There Is No Parity: Chien-Shiung Wu

Hold out your hands in front of you, palms forward. They look quite similar, but I’m sure you’re all too aware that they’re actually mirror images of each other. Your hands are chiral objects, which means they’re asymmetric but not superimposable. This property is quite interesting when studying the physical properties of matter. A chiral molecule can have completely different properties from its mirrored counterpart. In physics, producing the mirror image of something is known as parity. And in 1927, a hypothetical law known as the conservation of parity was formulated. It stated that no matter the experiment or physical interaction between objects – parity must be conserved. In other words, the results of an experiment would remain the same if you tired it again with the experiment arranged in its mirror image. There can be no distinction between left/right or clockwise/counter-clockwise in terms of any physical interaction.

Dr. Wu working with a particle accelerator via Biography.

The nuclear physicist, Chien-Shiung Wu, who would eventually prove that quantum mechanics discriminates between left- and right-handedness, was a woman, and the two men who worked out the theory behind the “Wu Experiment” received a Nobel prize for their joint work. If we think it’s strange that quantum mechanics works differently for mirror-image particles, how strange is it that a physicist wouldn’t get recognized just because of (her) gender? We’re mostly here to talk about the physics, but we’ll get back to Chien-Shiung Wu soon.

The End of Parity

Conservation of parity was the product of a physicist by the name of Eugene P. Wigner, and it would play an important role in the growing maturity of quantum mechanics. It was common knowledge that macro-world objects like planets and baseballs followed Wigner’s conservation of parity. To suggest that this law extended into the quantum world was intuitive, but not more than intuition. And at that time, it was already well known that quantum objects did not play by the same rules as classical objects. Would quantum mechanics be so strange as to care about handedness?

By the time the 1950’s rolled around, physicists were smashing subatomic particles into one another in high speed particle accelerators and analyzing the resulting explosion of new, sometimes previously undiscovered particles. One of these previously undiscovered particles was quite puzzling – the K meson. It appeared that there were two different versions – one would decay into 2 pi mesons and the other would decay into 3 pi mesons. Pi mesons are also called pions. All other properties of the K meson were identical, suggesting there was only one type.

Setting up the Cobalt-60 parity test via NIST.

After doing the math, it was determined that the pions in the two and three particle systems must have opposite parity. And according to Wigner’s conservation of parity, there therefor must be two types of K mesons – one that produced two pions and one that produced three pions. Conservation of parity would not allow both systems to come from a single particle.

But what if Wigner’s parity theory were wrong? The K mesons were produced during weak force interactions when protons were smashed into heavier nuclei. In 1956, physicists T.D Lee and C.N. Yang suggested that weak force interactions might not follow the conservation of parity, and that there was indeed a single K meson. That the two systems of pions were the result of a single K meson that had a definable parity, and that parity was not conserved in this particular case. Consider if the K meson had a spin, and that a clockwise spin produced the 2 pion system and a counterclockwise spin produced the 3 pion system. This would be an example of a violation of the conversation of parity. A violation that physicist were suggesting was occurring with the K meson.

An experiment was derived to put parity to the ultimate test.


As you can imagine, these kinds of experiments are a bit complicated. And it’s my mission to break complicated things down to the point that your everyday curious hacker can understand. The goal is to prove that Wigner’s conservation of parity did not hold water with weak force interactions. To do this, we’re going to need three things:

  1. Something that emits beta radiation (beta decay is caused by the weak force).
  2. Something that has two known physical states.
  3. A way to measure the radiation in each of the states.
Cobalt-60 parity test apparatus via NIST.

Conservation of parity would insist that the measured radiation be the same in either state. Because if parity is to be conserved, it should not be possible to get a different experimental outcome between different states, like spin for instance. This can be done by taking Colbalt-60, which is naturally radioactive and cooling it to a smidgen above absolute zero. Cooling it to this temperature takes away most molecular motion, and allows the atoms to arrange themselves in a crystal structure while in the presence of a very strong magnetic field. The magnet field also polarizes the Colbalt-60 nuclei, which means they spin in the same direction parallel to the magnetic field.

Now all we have to do is:

  1. Measure the beta radiation intensity.
  2. Reverse the magnetic field direction.
  3. Measure the beta radiation intensity.
  4. Compare the results.

