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jgourlay
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Why does Fusion work?
My boss and I were in an alternative energy debate today, and a "fundamental" question came up that has me completely stymied.
Why is fusion, in this case fusion of hydrogen to helium, exothermic? Put another way: so gravity makes the pressure really high at the core of the
sun, fine: why does that make it hot?
For that matter, it sounds like gravity is your perpetual motion something for nothing machine? Gravity has to do work to pull things together and
cram them ever harder into each other....
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12AX7
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So, you're asking something like this?
Let's say you have a big ball of gas out in space, initially fairly cool and at a low pressure. If there is enough mass present, the gas will draw
together under gravity. The gas compresses and heats adiabatically. Simple gasses won't radiate heat (e.g., H2, He) until they get a lot hotter,
while complex molecules and dust (which are probably present in small amounts) may radiate heat. As the gas ball draws together, it heats up; if it
heats up slowly, and radiates slowly, pressure falls and it can collapse faster. If it doesn't radiate much, it may reach an equilibrium of some
scale. Even so, it will eventually reach fairly high pressures in the middle, which will inevitably reach fairly high temperatures (especially if
there's so much dust that, down to the center, it reflects as much as it radiates -- a greenhouse effect). If the temperature and pressure is high
enough (around a solar mass of hydrogen), fusion can begin, creating additional heat. Soon, the gas ball comes to thermal and geometric equilibrium
and a star is born.
Is that what you were thinking? The basic ideas are, in space, 1. everything is so huge that gravity does some serious amounts of work, 2. all that
work goes into heat (due to compression), and 3. because the surface area to volume ratio is so low, the heat can't radiate away fast enough. So it
gets really hot and dense.
The limiting case is where there's enough mass together (> 1.44 solar masses, no fusion) that it collapses into a neutron star or black hole.
Incidentially, black holes are the most efficient mass-to-energy converters, throwing off something like 20% RME (as radiation from the plasma,
including black body radiation, bremsstrahlung and synchrotron radiation) when stuff falls in. The other 80% stays inside the black hole, unless it's
small and hot, giving off Hawking radiation, in which case it would be 100% over the life of the singularity.
Tim
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merrlin
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Fusion between atoms is energetically favored when the average binding energy for the nucleons in the product atom is greater than the average binding
energy in the reacting atoms. Search for "binding energy" on Wikipedia. Roughly speaking, elements above iron can release energy by fission and those
below iron can release energy through fusion. However, as 12AX7 has pointed out, fusing atoms takes a lot of work, even for the "easy" ones.
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JohnWW
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That's right, because of the high coulombic energy barrier that has to be overcome in fusing nuclei due to the repulsive charges on the protons, even
when attempting the fusion of the lowest-charged possible nuclei, two deuterons. But up to about iron, there is a net radiant energy release on fusion
where it results in another stable nucleus. This accounts for why iron, and to a lesser extent neighboring elements such as manganese, chromium,
vanadium, titanium, cobalt, nickel, and copper, are the most abundant metals in Earth's core. Because of their density they sink (along with most of
the other heavy metals including gold and the platinum metals) into the core, and their abundance is depleted on the surface. The interiors of "white
dwarf" stars, not massive enough to become neutron stars, are believed to also consist mostly of iron and neighboring elements.
Fe-56 is probably the nucleus in which binding energy is at a maximum, with no possible net energy release by fusion even with one additional proton
or deuteron, and no energy gain on fission even if it were possible.
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Pyrovus
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Actually, nickel 62 is the nucleus with the greatest binding energy per nucleon:
http://en.wikipedia.org/wiki/Nickel-62
Never accept that which can be changed.
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jgourlay
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12ax7: gravity does work, understood. But what is gravity that it does not run out of the ability to do work?
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Sauron
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Fusion works because God is Hungarian and He did not want Edward Teller to look stupid.
Sic gorgeamus a los subjectatus nunc.
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12AX7
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Good answer.
Gravity does not run out of the ability to do work because the universe has not ran out of the work put into it by the Big Bang (indeed, it's
projected that the universe will never shrink).
Tim
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hissingnoise
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I find the idea of an infinitely expanding universe curiously chilling. . .
And any way you look at it, the Big Bang Theory still remains a theory.
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12AX7
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Yeah, it would be elegant if the universe collapsed again, in an infinite(?) series of bangs and crunches. Sadly, the estimated universe is larger
than its Schwarzschild radius, precluding this. Indeed, evidence suggests some unknown expansive force, which so far has been fairly kind to us (if
it were much larger, the universe could only be a few billion years old, and the diversity of structure we see could not have developed), may
eventually rip apart the fabric of space (it seems to be increasing; if it increases without bound, eventually it will even dissociate atoms, nuclei,
perhaps even quarks, until a low pressure mist of nothingness remains).
