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Comment by redwood | Hacker News Reader

True but at the same time there are physical powers we witness in the universe that we cannot yet dream of harnessing but which, by virtue of their existence: do ultimately offer a whole new paradigm.

The main one that comes to mind is nuclear fusion... The power of the sun. We see and rely on it every day. We know it's possible to create with a trivial set-up: lot's of crap thrown together. We haven't figured out how to control it yet but once we do, it'll change everything.

So you're right: there are physical limits and some absolute of course, but we're not at the absolutes yet: it's just going to be a very step-wise function on the way up.

edit: while this may seem tangential to a discussion of jet engines: imagine if we did harness fusion... think about the aeronautical implications. We could probably have helicopter islands like in a Sci-Fi film. :)

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Just an amusing note: a useful fusion power plant on Earth would probably require conditions many times more extreme than those inside the Sun.

"The power production density of the core overall is similar to the metabolic production density of a reptile. The peak power production in the Sun's center, per volume, has been compared to the volumetric heat generated in an active compost heap. The tremendous power output of the Sun is due not to its high power per volume, but rather to its gigantic size." http://en.wikipedia.org/wiki/Solar_core

This.

People romanticize the notion of Fusion. It's a great thing, no doubt, But to say:

> We could probably have helicopter islands like in a Sci-Fi film.

Feels a little bit to me like people saying the same thing when they first discovered refined petroleum.

Having said that we've made massive progress towards energy positive fusion with both magnetic confinement, and inertial confinement, And it's not crazy to think they may provide good usable power into our grid within our lifetimes.

Deuterium/Tritium fusion is vastly more efficient than light hydrogen fusion.

I think you're misunderstanding the concept here. They're saying that temperature gradients determine power production and since the sun is reasonably smooth in its gradient, there is minimal power generation possibility her volume..

But if you were able to contain a golf ball of sun, the heat it produced would be able to power far more than a reptile's brain as its heat was transfered to the environment...

_Perhaps_ you're pointing out that the sun's size is what allows it to 'contain' these many otherwise-uncontainable golf-ball size chunks. If so, I hear you... obviously the engineering challenge is developing a system on earth to contain. We've clearly shown with fusion bombs that we can harness this effect for a net energy gain for an instant.

My point was that these are engineering challenges, rather than physical limits.

>But if you were able to contain a golf ball of sun, the heat it produced would be able to power far more than a reptile's brain as its heat was transfered to the environment...

It is you who is misunderstanding the concept here. A golf ball of Sun produces heat only about as much as the reptile brain. In other way - if you put as many reptile brains in one pile so that the pile will be as large as Sun, you'll get the pile radiating with Sun's black-body radiation temperature and intensity. The trick here is that amount of energy produced is proportional to volume, while radiating surface is just square-proportional, thus black-body radiation temperature (i.e. frequency, ie. energy of a given radiated photon) of a pile of reptile brains (or golf-ball sized chunks of Sun) would be much higher than that of a one brain or the same set of brains (or chunks of Sun) spread around with significant distances between each other.

Edit: it isn't to say that fusion power can't be used as a practical source of energy on Earth. It is just some forms of [stationary] containment are ruled out, mainly by bremsstrahlung (Sun overcomes it precisely because of volume to ratio being big enough). Dynamic forms of containment are proven to be working (net-positive) at least by H-bomb.

I'm sorry, but the core of the sun is 150 times as dense as a reptile's brain and far far hotter. Thus you would need far more than the equivalent volume of brains to create the same effect. I see what you mean about the energy of escaping radiation, however.

For your point to be persuasive you need to add why temperature and density would increase power. e.g. one could have a core of extremely hot, dense inert material.

Something hot should not be thought of as inert, because something hot by definition is something with incredible internal velocity. But then there are multiple definitions of inert..

I think what you're saying, though, is that if you contain something extremely hot and dense, you cannot get any useful energy out... Because the energy needed to contain it prevents you from getting anything out. Right?

Because I'm coming from a perspective that says: if you can contain something like a small bit of solar core: and if you can let some of that heat leak to the outside environment: you can derive terrific power from the thermal dynamics at work.

The sun exists in the pressure-free vacuum and thus extends to a far less dense outer layer where the heat gradient (the argument against power potential per volume) is low. However if you could contain the sun into a smaller volume, it would have the same massive heat flux but over a much lower surface area and thus with far more power potential per volume.

I assume we're not so much disagreeing: over focusing on different issues. Is it correct that you're saying that the act of containment itself prevents the ability to harness the flux?

We might be slightly talking at cross purposes. I agree that something hot contains a lot of available energy. I'm making the point that it may not produce any available energy (i.e. inert in the sense of nuclear fusion).

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