Snyder Space-Time: K-Loop and Lie Triple System
Different deformations of the Poincaré symmetries have been identified for various non-commutative spaces (e.g. κ-Minkowski, sl(2,R), Moyal). We present here the deformation of the Poincaré symmetries related to Snyder space-time. The notions of smooth ''K-loop'', a non-associati...
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irk-123456789-1465352019-02-10T01:24:55Z Snyder Space-Time: K-Loop and Lie Triple System Girelli, F. Different deformations of the Poincaré symmetries have been identified for various non-commutative spaces (e.g. κ-Minkowski, sl(2,R), Moyal). We present here the deformation of the Poincaré symmetries related to Snyder space-time. The notions of smooth ''K-loop'', a non-associative generalization of Abelian Lie groups, and its infinitesimal counterpart given by the Lie triple system are the key objects in the construction. 2010 Article Snyder Space-Time: K-Loop and Lie Triple System / F. Girelli // Symmetry, Integrability and Geometry: Methods and Applications. — 2010. — Т. 6. — Бібліогр.: 28 назв. — англ. 1815-0659 2010 Mathematics Subject Classification: 17C90; 81T75 DOI:10.3842/SIGMA.2010.074 http://dspace.nbuv.gov.ua/handle/123456789/146535 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України |
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Different deformations of the Poincaré symmetries have been identified for various non-commutative spaces (e.g. κ-Minkowski, sl(2,R), Moyal). We present here the deformation of the Poincaré symmetries related to Snyder space-time. The notions of smooth ''K-loop'', a non-associative generalization of Abelian Lie groups, and its infinitesimal counterpart given by the Lie triple system are the key objects in the construction. |
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Girelli, F. Snyder Space-Time: K-Loop and Lie Triple System Symmetry, Integrability and Geometry: Methods and Applications |
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Girelli, F. |
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Snyder Space-Time: K-Loop and Lie Triple System |
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Snyder Space-Time: K-Loop and Lie Triple System |
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Snyder Space-Time: K-Loop and Lie Triple System |
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Snyder Space-Time: K-Loop and Lie Triple System |
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Snyder Space-Time: K-Loop and Lie Triple System |
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snyder space-time: k-loop and lie triple system |
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Snyder Space-Time: K-Loop and Lie Triple System / F. Girelli // Symmetry, Integrability and Geometry: Methods and Applications. — 2010. — Т. 6. — Бібліогр.: 28 назв. — англ. |
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Symmetry, Integrability and Geometry: Methods and Applications |
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Symmetry, Integrability and Geometry: Methods and Applications SIGMA 6 (2010), 074, 19 pages
Snyder Space-Time: K-Loop and Lie Triple System?
Florian GIRELLI
School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia
E-mail: girelli@physics.usyd.edu.au
Received April 29, 2010, in final form September 13, 2010; Published online September 24, 2010
doi:10.3842/SIGMA.2010.074
Abstract. Different deformations of the Poincaré symmetries have been identified for
various non-commutative spaces (e.g. κ-Minkowski, sl(2, R), Moyal). We present here the
deformation of the Poincaré symmetries related to Snyder space-time. The notions of smooth
“K-loop”, a non-associative generalization of Abelian Lie groups, and its infinitesimal coun-
terpart given by the Lie triple system are the key objects in the construction.
Key words: Snyder space-time; quantum group
2010 Mathematics Subject Classification: 17C90; 81T75
1 Introduction
Snyder space-time has been introduced in 1947, and is one of the first examples of non-com-
mutative geometry [1]. The physical features of a quantum field theory living on this space are
not very well known since there have been very few attempts to construct such field theory on
this space (see [2] and references therein). It is only recently that a star product and a scalar
field theory have been constructed [3, 4]. Using a different route and doing the analysis at first
order, [5] reached similar results. In both of these works, the scalar field action is shown to be
invariant under a new type of deformed Poincaré symmetries. The main feature of this deformed
symmetry is the non-(co-)associativity, which explains why this deformation did not appear when
considering the (co-)associative deformations of the Poincaré group [6]. In this perspective, the
natural question to ask is what is the algebraic structure or “quantum group” encoding the
deformation of the Poincaré symmetries consistent with the Snyder non-commutativity? We
present here the answer to this question by defining a new type of quantum group.
Before explaining the strategy we shall use in the Snyder case, let us recall the construction
in the κ-Minkowski non-commutative space which is well understood and can serve as a gui-
ding line. In this case, the relevant deformation of the Poincaré symmetries, the κ-Poincaré
deformation [7, 8], is based on the Iwasawa decomposition of SO(p, 1) ∼ ANp .SO(p − 1, 1) ∼
ANp ./ SO(p−1, 1) [9, 10]. The (Abelian nilpotent) group ANp is interpreted as momentum space
when dealing with a scalar field theory living in κ-Minkowski. The deformation of the translation
symmetry can be traced back to the non-Abelian ANp group structure. The κ-Poincaré (or
bicrossproduct) algebra is encoded by B = C(ANp) I� kSO(p − 1, 1), where the algebra of
functions over ANp coacts on the group algebra kSO(p− 1, 1), and kSO(p− 1, 1) acts on ANp.
This quantum group encodes the deformation of the Poincaré Hopf algebra C(Rp) o kSO(p −
1, 1) κ−→ C(ANp) I� kSO(p−1, 1), by implementing the Planck scale κ. The κ-Minkowski space
is encoded into the coordinate operators Xµ which satisfy the same commutator relations as the
Lie algebra anp of ANp. Using the Weyl map, it is possible to construct a ∗-product between
c-number coordinates xµ which satisfy the same commutation relations as anp [11].
?This paper is a contribution to the Special Issue “Noncommutative Spaces and Fields”. The full collection is
available at http://www.emis.de/journals/SIGMA/noncommutative.html
mailto:girelli@physics.usyd.edu.au
http://dx.doi.org/10.3842/SIGMA.2010.074
http://www.emis.de/journals/SIGMA/noncommutative.html
2 F. Girelli
We can follow a similar route to construct the relevant non-commutative space associated to
Snyder space-time. The Snyder deformation will be based on the decomposition SO(p− 1, 1) ∼
L . SO(p−1, 1), where L is not a group but a K-loop, a non-associative generalization of Abelian
groups [12]. In this case, the deformation of the translation symmetry will be related to the K-
loop structure. The goal of the paper is to construct the quantum group S = C(L) o kSO(p −
1, 1) – where the group algebra kSO(p−1, 1) acts on the algebra of functions C(L) on the K-loop –
with the relevant Hopf structures. This quantum group can be seen once again as a deformation
of the Poincaré Hopf algebra C(Rp) o kSO(p − 1, 1) κ−→ C(L) o kSO(p − 1, 1), but not in the
Hopf or quasi-Hopf algebra setting, since the antipode of S will not be antimultiplicative. This
means in particular that this deformation has not been identified in Zakrzewski’s classification
of Poincaré deformations [6]. We will show that this quantum group S encodes the deformed
Poincaré symmetry identified in [3, 4]. The infinitesimal structure of a K-loop is a Lie triple
system and we will explain how Snyder non-commutative structure is naturally encoded in such
structure. Lie triple systems provide therefore a new type of non-commutative spaces. This is
the other main result of this paper. As a side result, one recovers that the ∗-product defined
in [3, 4] is actually a realization of the Lie triple structure.