By reversing the magnetic field, we cause the cobalt-60 nuclei to reverse polarization. Conservation of parity says there should be no measurable difference between the two physical states. And I’m sure you can guess by now what they found – the measured beta radiation intensity was greater in one direction. This was the nail in the coffin for Wigner’s parity theory. It allowed physicists to reexamine results of previous experiments involving weak force interactions and helped advance quantum theory and eventuality lead to the Standard Model of particles we have today.

Chien-Shiung Wu

As we mentioned above, Lee and Young received the Nobel Prize for the Wu experiment. Wu, unfortunately, didn’t. Wu was one of a handful of female physicists during that time whose name and research are not as well known as they should be. Gender-based injustice was widespread during her time, but she would receive worldwide recognition for her contributions to nuclear physics from the 70’s onward.

Indeed, she was awarded the inaugural Wolf Prize in Physics in 1978, partly to make amends for the Nobel slight, but also for her further work in experimental nuclear physics. Wu also worked on the Manhattan Project, held a prestigious chair at Columbia University, and was responsible for important experimental results in exploring the weak nuclear force and the first experimental confirmation of quantum photon entanglement.

Chien-Shiung Wu died in 1997 at 84. Let’s close with a fitting quote from Dr. Wu during an address at an MIT symposium in 1964:

“I wonder whether the tiny atoms and nuclei, or the mathematical symbols, or the DNA molecules have any preference for either masculine or feminine treatment.”

68 thoughts on “There Is No Parity: Chien-Shiung Wu

    1. Lee and Yang came up with the experiment. Wu actually *did* it.

      I’m not sure I’d actually say that her not receiving the Nobel Prize is really a slight. Her gender/nationality might’ve had something to do with it (we are talking about the ~50s-60s and physics, after all) but the Nobel tends to be awarded to the predictors and not the experimenter when something is predicted and confirmed. See Higgs/Englert, Glashow/Salam/Weinberg, Kobayashi/Maskawa.

      While many fundamental particle physics Nobels go to experimentalists, they’re experiments that discover something completely unexpected (e.g. CP violation, the solar neutrino problem, the CMB, etc.). An interesting counterexample to that is Kajita/McDonald (SuperK/SNO), but that’s a bit of an odd case since there wasn’t really anything groundbreaking about postulating that neutrinos can oscillate – the impressive part was figuring out a way to measure it.

  1. Typo detected: “remain the same if you tired it again”
    You tired to avoid typos, but alas :)

    As for the article, to me it is rather obvious that the reaction would not be the same and the surprising part is that anybody thought it would be. But of course if a physicist that had thought about it said it would not be different I’d assume it was as he said, but it’s counter-intuitive to my mind.

    1. “As for the article, to me it is rather obvious that the reaction would not be the same”

      Huh? Why the heck is it obvious?

      Gravity is parity-invariant. Electromagnetism is parity invariant. Even the strong force is parity invariant. The *only thing* in the Universe that gives a crap about parity is the weak force. And it’s completely bizarre that it does.

      If you have a ball, you can spin it left, you can spin it right. No problem. If you take the spinning ball and shove it into a wall, it caroms off in one direction. But if you spin it the other way, it caroms off in the other direction. But those two experiments look exactly the same in a mirror. So they’re reflection symmetric.

      Parity violation is like taking a ball, spinning it, and flinging into a wall, and it *always caroms off left* regardless of which direction you’re spinning the ball. It’s totally bizarre.

        1. It’s not an assumption. I’m giving an example. If you had a ball you bounced off a flat wall, you’d be damned surprised if you couldn’t make it carom off in one direction regardless of how you spinned it. But something like that is exactly what’s going on with parity violation.

          1. The examples assume a flat, featureless target because that’s what’s going at the subatomic level. You can set up an experiment such that you’ve got a particle with spin, incident on a uniform magnetic field (which is like the flat billiard table edge), and it always caroms in a specific direction. The weak force is literally the only force that acts like this, which is why Yang and Lee’s ideas were originally dismissed.

      1. Except that there’s nothing actually spinning, per say. “Spin” is just a classical metaphor for a quantum phenomenon. So to compare the two is somewhat, if not very, misleading, because we understand the underlying reasons for why the ball caroms. Physicists are particularly interested in symmetry, and it certainly does make things easier, but we have to be careful not to assume all our cows are spheres.