Tim
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JohnWW
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Quote: Originally posted by hissingnoise  | I find the idea of an infinitely expanding universe curiously chilling. . .
And any way you look at it, the Big Bang Theory still remains a theory. |
and that is how it will remain, -
just a theory, because it requires all matter in the universe to have been present in a "singularity", i.e. at a single point in space, which would
also have had to have been a gigantic "black hole", about 13.7 billion years ago. NONE of the proponents of the theory have been able to point to the
angular and distance coordinates of the specific location in space when the "big bang" occurred, or to explain where the energy came from to propel
all the matter in the universe outward in all directions from such a single location with enough force to reach mutual escape velocity. And at the
same time, thanks to academic hegemony and sloth in physics and astronomy, no salaried academic physicist or astronomer, of any influence, has dared
to question the theory on account of these obvious unsuperable flaws, for fear of losing their fat-salaried jobs or research grants.
Of course, something must have happened to result in the creation of all the matter in the universe about or not long before 13.7 billion years ago,
as free protons and electrons, because of the measurable rate of conversion by fusion of matter into increasingly heavy elements. The ages and
compositions of the oldest stars are crucial evidence in this regard. But the matter must have filled space uniformly to begin with, until the
propagation of an irregularity in distribution resulted in the coalescence under the influence of gravity and ongoing fusion processes. Although due
to gravitational coalescence the distribution of matter in space has become quite uneven on scales of up to millions of light-years, there is NO
evidence of any anisotropy in distribution on an infinite scale which points to the existence of any point at which a "big bang" may have occurred.
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not_important
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| Quote: Originally posted by JohnWW | | ... NONE of the proponents of the theory have been able to point to the angular and distance coordinates of the specific location in space when the
"big bang" occurred... |
This is because the proposed Big Bang is not only an explosion of matter/energy, but of space - the Monoblock was all of space, the Bang expanding
space and reducing the density of matter as it proceeded.
And past the Big Bang itself is the phase change and resulting hyperinflationary period, which evens out fluctuations in space/energy-densities save
for those resulting from quantum fluctuations at that time and results in a nearly homogeneous and isotropic universe. Again, it was space itself
expanding, with matter-energy going along for the ride, not matter expanding out into pre-existing space.
There are and have been other variants of the Big Bang theory, indeed hyperinflation replaced several earlier models of the BB. Out and out
alternatives to the BB include the Friedmann oscillatory universe, the Milne model (which similar to your concept of the BB does not include metric
expansion of space but simply an explosion of matter), the Hoyle steady state model, and Zwicky's tired light theory. None of these could
successfully explain observations and in general have been retired.
The steady state models for the most part fell when the comic microwave background was discovered, although there are still a few supporters; the
remaining form of this group of theories require a universal 'fog' of iron needles to produce the CMB. Steady state theories also run afoul of
nucleosynthesis and light element abundance issues.
The TeVeS gravity theory supports some alternative models, and removes the need for dark matter as well. However it has its own problems, such as
the need for a fairly massive neutrino.
Alternative cosmological models continue to be discussed, but so far all are noticeably less successful at explaining observations than is the current
set of Big Bang models.
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chemrox
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I think we're past the Big Bang and into the big bounces. It inflates . then it deflates to zero/pure energy but it does this over and over just as
the Buddhist cosmologists were saying 1600 years ago. Big Bang vs Big Bounce may be semantics to most of us. The physicists tell me its in making
the math work and bring quantum and relativity together. I don't think in equations so I have to stay at home and cook. Anyway, here's my simplistic
lay view: if all the space time is converted to energy at the singularity, then there are as many or few singularities as you choose because at that
moment there's no past no future. As far as ongoing and irreversible inflation goes recall that galaxies participate in that as accelerating units
with black holes at the centers. We only just proved black holes exist so we don't know what sort of localized contraction they may provide. Quasars
are throwing out huge amounts of energy that is conserved thus much matter and space time is consumed at the same time.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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tom haggen
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You've all got it wrong. The universe is less than 7,000 years old.
[Edited on 11-7-2009 by tom haggen]
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hissingnoise
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I know there are people who actually believe that tom, but I have to assume that you're joking. . .
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not_important
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Problem is that the boffins have done the maths, and entropy has an effect on 'bouncing' cyclic universes, each cycle expands longer and larger than
its predecessor until finally there it resembles and open universe - heat death, matter spread thin or decayed, all that.
