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Uncertain Principles

Physics, Politics, Pop Culture

Saturday, August 02, 2003

Happy Anniversary E-ZPass

This weekend marks the tenth anniversary of the E-ZPass automatic toll pay system:

Ten years ago today, the first few drivers with little tags on their windshields passed through the Thruway's Spring Valley toll barrier in Rockland County without rolling down a window or fumbling for change.

Four weeks later, the new wave of E-ZPass commuters washed across the Tappan Zee Bridge, and on April 17, 1995, it was the Capital Region's turn from Exit 23 in Albany to Exit 27 in Amsterdam.

By 1997, drivers could travel the entire state Thruway system without concern about the cash in their pockets.

Today, more than half of the millions of trips taken each year on the Thruway are paid with E-ZPass tags, and Capital Region commuters are among the technology's greatest fans.

My parents gave me one of these a few years ago, when Kate was living in The City for the summer (working for Rudy G.), and it's been a real lifesaver. It's a simple and obvious idea, and yet it's good for a "living in the future" moment every time I blow through a toll lane without rolling down my windows. Up here, they've even got it set up so you can put the parking charges from the airport on your E-ZPass...

Yeah, New Jersey made a hash of it when they first installed the system, but I figure what the hell, that's Jersey. And it does get me in trouble whenever I find myself in a car without an E-ZPass (leading to the $3.00 check I had to write to the Mass Pike when I moved up here...). But they've got the bugs worked out now, and I can drive all the way to Boston or DC without having to worry about cash for tolls.

Technology is Good.

Posted at 8:18 AM | link | follow-ups | 6 comments

If You Want Your Unqualified Offerings Back, Turn Yourself In

Sure, you've heard the buzz, and read people singing the praises of Mozilla Firebird. But think carefully before you consider switching. Consider the sorry tale Jim Henley, who announced a switch from Opera to Mozilla, and promptly vanished.

Opera is good. Opera is safe. You've got a nice website there, and you wouldn't want to link the Unlearned Hand, now, would you?

Posted at 7:57 AM | link | follow-ups | no comments

Thursday, July 31, 2003

Look Closer, and It's Easy to Trace...

At the end of my last post about the creation of pentaquarks, I left things hanging a bit. Specifically, I didn't explain how the experimenters go about figuring out that these things were created in the first place. In essence, this comes down to explaining what the graph at the bottom of this page is, and how they get it.

The basic process is the same for all particle physics type experiments, and traces its lineage back to the immortal Ernest Rutherford, who was the first physicist to earn fame by shooting small particles at the nucleus of an atom and finding surprising results. Granted, things have gotten a lot more complicated since Rutherford's day-- his shocking discovery was simply the fact that there is a nucleus in the first place (this was deemed one of the most beautiful experiments in physics)-- but the method is unchanged. Essentially, you get a bunch of small particles (protons, electrons, photons), fire them at some sort of target (atoms, protons, anti-protons), and look at what comes out after the collision.

Just by itself, that sounds like a pretty horrible way to do science-- it's akin to trying to figure out how to make a wristwatch by dropping one off a tall building and looking at the pieces that fly off. But the real story is actually even worse than that, for two reasons: 1) As noted in the previous post, what emerges from a collision isn't necessarily what you started with, because some of the energy of the colliding particles can be used to create new particles out of nothing, and 2) With very few exceptions, none of these particles last very long once they are created, with the more exotic particles falling apart into other things within a tiny fraction of a nanosecond. Reconstructing what happened in a particle collision is a very difficult process indeed, requiring whopping huge, intimidating detectors, and a horrible alphabet soup of acronyms (CEBAF, CLAS, SPring-8, LEPS).

I'm far from an expert on how this stuff is done, but I'll try to lay out the basic bit of apparatus that were used in one of the pentaquark experiments (specifically, the ones in Hall B of the Jefferson Lab in Virginia). The details vary a bit from accelerator to accelerator, and experiment to experiment, but the basic ideas are generally the same.

The central goal of the whole process is to identify the particles that come screaming out of the target area after a collision occurs- and they are screaming, moving at a fair fraction of the speed of light. Particle identification comes down to measuring a handful of properties-- charge, mass, total energy, and momentum-- and deducing what the particle was on that basis. To pick a simple example, if a positively charged particle with a mass of just under 1 GeV (1 giga-electron-volt, or 1,000,000,000 times the energy an electron acquires when accelerated across a 1 volt potential difference) passes through your detector, you can confidently say that that was a proton. A neutral particle with a similar mass would be a neutron, while a negatively charged particle of the same mass would be an anti-proton.

The clever trick here is that all these properties are worked out indirectly, by tracking the motion of the particles after the collision. You have to do it this way, for a number of reasons-- these are subatomic particles, after all, some of which don't last very long, so you can't just pin them down and interrogate them at your leisure. Instead, particle-physics experiments set up situations where they can record the tracks of large numbers of particles, and identify them later on from the way they move.

There are generally a few layers of detectors in any such experiment, each of which serves a slightly different purpose. In the specific case of the CLAS ("CEBAF Large Acceptance Spectrometer," where CEBAF stands for "Continuous Electron Beam Accelerator Facility." This is a government lab, so they need lots of acronyms...) detector used for the "pentaquark" results, these fall into three general categories: scintillators, calorimeter, and drift chambers. These are arranged in concentric layers around the central region where the collision occurs. (A spiffy exploded diagram of the CLAS detector exists, because I've seen it in talks, but I can't find it on the J-Lab site, the bastards, so you'll just have to use your imagination-- picture a big glass and metal onion, several stories high...)

