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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
But by any stretch I can't see how you can get antiparticle from particle by only a parity transformation.
Why would you get one? Of course you do not.
Look again at what I have written. The anti-particles are mirror images of the particles, but that does not apply to the weak interaction. Parity alone won't mirror everything out. You need charge conjugation as well for this to happen.
But that is exactly what I was saying: even assuming P-symmetry, you don't get a antiparticle by applying P to a particle.
I did not say that P gives you an anti-particle.
Most recent comments show all comments
@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.
+ 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 ]
@Jia Yiyang, I am sorry but you obviously are wrong. If you look later at the passage I explicitly mention Kaons and I am asking the person that made the question how he would determine if the alien is made out of matter or antimatter. Obviously I am implying that CP is not conserved. Read better. Any graduate knows this and this is a place for graduate students. Even so, I am explicitly asking the question: do Kaon oscillations say anything to you?

You are making a fuss without reason and I am really curious on why.
@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.
Is it not the whole point of Physics Overflow that only questions of grad level and above will be considered? This is done in undergrad "introduction to particle physics" course, not even QFT, and while, yes, usually people refer to mirror symmetry (not string theory) as space inversion it can be extended to include other meanings as well. As for the CP violation, it is indirectly stated via the Kaons, so to me, as well, this is just pointless as I mentioned earlier.
@Jia Yiyang, for another time and actually the last one. CP violation is implied through the Kaon question I make and this is obvious if you know your theory. Your conclusion is wrong and your "answer" is completely redundant.
@conformal_gk, @Outlander, I didn't and never intended to say conformal_gk doesn't know the fact that CP is not conserved, but I don't see your answer interpreting the quoted passage to be conjecturing CP-conservation by the author at the time the speech was given, does this clarify?
@Jia Yiyang You have to read the whole passage to draw any conclusions. I ve told you several times where I mention CP violation.
@conformal_gk, please be sure I sincerely tried.
By the way, this is not an answer to any part of the question.
@Outlander, this directly addresses question 2.

The second @downvoter, I would be rather pleased to hear you reason of the downvote, if possible.

I didn't vote on this, but I think suggesting a conservation law is interpreting too much into the original statement. It just says that to turn a particle into an antiparticle you need to apply at least C and P. In fact you need to apply CPT, as only this is a necessary symmetry.

@ArnoldNeumaier, thanks for the reply, but wouldn't 

 It just says that to turn a particle into an antiparticle you need to apply at least C and P

be just a wrong statement? By definition a charge conjugation operator is enough to take a particle to an antiparticle, precisely because of this I think the minimal interpretation that makes sense is that the author was suggesting CP conservation.

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.

@ArnoldNeumaier, that alone may make sense, but the sense would be lost in light of(an excerpt from OP's quote in main post)

 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.

If we interpret it like you suggested, then the highlighted text seems to infer

(1)He forgot to mention T transformation.

(2)Suppose (1) was just an innocent mistake, still this means the author seriously considered CPT violation as a possiblity, that CPT invariance was only "probable" and he only supposed it's true because there had been no experiment contradicting it.

However, as far as I know about the history, no serious doubt has really risen toward CPT invariance, on the contrary, there was indeed a period of supportive sentiment toward CP conservation. Hence the CP conservation interpretation seems more probable to me. 

Are you suggesting Weinberg implied CP conservation? 

@Outlander, I don't understand you, where did I mention Weinberg in this post?

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.

Ok, still, I think you are trying to play with words again. If I were you I would not worry so much. Additionally although as you say indeed CPT is a symmetry in principle you can look at it. It is nuts but you can and of course you will find no results. I know people at the University of Manchester (like a top university) who even look on electron-less double beta decay. Speechless I am..

@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.

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