1
00:00:00,970 --> 00:00:04,310
Yeah. Okay, so let's just go.

2
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So we were, we just began work on the parity. Introduced the parity operator p yesterday.

3
00:00:12,050 --> 00:00:16,580
So what does it do. It makes out of your left hand, your right hand,

4
00:00:16,580 --> 00:00:23,300
if you orient it correctly by reflecting by producing a state which is the same as the state you first had,

5
00:00:23,660 --> 00:00:32,270
but with everything reflected through the origin. So what you used to find the amplitude to be at minus x is now the amplitude to be at x.

6
00:00:32,570 --> 00:00:38,990
We showed, not surprisingly, that the square of P is the identity operator, because if you reflect something twice,

7
00:00:38,990 --> 00:00:42,980
you have it back where it was, which formally implies that P is P minus one.

8
00:00:43,290 --> 00:00:49,700
And the next item on the agenda is to check that P is a commission operator and therefore an observable.

9
00:00:51,020 --> 00:01:00,590
And the proof of that is that we we take two states, find cy and we evaluate this complex conjugate.

10
00:01:02,030 --> 00:01:05,870
Let's make sure that is what I exactly what I plan to do. Yes. So.

11
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All right. Yeah.

12
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So we need to do what we want to do because we know what P does with an X here on the left of P,

13
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we slide an identity operator in between the find the P so we right this is d cubed of x phi x x p ci.

14
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Then we can and we need to star the whole thing because I decided to start the thing on the left.

15
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Right. That star is that star X of course is real interesting is real.

16
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And then we can use that to replace this.

17
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So that becomes d cubed x phi x minus x oops minus x minus x sign.

18
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And then we need to do the starring operation. So that's the integral de.

19
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Q Dex, take the complex conjugate of that and it becomes by excuse me,

20
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minus X and then here we're going to have X and I could write just five but just

21
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for fun went into write p squared phi because p squared is the identity operator.

22
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So that's safe enough except I regard p squared is p times p and then I can take I can say, look,

23
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this p this out of p can be got rid of by replacing this by a minus x because I'm using x, p, some newfangled state phi primed which is PFI.

24
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So this can be written as the integral d cubed of x sine minus x minus x.

25
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A P that's this inner p that out p has been dealt with by changing that sign on fi and then I can change my variable of integration from x to x prime,

26
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which is minus x, and that's going to produce that.

27
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He's going to be. But this take away the identity operator story that's next.

28
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Primed, which is minus X, take away the identity operator.

29
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And we're looking at. Which says that p since fine and CYA arbitrary.

30
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That tells me comparing with the initial thing that P dagger is equal to P, which we already know is equal to P minus one.

31
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So first of all, this says that it's her mission. So it's an observable and this P dagger equals minus one says it's also at the same time unitary.

32
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So it leaves the length of all states the same.

33
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So since it's a mission. So. So what are its eigenvalues?

34
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We have if p upside is equal to M upside, this is its eigenvalue.

35
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Or maybe we should call it lambda more traditional. So if PFC is equal to emit size.

36
00:04:36,050 --> 00:04:46,930
So it's a nice sine eigen ket. Then we can apply again and get that piece squared of ASI,

37
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which is actually equal to upside because p squared is one is also equal to Lambda PSI, which is lambda squared CI.

38
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So so we have the size lambda squared CI.

39
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And that implies that lambda squared is one, which implies that lambda must be plus and minus one.

40
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It has two eigenvalues plus and minus one. And we say that.

41
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So if P upside is equal to CI, we say that size and even parity state.

42
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Correspondingly, of course, if PSI is minus of PSI.

43
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So that's the nagaon cat with the one with a plus one eigenvalue.

44
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This is an I can cat with the minus one eigenvalue.

45
00:05:39,790 --> 00:05:48,780
We say it's an it's an old parity state. What does that mean?

46
00:05:49,140 --> 00:06:00,970
From what we have up there, it means that. It means that when you from the top there the question is so so.

47
00:06:01,390 --> 00:06:05,450
So let's let's have a look at this. Sorry. From the wave function point of view, right?

48
00:06:05,870 --> 00:06:19,460
For even parity, we can say that s p psi which is equal to minus x of psi by the operation of the p thing.

49
00:06:19,670 --> 00:06:24,970
But since size equal to up psi is also equal to x abassi.

50
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In other words, the wave function.

51
00:06:29,170 --> 00:06:39,280
Isn't even a function of x and similarly odd parity states have way functions which are odd functions of x, etc. etc.

52
00:06:39,520 --> 00:06:44,470
And when we did the harmonic oscillator, we found that, for example, we found that.

53
00:06:48,410 --> 00:06:57,860
We found that n is even parity for n an even number.

54
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And correspondingly, it's odd parity for an odd number.

55
00:07:08,140 --> 00:07:13,960
Just as a concrete example. So we very often classify our states.

56
00:07:14,080 --> 00:07:18,700
It's very useful to know whether our states are even parity or odd parity.

57
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We like to work with ones that have well-defined parity. That is to say, our eigen functions of this parity operator.

58
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By no means all states are eigen functions of the parity operator.

59
00:07:27,790 --> 00:07:37,430
However. Okay. Now we do something considerably more interesting, which is transformations of operators.

60
00:07:48,890 --> 00:08:01,140
So we introduced this. We introduced the displacement operator loss yesterday.

61
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So it was called U of A and it was e to the minus I a dot p over h bar where p is the momentum operator?