        1. This is, essentially, what I’m getting at, and why it’s not really “insane” to me (just as wave-particle duality isn’t really all that “insane” if you take a view similar to this: It’s really more mysterious than insane:

          “If one insists on the image of a spinning object, then real paradoxes arise; unlike a tossed softball, for instance, the spin of an electron never changes, and it has only two possible orientations. In addition, the very notion that electrons and protons are solid ‘objects’ that can ‘rotate’ in space is itself difficult to sustain, given what we know about the rules of quantum mechanics.”

        2. “So to compare the two is somewhat, if not very, misleading,”

          It’s not misleading at all. It’s angular momentum. You can convert the spin angular momentum into actual orbital angular momentum if you want, and then it *really is* something spinning.

          The article you linked doesn’t help at all, and I *very much* disagree with the first professor linked in that article, and he’s just *completely* wrong on one comment – an electron’s spin doesn’t have 2 orientations. It has any orientation it wants. We just project them into a common basis when we talk about them, and quantized spin, being the pain in the neck it is, means that when you force them to interact in that basis the resulting interactions have complete components in that direction.

          The *third* professor (Vic Stenger, who I’ve had several disagreements with on other topics, both in person and via correspondence, but on this one we agree) is correct on this. It’s the intrinsic angular momentum of the field. However you want to “view” that doesn’t matter, because in the end, it’s angular momentum.

          Like I said, I can very easily contrive an experiment to convert ‘spin’ angular momentum into ‘orbital’ angular momentum. Happens all the time in decay experiments. Hell, via a *ridiculously* contrived experiment, you could actually make it work just like I said.

          “but we have to be careful not to assume all our cows are spheres.”

          This is not about an assumption at all. This is about finding out that something that was totally arbitrary – the handedness we assigned to angular rotations (and thus, to angular momentum) wasn’t actually arbitrary at all. Feynman’s talk on this is particularly instructive, although I don’t think he stressed enough how insane it was – especially given the fact that it had *just happened* when he gave the talk.

  2. Your title’s really funny. In fact, discovery of parity symmetry violation means that there *is* parity – that is, *the Universe itself has a handedness*. If parity symmetry were true, then clockwise/counterclockwise would be a completely arbitrary definition (much like ‘velocity’ is in simple special relativity examples). But it isn’t. If we had a way to communicate with an alien civilization, we would be able to define and agree on “clockwise” and “counterclockwise” just through fundamental physics.

    1. Yes, I’m sure all civilizations (including alien) have 2 things in common, clocks that rotate to the right for clockwise motion and to the left for counter-clockwise motion,
      and Gin and Tonics.

      1. what Pat is saying, is that even if the aliens don’t have clocks, the alien ‘civilization’ (implying they have either already studied and discovered the same physics, or are able to follow our instructions for the specific experiment) can be made to understand how to build a clock as humans are used to them, merely by sending information.

        If there were no such parity detection experiment, they might still be able to make our clocks as our welcome gift, but during construction they wouldn’t know which chirality we expect. So they would be forced to build both versions, and then risk slightly humorous humiliation with 50%-50% probability.

        1. The idea of how amazing this is (that you can define ‘left’/’right’ with physics, not with the shape your fingers make) comes from a lecture by Feynman, “Symmetry in Physical Law,” which can be found here.

      2. Not true. Reverse (mirror image) clocks exist. I think the story of one was that an emperor or king wanted to be able to read this clock in the mirror, because it could by some reason not be placed physically on the right spot.

  3. I think from a more a broad perspective, it’s easy to assume that a change in something like spin might effect how a particle behaves. However if you are in the shoes of a physicist arguing the counter, what physical phenomenon would you use to explain why you think spin direction will effect radioactive decay?

    1. Parity violation doesn’t say that spin affects how a particle behaves. Spin obviously affects how a particle behaves – magnets show you that in a heartbeat. But for almost all processes, that effect is symmetric in a mirror. Like I said above, it’s like spinning a billiard ball and throwing it at a wall. It hits the wall, and caroms off in one direction. Spin it in the other direction, and it caroms off in the other direction.

      Parity violation is like saying there’s a ball that always caroms off to the left, no matter what. Can’t make it go to the right. It’s completely insane. Parity violation actually says that what gets taught about magnets – that ‘north’ and ‘south’ are arbitrary – is actually wrong. You could call the south side of a magnet “beta-ray deflector pole” and the north side of a magnet “beta-ray attractor pole.”

