I don't actually understand what it really means, and from what little I've read since posting it doesn't seem applicable in the way I hoped.
However, a post in the article I posted does seem to indicate my original thought, that this is a possible way to get solid hydrogen.
"Negative-temperature systems have the opposite behaviour. Adding energy reduces their disorder, and hence their temperature."
The end of the latter sentence is incorrect. Temperature is the derivative of energy with respect to entropy. As one can see in the graph accompanying the article, in the negative temperature region, this derivative gets closer and closer to zero as the energy is increased. (In the graph, the slope is the inverse of this derivative so it goes to infinity.) So adding energy reduces the entropy but increases the temperature, bringing it to a less negative value. As the energy goes to the maximum, the temperature approaches zero from below.
I underlined the last part. I guess the question is if this would be more efficient than cooling the material top down, rather than bottom up with negative-temperature. But, I'm still missing a lot with what this really means.
Heat is an expression of how much energy a system has, as you increase heat disorder in the system increases, as the atoms have more energy to move around. Usually there's an infinite number of energy states for this, so heat and entropy can rise indefinitely. With a negative energy system though, there is not an infinite number of energy states, once you reach the maximum energy state and carry on adding heat to the system, instead of entropy increasing (because it can't) it starts decreasing. So in a negative temperature system acts like heat is being removed when it's actually being added.
So it's like a signed variable in computing. If you increase it beyond it's limit it suddenly turns negative.
The problem with applying it here is that negative temperatures only apply to systems with limited states, such as nuclear spin; a Hydrogen atom has infinite states to occupy, so can't act as a negative temperature system, unless some way can by found to limit it's available energy states. The article seems to suggest that is essentially what the researchers did, whether their method is theoretically practical, or even applicable to this usage, you'd probably have to ask them, but I would doubt it.
Part of the article did mention how the experiment had limited states and that someone is going to carry one out with more controllable states. If the assumption is made that more than one state can be controlled at once, then I think that keeps it as, at worst, semi-hard scifi?
semi hard scifi, wich seems to be Alnair's score, isn't it?
Increasing the density of the reaction mass is a new concept to me, and it's sounds really nice, but I have a [maybe nooby] question:
If you increase the density of something you'll reduce its volume but you'll also increase its mass. If you increase the mass then you'll need more power to move it and so you'll need more reaction mass... so would it be really "cheaper" to reduce the density of reaction mass instead of bringing some "tanks" ?
semi hard scifi, wich seems to be Alnair's score, isn't it?
Increasing the density of the reaction mass is a new concept to me, and it's sounds really nice, but I have a [maybe nooby] question:
If you increase the density of something you'll reduce its volume but you'll also increase its mass. If you increase the mass then you'll need more power to move it and so you'll need more reaction mass... so would it be really "cheaper" to reduce the density of reaction mass instead of bringing some "tanks" ?
No. It's not the mass of propellant that's important, it's the mass of propellant as a ratio to the mass of your payload. Payload here is everything from the engine, to the structure of the ship, tanks, crew etc. Very simply, the denser the propellant is, the less tank you need to store it, the higher your mass ratio, because you're still taking the same amount of propellant, but you have much less tank to lug around. It's not quite that simple, but it's the general idea.
I suspect that Alnair is more interested in the artistic element though, giving a ship that has a reasonable mass ratio without being all propellant tank. Last time I designed a ship using hydrogen as a propellant, it had a mass ratio of 3 (i.e. 3 grams of propellant to 1 gram of ship, which is quite low for a Hard Sci-fi spaceship), it was a long spindly line of tanks with a tiny ship stuck on the end. You could barely make out the ship section if you were zoomed out far enough to see the whole ship at once. The reason is simple, Hydrogen's density is so low you need a lot of tank volume for a relatively low mass of propellant. Up the density you get more propellant per gram of ship (higher mass ratio) and a ship that isn't a giant tank with a tiny crew compartment stuck on the front (more visually pleasing).
Eh, you can allways go with Zubrin's Nuclear Salt Water Rocket for propulsion. Of course, the downside for a combat ships is that if your propellant tank gets hit, you've got a nuclear explosion going off inside of your ship....
And you DO NOT want to point that **** at any real estate that you still want to use afterwards. Unless you LIKE living on glass-lined glow-in-the-dark parking lots...
Antimatter triggered fusion in hydrogen could shrink the load. But, there are gamma ray issues, and that might take the universe undesired places. Think, pea size fusion nukes fired from man portable weapons, with a 100 meter assured kill radius.
Posts
http://www.newscientist.com/article/mg20827893.500-how-to-create-temperatures-below-absolute-zero.html
However, a post in the article I posted does seem to indicate my original thought, that this is a possible way to get solid hydrogen.
I underlined the last part. I guess the question is if this would be more efficient than cooling the material top down, rather than bottom up with negative-temperature. But, I'm still missing a lot with what this really means.
So it's like a signed variable in computing. If you increase it beyond it's limit it suddenly turns negative.
The problem with applying it here is that negative temperatures only apply to systems with limited states, such as nuclear spin; a Hydrogen atom has infinite states to occupy, so can't act as a negative temperature system, unless some way can by found to limit it's available energy states. The article seems to suggest that is essentially what the researchers did, whether their method is theoretically practical, or even applicable to this usage, you'd probably have to ask them, but I would doubt it.
Increasing the density of the reaction mass is a new concept to me, and it's sounds really nice, but I have a [maybe nooby] question:
If you increase the density of something you'll reduce its volume but you'll also increase its mass. If you increase the mass then you'll need more power to move it and so you'll need more reaction mass... so would it be really "cheaper" to reduce the density of reaction mass instead of bringing some "tanks" ?
I suspect that Alnair is more interested in the artistic element though, giving a ship that has a reasonable mass ratio without being all propellant tank. Last time I designed a ship using hydrogen as a propellant, it had a mass ratio of 3 (i.e. 3 grams of propellant to 1 gram of ship, which is quite low for a Hard Sci-fi spaceship), it was a long spindly line of tanks with a tiny ship stuck on the end. You could barely make out the ship section if you were zoomed out far enough to see the whole ship at once. The reason is simple, Hydrogen's density is so low you need a lot of tank volume for a relatively low mass of propellant. Up the density you get more propellant per gram of ship (higher mass ratio) and a ship that isn't a giant tank with a tiny crew compartment stuck on the front (more visually pleasing).
And you DO NOT want to point that **** at any real estate that you still want to use afterwards. Unless you LIKE living on glass-lined glow-in-the-dark parking lots...