62
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And we understood that what it did was it made out of a state so so ci primed being U of a times that CI is a new state of the system,

63
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the state that it would have if it was identical in all respects, except it was shoved along by the vector.

64
00:08:37,430 --> 00:08:42,010
A If you shove your system on by the vector.

65
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A The expectation value of the position obviously has to increment by a right.

66
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So if so. So we can make the following.

67
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We can make the following statement that the expectation value in the state of SCI primed of x.

68
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Has to equal the expectation value in the state website plus a.

69
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Because we have we have displaced our system.

70
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This system is the same as this system, except its location has been incremented.

71
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It's been moved by the vector A and that that is logical necessity.

72
00:09:28,500 --> 00:09:40,550
But this but this we can write in a different way. This we can write using that expression as a ci times you dagger x you times of ci.

73
00:09:40,980 --> 00:09:44,070
All right. That's just a rewrite of that using this operator here.

74
00:09:44,730 --> 00:09:54,750
And this I can rewrite in a different way because I can say this vector A which is just an ordinary boring vector, I could multiply by one.

75
00:09:54,750 --> 00:10:04,260
I could say that this is this is the following. This is X plus a times the identity operator on its side.

76
00:10:04,260 --> 00:10:11,490
Right. Because it's clear that it's the expectation value of a times the identity operator is the vector A so these are equivalent expressions.

77
00:10:12,030 --> 00:10:19,740
So I found that the expectation value of this operator is equal to the expectation value of this operator for any sign whatsoever.

78
00:10:21,650 --> 00:10:25,270
And it it's shown in a box. It's in the book.

79
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It's a it's a little box which leads to the conclusion that not surprising conclusion, that if that's true,

80
00:10:31,400 --> 00:10:37,340
that this expectation is equal to this expectation of two operators have the same expectation for every state whatsoever.

81
00:10:37,550 --> 00:10:44,270
The two operators have to be equal. So this implies with a little bit of footwork that's that's relegated to a box in the

82
00:10:44,270 --> 00:10:56,389
book that you dagger X you is equal to X plus AA where this eye is perhaps understood.

83
00:10:56,390 --> 00:11:03,530
Well, let's put it in, because I want that to be an operator. Right. This is an operator on the right hand side here right now.

84
00:11:03,530 --> 00:11:10,340
Let's let's now make we'll make a small right.

85
00:11:12,800 --> 00:11:18,260
If a small we know we can expand this in terms of its of its generator.

86
00:11:18,290 --> 00:11:29,390
Right. So we can write this is this thing here can be written as the identity operator minus I a dot p over h bar plus order of h squared.

87
00:11:31,550 --> 00:11:36,740
So we we've done that before and we're going to do it again. So that becomes the identity operator,

88
00:11:36,740 --> 00:11:47,420
plus a dot p overage plus dot dot dot which we will ignore times x bracket one minus

89
00:11:47,420 --> 00:11:57,350
i a dot p over par dot dot dot that is equal to x plus a the I can be understood.

90
00:12:01,430 --> 00:12:08,200
So we multiply this all up to find what the terms on the order of a proportional to a also a small but still arbitrary.

91
00:12:08,210 --> 00:12:20,720
I mean, you can still fiddle around with it, but it's small. So this is going to give me the this is going to give me r, i x is going to equal that.

92
00:12:20,870 --> 00:12:27,139
So I'm going to get an X on the left hand side, which will cancel with that. And in the next order, we're going to get a dot P,

93
00:12:27,140 --> 00:12:36,410
which is small because I is small on x times I and then we'll have an I times x times minus eight dot P.

94
00:12:36,770 --> 00:12:45,170
So what we're going to left with, left with in the order of a is I have for a dot p,

95
00:12:45,950 --> 00:12:51,529
comma x the commentator because we're going to have this times this and also

96
00:12:51,530 --> 00:12:57,290
with a plus sign and we're going to have this times this with the minus sign.

97
00:12:57,530 --> 00:13:03,500
And what's that equal to? That's equal to a because that's the terms on the order of a on the right hand side.

98
00:13:03,800 --> 00:13:09,020
The higher order terms must all cancel that. We leave that to the magic of mathematics and not interested in it.

99
00:13:11,430 --> 00:13:16,530
So we have this relationship here and let's look what this looks like in terms of components.

100
00:13:17,550 --> 00:13:22,350
If I look at the this so this is a set of three equations, one for the X component of this,

101
00:13:22,350 --> 00:13:27,060
one for the Y components of this and one for the Z component of this. So what does it look like?

102
00:13:27,620 --> 00:13:39,990
It it looks like this multiply by I over each bar and swap the order here and this is going to be x comma.

103
00:13:40,320 --> 00:13:44,850
A dot p commentator is equal to i.

104
00:13:44,880 --> 00:13:51,170
H bar a. Now let's use write this out in its components.

105
00:13:51,180 --> 00:14:00,570
This is to say a set of three equations so I can say that x j comma the sum over k of a.

106
00:14:00,900 --> 00:14:03,959
Well, it's AJ sorry.

107
00:14:03,960 --> 00:14:13,160
It's sum of a k a.k.a k, but I can take the a.k.a outside the commentator because it's a man number is equal to i h far.

108
00:14:14,520 --> 00:14:22,010
I have to right now a j because this is the this this component to this vector here matches this here, here, right.

109
00:14:22,020 --> 00:14:28,680
This is a dot product which is the sum PKK. And now I can identify.