      If you’ve ever made a home-made compass, this is just insanely bizarre. You know that when you magnetize a compass, and float it in water, it’ll align north-to-south, right? But you don’t know which is north, and which is south. You have to use stars, or some other fixed-reference to figure it out. This says “nah, you don’t need any of that. You can build a little device that can figure out north from south all on its own, even if you’re dropped in an underground cave someplace on the Earth.” Again, it’s totally nuts.

  4. Will Sweatman (HaD) said:

    “If we think it’s strange that quantum mechanics works differently for mirror-image particles, how strange is it that a physicist wouldn’t get recognized just because of (her) gender?”

    Excuse me Mr. Sweatman: You are implying Gender-Bias! This has NOTHING to do with Gender-Bias. Please check your Political Correctness.

    The way SCIENCE usually works (especially when it comes to quantum physics) is THEORETICAL scientists posit the existence of something – then EXPERIMENTAL scientists (many of them hopefully) test the theory. Then ONLY after the results of MULTIPLE independent and objective experimental scientists match-up statistically, will the work of the theoretical scientists be declared a DISCOVERY!

    Let me dumb this down with a simple hypothetical:

    1. A theoretical chemist proposes that mixing vinegar and baking soda will produce a foamy mess. The theoretical scientist bases his/her work only on the theoretical knowledge of what baking soda and vinegar MAY do when mixed at a molecular level.

    It should be noted: Sometimes the theoretical scientist may also propose one or more methods experimental scientists may use to attempt to prove his or her the proposed result. But the actual testing at an experimental level IS NOT the job of the theoretical scientists. There’s a good reason for this:

    Without independent objective experimental testing of the theoretical scientist’s proposed output, there is significant risk of Confirmation Bias [5].

    2. Experimental scientists (multiple hopefully) attempt to prove the theoretical scientist’s expected result.

    3. The expected result posed by the theoretical scientist IS proven by the test results from the work of the experimental scientists.

    4. The theoretical scientist (not the multiple experimental scientists) is given a PRIZE for his/her DISCOVERY (yay!) All the experimental scientists are happy too, not matter if they are male or female.

    In the end, the point is: The theoretical scientist DISCOVERED something. The experimental scientists that verified the discovery are to be lauded, but they DID NOT discover anything in a strict-sense.

    Here’s is the (sadly) missing background information on this subject that was not included in the HaD article (excerpted below, but with references to full content at the end):

    Chien-Shiung Wu (a woman) [1] was a Chinese-American experimental physicist who made significant contributions in the field of nuclear physics. She is best known for conducting the Wu experiment [2], which contradicted the hypothetical law of conservation of parity. This discovery resulted in her colleagues Tsung-Dao Lee [3] and Chen-Ning Yang [4] winning the 1957 Nobel Prize in physics, and also earned Wu the inaugural Wolf Prize in Physics a mere two decades later in 1978.

    Tsung-Dao Lee and Chen-Ning Yang (both males), the theoretical physicists who originated the idea of parity nonconservation and proposed the experiment, received the 1957 Nobel Prize in physics for this result.







      1. Unless they are Chinese, who named a race after a place known for it’s diversity wall.
        There are those that claim they invented golf, the airplane, and Viagra — but it is only true if everyone believes in a lie.
        If someone didn’t contribute to the science theory being awarded, than they should not get credit for the discovery.

        There are still those with a nationalist entitlement, low self-esteem, and despotic political structure.
        The part that bothers us the most is property investment companies advertising “pure blood” only condos (we travel and read mandarin too you bigots) in our American town known for a rich pre-communist genocide Asian history. Seriously, people need to stop identifying themselves as Chinese given the actual country sill has some people with extreme xenophobic 1700s colonial ideals that make fascism seem ridiculously mild.

        Truly, science is the only thing that can save us from our own primitive bullshit with reality — meritocrats and despotism have nothing to do with actual science — other than a study subject exposing disappointing human biases.

        1. Should we even start on all of the god#%$& fake pay to play academic journals they are poisoning science with? Many of these contain fake experiments, made up or multi-journal submitted articles.

