Tuesday, November 25, 2008

A New Project for SETI

The Cepheid Galactic Internet

J.G. Learned, R.P. Kudritzki, S. Pakvasa, and A. Zee

arXiv:0809.0339v2

URL: http://arxiv.org/abs/0809.0339

This paper puts forward an interesting proposition for intergalactic communication: frequency modulation of cepheid variable stars.

Cepheids are 1,000 to 10,000 times brighter than our sun and have regular, detectable variations in brightness with a period of 1-50 days. The frequency and luminosity are strongly correlated, so one can measure the period of the stars, deduce the luminosity, compare with the observed brightness, and determine how far away the star is. Parallax is used to calibrate the distance scale on nearby cepheids.

The authors of this paper propose that an advanced race might intentionally modify the period of these stars to send information throughout the galaxy: FM at very low frequencies!

The mechanism by which the brightness varies is similar to the charging and discharging of a capacitor. According to the authors, as the star consumes hydrogen through nuclear fusion, ionized helium builds up on the surface of the star. This decreases the luminosity and increases the temperature of the star. This is followed by "violent expansion and deionization," after which the cycle repeats.

It is interesting that an avalanche process like this leads to a regular cycle of variations in the brightness. The same thing happens in circuits with a capacitor (if I remember my physics lab correctly). A build up of charge is followed by dielectric breakdown (a spark) after which the process repeats. The spark frequency is constant. What makes these systems different from, say, a pile of sand? The avalanches in a sand pile have a power law distribution in both size and frequency. What makes the two systems so different? I assume the circuit and the cepheid have nice, normal, linear equations of motion while the sand pile is governed by nonlinear equations. But what parameter differentiates the two? Is there some control parameter in the circuit that would lead to a crossover between periodic sparks and a frequency distribution more like the avalanches in the sand pile?

I digress. The reason I described the process that gives rise to the periodic variations in a cepheid is that the authors claim it can be influenced by external agents. They suggest that the deionization could be triggered early with a sufficient burst of power. The variation in the period could be used to transmit messages. To make the star fire early, the authors suggest that a neutrino beam focused on the star's core might do the trick.

The authors suggest the neutrino beam could be generated by a large space station orbiting the star running on solar power. We've got a long way to go from our handheld calculators to star-altering neutrino beams! For a society that could build such a device, it would seem a simple matter to build the beacon and leave it in place.

So are aliens broadcasting over the cepheid Internet? The authors say it wouldn't be too hard to find out. Normally astronomers measure the average period of these variable stars. Averaging the data would, of course, wash out any signal. Rather than determining the average, one would simply have to bin the data and look for a splitting of the signal around the fundamental frequency.

The authors don't mention it, but one might also perform an entropy analysis of the time series data. A periodic source would have zero entropy, while a modulated source would appear more ergodic. This would be a secondary analysis. If a splitting of the fundamental peak in the frequency spectrum were observed, one would then analyze the time series data to see if the splitting is also periodic, or if it has more structure to it.

Calling this system a galactic Internet is a bit misleading. It's more like the galactic telegraph. An early flash corresponds to a 1, an on-time flash corresponds to 0. Only one bit can be sent per period, so the maximum transmission rate is on the order of 1 bit per month. It would take forever to download a page from Encyclopedia Galactica!

I wonder what types of message would be transmitted. There seem to be two schools of thought about aliens: the Star Trek school, and the Hitchhiker school. The former envision alien civilizations as refined, peaceful, rational, and technologically advanced. The latter imagine alien civilizations to be more like our own society, with profit-driven pleasure-seekers, petty bickering, advertising, and bureaucracy. Each would certainly use the galactic beacon to transmit different kinds of messages.

The Star Trek variety of civilization might use the beacon as a lighthouse, a simple welcome message for young civilizations pointing them to a source of information with more bandwidth, or a galactic emergency broadcast system. If the cepheids were run by Hitchhikers, we might instead find messages like "Tresspassers will be shot on sight" or "Eat at Joe's." Imagine, after years of trying to crack the code, we discovered the message to be

"At the next pulse, the current time will be ..."

