Thorondor

At industrial scales, you can buy electricity much cheaper than $0.1/kWh if you locate your facility in the right place. For example, hydroelectric power near a large dam can cost as little as $0.02 to $0.05/kWh.

Sussman's Wikipedia contribution history is available here: https://en.wikipedia.org/wiki/Special:Contributions/BC1278

You're ignoring Alpha Centauri A and B, which are much larger. Also, even without the Alpha Centauri system, very distant orbits (more than 2-3 ly or so) would be destabilized by the galactic tide, i.e. the gravitational gradient of the Milky Way galaxy.

In general, the dielectric strength of a gas varies with the gap length, so for small gaps it's not strictly correct to say that air has a dielectric strength of 3 kV/mm or any other single value. [0] But at this scale, the gap is so small that the idea of "dielectric strength" isn't really meaningful anyway. There are so few atoms between the electrodes that electrons can flow with negligible resistance at any reasonable field strength.

[0] https://en.wikipedia.org/wiki/Paschen%27s_law

I think the BBC report that I submitted earlier this morning [0] did a better job of discussing the environmentalists' case. In particular, it linked to the Office of Hawaiian Affairs's civil complaint [1] which is probably the best version of the arguments against the telescope. Summary: the complaint alleges that mismanagement of the telescope site, including poor financial planning and procedural issues, has led to a history of hazardous material spills and car accidents, and that failure to implement the 2009 Comprehensive Management Plan constitutes a breach of contract. There is also a longer explanation of why Mauna Kea is sacred to native Hawaiians.

For the record, I strongly support building the TMT. But the opposition isn't totally illegitimate and I think it's worth covering both sides fairly.

[0] https://www.bbc.com/news/world-us-canada-46046864

[1] https://19of32x2yl33s8o4xza0gf14-wpengine.netdna-ssl.com/wp-...

There was an Ask HN about this yesterday: https://news.ycombinator.com/item?id=18323236

Most mainstream physicists believe that the polywell design cannot work due to bremsstrahlung and electron cusp losses, as described by Rider. [0] Bussard and some of his followers have argued against some assumptions made in the Rider paper, but as far as I know, no one has found any loopholes big enough to allow a practical, power-producing reactor.

If you're interested in alternative fusion reactor designs, though, you might want to look into the levitated dipole. [1] There is a lot of evidence suggesting that this design avoids most of the major problems that plague tokamak development. [2][3] The remaining obstacles (as far as I can tell, mainly interchange instabilities and engineering difficulties) don't seem insurmountable to me. MIT made a serious attempt to build a levitated dipole reactor in the late 2000's, but unfortunately, the US Department of Energy cut off all funding in 2011.

[0] http://fsl.npre.illinois.edu/IEC/Rider,%20Phys.ofPlasmas1995...

[1] https://en.wikipedia.org/wiki/Levitated_dipole

[2] https://pdfs.semanticscholar.org/92cb/3e1ec34a707b2920f1c03a...

[3] https://dspace.mit.edu/bitstream/handle/1721.1/95395/99ja014...

That's very, very unlikely. A rogue planet passing near a star wouldn't crash into the star, it would follow a hyperbolic trajectory out of the system.

No, because the transition function has to be applied at each time step as well as the convolution. In the source code, that's

    self.rules.s(N_buffer, M_buffer)

I'm also pretty skeptical about this story to be honest, but FWIW it was repeated by the BBC. http://www.bbc.com/news/world-europe-43463195 The link I submitted is the nearest I can find to an original source.

The truth is probably either that a smaller amount of californium was seized (possibly 1.4g) or the californium was part of a 1.4kg assembly including some other elements.

There's only a limited amount of bandwidth available, and if you transmit at a certain frequency, you interfere with other people operating near that frequency. It's like jamming their signals. If too many people do this, the entire radio spectrum becomes useless for everyone.

Why do you think the FCC should be involved in this, though?

Do you think the FCC should be able to make safety regulations for, say, airplanes or sailboats?

I think that's correct (this comment does not constitute legal advice, etc.). However, international telecommunications treaties would still apply and they'd have to deal with export restrictions (ITAR and EAR).

IMO the FAA should be the responsible regulatory agency.

Which is exactly my point. The FCC rejected Swarm's experimental license application in December 2017 (thus the letter) because they were worried about orbital debris. They should be focusing on the RF characteristics of the satellite and earth stations instead.

A lot of people are wondering why the FCC has jurisdiction over this. The reason is that Swarm is operating an unauthorized experimental radio station on American soil. [0] That's always been illegal, as it should be.

That shouldn't be the end of the discussion, though. Why didn't the FCC grant Swarm a license in the first place? Basically, the FCC claims the right to take action to minimize the danger of orbital debris created by US-operated satellites. [1] According to the FCC, this jurisdiction is granted by the Communications Act of 1934. [2] But that requires a very broad interpretation of the law. If promoting "the larger and more effective use of radio in the public interest" lets the FCC make regulations about orbital debris, then surely it also applies to just about anything that has a radio.