In the first section we recall the different notions of loop which can be found in the standard
literature, emphasising on the K-loop case. In the second section we introduce the main result
of the paper that is the construction of the quantum group encoding the symmetries of Snyder
spacetime. In the third section, we show how to construct a scalar field action invariant under
such symmetry, using the method described in [13]. In the fourth section, we recall the infinite-
simal notion of a smooth K-loop given by Lie triple system and explain how this structure can
be interpreted as encoding Snyder non-commutative geometry. The concluding section sets a list
of interesting points to explore.
2 K-loop: a review
We recall in this section the basic definitions of the most studied loops with particular emphasis
on the notion of K-loop, a (midely) non-associative generalization of Abelian group. Most of the
content of this section (except Proposition 2, Definition 4 and Example 2) can be found in the
standard reference on K-loops [12] (see also [14] for further references on the notion of loops).
Definition 1 (quasi-group and loop). A quasigroup (S, ·) is a set S with a binary opera-
tion “·” such that for each a and b in S, there exist unique elements x and y in S such that
a ·x = b, y · a = b. A quasigroup is a loop L if it has also an identity e such that e · a = a · e = a.
Note that this implies in particular that in a loop, we have a unique identity element and
that the left and right inverse are unique.
Definition 2 (left Bol loop). Let L be a loop. The left Bol identity is
a · (b · (a · c)) = (a · (b · a)) · c, ∀ a, b, c ∈ L.
The right Bol identity is
(c · a) · b) · a = c · ((a · b) · a)), ∀ a, b, c ∈ L.
A loop L is said to be a left Bol loop (resp. right Bol loop) if it satisfies the left Bol identity
(resp. the right Bol identity).
Proposition 1 (alternative loop). Let L be a left Bol loop, then it is left alternative, that is
a · (a · b) = a2 · b = (a · a) · b.
Snyder Space-Time: K-Loop and Lie Triple System 3
This proposition follows from the left Bol property with b = 1. The consistency relationships
between the product and inverse map of a loop will characterize different types of loop.
Definition 3 (inverse properties). Let L be a loop.
• The automorphic inverse property (AIP) is (a · b)−1 = a−1 · b−1, ∀ a, b ∈ L.
• The inverse property (IP) is (a · b)−1 = b−1 · a−1, ∀ a, b ∈ L.
• The left inverse property (LIP) is ∃ a−1 ∈ L, a−1 · (a · b) = b, ∀ a, b ∈ L.
• The right inverse property (RIP) is ∃ b−1 ∈ L, (a · b) · b−1 = a, ∀ a, b ∈ L.
Note that the LIP together with the RIP gives the IP. Let us give now some examples of
loops.
Definition 4 (Lie group). A group is a loop satisfying the inverse property and
a · (b · c) = (a · b) · c = a · b · c.
A group is therefore in particular a left and right Bol loop.
Definition 5 (Moufang loop). A Moufang loop is a left and right Bol loop. It satisfies in
particular the inverse property.
Definition 6 (left K-loop). A left K- loop (or left Bruck loop) is a left Bol loop satisfying the
automorphic inverse property.
In [15] the authors focused on the Hopf structures related to the Moufang case, which are
a natural non-associative generalization of non-Abelian groups. We are interested instead in the
K-loop case. In the following, we shall omit the term “left” since we shall always consider left
K-loops, unless specified otherwise. To generate loops, one can consider the factorization of Lie
groups.
Proposition 2. Consider the group SO(p, 1). The decomposition SO(p, 1) ∼ L . SO(p − 1, 1)
provides a unique decomposition
g = ah, g ∈ SO(p, 1), h ∈ SO(p− 1, 1), a ∈ L.
The product of SO(p, 1) induces a product “·” in L
ab = (a · b)hab, ∀ a, b ∈ L, hab ∈ SO(p− 1, 1).
The groupoid L is a smooth K-loop with product “·”. Its inverse map coincides with the group
inverse map.
The same proposition holds for the Cartan decomposition SO(p, 1) ∼ L′ .SO(p), and the
decomposition SO(p) ∼ L′′ .SO(p − 1), i.e. both L′ and L′′ are K-loops. The proof for these
cases is given in [12]. This proposition and its variants for the other decompositions can also be
proved using the notion of Lie triple systems [16].
There is a weak form of commutativity and associativity present in a K-loop. When con-
structed from a group factorization as in the above decompositions, it takes the shape
a · b = hab(b · a)h−1
ab , (2.1)
a · (b · c) = (a · b) ·
(
habch
−1
ab
)
.
Hence K-loops can be seen as non-associative generalization of Abelian groups. To conclude this
section, let us give two examples of K-loops.
4 F. Girelli
Example 1. Consider the orthochronous Lorentz group SO+(3, 1) and its Cartan decomposition
SO+(3, 1) ∼ L′ .SO(3). L′ is a K-loop [17, 18] which can be identified to the space of 3D speeds
in Special Relativity, and the loop product encodes the speed addition used when changing
reference frame. Let us recall its structure. L′ is isomorphic to the 3d upper hyperboloid H+
3 .
This space can be embedded in R4 as H+
3 = {Vµ ∈ R4, V µVµ = 1, V0 > 0} and is generated
by the boost a = ei η
2
~b· ~K , where Ki ∈ so(3, 1) are the boost generators, η is the boost angle, ~b is
the boost vector (~b 2 = 1), and we are using the spinorial representation of SL(2, C) [19, 4]. The
product of boosts gives the loop product:
ei
η1
2
~b1· ~Kei
η2
2
~b2· ~K = ei
η12
2
~b12· ~KR
ηi,~bi
⇐⇒ a1a2 = a12h12
↓ (2.2)
ei
η1
2
~b1· ~K · ei
η2
2
~b2· ~K ≡ ei
η12
2
~b12· ~K ⇐⇒ a1 · a2 = a12.
The rotation R
ηi,~bi
is called the Thomas precession. The 3d speed ~v in Special Relativity is
given in terms of the coordinates on H+
3 by
~v(a) ≡ ~v = c tanh η~b = c
~V
V0
,
where c is the speed of light. The loop product (2.2) induces the sum of speeds:
~v(a1)⊕ ~v(a2) ≡ ~v1 ⊕ ~v2 = c tanh η12
~b12 = ~v(a1 · a2)
=
1
1 + ~v1·~v2
c2
((
1 +
γ1
1 + γ1
~v1 · ~v2
c2
)
~v1 +
1
γ1
~v2
)
, γ1 =
1√
1− ~v2
1
c2
.
It is not difficult to check that the inverse coincides with the usual inverse ~v = −~v and this
addition satisfies the automorphic inverse property −(~v1 ⊕ ~v2) = (−~v1) ⊕ (−~v2). This K-loop
can also be interpreted as momentum space in which case one reconstructs an Euclidian Snyder
space-time [3, 4].
Example 2. Consider the decomposition SO(4, 1) ∼ L . SO(3, 1). In this case L is isomorphic
to the de Sitter space dS, which can be embedded in R5 as dS = {πA ∈ R5, πAπA = −1}.
