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jamesough

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Here's an intuitive explanation for L1:

Imagine you have two planetary bodies fixed in space. A free mass is attracted to both, but it's possible to find an unstable equilibrium somewhere between them. Put the mass here, and it'll stay still-- but it's like balancing a ball on the top of a sphere; it'll roll off towards one of the two planets unless you stabilize it.

Now imagine displacing the mass from the equilibrium, but in a direction perpendicular to the line between the planets-- it'll swing back and forth like one of those slingshot rides. There are two dimensions perpendicular to the line between the planets, so two dimensions with stable oscillations, therefore you can get circular orbits.

I got asked to be on a University Challenge team after someone saw me playing... but completely flopped when they asked independent questions at trials.

This kind of unlinked data leaks very fast, too: at my peak I had to top up with Anki for about 3 hours a day. I've forgotten almost all of it now.

A small consolation is that I can still tell you the 'exact number of gallons of water' in most major lakes.

The interesting meta-point here is the chicken-egg problem between cheap robotic hardware and large-scale data collection.

We don't have general-purpose robotics because there aren't enough robots being built for them to become cheap.

And the robots can't do anything in the real world because they cost too much for someone to gather 1m hours of training data.

Presumably this is the Softbank game plan -- build a ton of general-purpose robotic hardware platforms, and hope that they (or another group) figures out how to build general-purpose algorithms in the next few years.

It's a calculated risk that Softbank have the resources to bankroll.

Yep, it all ends up as heat. Energy is conserved, so if it didn't end up as heat, it would have to go somewhere, and there isn't really anywhere else it could go.

Related topic: reversible computing. You might wonder if a computation need take any energy at all: it turns out that an irreversible computation (e.g. NOR, XOR, AND etc) is physically guaranteed to waste energy, but if you make sure your compute steps are always reversible (i.e. each input maps to one and only one output) then you can theoretically compute for free (the Feynmann lectures on computation cover this well).

But these energies are negligible compared to the wattage that goes through a standard CPU.

Yeah this is why you'd use UWB -- the capacity is insane. Also the raw information theory calculation makes sense only if you have zero prior information about the sample you're reconstructing. But 'people in buildings' is a pretty strong prior. And it's not like you have to do any extra work, just put a simulated raw signal through a neural net and regress to groundtruth data.

But let's do the math, I'm curious!

Use ultra-wideband radio pulses, an array of a few thousand 3D printed UWB antennas and a phased-array scanning beam. You'll get a machine you could fit in the back of a truck that can scan a city block in real-time to mm precision. Sweet dreams, citizen.