in (7) cancel out. The use of D simplifies many expressions and is a convenient device.
Let now S =
∫
L(t, q(t), q̇(t)) dt be the action integral. The variation of S is then
δS =
∫
£X (Ldt) =
∫
(X (L)dt+ L£X (dt)),
where £X is the Lie derivative with respect to X .
One finds after a short calculation that
£X (Ldt) =
(
εiELi(X) +D
(
εi
∂L
∂vi
+ Lτ
))
dt+ L
∂τ
∂qi
(dqi − vi dt), (8)
where the first term is X (L)dt and the second L£X (dt). In spite of the presence of the acce-
leration ai in the element X of E(TM) and in D, these contributions cancel out from the full
expression, though present in the individual contributions, an important fact. Concerning the
coefficient of dt in the first term we have
εiELi(X) +D
(
εi
∂L
∂vi
+ Lτ
)
= XV (L) +D(Lτ), (9)
8 G. Svetlichny
a fact that we shall use below. The equality of the two expressions makes use of the fundamental
isomorphism Λ.
The usual statement of the Noether theorem is that if δS = 0 integrated over an arbitrary
interval, then the solutions of Euler’s equations satisfy a conservation law. Translated into our
language this means that if £X (Ldt) = 0 then solutions satisfy a conservation law. Indeed
in a pull-back of (8) onto an integral curve (q(t), v(t), v(t), a(t)) ∈ E(TM) of Euler’s equa-
tions, the second term vanishes since vi(t) = dqi(t)
dt and the pullback of εiELi(X) dt vanishes
because ELi(X) = 0 is precisely Euler’s equations. Thus we have on such integral curves that
D
(
εi ∂L
∂vi + Lτ
)
= 0 which is a conservation law.
If one is to replace variational calculus by a purely geometric formalism one would expect to
state Noether symmetries (the analog of δS = 0) purely by X (L) = 0 and deduce conservation
laws from this. From (8) we see that this would be the case if τ = 0. Now from the point of
view of an underlying fundamental theory, the introduction of δt along with δqi is seemingly
contradictory. Under the relational view of space-time, the time and space coordinates are noth-
ing more than constructs from events, which are governed by fundamental degrees of freedom.
Varying these degrees of freedom would bring as a consequence a variation of the space-time
coordinates and these should not have an independent variation. Thus one should relate our
variational calculation above to one in which τ = 0 and treat the integration variable as a mere
parameter. We achieve this by a “de-effectivation“ of L: Promote t to a dynamical variable
(think of it as q0) and let s be the integration variable. Since t depends on s we need also
introduce the “velocity of time”, that is w = dt
ds . Let v̂i = dqi
ds . One has, going back to the
integral∫
L(t, q(t), v(t)) dt =
∫
L
(
t(s), q(s),
v̂(s)
w(s)
)
w(s) ds.
One should now in principle consider the Lagrangian function L(t, q, v̂/w)w. This isn’t quite
right as now t is to be an arbitrary function of s making it a gauge variable but we don’t have
a gauge theory (variations with respect to t will impose restriction we don’t want). The way
out is the oft used trick of gauge fixing. Introduce yet another dynamic variable λ (think of it
as q∞) as a Lagrange multiplier to fix the gauge to w = 1 and thus use the Lagrangian
L̂(t, q, λ, v̂) = L(t, q, v̂/w)w + λ(w − 1).
Everything works out perfectly now. The variation with respect to the qi variables gives
Ei(L̂) = w
∂L
∂qi
(t, q, v̂/w)− d
ds
(
∂L
∂vi
(t, q, v̂/w)
)
.
If we divide the right-hand side by w and equate the result to zero we get the re-parameterized
(with s as independent variable) version of the original Euler equations.
The variation with respect to q0 = t gives
E0(L̂) = w
∂L
∂t
(t, q, v̂/w)− d
ds
(
L(t, q, v̂/w)− 1
w
∂L
∂vi
(t, q, v̂/w)vi + λ
)
,
whose vanishing defines λ up to a constant as a function of the other variables
d
ds
λ = w
∂L
∂t
(t, q, v̂/w) +
d
ds
(
1
w
∂L
∂vi
(t, q, v̂/w)vi − L(t, q, v̂/w)
)
.
