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  Confusing statement in Nobel Prize Physics 1957 Award Ceremony Speech

+ 4 like - 0 dislike

Here is an expert of the Nobel Prize Physics 1957 Award Ceremony Speech given by O.B. Klein, Nobel Committee for physics 


Let us assume that the magnetic field is created by means of a coil placed like a spool of thread on a table, and that the electric current is flowing counterclockwise in the wire. Then the north poles of the cobalt nuclei will be directed upwards. The experiment, now, gave the result that the electrons from the radioactive process with this arrangement were preferentially thrown downwards towards the floor. From this it follows unambiguously that the process lacks that right-left symmetry, which one had earlier assumed. Thus, by means of this experiment it could be explained to a person, who did not know it - let us say an inhabitant of a distant stellar system - what we mean by right and left. In fact, it would be sufficient to ask him to arrange the experiment so as to make the preferential direction of the electrons point downwards. The current will then have the same direction as that in which he has to turn at the command "left face".

However - and this is a thing of the utmost importance for the incorporation of the new discoveries into our edifice of physical laws - the person on the distant planet will be able to follow our prescriptions only if he knows what we mean by the direction of an electric current. And to know this he must know that our atoms and his are made up of the same elementary particles. We know, however, that not only are there double sets of electrons - positive and negative - but that the same holds for protons and neutrons, the building stones of atomic nuclei. It is therefore possible that his atoms contrary to ours would consist of positive electrons and negative nuclei. If they did, he would judge the direction of the current opposite to what we would do, with the result that he would call right left and left right. In stating this we have tacitly made an assumption which is not quite confirmed as yet but which, as far as the experiments go, seems probable, namely that the results of all experiments performed with the opposite kind of elementary particles would be just such as to reestablish the right-left symmetry. With other words, one should be able to regard the antiparticles not only as the electric opposites of the particles but also as their mirror images.

I find it a bit confusing. As the main point of the speech is trying to address popular physics understanding of parity violation.

Question no.1: But how does it relate to the key idea of parity violation?

Question no.2: And is this last statement even correct at all: "one should be able to regard the antiparticles not only as the electric opposites of the particles but also as their mirror images?"

Question no.3: To clarify the no.2, we should examine the concept of particles and anti-particles, Time reversal  (T) partner, charge conjugate (C) partner and the parity (P) partner, more carefully?

asked Jan 4, 2015 in Theoretical Physics by RKKY (325 points) [ revision history ]
retagged Mar 10, 2015 by Jia Yiyang

2 Answers

+ 2 like - 0 dislike

The alien example is a very typical one (including a joke with aliens in many variations) about how one can establish right and left if one can not exchange words. The idea is, I 'll give a similar example, that you can communicate with some alien only via radiation. The alien has two hands like us but his physiology is completely different. Also he has no causal connection with out part of the universe (or universe whatsoever), but the laws of physics are the same. So imagine you want to explain to the alien how humans introduce to each others, i.e. to explain that you extend the right arm and the other side does the same (or make the current go clockwise or anti-clockwise, its the same idea). Therefore you need to explain to the alien the notion of "right-hand-ness". Until 1957 it was believed that the laws of physics were invariant under parity transformations and that no physics experiment could show a preference for left-handedness or right-handedness. In 1957 it was found that the radioactive beta decay process breaks P symmetry. It was found that when a specific nucleus (cobalt-60) was placed in a magnetic field, electrons from the beta decay were preferentially emitted in the direction opposite that of the aligned angular momentum of the nucleus. When it is possible to distinguish these two cases in a mirror, parity is not conserved. As a result, the world we live in is distinguishable from its mirror image. If we made a nucleus out of antimatter (antiprotons and antineutrons), its beta decay would behave in almost the same way but not quite the same because of parity violation. Therefore until 1957, there would have been no way of answering that question. But now the alien could make parity experiments. The alien could turn the experiment in a specific direction until the electrons come out in the upward direction (opposite to gravity), and the front edge of the rotating nucleus will move from right to left or clockwise to make the angular momentum.

This is a way to show that the parity violation of the weak interaction allows us, at a fundamental level, to distinguish right from left. Now, how would you determine if the Alien is made out of matter or anti-matter? Do Kaon oscillations say anything to you? Have I made you Q1 clear?

Finally as far as the particles and anti-particles are concerned they are indeed mirror particles. It is the weak interaction that is not right-left symmetric, not the particles (and I think people suspect/have found that also some strong interactions violate parity). Physicists have studies extremely well particles and anti-particles, C, T and P symmetries and they continue to do so. Do not expect though something dramatic to be found.

