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Underground Experiment Confirms Fusion Powers the Sun
sciencehabit writes: Scientists have long believed that the power of the sun comes largely from the fusion of protons into helium, but now they can finally prove it (abstract). An international team of researchers using a detector buried deep below the mountains of central Italy has detected neutrinos—ghostly particles that interact only very reluctantly with matter—streaming from the heart of the sun. Other solar neutrinos have been detected before, but these particular ones come from the key proton-proton fusion reaction that is the first part of a chain of reactions that provides 99% of the sun's power.
Obvious is different from proven.
Nothing has been proven. Scientists have long had a theory about how the Sun powers itself. That theory can be used to make predictions, such as the type of neutrinos that we should expect to see emanating from the Sun. An experiment was devised to test such a prediction, the hypothesis being that this type of neutrinos is being produced and thus will be detected. Having performed the experiment, we see that the results match what we expected, validating the hypothesis. This is important and significant, and it provides further evidence suggesting the widely accepted theory is accurate, but it does not -- nor can it -- constitute a proof.
The other interesting result would be if the expected neutrino type was not detected by this experiment, invalidating the hypothesis. This would raise further questions such as: is there some other mechanism powering the Sun? Is there something deficient in our understanding of neutrinos that prevented us from detecting them despite them being there? Was there an error in the test setup (i.e. is it repeatable by other parties)?
it caught me by surprise as well. but thinking about it more it's mind blowing to think that there are a lot of things we take as fact when they may just be assertions. like fusion powering the sun, for example. I call this the Wikipedia phallacy.
. . . the last professor in the once-prestigious Solar Combustion Sciences department clutches his chest, winces, and slumps face-down on his desk.
It has been known since the 1960's that the Sun produces energy from fusion, but the actual neutrino's observed then (and until now) were high energy electron neutrinos that actually came from relatively unimportant fusion chains (from the standpoint of energy production), not the proton-proton chain though to produce most of the Sun's energy. Since there was a "neutrino problem" (the Sun appeared to produce only 1/3 of the neutrinos predicted by theory), some people did think that for whatever reason the main energy source - the proton–proton chain reaction - was for some reason mostly shut down, presumably as part of some long period oscillation in the Sun's deep interior (although Arthur C Clarke wrote a novel, "The Songs of Distant Earth," in which it was a permanent shutdown of the Sun's fusion, and a prelude to our Sun going supernova). At that time, the inability to directly see the pp chain seemed like a big deal, but since the discovery of neutrino oscillations (which nicely explain the factor of 1/3), and also with solar interior modeling from helioseismology, there has been a pretty solid consensus that the pp chain was running the Sun, even if there was no direct observation of it.
Now it has been proved. In 1990 that would have been a big deal, but now it is more a matter of just being satisfyingly complete in our observations of the Sun.
secondary reactions based around contaminants (as in non-H or He atoms).
One is the CNO cycle which is about 0.4% of the total solar energy.
One of these days, I'm going to RTFA.
it caught me by surprise as well. but thinking about it more it's mind blowing to think that there are a lot of things we take as fact when they may just be assertions. like fusion powering the sun, for example. I call this the Wikipedia phallacy.
I think (hope) you mean "fallacy"...
No, he has it right. By analogy with "democracy", "oligarchy", "anarchy", and so forth, naturally Wikipedia is a "phallacy" since it's quite well established that many of the editors there who run the place are pricks.
Making huge discoveries about the universe without leaving mom's basement? Nerdgasm!
Table-ized A.I.
Yeah, but we didn't need to detect proton-proton neutrinos to know that fusion powers the sun, because we have myriad other indicators (spectrum, energy output, solar wind) that agree with the current theory. The fact that we have now seen proton-proton neutrinos is cool as hell, but this will never be "proven" significantly more than it currently is, unless science changes drastically to allow for deductive facts. Science allows for an inductive form of "proof" (something being so probable it will likely never be demonstrated wrong) that's less rigorous than the logical kind, and fusion in the Sun has long been under that label. For analogy, we didn't have to wait until Sputnik had orbited Earth to know that Earth was round (since that was known to academics 2000 years earlier), but it certainly made people feel more confident in that fact when it happened.
