12.14.2019

10.05.2004

4:00am

Filed under: neat!,news,physics @ 17:17

About the time I was finally able to get to sleep last night, a certain member of the Caltech faculty was abruptly awakened by the ringing of his telephone. Professor of Theoretical Physics David Politzer must have first awakened in annoyance when he heard the phone, then fear as he subconciously prepared himself for some sort of emergency, and finally elation when he realized what day it was. 10.05.2004: the day the Nobel Prize in Physics is announced.

He lifted the receiver and was connected with Stockholm.

I like to think the King himself was on the phone, but, as far as I know, the Royal Swedish Academy of Sciences, though founded by the Crown, is entirely independent; I do not believe the current King is a member.

Dr. Politzer was awarded the prize for his earlier work in on the concept of asymptotic freedom. The strong nuclear force is responsible for holding together such “everyday” particles as the proton and the neutron (together known as nucleons). These nucleons are in turn comprised of other, more fundamental particles called quarks and gluons. The odd thing (at least in the early 1970’s) is that particle collision experiments suggest that quarks are held together only very weakly inside a nucleon, however they have never been observed on their own outside of a nucleon. How can particles held together so very weakly not be pulled apart and examined individually?

Dr. Politzer is (in part) responsible for the answer to this question. It turns out that gluons are the key. These gluons are the force mediating particles for the strong nuclear force, just as photons are the force mediating particles of the electromagnetic forces. However, unlike photons (which have no electromagnetic charge), gluons do carry strong force “charge”, which is called “color”. Naively, Quarks are always “surrounded” by gluons (whether you like it or not, electrons are always “surrounded” by photons), and since gluons themselves carry color, there’s an awful lot of color going around. Pull two quarks away from each other, and you get more gluons, more color, and so more force keeping them together. In fact, it requires so much energy to pull quarks apart because of all the colored gluons, that, once you reach a certain distance between the two quarks in question, it becomes energetically more favorable for nature to create two more quarks inbetween (so you get two paris of closely spaced quarks instead of two quarks far apart from one another). Thus, quarks are never seen individually, but only with other quarks. This is called “quark confinement”. (In the above I have made no distinction between quarks and anti-quarks.)

On the other hand, if you had a really tiny stick and could weave it past all the gluons and start poking the just the quark, you would find a very weak color charge; the quark is just one particle and only carries one unit of charge (In contrast to when you have a thicker stick and are poking the quark and all its gluons. In this situation, you are poking many particles and you find a lot of color). So if two quarks are really close to one another (close enough to “get past” all the gluons), they see only a small amount of color and so feel only a very weak force between them. This is “asymptotic freedom”: the closer two quarks get to each other, the less color they see, the less force the feel.

The difference is one of length scales. At large lengths (poking with a fatter stick – seeing lots of color – quark confinement), the strong nuclear force between two quarks is enormous because of all the gluons. At small lenghts (poking with a skinny stick – seeing little color – asymptotic freedom), the strong force seems much weaker. The paricle collision experiments mentioned above probe the system at very high energies, which translate into very small length scales (analogous to poking with a very skinny stick), which is why they suggest a very weak force.

Dr. Politzer shares the prize with Professor David Gross (now at UCSB) and Professor Frank Wilczek (now at MIT). Did I mention that Politzer published the relevant paper in grad school (Harvard)? Did I further mention that this was his very first paper? And here I am – blogging.

So I finally got to sleep, but I guess being awake at 4:00am isn’t always so frustrating.

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