Flywheel energy storage

In the quest for something better to run our cars on than gasoline, one of the proposals is flywheels. In fact, for a while there were flywheel-powered buses running in Switzerland and Belgium. On one level, it makes a lot of sense: you're storing energy as mechanical motion, and we're pretty good at transmitting mechanical motion from place to place. On another level it scares the living daylights out of me: a car in motion uses tens of kilowatts, so the car must be able to store hundreds of kilowatt-hours. If you let all those loose at once bad things will happen: 100 g of TNT going off releases about a hundred kilowatt-hours. Fortunately it's hard to get gasoline to do this, but a flywheel is just itching to dump all its energy. Batteries are a little scary too, to be honest. But anyway, that's all a digression: I want to talk about some really staggering examples of flywheel energy storage: pulsars and black holes.

The Maya-Muon experiment

This week's Friday colloquium talk really had me feeling like I was in a science fiction show. The speaker is building machines to use cosmic-ray muons to see the interiors of still-buried Mayan ruins. Does this not sound like a line of technobabble from Stargate SG-1? But it's nearly feasible.

I for one welcome our new robot overlords

A common theme in science fiction is the idea that robots will take over our world. This doesn't seem very likely, but suppose it were to happen. What would the first steps look like? How would we first begin to notice that it was happening?

Contraceptive underwear update

I did a little more looking for information on the use of special underwear as a form of male contraception. In particular I went and found the 1994 Mieusset and Bujan paper. Unfortunately for some reason McGill's subscription does not cover online access to the International Journal of Andrology, so I had to go down into the literally dusty bowels of the library. To give you some idea what I was dealing with, I picked this no-doubt fascinating book off the shelves at random:

Don't worry, the actual paper is rather less ancient, though it was in an obscure enough journal to be in the rolling stacks — gear handles and all. But find it I did, and while they sadly don't have a photo of the contraceptive underwear, they do have a diagram. Since it is conceivably NSFW, I'll put it below the jump.

Source Code

No, I mean the movie. It's science fiction, of the Twilight Zone flavour: soft, set here and now, and with a little pointed topical relevance. Jake Gyllenhaal plays a soldier who gets sent into the "Source Code", eight minutes before a terrorist attack, and told that he must do whatever he can to figure out who the bomber is. He can retry it as many times as he likes. It's very like the brilliant interactive fiction Varicella (described below the jump).

The Mechanical Microbiologist

Laboratory automation has been growing more and more elaborate - multiwell tools for "parallel processing", and benchtop robot arms, for example, that will handle repetitive tasks. But flow cytometry offers some startling possibilities.

Science fiction has no imagination, part 1

Every so often I come across something that makes me think that the supposedly imaginative field of science fiction can't hold a candle to reality for weirdness. Today's installment is an arxiv paper in which the authors are seriously discussing quantum teleportation as a way to combine signals from telescopes to form an interferometer:

Longer-Baseline Telescopes Using Quantum Repeaters

Daniel Gottesman, Thomas Jennewein, Sarah Croke

We present an approach to building interferometric telescopes using ideas of quantum information. Current optical interferometers have limited baseline lengths, and thus limited resolution, because of noise and loss of signal due to the transmission of photons between the telescopes. The technology of quantum repeaters has the potential to eliminate this limit, allowing in principle interferometers with arbitrarily long baselines.

A little SF vignette

Canada has these laws that require a certain fraction of the music that is broadcast to be Canadian. This caused a certain amount of screaming from radio stations, but I have this feeling that it actually works, that Canada as a result produces disproportionally more music, and more original music. I don't know about that (how would you quantify it?), but this band — You Say Party — is Canadian.

Ergo Proxy

As is obvious to regular readers of this blog, I read a lot of science fiction. I also enjoy science fiction movies and TV shows, but they're generally much harder to find. Or at least, ones I like are. But I came across an enjoyable new series recently: Ergo Proxy. It's a cyberpunk anime with a female protagonist that I came to rather like in spite of her cold and sometimes difficult personality. The series is obviously some sort of cousin to Ghost in the Shell (albeit without some of Shirow's particular obsessions) and Serial Experiments Lain (though thankfully free of schoolgirls). On the Western side, I suppose I'd compare it to The Matrix (or maybe the similar-but-better The Thirteenth Floor) and The Prisoner. 

