February 6, 2010

Particle Physics is Basically Awesome (historical overview)

Yes, another Columbia SHP related post. I seem to have a lot of those.

I've got particle physics this semester, and it's going to be good. My teacher (I could call him a professor, but the teachers are grad students, so it seems a bit weird) is British and drawls his words just enough that's it's very easy to consign him to the background and zone out but, when I actually listen, he's really very interesting.

As proof of it's interestingness, I actually took notes. He started the class with three particle physics related websites we should check out, but they also have other physics related stuff, and I think Julie would find them interesting. The sites are Interactions.org, CERN Courier, and Symmetry. The second site is reportedly the most "substantive," whatever that is meant to mean.

During the class itself, we took a somewhat circuitous route through the development of particle physics. I always like learning science like that, because it makes you see the thought process of how and why the science works, rather than just what it is.

The lecture opened with the end of the 1800s, at which point atoms were considered as small as it got. These various atoms were categorized by the properties, and the periodic nature of these categorizations hinted at some form of internal structure.

During the 1890s, radioactivity was intensely studied by Curie, Becquerel, Rutherford, and others. Radioactivity is the emission of particles. The existence of radioactivity implies that atoms must have an internal structure, as they are able to be broken into bits. There are three particles associated with radiation: alpha, beta and gamma. Alpha particles are the equivalent of a helium nucleus and have a charge of +2 and a mass equal to that of four protons. Beta particles (electrons) have a charge of -1 and a mass of 1/1800 protons. Gamma particles (photons) are electrically neutral and don't have mass. These particles were identified using deflection in a magnetic field (electrons, protons) and film sheets (gamma).

At this point in the lecture, Allana and I took a break to pass notes:

Tea: the clock is 1 hr fast
Allana: shh don't tell him. his face is smug.

Then we returned to 1897, when electrons were discovered by analyzing cathode rays responses to magnetic fields. These responses found that the electrons responeded to Loretz's force law, indicating that they were particles. This law can also, by breaking force into mass*acceleration, be used to find the charge/mass ratio of various particles through the measurement of only acceleration. This ratio was obtained by establishing the energy equilibrium of charged droplets of oil when they were suspended over electric fields, a concept which, two days ago, I would not have understood. These charges are always integer multiples of a fundamental charge, the charge on an electron. The mass was then able to be calculated.

The mass of an electron is .511 MeV/c^2, or a really fucking tiny amount, or 500000 electron volts. This unit of measurement is derived from the famous E = mc^2, and, when written in literature, the c^2 is often implied, because typists are lazy.

Then we move onwards to the plum pudding model (!!!), which Julie and Gretchen will remember from chem. This was created by Thomson, and involves raisin studded pudding. The model was tested by Rutherford's graduate students, who spent long, painful hours in dark rooms waiting for their eyes to adjust to dark enough conditions for them to observe the changes in a sheet that fluoresced when hit by particles. When alpha particles were accelerated at a gold sheet, those that didn't hit the nuclei of the various atoms went through with only slight deflections, but those that collided were refracted at angles greater than 90%, demonstrating that the negative charges could not feasibly be scattered within a positive goo, as Thomson had predicted.

This motion is represented by the equation N(theta) is directly proportional to sin (theta/2) to the negative fourth power. How they derived this equation in a time before the TI-89 is entirely beyond my ability to comprehend.

In 1911 the nucleus was discovered, with hydrogen atoms standing as some elementary particle. The we went into photons, and I got a bit lost, since I only vaguely comprehend sinusoidal motion.

in 1914, Bohr did stuff about electrons moving in circular paths.

Centripetal Force= m v^2 / r, as we learned in physics, and it is directly proportional to e^2/r^2 (the coulombic attractive force between the protons in the nucleus and the electrons). What Bohr tried to examine was why the electrons didn't simply spiral in towards the nucleus. He also did some quantization, noting more things that come in discreet chunks/packets, indicating that a particle of a constant mass is involved.

Quantize is my new favorite word.

1927 was antimatter, by P. Dirac. He used to full form of E = mc^2 to show that equations have positive and negative energy solutions, but that, rather than negative energy existing, antimatter had positive energy. In 1932 C. Anderson observed the antimatter equivalent of an electron, the positron. This particle had the same mass as an electron, but opposite quantum numbers such as charge, lepton number, and spin. The positron is notated using an e with a bar over it.

1932 was the discovery of the neutron, as elements were heavier than their protons. Still- what holds them together? Some other force must be involved, and the particles that carry it buts be neutral but massive. This neutral charge made them difficult to detect experimentally, as one can't simply measure deflection in an electric field.

This force was the strong force, which is "strong" in the nucleus but decreases in strength very rapidly over distance. This was predicted to depend on an exchange of particles between the protons and neutrons, and these particles were called mesons, as the mass was between that of protons and electrons.

Then we went into Heisenburg's OTHER uncertainty principle, and light waves, and I got rather lost, and class ended.

It was all jolly good fun.

3 comments:

Nyx said...

When I started reading this, I didn't realize you actually meant it when you said "historical."

Now I am wiser. I am glad you enjoy nerding out at SHP. Nothing like learning that is actually interesting! Wish we could say the same about some Staples classes...*cough*french*cough*

Gretchen said...

haha, bruney remember rutherford?

Tea said...

she did?