          It is hard to be creative if your whole life is rote memorization I guess. Just copy the shit out of everything. This is what really steams me. Open source is great but academic and industrial thievery is something we should be aware of and try to head off and definitely prosecute. It is flat out going to smash any effort put forth by good STEM students. Also international “sister university” peer review would go a long ways to combat this kind of crime.
          So I don’t sound like a total jerk, there are very smart people in China, but this academic scandal taints their accomplishments as well. Kinda like jumping a border rather than even bothering for a tourist visa to overstay smdh.
          Kinda shitty to think I can pay $80.00 to get an academic article published on Cat Poop as Nuclear Fuel whereas 6+ years of research and focus yields an RnR on submission to a real journal over here.
          India, I am also looking in your direction…

    1. “The theoretical scientist (not the multiple experimental scientists) is given a PRIZE for his/her DISCOVERY (yay!) All the experimental scientists are happy too, not matter if they are male or female.”

      This isn’t the way the Nobel Prize works. Nor should it – it’s not like theoretical scientists are the only ones who can make contributions.

      If you’ve got multiple experiments confirming a prediction short after it was proposed, the experiment wasn’t that hard. If you’ve got multiple theorists proposing similar ideas, and it’s really just a minor change from existing theory, the prediction wasn’t that hard. The prize goes to the breakthrough, whichever it is.

      1. Discoveries by Experimental Scientists and/or Engineers DO get Nobel prizes, on their own with NO input from Theoretical Scientists. (a classical case reference is included below)

        But what happened in the specific case of Chien-Shiung in this HaD article does NOT apply here, she did not DISCOVER anything. This was the point of my post on this HaD article.

        The emphasis HaD put on the topic was FAKE Politically Correct Gender Bias. Check your BIAS HaD, I’m “Triggered”!

        One example of Experimental Scientists and/or Engineers that earned a Nobel Prize for their actual discovery:

        Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery of the Cosmic Microwave Background (CMB) [1].

        Their discovery was purely through experimental means. Rigorous methods to qualify their findings were needed, which led to the results being duplicated by other invesigators.



        1. Woah, that’s way too extreme. She did discover something. She discovered parity violation. Before that, it was just a theoretical prediction. But in that case, the prediction and experiment were proposed by Yang and Lee, so it’s not *that* surprising that Wu wasn’t included. But you don’t say “Yang and Lee discovered parity violation.” Wu did, absolutely. Just like you don’t say “Higgs discovered the Higgs boson.” ATLAS and CMS discovered it: Higgs predicted it.

          *Most* experimentalists get it when they discover something totally unexpected. Penzias/Wilson, or the discovery of the accelerating universe.

          However, there are plenty of examples of other experimentalists who confirmed something that was already well-predicted (neutrino oscillation, COBE, or gravitational waves when LIGO gets it in the next years, if not this one). They get it when the breakthrough was in the measurement, rather than the prediction. It’s about the breakthrough, not the discovery.

  5. There is no doubt that there has been an awards bias toward men. However, there is a second issue here, which is that Wu was an experimenter, and the men who got the award are theorists. Experimenters make a critical observation, providing evidence to falsify or verify a theory or which calls for a new one to explain the observation. Theorists explain why things happen. So, we value Einstein more than the scientists who verified his time-dilation theory.

    1. That’s not true. Plenty of experimentalists have won the Nobel Prize. The prize tends to go to the group that was really innovative in the scientific process. If a theorist proposes an idea, and an experimentalist *figures out a way to test it*, a lot of times the experimenter will get as much credit or more.

      Similar issue with SNO/SuperK and neutrino oscillation. Pontecorvo first proposed the idea of neutrino oscillation, but it was in more of a “these are all the possible ways lepton charge conservation can work” paper. Measuring it was really the innovative part.

  6. All electronic controls are clockwise. There exceptions, ugh. But because right handed threads were used on water taps, all faucets are bas awkards. This is copied to gas valves and water valves that are not threaded (gate type).
    What madness! I converted my gas stove. It’s so much easier now to adjust while cooking.

  7. Meh, the lack of parity is self evident, otherwise the universe would not have ended up filled with mostly matter. The entire thing is asymmetric at all scales, even time is in practice asymmetric. The Wu experiment was a remix of one done by Luis Walter Alvarez furthermore Lee and Yang’s theoretical calculations were done before she ran her improved version of the experiment. Science is like that elite practical skills are great (and nobody doubts she had them), but theory and being first are paramount, they give no prizes for second place. Publish or perish and all that. As for her quote, I hope she saw the irony in that, after all the reality if the situation still is that there is no parity.

