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So as an exercise in using this notation let’s look at. The indices indicate very specific matrix or vector components/elements. These are not matrices themselves, but just numbers, which we can reorder as we wish. We still have to respect the summations over repeated indices!. And remember - PowerPoint PPT Presentation
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So as an exercise in using this notation let’s look at
)()( yx
)()( ygx
ygx
yxg
The indices indicate very specific matrix or vector components/elements. These are not matrices themselves, but just numbers, which we can reorder as
we wish. We still have to respect the summations over repeated indices!
yxg??
(g) = gAnd remember we just showed
xyyx
i.e.
yxyx )()( All dot products are INVARIANT
under Lorentz transformations.
yxg
even for ROTATIONS as an example, considerrotations about the z-axis
331212
212100
)sincos)(sincos(
)sincos)(sincos()()(
yxyyxx
yyxxyxyx
332111221222
222122121100
sincossincossincossincossincossincos yx
yxyxyxyxyxyxyxyxyx
33221100 yxxxxxyx yx
The relativistic transformations:
)(cEpp xx
)( xpcE
cE )( xcpEE
suggest a 4-vector);( p
cEp that also
transforms by
pp )(
so pp should be an invariant!
22
2p
cE
Ec
In the particle’s rest frame:px = ? E = ? pp = ?0 mc2 m2c2
In the “lab” frame:
)(cEpp xx
)( xpcE
cE
)0( mc = mv
= = mc
2222222)()( cmcmpp
so
2222 )1( cm
22cm
Limitations of Schrödinger’s Equation
1-particle equation ),()(),(
2),( 2
1
22
txxVtxxm
txt
i
),,(),()()(
),,(2
),,(2
),,(
212121
2122
2
2
2
2121
2
2
2
21
txxxxVxVxV
txxxm
txxxm
txxt
i
2-particle equation:
mutual interaction
But in many high energy reactions
the number of particles is not conserved!
np+e++e
n+p n+p+3
e+ p e+ p + 6 + 3
i.e. a class of differential eq's
to which Schrodinger's
equations all belong!
Sturm-Liouville Equations
then notice we have automatically
a class of differential equations that include:
Legendre's equationthe associated Legendre equationBessel's equationthe quantum mechanical harmonic oscillator
whose solutions satisfy: 3** )( drmnmn for different
eigenfunctions, n0
If we adopt the following as a definition of the "inner product"
3* dr compare this directly to the vector "dot product"
nnnuv*
*
eigenvalues are REALand
different eigenfunctionsare "orthogonal"
Recall: any linear combination of simple solutions to a differential equation is also a solution,
and, from previous slide:
nmnm mn
Thus the set of all possible eigenfunctions (basic solutions) provide an "orthonormal" basis set and any general solution to the differential equation becomes expressible as
n
nna where 3* dra nnn
any general solution will be a function in the "space" of all possible solutions (the solution set) sometimes called a Hilbert Space (as opposed to the 3-dimensional space of geometric points.
What does it mean to have a matrix representation of an operator? of Schrödinger’s equation?
nnn EH
nEnH nwhere n represents all distinguishing quantum numbers
(e.g. n, m, ℓ, s, …)
nmnn EnmEnHm
Hmn
HHnEnH
mEHm
n
m
since†
E1 0 0 0 0 . . .H = 0 E2 0 0 0 . . . 0 0 E3 0 0 . . .
0 0 0 E4 0 . . .:
.100 : ·
010 : ·
001 : ·, , ,
...with the “basis set”:
This is not general at all (different electrons, different atoms require different matrices)Awkward because it provides no finite-dimensional representation
That’s why its desirable to abstract the formalism
azreaz /
23
1001
aZrea
Zra
Z 2/23
200 21
21
iaZr ereaZ
sin
81 2/
25
121
cos2
1 2/25
210aZrre
aZ
iaZr ereaZ sin
81 2/
25
211
aZrea
rZaZr
aZ 3/
2
2223
300 21827381
1
Hydrogen Wave Functions 10000 ::
01000 ::
00100 ::
00010 ::
00001 :: 0
0000
Angular Momentum|lmsms…>
l = 0, 1, 2, 3, ...Lz|lm> = mh|lm> for m = l, l+1, … l1, lL2|lm> = l(l+1)h2|lm>
Sz|lm> = msh|sms> for ms = s, s+1, … s1, sS2|lm> = s(s+1)h2|sms>
Of course |nℓm> is dimensional again
But the sub-space of angular momentum(described by just a subset of the quantum numbers)
doesn’t suffer this complication.