It is generated by the de Sitter boosts a = ei η
2
B·J4 , where J4 ≡ J4µ ∈ so(4, 1) (µ = 0, . . . , 3),
η and Bµ are the angle and the boost vector (B2 = −1) respectively and we are using the
spinorial representation of SO(4, 1). The de Sitter space is covered by two coordinate charts
given by π4 > 0 and π4 < 0. As in the previous example, we restrict to the upper part of the
de Sitter space, i.e. the sector with π4 > 0 (which is stable under the Lorentz action). Snyder
used the de Sitter space to define momentum space and momentum is given in terms of the
coordinates on de Sitter space [1]
pµ(a) ≡ pµ = κ tanh ηBµ = κ
πµ
π4
, (2.3)
κ is the Planck mass (~ = c = 1 in this example). He did not define the notion of momenta
addition, but we can use the K-loop structure to define it
ei
η1
2
B1·J4ei
η2
2
B2·J4 = ei
η12
2
B12·Ληi, ~Bi
⇐⇒ a1a2 = a12h12
↓
ei
η1
2
B1·J4 · ei
η2
2
B2·J4 ≡ ei
η12
2
B12·J4 ⇐⇒ a1 · a2 = a12,
Snyder Space-Time: K-Loop and Lie Triple System 5
where Ληi, ~Bi
is Lorentz transformation determined by the coordinates (ηi, Bi). Using the ex-
pression (2.3), we obtain the addition of momenta pµ(a1)⊕ pµ(a2) ≡ (p⊕ q)µ = κ tanh η12
~B12 =
pµ(a1 · a2) given in the appendix. Since it is quite complicated we give here the first order
contribution in κ2 (with p · q = pµqµ, p2 = pµpµ)
(p⊕ q)0 = p0 + q0 +
1
2κ2
(
(p0 + 2q0)p · q + q0p
2
)
+ O
(
1
κ3
)
, (2.4)
(p⊕ q)i = pi + qi +
1
2κ2
(
(pi + 2qi)p · q + (qi − pi)p0q0 − piq
2
0 + qi~p
2
)
+ O
(
1
κ3
)
. (2.5)
Once again, it is easy to check that −p is the inverse of the addition ⊕. A direct calculation
shows that the addition ⊕ satisfies the automorphic inverse property law, that is −(p1 ⊕ p2) =
(−p1)⊕ (−p2).
3 Snyder quantum group
In this section we present the main result of this paper, that is the construction of the quantum
group encoding the symmetries of Snyder spacetime. Firstly, we define the notion of K-Hopf
loop, which will be used to define the K-loop algebra kL, the analogue of the group algebra.
Secondly, we define K-Hopf coloop which is the dual notion of K-Hopf loop. A specific example
will be the algebra of functions on the K-loop C(L). Finally, we introduce the Snyder quantum
group C(L) o kSO(p− 1, 1) built out from cross product of the Lorentz group algebra with the
algebra of functions on the K-loop.
3.1 K-Hopf loop
We define here the relevant Hopf structure for the K-loop. We use the Sweedler notation for
the coproduct 4a = a(1)⊗ a(2).
Definition 7 (K-Hopf loop). Let k be a field of characteristic 6= 2, 3 and a, b ∈ H. A K-Hopf
loop is a unital algebra H, equipped with algebra homomorphisms 4 : H→H⊗H, ε : H→k
forming a coassociative coalgebra, and a map S : H→H such that
a(1) · (b · (a(2) · c)) = (a(1) · (b · a(2))) · c, (3.1)
a(1) · (S(a(2)) · b) = b = S(a(1)) · (a(2) · b). (3.2)
The equations (3.2) are the alter-ego of the left inverse property, whereas (3.1) is related to the
left Bol property. The antipode S is furthermore “multiplicative” and “comultiplicative”
S(a · b) = S(a) · S(b),
4(S(a)) = S(a(1))⊗S(a(2)).
This antipode is very different to the one met in a Hopf algebra [9] and in a Moufang Hopf
quasigroup [15], since in these cases it is “antimultiplicative” and “anticomultiplicative”.
Proposition 3. A K-Hopf loop satisfies
a(1) · (a(2) · b) = (a(1) · a(2)) · b, (3.3)
S(a(1)) · a(2) = a(1) · S(a(2)). (3.4)
Proof. (3.3) says that the K-Hopf loop is left alternative [15]. This is obtained from (3.1) by
setting b = 1. (3.4) is obtained from (3.2) applied on a⊗ 1. �
6 F. Girelli
The group algebra kG for a group G provides a natural example of Hopf algebra [9]. In
a similar way, the K-loop algebra kL provides an example of K-Hopf loop.
Proposition 4. If L is a K-loop, then H = kL is a K-Hopf loop with linear extension of the
product and on the basis elements a, b we have
m(a⊗ b) = a · b, ε(a) = 1, 4a = a⊗ a, S(a) = a−1, (3.5)
and the unit is given by e.
The proof is straightforward. We notice in particular that kL is both co-commutative and
co-associative. The infinitesimal version of smooth K-loops is a Lie triple system [16]. The
enveloping algebra for a Lie triple system has been constructed in [20]. This is another example
of K-Hopf loop and can be seen as the infinitesimal version of kL, when L is a smooth K-loop.
3.2 K-Hopf coloop
Once we have linearised the concept of K-loop into the K-loop algebra kL, one can reverse the
arrows on all the maps, and obtain a K-Hopf coloop. We note m the multiplication.
Definition 8 (K-Hopf coloop). Let k be a field of characteristic 6= 2, 3 and f, fi ∈ H.
A K-Hopf coloop is a unital associative algebra H, equipped with counital algebra homomor-
phisms 4 : H→H⊗H, ε : H→k and a linear map ι : H→H
m(f(21)⊗ f(1))⊗ f(221)⊗ f(222) = m(f(121)⊗ f(11))⊗ f(122)⊗ f(2),
(m⊗ id)(ι⊗ id⊗ id)(id⊗4)4 = 1⊗ id = (m⊗ id)(id⊗ ι⊗ id)(id⊗4)4,
ι(m(f1⊗ f2)) = m(ιf1⊗ ιf2),
4(ιf) = ιf(1)⊗ ιf(2).
By counital, we mean (id⊗ ε)4 = (ε⊗ id)4 = id.
A K-Hopf coloop satisfies the analogue of Proposition 3, where the arrows are reversed.
Proposition 5. A K-Hopf coloop satisfies
f(1) · f(21)⊗ f(22) = f(11) · f(12)⊗ f2,
m(ι⊗ id)4 = Iε = m(id⊗ ι)4.
Proof. From (3.3), we have for f ∈ k(L), a, b ∈ L,
(f, a(1) · (a(2) · b)) = (f, (a(1) · a(2)) · b)
⇔ (f(1) · f(21)⊗ f(22), a⊗ b) = (f(11) · f(12)⊗ f2, a⊗ b).
On the other hand, we have from (3.4)
(f, a(1)Sa(2)) = (f, Sa(1)a(2)) = ε(f)ε(a) ⇔ (m(ι⊗ id)4f, a) = (m(id⊗ ι)4f, a). �
As an example, we construct the K-Hopf coloop k(L) – which is identified to the algebra
of functions on L – by duality, from the K-Hopf loop kL. We use the duality (f, a) ≡ f(a),
∀f ∈ k(L), a basis element in kL, and it is extended by linearity to all elements in kL.
Proposition 6. If L is a K-loop, then H = k(L) is a K-Hopf coloop, with
(m(f1⊗ f2), a) = f1(a)f2(a), ε(f) = f(e),
(4f, a⊗ b) = f(a · b), (ιf, a) = f(a−1), (3.6)
with a, b elements in the basis of kL and fi ∈ k(L), and the unit function is I such that I(a) = 1,
∀ a ∈ L.
Snyder Space-Time: K-Loop and Lie Triple System 7
The proof is straightforward. We notice that k(L) is a commutative and associative algebra,
but is not co-commutative nor co-associative.
The general Hopf coloop satisfies similar properties as in Proposition 3, which can be simply
shown by reversing the arrows of the maps.
