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Comment by mannykannot | Hacker News Reader

This is somewhat tangential to the specific issue here, but I was surprised by this:

At the center of the Sun, fusion power is estimated by models to be about 276.5 watts/m3. Despite its intense temperature, the peak power generating density of the core overall is similar to an active compost heap, and is lower than the power density produced by the metabolism of an adult human.[1]

So practical fusion power requires producing much higher power densities than are found in the Sun.

[1] https://en.wikipedia.org/wiki/Solar_core#Energy_conversion

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> Despite its intense temperature, the peak power generating density of the core overall is similar to an active compost heap...

I've seen this comparison before and it also seemed quite surprising to me. But in hindsight maybe it shouldn't: The sun is already billions of years old and it hasn't burned out yet.

Another way to think of it: A larger compost pile has to radiate more heat out of the same surface area as a smaller heap because of the Square–cube law [1]. Now imagine a compost heap more than 800,000 miles in diameter.

[1] https://en.wikipedia.org/wiki/Square%E2%80%93cube_law

That also shows why fusion power always centers around exotic isotopes like deuterium and tritium rather than plain hydrogen. The reaction paths needed to fuse plain hydrogen are so improbable that they don’t happen fast enough to be useful.

Is this an another way of saying that the only way to fuse to Hydrogen is to create a structure as large and with gravitational features that of a star?

Then a star/solar energy is the only way you will ever have large scale fusion energy generation and harvesting.

I suppose the universe doesn't owe us a cheap, easy, unlimited source of energy. A lot of people seem to think that it's inevitable we will achieve amazing, magical feats of technology given sufficient time and effort that are unthinkable today. Maybe. But it's also just as possible that we already know most of the general parameters of what is possible in the universe, and the peak practical feats of technology that will ever be open to us are not all that far off from what we have now. How can we tell?

Nuclear tech is not even a century old. The idea that we can't do any better in a thousand or a million years seems so improbable to me that I can't take it seriously. There is some kind of extrange pessimism around that mindset: that we will be extinct much sooner, or that the Sun will die and thus Humanity, as if there's no obvious way to escape that apocalypsis.

I think that significant swathes of human kind throughout history have assumed that their current state of existence is the peak and final form of humanity. While it is always logically possible that this is the case, it has never yet been true.

I don't know what it is about the idea of the far future that makes people so willing to throw out fundamental physical laws. The relevant point here would be that there are fundamental limits to heat transfer, and a finite amount of work which can be done on Earth-as-we-know-it for a given unit of time, irrespective of the energy source. An "unlimited" source of energy would be in practice limited by its heat output.

Improbable is not the same as impossible. We didn't even know about atoms for a huge stretch of science, and now we're smashing them together to break them apart looking for smaller things.

Is it completely out of the question that some discovery will be made that invalidates our current understanding of physics?

Even without new physics, processes can be improved with new tech. As far as I know current computers are based in the same foundations as in the past century, but they're many orders of magnitude more powerful. Then, after some threshold, quantitative changes become qualitative ones.

Physical processes cannot be made arbitrarily more efficient. There are thermodynamic limits to computation, the same as any other form of work.

You are beating a strawman with great dedication, congrats.

Yes, it's absurd to consider that there will be any large-scale violation of thermodynamics in the indefinitely far future. Literally, nothing is less probable.

Literally nothing is less probable? Alright then. I don't know why we bother with science any more since you've apparently got this one in the bag.

Non sequitur, and rude.

> do any better in a thousand or a million years

From what I can tell, there is a much bigger chance that we'll do much worse - and much sooner.

And I was afraid someone would call me Dr. Freud for mentioning the pessimism...

Agreed, the accepted models in physics are laughably incomplete. There is still a tremendous amount we still don't know. Although it's possible we've peaked, I have doubts.

It is also a self fulfilling prophecy to declare that we have reached our technological apex, and not striving to progress further.

No, it's saying that large-scale fusion using hydrogen fuel requires a star, because hydrogen fusion reactions are very improbable. That reaction only happens when four nuclei collide at once.

Fortunately we have other fusion fuels that are much more likely to fuse, including deuterium, which makes up 1/2500 of the hydrogen atoms in the ocean. Most projects target the easiest fuel, deuterium-tritium; while tritium isn't naturally available on Earth, it can be bred from lithium.

Nope! The p-p chain only has pairwise interactions. The issue is the p+p->d step is very disfavored, thanks to the weak step. D+T is all strong interaction.

Could that be explained with less jargon? I’d like to understand it.

There is a chain of individual steps that need to happen to create the final product of hydrogen fusion, helium. One of the crucial steps is the production of deuterium from two protons. This step is incredibly slow and limits the rest of the chain. However if you skip the slow step and start by fusing deuterium and tritium you can have a much faster and efficient reactor.

Indeed. After a D nucleus forms in the sun, it reacts (with a proton, to form 3He and a photon) within a few seconds.

The 3He+3He reaction is also slow, but not as slow as that first step.

Thanks, that was interesting. Apparently the five-minute video I got my statement from led me astray.

Ah, one of my favorite astronomy-related trivia.

It also shows how a supernova can (briefly) shine as brightly as its whole host galaxy (i.e. some 10-11 orders of magnitude stronger than an average dwarf star - like you wrote, those basically "smoulder").

It continues like this though:

> The low power outputs occurring inside the fusion core of the Sun may also be surprising, considering the large power which might be predicted by a simple application of the Stefan–Boltzmann law for temperatures of 10 to 15 million kelvins. However, layers of the Sun are radiating to outer layers only slightly lower in temperature, and it is this difference in radiation powers between layers which determines net power generation and transfer in the solar core.

So if a m^3 of sun core was moved to a powerplant it would generate a lot more energy.

Not exactly. If, using your Star Trek-style transporter, you instantaneously transported a cubic meter of sun core into empty space, it would radiate a lot of power at first, but by the very fact of radiating all that power, it would quickly cool, especially if you didn't have a good way of keeping it from expanding. But even if you did have some way to contain it, it would still cool pretty fast, since it would no longer be receiving the power input of all the other core stuff around it.

> So if a m^3 of sun core was moved to a powerplant it would generate a lot more energy.

Yes, for a tiny fraction of a second as it exploded. Its pressure would far beyond what any material could withstand (and magnetic confinement does that change that, as the outward pressure has to eventually be carried by the magnet support structure).

Just to expand on that: the kinetic energy of the particles in 1 m^3 of the material at the core of the Sun is equivalent to the yield of a 5 megaton bomb. So when I say "exploded" I really mean it.

> So if a m^3 of sun core was moved to a powerplant it would generate a lot more energy.

Would it? Or would it expand into basically normal hydrogen and helium?

Provided you could keep the plasma contained in magnetic fields, it would certainly generate plenty of free neutrons to provide heat for substantial power generation.

If you let it cool, it would stop fusing. The fusion power wouldn't magically go up.

fusion halts harmlessly when conditions are unmet

Halting harmlessly is the opposite of "producing a lot more energy".

It would produce negligible quantities of neutrons.

My understanding is that neutrons are the primary component for heat generation and therefore steam and therefore energy. At low energy fusion reactions, is this not the desired outcome?

You seem to think that the outcome of an experiment is determined by desirability, not by physics. This is called wishful thinking.

Uh my counterexample is the Sun itself. Uses fusion on a daily basis to provide vast quantities of light and heat. What a fusion reactor does is it decouples the plasma superhot gravity-laden center of a sun with a magnetic field strong enough to hold tiny pockets of plasma to generate energy (once we can go straight to fusion reactions using Boron) or to generate other particles which will create heat.

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