  8. Why is her gender more important than her work?

    I’ve never, ever met a scientist who believes that a persons ability is defined by their gender. Most of those I know, don’t even consider the gender of an author when they read their papers. Gender bias in science is an anomaly, not the rule.

    On the other hand, in humanities and administration, gender bias is common. That’s the sort of people who typically sit in awards committees.

    The article could just as well harp on and on about how women in the Islamic world still suffer from that kind of gender bias, and even hint that it might be the reason the contribution from that part of the world is so disparate today. Ah, but I forget: that would be racism, wouldn’t it? (To a logical person, describing negative effects of a set of common habits or cultural rules is not, but logic isn’t in favour in today’s socio-politically aware world.)

      1. Wow, you’ve really been very helpful. Since you brought up a strict definition of “race” I’ve realized that as someone with European heritage, I can now say anything I want about any group of Caucasians without it being possible to be “racist”. It’s amazing. Good parts of the middle east, asia minor, western asia, south asia, central asia, north africa, and northeastern africa are now on the table.

        I can call them all kinds of denigrating names and no one can see it as racist because I’m the same race. I even capitalized “European” and left all those other groups lowercase as a completely non-racist insult.


    1. “I’ve never, ever met a scientist who believes that a persons ability is defined by their gender. ”

      There’s a scene in Hidden Figures where the (fictitious) obviously racist supervisor of the white computing group says to one of the black computers “Despite what you may think, I have no problem with y’all.” And her response is “I know. I know you probably believe that.”

      The problem isn’t what they believe. It’s what the results of their actions show. And speaking as a male scientist who’s seen the way that other scientists interact with female scientists… it is not an anomaly. At all. It’s really, really pervasive. It’s just subtle.

  9. When was it discovered that mirror image molecules often don’t work properly, or at all ,or have undeseriable/toxic effects in biology? For example, D-glucose vs L-glucose. L-glucose can’t be used by humans or any other Earth life form that uses glucose for energy. D-glucose is the naturally occurring form, with L-glucose having to be manufactured. It still activates taste receptors for sweetness but can’t be metabolized.

    1. From Wikipedia “In 1894 Fischer suggested that enzyme-substrate interactions take the form of a “lock and key”, the fundamental principles of molecular recognition and host-guest chemistry.” i.e. the entire thing about the shape of molecules is irrelevant. Molecular symmetry and interactions are mechanistic (without getting lost in quantum details) and particle symmetry and interactions are at a completely different level.

    2. Chirality has a tremendous effect on not only chemical interaction with other molecules in general, but the way the molecule will (or won’t) interact with receptors. For example the L and D forms of methamphetamine have very different potency in mammals.

    3. The answer that applies to your sugar example is 1886 (Puitti). Because receptors are proteins, and the proteins are chiral, the protein formation in tissues tends to not be a racemic mixture due to the nature of their creation.. i.e.: the proteins have only one handedness, and they only interact with molecules of a mating chirality. So, the handedness of the sugar is very important in the interaction, whether it is in a simple “taste” interaction, or in the activation of a receptor that releases an enzyme. Enzymes themselves are chiral also, and might not interact at all with a particular handedness of sugar.

  10. Can somebody explain why electromagnetic fields follow a right-hand rule and not a left-hand rule? This always seemed far more strange than any of this modern quantum stuff. There’s lots of descriptions of the rule, and how to apply it, but I’ve never seen a good common-sense explanation of why. I know its a cross-product and all that, but why the particular handedness?

    1. Speaking of “moot”…

      The right-hand rule is just definitional. We say that the “north” pole is where the thumb goes. It could have easily been “south”. Same same.

      I had an eccentric astronomy prof who redid all of the sign conventions to fit a left-hand rule — it works out. He did this so that he wouldn’t have to drop his pencil to make the grippy gesture. Makes more sense, honestly. Not worth the hassle. Sign errors….ugh.

      1. No! This is actually wrong, which is one of the crazy physics discoveries in modern times.

        The original choice was arbitrary, but you actually *can’t* define things with a left-hand rule and keep physics the same. This is utterly insane to believe, but it’s completely true. You can do it if you stay within electromagnetism, but once you bring in the weak force, it doesn’t work anymore.

