1
00:00:04,717 --> 00:00:05,550
MARKUS KLUTE: Hello.

2
00:00:05,550 --> 00:00:07,170
Welcome back to 8.701.

3
00:00:07,170 --> 00:00:11,370
In this short video, we will
talk about charge conjugation.

4
00:00:11,370 --> 00:00:13,500
Charge conjugation
is the transformation

5
00:00:13,500 --> 00:00:17,010
which switches all particles
to their corresponding

6
00:00:17,010 --> 00:00:19,570
antiparticles and vice versa.

7
00:00:19,570 --> 00:00:25,410
So you have a
particle p, you apply

8
00:00:25,410 --> 00:00:27,630
charge conjugation
on this particle,

9
00:00:27,630 --> 00:00:30,600
and you receive
its antiparticle.

10
00:00:30,600 --> 00:00:34,980
This changes all signs of
internal quantum numbers--

11
00:00:34,980 --> 00:00:39,210
the charge, the baryon number,
the lepton number, strangeness,

12
00:00:39,210 --> 00:00:41,410
charmness, and so on.

13
00:00:41,410 --> 00:00:44,460
But at leaves the mass,
the energy, the momentum,

14
00:00:44,460 --> 00:00:47,010
and the spin untouched.

15
00:00:47,010 --> 00:00:49,080
The electromagnetic and
strong interactions,

16
00:00:49,080 --> 00:00:50,580
they obey charged symmetry.

17
00:00:50,580 --> 00:00:53,345
But the weak interaction
violates charge symmetry.

18
00:00:55,970 --> 00:00:59,500
So charge conjugation, it's a
multiplicative quantum number,

19
00:00:59,500 --> 00:01:01,690
like parity.

20
00:01:01,690 --> 00:01:06,850
You get identity if you apply
charge conjugation twice.

21
00:01:06,850 --> 00:01:08,680
You make an
antiparticle, and then

22
00:01:08,680 --> 00:01:10,360
you apply this to
the antiparticle,

23
00:01:10,360 --> 00:01:12,460
you get the particle back.

24
00:01:12,460 --> 00:01:15,700
Only particles that are
their own antiparticles

25
00:01:15,700 --> 00:01:18,990
can be eigenstates
of this symmetry.

26
00:01:18,990 --> 00:01:21,550
You can see this here.

27
00:01:21,550 --> 00:01:25,450
When you apply this, you either
get a positive or negative

28
00:01:25,450 --> 00:01:26,920
sign.

29
00:01:26,920 --> 00:01:29,740
But this is only valid
for particles who

30
00:01:29,740 --> 00:01:33,530
are their own antiparticles.

31
00:01:33,530 --> 00:01:37,760
As elementary particles,
that only leaves the photon.

32
00:01:37,760 --> 00:01:39,950
As composite particles,
you will see later

33
00:01:39,950 --> 00:01:43,460
that there is a
number of mesons which

34
00:01:43,460 --> 00:01:46,900
can be their own antiparticles.

35
00:01:46,900 --> 00:01:51,020
But we'll discuss this
in subsequent lectures.

36
00:01:51,020 --> 00:01:55,130
By itself, there's limited
use to the symmetry

37
00:01:55,130 --> 00:01:56,345
in order to learn things.

38
00:01:56,345 --> 00:01:58,440
There are some examples,
and we'll discuss them

39
00:01:58,440 --> 00:02:03,890
in a recitation, where you can
learn about possible decays,

40
00:02:03,890 --> 00:02:06,860
for example, in
pions, neutral pions,

41
00:02:06,860 --> 00:02:11,630
from applying this symmetry,
without knowing really

42
00:02:11,630 --> 00:02:14,300
what is the underlying physics.

43
00:02:14,300 --> 00:02:16,400
But in the next
lecture, we then talk

44
00:02:16,400 --> 00:02:23,300
about CP, the multiplication of
parity and charge conjugation,

45
00:02:23,300 --> 00:02:27,490
and some of the interesting
effects which occur from this.