Sunday, June 14, 2015

Nanotube memory

Followers of this blog may remember some previous posts I'd made about new non-volatile memory that looked like it could also fulfill the same role that RAM does now.  That is, it's reasonably dense and fast and so could be used as the working memory of your computer but also doesn't lose the information it holds when you turn off the power.

Well, yet another type of memory with these properties has been in the news recently.  The stuff is called NRAM after "nanotube."  The nanotubes in the name are the same carbon nanotubes that people talk about making a space elevator with if they can be made long enough and in sufficient bulk.   People have been trying to make transistors out of nanotubes for a while and they work but there's a big problem with manufacturing them at scale.  You make a big silicon chip with conventional techniques with a bunch of pads on it which you want the nanotubes to stick to.  You wash it with a solution containing the tubes and some bonding agent but you'll be lucky to get a tube at 90% of the places you want them.  That's enough to do experiments but it's still way too high to build any sort of computer out of.  When I first read the EE Times article on this I thought that NRAM would work in a similar way and that they might have manufacturing difficulties.  But then a new article with a bunch of presentation slides came out at Anandtech showing that each connection is made up of several nanotubes so you don't have to have every junction have the same configuration and some variation shouldn't prevent it from functioning.

Then I managed to find a link to an actual PowerPoint presentation form a Real World Tech forum thread.  It has a lot of details, including the actual current defect rate of .00003 which ought to be fine.

So how does this stack up against the other potential non-volatile memories people have been working on?  Well it does seem to be pretty tough.  There are actual molecules being shifted into different configurations so it's resistant to electrical damage which is why NASA was interested enough to fly some up to test it in space.  Cosmic rays flipping bits is a big problem for computers in space and even if NRAM doesn't have anything else that might be a niche for it.

The write endurance also seems to be pretty good, just like with RRAM.  With at least 10e11 writes you could be writing across typically sized RAM card at typical max speeds and it would take you a millennium to exhaust the writes.  Even if you write to the smallest area that won't be absorbed by the cache system on a modern CPU you've got a year of writing before you burn a hole there.  For typical usage I'd guess it would last something like 20 years which ought to be good enough.

Which of this and MRAM and RRAM will win out if any does?  I've got no idea but I'm finding it interesting.

Insurance and Driverless Cars

There was an article in IEEE Spectrum recently about insurance and driverless cars.  It was basically talking about how you'd expect the manufacturer of the driverless control to take over the job of insuring the cars when they operate in driverless mode because they're big enough to and they have more information than anyone else.  The price of this would probably be part of the cars price.  It occurs to me that if insurance for a car's automated and manual modes become separated and if the automated insurance is bundled then this will be a force pushing for 100% automated roads if driverless cars take off.

Now, clearly this would be a major hit for the insurance industry and all those people whose livelihood depends on insuring vehicles are going to be putting pressure on politicians to stop these forces from affecting them somehow.  I'd expect a major push to require all cars to have manual driving insurance whether the car has a manual mode or not.  But after that I'd expect insurance companies to offer cheaper insurance for those cars and there to be a race to the bottom that makes the whole issue nearly moot.  Or quite possible there's some side of this that I'm not seeing.

It's Alive!

Remember Philae, that robotic probe that the Rosetta mission dropped onto that comet?  It turns out that harpooning comets is tricky as this rather cute xkcd comit sequence explained and Philae bounced a few times after landing and came to rest in a shadowed cleft where it couldn't power itself (pictures of journey and cleft here).  Scientists had hoped that as the comet got closer to the sun the increased light would be enough for Philae to power itself up and resume communications.  Luckily that's exactly what happened.  It looks like Philae is working well enough to give us all the measurements we wanted as it passed near to the sun.

Sunday, June 7, 2015

Seveneves and the Roche limit

For an author it's important to get things that the reader might find hard to swallow out there and dealt with as soon as possible.  I didn't really enjoy Neal Stephenson's last book, REAMDE, past the halfway point because too many improbably occurrences had piled up and my suspension of disbelief didn't recover.  By contrast his newest novel, Seveneves, seems to be doing an excellent job of getting the improbable stuff dealt with quickly and I've been enjoying the book without any hangups.  I've only gotten through chapter 7, acknowledged, but I've got a feeling I'll continue to enjoy this one.