Another surprising fact about fusion in the Sun is that the fusion power generated is about 1.5 watts per ton of core. Even in conditions in the core of the sun, fusion is hard, and the particular reaction process just confirmed was at the end of a long chain of reasoning explaining what we do see. So I think this actually give evidence that a bunch of stuff in Wikipedia about processes in the Sun is also true. (If a different fusion process was found, then we'd likely be wrong about how much power is generated, and thus about the rate and manner that that power eventually makes it to the surface and gets radiated).
Socialism: a lie told by totalitarians and believed by fools.
A theory is something that has strong supporting evidence, and if you agree with Popper and Kuhn and various" Historians or Philosophers of Science", something that skilled people have tried to come up with alternatives, tested them, and the theory has survived where they didn't. Ideas that have been proposed, and maybe have a little supporting evidence, but are considered not tested enough, and not studied rigorously to see if they can be falsified, or if some other idea better fits Occam's razor, are called hypothesi (or often just interesting ideas until they get at least a little support). Yes, just who qualifies as skilled, which idea is actually simpler by the razor, how much testing is enough, and 'how much better at predicting what than the competing ideas are' are all somewhat subjective, and individual scientists are not exceptionally flawless at making those judgement calls. But that's true of just about everything. Science works because the method tends to correct for those subjective aspects, not make them more powerful as in so many other areas of human activity.
By this era, the theory that the sun was powered by Fusion of Hydrogen into Helium had a lot of evidence supporting it, such as the abundance of various elements in it and other stars, as determined spectrally. Try a web search for Hans Bethe if you want to know about the first evidence that helped raise this hypothesis to the status of theory, in 1930, although he didn't get the Nobel for his work until 1967. It's interesting to me that people are debating just what counts as a theory, and for this particular case, there's an exact date when a particular paper was published, and widespread agreement that this date and event is when the hypothesis got enough support to start calling it a theory. This is additional evidence that adds more support, and by the Philosophers of Science, ought to mean anyone who thinks they have a better idea will have to gather even more evidence and work even harder if they want their alternative to be taken seriously.
Who is John Cabal?
To be completely accurate, the sun doesn't produce any energy, it converts energy from one form (rest mass) to another form (electromagnetic radiation), increasing entropy in the process in keeping with the second law. That conversion process itself requires an input of energy (though one less than the energy output by said process) to initialize and sustain, and that energy is in turn supplied, in the form of kinetic energy, by conversion from yet another form (gravitational potential energy) spontaneously, precisely because of the second law of thermodynamics.
At one time in the history of science, it was thought that all of the energy of the sun was converted more or less directly from gravitational potential energy: a cloud of hydrogen collapses under gravity, converting its potential energy into kinetic energy, rendered macroscopically as temperature, causing the ball of collapsing gas to glow incandescently. The problem was that that process can't last for very long, so the sun (and consequently the whole solar system) would have to be pretty young, relatively (still massively old on a human scale) if that's what's making the sun glow. When we discovered that the Earth itself, and space rocks, are much older than the sun would have to be according to that theory, it required that something else be powering the sun on a longer scale. The introduction of nuclear fusion to the model solved that problem, and nowadays almost nobody even remembers that we once thought the sun was just, in effect, gravity-powered.
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Another surprising fact, the Sun's core is so dense (150 g/cc) that a metric ton of core only needs the volume of a cube 19cm per side to occupy.
Yeah, but we didn't need to detect proton-proton neutrinos to know that fusion powers the sun, because we have myriad other indicators (spectrum, energy output, solar wind) that agree with the current theory. The fact that we have now seen proton-proton neutrinos is cool as hell, but this will never be "proven" significantly more than it currently is, unless science changes drastically to allow for deductive facts. Science allows for an inductive form of "proof" (something being so probable it will likely never be demonstrated wrong) that's less rigorous than the logical kind, and fusion in the Sun has long been under that label. For analogy, we didn't have to wait until Sputnik had orbited Earth to know that Earth was round (since that was known to academics 2000 years earlier), but it certainly made people feel more confident in that fact when it happened.
Science allows for deductive facts to carry scientific weight. I have no idea why you would think it does not.