Now λ has no conjugate momentum and this theory is thus one with constraints (in the Dirac
sense).
Equivariance, Variational Principles, and the Feynman Integral 9
The variation with respect to q∞ = λ gives
E∞(L̂) = w − 1,
whose vanishing fixes the gauge and forces w = 1, or in other words t = s+ t0, with t0 another
constant of integration.
This theory therefore is equivalent to the original one modulo the trivial freedom of choosing
the integration constants for λ and t.
One has to now check if the variational symmetries of the two theories are equivalent. In
terms of the original τ(t, q) and η(t, q) we now have a new vector field (no further terms will be
necessary)
X̂ = τ
∂
∂t
+ ηi
∂
∂qi
+ η∞
∂
∂λ
+ D̂τ
∂
∂w
+ D̂ηi
∂
∂v̂i
, (10)
where we have the new total derivative
D̂ =
∂
∂s
+ w
∂
∂t
+ v̂i
∂
∂qi
(no further terms will be necessary) and where η∞ is the yet to be discovered variation of λ
(δλ = ωη∞, ω the infinitesimal). Note there is no ∂/∂s term in (10) meaning that τ̂ , the new τ
is zero. Likewise the new εi functions given by ε̂i = η̂i− v̂iτ̂ coincide with the η̂i = ηi functions;
also ε̂0 = η0 = τ and ε̂∞ = η̂∞ = η∞. We now argue for the new action integral Ŝ =
∫
L̂ ds that
δŜ = 0 ⇔ X̂ (L̂) = 0 after a choice for η∞. Referring to (9) one finds
XV (L) +D(τL) = τ
∂L
∂t
+ ηi
∂L
∂qi
+ (Dηi − viDτ)
∂L
∂vi
+ (Dτ)L. (11)
Also
X̂ (L̂) = wτ
∂L
∂t
+ wηi
∂L
∂qi
+ (D̂ηi − v̂i
w
D̂τ)
∂L
∂vi
+ (D̂τ)L+ wη∞ + (D̂τ)λ, (12)
where all the derivatives of L are to be evaluated at (t, q, v̂/w).
Let now σ(t) denote the triple (t, q(t), v(t)) and similarly σ̂(s) the triple (t(s), q(s), v̂(s)/w(s)).
If F (t, q, v) is any function and F̃ (t, q, v̂) = F (t, q, v̂/w) then one has
(DF )(σ(t)) =
d
dt
(F (σ(t))) =
1
w(s)
d
ds
(F̃ (σ̂(s))) =
1
w(s)
(D̂F̃ )(σ̂(s)),
where the t on one side and the s on the other are related by their functional dependence t(s).
Keeping track of what’s a function of what and make appropriate use of the chain rule one can
set D̂ = wD and v̂/w = v. Now in view of (11), we can write (12) as
X̂ (L̂) = w(XV (L) +D(τL) + η∞ + (Dτ)λ)
and so the two theories have the same variational symmetries if we define the variation of the
new variable λ as
η∞ = −(Dτ)λ.
A remark is in order about this. Expanding one has
η∞ = −(∂τ/∂t+ vi∂τ/∂qi)λ = −(∂τ/∂t+ (v̂i/w)∂τ/∂qi)λ
10 G. Svetlichny
and so this variation in general depends on the velocities (of q and of t). In general such
a situation leads to an infinite regress needing to compute the variations of ever higher derivatives
of the variables involved, placing the problem in an infinite dimensional jet space. But, in certain
circumstance this may not be the case and here this doesn’t happen as there are no loops of
dependencies (that is, the variation of A depending on the variable B whose variation depends
on the variable C etc., leading back to A) in which a dependence on a velocity appears in each
step (the infinite regress stems from such loops) [6]. Now η∞ depends on the derivative of the qi
and t but the ηi and τ don’t depend on λ or any of its s-derivatives at all and so we are thus
safe from the infinite regress.
We have thus come to our first conclusion: under an appropriate “de-effectivation” a La-
grangian can be assumed to be time independent without sacrificing Euler’s equations or Noether
symmetries. This justifies our assumption of time-independent Lagrangians in Sections 2 and 3.