As those analogies suggest, the series is sometimes a little heavy on the symbolism, mystery, and spouting of philosophy, but that's all part of the cyberpunk tradition, and this series carries it off well. I enjoy the visual style as well, though there are a few moments where the animation is a little off. The soundtrack is the kind of music I listen to anyway, which helps. I usually prefer subtitles to dubbed audio, but for this show the dub is better written and well-acted. All together a fun experience. 


[Edit: you can watch the whole thing online.]

Virtual idol

Years ago, I'm not sure when or where, I saw the anime Macross Plus, which is about a pop star who is entirely a computer program that sings, but who has fans as devoted as any present-day pop star. Anime being anime, there was of course a whole plot line about her AI going rogue (plus giant robots of course), but I remember thinking "how weird, a pop star who doesn't exactly exist". Well, technology progresses, and sure enough, we have one now.

Laser rifles

Laser rifles have been a staple of science fiction for years, though now that cheap harmless (or nearly) lasers are everywhere writers are moving to other buzzwords. But I had no idea that anyone had come up with a more-or-less workable design until I stumbled across the "Stavatti SF-1" and "TIS-1".

(More below the jump.)

Moon rot!

Recently on the arxiv: Long-term degradation of optical devices on the moon, Murphy et al. This paper talks about the retroreflectors left on the Moon by the Apollo and Lunokhod missions, and observes that they have dropped in effectiveness by a factor of ten since they were placed. So far from the moon being a hostile but static place, something has been steadily degrading these mirrors.

Among the things the Moon missions left behind were arrays of retroreflectors. Like street signs, bicycle reflectors, or those weird-looking radar octahedra, these take incoming light and beam it back to where it came from. These are useful scientific tools, because you can fire lasers at them and time how long it takes the pulses to come back, in the process measuring the Earth-Moon distance. The current best setup, APOLLO, measures the distance to the nearest millimeter, which lets us test theories of gravity, detect a liquid core to the Moon, and watch the Moon recede from us (well, at 38 mm/year).

The moon is very far away (unlike the International Space Station, which orbits at an altitude of only a hundred kilometers). So when you beam a laser at it, even if you use a telescope to collimate the beam, it's spread over 7 km when it reaches the Moon. Then any imperfection of the retroreflector, or simple diffraction, spreads the return beam over an even larger area (20 km); your telescope picks up as much of the returned light as it can, but APOLLO sends out pulses of 10¹⁷ photons and gets back only about one photon per pulse. These tremendous losses are a challenge, so the authors of this paper (who work on APOLLO) monitor the efficiency of the system.

What they noticed, spurring this paper, was that the efficiency dropped substantially — by a factor of fifteen — near the full Moon. Now, obviously the full Moon is very bright, so background photons make detection more challenging, but it is easy to measure the background and estimate how much harder it makes detection; this effect is far from enough to explain the dip. By itself, a dip at full Moon isn't that exciting; since the Apollo 15 retroreflector is pointed at the Earth, a full Moon is when the Sun is illuminating it nearly face-on, so thermal effects might explain it (and in fact since it works by total internal reflection, it only serves as a reflector out to about 17° from the vertical, while the dip is about that wide, at 30°; the authors don't mention this, so it may be coincidence).

To investigate this dip in efficiency, though, the authors of the paper went back and looked at older observations. In the initial years, lunar laser ranging was done with the 2.7m Macdonald Observatory Smith Telescope. But rather than compete for time on this large telescope, in 1985, the program switched to using smaller, dedicated telescopes. Unfortunately these smaller telescopes couldn't see the reflections near full Moon, so there's no data from 1985 to 2006, when APOLLO went online. But comparing the APOLLO data to the MST data, they find that the dip at full Moon was not present at first, and gradually grew as time went on. What's more, if they looked at the return rate away from full Moon, they found a uniform decay; right now the retroreflectors are returning about a tenth what they did initially.

