## Search found 512 matches

- Thu Sep 24, 2015 1:10 am UTC
- Forum: Science
- Topic: Nuclear explosion powerplant
- Replies:
**31** - Views:
**5404**

### Re: Nuclear explosion powerplant

Hmm, it seems, unless i'm mistaken, that geothermal plants are only limited by how fast they can generate energy from the heat difference. This means the lake idea won't be very useful. I'm not sure how I would convert a nuclear blast to plasma for the MHD. I still have the containment problem, and ...

- Wed Sep 23, 2015 3:56 pm UTC
- Forum: Science
- Topic: Nuclear explosion powerplant
- Replies:
**31** - Views:
**5404**

### Re: Nuclear explosion powerplant

1. I was kind of hoping the plug would relieve the pressure, saving the chamber 3. Yes, but luckily a 7 meter radius cable out of 2800 Maraging steel will withstand the 400 GN of gravitational force 4. Actually I went with a radius of 100 because I was afraid with anything larger, I wouldn't get any...

- Wed Sep 23, 2015 2:08 pm UTC
- Forum: Science
- Topic: Nuclear explosion powerplant
- Replies:
**31** - Views:
**5404**

### Nuclear explosion powerplant

You have an unlimited supply of thermonuclear weapons, each with a cost of $1 million and with a yield of 1 MT (~4E15 J). Disregarding safety concerns and public outcry, design a powerplant which detonates the bomb and generates electricity. The powerplant should be both economical, and extract a re...

- Fri Sep 18, 2015 3:51 pm UTC
- Forum: Mathematics
- Topic: Bayes theorem applied to sports betting
- Replies:
**6** - Views:
**2024**

### Re: Bayes theorem applied to sports betting

So what you're looking for is the probability distribution of how defective the cars are. Bayes' Theorem: P(H|E) = P(H)*P(E|H)/P(E) In this case H is continuous and E is discrete, so when I say P(H) and P(H|E) that's really a PDF. P(H) is your prior -- your guess on how defective the cars are. Say t...

- Fri Sep 18, 2015 1:04 pm UTC
- Forum: Mathematics
- Topic: Bayes theorem applied to sports betting
- Replies:
**6** - Views:
**2024**

### Re: Bayes theorem applied to sports betting

what you need, which is missing, is the probability that those cars are defective, and the probability that a defective car can win a race, and how certain you are on those previous two items.

- Mon Sep 14, 2015 1:52 pm UTC
- Forum: Science
- Topic: space fountain energy fermi estimation
- Replies:
**14** - Views:
**2980**

### Re: space fountain energy fermi estimation

according to wikipedia, on the rising side of the pellet stream, a series of devices are used to extract energy and slow down the pellets. on the falling side, that energy is used to accelerate the particles downward. this setup exerts the upward forces necessary to keep the tower up. the tower can'...

- Sun Sep 13, 2015 7:21 pm UTC
- Forum: Science
- Topic: space fountain energy fermi estimation
- Replies:
**14** - Views:
**2980**

### Re: space fountain energy fermi estimation

one thing that concerns me is that the 1 percent figure is just when the flywheel is spinning. whereas the roundtrip energy efficiency is more like 90% with 10% loss. and since the fountain is constantly converting between kinetic and potential energy, it seems like the second scenario is the better...

- Fri Sep 11, 2015 8:40 pm UTC
- Forum: Science
- Topic: space fountain energy fermi estimation
- Replies:
**14** - Views:
**2980**

### Re: space fountain energy fermi estimation

i'm no upper-atmospherologist but that sounds like placing an awful amount of trust in the constant stability of the wind

- Fri Sep 11, 2015 5:34 pm UTC
- Forum: Science
- Topic: space fountain energy fermi estimation
- Replies:
**14** - Views:
**2980**

### Re: space fountain energy fermi estimation

It would only be the same if you had a turbine below collecting wind energy from the helicopter and blowing air back up and the helicopter was able to run on the electricity generated by that upward stream

A helicopter with frictionless bearings cannot stay up indefinitely. This will.

A helicopter with frictionless bearings cannot stay up indefinitely. This will.