        Imagine you say, I’m going to use the left-hand rule to define the direction of the magnetic field. But that means that what *used* to be the ‘south’ magnetic pole now points geographic north, and the ‘north’ magnetic pole points geographic south. So what, you say? It’s an arbitrary definition, right? A needle aligns to the magnetic field just north/south. There’s no way to tell which end is which.

        Except… there *is*. Beta rays (electrons from beta decay) used to deflect away from what you called the ‘south’ pole. Now they deflect from what you call the ‘north’ pole. The physics isn’t the same!

        OK, you say, well, I can fix *that* by flipping the sign of the electron, too. After all, you can use the ‘left hand rule’ to figure out the direction of an electron’s deflection, too. So that reorients the magnetic field to ‘north’ again. Flip the sign of the electron, flip parity, and *now* everything stays the same, right? (In physics, that’s a ‘CP’ operation)

        Nope! It turns out that there’s *another* problem with this. There’s a particle called the neutral kaon, and there’s two ‘types’ of it – effectively, one that’s right handed, and one that’s left handed, and they’re both effectively opposite ‘charges’ of the other (yes, they’re neutral, but they’re combinations of two charged quarks – swapping signs means you change the charges on those quarks). If you swap the sign of the electron, and swap handedness, that means you change the definition on those kaons. No big deal, right? The definition of those kaons was arbitrary, right?

        Nope. You can tell the difference between those two, because one of them is short-lived, and one of them is long-lived. That alone doesn’t help, because the long-lived one mostly decays into three pions, and flipping charge and handedness flips charge and handedness of those decay products.

        Except…. it *doesn’t always decay to 3 pions*. It rarely decays to *two* pions. Which have *opposite* charge and handedness from the other one. When you try to write down the math that determines how often that handedness switch happens (you describe it by something like an angle) – that angle would have to *change sign* in your ‘charge-mirror’ world.

        Ridiculously, the only way you could make a ‘left-hand’ rule actually work is to actually do three things. Flip handedness, flip the sign of the electron, and *flip the direction of time*. That’s CPT symmetry, and *that* looks like it’s fundamental to electromagnetism, the weak force, and the strong force. (Gravity, though… that’s a different story.)

        1. Beh, I just realized there’s a better way to explain how CP screws up physics. Right now, you can say “the long-lived neutral kaon has a 2-pion decay where the positive pion is spinning right’.

          If you swap charge and handedness, you might think that you’d end up with the positive pion spinning right, still. Because you swapped positive for negative, and left for right. Except if you went and did the experiment… you’d be wrong. Your positive pion would be spinning *left*. The physics isn’t the same.

          That answer is really, really hard to wrap your head around, but it’s the way the Universe works. Our initial choices of handedness and charge sign were arbitrary, but now we could replace them with the results of experiments.

          Instead of saying “oh, the right hand rule is arbitrary” you could say “the right hand rule is the orientation that makes field lines go outward on the beta-ray attracting pole”. Likewise, instead of saying “charge is arbitrary” you can say “positive charge is the charge of the pion that spins right in the 2-pion decay of the long-lived neutral kaon.”

          Those *choices* are arbitrary, but you can’t flip the choices without changing the results of those experiments.

      2. yes, changing the ‘sign’ (or the arbitrary coordinate definitions) flips the handedness, but is there a way to explain WHY there is a handedness to the electromagnetic field/force relationship in the first place? (without using either circular arguments based on the empirical rules or complicated mathematics). At school, the teacher told us ‘thats just the way it is’. I’ve heard people talk about helicity or particle ‘drift’ but I’ve never heard a simple explanation that I could believe. Its like having somebody pull you in the forward direction whilst gravity pulls you straight down and that somehow makes you move to the left (but not the right).

        1. Not sure what you’re looking for here. I think you’re asking “why do magnetic forces act at right angles to currents” (or in reverse, why are magnetic fields generated at right angles to currents)? Because that’s what gives something a handedness: any time you’ve got a force that acts at a right angle, you need to choose left or right.

          And there *is* a reason for that, although it’s not easy to explain.

          It’s because magnetism and electricity are the same forces, but different components. Electricity is the “timelike” component – it’s what you get when two charges are at rest. Magnetism is what you get when the other charge is moving. Relativity says that the physics can’t change with a boost in velocity, you just end up mixing the time component and the space component. Which means the time component (electricity) has to mix into a directional component, and that gets you the right angle.

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