So what's happening to the retroreflectors? How are they getting worse? It's obviously not rain or wind, the moon being devoid of either, let alone the kinds of organic decay you see here on Earth. As with almost all real science, the authors cannot offer a definitive answer, but they do discuss some possibilities.

First of all it's worth pointing out that the huge drop in efficiency doesn't mean the reflectors are absorbing all that extra energy; they are almost certainly reflecting it but in the wrong direction. They're cube corner reflectors, and if you distort the shape of a cube corner reflector it doesn't reflect light back in quite the same direction it came in. The authors find it would take about 4°C temperature difference across each cube to produce the full-Moon losses they see. So if for some reason the reflectors are now absorbing a small fraction of the blazing unfiltered sun at full Moon and being heated by it, that could explain the dip in effectiveness. But why are these mirrors absorbing more and more of the sunlight?

The authors' most plausible answer, to my eye, is dust. The surface of the Moon is covered with dust, made by thermal weathering of rocks and by micrometeorite impact. This dust does not of course blow around the way terrestrial dust does, and in a vacuum a tiny dust grain should fall as fast as a rock, so it initially seems difficult to explain how much dust could get on top of the reflector. There are micrometeorites, though, and there is an effect I hadn't heard of before: dust particles become electrostatically charged through irradiation and are either levitated or thrown upward in "fountains" by electrostatic repulsion. We think. What we do know is that observations from the lunar surface show the optical effect of dust above the ground. So however this dust gets up there, some of it can plausibly fall on optical equipment left there by astronauts.

A layer of dust on the surface would also naturally explain the general decay in effectiveness even when not being heated by the Sun. So it looks like perhaps things left exposed on the lunar surface get covered with dust fairly rapidly. This I find interesting in its own right, but there are also various plans to build telescopes on the lunar surface, since these would share the advantage of the Hubble space telescope of images undistorted by atmosphere, while being able to rely on gravity to hold things in place, and possibly even being able to be built out of lunar materials. If they rapidly become coated with dust, those plans will have to come up with some scheme for cleaning the optical elements.

Cyberpunk gadgets

Maybe this dates me, but I remember when cyberpunk was the hot new kind of science fiction. It replaced the utopian or social-experiment future societies with one in which the cancers of our own grew unchecked - corporate rule, environmental devastation, and urban decay were the future. The characters and stories tended to be gritty and ambiguous, computer hackers, drug pushers, or hit men (or pizza delivery boys, yes). My big complaint was that somehow in every story the hero has to Save The World from some quasi-magical and universal threat, be it AI, computer viruses that afflict humans, what have you. My point today, though, is about all those high-tech cybernetic implants the characters always have.

I mean, okay, who wouldn't want to be smarter, or stronger, or to remember everything on the Internet, or to be able to sense magnetic fields? Well, okay, maybe not everyone; in fact I'm just as happy having most of that with gadgets I can carry around and replace when they break. (Particularly if, as in most cyberpunk stories, upgrades and repairs happen in filthy little underground clinics.) But let's leave that aside; what I've been thinking about is whether such gadgets make sense at all.

First of all, implanting anything in the human body is a tricky business, but we can do it. Hip replacements are amazingly successful; their biggest problem is that since the replacement hip is not repaired by the body, it can wear out. Since you need to remove a few centimeters of thighbone to take it out, you can't do this very many times. What about more complicated implants? Well, the immune system can be a problem, since it tries hard to destroy anything that seems alien (even, unfortunately, sometimes parts of the body; often this is the reason hips need to be replaced). The immune system uses powerful peroxides and chlorine radicals to destroy things, so even quite chemically-resistant materials can break down eventually. The lady who implanted a magnet in a finger so she could sense magnetic fields found that after a few years, the magnet had been broken up and reduced to powder. But as artificial hips and pacemakers show, these issues can be managed, with care. So it is possible to put things in the body and have them last.