The point is that from 1957 we know that the laws of physics (and in specific the weak interaction) is not left-right symmetric (this can also be seen directly from the Standard Model Lagranagian which is chiral) although the particles and anti-particles are mirror images of each other. 

answered Jan 4, 2015 by conformal_gk (3,625 points) [ no revision ]
reshown Jan 4, 2015 by Dilaton
Most voted comments show all comments
Why would you get one? Of course you do not.
@Jia Yiyang I think your comments are pointless. "Mirror" in physics is not all about space coordinates reflection but even if it were the answer above clearly distinguishes it and uses it in a more "free" way.
@conformal_gk, but isn't the whole article directed to general audience rather than students in a QFT course?

Also, your comment " I did not ask you if you know how to find its charge conjugate rather its antiparticle" is genuinely confusing, to me you are exactly asking me this question. If I misunderstood the physics, please be explicit; if I misunderstood your words, please also be explicit.

@Outlander, to me it is exactly in this context that it has become confusing, where parity and charge conjugation are both mentioned. In particular, here it looks like he's trying to convey that CP is conjectured to be conserved, and to me the perfect place to stop is right after "...reestablish the right-left symmetry."

But I'll concede that it's possibly a mere issue of personal psychology.
@Jia Yiyang no it is not directed to a general audience rather to people who have done QFT and understand that I am too lazy to write down equations. I agree with Outlander. You are making a whole discussion pointless because you do not like the word "mirror". I have explicitly stated everything. I am not going to explain further trivial stuff like C,P and T symmetry and what mirror is, and to what it can refer to. You obviously did not understand my answer and my comments if you write down things like "here it looks like he's trying to convey that CP is conjectured to be conserved".
@conformal_gk, ok, I made my opinion into an answer.
Most recent comments show all comments
Take an electron given in the state $|\psi(x), s ; m, e\rangle$ where $s$ is the 3rd component of the spin $J$, $m$ it the electron's mass $e$ is the $U(1)$ charge. Can you find its mirror which in this sense means its anti-particle? Do not confuse what I mean here with "mirror" with an actual mirror that exchanges only $\vec{x} \to -\vec{x}$.
Sure I know how to do charge conjugation, but in that case it just make "mirror image" a completely useless way of delivering anything, in the sense that not only it does not seem to convey anything like the concept of charge conjugation, also it will confuse people what parity transformation is about.
+ 3 like - 1 dislike

On question number 2, I disagree with conformal_gk 's answer. Given

 In stating this we have tacitly made an assumption which is not quite confirmed as yet but which, as far as the experiments go, seems probable, namely that the results of all experiments performed with the opposite kind of elementary particles would be just such as to reestablish the right-left symmetry. With other words, one should be able to regard the antiparticles not only as the electric opposites of the particles but also as their mirror images.

it is more likely that the author was conjecturing a universal CP conservation, which was not directly contradicted by the Cobalt-60 experiment. The general sentiment toward CP-conservation was supportive but not with complete confidence, and the conjecture was falsified by K-meson experiment in 1964. 

My personal gripe with the highlighted sentence is that it certainly might confuse physics students, and does not really increase the understanding for lay people. 

answered Jan 5, 2015 by Jia Yiyang (2,640 points) [ revision history ]
Most voted comments show all comments
@conformal_gk, I've tried my best to read your answer, to me you certainly don't interpret that passage as conjecturing CP-conservation, which is why I need to add this answer. This answer might be wrong, but certainly not redundant.

If you consider Feynman diagrams, an antiparticle is conventionally just a particle line traversed in the other direction. I believe that this corresponds to CPT and not to C.

@Outlander, my answer is probably not a particularly interesting one judging from how much physics it contains, but potentially interesting for historical perspective. Besides, the question itself is asking about the interpretation of specific sentences, so I plead not guilty on taking too much effort on words.

Reading people's mind is difficult, especially after almost 70 years. I don't think one can weigh every word of a lecture, not even of a Nobel lecture. So let the result remain conjectural.

Jia is right--- the anecdote in 1957, and in the Feynman lectures repeating this anecdote, explicitly assume CP conservation. Now we would need to do CPT to get a symmetry, and this is exact, so you can still tell a story, just not the same story as in 1957. So it is important to say that the 1957 story assumes CP is exact, and if you omit this, important historical information is omitted, namely that the physics community believed CP was exact from 1954 until 1970 or 1980, whenever the heck the Kaon violations were accepted. I remember this confusing me too as a young student.

Most recent comments show all comments

My bad. To whom are you referring this time? To Klein?

@Outlander, yes, the quote is from the OP's quote of Klein, in the main post. I'll edit my last comment to make it less confusing.

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