Concerning Noether conservations laws there is another situation called “quasi-invariance”
meaning that δS is not zero but an integral of a total derivative δS =
∫
DΦ dt. One still deduces
a conservation law for solutions of Euler’s equation which now is D(εi(∂L/∂vi) + Lτ − Φ) = 0.
We now show that this too can be subsumed under simple invariance X (L) = 0 under an
appropriate “de-effectivation”. Taking into account the first part of this section the context
now is a Lagrangian L(q, v) that is time-independent and variations δqi = ωηi, ω infinitesimal,
and δt = 0. Suppose now that X (L) = DΦ. By our assumption, ηi, can only depend on qi as the
integration variable is just a parameter, hence Φ is a function only of qi. As before introduce
now a new dynamic variable q0 = ξ with velocity v0 = ν and another dynamic variable q∞ = λ
whose velocity we’ll not need. Consider the Lagrangian
L̂(q, v, ξ, ν, λ) = L(q, v) + νDΦ + λ(ξ − 1).
Variations with respect to qi gives ( since E(DΦ) = 0)
Ei(L̂) = Ei(L),
and we recover the old Euler equations. Variations with respect to q0 gives
E0(L̂) = λ−DDΦ
the vanishing of which defines λ in terms of the original dynamical variables λ = DDΦ. Varia-
tions with respect to q∞ gives
E∞(L̂) = ξ − 1
the vanishing of which fixes the new variable ξ to be the constant 1.
Again, as far as the dynamics is concerned we can consider L̂ as defining an equivalent theory.
Concerning Noether’s theorem, the new vector field defining the new variation has to be of the
form
X̂ = η
∂
∂q
+Dη
∂
∂v
+ η0 ∂
∂ξ
+ D̂η0 ∂
∂ν
+ η∞
∂
∂λ
,
where η0 and η∞ are two new variations to be determined: δξ = ωη0 and δλ = ωη∞, and D̂ is
the new total derivative taking into account the new variables. One finds
X̂ (L̂) = X (L) + νX (DΦ) + η0λ+ D̂η0DΦ + η∞ξ.
Now for the quasi-invariance of L to be equivalent to true invariance of L̂ one need have
DΦ + νX (DΦ) + η0λ+ D̂η0DΦ + η∞ξ = 0.
Equivariance, Variational Principles, and the Feynman Integral 11
There are seemingly many ways to achieve this, but a simple one is to take η0 = 0 and
η∞ = −1
ξ
(DΦ + νX (DΦ)).
Again, this is a variation that depends on velocities, but again, there is no problem.
The second conclusion is that conservations laws coming from quasi-invariance can be realized
as coming from true invariance after an appropriate “de-effectivation” of the Lagrangian.
5 Conclusions and the Feynman integral
The strange effectiveness of Lagrangian theories and the formal use of variational calculus sug-
gests that one should try to achieve the same results without recourse to the action integral and
its variation. In this respect we have shown:
1. Euler’s equations can stand on their own as they arise from an equivariance principle as
stated in the conjecture (6). True, there are two other possible terms and one would like
to be able to identify just the Euler one-form in some canonical manner. In a sense this
is possible for if ∂2L
∂vi∂vj 6= 0 then differentiating (6) with respect to the acceleration a we
can find γ and so just the Euler term. This is not exactly a canonical identification, but
is already progress in the right direction.
2. Variational symmetries can stand on their own. A variational symmetry is invariance of the
Lagrangian under an infinitesimal diffeomorphism of the manifold M lifted canonically
to TM , provided the Lagrangians are of a special class. Any Lagrangian can be “de-
effectivated” to one in such a class.
The action integral can now be viewed as a convenient short-cut to arrive at some purely
geometric results. Its existence as a true integral, that is, as a number obtained by integrating
an integrable function, is now seen to be irrelevant to the use to which it is put. What is still left
up in the air is why should there be Lagrangians at all. It’s all well and good that equivariance
and invariance lead to the usual variational results, but why start with a Lagrangian anyway?
It seems that the Feynman integral can offer some insight. One has:
Z =
∫
eiS(φ)Dφ, (13)
where φ stands for a set of fields and S(φ) is the action integral
∫
L(φ, ∂µφ) d4x. It is instructive
to see how effective theories arise in this context. To get an effective theory out of (13) for
some independent quantities ψ that depend on the φ one then chooses further independent
quantities φ̃ so that one can view the transition φ 7→ (ψ, φ̃) as a “coordinate change in φ space”.