Scintillators are basically blocks of glass, which are prepared in such a way that a particle slamming into the glass will produce a small flash of light, which gets picked up by a detector. These are the outermost layer of detectors, and are mostly used for timing purposes: when you see the flash, you know that a particle hit the detector. If you know when the collision occurred at the center of the detector, you can use this to work out the speed of the particle.

Calorimeters are used to measure the energy of some types of particles. Basically, when the particle enters the calorimeter, it loses some or all of its energy to the calorimeter, which tallies up the energy gain, and calculates the total energy of the particle. These are the penultimate layer in the CLAS detector, just inside the scintillators.

The most important components of the detection system are the drift chambers. These consist of large volumes of space divided into small cells with an array of fine wires. When a charged particle passes through one of these cells, it causes a little "blip" of current in one of the wires. By keeping track of successive "blips," you can stitch together a map of the particle's track through the chamber-- it passed this wire, then that one, then that one over there, and so on.

"Big deal," you say. After all, knowing exactly what path the particle took on its way out doesn't necessarily get you any information that you don't already have. That's why you put the whole thing in a large magnetic field.

A charged particle (an electron, say) moving in a magnetic field will feel a force that depends on three things: 1) the velocity of the particle, 2) the magnetic field, and 3) the charge of the particle. An electron moving horizontally through a vertical magnetic field will feel a force to the left, while a positron would feel a force to the right. The faster the particle, the bigger the force, and the stronger the field, the bigger the force.

This means that particles passing through a drift chamber in a magnetic field will follow curved tracks, and by following the tracks, you can get a lot of information about the particle, and putting this together with the information from other detectors gets you all the information you need. You can get the sign of the charge from noting which direction the particle curves (positive charges go one way, negative charges the other). If you know the strength of the field and the speed of the particle (from the scintillators), you can figure out the mass (heavier particles curve more slowly than light ones) from Newton's Laws (at least, the relativistic analogue thereof). (Technically speaking, what you get is the ratio of the charge to the mass, which gives you the mass for known charges-- if you don't know what the charge is, you need the mass to complete your knowledge, and you can get that from other detectors.)

The CLAS detector actually has three layers of drift chambers. Particles leaving the collision region pass through one chamber with no magnetic field, then one with a large field applied, then a third with no field again. I'm not entirely sure why they use three layers-- possibly to provide a couple of bits of straight track that can be traced back to the source, but I really don't know.

Putting all of this together gives you a complete description of the collision-- you know what two things hit each other (because you set it up, choosing the particles that make up the beam and the target), and by stitching together all the information from the detectors described above, you can work out what came out of the collision region (what you can detect, anyway-- some of the products may be things like neutrinos that pass right through the detector without leaving a track). That's where the difficult part starts...

But a crude sketch of how you use this to identify new types of particles will have to wait for another post...

Posted at 9:37 AM | link | follow-ups | 8 comments

Tuesday, July 29, 2003

We Have Met the Enemy and He Is Us

I'm having a Bad Morning. It's not that anything spectacularly awful has happened, it's just that I can't quite seem to wake up. I got out of bed just fine, and went through my normal morning routine, but I'm still in a fog. I did the usual morning blogroll-- a bit more than usual, in fact, as I didn't read much yesterday-- but I realized just a few minutes ago that I don't remember any of it. Maybe I just need more caffeine.

One thing did penetrate, though. From the Washington Post:

Col. David Hogg, commander of the 2nd Brigade of the 4th Infantry Division, said tougher methods are being used to gather the intelligence. On Wednesday night, he said, his troops picked up the wife and daughter of an Iraqi lieutenant general. They left a note: "If you want your family released, turn yourself in."

I'm speechless. Fortunately, Jim Henley is not:

Stamp it on our coins. Include it in the prayers that open Congress. Add it to the instruction block of all triplicate government forms. Use it as the description line for your warblog.

. . . and to the Republic for which it stands, One Nation, Under God, Indivisible. If you want your family released, turn yourself in.

OK, one addition, completing the paragraph quoted above:

Such tactics are justified, [Hogg] said, because, "It's an intelligence operation with detainees, and these people have info." They would have been released in due course, he added later.

Well, that's all OK, then-- you can invoke the "Just Kidding" clause of the Geneva Convention.

Coming soon from the legal geniuses of the Bush Administration: "It would've been a war crime, but we had our fingers crossed, so it doesn't count."

Posted at 10:16 AM | link | follow-ups | 7 comments

Monday, July 28, 2003

Washing the Car to Make It Rain

It's been a mostly blog-less weekend (Saturday was taken up with an ill-fated attempt to get a dog, followed by a family visit, and Sunday was eaten up by potential-dog-inspired home construction), and this seems likely to continue for the next few days (more family visiting today, students coming over for dinner Tuesday, lots of actual physics-type stuff to be doing...).

While I'm being lame here, you can always amuse yourselves elsewhere, say by reading about the new Iain Banks novel over at The Library of Babel, or the repost at Blogcritics, where a couple of commenters talk about the differences between "M" and "non-M" Banks.

Or maybe not-- in keeping with blog tradition, the formal announcement of a brief interruption in regular service may spark a veritable torrent of posts over the next few days. Hard to say, really.

Posted at 9:49 AM | link | follow-ups | no comments

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