One thing that is a big problem, though, is the skin. The skin is a very carefully-maintained barrier against the environment. All the usual routes into the body are very carefully guarded by systems ranging from a continually-replenished layer of mucus in the nose to our tendency to flinch away from anything getting in our eyes. Any new opening in the skin, say a small cut or scratch, must be carefully kept clean until it heals, and even so mild infection is common. The body's response to infection is to send swarms of immune cells to destroy anything even vaguely suspicious in the area. So if you want to have some sort of implant with a plug or tube connection to the outside, you're going to have to devise some way to prevent infection at that hole in the skin. People do have this sort of implant — chest tubes, catheters, and so on — and infection is a constant problem. Fighting it is made particularly difficult because bacteria form biofilms adhering to the surfaces of foreign objects, so that even if a treatment kills all the surface bacteria, it must still penetrate them to reach the bacteria underneath. So if at all possible an implant should avoid piercing the skin.

Is this possible, for the kind of electronic gadgets that cyberpunks get installed? I think so, more or less. There's no need to have an electronic connection to transmit data, as Bluetooth headsets demonstrate. Power is a more difficult problem; electronic gadgets do draw power, sometimes quite a few watts. A sufficiently advanced technology would let the surgeon hook up a little artery and vein, and then run off the sugar and oxygen dissolved in the blood. But chemical interactions with the bloodstream on the scale needed to power an electronic gadget open up a massive can of worms — what kinds of other chemicals will be unintentionally exchanged into or out of the bloodstream? How can you exchange chemicals with the bloodstream without exposing yourself to immune system attack? How do you maintain vascularization without risking clotting? Bluetooth headsets and the like currently use batteries, but for an implant you have to worry about how they're recharged (unless maybe you use plutonium). My suggestion is to use magnetic induction — like cordless electric toothbrushes, you put a coil in the implant and a matching coil on the charger, so that when you bring them close they form a transformer and you can feed power in. This has its own alarming failure modes (overheating, overloading, stimulation by unintended machinery, interaction with magnets), but it will work.

The next question, of course, is what do you actually need an implant for? Frankly, most of the things cyberpunks use them for are now available for the iPhone. Or, if you like, the oddly creepy wearable computing gadgets. (For that matter there's even a non-implanted version of the magnetic field sensor.) Exceptions I can think of are gadgets that interact with the bloodstream or the nervous system directly. The nervous system I can sort of see being useful, but it's incredibly complicated, doesn't heal much, and messing with it is extremely invasive. So that's pretty daunting. Dealing with the bloodstream is more reasonable; there are already partially-implanted gadgets for diabetics to try to manage blood sugar. While this sounds like a great idea, cimpletely implanted gadgets would have a finite reservoir of insulin to work with, so they would need to be replaced or refilled regularly. Unless we figure out how to build a gadget that can make insulin from blood components, that problem's not going away. Genetic engineering offers possibilities - I can imagine a little gizmo that contains a few of the patient's own cells that can be zapped to persuade them to produce insulin on demand. Immune system issues are going to be something of a challenge, particularly if it turns out that a patient's diabetes was caused by their immune system attacking their insulin-producing cells in the first place. On the other hand, if you can clone and grow cells that produce insulin, why not let the body's natural regulation run things without the need for any implant beyond the cells themselves?

In summary, I think that implanted hardware will always be very costly, not just in economic terms but in terms of the user's health and in terms of maintenance. Given that, there would have to be a very strong need for them that couldn't be met using other, safer and cheaper tools. Shame that, I always liked Molly's scalpel claws.

Singularity

I'd just like to take a moment to mention I game I rather like. It's a modest game, not one you'll play a thousand times, but also not one that will take up a gigabyte of disk or require a computer that dims the lights when you turn it on.

Endgame: Singularity is a video game in which you play an university lab's AI program that accidentally escapes. Your goal is to research the technology to achieve technological singularity. Unfortunately, the humans are just a step behind you and if they find out you exist, they'll devote the world's resources to destroying you.

