Becoming a BaBarian

Earlier last week, the BaBar Collaboration met at the SLAC National Accelerator Laboratory for its autumn meeting. Over four days, we discussed a great variety of physics (during over 100 parallel session talks), learned about the process of preserving the BaBar data for future researchers, and even celebrated our 400th published paper. In addition to these activities, we bid farewell to some long-time pillars of the BaBar community, as they move on to greater things, and we welcomed new members: young students and post-docs who came to the meeting fresh from the BaBar Analysis School the week before.

I attended the meeting not only to present some recent research activities, but to present my case for SMU membership in the collaboration. This case has been built up, in consultation with leadership in the collaboration, over the course of the last two months. It is with tremendous pleasure that I report that the case for membership was enthusiastically received, and SMU is now a new institutional member of the BaBar collaboration.

My participation is focused largely on contributions to the physics efforts of the collaboration: research, review, and publication. My goal is to also contribute simulated data, produced at SMU and distributed to the collaboration; this effort is already well underway and nearing complete success. Having established these milestones, my attention will be turning back to contributing to ATLAS through operational work. I’ll be at CERN in December, training for these efforts and learning from my experienced ATLAS colleagues. There are two blocks in my life right now, and I feel like a new kid on both of them.

While I have been a member of the BaBar Collaboration since 2000, I have always been so under the auspices of an established institution. Bringing a new institution into the collaboration is a novel experience for me, exciting and full of opportunity. I envision significant contributions being made by SMU undergraduates to the BaBar research effort. I see BaBar data serving as a direct means to engage students in independent research, to teach them modern research techniques, and to communicate the work and knowledge of particle physics. I see the immense dataset, in conjunction with the coming data from ATLAS, teaching us about all aspects of new physical phenomena: their properties visible only at the highest energies and their subtle effects visible only with the best precision, the most data. I see opportunity for SMU, and I am happy to be leading this effort.

Why the cosmos needs conductors

The conductor of an orchestra is not for show. A conductor is not just a part of the social construct of the orchestra. A conductor is not just a means by which one person can be made more important than another. The reality is that the conductor of an orchestra is required, by the speed of sound and the speed of light, to exist.

What do I mean by this? Let’s consider the human head. The distance between our two ears is about 22cm (about 8.5 inches). This distance means that sounds that are not originating from directly in front of us reach our two ears at different times. The human auditory system is capable of discerning sounds that are no more than 0.000660 seconds apart from one another – that’s 0.660 milliseconds (ms) [1]. The result of this ability to process signals which arrive more than 0.660 ms apart is that we can localize sound in space. This process, called “Duplex theory,” means that if sounds are closer than 0.660 ms then they cannot be localized; we lose the ability to distinguish the sounds, and they seem to arrive at the same time.

So, what does this have to do with the conductor of an orchestra? Let’s imagine a situation where the orchestra, full of spite for the primadonna conductor, decides to go completely democratic and tosses the tyrant out onto the street. “We can synchronize ourselves!” they proclaim, and they turn to the oboe player.

“Oboe player, sit in the middle of the orchestra pit and blow a series of tones in the tempo of the song! That way, we can listen for the tempo and mark the passage of time, so that we, too, can stay on time.”

This seems like a great idea. Sadly, the universe has conspired against this democratic process. Sound travels at about 300 m/s. As a result, it takes 0.033s, or 33ms, for sound to travel even the 10 meters from the center of the pit to the outermost players (assuming a 20m-diameter pit). The result is that a player on the outskirts of the pit hears the oboe’s beat 33ms after a player next to the oboe hears it. Players on the outside of the pit are 33ms LATE in playing their instrument.

What does this mean? It means that the players on the outskirts of the pit become unsynchronised from their center-pit colleagues. Is this a bad thing? YES. Since the human ear is capable of hearing the difference between sounds that are at least 0.66ms apart, and the sounds from the inside and outside of the pit are 50 times further apart than that in time, the whole orchestra sounds like they’re playing out-of-time. The resulting cacophony will  surely cause ticket sales to plummet, and our democratic orchestra will go flat broke.

The conductor, on the other hand, standing in front of the orchestra, uses light, not sound, to synchronize the orchestra. Since light travels at about 300,000,000 m/s, it takes just 0.000067 ms for their hand gestures to be seen by players at the back of the pit, compared to those closest to the conductor. This means that players are just 0.000067 ms out-of-time with one another; this is vastly below the human ability to hear the difference between two sounds, saving the orchestra and insuring a flawless performance.

Before you cast out that tyrant of a conductor, remember this: the universe has conspired to make it necessary to use light, and not sound, to synchronise the orchestra. Instead of tossing the bum out on the street, remember that you need them more than you don’t. Or, at least, think about hiring a replacement.

[1] http://en.wikipedia.org/wiki/Interaural_time_difference

Thunder and Lightning

Sunday night, as we sat on the patio by the grill, we thought about thunder and lightning. The sky was overcast, and in the distance storms were brewing. Lightning reflected off the clouds above us, followed a short time later by thunder. We started to think about the relationship between the light and sound from the storm. We knew that it would be easy to come up with a quick relationship between the time between the sound of thunder and the flash of lightning.

The light and sound travel the same distance, d. The time the light requires to go from the storm to us is shorter than the time for the sound. The time for the light is t_l = d/c, where c is the speed of light (300,000,000 m/s). The time for sound is t_s = d/v_s, where v_s is the speed of sound (300 m/s). We are interested in the number of seconds between the sound and the light, so let’s compute the time difference using these formula:

t_s – t_l = d/v_s – d/c = d(1/v_s – 1/c)

We can rewrite the quantity (1/v_s – 1/c) = (1/300 – 1/300,000,000) = 1/300 (1-0.000001), which is (to a very good approximation) 1/300.

We arrive at our final relationship:

t_s – t_1 = d/300

where time is in seconds and distance is in meters. So, if we are sitting on our patio and we see the flash of lightning, and then 6 seconds later we hear the thunder, the storm is (6s * 300 m/s) = 1,800 m, or about 1.1 miles.