I don't like what Swarm did here -- I think they are legitimately endangering other satellites -- but I don't like the precedent the FCC is setting either. This seems like a classic case of bureaucratic overreach to me.

Primary sources:

[0] Swarm's license application: https://apps.fcc.gov/oetcf/els/reports/442_Print.cfm?mode=cu...

[1] FCC experimental license rules, section 5.64: http://www.fr.com/files/Uploads/attachments/fcc/FCC_Part%205...

[2] "Mitigation of Orbital Debris", section (III)(A): https://apps.fcc.gov/edocs_public/attachmatch/FCC-04-130A1.p...

It's not really possible to derive temperature from other more fundamental units. For example, you can't define entropy as an absolute number; it needs to be assigned a unit, and the standard way of doing this is to multiply it by the Boltzmann constant, which depends on a unit of temperature. The mean kinetic energy of gas particles (3/2 kT) also depends on the Boltzmann constant.

I signed, too, but I didn't see an update in the signature count either. According to https://www.buzzfeed.com/blakemontgomery/signatures-on-the-w... We the People is experiencing technical difficulties at present; the Trump administration said it was "a question of high volume."

Not yet, but Breakthrough Starshot is working on it. https://en.wikipedia.org/wiki/Breakthrough_Starshot

The numbers they are putting out are probably more than a little on the optimistic side, but the project is definitely a serious attempt and I'd love to see it succeed.

I would add that by Noether's theorem, a violation of the conservation of momentum implies that the laws of physics are different from one place to another. So these results are even more suspicious than they might sound to a non-physicist.

Probably safer to let somebody else try this... ;) A couple of Ukrainians made a cool YouTube video recently where they built something similar and used it to blow up some cell phones. https://www.youtube.com/watch?v=FIU8WZR9DNA

The Nyquist signaling rate only applies to binary/two-level signals. Most optical communications systems use many levels to improve the data rate. StackExchange discussion: http://electronics.stackexchange.com/questions/21854/why-is-...

Technically, 1.22λ/D is the angle for the first dark circular ring of the Airy disc (first zero of the relevant first-order Bessel function [0]). But you are still right that the host star needs to be blocked out in some way to produce a useful image. I think NASA is working on some ways to do this, see [1].

[0] https://oeis.org/A245461

[1] https://exoplanets.nasa.gov/resources/1015/

Hubble's angular resolution is about 0.05 arcsec = 2.4e-7 radians. And you're absolutely right that angular resolution isn't the only challenge here - some way of filtering out the extremely bright (relatively) light from the host star is needed as well, for starters.

The planet is about 0.05 AU from Proxima Centauri, meaning we need an angular resolution of about 1.9e-7 radians to even distinguish it from its host star. Is that realistic?

In theory, an orbiting space telescope has a diffraction-limited resolution of approximately 1.22λ/D (λ = wavelength, D = aperture size). Modern image processing techniques can improve on this somewhat, but it makes a good order-of-magnitude estimate. Anyway, this formula tells us a 4-meter telescope has a maximum angular resolution of about 1.8e-7 radians at a typical visible light wavelength of 600 nm. That would be just good enough... except that we don't actually have a 4 meter orbiting space telescope. Resolving even large features on the planet would require a much larger telescope, probably kilometers or more.

For ground based telescopes, the situation is even worse because of atmospheric effects. Despite being 10 meters in aperture, the Keck telescopes in Hawaii are limited to an angular resolution of about 2e-7 radians because of the atmosphere. However, there is reason to hope that the even larger European Extremely Large Telescope will have enough resolution (about 5e-8 radians? hard to tell from their official publications) to image Proxima b directly. https://www.eso.org/sci/meetings/2011/VLTI2011/presentations... Again, this is still not enough to resolve surface features.

So, long story short the answer is unfortunately no at present. Maybe space-based manufacturing will let us build a big enough telescope someday?

No, deuterium-deuterium fusion reactions usually produce helium-3 and a neutron. Due to the conservation of energy, producing helium-4 requires the emission of a gamma ray. This happens rarely because, since the strong nuclear force is stronger than the electromagnetic force at small distances, fusion reactions tend to release energy as protons and neutrons rather than gamma rays.

Since a fusor normally runs on deuterium fuel, wouldn't the output be helium-3 with a mass of 3 grams/mole?

Not that it makes a big difference - producing helium with a fusor remains extremely impractical.

Skylon does have the advantage of burning atmospheric oxygen at low altitudes, meaning it would have a significantly higher payload fraction than the Falcon 9.

It's SoS in Klingon.

I wonder what this tells us about Klingon physiology? After reading the article, the words for 'mother' and 'father' actually seem like interesting pieces of information to learn about any newly discovered race of aliens.

The paper http://arxiv.org/abs/0708.0261 is a reasonably good introduction with a fair amount of math.

If you're looking for a non-technical overview, you might try https://uwaterloo.ca/institute-for-quantum-computing/quantum... but I don't think English alone is precise enough to explain anything really interesting about quantum computing.