Example 3. Consider the K-Hopf loop generated from L ∼ SO(4, 1)/SO(3, 1) as given in
Example 2. The dual structure is given by the set of functions C(L). The coordinate func-
tions pµ ∈ C(L) is a natural example to consider. The coproduct 4pµ is then the dual of the
sum (2.4), (2.5)
〈(4pµ); a1⊗ a2〉 = 〈pµ; a1 · a2〉 = pµ(a1 · a2) = (p⊕ q)µ, ai ∈ L.
3.3 Snyder Hopf loop
We are now presenting the algebraic structure that is encoding the deformation of the Poincaré
symmetry consistent with the Snyder space. We focus on the field k = C. Considering the
decomposition SO(p, 1) ∼ L . SO(p − 1, 1), L is isomorphic to the de Sitter space. There is
a natural action of SO(p − 1, 1) on L given in terms of the adjoint action a→uau−1 = u . a,
∀ a ∈ L, u ∈ SO(p− 1, 1). This action is naturally lifted to the action of kSO(p− 1, 1) on k(L),
with (u . f, a) ≡ f(u . a) and u basis element of kSO(p − 1, 1). We recall that the group Hopf
algebra kSO(p − 1, 1) is specified by the coproduct, antipode, counity and unity respectively
defined on basis element u ∈ SO(p− 1, 1) (and extended by linearity),
4u = u⊗u, Su = u−1, ε(u) = 1, e. (3.7)
Proposition 7. k(L) is a kSO(p− 1, 1)-module algebra and coalgebra, i.e.
u . (f1.f2) = m(4u . (f1⊗ f2)), u . I = ε(u)I = I, (3.8)
4(u . f) = 4u .4f, ε(u . f) = ε(u)ε(f) = ε(f), (3.9)
with u basis elements of kSO(p−1, 1) and we used (3.7). This is naturally extended by linearity.
We have moreover
ι(u . f) = u . (ιf). (3.10)
Proof. We shall consider a, b basis elements of kL before extending naturally by linearity. First
we prove (3.8),
(u . (f1.f2), a) = ((f1.f2), u . a) = (f1⊗ f2,4(u . a)) = ((f1⊗ f2, (u . a)⊗ (u . a)))
= ((f1⊗ f2, (4u) . (4a)))) = (4u . (f1⊗ f2),4a)
= (m(4u . (f1⊗ f2)), a).
We have moreover that
(u . I, a) = (I, u . a) = ε(u . a) = 1 = ε(a) = (I, a),
where ε(u . a) is the counity on kL. Then we prove (3.9)
(4(u . f), a⊗ b) = (u . f, a · b) = (f, u . (a · b)) = (f, (u . a) · (u . b))
= (4f, (u . a)⊗ (u . b)) = ((4u) . (4f), a⊗ b).
We have also
ε(u . f) = (u . f, e) = (f, euSu) = (f, e)ε(u) = ε(f).
8 F. Girelli
Finally the proof of (3.10) goes as follows
(ι(u . f), a) =
(
u . f, a−1
)
=
(
f, u . a−1
)
= (ιf, u . a) = (u . ιf, a),
where we used that the inverse of L and SO(p − 1, 1) actually coincides with the inverse
in SO(p, 1). �
Since k(L) is a kSO(p − 1, 1) module and co-module, we can consider the semi-direct pro-
duct [9].
Definition 9 (Snyder Hopf loop). The Snyder Hopf loop S ≡ C(L) o kSO(p− 1, 1) is given
in terms of the vector space C(L) ⊗ kSO(p − 1, 1), and respectively, product, coproduct, unit,
counit, antipode and complex conjugate
Product (f1⊗u1)(f2⊗u2) = (f1.(u1 . f2)⊗u1u2),
Coproduct 4(f ⊗u) = (f(1)⊗u)⊗ (f(2)⊗u),
Unit and counit I⊗ e, ε(f ⊗u) = f(e),
Antiopde s(f ⊗u) =
(
u−1 . ιf ⊗u−1
)
,
Complex structure (f ⊗u)∗ = (u−1 . f ⊗u−1).
These relations are naturally extended by linearity when u is a general element of kSO(p−1, 1).
The coproduct 4f = f(1)⊗ f(2) is given in (3.6).
This construction can be extended to the other cases generated by the decompositions we
mentioned above: C(L′′) o kSO(p− 1) and C(L′) o kSO(p) are also Snyder Hopf loops. We have
the following proposition
Proposition 8. The antipode in the Snyder Hopf loop is antimultitplicative but comultiplicative
s ((f1⊗u1)(f2⊗u2))) = s(f2⊗u2)s(f1⊗u1),
4s = (s⊗ s)4,
m(s⊗ id)4 = m(id⊗ s)4 = Iε⊗ eε. (3.11)
Proof.
s ((f1⊗u1)(f2⊗u2))) = s(f1.(u1 . f2)⊗u1u2) =
(
(u1u2)−1 . ι(f1.(u1 . f2))⊗ (u1u2)−1
)
=
(
(u1u2)−1 . (ιf1.(u1 . ιf2))⊗ (u1u2)−1
)
=
(
(u1u2)−1 . (ιf1).(u−1
2 . (ιf2))⊗ (u1u2)−1
)
, (3.12)
where we used Proposition 7, and the properties of the action
s(f2⊗u2)s(f1⊗u1) =
(
u−1
2 . (ιf2)⊗u−1
2
)(
u−1
1 . (ιf1)⊗u−1
1
)
=
(
u−1
2 . (ιf2).(u−1
2 u−1
1 ) . (ιf1)⊗u−1
2 u−1
1
)
.
This is equal to (3.12) if we recall that the algebra k(L) is commutative. Finally we show that
the the antipode is comultiplicative
4s(f ⊗u) = 4
(
u−1 . ιf ⊗u−1
)
=
((
4u−1
)
. (4ιf)⊗4u
)
=
((
4u−1
)
. ((ι⊗ ι)4f)⊗4u
)
= (s⊗ s)4(f ⊗u),
where we used the consistency relations between the coproduct and antipode for k(L) and
kSO(p, 1) inherited from Proposition 7. The proof of (3.11) follows directly from Proposi-
tion 5. �
Snyder Space-Time: K-Loop and Lie Triple System 9
4 Scalar field theory
Once we have defined the fundamental algebraic structure, we can proceed to construct a scalar
field theory, invariant under such symmetry group, following the same route as [13]. We focus
here on the 4d Lorentzian case, i.e. L ∼ dS and we are going to show how to recover the results
of [3, 4].
As in Example 2, we focus on the upper part of the de Sitter space L+. We consider therefore
the Snyder Hopf loop S+ = C(L+)o kSO(3, 1) which is naturally obtained from S by restricting
the set of functions C(L) to C(L+). This restriction is consistent with the Hopf structures given
in Definition 9.
The scalar field is an element of the dual D∗ of the coset k(L+) ∼ S+/kSO(3, 1), which can
be specified as the set of distributions with compact support k∗(L+). ∀ f ⊗u ∈ S, v ∈ SO(3, 1)
D∗ = {F ∈ S+∗ |〈(f ⊗u)(I⊗ v), F 〉 = 〈f ⊗u, F 〉}
∼ k∗(L+) = {Φ|〈f,Φ〉 = 〈f ⊗ e, F 〉, f ∈ k(L+)}.
This set of distributions is equipped with the convolution product which makes k∗(L+) an
algebra. The convolution product is constructed using the coproduct on S+,
〈f,Φ1 ◦ Φ2〉 ≡ 〈4S(f ⊗ e), F1⊗F2〉 = 〈4k(L)f,Φ1⊗Φ2〉.
