# Category Archives: engineering

## Cheating on Tests

I’m adding Parkway Rest Stop to my blog roll. The blog roll here is not a list of stuff that I approve of, but just a convenient place to keep a list of blogs I like to read. I ended up directed to Parkway because of a comment at The Mind of Dr. Pion that pointed to this post, about Army tests, and cheating on them.

And I’m wondering if Global Pioneering has moved on. His (well, her, according to Kea) website is coming up 404 now.
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## Carl’s new blog, “Ethanol Fuel”

During the past 24 hours, I heard, I swear!, 6 commentators blame biofuels for one of the world’s pains or another. What bothers me about this is that the complaints are, by and large, misguided in one way or another. So I’m starting a blog on the subject, “Ethanol Fuel”, or “Moses Lake”. It’s gotten to the point where the people in the industry joke with each other by pointing out yet another news article blaming biofuels for one thing or another.

So I’m starting another blog. I’m going to provide informed commentary on the energy situation, the food supply, land use, regulation, the economics of biofuels and fuels in general, the engineering, the politics of the subject, etc. And perhaps posting will leave me feeling that I’ve made a little effort towards stopping our country from being led by ignorance into policies that will take years to retract. I don’t think these belong on Mass which is devoted more to physics.

Producing fuel and electricity are engineering problems, the best decisions will be made by engineers who specialize in those areas, not by the general public. Nevertheless, the general public has the ultimate political power and their decisions, rational or otherwise, could effect the future of this country and this planet. And so I will write on the issues with the objective of education.

And it’s not like the US (or any other country) is immune to engineering decisions made on the basis of emotion. As a youth, I saw the nuclear power industry in the US destroyed by ignorance and fear masquerading as wisdom. Now, with the benefit of 30 years of hindsight, and another wave of ignorance and fear (this time related to global warming rather than radiation) support for nuclear power is increasing.

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## MUBs and the 0.7 Anomaly in QPC conductance

Thank Nanoscale for bringing to my attention a long standing puzzle in mesocopic scale condensed matter physics, the “0.7 anomaly”. The problem is the behavior of the conductance (inverse of resistance) for a quantum point contact (QPC). Two experimental papers showing the effect are 0706.0792 and cond-mat/0005082, which see. Shot noise decreases at the 0.7 anomaly just as it does at the integer conductance points, see cond-mat/0311435. A recent perturbation analysis of the situation is given by 0707.1989. This blog post gives a non perturbational calculation.

Such a point contact is a region which is so small that it can hold at most a single electron. One controls the size of the conductance region by applying a voltage to a gate. One expects that the conductance will be a multiple of G0 = 1/13K ohms; this works for high conductance values (increasing to saturation) but an anomaly appears near cutoff.

This really is basic physics and should be well understood. The computer that I (and you, assuming you’re reading this in 2008) are using are built from CMOS logic gates. The QPC effect occurs when such a gate is operated near its cutoff point. There are billions of billions of these gates currently in operation on this planet (operating in cutoff and saturation); by all expectations they should have been well understood many decades ago.

However, it turns out that the situation is difficult to analyze with the usual tools of quantum mechanics. One ends up with a highly nonlinear situation involving “quasibound” states. Since the density matrix formalism I work with is specifically designed to solve highly nonlinear bound state QFT problems without recourse to the usual perturbation theory, this is a natural place to explore its application. In addition, this could be a good first application of MUB (mutually unbiased bases) to quantum theory.
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## Measuring the Speed of Gravity (Waves)

Newton’s equations give the speed of gravity as infinite. For example, in Cartesian coordinates, suppose a gravitating mass 2M is at the origin up until time t=0.  At that time, the mass splits into two masses of mass M, one going in the +x direction at speed v the other in the -x direction at speed v. For times greater than 0, the gravitational potential is given by the sum of the two gravitational potentials:

(1) $\;\Phi(x,y,z,t) = \frac{GM}{\sqrt{x^2+y^2+(z-vt)^2}} + \frac{GM}{\sqrt{x^2+y^2+(z+vt)^2}}.$

At any distance, the above depends on t so the gravitational potential (and it is easy to show the gravitational force) is instantaneously changed at all distances from the origin. The speed of gravity is therefore infinite in Newton’s theory.
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Filed under aether, anomalies, engineering, gravity, heresy, physics, seismology

## Why does DNA only use 4 nucleotides?

A few days ago Kea wrote post #66 in her long and fascinating series of M-theory posts. Seems like she’s giving away the text to a book here. She wrote:

I was quite intrigued when a mathematical biologist at a conference told me recently that no one really knew why DNA had four bases rather than two. Apparently it isn’t clear why self-replicating molecules fail to adopt a binary code in X and Y.

The context of the problem needs some explaining. DNA is a long chain molecule that is built from a series of nucleotides. The strange things is that exactly four nucleotides are used. This is strange because there are at least a dozen different nucleotides, why use just four?

[edit Feb 8, 2009]This question is discussed in the scientific literature. For example, I just now found the article: Why Are There Four Letters in the Genetic Alphabet?, which describes the question generally, but I think the reader will find the chemical and information content argument given here to be more precise.[/edit]
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