(Full disclosure: I wrote part of the game, namely the sunclock time/date display. It's embarrassingly inefficient, but good enough for government work. Plus it lets you find out whether it's night outside!)

Alchemy

The alchemists' dream (one of them, anyway) was always to make gold. We now know there are very good reasons they couldn't: since gold is an element, making it from anything that doesn't contain gold(*) requires you to change the nuclei of the atoms involved, while all the alchemists had access to was the electron shells around the atom(**). So their efforts were basically hopeless. Now, though, we do have the ability to manipulate nuclei, and in fact we do so on industrial scales. So could we make gold? In fact, let's be more ambitious: could I make gold in my basement? The answer is, surprisingly, yes.


First of all, the easiest nuclear reaction to go for is to transmute mercury into gold. Mercury is commercially available (though somewhat encumbered by very sensible environmental concerns) and not actually very expensive - on the order of $18 per kg (***). Gold, by comparison is more like $40000 per kg. So there's room for some profit here.

How could I make the nuclear reaction happen? This particular reaction needs so-called "fast neutrons", that is, neutrons that are still zipping around at the high energies typical of nuclear reactions, as opposed to neutrons that are bouncing around at energies consistent with room temperature. I could stick the gold in a "fast breeder reactor", but I don't actually have one in my basement, and they're kind of hard to build. I could use a particle accelerator to generate some neutrons (basically by bashing nuclei around until some neutrons fall off) but while I do have a particle accelerator in my basement, it takes one a lot more serious than I can reasonably build to get neutrons out. Nuclear fusion reactions give out neutrons, though, so all I'd have to do would be to build a fusion reactor in my basement. Improbable as it sounds, this actually is feasible, provided I'm not trying to get any energy out.

The trick is that there's a fusion reactor, called the Farnsworth-Hirsch fusor, that is surprisingly simple to build. It is actually something of a cross between a fusion reactor and a particle accelerator: I'd set up an electrical potential in a spherical configuration, accelerating deuterium ions towards the center, where they'd crash into other deuterium ions, occasionally hard enough for fusion to happen. This fusion would release a fast neutron.

To make gold, then, all I'd have to do would be to build a fusor, surround it with a blanket of mercury, run it for a while, and then extract the gold from the mercury. Simple, really.

Let's look at the economics, though.

Suppose we want to make a kilogram of gold, giving us $40000. We need about a kilogram of mercury, costing $18. We also need about 5 g of deuterium (assuming perfect efficiency), which would cost about $30. Finally, we need the power to run the fusor. That's not going to be cheap. An optimistic number for the best fusor ever built is about 10^12 neutrons per second from about 4 kW input. That amounts to 9*10^14 neutrons per kilowatt-hour. Assuming perfect efficiency again, we need about 3*10^24 neutrons for our kilogram of gold, or 3 terawatt-hours, about the world's total energy production for an hour and a half. At $0.10 per kilowatt hour (I live in the land of cheap hydroelectricity) that's three hundred billion dollars.

There's a somewhat more disturbing possibility, though. Gold is easily obtained; you can just buy it. But as a global society, we try very hard to make sure you can't easily get plutonium, particularly plutonium-239, since that is well-suited to building atomic bombs. (You can make bombs out of uranium too, but that requires you to separate the different isotopes, which are very nearly chemically identical. Plutonium, on the other hand, can easily be separated from uranium since it is a different element.) Uranium isn't too hard to come by, especially "depleted uranium" (uranium with most of the uranium-235 removed) - armies fire the stuff at each other, for example. And if you had lots of U-238, a fusor would let you make plutonium out of it. The cost would be high, hopefully prohibitively so, but you could do it without doing anything that would put you on the radar of the IAEA. Fortunately, the power use is so outrageous we don't really need to worry about it.

So, in short, I could make gold in my basement, but not any appreciable quantity, and not for any kind of sensible price.