110
00:14:31,500 --> 00:14:38,550
Okay, so, so this has to be true for all small a k.

111
00:14:43,460 --> 00:14:48,050
So I can write this right hand side as the sum of a k if I want to.

112
00:14:49,580 --> 00:14:59,270
Sorry, I h bar the sum of a k of delta j k a k posh way of writing it.

113
00:14:59,720 --> 00:15:07,070
And now I can say because a k is arbitrary, the coefficient of k on the right side has to equal the coefficient of K on the left side.

114
00:15:07,400 --> 00:15:16,400
So that leads to the conclusion that x j comma k is equal to h bar delta j k.

115
00:15:17,030 --> 00:15:22,840
So we've recovered the canonical. Commutation relation.

116
00:15:26,700 --> 00:15:34,260
Between X and P as a consequence of P being the operator, which generates translations.

117
00:15:35,590 --> 00:15:41,590
So we've we've come at this in rather a roundabout way just to review how this has happened.

118
00:15:42,220 --> 00:15:45,730
I wrote down a rather arbitrary rule.

119
00:15:45,730 --> 00:15:57,710
I introduced P by arbitrary rule. I said that I said that x p psi is minus h bar d by x x ci.

120
00:15:58,360 --> 00:16:06,760
Using Ehrenfest theorem, I tried to persuade you that this wasn't completely crazy, but really it wasn't a very satisfactory job to start in that way.

121
00:16:08,050 --> 00:16:17,620
Then we showed that because P has this DVD structure, it is the generator of translations.

122
00:16:20,110 --> 00:16:25,810
And as a consequence of its being the generator of translations, it must have this commutation rule.

123
00:16:26,920 --> 00:16:34,900
And what we should have done, really, is we should have said, look, there must be some operator which generates translations.

124
00:16:35,680 --> 00:16:41,530
This operator is going to have this commutation relation and we should have worked our way down to finding out that in the position,

125
00:16:41,530 --> 00:16:48,490
representation is represented like this. And for the angular momentum operations, this is the line of argument we're pursuing.

126
00:16:49,390 --> 00:16:53,890
We are using we are introducing them as the generators of rotations.

127
00:16:54,160 --> 00:16:59,200
And then we're going to find out what they look like in the position rotation and the position representation later on.

128
00:17:00,980 --> 00:17:08,930
So we've come at this in a slightly torturous way. This is the main job, the momentum the momentum operator has.

129
00:17:09,980 --> 00:17:15,560
It's interesting. So it's probably worthwhile just checking that.

130
00:17:18,680 --> 00:17:27,320
This commutation relation guarantees that rule up there that you dagger x you is equal to x plus a.

131
00:17:27,800 --> 00:17:34,460
Even when A is big. Right. So we've this this stuff has all been for an infinitesimal a.

132
00:17:35,980 --> 00:17:39,670
And it's good to check that the other thing works.

133
00:17:40,120 --> 00:17:51,730
It sorts us even for a big. So now is just talk about for any a including a big one any big displacement.

134
00:17:55,400 --> 00:17:59,360
So we're going to be looking at you. Dagger. Sorry.

135
00:18:00,500 --> 00:18:05,209
Excu. You just checked my. Yeah. Which I can.

136
00:18:05,210 --> 00:18:10,670
Right. Of course. As you dagger you x plus you dagger.

137
00:18:13,590 --> 00:18:21,880
Excuse me of you. So I've just swapped the order of those two and put it in the commentator that compensates.

138
00:18:22,300 --> 00:18:26,500
This of course, is going to be ex because you dagger you is one.

139
00:18:27,760 --> 00:18:38,080
And what's this going to be? So it's going to be x plus u dagger of x comma e to the I minus i p excuse me.

140
00:18:38,620 --> 00:18:42,310
A dot p over h bar.

141
00:18:44,070 --> 00:18:52,370
Close brackets. So this is a this is a classic example.

142
00:18:52,520 --> 00:18:57,259
We studied this problem before. We're doing the commentator of X and a function of P.

143
00:18:57,260 --> 00:19:04,520
This is the function of P. We're doing it. And do you recall that the answer to that problem was that we could write that the commentator

144
00:19:04,520 --> 00:19:10,340
is the rate of change of this function with respect to t p sorry times x comma p.

145
00:19:10,670 --> 00:19:14,510
So this can be written as x plus u dagger.

146
00:19:14,840 --> 00:19:22,790
The rate of change of this with respect to the derivative of this with respect to P, which is going to be of course is going to be U,

147
00:19:23,660 --> 00:19:29,750
because the derivative of an exponential is the exponential times, the derivative of what's up here with respect to P.

148
00:19:30,140 --> 00:19:39,920
So it's going to be u d by well, DVT, P of this is a minus.

149
00:19:41,050 --> 00:19:44,450
I. Now.

150
00:19:49,820 --> 00:19:58,630
But. Sorry.

151
00:19:58,660 --> 00:20:03,030
Let us do this as the derivative with respect to a dot p right.

152
00:20:03,070 --> 00:20:05,370
We regard this as a function of a dot P.

153
00:20:05,380 --> 00:20:10,840
I'm worried about components in the way I can get out of that is considering this to be a function a dot p, which is just one thing.

154
00:20:11,320 --> 00:20:13,990
So if I take the derivative of this with respect to a dot p,

155
00:20:14,200 --> 00:20:22,090
I get you because I get the exponential back and then I have times minus I over bar, right?

156
00:20:22,120 --> 00:20:26,139
That's the derivative of the argument of the exponential with respect to a P.