- Fri Sep 11, 2015 3:48 pm UTC
- Forum: Science
- Topic: space fountain energy fermi estimation
- Replies:
**14** - Views:
**2980**

### space fountain energy fermi estimation

The design for a space fountain looks like it could be applied to constructing extremely tall skyscrapers. I'm looking for a fermi estimation to how many watts it would take to support a skyscraper weighing m kilograms. There are two main sources of energy loss 1. Drag between particles and air in t...

- Fri Sep 11, 2015 4:41 am UTC
- Forum: Mathematics
- Topic: I don't seem to understand my own brain teaser, please help!
- Replies:
**22** - Views:
**4546**

### Re: I don't seem to understand my own brain teaser, please h

the average case was, when you said, "There is a 55% chance of heads, which will double your remaining earthly time, and a 45% chance of tails, which will reduce your remaining earthly time by 70%." This means that for every flip there is a 55%/45% chance of heads versus tails; and that a...

- Fri Sep 11, 2015 1:52 am UTC
- Forum: Mathematics
- Topic: I don't seem to understand my own brain teaser, please help!
- Replies:
**22** - Views:
**4546**

### Re: I don't seem to understand my own brain teaser, please h

The expected value is still 10*(1.235)^1000 days. However, this is made up of a very small chance of living a very long time, and a very large chance of living a very short time. Wait, isn't this wrong? If I win 55 times and lose 45 times I end up behind. Isn't the only valid way to do this with lo...

- Thu Aug 13, 2015 3:18 pm UTC
- Forum: Mathematics
- Topic: maximization on a function with constraints
- Replies:
**4** - Views:
**1459**

### Re: maximization on a function with constraints

I'm confused by this sentence. The first half defines g as a derivative of f, then the second half looks like a new and different definition of g? Also, f(1, q) is a constant if q is a constant, so I'm not sure how you would maximize it. g is a derivative on f, but f(x,t) isn't fixed for t != 0 yet...

- Wed Aug 12, 2015 6:30 pm UTC
- Forum: Mathematics
- Topic: maximization on a function with constraints
- Replies:
**4** - Views:
**1459**

### maximization on a function with constraints

I'm given a function f(x,0) which is f(x,t) at t = 0, and T(x,t) Both of these functions map the [0,1] x R+ to R+ How can I find g(x,t) = d/dt f(x,t) which maximizes f(1,q), for a constant q in R+? Also subject to the constraint that for all t, f(1,t) = ∫ 0 1 g(x,t)/T(x,t) dx I'm not sure if it matt...

- Mon Aug 10, 2015 3:37 pm UTC
- Forum: Mathematics
- Topic: Powertrains
- Replies:
**1** - Views:
**1139**

### Re: Powertrains

yeah if you take that definition of indestructible then 2534 is, but the book specifically mentions the two numbers as fixed points, which 2534 would not be. also reduce doesn't necessarily mean to reduce in size in general, mathematically speaking. you can say P(2534) reduces to 2592 in the same wa...

- Fri Aug 07, 2015 4:48 am UTC
- Forum: Science
- Topic: A million years
- Replies:
**59** - Views:
**7504**

### Re: A million years

Slightly off topic, but would you happen to know how temperature corresponds to intensity or power? http://www.setileague.org/photos/miscpix/waterhol.gif this image seems to suggest that the CMB has an intensity of around 3K, but from what I know, doesn't temperature just specific a specific black b...

- Thu Aug 06, 2015 3:16 pm UTC
- Forum: Science
- Topic: A million years
- Replies:
**59** - Views:
**7504**

### Re: A million years

sure, i just wasnt sure if it might get drowned out by other sources like cmb or exoplanets' magnetospheres or blocked by objects in space. As i said before, it would be nice if there was data somewhere on the "brightness" of space in all frequencies of em

- Thu Aug 06, 2015 2:47 am UTC
- Forum: Science
- Topic: A million years
- Replies:
**59** - Views:
**7504**

### Re: A million years

well then, with an alternate angle of attack, why not launch a new probe every few years to act as relay stations. the planets might not be aligned properly, making probe launch tricky, but that's nothing that can't solved with brute force. OR, just wait a million years, and when the probe has info ...