But of course the second improbable thing gives me a chance to talk about some physics I find interesting so I'm going to dissect what I think Stephenson gets wrong.  Not because I think the author is a bad person or wrote a bad book but just because I think the physics is nifty and reading this prompted me to share it.

The basic setup of the book is that the moon is hit by some high energy cosmic event that basically blows it up.  That's improbable thing #1 but hey, we haven't totally figured out physics yet.  After that the moon breaks into seven fragments that go into orbit around each other in a cluster at several times the original diameter of the moon.  The periodically collide with each other and break into more and more fragments.  Our protagonists figure out that eventually they'll break up so much that they'll get away from the old lunar orbit and either collide with Earth or form a new ring.  That's improbable thing #2 and also the driver of the plot.

There are a few things wrong with that but they'll take some explaining.  The first is that size matters and large objects don't behave the same way as small objects.  Way back in 1638 Galileo wrote Discourses and Mathematical Demonstrations Relating to Two New Sciences which is the first I've ever heard of anyone talking about this principle.  Galileo observed that small animals like cats can survive falls much better than large animals like horses even over a couple of meters.  The thing is that a things strength increases in proportion to its cross section but its weight increases as its volume.  So as an animal's length increases its ability to support itself against falls decreases as its length squared over its length cubed, or just inversely proportionally to its length all things being equal.  That's how an ant, say, can lift 40 times its body weight but a human could never do that.

This applies to structures and moons as well as animals.  If you're familiar with 10 cm cubes of stone and want to think about how 1 m cubes behave just pretend that the larger cube is like the smaller but with only 1/10th the strength.  Stone is pretty strong but a humongous cube wont be able to support itself.  The edges will fall off under their own weight and you'll have a pile rather than a cube.  The radius of the moon is over 1,700,000 meters so it would be incredibly fragile if you did something like bring it to the surface of the Earth.  A merely mountain sized mass of rock would crumble into a pile.  Something the size of the moon would flow like water.  That's why all celestial bodies over a certain size end up as spheres rather than the bumpy shapes of smaller moons and asteroids.

So if you were to hit the moon with a big jolt and cause it to break up it wouldn't break up into 7 pieces that would then maybe split in half if they hit each other.  It would be more like the breakup of a mass of dust.

This sometimes goes against our intuitions.  I know I've read 9/11 truther websites where they say that the twin towers can't have just collapsed because when a 10 story building collapses in an earthquake it doesn't look anything like what happened to the World Trade Center.  But really the Twin Towers were 10 time taller and so only one tenth as able to withstand their own weight once it had been unleashed on them and it's no surprise that they could be reduced so completely to rubble.

The other problem is the way the moon overcomes its own gravity when it fragments too far.  When you've got a moon orbiting a planet you've got its own gravity trying to keep it together but you've also got the tidal forces of the planet's gravity trying to pull it apart.  A moon's gravity is just as strong no matter where it is but the planet's pull on the moon and the resulting differential get stronger the closer a moon is to its planet.  There's this thing called the Roche Limit that tells you when a moon is far enough away to stick together and when it's gotten so close that it is pulled apart into a ring system.  And the Earth's moon is way, way out beyond that limit.  It's about 500 km for the Moon when the Moon's actual orbit is way out at 400,000 km.

If you've got a few big hunks of moon orbiting their mutual center of mass then there isn't an obvious path from that to their fragments escaping that orbit.  Now, when rigid objects hit each other and shatter you often have pieces fly off going faster than the objects that collided.  But again that tends to happen with hard objects rather than the effectively very soft objects that the moon chunks would be.

So basically I don't buy the setup in Seveneves.  But it's ok since it's something I swallowed at the start and the rest of the book is good enough to make me forget it so far.

UPDATE:  Finished the book and I really enjoyed it up 'til the timeskip at the 2/3 point and enjoyed it enough after.  I think the book would have been better with an Ender's Game style "And this is what happens in the future" last chapter instead of that last third but I'm still happy with the book overall.

We're trying to make our SARS-2 tests better than we should

Ok, that's a somewhat provocative title but I think it's basically accurate.  During this pandemic the US in particular has had a pr...