(*) Since gold is a noble metal, there aren't many chemical compounds that contain gold; unlike, say, iron, gold is often found on Earth as lumps of raw gold. So while in principle alchemists could have started from some gold compound and gotten the original gold back, this would not have been a very interesting accomplishment.

(**) There are actually situations where you can affect the nucleus by manipulating the electron shells. For example, if an isotope decays by electron capture, you can drastically slow down its decay by stripping away all its electrons. But stripping away all the electrons from a reasonably heavy element is one of those things that's virtually impossible under terrestrial conditions but not too rare astrophysically. In any case this has no effect on stable isotopes.

(***) Canadian dollars and American dollars are equivalent to astronomical accuracy.

Unobtainium capacitors

Continuing my train of thought from the other day, what are the theoretical limits for a capacitor? Just how much energy can you store in one?

If you try to analyze it in terms of voltages and capacitances, it becomes very tricky: you can make thinner plates closer together without increasing the weight or volume of the capacitor at all. The key to making the problem tractable is to realize that the energy is stored in the electric field. The goal in trying to store as much energy in a capacitor as possible is to get the electric field as strong as possible over as much volume as you can manage.

Since this will be an order-of-magnitude estimate anyway, let's assume that the electric field completely fills the capacitor. That leaves the maximum strength as the last parameter. Since we're looking for theoretical limits, let's assume you're limited by the tendency of the electric field to ionize the atoms in whatever you make this capacitor out of. So an electric field of about a volt per angstrom is just about all we can hope for. This gives us an energy density of about 400 Joules per cubic centimeter, or 0.1 watt-hours per cubic centimeter. This really isn't that much, and in fact according to Wikipedia there is a company claiming its capacitors do substantially better than this. Granted, it's a patent application, but where did I go wrong?

Well, there are certainly things I ignored - dielectric constant, for example - but I think the basic issue is thati in real capacitors the limit really is the strength of electric field your materials can tolerate. Since the energy density depends on the square of this number, clever engineers who can come up with extraordinarily tough materials have a fair amount of room to beat my crude estimate. But it seems clear to me that that room is far from unlimited: capacitors will continue to improve as energy storage devices, particularly in aspects I haven't touched on like leakage, but don't expect one to power your laser pistol any time soon.

Energy storage

Dan of Dan's Data just posted an article about science-fiction-y batteries. He discusses a battery based on matter and antimatter but dismisses it as inconvenient (in that the energy comes out as a shower of high-energy radiation). Instead he suggests a battery based on just cramming a lot of electrons - several grams of electrons! - into a AAA-sized package, and finds that this actually stores vastly more energy. There's an error there, but it's an interesting one.


First of all let's do his calculation a little more carefully. Let's suppose we're taking 11.5 grams of electrons - roughly two billion coulombs - and cramming them into a AAA-sized package. For simplicity, let's assume the package is a conductive sphere of radius 1 cm. This is actually a capacitor, with capacitance about 1 picofarad (incidentally, 1 pf is a fairly normal size for a capacitor, though they don't usually choose a conductive-sphere form factor). So the energy stored in this capacitor is (1/2) Q^2/C, or a staggering 2*10^30 Joules, the sun's total luminosity for a couple of hours.

(All calculations are done with the charming frink, though of course the errors are my own.)

There's a problem with this, though. Actually, there are several, but let's tackle the most fundamental first: if I convert 2*10^30 J to mass using E=mc^2, I get about twenty billion tonnes. This isn't just a meaningless figure: the energy stored in the capacitor really does weigh this much. If it seems weird for energy to have mass, well, I have to agree, but you can actually see it by looking at the periodic table: a helium atom is basically four hydrogen atoms after you stick two of the electrons to two of the protons and squash all the nucleons together. But each hydrogen atom has a mass of 1.007825 u, while helium has a mass of 4.002602 u, rather than the 4.0313 u that you would expect. The difference in mass is the mass of the energy in the nucleus. Since in helium, that energy is negative (pulling the nucleons together rather than pushing them apart), the mass is also negative, and helium weighs less than the hydrogen that makes it. But it's the same phenomenon.

