# Tag Info

199

Proton-proton fusion happens at energies around 15 keV. The LHC currently operates at an energy of 13 TeV, which is literally one billion times larger. Fusion is one of the lowest-energy processes that could occur at the LHC, and most of the interesting reactions that are studied there go way beyond nuclear fusion. A proton is made up of fundamental, ...

175

In this occasion Vsauce rather dropped the ball, I should think. As the other answers show, the claim as stated doesn't make much sense when you put in the numbers, and if you chase the source to its origin there's some crucial context that got dropped along the chain. The video description attributes the quote to the book The Universe and the Teacup: The ...

89

The fusion that occurs in the core of the Sun occurs in nothing like the conditions you might be thinking of in a bomb, or a fusion reactor. In particular, it occurs at much lower temperatures and at a much lower rate. A cubic metre of material in the solar core is only releasing around 250 W of power by fusion. The fusion rate is set by the temperature (and ...

81

In general, both fusion and fission may either require or release energy. Purely classical model Nucleons are bound together with the strong (and some weak) nuclear force. The nuclear binding is very short range; this means that we can think of nucleons as "sticking" together due to this force. Additionally the protons repel due to their electric charge. ...

76

Not an expert, but I believe the answer lies in the hairy ball theorem. You see, for a magnetic field to turn charged particles back from a surface, the field must be parallel to the surface, which means that to have a fully confining geometry you must have a smooth, everywhere non-zero, and continuous vector field mapped onto a surface. But the theorem ...

72

Heavy elements couldn't form right after the Big Bang because there aren't any stable nuclei with 5 or 8 nucleons. Source: Wikipedia (user Pamputt) In the Big Bang nucleosynthesis, the main product was $^4He$, because it is the most stable light isotope: 20 minutes after the Big Bang, helium-4 represented about 25% of the mass of the Universe, and the ...

64

The example of a Molotov bomb, a favorite of anarchists, and a car engine are a good analogy. The technology needed to contain the energies in a fusion reaction is much harder than the one needed for a car engine because of the MeV energies needed to initiate fusion. Once initiated it is explosive, so it must be engineered into small explosions from which ...

64

Fission releases energy, because a heavy nucleus (like Uranium-235) is like a cocked mouse trap: it took energy to squeeze all those protons and neutrons hard enough together to make them barely stick (by the nuclear force) against the natural tendency for all those protons to fly violently apart because of their electrostatic repulsion. When struck by an ...

58

Elements heavier than iron are produced mainly by neutron-capture inside stars, although there are other more minor contributors (cosmic ray spallation, radioactive decay). They are not only produced in stars that explode as supernovae. This has now been established fact since the detection of short-lived Technetium in the atmospheres of red giant and AGB ...

45

While a free neutron does have more mass than a free proton, a bound helium-4 nucleus has less mass than two free protons and two free neutrons. In fact, the helium-4 nucleus has less mass than four free protons. The difference goes into the binding energy of the nucleus. Therefore, as the other answers state correctly, stars are constantly losing mass, not ...

39

Matter-antimatter annihilation, such as an electron annihilating with a positron to form two high-energy photons, can convert 100% of the mass into radiation. So fission and fusion are far from the most efficient ways to convert mass into other forms of energy. Unfortunately, the universe appears to contain almost no antimatter.

37

A pin head is maybe equivalent to a spherical piece of iron with a diameter of 2 mm. That gives it a volume of about 4 mm$^3$ and a mass of $3.2 \times 10^{-6}~\rm{kg}$; computing the heat capacity of matter at these kinds of temperatures is hard, but whatever method you use, the energy content of the pin that you calculate would be insufficient to kill all ...

35

In the case of a supernova explosion it is possible to create heavy elements through fusion. Supernovae have a tremendous amount of energy in a very small volume but not as much energy per volume as there was in our early universe. So, what is the major difference? Why didn't the Big Bang create heavy elements? I just want to point out, too much ...

34

Hydrogen and helium can briefly bind together to make lithium-5, but this is an extremely unstable nuclide which falls apart instantly (with a half-life of ${\sim}4\times 10^{-22}\:\rm s$) and which actively requires energy to make (i.e. it is an endothermic process, as opposed to how we normally think of nuclear fusion). The reason for this is that helium-4 ...

33

Has Musk done his homework? With regard to the basic idea of using nuclear weapons to release CO2 and thereby warm Mars, no, he hasn't. I suspect this was either Bored Elon Musk speaking, or perhaps the Elon Musk who didn't quite deny being a super villain ( 1-900-MHA-HAHA Elon Musk?) in that interview with Colbert. CO2's enthalpy of sublimation is about ...

