I just got the following email from my sister. “C” is her five-year-old son, who is aggressively curious about the world:
Now, I understand fusion pretty well. I could probably explain it to a layperson. But the history behind its discovery? I’m not too sure about that.
As for the second part, my first instinct was to tell him that they figured it out because they found helium in the sun’s spectrum. But then I realized that I know that the helium in the sun comes from fusion of hydrogen, and I know they discovered helium in the sun before they ever found it here, but I don’t know if that was really how the discovery process went…
I might be way off but I think most of the initial research done w/ fusion was by Edward Teller. A hydrogen bomb (fusion bomb) was his pet project. research into civilian uses for fusion took off from there. Again, I might be way off in my recollection!
I’m not a historian, but I can answer the second one. Basically, we know what the sun’s made of because of the colors that show up in sunlight when we pass it through a prism. We also know how much of it there is from how our planet’s orbit behaves (mass and radius determine orbital period (roughly)). We also know how gravity behaves (more or less), so what happens when you get that much stuff together in one place? It gets all pressed together close enough that the nuclei of the atoms start fusing together.
Damn. I wish I’d been asking questions that worthwhile when I was five.
If you’re asking who first came up with the idea for fusion, then (according to Wikipedia) two guys named Atkinson and Houtermans were the first to publish a paper on the subject. However, firguring that out was really just an application of Einstein’s famous E = mc[sup]2[/sup] theorem: basically, if you can write down two batches of nuclei with the same number of baryons (protons & neutrons), the same number of leptons (electrons & neutrinos), and the same electric charge, but different masses, then it should be possible for a nuclear process to take the heavier batch to the lighter batch. You’ll have to add in enough energy (sometimes a huge amount) to get it started, but you’ll get out more in the end than you put in to start with.
The guy usually credited with figuring out how the Sun uses nuclear fusion is Hans Bethe. (Eddington suggested it before that, but Bethe was the one who worked out all the details.) He basically took what was known about fusion reactions at the time (1939) and figured out which ones fit the data we knew about the sun: its age, its mass, its density, its temperature, and so forth. He won the Nobel Prize for it in 1967.
The initial reasoning behind saying that fusion powered the Sun, by the way, was simple: nobody knew anything else that could do it. If the Sun was just a big glowing ember or a burning lump of carbon, it would go out in a few thousand years; once people figured out that the Earth (and, by extension, the whole Solar System) was much, much, older than that, the discrepancy became one of the bigger problems in physics. More recently, we’ve also detected some of the “by-products” of fusion in the sun: neutrinos, which are produced when two protons are fused into a deuteron. They’re light enough to escape from the Sun and zoom to the Earth at almost the speed of light; but they’re also very hard to detect. (The first person to do it used 470 tons of drycleaning fluid to do it.)
We know that the Sun works by nuclear fusion because there really isn’t any other way to make it put out the amount of energy that it does for as long as it’s been there. There have been various theories over the years before fusion was discovered, but they all ran into the problem that, if those were the source of the Sun’s energy, the Sun would have to be younger than some of the rocks on Earth.
We also have some direct evidence of fusion occurring in the Sun in the form of solar neutrinos. The theory predicts that the fusion reactions in the Sun should release neutrinos, and we observe neutrinos coming from the Sun (with some difficulty- they’re not easy to detect). We don’t observe as many as we think we should, and that’s been a problem of interest to astronomers and physicists for many years, but we do observe some.
Another competing theory was gravitational contraction: the supposed conversion of the sun’s gravitational energy into heat, and from that radiation. This idea eventually had the same sort of time-scale problem as chemical reactions, however.
And hey, I just noticed…
What an odd name to give a child. Doesn’t he get teased a lot?
This one has been resolved. We only detected a third as many as we expected. Now, it happens that there are three different types of neutrinoes, only one of which was detected by the first generation detectors. This also happens to be the type produced in the Sun, but there’s new evidence that one type can turn into another, such that by the time they reach the Earth, all three are jumbled together. Newer detectors can detect all three, and find about equal amounts of each, so it all adds up.
I didn’t know that! I had heard the neutrino oscillation theory, but didn’t know it was confirmed. That’s what I get for not keeping up with solar neutrino news since I left grad school… :o
The person experimenting with neutrino detection in a huge tank of naphtha is Ray Davis who shared the Nobel Prize in Physics in 2002 http://physicsweb.org/articles/world/15/11/1.
He was a spry 88 years old upon receiving the prize and even then was in the early stages of Alzheimer’s disease. I can understand the Nobel Prize Committee being cautious about awarding prizes particularly in the sciences but geez !!!
The first entry in the Wikipedia timeline is misleading. Aside from omitting Eddington’s 1920 paper, it seriously underplays the sophistication of what Atkinson and Houtermans were doing in their 1929 paper.
The person who applied E = mc[sup]2[/sup] to the problem and got no further was Eddington; Francis Aston had already pointed out that some sort of fusing together of protons into helium (this before the discovery of the neutron) would release energy, but hadn’t connected this with stars and that suggestion was left to Eddington. But he could get really get no further than that.
Atkinson and Houtermans were at Gottingen and so were exposed to Gamow ideas about explaining alpha decays as quantum tunnelling. With Gamow, they realised that there might be similar processes working in reverse. Their paper was thus the first quantitative study of fusion reaction rates and they proposed several specific reactions that might be taking place at the temperatures inside stars. They turned out to be wrong about the detailed reactions, but this was all far more ambitious than simply pointing out that fusion reactions would release energy and it was the first step towards what Bethe did.
It’s something of an aside, but I’ll take the opportunity to point out that “Fritz” Houtermans is one of the most underestimated physicists of the 20th century. There’s no adequate biography of him, despite him both cropping up all over the place with significant contributions and having one of the more complicated paths through the 1930s and 40s: a partially-Jewish Communist, he managed to be imprisoned by both Stalin and Hitler and then arguably got closer to giving the Nazis a working nuclear reactor than Heisenberg did. No account I’ve seen of his life has managed to even touch on all the twists, turns and ambiguities of it.
The Wikipedia entry on him is strikingly bare, though the Science World entry at least covers the outlines.
To return to the original question, if you want to read more about how it was figured out how stars work, Arthur I. Miller’s recent book Empire of the Stars (Little, Brown, 2005) is mainly about Chandrasekhar and Eddington, but inevitably gets into the wider story.
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>> I just got the following email from my sister. “C” is her five-year-old son …
What an odd name to give a child. Doesn’t he get teased a lot?
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Yes, but eventually he’ll head a department in the British Secret Service, so it works out.