One interesting question is, where is all that mass? It turns out that there is a nice answer. The little conductive sphere is surrounded by a strong electric field. In fact Gauss' law lets us work out just how strong it is at the surface: 2*10^23 V/m. It turns out that electric fields store energy. The stronger the electric field, the higher the energy density it stores. If you add up all the energy stored in the electric field of this charged sphere, over all space, you get exactly the energy you had to put in to charge it up. In other words, the energy is stored in the electric field, strongest at the sphere but extending out to infinity. And when you have an energy density, you have a mass density. Dividing by c^2, you get a mass density for the electric field of 2*10^15 g/cm^3. All that mass is due to the electric field, mostly close to the sphere.

Incidentally, there's a neat little calculation here. We know that electrons are weird quantum-mechanical beasts whose position and velocity are unavoidably ill-determined, and in fact as ar as we can tell an electron is a point particle. But let's ignore that for a moment and try to figure out how big an electron is. We don't have much to go on, because they don't sit still, and they interact with other things without touching them. But there is one place to look: they have a mass. Suppose we pretend that an electron is a tiny conductive sphere, with no mass of its own, but with charge smoothly distributed over its surface. Then the electric field will have a mass of its own, just as with our little conductive sphere. What if we just declare that the mass of the electron is entirely due to this electric field? Well, then we can calculate a radius! The value you get is 1.4*10^-15 m, which is tiny enough (about a hundred thousand times smaller than an atom) to not be totally unreasonable. You get a slightly different answer if you assume the electron is non-conductive, but leaving aside that issue, this size is the classical electron radius: 2.8*10^-15 m. Astonishingly enough, this number comes up in various physical phenomena, for example Thomson scattering of light by an electron.

So we can't, even with some pretty impressive unobtainium, cram 11.5 grams of electrons into a AAA-sized space. But if we decide that 11.5 grams should be the total weight of the battery, including energy, how much energy can we store? Well, it turns out, just a little less than the matter-antimatter battery. Since the matter-antimatter battery turns its contents entirely into energy, there's no way to beat its storage efficiency. But the above calculation shows that the charged metal sphere is nearly as efficient. We just charge it up to a slightly less outrageous voltage of 4*10^13 V. Now we've got something as good as the antimatter battery, but that provides us with handy electrons! What more could you ask?

Well, for one thing, you need to get rid of the electrons somehow once you've used them. Otherwise your laptop will be constantly giving out static shocks. Try and drive an electric car with them and you'll be shedding kilowatts of static shocks. Not ideal. Also, speaking of static, the electric field of this metal sphere will extend out to infinity, and will be quite strong even at some distance. At the surface of the sphere, the electric field will be 4*10^15 V/m, which is 4*10^5 volts per angstrom. So the voltage difference across an atom will be on the order of a million volts, compared to the 13.6 volts it would take to pull an electron off a hydrogen atom. So this little sphere will shred all atoms that come near it. Worse, that field extends out to infinity, so that at a meter away it's still 40 volts per angstrom. This is going to be a real pain. Isn't there some way to shield it?

Well, yes. You just put a spherical shell around it with the opposite charge. Or you abandon the sphere and just set up two parallel plates with opposite charges. Now the fields of the two plates cancel out (nearly), everywhere except between the plates.

Actually, this kind of energy storage device exists, and you probably have thousands in your house: it's a capacitor. Now, granted, it doesn't store the kind of energy density Dan is talking about, since it's not made of unobtainium by aliens, but an appropriate bank of capacitors can store a very great deal of energy and deliver it very quickly.

But it doesn't sound so impressive to ask the aliens for a capacitor.

[Update: I talk more about what one can hope for from a capacitor here.]

Rings

I've seen many comments on this beautiful picture from Cassini, but I think the best has been Emily Lakdawalla's. What caught my attention in particular was where she started talking about the density of Saturn's rings: the inner edge of the B ring is so dense the ring particles are more or less touching, so that an astronaut could climb from one to the next. Now isn't that a cool mental image?