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Comment by mikeash | Hacker News Reader

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.

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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.

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.

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

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.

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.

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.

> 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...

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.

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