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Comment Re:Why does anyone care? (Score 2) 117 117

You can use high powered lasers in short pulses to compress and heat a fuel pellet to achieve fusion. A particular approach called fast ignition requires a petawatt pulse. Given that the laser is named LFEX for "Laser for Fast Ignition Experiments", it is a good bet this is what it is for.

(My expertise in this is limited to having had an inkling which Wikipedia article to look in for the answer. Further input from real experts is welcome.)

Comment This would look really cool (Score 4, Interesting) 118 118

If you've ever played with a normal monochrome laser in a dark room, you'll have seen how laser illumination makes things look speckly. Illuminating with this "white" laser will make superimposed speckly in three colours, with the locations of the speckles not coinciding, so it would be iridescent speckly.

Comment Re:Resistance to Power (Score 1) 90 90

Ohm's law: V=IR => I = V/R
Power dissipated in a resister: P=VI
Substituting for I from Ohm's law: P=V^2/R
So for a fixed voltage, you dissipate more energy with a low resistance. This would be what you're remembering from electronics. For example if you attempt to short high tension power lines with a dry kite string, the effect will be unimpressive. On the other hand if you short them with a solid copper bar, expect to be rained with molten copper.

However it is not the case that you're going to take a circuit and replace a Si device with a GaN device and leave everything else (including the voltage) unchanged. You design your circuit to provide voltages appropriate to the components it contains. If you need a certain current to make a device work, and can adjust the voltage to provide that current, then instead you get P=I^2R, and lowering R lowers the power dissipation.

Comment Re: Looking more and more likely all the time... (Score 2) 511 511

It will take a demonstration satellite accelerating through space before the physics community goes into party mode. Until you're in free fall and vacuum, there is too much scope for systematic errors to accept a result of this level of importance.

Comment Compare to the Higgs boson (Score 5, Informative) 511 511

Looking at this another way:

When LHC were looking for the Higgs boson - a particle entirely expected by modern physics - they required a five sigma signal before they were satisfied that they had really found something.

This is a result not only entirely unexpected, but contradictory to almost all known physics. A two sigma (NASA) and three sigma (Germany) signal is not remotely enough to be convincing. At best it is convincing enough for someone to spend the money to further and better test it.

Comment Re: Looking more and more likely all the time... (Score 5, Informative) 511 511

People are so sceptical of this one because if true the implications are universe-shaking. It would completely overturn not just modern physics but all of physics since Newton. The claim is that the device violates conservation of momentum. Then via Noether's theorem this implies that the laws of physics are not independent of location in space. (Alternatively, the device is creating a beam of hard to detect particles via some completely unknown but low energy mechanism.)

Also, the device was first designed using a provably incorrect analysis - an analysis using standard physics determined that the device would produce thrust without reaction mass, violating conservation of momentum. As all the standard physics used in the analysis conserves momentum, the analysis must be incorrect. If someone adds up many even numbers and comes up with an odd total, we know they have made a mistake, even without examining their calculations to find out where. This case is exactly analogous. So if this device really does violate momentum conservation, it is a complete and utter fluke, and not by design.

Comment A self limiting problem (Score 4, Interesting) 312 312

So visitors to his website:
* Must have been sequenced by 23andMe
* And be so interested in his website that they are willing to give him access to their genetic data
* And meet whatever genetic filter he has imposed.

At this point, what he is running is less of a 'website', more of a 'diary', as it will have only one reader.

Comment Two caveats (Score 1) 248 248

Water on the moon is a non-renewable resource. The rest of the world is likely to say "Hey, that belongs to all of us, not just to the nation who first has the technology to extract it."

The article says "Although NASA paid for the $100,000 report it is unlikely to immediately embrace its conclusions." $100,000 is perhaps half an engineer-year of analysis. It may be a good start, but I'd want to be a whole lot more thorough before deciding how to spend tens of billions of dollars.
 

Comment Re:No local intelligence (Score 3, Interesting) 431 431

"a cubic foot of natural gas is about equivalent to four sticks of dynamite."

This seems implausible to me. Using as sources
https://en.wikipedia.org/wiki/...
https://en.wikipedia.org/wiki/...

1 cubic foot of methane -> 28.3 litres -> 18.6g (at 25 C, 1 atmosphere) -> 1.16 mole -> 1.03 MJ combustion energy (at 890 kJ/mol).
4 sticks dynamite -> 0.744 kg -> 3.72MJ (at 5MJ/kg, 186g sticks)

So it is more like a cubic foot of methane = 1 stick of dynamite -- still much more than I expected.

Comment I don't see the logic here (Score 3, Insightful) 86 86

A launch site at latitude L can launch into an orbit of inclination L *or higher*. You can launch into a polar orbit from anywhere on the planet. You can only launch into an equatorial orbit from the equator. Equatorial sites have the advantage, not high latitude sites. (Also, the hemisphere doesn't matter. Something launched into low Earth orbit from 45 degrees south will be at 45 degrees north in about 45 minutes time.)

Some technicalities:
Yes, you can launch into one orbit then change plane to a lower inclination later - but doing so in LEO is very expensive. (I think the cheapest way to do it is to put yourself into a high eccentricity orbit, do the plane change at max distance from Earth, then recircularize your orbit into LEO.) ('expense' = delta-v.)
Launching from latitude L also can't launch into retrograde orbits closer than L to 180 degrees. E.g. from latitude +/- 30 degrees, you can launch directly into orbits with inclination between 30 and 150 degrees.
If you specifically want a 45 degree inclination orbit, I don't know whether launching due east from a 45 degree latitude is cheaper or more expensive than launching either NE or SE from an equatorial site. I suspect there is no difference.

Comment Overpressure in upper stage oxygen tank (Score 1) 49 49

Now he says "There was an overpressure event in the upper stage liquid oxygen tank. Data suggests counterintuitive cause."
"That's all we can say with confidence right now. Will have more to say following a thorough fault tree analysis."
https://twitter.com/elonmusk

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