How long do you need for a party balloon to stay filled? A normal party balloon will hold hydrogen just fine if the relevant timeframe is on the order of a few days. How do I know? I run an organic chemistry laboratory. My students use normal party balloons (much cheaper than balloons sold by lab supply companies, but equally effective) to set up reactions in a hydrogen atmosphere. Place the reaction flask under vacuum. Backfill the flask by connecting a balloon that was filled from a hydrogen tank. Voila... a reaction under an atmosphere of hydrogen. If everything is well sealed, then the party balloon will not leak an appreciable amount of hydrogen in a day. Mind you, these are party balloons being exposed to harsh organic solvents. In a typical home environment, they should hold up even better.

The power of the plutonium RTGs continually declining is one issue, as already noted. Another issue is the finite amount of hydrazine on board for what little maneuvering may need to be done. See the last paragraph of this page and this article.

The problem with this argument is that it assumes there is but one government. Why shouldn't a small nation somewhere choose to offer financial advantages in order to lure business to its locale? (Hint: Several already do.) Why shouldn't this extend potentially to anonymous currency? If there is enough benefit to the nation (infusion of capital, prestige, whatever), then it just might happen.

In case anyone is looking for the aforementioned Youtube video of the Sousa march: http://www.youtube.com/watch?v=-rNQFUqt49Q

You don't need to see the sun to track it. Its location is predictable, so you only need to know your current location (easily determined by GPS at installation) and the current date and time.

If the matter is that dense, then it may be the size of a portable device but it still wouldn't be the mass of a portable device. Your basic assumption is that you need x amount of material to detect, and so if you pack the same amount of material tighter you should be able to detect in less space. That seems reasonable to me, but packing into less space won't reduce the mass any.

Furthermore, the store acknowledges that returned iPads appears to be the way the clay fakes got there in the first place. It says so right in the summary.

Oops... messed up the number of dimensions in my estimation. Frankly, that type of error in mental arithmetic could happen with any unit of measure, metric or not.

One square foot packed from floor to ceiling is about one cubic meter. One medium-sized bedroom is about 100 square feet (maybe 120, but close enough for this purpose). So if you packed your stuff from floor to ceiling with no packing materials and no waste of space, could you fit it in half of a bedroom? If so, you've got under 50 cubic meters of stuff. If not, you've got more.

I am all for space exploration, but we know about all isotopes from right here on earth. From right here on earth, we can study stable isotopes, isotopes so light that their half lives are fractionths of a second, and isotopes so heavy that their half lives are fractionths of a second. Isotopic abundances will vary by location, but the properties of the individual isotopes will be the same.

Historical definitions and modern definitions have little in common. The historical definition of the meter was 1/10,000,000th of the distance from the equator to the north pole along a certain longitude line. The modern definition of the meter is based on the distance a certain wavelength of light travels in a perfect vacuum in a measured amount of time. That's (indirectly, through the meter) the modern definition of an inch, too. It just happens to be a different measured amount of time. See http://www.npl.co.uk/reference/faqs/on-what-basis-is-one-inch-exactly-equal-to-25.4-mm-has-the-imperial-inch-been-adjusted-to-give-this-exact-fit-and-if-so-when-(faq-length)

An inch is defined as exactly 2.54 centimeters. I don't know how much more based on SI units it could be. It's not really any different from how we define minutes, hours, days, and years. Those aren't all factors of 10, either.

Interesting. I just learned tonight from reading /. that I have not been doing proper science this whole time. My notebook of organic reactions is full of measurements in minutes, hours, and days. In over a decade of organic chemistry research, measurements recorded in seconds account for under 1% of my time measurements. Indeed, my raw time data usually takes the form of hours and minutes from a clock. Quick... how many seconds into the day am I at the exact moment when the second hand reaches 12 o'clock (when I usually do something if I want high precision in my timing) at 15:47? Equally importantly, why should I care?

That's silly. We all know that heat rises. It's true that you want to cut a hole in the floor, but it's to get the cold air. (Don't worry... that heat engine will still work.) You need another hole in the ceiling for the heat to escape. Straddle the hole and hold your laptop right there, and you'll get the best cooling. If you don't believe me, just try it!

Left foot on clutch. Left side of right foot on brake pedal. Right side of right foot partial throttle. Let off clutch pedal far enough for the clutch to bite and transmit enough power to have the car straining forward just a bit. Roll right foot gradually off the brakes and onto the gas while continuing to let off the clutch. With just a little practice, you can pull this off with _zero_ backward rolling. You can do it quickly to get a fast start, or slowly to get a nice slow roll.