Activity for dmckee‭

Type	On...	Excerpt	Status	Date
Comment	Post #276696	Also of note, the preference for higher exhaust velocity is predicated on having sufficient power available. You get more $\Delta v$ per gram of propellant from higher exhaust velocities, but it takes more energy per unit $\Delta v$ (and of course a photon drive represents the ultimate in this trade-... (more)	—	almost 4 years ago
Comment	Post #276696	On the other hand, this answer is also neglecting space-charge effects which will cause the beam divergence to grow after leaving the confines of the engine. If I find time I'll do a BoTE estimate. (more)	—	almost 4 years ago
Edit	Post #276232	Post edited:	—	almost 4 years ago
Comment	Post #276232	BTW -- I've discussed various issues brought up in this answer in rather more detail in various posts on physics stack exchange. But I don't feel like sprinkling my first post here with links pointing back there. (more)	—	almost 4 years ago
Edit	Post #276232	Initial revision	—	almost 4 years ago
Answer	—	A: Is it possible to create a beam of non-relativistic neutrinos? Any mechanism for generating a neutrino beam starts with a way to generate neutrinos, and there is only one underlying means right now: weak decay (including beta decay in radioactive nuclides). These are three body processes (at tree level) which means that they generate a non-trivial distributio... (more)	—	almost 4 years ago
Answer	—	A: How could humanoids master forge fire without using vegetal materials? Dung Charcoal A staple fuel for grassland people is dried dung. It generally doesn't burn particularly hot, but if you a assume a source of dung that is both unusually dense and comes in cohesive chunks of significant size, then activating it as wood can be made into charcoal or coal into coke offe... (more)	—	over 5 years ago
Answer	—	A: What are the effects of slicing the Earth in half with a particle beam? Practical issues with the whole cut-a-planet-with-a-beam idea. Your fundamental problem is that if you dig the trench by sequential applications of a beam with shallow penetration the time scale is dominated by how long it takes the already heated material to clear out of the way and that scale is l... (more)	—	almost 6 years ago
Answer	—	A: Fact-based fiction: Electric, no-fuel propulsion for autonomous space drone Any light source is a propellant-less drive (often called a "photon drive"), it's just extremely energy inefficient. With perfect conversion efficiency, the impulse of the device is (energy supplied)/(speed of light) and the thrust is therefore (power supplied)/(speed of light). That is, a 1 kilowat... (more)	—	over 6 years ago
Answer	—	A: Is using thermal energy an effective method to powering an implanted tracking device? Using thermal energy for power generation is a thing, but you need a temperature difference to make it work. You implanted device is likely to be essentially uniform in temperature meaning that it won't be able to generate any power this way. Even if you do have a minor thermal gradient you can't ge... (more)	—	over 6 years ago
Answer	—	A: Would a galaxy-wide civilization have any reason to build a solar probe? Preconditions Bank's The Algebraist and many other works feature galactic scale civilizations connected by wormhole networks, but with only relativistic (i.e. slow and expensive; possibly very slow and expensive) real-space travel. Systems off the network are isolated at best, and not worth bother... (more)	—	over 6 years ago
Answer	—	A: What would happen if all dark energy instantly converted to radiation due to vacuum metastability event? Convert mass density to energy density, to get $9 \times 10^{-11}\,\mathrm{J/m^3}$ which isn't very much. After all, an adult human is on order of one eighth to one quarter of a cubic meter. So, assuming a blackbody spectrum so that the radiation is essentially all low energy and does not ionize the... (more)	—	over 7 years ago