The scalar field can be seen as a function on L+, which is then interpreted as momentum
space. Hence, it is convenient to consider the subalgebra of distributions given by the algebra
of functions (with compact support) k(L+) ⊂ D∗. For φi ∈ k(L+), we have
φ1 ◦ φ2(a) =
∫
[dai]2φ1(a1)φ2(a2)δ
(
a−1 · (a1 · a2)
)
,
where we are using the measure [da] = d5π δ(πAπA + 1)θ(π4) on L+, induced from the group
decomposition and the loop product. This convolution product between functions is used to
generate the relevant terms in the scalar field action. The propagating term and the φ3-like
interaction term are given as
Ψ ◦ φ(e) =
∫
[da]2
(
p2(a1) + m2
)
φ(a1)φ(a2)δ(a1 · a2), Ψ(a) =
(
p2(a) + m2
)
φ(a), (4.1)
φ ◦ (φ ◦ φ)(e) =
∫
[da]3φ1(a1)φ2(a2)φ2(a2)δ(a1 · (a2 · a3)), (4.2)
where the delta function encodes the generalization of the momentum conservation law and pµ(a)
is the Snyder coordinates associated to the loop element a. Note that the convolution product
is not associative, since it is based on the loop product. It is therefore important to keep track
of the order of the brackets. The action Σ(φ) for the scalar field φ reads explicitly
Σ(φ) = Ψ ◦ φ(e) +
λ
3!
φ ◦ (φ ◦ φ)(e) =
∫
[dp]2 φ(p1)
(
p2
1 + m2
)
φ(p2)δ(p1 ⊕ p2)
+
λ
3!
∫
[dp]3 φ(p1)φ(p2)φ(p3)δ(p1 ⊕ (p2 ⊕ p3)).
In the second line, we have expressed the action in terms of the Snyder coordinates, in par-
ticular the measures reads κ4[da] = [dp] = 1
2d4p
((
1 − p2
κ2
))− 5
2 . The sum of momenta is given
by (2.4), (2.5).
We need to check that the Snyder Hopf loop S+ encodes the right symmetry for this scalar
action. We need therefore to define how the field φ is transforming under the quantum group
action. The symmetry action of S+ on k∗(L+) is induced by its action on S∗
〈f, (a⊗u) . φ〉 ≡ 〈f ⊗ e, (a⊗u) . F 〉 = 〈(a⊗u)∗(f ⊗ e), F 〉.
10 F. Girelli
This means concretely that the Lorentz group SO(3, 1) sector acts by the adjoint action, whereas
the k(L+) sector acts by multiplication. This last sector encodes the deformed translation
symmetry
φ(a)→φ(u . a), φ(a)→f(a)φ(a). (4.3)
To determine the exact realization of the translation symmetry, one needs to introduce space-
time, through a Fourier transform. We consider therefore the c-numbers xµ ∈ R4 and the
plane-wave eip·x. To keep track of the modified sum of momenta (2.4), (2.5), we introduce the
modified product ∗ between plane-waves
eip1·x ∗ eip2·x ≡ ei(p1⊕p2)·x.
We can then introduce the Fourier transform of the field φ ∈ C(L+)
φ̂(x) ≡
∫
[da] eip(a)·xφ(a) =
∫
[dp] eip·xφ(p).
The product between fields is given by the ∗-product, which is the dual of the convolution
product
(φ̂1 ∗ φ̂2)(x) =
∫
[dp]2 eip1·x ∗ eip2·xφ1(p1)φ2(p2)
=
∫
[dp]3 eip·xφ1(p1)φ2(p2)δ(−p⊕ (p1 ⊕ p2)) =
∫
[dp] eip·xφ1 ◦ φ2(p).
With this Fourier transform, the scalar field action becomes
Σ(φ) =
∫
[dx]
(
−(∂µφ̂) ∗ (∂µφ̂) + m2φ̂ ∗ φ̂ +
λ
3!
φ̂ ∗ (φ̂ ∗ φ̂)
)
.
The translation action is now given as x→x + ε and the field φ(p) is transforming therefore as
φ(p)→ eip·εφ(p), which gives us therefore f(p) = eip·ε for the symmetry action (4.3).
To prove invariance of the action under S+, we also need to know how it is acting on convo-
lution product. This is constructed naturally using the coproduct of S+ and the coproduct on
k(L+) as in (3.6)
(f ⊗u) . (φ1 ◦ φ2) ≡ ((f(1)⊗u) . φ1) ◦ ((f(1)⊗u) . φ2).
It is straightforward to check that the action Σ(φ) constructed from the terms (4.1) and (4.2)
is then invariant under such transformations. The coproduct of S+ also provides the action of
the Lorentz group and the translations on a tensor product of fields
φ(p1)φ(p2)→φ(u . p1)φ(u . p2), φ(p1)φ(p2)→ ei(p1⊕p2)·εφ(p1)φ(p2).
The deformation of the translation symmetry is therefore naturally encoded in the loop product.
Before closing this section, we can evaluate explicitely the star product between different
typical functions, using the properties of the Fourier transform we have introduced. For example,
we can calculate the commutator of coordinates with the ∗-product [3, 4]
[xµ, xν ]∗ = 0. (4.4)
The commutator of monomes of the first degree is zero. This is quite different than the usual non-
commutative spaces. However, this does not mean that we have a commutative space. Indeed,
for example the commutator of a monome of the second degree with one of the first degree
Snyder Space-Time: K-Loop and Lie Triple System 11
is not zero due to the non-associativity. As an example, let us look at the more complicated
combination:
〈xµ, xν , xα〉 ≡ xµ ∗ (xν ∗ xα)− xν ∗ (xµ ∗ xα) = − 1
κ2
(ηναxµ − ηµαxν) =
i
κ2
Jµν . xα, (4.5)
where Jµν ∈ so(3, 1) acts in the usual way on the coordinates xα. By introducing a Weyl map,
we are going to show in the next section how this star product is the realization of the non-
commutative structure implemented by a Lie triple system, the infinitesimal version of a smooth
K-loop.
5 Snyder space as a Lie triple system
In the first subsection, we first recall the different examples of Bol algebras which are the
infinitesimal version of the different smooth Bol loops we considered in Section 2. We focus
on the Lie triple system case and show how given a Lie triple system, we can recover Snyder
commutation relations, through Jacobson’s embedding theorem. In the second subsection, we
define the Weyl map and recover the star product we have introduced.
5.1 Lie triple system
In Section 2, we have first introduced the concept of Bol loop. The concept of Bol algebra is
the infinitesimal version of a smooth Bol loop.
Definition 10 (left Bol algebra, see [20] and references therein). A left Bol algebra is
a vector space B equipped with a bilinear and a trilinear product, noted respectively [ , ]B and
〈 , , 〉, satisfying the following properties X, Y, Z, U, V ∈ B
〈X, Y, Z〉 = −〈Y, X,Z〉, [X, Y ]B = −[Y, X]B,
〈X, Y, Z〉+ 〈Y, Z, X〉+ 〈Z,X, Y 〉 = 0, (5.1)
〈U, V, 〈Z,X, Y 〉〉 = 〈〈U, V, Z〉, X, Y 〉+ 〈Z, 〈U, V, X〉, Y 〉+ 〈Z,X, 〈U, V, Y 〉〉, (5.2)
〈U, V, [X, Y ]B〉 = [〈U, V,X〉, Y ]B + [X, 〈U, V, Y 〉]B + [X, 〈U, V, Y 〉]B
+ 〈X, Y, [U, V ]〉+ [[U, V ], [X, Y ]]B.