Monday, November 24, 2008

Hidden Interactions?

Hidden One-Electron Interactions in Carbon Nanotubes Revealed in Graphene Nanostrips

C.T. White, J. Li, D. Gunlycke, and J.W. Mintmire

Nanoletters 7, 825--830 (2007)

URL: http://pubs.acs.org/doi/abs/10.1021/nl0627745


The authors explore the band gap of graphene nanoribbons. I was disappointed to discover that the "hidden interactions‚" they refer to are second and third nearest neighbor couplings that must be added to the tight binding model to reproduce the results of a density functional calculation. They predict that all armchair edge nanoribbons will have a gap.

These interactions are hidden in nanotubes because the curvature of a nanotube reduces the coupling between more distant neighbors. However, they should have been observable in large-radius nanotubes, in which curvature effects are very weak. The authors do not appear to have addressed this point.

I realize that density functional theory is a powerful tool for investigating the electronic structure of many materials. However, I do not know that it has been used successfully in graphene. Moreover, the authors state that the particular DFT package they used is "especially tailored to take advantage of helical symmetry"--- a symmetry possessed by carbon nanotubes, but not graphene nanoribbons. Moreover, claims that the method "has been successfully used in wide-ranging studies of single walled carbon nanotubes" only refer to studies carried out by the authors.

I am uncomfortable with the idea of taking an empirical model -- the tight-binding model for electrons in graphene -- and adjusting its parameters to fit the results of a density functional calculation. It would be a different matter entirely if the “hidden interactions” were necessary to fit experimental data.

I was discussing a similar idea with my adivsor the other day. We had just listened to a talk in which a biophysicist described the molecular dynamics simulations he had developed to study fluctuating membranes. Because the model reproduced some features of the experimental data, he inferred that the model captured some important physical property of the real system. White et al. do the same thing here, making the claim that because the amended tight-binding model reproduces the results of the DFT calculation, there must be 2nd and 3rd nearest neighbor interactions in graphene.

What this type of reasoning lacks is a proof of a one-to-one correspondence between models and the physical properties of the system. I.e., one would have to establish that a particular model is the only one they could generate a particular physical property before making any conclusions about real graphene samples or fluctuating membranes. I do not believe this type of correspondence exists. At best, one might be able to argue that the model and the physical system fall into the same universality class in the sense of the renormalization group.

In the end, experimental studies will determine whether or not these hidden interactions are physically relevant. The same experiments will also assess the validity of the DFT calculation. It will be interesting to see whether all armchair nanoribbons are semiconductors.

End Interlude

Well, another 9 months have passed since my last post. Although, judging from the comments, I'm the only one who noticed.

I'm back now, and I've read some interesting articles. Let's not delay any longer.

Monday, February 18, 2008

Welcome Back

It's been quite some time since my last post. All of my writing efforts have gone toward producing the first draft of my Ph.D. dissertation, The Effects of Static Electric Potentials on Single Electrons and Excitons in Carbon Nanotubes: A Theoretical Study. I'm just about done --- with the first draft. I've got a 200 page monster with no figures and incomplete references that I've got to hammer into shape by the middle of May, but it's a lot better than staring at a blank page!

Although I haven't posted anything recently, I've been reading a lot. Going over background materials for my dissertation reminded me of how fascinating semiconductor physics, quantum field theory, and excitons are! I've had some time for deviations from my primary research focus as well. My pile of interesting papers continues to grow, plus I've started reading some textbooks on econophysics and nonlinear dynamics. Fascinating stuff.

Hopefully I'll get back in the habit of posting regularly. I hope to make the blog more friendly to others too. It will still primarily focus on interesting papers I've read (I'm still worried that once they go into my filing cabinet, I may never be able to find them again...), but I'd like to make the blog into more of a journal than a database. Maybe we can get a little dialog going on some topics. Maybe I can sharpen my skills at explaining complicated things in simple terms. Maybe I'll be so busy finishing my dissertation and finding a job that I won't have time for anything else. Who knows?

Check back in from time to time to see what's new. And drop me an e-mail if you like.


Jesse