157
00:20:26,140 --> 00:20:30,460
And now I have to write down the commentator of X with respect to a p.

158
00:20:34,860 --> 00:20:39,360
So this, of course, produces one. And what does this produce?

159
00:20:39,600 --> 00:20:47,160
This can be. Let's let's write that down. That's X plus groups minus because of this I over h bar.

160
00:20:48,240 --> 00:20:54,870
This is producing a one. And now I need the derivative of this, which is the sum of a k of x.

161
00:20:58,210 --> 00:21:03,510
Comma. Peak times AK.

162
00:21:03,970 --> 00:21:08,400
Right. It doesn't matter what order I put on because it's a number and this.

163
00:21:12,230 --> 00:21:16,640
It may be that we ought now to introduce an index on X, otherwise we're going to get into confusion.

164
00:21:16,650 --> 00:21:21,889
So let's make that I. So I'm making this was a vector X, an arbitrary component.

165
00:21:21,890 --> 00:21:25,070
Let me call that component I, then this becomes I,

166
00:21:25,280 --> 00:21:39,560
then this becomes this becomes delta h bar of delta i k the I and the I make a minus one which cancels this, the bar kills that.

167
00:21:39,950 --> 00:21:44,209
So this is equal to x and then this is nothing.

168
00:21:44,210 --> 00:21:48,110
This, this, this is nothing is k is summed except when k equals I.

169
00:21:48,350 --> 00:21:51,380
So that becomes an I. So this book just becomes an I.

170
00:21:51,560 --> 00:21:59,030
And yes, it does sort us. That thing is equal to x plus a as advertised at the top that.

171
00:22:04,540 --> 00:22:19,590
So now let's think about rotations. So we have we introduced these operators, we had J, X, j y and Jay Z.

172
00:22:19,890 --> 00:22:24,330
So that alpha dot j generated.

173
00:22:27,850 --> 00:22:41,520
Rotation. Through Model Alpha about the unit vector in the direction of alpha.

174
00:22:42,090 --> 00:22:45,990
That's that's what we establish. We use that notation. We said there had to be such a thing.

175
00:22:49,500 --> 00:22:54,330
And what we want to do now is talk about is apply is is is adapt that argument to this case.

176
00:22:54,900 --> 00:22:58,260
So the thing is the expectation value.

177
00:23:01,020 --> 00:23:10,740
It so sorry we've let us let it slip primed be the state that you get when you use the ulfa on its side.

178
00:23:11,130 --> 00:23:13,890
So this is the state of the system which is identical to this state,

179
00:23:13,890 --> 00:23:19,560
except it's been turned around through an angle around the axis as advertised up there.

180
00:23:19,590 --> 00:23:19,830
Right.

181
00:23:21,780 --> 00:23:29,639
So we can say something about the expectation value of x of this system must be the same as the expectation value of that system, but rotated too.

182
00:23:29,640 --> 00:23:32,730
If you've rotated the system, you've rotated the expectation value of X.

183
00:23:33,270 --> 00:23:43,349
So we can say that upside primed X upside primed, which is now this thing is a boring vector, right?

184
00:23:43,350 --> 00:23:47,000
It's the expectation value of a vector operator. So it is a boring vector.

185
00:23:47,010 --> 00:23:55,370
It's a set of three numbers. And instead of three numbers, we can use a boring rotation matrix on which I'm going to call R Alpha.

186
00:23:56,120 --> 00:24:03,139
So this is a three by three rotation matrix, an ordinary boring rotation matrix, such as?

187
00:24:03,140 --> 00:24:11,180
I think you must have studied in Professor this course operating on a sci x sign.

188
00:24:12,290 --> 00:24:16,760
So this was the old expectation, the expectation value in our around rotated system.

189
00:24:17,000 --> 00:24:21,800
If you rotate that expectation value, you must get the expectation value in the rotated system.

190
00:24:23,910 --> 00:24:27,570
So that's the analogue for the rotational case of.

191
00:24:30,290 --> 00:24:33,320
Of that statement. You dagger x u is x plus a.

192
00:24:35,740 --> 00:24:40,240
Well, except I haven't yet written what this is. So I'm going to write this as a.

193
00:24:41,500 --> 00:24:48,370
You dagger of Alpha. X you of alpha upside.

194
00:24:48,740 --> 00:24:52,940
All right. So so I'm replacing this with that.

195
00:24:54,290 --> 00:25:02,150
And now I'm saying that for any state upside, this expectation value equals this expectation value.

196
00:25:05,550 --> 00:25:12,270
Ergo, since this is a set of boring numbers, it can go inside the inside the expectation and just rotate the operator.

197
00:25:13,320 --> 00:25:18,170
So it's just taking a linear this thing. This is a boring three by three rotation matrix.

198
00:25:18,170 --> 00:25:26,610
So if I allow it to go inside there, it's simply going to be taking a linear combinations, combination of the X, Y and Z position operators.

199
00:25:27,240 --> 00:25:40,920
So I'm going to be able to say that u dagger, uh, x u is equal to oh alpha of x.

200
00:25:45,820 --> 00:25:54,820
That's a matrix. And now, of course, I'm going to express this as one plus alpha.

201
00:25:55,360 --> 00:26:00,470
Dot J. Dot, dot, dot. So this is now.

202
00:26:00,510 --> 00:26:03,820
Now we're now making small alpha.