(5.1) encodes the Jacobi identity for the trilinear operation. (5.2) means that the map
δX,Y : B→B defined as δX,Y (Z) = 〈X, Y, Z〉 is a derivation for the ternary operation. Explicitly,
if D is a derivation D for the trilinear product one has
D〈X, Y, Z〉 = 〈DX, Y, Z〉+ 〈X, DY,Z〉+ 〈X, Y,DZ〉.
Proposition 9. Given a Bol loop L, the bilinear and trilinear products [ , ]B, 〈 , , 〉of the Bol
algebra are obtained as follows [21]:
[X1, X2]B ≡ −1
2
d2
dt1dt2
(
(a1 · a2) · (a2 · a1)−1
)
,
(X1, X2, X3) ≡ i
1
3!
d3
dt1dt2dt3
(
((a1 · a2) · a3) · (a1 · (a2 · a3))−1
)
,
〈X1, X2, X3〉 = 2(X1, X3, X2) + [[X, Y ]B, Z]B, (5.3)
with Xi = d
dti
ai
∣∣
ti=0
= d
dti
eitiXi
∣∣
ti=0
.
We can identify now the infinitesimal structures behind the most well known examples of
Bol loops.
12 F. Girelli
Example 4. A Lie algebra g with product [ , ] is a Bol algebra such that
[X, Y ]B ≡ [X, Y ], 〈X, Y, Z〉 ≡ 0.
This can be derived from Proposition 9 using the loop given by a Lie group. We recover that
a Lie algebra is the infinitesimal version of a Lie group.
Definition 11 (Mal’tsev algebra, see [20] and references therein). A Mal’tsev algebra
is a left Bol algebra such that
〈X, Y, Z〉 = [[x, y]B, z]B −
1
3
J(x, y, z), [J(x, y, z), x]B = J(x, y, [x, z]B)
with
J(x, y, z) = [[x, y]B, z]B + [[z, u]B, y]B + [[y, z]B, x]B.
Such algebra can be seen as the infinitesimal version of a smooth Moufang loop as it can be
checked using Proposition 9.
We introduce now the relevant example for studying Snyder space-time.
Definition 12 (Lie triple system, see [20, 22, 16] and references therein). A Lie triple
system ` is a left Bol algebra such that
[X, Y ]B = 0, ∀x, y ∈ `.
It is therefore totally determined in terms of the trilinear product which has the following
properties
〈X, Y, Z〉 = −〈Y, X,Z〉,
〈X, Y, Z〉+ 〈Y, Z, X〉+ 〈Z,X, Y 〉 = 0, (5.4)
〈U, V, 〈Z,X, Y 〉〉 = 〈〈U, V, Z〉, X, Y 〉+ 〈Z, 〈U, V, X〉, Y 〉+ 〈Z,X, 〈U, V, Y 〉〉.
In fact a direct calculation from (5.3) shows that we have
〈X, Y, Z〉 = 2(Z,X, Y ) = [[X, Y ], Z] = (XY − Y X)Z − Z(XY − Y X). (5.5)
This structure can be seen as the infinitesimal version of a smooth K-loop [16], once again using
Proposition 9.
Lie triple systems have been studied by mathematicians and most of the results found in the
context of Lie algebras have been analysed in the Lie triple system context, see [16, 23, 24] and
references therein.
The property (5.5) indicates that the trilinear product can be expressed in terms of some Lie
algebra bracket. The following theorem makes this statement more precise.
Theorem 1 (standard imbedding [22, 24]). Let ` a Lie triple system, and for X, Y ∈ `, we
note δX,Y (Z) the linear transformation Z→〈X, Y, Z〉 and m the set of all such linear transfor-
mations. Then m is a Lie algebra and moreover the vector space G ≡ `⊕m is a Lie algebra with
bracket
[X, Y ] = δX,Y , [A,B] = AB −BA,
[A,X] = −[X, A] = AX, ∀A,B ∈ m, ∀X, Y ∈ p.
The map σ : G→G, such that σ(X) = −X, σ(A) = A is an involutive automorphism of G.
Snyder Space-Time: K-Loop and Lie Triple System 13
Notice that the bracket appearing in this construction has nothing to do with the Bol brac-
ket [ , ]B.
Remark 1. From the property (5.2), we notice that δX,Y is a derivation. One can restate the
previous theorem by saying that one can make Der` ⊕ ` a Lie algebra, where Der` is the set of
(inner) derivations of `.
Proof. To prove the Theorem 1, one has to check that the Lie bracket satisfies the Jacobi
identity. Following the property (5.4), it is clear that we have for example
[[X, Y ], Z] + [[Z,X], Y ] + [[Y, Z], X] = 0.
The relation
[[A, Y ], Z] + [[Z,A], Y ] + [[Y, Z], A] = 0,
states that A = δX,Y is a derivation. This relation is shown by straightforward calculations.
The other relations follow also by direct calculations. �
Conversely, given a Lie algebra G and an involutive automorphism σ, one can easily construct
a Lie triple system. Indeed, since σ is an involution, it has two eigenvalues −1, 1, with respective
eigenspaces noted `, m, such that G = ` ⊕ m. σ is an automorphism, therefore using the
compatibility with the product, it is easy to check that [`, `] ⊂ m, [`,m] ⊂ ` and [m,m] ⊂ m.
The vector space ` equipped with the trilinear product 〈X, Y, Z〉 = [[X, Y ], Z] constructed from
the Lie algebra bracket 〈 , , 〉 is a Lie triple system.
This theorem and its converse are the key to understand the nature of Snyder spacetime. The
following example shows how to construct a Lie triple system from so(p, 1) and recover Snyder
non-commutative structure.
Example 5. The decomposition of the Lie algebra so(p, 1) ∼ ` ⊕ so(p − 1, 1) is given by
the involutive automorphism σ(Jpµ) = −Jpµ and σ(Jµν) = Jµν with µ = 0, . . . , p − 1 and
Jµν ∈ so(p− 1, 1). The sector ` is generated by the “de Sitter boosts” Jpµ and is therefore a Lie
triple system. We have following Jacobson’s theorem
〈Jpµ, Jpν , Jpα〉 = ηναJpµ − ηµαJpν .
Conversely, given the Lie triple system `, we recover the Snyder commutation relation by intro-
ducing the dimensionful coordinates
Xµ =
1
κ
Jpµ,
which satisfy therefore, still thanks to Jacobson’s theorem
[Xµ, Xν ] ≡ δXµ,Xν = 〈Xµ, Xν , .〉 =
1
κ2
Jµν .
This can be naturally extended to the case so(p, q). Coming back to the example we have
considered in Section 4, the smooth K-loop L ∼ SO(4, 1)/SO(3, 1) is associated with the Lie
triple system of Example 5. The operator Xµ ∼ 1
κJpµ can be viewed as a distribution (with
support the identity element) as follows. Given a test function f ∈ C(L+),
(Xµ, f) ≡ −2κi
d
dη
f
(
ei η
2
Jpµ
)∣∣∣
η=0
.
14 F. Girelli
The bilinear and trilinear products are then constructed in a similar way as in Proposition 9
([X1, X2]B, f) ≡ i
1
2
d2
dt1dt2
f
(
(a1 · a2) · (a2 · a1)−1
)
,
(〈X1, X2, X3〉, f)i ≡ 1
3
d3
dt1dt2dt3
f
(
((a1 · a2) · a3) · (a1 · (a2 · a3))−1
)
.