- Wed Aug 05, 2015 4:10 pm UTC
- Forum: Science
- Topic: A million years
- Replies:
**59** - Views:
**7504**

### Re: A million years

if not elf radiowaves, how about gamma rays. Takes about 1E20 times more energy but if you increase telescope area to 1E9 sq km, you get a manageable 1E7J/photon

- Tue Aug 04, 2015 11:36 pm UTC
- Forum: Science
- Topic: A million years
- Replies:
**59** - Views:
**7504**

### Re: A million years

I haven't been able to find any data on this but are there any EM spectrum wavelengths that just don't appear at all from natural sources? If there is such a frequency, sending just one photon back should be enough to transmit data. For example, ELF radio waves have energy on the order of 1E-32 joul...

- Wed Jul 29, 2015 2:16 am UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### Re: N-Body Simulation questions

I implemented metric expansion and also leapfrog integration. For some reason, the simulations with leapfrog integration seem to collapse much slowly.When I add metric expansion (linear expansion from 5E23 m to 1E24 m), barely any clusters have time to form. However, when I computed the error in the...

- Thu Jul 23, 2015 11:54 am UTC
- Forum: Science
- Topic: A million years
- Replies:
**59** - Views:
**7504**

### Re: A million years

10^12 kilometers is a fraction of a light-year -- it's nowhere near enough to get to any star. The communication at 10^12 kilometers should be easy -- we could probably do it now. Communication at tens of light-years is harder, but we can simply wait a million years for the technology to get there b...

- Wed Jul 22, 2015 11:32 pm UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### Re: N-Body Simulation questions

If it's necessary I can try a run with 16384 or 65536 particles. I looked up the zel'dovich approximation and the math seems to be a bit beyond me. By power spectrum do you mean the distribution of the magnitudes of the displacements? Can it be described as a normal/poisson/gamma distribution? Curre...

- Wed Jul 22, 2015 1:14 pm UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### Re: N-Body Simulation questions

I was actually trying to find the equation for the repulsive/radiation force, but ended up only implementing plummer softening. Since radiation drops off proportional to distance squared, I reasoned modeling it would be equivalent to weakening the gravitational constant a little. Plus, as icicletast...

- Tue Jul 21, 2015 9:28 pm UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### Re: N-Body Simulation questions

Actually it turned out to be a pretty severe bug in my Barnes Hut implementation which I've fixed. :oops: Here's the corrected simulation , covering just 15 billion years Here's frame four hundred . There's definitely clusters and voids, but I'm not sure about the filaments. With a bit of creative l...

- Tue Jul 21, 2015 7:30 pm UTC
- Forum: Science
- Topic: Puzzling artifacts and reasonable reasoning
- Replies:
**79** - Views:
**13284**

### Re: Puzzling artifacts and reasonable reasoning

If you only need a few thousand years and don't mind a bit of futuristic technology, why not encode your message of choice into the genes of a bunch of organisms? You'd need some error resistant encoding, but nothing too complex

- Mon Jul 20, 2015 2:19 am UTC
- Forum: Science
- Topic: Pluto down, next stop...interstellar?
- Replies:
**30** - Views:
**5538**

### Re: Pluto down, next stop...interstellar?

How would planets create interference? I know Jupiter produces radio waves but their wavelength isn't anywhere near 10 km. And if the planets in the destination system do produce radiowaves at that wavelength, we can examine it and pick a different frequency beforehand. If it's really neccessary, wi...

- Sun Jul 19, 2015 6:14 pm UTC
- Forum: Science
- Topic: Pluto down, next stop...interstellar?
- Replies:
**30** - Views:
**5538**

### Re: Pluto down, next stop...interstellar?

cant we just send signals as radio waves, or anything not in the center of the blackbody spectrum. the spectral radiance of a 5790K blackbody is 1.4E-100 W/m^2/sr/m at 10km wavelength, so we shouldn't get any interference from the stars at 4 lightyears and with a 1kW transmitter, the aercibo telesco...

- Sun Jul 19, 2015 12:55 am UTC
- Forum: Mathematics
- Topic: sum and product of natural number
- Replies:
**9** - Views:
**2547**

### Re: sum and product of natural number

I understand it a bit more now -- though thinking about it hurts my brain. I was hoping to compute the pair as a function of the number of times the mathematicians ask each other but that seems out of the question now.