203
00:26:05,310 --> 00:26:18,810
So we're rotating through a small angle and this can be written. Plus, here is an x, here is one minus I alpha dot j plus dot dot dot.

204
00:26:20,520 --> 00:26:25,520
And that is equal to the. To this vector.

205
00:26:25,520 --> 00:26:30,020
It is a vector of operators, but still it is a vector rotated by a small angle.

206
00:26:30,830 --> 00:26:34,460
Now we know what that is. At least I hope we do. Come on.

207
00:26:34,550 --> 00:26:38,100
Come on. Oh, no. Go to sleep.

208
00:26:38,110 --> 00:26:41,460
Do I have to draw? We're.

209
00:26:43,220 --> 00:26:50,160
Yeah. System seems to. Just. No, it's gone to sleep.

210
00:26:50,620 --> 00:26:54,880
So this requires a bit of this is this is a piece of just standard geometry.

211
00:26:55,570 --> 00:26:58,660
What I want to do is write the action of a rotation matrix.

212
00:27:02,130 --> 00:27:05,940
When for a small angle, if I rotate something through a small angle.

213
00:27:06,490 --> 00:27:09,870
And I hate drawing these diagrams. It's going to be something as it come alive.

214
00:27:10,110 --> 00:27:14,819
Oh, right. Yeah. Okay. It just went to sleep. It needs warming up. So we're looking at this second diagram.

215
00:27:14,820 --> 00:27:18,480
Can you see it? Because I'm sure it will count. It's too faint anyway.

216
00:27:18,690 --> 00:27:24,120
So the point is that this is the vector V here is the rotation axis alpha.

217
00:27:24,360 --> 00:27:29,130
We're rotating through a small angle. Therefore, this distance there is small.

218
00:27:29,550 --> 00:27:33,270
The the, the displacement that you have up there.

219
00:27:33,270 --> 00:27:36,540
This is the rotated vectors on the right, the unrooted vectors on the left.

220
00:27:36,870 --> 00:27:43,560
The displacement is, is this thing here, which is the, the vector delta alpha or the vector.

221
00:27:44,010 --> 00:27:45,240
So the small,

222
00:27:45,900 --> 00:27:56,190
the small rotation vector crossed with the original V so that we can say that V primed the rotated vector is equal to the original vector.

223
00:27:56,190 --> 00:28:02,430
Plus this, this, this, this small rotation vector crossed into into V.

224
00:28:02,910 --> 00:28:10,020
So the right side. So I'm not going to draw this horrible diagram. It's the right side.

225
00:28:23,260 --> 00:28:26,610
So this is going to become X.

226
00:28:27,160 --> 00:28:33,640
That's the V up there. Plus Alpha Cross X.

227
00:28:33,970 --> 00:28:38,260
So our alpha is small, so we don't need the Delta Alpha. It's just it's just alpha.

228
00:28:38,260 --> 00:28:43,270
We've made it small to get rid of symbols. So we do the same old stuff.

229
00:28:43,270 --> 00:28:47,559
We multiply this out on the left to the to two, up toward Alpha.

230
00:28:47,560 --> 00:28:52,240
We notice that the eye, the x of the eye produces an x, which cancels with the eye on the right.

231
00:28:52,720 --> 00:29:01,450
And we find that what we're left with toward alpha is alpha dot g times x minus from the eye,

232
00:29:01,450 --> 00:29:04,450
the x and the alpha dot j the thing the other way around.

233
00:29:04,630 --> 00:29:17,050
So we find it eye alpha whoops alpha dot j comma x is equal to alpha cross x.

234
00:29:19,000 --> 00:29:28,270
Now we need to write this in, uh, we need to introduce indices in order to disentangle what's going on around here.

235
00:29:28,630 --> 00:29:39,760
So this, this is going to be the sum over k i times the sum of the k of alpha k which will come out times j k.

236
00:29:39,790 --> 00:29:44,020
All right, that's alpha k k, comma x j.

237
00:29:44,200 --> 00:29:49,880
Now this is sorry. Let's change that. Let's just change that to something we some of it to be.

238
00:29:51,200 --> 00:29:55,050
It doesn't matter what we call it, but let's call it that and call that K.

239
00:29:55,080 --> 00:30:01,320
All right. What's that going to be? That is going to be the.

240
00:30:04,630 --> 00:30:08,710
The the case component of this vector on the right.

241
00:30:09,890 --> 00:30:14,740
Oh, sorry. We should call that I. Right. This is going to be the IV component of the vector on the right.

242
00:30:15,100 --> 00:30:21,970
Now, across product can be written as the sum over j and K of epsilon i.

243
00:30:22,090 --> 00:30:25,719
J k. This is the thing which keeps changing its sign.

244
00:30:25,720 --> 00:30:28,959
If you swap any two indices, it changes its sine and epsilon.

245
00:30:28,960 --> 00:30:32,800
One, two, three is one. Which I hope you've met in Professor Restless course.

246
00:30:35,600 --> 00:30:41,629
So this is just writing a cross product intensive note in Cartesian tends to notation nothing to do with quantum mechanics,

247
00:30:41,630 --> 00:30:45,290
it's just standard vector algebra.

248
00:30:46,580 --> 00:30:53,840
And we have arranged it so that we have the eyes components of the left side here and the eyes component on the right side there.

249
00:30:54,140 --> 00:31:01,010
Now we play our trick of saying that, look, alpha is arbitrary, it needs to be small, but otherwise it's arbitrary.

250
00:31:01,010 --> 00:31:04,010
You can choose this direction any which way you like and its magnitude in detail.