The other thing is that if we were in a contraction phase we should see the nearer galaxies slowing their flight 'from us', pausing, and starting
towards us - the nearest ones being blueshifted, then a shell of them not really moving relative to use, the the standard increasing red shift as you
look further out. The transition from red shift to blue shift would expand further and further out as time passes, the CMB would show a similar
increasing blue shift as photons from further and further away from us reach us (provided that the shrinking is sufficiently slower than light - as it
is the space-time metric that expands and contracts it can do so 'faster than light' without violating the rules.)
Membrane models can support oscillatory universes without running into the entropy boost problem, I believe, but it is rather different than the
simple Big Bang+Crunch models.
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tom haggen
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Actually to be honest I dont believe there is 100% proof that the universe is older than 7,000 years. Even you guys say that the big bang theory is
just a theory and not concrete evidence. Im not sure how old the universe is.
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12AX7
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Do you have proof that it is 7000 years old? Is that a testable hypothesis?
Oddly, I can test that the universe is more than 0 seconds old, but how much older remains to be seen. I can at least tell with certainty that it is
not -1 seconds old!
Tim
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not_important
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| Quote: Originally posted by tom haggen | | Actually to be honest I dont believe there is 100% proof that the universe is older than 7,000 years. Even you guys say that the big bang theory is
just a theory and not concrete evidence. Im not sure how old the universe is. |
Every 'law' in science is a theory, granted a theory that has much evidence that it is true. Newtons laws were shown to be incorrect once you got
moving fast enough, they exists as a close approximation at low speeds of their replacements, but still not correct as originally stated.
There is plenty of evidence that the Earth is much older than a few thousand years. Erosion, radioisotope dating, things like repeated beds of coal -
marine sediments - shoreline soils - soils supporting heavy vegetation - coal for dozens of cycles, seafloor spreading, and so on. And there's much
more astronomical/cosmological evidence that doesn't depend on the Big Bang. Recently genetic evidence has been added as well.
You can say the the universe was created that way, with the geological conditions in place, with stars just appearing to be thousands to millions of
light years away but really aren't, or that they are that distant but light was created so as to appear to have left those stars in a distant
non-existent past.
However that seems to add complexity, of an ever increasing nature. And would you want to live in a created universe were the creator went to the
efforts of hiding the created nature of that universe, and then states you will be punished for believing those skillfully crafted lies? Wanting such
a state of being goes way beyond masochism.
[Edited on 12-7-2009 by not_important]
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497
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| Quote: | | Do you have proof that it is 7000 years old? Is that a testable hypothesis? |
So true.. Seriously, can anyone show me ANY decent evidence that the earth isn't billions of years old?
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hissingnoise
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No one is, but current estimates put it at around 14 billion years, but it could conceivably be much older than that. . .
Time is only as old as the universe!
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tom haggen
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Radioisotope dating is completely flawed. The only way radioisotope dating would be accurate is if you assumed that the level of radio active
isotopes that you are using to date things with have remained at a constant level in the earth's atmosphere indefinitely. To assume we know what the
composition of the earth's atmosphere was even 50,000 years ago is complete rubish. I have no proof that the earth and the universe are less than
7,000 thousand years old, I'm simply saying that theres no proof that the universe and the earth are billions of years old. Using geometry and the
physics of light to guess how far away stars are, to prove how old the universe is, cannot be trusted either. People can't prove how long stars have
been in exsistence by measuring there distance from us. If you have no idea how something came into exsistance, there's no way you can prove how long
it has exsisted for. For all you know a magical smurf who can fly on a surf board made it all appear. The dating of the universe and the earth is a
complete pseudoscience and has no more scientific credibility than the nazi racial sciences.
[Edit Ramiel: Godwin's law just turned the argument over to the blue team! reductio ad Hitlerum, you just lost the game]
[Edited on 13-7-2009 by tom haggen]
[Edited on 14-7-2009 by Ramiel]
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Lambda-Eyde
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http://en.wikipedia.org/wiki/Ice_core
At first I was having a hard time determining whether you were trolling or not.
Now I see that you're actually serious about this complete bullshit, and I can't understand why you even bother - it's
ScienceMadness!
It's people like you who obstruct the advancement of science, and we need some serious change in attitude if that's going to change.
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hissingnoise
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| Quote: Originally posted by tom haggen | | Using geometry and the physics of light to guess how far away stars are, to prove how old the universe is, cannot be trusted either.
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And star distances, tom, are calculated, not guessed at. . .
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setback
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| Quote: Originally posted by 497 | | Quote: | | Do you have proof that it is 7000 years old? Is that a testable hypothesis? |
So true.. Seriously, can anyone show me ANY decent evidence that the earth isn't billions of years old? |
Some people are willing to blindly cling to ignorance in the face of overwhelming evidence.
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