Write an equation in slope-intercept form of the line that bisects the angle formed by NM and NO.

=0
d
F
=
0

⋆=⋆
d
⋆
F
=
⋆
J

where in fact the second equality is not needed because of the identity 2=0
d
2
=
0
. In abstract tensor notation, these equations expand to
=2∇[]
F
a
b
=
2
∇
[
a
A
b
]

∇[]=0
∇
[
a
F
b
c
]
=
0

∇=
∇
a
F
a
b
=
J
b

where in the above, additional compact notation has been used to simplify things: square brackets mean antisymmetrizing indices, and the covariant derivative ∇
∇
a
is related to a flat derivative ∂
∂
a
and a connection Γ
Γ
b
a
c
. Now, performing a 3+1 decomposition (here in flat space-time) Maxwell's equations break down as
∇⋅⃗=0
∇
⋅
B
→
=
0

∇⋅⃗=4
∇
⋅
E
→
=
4
π
ρ

∂⃗∂=−∇×⃗
∂
B
→
∂
t
=
−
∇
×
B
→

∂⃗∂=+∇×⃗−4⃗.
∂
E
→
∂
t
=
+
∇
×
B
→
−
4
π
J
→
.

In the above, there is yet more compactifying notation: the divergence is defined as
∇⋅⃗=∂∂
∇
⋅
v
→
=
∂
v
i
∂
x
i

and the curl is defined as
(∇×⃗)=∂∂.
(
∇
×
v
→
)
i
=
ϵ
i
j
k
∂
v
k
∂
x
j
.

In the above, still more compactifying notation has been used, namely the Einstein summation convention. Written out in components, the divergence is
∇⋅⃗=∂∂+∂∂+∂∂
∇
⋅
v
→
=
∂
v
x
∂
x
+
∂
v
y
∂
y
+
∂
v
z
∂
z

and curl is defined as
(∇×⃗)=∂∂−∂∂
(
∇
×
v
→
)
z
=
∂
v
y
∂
x
−
∂
v
x
∂
y

and similarly for the other two components.

As you can see, the extremely compact and powerful notation
=0
d
F
=
0

⋆=⋆
d
⋆
F
=
⋆
J

expands out to a large set of component equations—but they have the same content, so it's not very meaningful that the equations in components are much longer than in terms of differential forms.

As another example, Elson Liu answered with an exploded-out form of the Standard Model Lagrangian. However, it can be written much more compactly:
=+Higgs+lep.+qrk.+Yuk.
L
=
L
Y
M
+
L
H
i
g
g
s
+
L
l
e
p
.
+
L
q
r
k
.
+
L
Y
u
k
.

where
=−14Tr
L
Y
M
=
−
1
4
T
r
F
a
b
F
a
b

Higgs=−(†)()+14(†−122)2
L
H
i
g
g
s
=
−
(
D
a
ϕ
†
)
(
D
a
ϕ
)
+
1
4
λ
(
ϕ
†
ϕ
−
1
2
v
2
)
2

lep.=ℓ†⧸ℓ+¯†⧸¯
L
l
e
p
.
=
i
ℓ
I
†
⧸
D
ℓ
I
+
i
e
¯
I
†
⧸
D
e
¯
I

qrk.=†⧸+¯†⧸¯+¯†⧸¯
L
q
r
k
.
=
i
q
I
†
⧸
D
q
I
+
i
u
¯
I
†
⧸
D
u
¯
+
i
d
¯
I
†
⧸
D
d
¯
I

Yuk.=−ℓ¯−′¯
L
Y
u
k
.
=
−
y
I
J
ϕ
ℓ
I
e
¯
J
−
y
I
J
′
ϕ
q
I
d
¯
J

−″†¯+ℎ..
−
y
I
J
″
ϕ
†
q
I
u
¯
J
+
h
.
c
.

and specifying that F is the curvature on the gauge group ≃(3)⊗(2)⊗(1)
G
≃
S
U
(
3
)
⊗
S
U
(
2
)
⊗
U
(
1
)
,
D
a
is the gauge covariant derivative on G, ⧸
⧸
D
is the gauge covariant Dirac operator on G, and specifying the representations of the fields: the scalar
ϕ
in the rep (1,2,-1/2), and the fermions ℓ
ℓ
in the rep (1,2,-1/2), ¯
e
¯
in (1,1,+1),
q
in (3,2,+1/6), ¯
u
¯
in (3¯,1,−2/3)
(
3
¯
,
1
,
−
2
/
3
)
, and ¯
d
¯
in (3¯,1,+1/3)
(
3
¯
,
1
,
+
1
/
3
)
; where I is a generation index running over 1,2,3, and where everything else is a constant.
42.7K viewsView 50 upvotes · Answer requested by Mario Ng

50

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