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Comment Re:Why not coax? (Score 1) 140

Coax is half-duplex too

No, it's not.

With proper impedance matching networks and reasonable termination at the ends of a run you can send separate signals at the same frequency/band of frequencies down a cable in each direction. (Impedance discontinuities DO reflect some of the signal going one way back the other way, causing some interference. But even that can be "tuned out" by suitable corrections if it's too severe to just ignore.)

You can do it on a balanced pair, too. Telephones have done this with audio for more than a century, and I recall encountering a simple hack to do it all the way down to DC back in the days of discrete-transistor logic. (And it has nothing to do with two wires being involved, either. With N (= any power of 2) conductors and "phantoming" you can have up to N-1 balanced and one unbalanced two-way transmission lines on N wires.

Time Domain Reflectometry does this to FIND and MEASURE discontinuities in a cable, essentially firing a pulse down the cable and listening to the reflections, radar-style.

Comment Re:Coming from Detroit (Score 1) 76

There is no security on the CAN communications of any modern vehicles that I know of. Any person connected to the bus can masquerade as anyone else.

That's why Tesla has several layers of bus, with firewalls between them, inside each car.

Get on one of the buses, you get to tweak the stuff on THAT bus. But you have to convince a firewall you're cool (i.e. doing something the firewall recognizes as legitimate) before it forwards your transaction to anything on even an adjacent bus.

Comment Not quite the end of the story. (Score 1) 326

In most countries the government is in charge of health care and they have a VERY easy way to regulate price gouging such as this. In any single payer system the national health service basically sets the price they are willing to pay and that's what it costs. End of story.

Well, not quite.

In any price control regime, the authority sets the price, and there are three options:
  1. They HAPPEN to hit the "market clearing" price on the nose.
  2. They set the price lower.
  3. They set the price higher.

1. is a small target, and very hard to get right even if you're trying. (Even market economies only get there by constant feedback in the form of purchase decisions.) Further, there are strong political pressures on regulators on where to set prices, so they aren't even trying. So 1 just doesn't happen.

2. means the consumer gets gouged. (But now he can't go to some competitive supplier to get the product or service at a better price. EVERYBODY who is selling is selling at that price. So the gouging is institutionalized. The only way to get a lower price is to apply pressure to the regulators (see 1.) or go to a black market (with lots of risks, including issues of quality, reliability, contract enforcement, and bad encounters with law enforcement and the rest of the legal system).

3. is where the regulators usually end up. But a price lower than market-clearing means suppliers chose to spend their resources supplying something else, so the supply dries up. You could buy it at a sale price IF you could buy it at all. But it isn't available, so you can't buy it at any price.

A free market has its own problems. For starters, with a single supplier (a monopoly) market forces encourage gouging. With two suppliers they encourage an approximately even division of the market (a duopoly) and, again, gouging, with only price signals, not collusion, to coordinate their behavior. The incentive to engage in competition that drives the prices down to market-clearing level doesn't appear until there are three players, and doesn't become strong until there are four or more.

(Unfortunately, US regulations generally have a built-in assumption that two suppliers are "competition". Thus you get things like the landline/cable internet duopoly, or the built-into-channel-allocations local duopoly (collapsing to local monopolies) of the early, analog, cellphone system.)

Comment Knew a math professor without eyes ... (Score 1) 69

Back in the 1970s I was an undergraduate at a highly-ranked math department. One of the professors there had no eyes. (It was a birth defect - they had not formed, and his face was slightly collapsed where they should have been.)

When a student would try to skip doing some part of a rigorous proof by substituting a geometric drawing, the other profs would ask "How would you explain it to [him]?".

This guy was VERY good. But he had a "blind spot" occasionally when a graphic analogy would have pointed him to some existing proof that would apply. (I recall once when he was discussing some bottleneck in what he was working on and another professor pointed out that the troublesome piece of the problem was equivalent to an angle trisection with compass and ruler.)

Comment Re:Seven phucking photons? (Score 2) 106

Can you please convert that to Olympic swimming pools or football fields? I am american. Thanks!

So am I. Let's see...

10,000 gram moles of x-ray photons...

Take 22 pounds of hydrogen. Turn each atom of hydrogen into an x-ray photon.

Hydrogen bombs do something like that... But let's use total annihilation because the numbers are easier to find.

1 gm of antimatter + 1 gm of matter -> 43 kilotons of TNT equivalent. So call it 21.5 kilotons per gram.

Energy equivalent of a proton's mass is really close to 1 GEv. We don't know what energy x-rays they were detecting, so let's use the energy of photons from a typical dental x-ray machine: 70 kEv. So 10^4 * 7*10^4 / 10^9 = 0.7 grams of energy, or about 15 kilotons of TNT-equivalent emitted per measurement interval.

The Hiroshima bomb was estimated at 15 kilotons, Nagasaki at 20. So call it "Almost exactly one Hiroshima bomb" or "3/4 of one Nagasaki bomb" of x-ray energy released during the observation interval.

(Or maybe boost it up a bit, because I assumed perfect efficiency for the x-ray telescope's mirrors and detector, which I suspect is quite optimistic.)

How's that?

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There is no royal road to geometry. -- Euclid