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We once had a Nuclear Engineering proposal on Area 51, but it failed. The space technology proposal might be interesting.

My academic interests are in power plants of all types, shapes, and sizes, and how well they play with each other. My hobbyist interest are kind of all over the place. I like the maker movement, I'm most interested in the sensors and data, the "internet of things" kind of stuff. In physics, I'm probably the most curious about general relativity topics, but my curiosities wind up all over the place.

request sent to Physics Overflow for account creation, 7/11/2014, 1:32pm


1d
comment Can a human size object move so fast that it ceases to be observable?
My first assumption was going to be that they're in a vacuum. The energy would be absurd, but it might be possible in principle.
Aug
24
comment Will this achieve linear movement of a closed container?
Is this in a closed container? I'm confused what setup you're trying to communicate.
Aug
19
comment Do electrons flow only on the surface of a wire?
Good point about the charge gradient. This accelerates charges, but it is balanced against the internal losses which equates to resistance. The steady state model is sufficient for this question as I read it.
Aug
19
comment Do electrons flow only on the surface of a wire?
@JanDvorak With semiconductors it certainly does. But those are engineered to have a razor thin population of mobile charges so that they readily reach saturation. The same effect can exist for conductors to a much lesser degree, sure, I don't deny that.
Aug
19
answered Do electrons flow only on the surface of a wire?
Aug
14
comment Does large acceleration have to cause damage to the human body?
Yes, this comes up as a valid space exploration topic. The solution you offer does increase the maximum sustained acceleration that a person can tolerate. But humans have this tricky need to breathe air, and the lungs will not have equal density. This becomes the limit. To push things further, you'll have to submerge the lungs and carefully evacuate pockets of air in a tremendously uncomfortable process that leaves a human surviving on life support, but might be able to live through much larger accelerations. Short of a nuclear space cannon (I'm serious btw), this doesn't look practical.
Aug
12
awarded  Necromancer
Aug
8
comment Trebuchet launch into space
This sounds like basically a means of velocity redirection. I can't clearly identify how it could be helpful, but I'm sure it could. Direct contact forces can reduce gravity drag in a wide variety of configurations, and that might be true here. But the problem is that tethers are no good for speeds much more than 2 km/s or so, and orbital speed is much higher. Materials aren't strong enough, so the taper ratio diverges to absurd numbers. You can't get around the taper mass absurdity with any adjustments to the configuration. The constraint is in the units.
Aug
6
answered For which temperatures are the ENDF cross-sections given?
Aug
1
comment Energon: is it possible?
Antimatter would solve your problem up to an energy density of twice the energy of the matter itself. But you can't do any better, if you increase the energy any more you'll also increase the inertia and weight. Unless some particle physicist has some other creative solution.
Jul
31
comment Why is binding energy $\Delta mc^2$?
I think you would help your question by elaborating more on the "special relativity" part about the nucleus. The binding energy has several things lumped into it. It's not only from some distance-potential relationship. Kinetic energy within the nucleus is also a part of it to some degree. There are some finer accounting details you can get into. However, there's not any singular confusion that obviously needs to be answered as far as I can tell here.
Jul
31
comment Birds inside of a closed box
Not really clear enough about the construction of the box to answer.
Jul
27
comment Critical size and Radioactive Nuclei
This question is expressing interest in the background fission rate / neutron flux, or alternatively the neutron trigger in bombs. In commercial reactors, an AmBe neutron source is often used. However, these are never necessary to start, it's just economical because you avoid having to waiting to climb so-many orders of magnitude of neutron flux above background. To the other points, fission itself does not require critical mass, you only need that for a self-sustaining reaction, or an increasing reaction. Subcritical configurations still amplify the background neutron flux.
Jul
26
comment speed of gas molecules when they escape to space
7.8 km/s is for orbit, and escape velocity is sqrt(2) times larger. If it's moving at orbital speed, it could remain suspended up there if neutral, if we ignore the Newton's cannonball issue (which is possible since it can start out in the exosphere to begin with). However, it's unlikely to stay neutral for long, and it will be acted upon by the magnetic field.
Jul
21
answered Optimal speed for the water wheel
Jul
19
comment Evolution and the arrow of time
@ticster There is an inflow of energy at entropy s1, and there is an outflow at entropy s2, where s1<s2. If you reverse time, the inflow becomes the outflow and the outflow becomes the inflow... and it would violate the 2nd law. So the arrow of time is very real and identifiable for such a subsystem. Is your question about how this relates to the universal entropy progression? Even if that is your question, I'm unclear about what needs to be answered about it.
Jul
19
comment Evolution and the arrow of time
@ticster It sounds like you're looking for a source for the low-entropy energy. That source is the big bang. This is scientific fact. The big bang had tremendously low entropy, and that's why the sun is able to shine. No one knows why the big bang was like that, but subsystems like the Earth are consequences of this initial condition.
Jul
19
comment Evolution and the arrow of time
@ticster I argued very specifically that entropy of the system is constant over time. However, it sees an energy transition which enables things to happen. High quality energy goes in, low quality energy goes out. It is no secret that this is what enables evolution. It also forces the time direction, since obviously the thermo 2nd law would be violated if ran in reverse. But the universal arrow of time isn't a matter of evolution alone. That is broader and has a cosmic explanation. That's what provides the high quality energy source to start with.
Jul
18
answered Ziolkowsky Rocket Equation
Jul
18
answered Evolution and the arrow of time