One then has Dφ = |det(J)|DψDφ̃ where J is the “Jacobian matrix of the inverse coordinate
change”. The effective theory for the variables ψ is then given by
Zeff =
∫
eiSeff(ψ)Dψ,
where
eiSeff(ψ) =
∫
eiS(ψ,φ̃)|det(J)|Dφ̃
defines the new effective action. The Feynman integral is thus a neat machine for getting
effective theories: just change variables and do a partial integration. If S is given by an ac-
tion integral and one is lucky then Seff will also be given by an action integral of the effective
12 G. Svetlichny
Lagrangian. If Feynman integration is the essence of quantum mechanics, then quantum me-
chanics has the enviable property that it allows any set of variables that you may chose to obey
an effective theory that is also quantum mechanical in principle, though it may not seem so.
This general situation also explains why macroscopic effective quantities (such as the Landau
phase in superconductivity) do exhibit quantum behavior when the conditions are right (as in
biased Josephson junctions). No quantity truly looses its quantum character and will exhibit it
under the right conditions.
The use of the Feynman integral to create effective theory seems to beg the question of the
need for Lagrangians, isn’t there then some “fundamental Lagrangian” from which all other
theories will then be effective theories. Some people do search for this fundamental Lagrangian
(a string-theorist will probably even exhibit his favorite, and there are various sums (discrete
integrals) over combinatorial objects proposed for quantum gravity). Why should there be such
a fundamental Lagrangian? Furthermore, the Feynman integral seems to give importance to the
action integral, after all, it is the exponential of such that one is called to integrate.
Now nobody has ever succeeded in defining the Feynman integral as a true integral in the
measure-theoretic sense. If we start questioning the action integral as a fundamental ingredient
in physical theories and begin to consider it as a mere expedient tool for expressing geometric
relations, then one can question whether Feynman’s integral is really about summing phases to
calculate transition probabilities. Maybe it also is a short-cut expression for a construct that can
be defined otherwise. In separate papers [3, 7] we present exactly such an idea, that Feynman’s
integral is about the existence of mutually unbiased bases somehow related to causality. In
a finite dimensional Hilbert space two bases ea and fb with a, b = 1, . . . , N are called mutually
unbiased [8] if |(ea, fb)|2 = 1
N . This means that knowing the result of a measurement in one of
the bases gives no information about what the result of a subsequent measurement in the next
basis. One then has
(ea, fb) =
eiL(a,b)
√
N
. (14)
Here one sees the appearance of the “Lagrangian” L(a, b). It is constrained by the requirement
that (14) be a unitary matrix. Seeing that the Feynman integral is an integral of phases,
it can be viewed as the requirement of the existence of a certain system of mutually unbiased
bases (interpreted appropriately in infinite-dimensional Hilbert spaces), or approximates of such.
Lagrangians are then the phase information carried in the inner product of eigenvectors taken
from the two bases. This would explain the physical origin of Lagrangians and appropriate
geometric principles would take care of variational calculus results.
Certain aspects of the choice of Lagrangians as practiced by physicists get suggestive clarifica-
tions from the idea that they are phases arising from inner products of mutually unbiased bases,
or ones nearly so. This is especially true if one considers bases such as position or field-strength
at two times with very small separation. From the positions (for field-strength analogous ob-
servations apply) at two times, in the limit of zero separation, one can construct a position
and a velocity and so the phase (Lagrangian) in this limit would be a function of position and
velocity. This suggests why phase space is important and why first-order lagrangians seem to be
of particular worth (one cannot deduce acceleration from two positions and a time difference).
Another common requirement is that the Lagrangian (or better yet, the action integral) ought
to be invariant under whatever symmetry group one feels governs the physics, or its description
(as in gauge theories). This of course is obviously natural and need not be justified, however,
in thinking of Lagrangian theories as being effective ones of some fundamental underlying one,
and adopting the relational viewpoint of space-time, one is naturally led to diffeomorphism
invariance (or better yet, equivariance) as a fundamental principle. It is in this scenario that
the purely geometric “variational calculus” in integral-free terms should have its expression. If
Equivariance, Variational Principles, and the Feynman Integral 13
one can achieve this, one would surely be able to answer some of the questions posed at the
beginning of this paper.
