MadSci Network: Physics
Query:

Re: Is counterclockwise rotation by a proton a significant property?

Date: Thu Feb 28 16:04:02 2002
Posted By: Benjamin Monreal, Grad student, Physics, MIT
Area of science: Physics
ID: 1014052079.Ph
Message:

Hi Tim,

Reversed electrical charges on the protons and electrons? That's more commonly known as antimatter. I hope you are taking good care of your antimatter: a) one gram of antimatter will explode with the energy of 10,000 tons of TNT, if brought into contact with matter. b) Given that it can only be "manufactured" one atom at a time at billion-dollar particle colliders, its estimated cost is $6400 trillion per gram. So be very careful with it. :)

Let me start with the short answer: no, backwards-spinning antinuclei are not "significant" or unusual in particular. However, they have some interesting symmetries when you compare them with forwards-spinning regular nuclei, especially when you force them to undergo "weak force" interactions.

While we've only recently succeeded in actually collecting anti-atoms in the lab, we have been studying their properties for decades. So let's talk about this stuff. I'm not sure what your question is really looking for, but it sounds to me like a parity violation experiment. There is a topic of particle physics called "CP violation" which I can try to sum up: Until around 1960, physicists thought that the laws of physics themselves couldn't tell left from right. Any physical law that applied to, say, a pool table, would look perfectly valid if the pool table was shown reflected in a mirror.

This was first illustrated in an experiment done by Madame Wu. The setup is as follows: you wind a spiral of wire around a block of radioactive cobalt. Putting some current through the wire generates a magnetic field; this aligns the nuclei (say, along the up-down direction). Some of the nuclei decay, emitting electrons in various directions. The question is: Will the electrons get emitted a) up "along" the magnetic field, b) down "against" the magnetic field, or c) equally in both directions?

It turns out that case A is true - more nuclei decay ALONG the field. (at least I think so. Maybe it's B, though.) So let's watch this experiment in a mirror; a special mirror, actually, that reverses up-down, north-south, AND east-west. If you like, you can imagine that someone has gone and "turned all of the signs around" - things are still in the same places, but what used to be labelled "north" is now labelled "south" and so on. It turns out that the magnetic field looks the same in the mirror, (It's true! But a bit surprising at first.) so the magnetic field still seems to be pointing along the new "up". However, if the decay electrons were expected to come out up, in the mirror they'll appear to come out "down". So someone in the mirror world WILL notice something amiss! The laws of physics, and the results of this particular experiment, are different when viewed in a mirror!

Instead of reflecting up-down, north-south, east-west (a parity change) we can swap the definitions of "positive" and "negative". This is equivalent to making the whole experiment out of antimatter. So now "up" is still "up", so there's no question about where the electrons are going. However, now the electric current creating the magnetic field is labelled backwards, so the field is backwards, so the nuclei are aligned the opposite of what you'd expect - and again, in this "mirror" the decays seem to be in the wrong direction. The laws of physics look different when viewed in this "charge conjugation" mirror.

There's only one law of physics that behaves this way, though: the so-called "Weak Interaction". The weak interaction one of the fundamental forces of nature. It's sort of an oddball, mostly what it does is change particles from one type to another: the weak interaction can turn neutrons into protons (which is what makes your Cobalt nuclei decay), muons into electrons and neutrinos, neutrinos into tau leptons, etc. Whenever you look at weak interactions, you'll find these "parity" and "charge conservation" violations. For example, if you were trying to scatter neutrinos off of your material, you would find very different interaction rates depending on how the nuclei are spinning, and the experimental effect will be "backwards" if you use nuclei and neutrinos, as compared to if you used antinuclei and antineutrinos. (An upgoing neutrino beam, I think, would prefer to scatter off of down-spinning nuclei, but an upgoing antineutrino beam would prefer up-spinning antinuclei.)

Interestingly, though, if you build your apparatus out of antimatter, AND reverse all of the directions, the experiment will again look the same as the non-mirror/normal matter experiment! Almost the same, at least - there are differences which are difficult to detect. This is called "CP conservation", and the differences are called "CP violation".

Hope this is helpful. CP violation and symmetry are an important and subtle part of particle physics - there are a few older MadSci questions on this topic, "What is the BaBar experiment?", "Why don't neutrons have temporary charge?", which you may enjoy reading. To understand CP violation in detail, you need to know some particle physics (I don't know how much you already know?), which you can get from a website like The Particle Adventure or a textbook like David Griffiths' or Donald Perkins'.

-Ben Monreal


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