can measure all the spatial (x,y,z) components (and thus L itself) of vmrL
not even possible in principal !
rixyx
irL
,,
ix
yy
xiLzSo, for
example
azimuthalangle inpolar
coordinates
Angular Momentum nlml…
Lz lm(,)R(r) = mħ lm(,)R(r)for m = l, l+1, … l1, l
L2lm(,)R(r)= l(l+1)ħ2lm(,)R(r) l = 0, 1, 2, 3, ...
Measuring Lx alters Ly (the operators change the quantum states).The best you can hope to do is measure:
States ARE simultaneously eigenfunctions of BOTH of THESE operators!We can UNAMBIGUOULSY label states with BOTH quantum numbers
ℓ = 2mℓ = 2, 1, 0, 1, 2
L2 = 2(3) = 6|L| = 6 = 2.4495
ℓ = 1mℓ = 1, 0, 1
L2 = 1(2) = 2|L| = 2 = 1.4142
2
1
0
1
0
Note the always odd number of possible orientations:
A “degeneracy” in otherwise identical states!
Spectra of the alkali metals
(here Sodium)all show
lots of doublets
1924: Pauli suggested electrons posses some new, previously un-recognized & non-classical 2-valued property
Perhaps our working definition of angular momentum was too literal…too classical
perhaps the operator relations
yzxxz
xyzzy
zxyyx
LiLLLL
LiLLLL
LiLLLL
may be the more fundamental definition
Such “Commutation Rules”are recognized by mathematicians as
the “defining algebra” of a non-abelian
(non-commuting) group[ Group Theory; Matrix Theory ]
Reserving L to represent orbital angular momentum, introducing the more generic operator J to represent any or all angular momentum
yzxxz
xyzzy
zxyyx
JiJJJJ
JiJJJJ
JiJJJJ
study this as an algebraic group
Uhlenbeck & Goudsmit find actually J=0, ½, 1, 3/2, 2, … are all allowed!
ms = ± 12
spin “up”spin “down”
s = ħ = 0.866 ħ 3 2
sz = ħ 12
| n l m > | > = nlm12
12
10( )
“spinor”
the most general state is a linear expansion in this 2-dimensional basis set
1 0 0 1( ) = + ( ) ( )
with 2 + 2 = 1
spin : 12p, n, e, , , e , , , u, d, c, s, t, b
leptons quarks
the fundamental constituents of all matter!
SPINORBITAL ANGULAR
MOMENTUMfundamental property
of an individual componentrelative motionbetween objects
Earth: orbital angular momentum: rmv plus “spin” angular momentum: I in fact ALSO “spin” angular momentum: Isunsun
but particle spin especially that of truly fundamental particlesof no determinable size (electrons, quarks)
or even mass (neutrinos, photons)
must be an “intrinsic” property of the particle itself
Total Angular Momentumnlmlsmsj… l = 0, 1, 2, 3, ...
Lz|lm> = mħ|lm> for m = l, l+1, … l1, lL2|lm> = l(l+1)ħ2|lm>
Sz|lm> = msħ|sms> for ms = s, s+1, … s1, sS2|lm> = s(s+1)ħ2|sms>
In any coupling between L and S it is the TOTAL J = L + s that is conserved.
ExampleJ/ particle: 2 (spin-1/2) quarks bound in a ground (orbital angular momentum=0) stateExamplespin-1/2 electron in an l=2 orbital. Total J ?