Science uses both deduction and induction, because while induction is not as absolutely rigorous as deduction, its not possible to deduce the entire cosmos from first principles. One of the axioms of Science is that the universe is not completely random and operates on the basis of rules that govern its behavior, and those rules can be discovered through observation. Universal gravitation is an induction, because there's no way to deduce that gravity operates in the same way everywhere. The presumption is that its highly unlikely that gravity operates the same way in every place we directly measure it, and also operates in a consistent way in every place we can indirectly measure it, but somehow operates differently in every place we just happened to not directly or indirectly observe it.
All logical deduction must start with fixed axioms, and depending on what physical scientific laws you consider strong enough to be axiomatic, you can deduce a lot which is considered scientifically rigorous. For example, when we observe photons striking a detector, we *deduce* they were emitted from somewhere along the path the photon struck the detector. You could argue that's just an induction; that we're only guessing because that's how we've observed photons to behave in the past, but at some point that's sophistry, because it rejects the notion that Scientific reasoning can contain any axioms. Without some reasonable starting point - like being able to trust observations at all - you can't do Science at all.
Also, I doubt there exists any significant number of people who were suddenly more confident the Earth is an oblate spheroid after the launch of Sputnik than before. Nor am I sure how the launch of Sputnik demonstrates the Earth is approximately spherical better than all other demonstrations of that fact prior to Sputnik. Sputnik did not itself observe the Earth, and observers of Sputnik did not get significantly more information from Sputnik in a scientific sense that other observations of Earth's curvature prior to that point.
Can't actually "prove" it, this just makes it much more likely.
Phone back when they actually go to the sun and check inside.
Sleep your way to a whiter smile...date a dentist!
The other interesting result would be if the expected neutrino type was not detected by this experiment, invalidating the hypothesis. This would raise further questions such as: is there some other mechanism powering the Sun? Is there something deficient in our understanding of neutrinos that prevented us from detecting them despite them being there?
That almost happened, in the early days of neutrino dectection - before things like old mines full of purified water and 3-D arrays of photodetectors running for months at a time, and you could count the number of detected neutrinos on two hands (in bi-quinary so you could go a bit higher than ten). This was when the detectors could only detect the type of neutrino directly generated by fusion reactions, and before the discovery of neutrino oscillation, when it wasn't yet clear whether neutrinos had no, or very very little, rest mass.
Early numbers, and their error bounds, made it clear that there weren't enough neutrinos being detected. (This was known for years as the "missing neutrino problem".) But the earliest ones WERE about right for a situation where all the stars EXCEPT the sun were running by fusion and the sun was out.
That may sound odd. But there was a very cute explanation that made it plausible:
The gradual gravitatonal collapse of the sun, as heat is radiated away, could power it for millenia. It's nowhere near enough to power it long enough to explain the fossil record, but it IS enough to have kept it running for historic time. Meanwhile, if a fusion reaction were to start up near the center of such a ball of collapsing gas, it would also take many years for the heat to make it to the surface. Neutrinos (which go through the sun like marbles through a light mist) are about the only signature of what's going on in there NOW.
But suppose, instead of fusing continuously, stars were reciprocating engines. They might run without fusion for centuries, or millenia, until they were compressed enough to "light up" at the center. Then the fusion heat and reaction products might make the reaction ramp up. They'd burn for a little while (which would heat them up and expand them mabye a few inches), until the decreased density and/or reduction in fuel and/or accumulation of reaction products "put the fire out" again. Repeat for the life of the star.
In this scenario, if our sun happened to be between "putts (and the very nearest stars didn't happen to have an unusual distribution of where they were in their cycles), you'd see the same neutrio flux from the rest of the sky as if all the rest of the stars were running continuous fusion. That's because it's the average of stars that are "on" and "off", and comes out to the same amount of total fusion and neutrinos.
Of course later data, both larger samples and detectors that could "see" the other neutrino types, put the kibosh on that model. A big part of it was the discovery of neutrino oscillations, allowing a stream of neutrinos that started out as one type in the sun to arrive as a mix of the three types. (This means that neutrinos have a non-zero rest mass, fly slightly slower than light, and thus experience time and are ABLE to change from one type to another.)
A pitty, thugh. By the time this was discovered I had done an outline for a five-volume fiction cycle, working through at least four genres, based on the sun going "putt" from time to time. B-b
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