251
00:31:04,010 --> 00:31:07,760
You can choose any which way you like. So we can compare.

252
00:31:07,760 --> 00:31:14,360
We can equate the coefficients on the two sides. Multiply by through through both sides by i to get rid of this,

253
00:31:14,630 --> 00:31:18,470
you'll get a minus sign swap the order of these in order to clean it up and we will

254
00:31:18,470 --> 00:31:29,270
have xy comma JJ is equal to I times that's this I brought across the sum of a k.

255
00:31:29,570 --> 00:31:39,770
The sum of a j will go away because we're awaiting the coefficient of j on the two sides of epsilon i j k x k.

256
00:31:39,800 --> 00:31:41,600
This is a terribly important relation.

257
00:31:43,550 --> 00:31:51,410
It tells us how j commutes, how the Jth component of angular momentum computes with any component of the position operator.

258
00:31:53,030 --> 00:32:02,149
But crucially in this argument here, we have used nothing about the position operators except that the components form that the three,

259
00:32:02,150 --> 00:32:05,900
the X, Y and Z operators are the components of a vector.

260
00:32:06,980 --> 00:32:15,260
So all of this argument could be repeated for the three component operators of any other vector, for example, for P.

261
00:32:16,220 --> 00:32:20,690
So it follows immediately. We've only used.

262
00:32:25,110 --> 00:32:31,650
Only property. Of X used if the operator x used.

263
00:32:34,520 --> 00:32:44,640
Is that it's a vector. So we've really shown that for any vector this relationship holds.

264
00:32:45,780 --> 00:33:02,010
So we've shown that v i j j equals i epsilon some of a k of epsilon i j k vk for any vector operator.

265
00:33:03,630 --> 00:33:07,740
So a vector operator is just a set of three operators, if you like. Whose components?

266
00:33:09,970 --> 00:33:14,260
The whose expectation values will be the components of some classical vector.

267
00:33:18,840 --> 00:33:26,850
Okay. So we can apply what we can immediately apply this as well as to X to VII is p the momentum.

268
00:33:30,500 --> 00:33:34,880
And we can also apply it to VII is equal to G.

269
00:33:35,030 --> 00:33:39,990
The angular momentum. Why is that?

270
00:33:43,450 --> 00:33:46,570
And it would be the other way round, which is right.

271
00:33:47,630 --> 00:33:52,810
Oh, sorry. In which one? This one. And this one just before the act.

272
00:33:55,160 --> 00:34:00,540
Okay. I lost a sign somewhere.

273
00:34:02,820 --> 00:34:08,370
No, I think. I think this is right. But I.

274
00:34:09,090 --> 00:34:13,430
Yeah, I multiply through by I and I swap the order of these two. Today is the boat from.

275
00:34:14,280 --> 00:34:20,790
No, I don't think so. No.

276
00:34:20,810 --> 00:34:26,510
But this order is the same as this order. Surely to goodness. I don't think you invoke the other way around.

277
00:34:27,440 --> 00:34:32,750
Okay. Let me let me take advice on that. I'm. Yeah, I can't help being sceptical, but I suppose I should look here.

278
00:34:35,180 --> 00:34:57,020
I suppose I should look. Yeah.

279
00:34:58,090 --> 00:35:02,680
True. You know the thing I'm thinking of.

280
00:35:02,700 --> 00:35:06,210
Okay, so maybe I have, dear. I think we.

281
00:35:10,370 --> 00:35:14,290
Have I drifted a sign somewhere? I feel that that's what they.

282
00:35:17,130 --> 00:35:21,120
Well, this is definitely the ice component of that cross product.

283
00:35:21,810 --> 00:35:30,240
This is definitely so. Do we agree about that, that should we just check whether that is whether this ordering is as advertised?

284
00:35:30,730 --> 00:35:44,580
Uh, uh. This is the closest thing.

285
00:36:09,040 --> 00:36:15,250
I can't see it at the moment. I think I think it's probably it's incredibly hard to do these sine problems on blackboards.

286
00:36:16,150 --> 00:36:19,360
Let me leave that and I will confirm tomorrow what the case is.

287
00:36:19,360 --> 00:36:23,649
I imagine the book is right and I'm wrong, but I do not see where I have made the mistake.

288
00:36:23,650 --> 00:36:28,750
As things stand, everything looks respectable.

289
00:36:32,520 --> 00:36:40,960
No, I can't see. I cannot see an errant sign. Oh, that's nasty.

290
00:36:43,690 --> 00:36:51,890
And it does, as you say, matter. Yeah.

291
00:36:53,210 --> 00:36:58,040
Yeah. I'll try and sort that and write up a for tomorrow's lecture the way it should be.

292
00:37:00,670 --> 00:37:05,590
We need to be persuaded that this will. The next thing I want to do is be persuaded that this is.

293
00:37:06,340 --> 00:37:10,930
So can we apply this to the Anglo momentum operations is going to be very important that we can.

294
00:37:11,320 --> 00:37:21,060
And what's the argument? The argument is that. Alpha J has to be a scalar.

295
00:37:21,910 --> 00:37:31,000
Why? Because the operator ulfa which is e to the minus eye alpha dot j.

296
00:37:32,470 --> 00:37:36,580
This thing is the rotation around a certain vector.

297
00:37:37,990 --> 00:37:43,150
What this operator is this is a physically meaningful thing,

298
00:37:43,630 --> 00:37:51,700
and it is defined not by the three numbers that we happen to use to define the vector, the direction of the vector.