Either3/2 or 5/2possible
BOSONS FERMIONS
spin 1 spin ½ e,p, n,
Nuclei (combinations of p,n) can haveJ = 1/2, 1, 3/2, 2, 5/2, …
BOSONS FERMIONS
spin 0 spin ½
spin 1 spin 3/2
spin 2 spin 5/2 : :
“psuedo-scalar” mesons
quarks and leptonse,, u, d, c, s, t, b,
Force mediators“vector”bosons: ,W,Z“vector” mesonsJ
Baryon “octet”p, n,
Baryon “decupltet”
Combining any pair of individual states |j1m1> and |j2m2> forms the final “product state”
|j1m1>|j2m2>
What final state angular momenta are possible?What is the probability of any single one of them?
Involves “measuring” or calculating OVERLAPS (ADMIXTURE contributions)
|j1m1>|j2m2> = j j1 j2;m m1 m2 | j m >
j=| j1j2 |
j1j2
Clebsch-Gordon coefficients
or forming the DECOMPOSITION into a new basis set of eigenvectors.
Matrix Representationfor a selected j
J2|jm> = j(j+1)h2| j m >Jz|jm> = m h| j m > for m = j, j+1, … j1, jJ±|jm> = j(j +1)m(m±1) h | j, m1 >
The raising/lowering operators through which we identifythe 2j+1 degenerate energy states sharing the same j.
J+ = Jx + iJy
J = Jx iJy
2Jx = J+ + J Jx = (J+ + J )/2
Jy = i(J J+)/2
adding
2iJy = J+ J
subtracting
The most common representation of angular
momentum diagonalizes the Jz operator:
<jn| Jz |jm> = mmn
1 0 00 0 00 0 -1
Jz =(j=1)
2 0 0 0 00 1 0 0 00 0 0 0 00 0 0 -1 00 0 0 0 -2
Jz =(j=2)
J | 1 1 > =
J±|jm> = j(j +1)m(m±1) h | j, m1 >
J | 1 0 > =
J | 1 -1 > =
J | 1 0 > =
J | 1 -1 > =
J | 1 1 > =
| 1 0 > 2| 1 -1 > 2
0
| 1 0 > 2| 1 1 > 2
0
J =
J =
< 1 0 | 0 0 0 0 00 02
2
0 00 0 0 0 0
22
< 1 -1 |
< 1 0 |
< 1 1 |
0210
210210210
020202
020
21 xJ
020
202020
020202
020
21
iii
i
ii
iiJ y
100010001
zJ
For J=1 states a matrix representation of the angular momentum operators
Which you can show conform to the COMMUTATOR relationship
you demonstrated in quantum mechanics for the differential operators
of angular momentum
[Jx, Jy] = iJz
Jx Jy Jy Jx =
100000001
0000
0
0000
0
22
22
22
22i
ii
ii
ii
ii
= iJz
100010001
1 zJJ
2/1002/12/1 z
JJ
2/300002/100002/100002/3
2/3 zJJ
2000001000000000001000002
2 zJJ
x
y
zz′R(1,2,3) =
11
11cossin0sincos0
001
1
y′1
=x′
x
y
zz′R(1,2,3) =
11
11cossin0sincos0
001
1
y′1
=x′
2
2
2 x′′
z′′
=y′′
22
22
cos0sin010
sin0cos
x
y
zz′R(1,2,3) =
11
11cossin0sincos0
001
1
y′1
=x′
2
2
2 x′′
z′′
=y′′
22
22
cos0sin010
sin0cos
3
y′′′
z′′′ =
x′′′
3
3
1000cossin0sincos
33
33
R(1,2,3) =
11
11cossin0sincos0
001
22
22
cos0sin010
sin0cos
1000cossin0sincos
33
33
These operators DO NOT COMMUTE!
about x-axis about
y′-axis about z′′-axis1st
2nd3rd
Recall: the “generators” of rotations are angular momentum operators and they don’t commute!
but as nn
Infinitesimal rotations DO commute!!
10010
01
1000101
2
2
3
3
1000101
10010
01
3
3
2
2
1001
1
2
3
23
1001
1
2
3
23