Dr. Bortrum
05/14/2009
Fusion and Roots
My last column ended with a reference to the forthcoming attempt to initiate nuclear fusion by focusing laser beams on a pellet of material containing hydrogen fuel. In effect, this experiment will try to duplicate some of the conditions present in stars such as our Sun. After posting that column, I read an article by Daniel Clery in the April 17 issue of Science titled "Fusion’s Great Bright Hope" on this future laser experiment at the National Ignition Facility in California. I had known that this was an enormous project but didn’t realize how huge until reading this article. For example, the building housing the laser is 10 stories tall and covers an area the size of three football fields!
In a complicated process of splitting the beams, amplifying the power of the beams and changing their frequencies, the resulting 192 laser beams are to be directed at a BB-size beryllium sphere filled with hydrogen isotopes. The plan is that the beryllium sphere will implode, crushing the hydrogen at a temperature and pressure so high that the hydrogen atoms will spontaneously "ignite" in a sustained fusion reaction to form helium, releasing more energy than was put in. The power of the laser beams focused on that little pellet is expected to reach 500 terawatts (500 trillion watts), more than the power generating capacity of the whole United States!
This may sound like an impossible feat, at least if you are unfamiliar with the concepts of power and energy. The answer is that the laser pulse is going to be very short in duration. Power is the rate of change of energy. Let’s take a plain old light bulb. If we have a 100-watt bulb, the power is 100 watts. Energy is power times the time. If our 100-watt bulb is lit for 1 hour, the energy consumed is 100 watts times 1 hour, or 100 watt-hours, the energy for which you are billed by your power company.
If energy equals power times time then power equals energy divided by time, the rate at which the energy is delivered to our light bulb or some other load. So, let’s say you want to zap something with a laser pulse; possibly you want to weld something, one application of lasers. Let’s assume that you generate a laser pulse carrying an energy 100 watt-hours. Now suppose the pulse is only 36 seconds long. There are 3600 seconds in an hour. What is the power of that pulse of laser light? Answer: 100 watt -hours divided by 0.01 hour (36/3600) equals 10,000 watts.
The shorter the pulse, the higher the power. At the National Ignition facility the projected blast of laser energy from the 192 laser beam is only going to last for nanoseconds, probably less than about 25 nanoseconds (25 billionths of a second). If our 100 watt-hour pulse had only lasted 36 nanoseconds, the power would have been a billion watts! I was shocked to find that when I converted the projected energy quoted for the laser beams converging on the pellet in the fusion experiment, I only came up with a figure of about 500 watt-hours. That’s only enough energy to light a 100-watt bulb for five hours! But, all that energy is going to be blasted into that small pellet in a very short time. It’s a stretch, but it’s sort of like focusing the Sun’s rays on a piece of paper or wood with a magnifying glass to initiate a sustained burning.
While the researchers at the National Ignition Facility are working toward the goal of sustained nuclear fusion, headlines were made recently when the results of a slowing down of nuclear fusion was detected by astronomers. As we’ve discussed in the past, when a star’s nuclear fuel runs low or runs out, depending on the size of the star, an explosion may occur that gives rise to a supernova, accompanied by an intense burst of gamma rays. When the material remaining in the star collapses, a black hole or neutron star may be formed. Also formed in a supernova are the heavier elements, some of which allow the formation of rocky planets that in turn nourish life such as ourselves. Without those supernovae, we might not exist.
On April 23, a gamma ray burst was observed from the oldest, most distant supernova ever recorded. In a sense, this is delving back in time to our roots. The burst, which lasted only 10 seconds, was first detected by NASA’s Swift satellite. The satellite’s X-ray and visible light/ultraviolet telescopes were immediately trained on the burst site and a fading X-ray afterglow was detected but no visible light. This lack of visible light suggested that the supernova was old since older visible light is shifted to the invisible infrared part of the spectrum.
Telescopes in the U.K., Chile, Hawaii and the Canary Islands quickly came on board and their measurements indicated the supernova was indeed very old. The Chile and Canary Island data on the afterglow’s redshift showed the supernova to be 13 billion light-years away. The star had exploded a mere 630 million years after the Big Bang. Fittingly, in this 400th anniversary of Galileo pointing his telescope skyward, the Galileo National Telescope on La Palma in the Canary Islands was one of the two telescopes (the other in Chile) to nail down the age of the supernova.
Who knows? Perhaps we or our surroundings contain a few atoms of elements originating in that supernova. Probably not, but it’s always fun to fantasize about our roots. In keeping with the roots theme, one of science’s more recent quests has been to attempt to find out what it is in our DNA that makes us human. We keep hearing that the DNAs of us and our cousin, the chimpanzee, are 99 percent the same. In the May issue of Scientific American, Katherine Pollard, a biostatistician involved in the sequencing of chimp DNA, has authored an article titled "What Makes Us Human?".
One possible "humanizing gene" is the so-called FOXP2 gene, associated with speech, first identified in 2001 by workers at the University of Oxford in England. This gene is apparently related to the form of certain facial muscles that permit the type of movements needed for human speech. More recently, a group in Germany found that Neandertals had the same human version of the FOXP2 gene that we do, indicating the possibility they also could converse with each other.
Pollard wrote a computer program designed to scan human and chimp DNA for regions that have changed most since we split off from a common ancestor some six million years ago. The region that changed the most was a region they designated as "human accelerated region 1", or HAR1. This region in the DNA has 118 "letters" (bases) in the 3 billion letter DNA code. Pollard and her mentor, David Hausler, were delighted when they found that others had found evidence of HAR1 activity in human brain cells, although nobody had yet named or studied the sequence in the HAR1 region.
The researchers then spent a year studying the evolution of HAR1 in other species, notably the chicken. It was some 300 million years ago that the chicken and common ancestor to humans and chimp lines split off from the common ancestor to all three. Today, only two letters differ in the 118-letter sequences of the chicken and chimp HAR1 regions. That’s not much change over 300 million years.
On the other hand, comparing the chimp and human regions of HAR1, there are 18 letters that are different, 20 if you compare human with the chicken. The change in chimp versus human HAR1 is obviously much more pronounced than that in chimp versus chicken. Other studies have shown possible connections between the HAR1 region and the development of the cerebral cortex in the brain. Just the sort of thing that might differentiate us from our chimp cousins. In addition, the HAR1 region is apparently a great example of what used to be called "junk" DNA, which increasingly is being shown to be far from being junk. At any rate, it looks as though we are making substantial progress to solving the question as to what makes us human.
If any of you are surprised that I started out writing about laser induced nuclear fusion and have ended up on DNA, I’m as surprised as you are. When I started the column, I had planned to continue on the power theme and discuss the need for a smart electrical grid in the U.S.A. However, longtime readers will know that I’m a sucker for anything that could be termed related to my roots, especially if it involves something astronomical.
Next column on May 28.
Allen F. Bortrum