This means in particular that there is two types of bracket one should not confuse. On one
hand there is the Bol bracket [Xµ, Xν ]B = 0 which is trivially zero since we are dealing with
a K-loop (and especially the automorphic property). On the other hand, there is the bracket
associated with the imbedding Lie algebra [Xµ, Xν ] ≡ δXµ,Xν from which the trilinear product
is constructed as in (5.5). The Snyder commutation relations are encoded in this bracket and
the natural algebraic framework to describe the Snyder space-time given by the Lie triple system
structure.
5.2 Weyl map
We have seen in the last section that given a Lie triple system, Jacobson’s theorem allows to
recover the Snyder commutation relation. It is therefore natural to consider the set of functions
on Snyder spacetime as the set of functions on the Lie triple system.
The notion of enveloping algebra U(B) has been constructed for Bol algebras, and in par-
ticular it satisfies the Poincaré–Birkhoff–Witt theorem; we refer to [20] for the details. As
a consequence, such enveloping algebra U(`) exists for a Lie triple system `. This enveloping
algebra U(`) can be interpreted as the algebra of non-commutative functions C(`) on Snyder
space-time, generated from the coordinates operators Xµ.
We want to check now that the star product we have introduced in Section 4 is related to
the non-commutative structure encoded in the Lie triple system. For this we need to introduce
a Weyl map, an algebra isomorphism between the algebra of functions on the Lie triple system
C(`) and the algebra of functions with the star product C∗(R4) .
W : C(`)→C∗
(
R4
)
,
f̂(X) ≡
∫
dS+
[dp] f(p)eip·X→W(f̂)(x) ≡
∫
dS+
[dp] f(p)eip·x, (5.6)
where p is a momentum coordinate choice on the upper part of the de Sitter space, [dp] the
measure on the upper part of the de Sitter space, eip·X ∈ L and f(p) ∈ C(L). For example,
for the generator Xµ, we have W(Xµ)(x) ≡ xµ. The Weyl map is an isomorphism of algebra
therefore by definition we need to have W(f̂ · ĥ)(x) ≡ (W(f̂) ∗ W(ĥ))(x). The definition of
the Weyl map (5.6) makes sure that the ∗-product we are using is precisely the one defined in
Section 4. We have in particular
W(XµXν)(x) = xµ ∗ xν .
It is then immediate to check that
W([Xµ, Xν ]B) = 0 = W(XµXν −XνXµ) = xµ ∗ xν − xνxµ = 0,
in accordance with (4.4). To consider the mapping of a product of three operators, we need
to be careful since we have to take into account the non-associativity. Indeed, the position
operator Xµ acts by (∗-)multiplication of xµ
Xµ . f̂(x) ≡ xµ ∗ f̂(x),
Snyder Space-Time: K-Loop and Lie Triple System 15
and we use the natural action on the left, so that when considering the product of operators,
we have
(XµXν) . f̂(x)) ≡ Xµ . (Xν . f̂(x)) = xµ ∗ (xν ∗ f̂(x)).
This is important to keep in mind since we are using now a non-associative structure: order
does matter. The product of operators is therefore naturally ordered from the right to the left.
We have then
W(XµXνXα) = xµ ∗ (xν ∗ xα).
When considering the triple product, the commutator does contribute in a non trivial way due
to the non-associativity
W(〈Xµ, Xν , Xα〉) = W([[Xµ, Xν ], Xα]) = W
(
i
κ2
[Jµν , Xα]
)
= W((XµXν −XνXµ)Xα −Xα(XµXν −XνXµ))
= xµ ∗ (xν ∗ xα)− xν ∗ (xµ ∗ xα)− xα ∗ (xµ ∗ xν − xν ∗ xµ)) (5.7)
=
i
κ2
[Jµν , xα] = 〈xµ, xν , xα〉,
where we have used (4.4), so that the last term in (5.7) is zero and we have used the result
in (4.5). The Weyl map preserves, as required, the Lie triple system structure.
The Snyder commutation relation [Xµ, Xν ] can also be obtained from the ∗-product using its
action a function f̂(x), which we take to be xα for simplicity [3, 4]
[Xµ, Xν ] . xα = (XµXν −XµXν) . xα = xµ ∗ (xν ∗ xα)− xν ∗ (xµ ∗ xα) =
i
κ2
Jµν . xα.
This shows that the commutator of the position operators does satisfy
[Xµ, Xν ] = i
1
κ2
Jµν . (5.8)
Concluding remarks
There are two ways to interpret Snyder commutation relations. The first one is to say that
this non-commutative space is actually a subspace of a bigger non-commutative space generated
from the coordinates algebra (Xα, Jµν). The coordinates Jµν are interpreted as coordinates
describing extra dimensions. This is the perspective followed in the Doplicher–Fredenhagen–
Roberts model [25] which can be seen then as an abelianization of the Snyder model [4, 26]
with this interpretation. The second possibility is to consider space-time as only generated by
the Xµ. In this case, we have to face the issue that the coordinates commutations do not close.
We argued in this paper that the solution of this issue is to consider Snyder space-time given
in terms of a Lie triple system, that is instead of the bilinear product (5.8), one considers the
trilinear product
〈Xµ, Xν , Xα〉 ≡ [[Xµ, Xν ], Xα] =
1
κ2
(ηναXµ − ηµαXν) .
In this case, one can construct a meaningful notion of non-commutative algebra of functions
using the notion of enveloping algebra for Lie triple system which is dual, using a generalized
Fourier transform, to the algebra of functions on the K-loop L ∼ SO(p, 1)/SO(p− 1, 1). On the
16 F. Girelli
other hand, given the Lie triple system, Jacobson’s theorem tells us how to embedd it into a Lie
algebra and to recover the Snyder commutation relations, from the trilinear product
[Xµ, Xν ] ≡ δXµ,Xν = 〈Xµ, Xν , .〉 =
1
κ2
Jµν .
Lie triple systems provide therefore a new type of non-commutative geometry. This non-
commutative geometry can be seen as flat since one can identify a quantum group, a deformation
of the the Poincaré group, which acts on this space in a consistent way. We provided here the
definition of this new quantum group, which we called Snyder quantum group, using the struc-
ture of K-loop. This latter can be be interpreted as momentum space as in Snyder’s initial idea.
The K-loop structure provides a rule to add momenta, an addition which is non-commutative
and non-associative, just like the speed addition in Special Relativity. We have also presented
how we can construct a scalar field action on a Lie triple system using a star product realization
and how this action is invariant under the Snyder quantum group, encoding the analogue of the
flat symmetries.
This work opens up a number of questions which are of interests for either mathematicians
or theoretical physicists.
• The Snyder quantum group needs to be studied in details: the analysis of the representa-
tion theory should be performed. It would be interesting to check if the weak notion of
commutativity (2.1) is related to a notion of braiding.
• The classification of the bi-covariant differential calculus [27] for Snyder space-time should
be performed to make the analysis of the conserved currents [11].
• Now that we have a well defined classical action for a scalar field, it would be interesting
to check if this non-commutative space fullfils Snyder’s hope, that is whether a quantum
field theory living in this space has no UV divergences.