Acknowledgements
This research was partially supported by the Conselho Nacional de Desenvolvimento Cient́ıfico
e Tecnológico (CNPq), and the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro
(FAPERJ).
References
[1] Saunders D.J., The geometry of jet bundles, Cambridge University Press, 1989.
[2] Olver P.J., Equivalence, invariants and symmetry, Cambridge University Press, 1995.
[3] Svetlichny G., Feynman’s integral is about mutually unbiased bases, arXiv:0708.3079.
[4] Kolar I., Michor P.W., Slovak J., Natural operations in differential geometry, Springer, New York, 1993,
available at http://www.emis.de/monographs/KSM/.
[5] Olver P.J., Applications of Lie groups to differential equations, Springer, New York, 1986.
[6] Otterson P., Svetlichny G., On derivative-dependent infinitesimal deformations of differentiable maps, J. Dif-
ferential Equations 36 (1980), 270–294.
[7] Svetlichny G., Why Lagrangians?, in Proceedings XXVI Workshop on Geometrical Methods in Physics (July
1–7, 2007, Bialowieza, Poland), AIP Conference Proceedings, Vol. 956, Editors P. Kielanowski, A. Odzije-
wicz, M. Schlichenmeier and T. Voronov, AIP, New York, 2007, 120–125.
[8] Bengtsson I., Three ways to look at mutually unbiased bases, quant-ph/0610216.
http://arxiv.org/abs/0708.3079
http://www.emis.de/monographs/KSM/
http://arxiv.org/abs/quant-ph/0610216
1 Introduction
2 Geometry of the variational calculus
3 Equivariance
4 Noether's theorem
5 Conclusions and the Feynman integral
References
|
| id | nasplib_isofts_kiev_ua-123456789-148982 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1815-0659 |
| language | English |
| last_indexed | 2025-12-07T13:23:57Z |
| publishDate | 2008 |
| publisher | Інститут математики НАН України |
| record_format | dspace |
| spelling | Svetlichny, G. 2019-02-19T12:40:37Z 2019-02-19T12:40:37Z 2008 Equivariance, Variational Principles, and the Feynman Integral / G. Svetlichny // Symmetry, Integrability and Geometry: Methods and Applications. — 2008. — Т. 4. — Бібліогр.: 8 назв. — англ. 1815-0659 2000 Mathematics Subject Classification: 49N99; 49Q99; 58D30; 58K70; 70S05; 70S10 https://nasplib.isofts.kiev.ua/handle/123456789/148982 We argue that the variational calculus leading to Euler's equations and Noether's theorem can be replaced by equivariance and invariance conditions avoiding the action integral. We also speculate about the origin of Lagrangian theories in physics and their connection to Feynman's integral. This paper is a contribution to the Proceedings of the Seventh International Conference “Symmetry in Nonlinear Mathematical Physics” (June 24–30, 2007, Kyiv, Ukraine). This research was partially supported by the Conselho Nacional de Desenvolvimento Cientıfico e Tecnologico (CNPq), and the Fundacao de Amparoa Pesquisa do Estado do Rio de Janeiro (FAPERJ). en Інститут математики НАН України Symmetry, Integrability and Geometry: Methods and Applications Equivariance, Variational Principles, and the Feynman Integral Article published earlier |
| spellingShingle | Equivariance, Variational Principles, and the Feynman Integral Svetlichny, G. |
| title | Equivariance, Variational Principles, and the Feynman Integral |
| title_full | Equivariance, Variational Principles, and the Feynman Integral |
| title_fullStr | Equivariance, Variational Principles, and the Feynman Integral |
| title_full_unstemmed | Equivariance, Variational Principles, and the Feynman Integral |
| title_short | Equivariance, Variational Principles, and the Feynman Integral |
| title_sort | equivariance, variational principles, and the feynman integral |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/148982 |
| work_keys_str_mv | AT svetlichnyg equivariancevariationalprinciplesandthefeynmanintegral |