299
00:37:52,510 --> 00:37:56,799
But by exactly what that vector is, if you change your coordinate systems,

300
00:37:56,800 --> 00:38:01,270
you use a new coordinate system, you'll be using a new set of three numbers to define this right.

301
00:38:02,530 --> 00:38:06,670
But you must get the same the same direction in space.

302
00:38:07,480 --> 00:38:13,450
And that will be the case if the if the these three operators also transform.

303
00:38:14,230 --> 00:38:18,810
So the new operator is the operator associated with J x primed.

304
00:38:18,820 --> 00:38:23,260
Where X prime does your new x axis will be a linear combination of the old operators,

305
00:38:23,260 --> 00:38:28,420
the operators associated with the old axes using the proper rule for rotation.

306
00:38:28,930 --> 00:38:33,759
Then this dot product will stay the same and this operator will stay the same as we require.

307
00:38:33,760 --> 00:38:40,000
So this operator will be independent of your coordinate system only if these three things transform amongst themselves.

308
00:38:41,840 --> 00:38:44,900
As for a vector. So J must be a vector.

309
00:38:45,140 --> 00:38:48,650
And that means we can use it in here. That is to say.

310
00:38:49,310 --> 00:38:58,340
We can say that j i comma j j commentator is equal to I epsilon sum.

311
00:38:58,430 --> 00:39:03,380
Some of it i some have a k epsilon i.

312
00:39:03,410 --> 00:39:07,340
J k. J k.

313
00:39:07,760 --> 00:39:18,440
So this is a crucial relationship. And from that, we will find out what the eigenvalues can be of these operators J, R and JJ,

314
00:39:18,440 --> 00:39:22,670
and then we'll be able to find what they what the states are well defined, angular momentum and everything else.

315
00:39:24,480 --> 00:39:30,120
This expression is right. Right. Because it's independence of of of any swap.

316
00:39:31,850 --> 00:39:36,050
Can it be that both expressions are right? Oh, I can't. I must not take time to think about it.

317
00:39:49,170 --> 00:39:54,270
Okay. Let's consider. Oh, what's a scale?

318
00:39:54,280 --> 00:40:00,360
Let's consider scalar scalar operators. So what is a scalar operator?

319
00:40:00,540 --> 00:40:04,170
It's a it's an operator. Which.

320
00:40:06,730 --> 00:40:12,580
Well, scalar, sorry. A scalar in ordinary physics is a number whose value is unaffected by a rotation of your coordinates.

321
00:40:13,450 --> 00:40:17,320
Right. Like a dot product. It's unaffected by rotation of the coordinates.

322
00:40:18,850 --> 00:40:25,720
So what can we say is that if X is a scalar operator and we then the expectation value,

323
00:40:27,760 --> 00:40:38,140
the expectation value of a scalar operator between rotated states must be the same as the expectation value between the and rotated states.

324
00:40:39,100 --> 00:40:47,740
But because, because this is a boring number and it's evidently by definition a scalar, something is unaffected by rotation.

325
00:40:47,750 --> 00:40:50,860
So the fact that you've rotated your system shouldn't have any effect.

326
00:40:51,430 --> 00:40:59,620
So when we ask ourselves, so what does that what implications that have is that you'd like to ask you is equal to X?

327
00:41:02,260 --> 00:41:09,310
We can multiply on the left by you, which is the inverse of you dagger because you is a unitary operator.

328
00:41:09,760 --> 00:41:19,930
So we have then that S2 is equal to us, which means that s comma u equals nought.

329
00:41:20,860 --> 00:41:26,500
But this of course is U of alpha, the rotation operator throughout.

330
00:41:27,490 --> 00:41:38,370
So scalar operator commutes with this rotation operator and it's easy to see by expanding this is one plus so if we write you is the identity minus I,

331
00:41:38,770 --> 00:41:48,400
alpha dot j, etc. that immediately goes to the statement that s comma j i equals nought.

332
00:41:48,430 --> 00:41:51,430
So scalar operators compute with all the angular momentum operators.

333
00:41:53,820 --> 00:42:10,610
There's a very important and interesting scalar operator. And that's j squared, which means the sum of a k jc j k a.k.a also known as J.

334
00:42:10,810 --> 00:42:17,530
J. Right. That's a scalar operator. Every product is a scalar operator.

335
00:42:17,530 --> 00:42:23,319
So we have statements like J squared, comma, GI is nought.

336
00:42:23,320 --> 00:42:30,340
We have statements like x squared, comma, GI equals nought.

337
00:42:30,340 --> 00:42:35,390
We have statements like p squared, comma, gi equals nought.

338
00:42:35,410 --> 00:42:37,220
These are all important results that we will use.

339
00:42:37,240 --> 00:42:47,740
Many times we have statements like x dot P that's a scalar operator, comma, gi equals nought and so on and so forth.

340
00:42:47,740 --> 00:42:55,420
So there are many operators we can make out of the operators already on the table which compute with the all the angular momentum operators.

341
00:42:58,290 --> 00:43:03,750
So just just a summary now of the angular momentum commutation relations.

342
00:43:04,410 --> 00:43:17,430
We've got that j i comma j j is equal to I some have a k excellent i j k j k.

343
00:43:17,550 --> 00:43:21,000
So the individual components is a somewhat strange state of affairs.

344
00:43:21,000 --> 00:43:24,210
The individual components of angular momentum do not compute with each other.