A Snyder sum
In this appendix we present the calculations to determine the modified sum of momenta (2.4),
(2.5) in Snyder’s coordinates. We calculate the product of de Sitter boosts ai = cosh ηi
2 Id +
2i sinh ηi
2 BµJ4µ
a1a2 = aΛR,
where Λ = cosh α
2 Id + 2i sinh α
2 biJ0i is a boost and R = cos θ
2 Id + i sin θ
2 riJi is a rotation. The
term a will encode the sum of momenta and ΛR is the Lorentz precession, the generalization of
the Thomas precession to the 4d Lorentzian case [28]
cosh
η1
2
cosh
η2
2
− sinh
η1
2
sinh
η2
2
B1 ·B2 = cosh
η
2
cosh
α
2
cos
θ
2
(1l), (A.1)
sinh
η1
2
sinh
η2
2
( ~B1 ∧ ~B2)i = cosh
η
2
cosh
α
2
cos
θ
2
ri (Ri), (A.2)
sinh
η1
2
sinh
η2
2
(B0
1Bi
2 −Bi
1B
0
2) = cosh
η
2
sinh
α
2
(
cos
θ
2
bi + sin
θ
2
(~b ∧ ~r)i
)
(K), (A.3)
cosh
η1
2
sinh
η2
2
B0
2 + cosh
η2
2
sinh
η1
2
B0
1 = cosh
α
2
sinh
η
2
cos
θ
2
B0
+ sinh
η
2
sinh
α
2
(
cos
θ
2
~B ·~b + sin
θ
2
εjkir
iBjbk
)
(J40), (A.4)
cosh
η1
2
sinh
η2
2
Bi
2 + cosh
η2
2
sinh
η1
2
Bi
1
Snyder Space-Time: K-Loop and Lie Triple System 17
= cosh
α
2
sinh
η
2
(
cos
θ
2
Bi − sin
θ
2
εi
mkr
mBk
)
+ B0 sinh
α
2
sinh
η
2
(
cos
θ
2
bi − sin
θ
2
bkrjε
i
kj
)
(J4i). (A.5)
The other equations state that we have ~r · ~b = 0 and ~r · ~B = 0. This all together allows to
reconstruct the sum of momenta given the choice of coordinates kµ = κ f(η)Bµ. we are going to
determine this sum in the Snyder’s choice, i.e. pµ = κ tanh η Bµ. With this choice, we have in
particular cosh η ≡ Γ = 1√
1+ p2
κ2
. Considering the ratio of the equation (A.2) with (A.1), together
with tanh η
2 = tanh η
1+Γ−1 , we have
tan
θ
2
~r =
~p1 ∧ ~p2
κ2(1 + Γ−1
1 )(1 + Γ−1
2 )− p1 · p2
⇔ ~ρ = ~A(p1, p2),
with ~ρ = tan θ
2 ~r. Since ~r 2 = 1, we have in particular that
tan2 θ
2
= | ~A(p1, p2)|2. (A.6)
Considering the ratio of the equation (A.3) with (A.1), we obtain
tanh
α
2
(
~b− tan
θ
2
~r ∧~b
)
=
p0
1~p2 − p0
2~p1
κ2(1 + Γ−1
1 )(1 + Γ−1
2 )− p1 · p2
⇔ tanh
α
2
(
1l− tan
θ
2
~r∧
)
(~b) =
p0
1~p2 − p0
2~p1
κ2(1 + Γ−1
1 )(1 + Γ−1
2 )− p1 · p2
⇔ ~F (~β) = ~C(p1, p2),
with ~β = tanh α
2
~b. The map ~F is a linear map and bearing in mind that ~r ·b = 0, its inverse ~F−1
is given by
~F−1 = cos2
θ
2
(
1l + tan
θ
2
~r∧
)
= cos2
θ
2
~D,
and is also a linear map. We can therefore define ~β = tanh α
2
~b in terms of p1 and p2 since
~β = cos2
θ
2
~D(~C(p1, p2)) = cos2
θ
2
(1l + ~ρ∧)
(
p0
1~p2 − p0
2~p1
κ2(1 + Γ−1
1 )(1 + Γ−1
2 )− p1 · p2
)
.
Now since cos2 θ
2 =
(
1 + tan2 θ
2
)−1 and tan2 θ
2 is given in (A.6), we can determine cos2 θ
2 in terms
of A(p1, p2) and hence ~β in terms of p1 and p2.
Considering the ratio of the equation (A.5) with (A.1), we obtain
κ
(
(1 + Γ−1
2 ) ~p1 + (1 + Γ−1
1 ) ~p2
κ2(1 + Γ−1
1 )(1 + Γ−1
2 )− p1 · p2
)
= tanh
η
2
( ~B − ~ρ ∧ ~B) + tanh
η
2
B0(~β + ~ρ ∧ ~β),
~E(p1, p2) = tanh
η
2
~F ( ~B) + tanh
η
2
B0 ~D(~β).
We can use the inverse map ~F−1 = cos2 θ
2
~D to determine ~B in terms of B0, p1, p2 and η, since
we have that ~r · ~B = 0,
tanh
η
2
~B = − tanh
η
2
B0 cos2
θ
2
~D( ~D(~β)) + cos2
θ
2
~D( ~E(p1, p2))
18 F. Girelli
⇔ tanh
η
2
~B = − tanh
η
2
B0 ~V + ~W, (A.7)
where ~V ≡ cos2 θ
2
~D( ~D(~β)) and ~W ≡ cos2 θ
2
~D( ~E(p1, p2)) are vectors totally determined in terms
of p1 and p2.
Considering the ratio of the equation (A.4) with (A.1), we obtain
κ
(
(1 + Γ−1
2 ) p0
1 + (1 + Γ−1
1 ) p0
2
κ2(1 + Γ−1
1 )(1 + Γ−1
2 )− p1 · p2
)
= tanh
η
2
(
B0 + ( ~B · ~β + ~B · (~ρ ∧ ~β))
)
⇔ I(p1, p2) = tanh
η
2
(
B0 + ~B · ~D(~β)
)
.
We can used now the value for ~B identified in (A.7)
tanh
η
2
B0 +
(
− tanh
η
2
B0 ~V + ~W
)
· ~D(~β) = I(p1, p2)
⇔ tanh
η
2
B0
(
1− ~V · ~D(~β)
)
= − ~W · ~D(~β) + I(p1, p2)
⇔ tanh
η
2
B0 =
− ~W · ~D(~β) + I(p1, p2)
1− ~V · ~D(~β)
.
We can then plug the value of B0 into (A.7) to obtain Bi in terms of the pi
tanh
η
2
~B =
− ~W · ~D(~β) + I(p1, p2)
1− ~V · ~D(~β)
~V + ~W.
The final step to recover the sum is to recall that tanh η
2 = tanh η
1+Γ−1 and Γ can be expressed in
terms of Γi
Γ = Γ1Γ2
(
1− p1 · p2
κ2
)
.
Hence we have that
~ptot = −−−−→
p1 ⊕ p2 =
(
1 + Γ−1
1 Γ−1
2
(
1− p1 · p2
κ2
)−1
)(
− ~W · ~D(~β) + I(p1, p2)
1− ~V · ~D(~β)
~V + ~W
)
,
p0
tot = (p1 ⊕ p2)0 =
(
1 + Γ−1
1 Γ−1
2
(
1− p1 · p2
κ2
)−1
)
− ~W · ~D(~β) + I(p1, p2)
1− ~V · ~D(~β)
.
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1 Introduction
2 K-loop: a review
3 Snyder quantum group
3.1 K-Hopf loop
3.2 K-Hopf coloop
3.3 Snyder Hopf loop
4 Scalar field theory
5 Snyder space as a Lie triple system
5.1 Lie triple system
5.2 Weyl map
A Snyder sum
References
|