33

The most efficient non-gravitational way of extracting energy from ordinary matter is indeed to convert it into elements in the $^{56}$Fe region. There is a fairly broad plateau of nuclides with binding energies of about $8.7$ MeV per nucleon, so it does not matter very much which of these you actually turn the source matter into. (However, $^{56}$Fe is ...

32

You have several questions there, let's first focus on the main question: why is there no working fusion reactor on earth as we already have the hydrogen bomb? This is an interesting question, as a lot of people had similar expectations when the first hydrogen bomb, Ivy Mike, was ignited in 1952. They probably had the first fission bomb, Trinity, ignited in ...

30

A back of the envelope calculation (and that is all this is) would go along the lines of assuming that the white dwarf is made entirely of $^{12}$C (it isn't) and is entirely converted into $^{56}$Ni (it isn't). The appropriate mass to use would be $\sim 1.4M_{\odot}$ (it is actually a touch lower - the real "Chandrasekhar mass" at which instability sets ...

30

Your assumption about the lowest energy state when everything is tightly stuck together is incorrect. It only goes this way until you get iron nuclei - and this is why iron is the heaviest element created by fusion. Creating nuclei heavier than iron consumes energy rather than releasing it. And this is why these elements are only created in supernova ...

30

In fact, the Sun is losing mass all the time. It radiates large amounts of energy, and through the energy-mass relationship $E = m c^2$, radiating energy means radiating mass. Since the mass of a helium atom is less than the mass of the four free protons which enter the fusion process, one can consider fusion the process of converting mass into energy.

27

Of course the reaction is possible. It doesn't even require special environmental conditions. Having no charge the neutrons don't need to overcome a strong Coulomb barrier to interact with atomic nuclei and will happily find any nuclei that can capture them at thermal energies. KamLAND (for instance) relies on this reaction as the delayed part of the delay-...

26

Suppose you start with a linear solenoid. Due to the Lorentz force charge particles travel in circles (or helices) inside the solenoid so they can't reach the walls of the solenoid. But obviously the trouble is that they will leak out of the ends. Now we curve the solenoid round and join its ends together to make a torus so now the particles can't leak out ...

26

Yes. See for example this table of energy densities. Let's take 30 MJ/kg for coal (the middle of the range in the table), then 400,000 tonnes of coal gives 1.2*1016 Joule. Assuming they're talking about deuterium-tritium fusion (which is the easiest form of nuclear fusion), we have 340,000,000 MJ/kg, and the 60 kg gives us 2.04*1016 Joule. Of course, both ...

26

The plasma in a fusion reactor is typically "optically thin"; the radiation isn't really in equilibrium with itself and the plasma particles. Generally, instead of just modeling the plasma as a black body, people look at specific radiation processes. Kenneth Gentle (UT) has a nice set of slides that works through that. Hot plasmas are almost transparent -...

23

The Sun fuses protons, and this is a very slow process because there is no bound state of two protons. Hydrogen bombs fuse deuterium and tritium, and this is much, much faster because there is a bound state of these nucleides. You might like to have a look at: How much faster is the fusion we make on earth compared to the fusion that happens in the sun? ...

23

I am assuming you have had no rigorous mathematical exposure to quantum mechanics. Let me know if you do, and I can point you towards more specific material. It is difficult to tell what it was exactly that you read, but here is my guess. Why Fusion is Classically "Impossible" There is Coulomb repulsion between two protons whose magnitude is: $F=k\frac{e^... 23 The external casing belongs to the cryostat that maintains the superconducting magnets at 4 K. It has 254 ports to heat and diagnose the plasma. For heating with neutral beams, you need relatively large ports. For doppler backscattering diagnostics, waveguides with a few centimeters diameter carry coherent electromagnetic radiation into the plasma chamber. ... 21 We can only give an answer on the basis of what we currently know about cosmological parameters. If indeed these have been correctly estimated, and that the cosmological constant is constant, then the universe will continue to expand at an accelerating rate. Given that about half the baryons in the universe currently exist outside galaxies in the "warm/... 20 The Sun obviously produces far more energy per second than is required to fuse an iron nucleus with some other nucleus. The problem is concentrating all that energy on the iron nucleus. It's not enough to know that it takes the energy from$n$hydrogen fusions to fuse one iron nucleus, it's getting the energetic products from those$n\$ hydrogen fusion events ...

20

This question is answered in detail by the so-called "Big Bang Nucleosynthesis", the theory about the creation of the nuclei in the early Universe. Almost out of nothing, it allows one to determine that 75% of the nuclear mass was coming in hydrogen, 25% in helium, and some small traces of lithium appeared, too. Even though Gamow used to think that all ...

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