345
00:43:24,720 --> 00:43:31,720
So you can't expect to know simultaneously the angle we're on the x axis in the engagement.

346
00:43:31,740 --> 00:43:38,460
Mm. Around the z-axis. I mean, in individual cases you can, but as a general rule, you can't expect to know that.

347
00:43:38,660 --> 00:43:42,979
And so there isn't a complete set of states which are simultaneously in states of checks.

348
00:43:42,980 --> 00:43:52,010
And Jay Z, for example. But we do have j squared comma j ii equals zero,

349
00:43:52,010 --> 00:43:58,940
and therefore there is a complete set of mutual eigen states of the total angular momentum and the Anglo mentum along any axis.

350
00:44:01,090 --> 00:44:11,049
And that's what one or what we we have to work with that we have to consider states which we have to work with, states which are mutually agreed.

351
00:44:11,050 --> 00:44:16,300
States have j squared and usually the axis we choose. We have to choose one because of this business.

352
00:44:16,300 --> 00:44:28,930
And the access we usually choose is the z axis. An important result about about Paris's.

353
00:44:28,930 --> 00:44:52,390
Go back to the Paris operation now. So in the same spirit, if I consider so the expectation value of X, if I reflect my system through the origin,

354
00:44:52,450 --> 00:44:57,700
right by using the power of operations, I make a system which is like my existing system but reflect it through the origin.

355
00:44:58,120 --> 00:45:08,650
It's obvious that the reflected system is going to have an expectation value which of x, which is minus the original expectation value, right?

356
00:45:09,310 --> 00:45:10,690
Because you've reflected everything.

357
00:45:10,690 --> 00:45:19,600
And therefore the if there was an average value of X of it in the original system, the reflective system will have minus that value.

358
00:45:21,150 --> 00:45:28,200
So this can be written as a CI p dagger xp up ci.

359
00:45:28,950 --> 00:45:33,089
Right. Because of ci prime just by definition p c p dagger here.

360
00:45:33,090 --> 00:45:46,710
But we know that p dagger is p, so we have the expectation value for any state whatsoever of minus x is equal to the expectation value of p xp.

361
00:45:47,310 --> 00:45:52,160
So it follows that minus x is equal to p x.

362
00:45:52,480 --> 00:46:07,020
P multiply through by p and use. The fact that p squared is equal to is equal to one and we conclude that p x plus xp is not is nothing very much.

363
00:46:07,020 --> 00:46:16,810
So you can say now that the the the parity or p anti commutes this condition with M

364
00:46:16,830 --> 00:46:20,190
plus sign there right with the minus sign it will be a commentator with a plus sign.

365
00:46:20,190 --> 00:46:29,160
It's anti commutation, anti commutes. With X and in fact, with any vector.

366
00:46:31,880 --> 00:46:39,710
Right, because this argument here really only exploited the fact that we were talking about a vector, not necessarily the position vector.

367
00:46:42,680 --> 00:46:47,720
Now, why is this stuff important? The practical importance of this is as follows.

368
00:46:49,070 --> 00:46:53,270
Suppose we have a state of well-defined parity.

369
00:46:55,520 --> 00:47:01,459
Okay. So let's let Pepsi equal either plus or minus upside.

370
00:47:01,460 --> 00:47:05,150
Don't care which, but it's going to be so size a state of well-defined parity.

371
00:47:10,740 --> 00:47:16,170
And we've seen that the Asian states of the harmonic oscillator Hamiltonian are actually.

372
00:47:17,760 --> 00:47:25,400
States are well defined parity. And now let's consider Asi x asi.

373
00:47:27,900 --> 00:47:32,560
Well, that we've just seen is equal to minus ACI.

374
00:47:33,420 --> 00:47:38,340
P x p of sizes is a pure rewrite of a line higher up there.

375
00:47:38,730 --> 00:47:42,960
Well, except I've taken the dagger of the P, but as we know, p dagger is P, so who cares?

376
00:47:44,760 --> 00:47:47,160
So. But we've acquired a minus sign. That's that minus sign.

377
00:47:48,690 --> 00:47:56,459
But P on up CI is equal to either plus or minus up ci mean C plus up ci and p on this upside is

378
00:47:56,460 --> 00:48:02,940
equal to plus upside so these two P's can be got rid of if we put in a couple of plus signs.

379
00:48:10,930 --> 00:48:18,309
Or if size minus, we get that we have we have an extract, we take a minus sign out, but then we get another minus sign from there.

380
00:48:18,310 --> 00:48:23,080
So either way, we're taking out two, some sign, and therefore this is definitely one.

381
00:48:23,830 --> 00:48:32,940
So we have but it's inevitably the case. This expectation value is equal to minus itself.

382
00:48:32,970 --> 00:48:42,480
The only number equal to minus itself is zero. So that implies that the expectation value of X vanishes for all states and all states.

383
00:48:44,390 --> 00:48:54,690
Of well-defined parity. This is a result we use very often and it doesn't just apply to X, it applies to any vector operator, right?

384
00:48:55,320 --> 00:48:58,110
I could have made x any vector operator and repeated the argument.

385
00:48:59,220 --> 00:49:05,700
So when you're in a state of well-defined priority, the expectation values of all parity operators are nothing.

386
00:49:06,420 --> 00:49:08,520
And I think that's that we've done 2 minutes in hand,

387
00:49:08,520 --> 00:49:13,410
but I think that is the moment to stop because the next section is on symmetries and conservation laws.