- Sun Jul 19, 2015 12:40 am UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### Re: N-Body Simulation questions

I tried it out, and it seems to work ok. But I'm still not seeing filaments. The uniform grid seems to collapse, forming some geometric patterns (probably because of the barnes-hut quadtree). Then, large areas of high and low density temporarily appear before the simulation reverts to a more chaotic...

- Fri Jul 17, 2015 11:42 pm UTC
- Forum: Mathematics
- Topic: sum and product of natural number
- Replies:
**9** - Views:
**2547**

### sum and product of natural number

I was reading this post: https://mathoverflow.net/questions/9754/magic-trick-based-on-deep-mathematics/31656#31656 But I'm a bit confused what the apparent "strategy" is. Can anyone explain how the mathematicians can figure out what number it is? I think it may be related to another puzzle...

- Fri Jul 17, 2015 12:56 pm UTC
- Forum: Science
- Topic: Puzzling artifacts and reasonable reasoning
- Replies:
**79** - Views:
**13284**

### Re: Puzzling artifacts and reasonable reasoning

While there may not be another ratio within 1% of the stones, it is relatively likely that the mass of stones were altered after creation by around 10% from erosion or some human activity, meaning any explanatory ratio off by up to 10% is good enough.

- Wed Jul 15, 2015 12:39 pm UTC
- Forum: Science
- Topic: Puzzling artifacts and reasonable reasoning
- Replies:
**79** - Views:
**13284**

### Re: Puzzling artifacts and reasonable reasoning

But you can assign odds to whether time-travel/FTL are possible and it makes sense to assign odds to whether they exist after that. If a rich man had amnesia and forgot how he acquired his wealth, he might reason winning the lottery was highly unlikely, therefore he probably didn't win the lottery. ...

- Tue Jul 14, 2015 10:02 pm UTC
- Forum: Science
- Topic: Puzzling artifacts and reasonable reasoning
- Replies:
**79** - Views:
**13284**

### Re: Puzzling artifacts and reasonable reasoning

Whoever was weighing the stones must have been hallucinating (if that doesn't fall under the hoax category).

- Tue Jul 14, 2015 1:08 pm UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### Re: Equation for repulsive forces in N-body simulation?

thanks! By the way, a bit off-topic, but would you happen to have any idea on what would be a good initial starting condition? I've read that a lot of large scale simulations use actual cosmological data, but if I just wanted to place the particles, how should I? I was thinking the mass of the parti...

- Tue Jul 14, 2015 12:52 pm UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### Re: Equation for repulsive forces in N-body simulation?

Just for fun, I'm trying to simulate the large scale structure of the universe by placing a bunch of particles and hoping they collapse into filaments and clusters and such. I wanted a repulsive force both to prevent everything from clumping together and to prevent force from going to infinity as ra...

- Sun Jul 12, 2015 3:22 am UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### Re: Equation for repulsive forces in N-body simulation?

Thank you. Just to be clear, u is the speed of the stellar wind?

- Sat Jul 11, 2015 7:00 pm UTC
- Forum: Science
- Topic: N-Body Simulation questions
- Replies:
**19** - Views:
**3457**

### N-Body Simulation questions

I'm trying to model an N-body system, and in addition to gravity, I want to have a force which models the repulsive forces of stellar wind, star formation, and other phenomenon. Hopefully something of the form F(m1, m2, distance). I tried using PV = nRT to solve for T, then using Stefan-Boltzmann, b...

- Thu Jul 09, 2015 12:15 pm UTC
- Forum: Mathematics
- Topic: variance of weighed sum of random variables
- Replies:
**7** - Views:
**2023**

### Re: variance of weighed sum of random variables

Thanks, that's the word I was looking for, and http://stats.stackexchange.com/a/16609 gives a nice expression for it.

- Thu Jul 09, 2015 3:40 am UTC
- Forum: Mathematics
- Topic: variance of weighed sum of random variables
- Replies:
**7** - Views:
**2023**

### Re: variance of weighed sum of random variables

I just realized what I really needed wasn't what I thought I was looking for here. What I actually want is the variance of the weighed average of two probability mass functions X and Y. In other words, the variance of the probability mass function Z = aX+bY, given a+b = 1. I'm not exactly sure what ...