BearOfNH

I can see the pictures OK. But I don't think there's anything special. The weather picture is far more dense. You might do better with a rotary lawn sprinkler, shot from above.

Nobody knows for sure. Hawking thinks there are a lot of universes, but doesn't have (nor claims to have) any proof. It's also a matter of definition. Where one physicist says there are many universes, another may claim those are all "sub-universes" and there's only one "universe" that contains them all. One man's meat is another man's poison...

Let's say you start off with a sample of a billion atoms of pure 222Rn, which has a half-life of 3.8 days. I can see you building a radioactive decay model based on just the Radon. Trouble is, it decays to 218Polonum, with a half-life of 3.1 minutes. So in addition to Radon decaying to Polonium (by emitting an α-particle), you've got Polonium decaying to 214Lead, also via α-decay. Furthermore, the Lead decays to radioactive Bismuth, which can then decay to either Polonium or Thallium. C'mon, I don't see how you can model all this with a single variable. I think you need to look at each element in the decay chain, at least until you hit an element that decays in microseconds or megacenturies, and sum up the individual components. I think it would be interesting to write a program to simulate all this and plot the combined half-lives of all the atoms as time progresses.

Why do you care about the location of the atom that decayed? OTOH, you should care about what decayed. Depending on the isotope, U-->Th-->Ra-->Rn-->Po etc. (where --> means "decays to"). Each of these has different half-lives so your box needs to display which isotope of which element just decayed.

First you need to assert the universe is convex. If it is not convex, say instead a Torus, then the predicted collapse may not happen the way you envision.

1. So for 10,000 memory references there are 9910 L1 hits, or 49,550 ns. The remaining 90 accesses break down into 80 L2 hits (totaling 1600 ns.) and 10 L2 misses (totaling 1000 ns.) for a total of 52150 ns. spent accessing memory. That should get you started. Don't let your teacher say there are only 9900 L1 hits...to get an L2 miss you need an L1 miss, and there are only 90 of those, not 100. 2. As Ed said, since we don't know the machine we can't help at the moment. I hope to heck your imaginary machine is at least 64 bits wide.

Are you saying Deimos and Phobos share the same orbit?

I may be wrong, I often am, but as I understand it: First there was a big bang and inflation and an expanding universe. Then at some time t1 the universe stopped expanding and started contracting. Then at time t2, about 7 billion years ago, the universe stopped contracting and began expanding again. If that's the case then I doubt the current expansion can be blamed on the big bang.

A photon traveling near a star will undergo a change of direction a la gravitational lensing. This does not happen because the star "pulls on" the photon, but rather because the star's gravity warps space itself. The photon thinks it's going in a straight line, and indeed it is, but a distant observer can see it appears to be curving in towards the star. This is why the orbit of Mercury was impossible to get right with the Newtonian equations alone. Einstein discovered his GR equations nailed the orbit precisely.

Not necessarily. Planets can be ejected from a solar system by orbital dynamics involving heavier planets or rogue stars. There are many "orphan" planets out there, hurtling through space with no sun. The future of such a planet is independent of the former parent star. Current best models predict the universe will eventually suffer a heat death. Such a universe may be perpetual but there's nothing worthwhile going on.

(Ignoring the temptation to riff on "hooter") Would that then lead to a feedback arrangement, where the hotter star would emit more light/heat, which would then be reflected back to further heat the star, which would then emit more light, etc. I think we're summing a geometric series with a multiplier < 1.0, so it converges. But maybe I'm wrong and it diverges, in which case we've succeeded in accelerating the star's death.

Would you see the same effect with a Dyson sphere?

A twofer: 1. By size I assume you mean mass, right? Presumably you can infer mass from observation once you have several points to go on. Isn't a month, or two months, enough time delta? You have what, three or four equations and one unknown? 2. I don't know where you got the figure "but 2500 kg" for a nuclear device. The venerable W80 warhead (still in service, cruise missiles mostly) yields 150kt (that's 10+ Hiroshima's) for a warhead mass of 130kg, about 5% of your figure. Wikipedia lists other heavier warheads with correspondingly higher yields. The formula seems to be: yieldkt ~= 1.2*masskg. So the W87 booms 300kt for 250kg, and the W88, 475kt at 360kg. Question then becomes: does this affect your calculations for the Delta IV Heavy?

astrolog says on sunset 9/5, Saturn was about 2 degrees off the ecliptic. Venus, < ½ degree. No other planets in the clear blue of the western sky.

Imagine a photon zipping along in space until CRASH it runs into an atom and is absorbed by an electron, sending the electron to a higher orbital. What happened to the photon's momentum? If momentum is conserved, something else must be going on that I don't see. Does the entire atom inherit the momentum and start moving, ever so slowly, in the photon's direction? Similarly when the electron falls back to a lower level, it's going to emit a photon that has a certain momentum. But you can't just materialize momentum out of thin air empty space -- that would violate conservation. Again, something else must happen ... but what? This was nearly touched on in a previous article titled "Where are the photons ?" but I think a new topic is the best way to go. I don't care about photons, I care about momentum.

We seem to have come to agreement on what the first sentence means, even if we don't like the phrasing. I don't see how the second sentence follows from the first. Is there any reasoning (other than "QM is strange") that allows one to make that inference?

This post led me to check out my camera's capabilities. It's a standard digital camera that images to an SD card, and has a USB port for image transfer to PC. It turns out that many cameras with USB ports can do an awful lot via those ports. In particular, there's a USB-controlled switch that allows you to activate the shutter without moving the camera.

Pardon my asking, but English slang is foreign to me. Does the above mean comet >> GRB, or comet << GRB? I'd guess a GRB starts out with tremendous energy which then rapidly dissipates over time and space. Comets, different.

The elements of a sequence are usually presented in a canonical form, viz: {a1, a2, a3,...}. Everybody understands what the elements of the sequence are. In this case the equal signs clearly don't mean what they normally mean, so I assume their use is some form of syntax. Do the slashes mean what we normally think, or is their use purely syntactic as well? Are we supposed to deduce the meaning of the operators in order to figure out the elements of the sequence?

But dust particles have only been around a few billion years, and there aren't an infinite number of stars. I speculate oops, suggest, the temperature of a dust particle in interstellar space tends to absolute zero, in which case you get a darkening effect. Isn't this what we observe, especially when observing distant galaxies?

Nobody seems to have mentioned it yet, but there's a lot of dust in the interstellar medium. I don't know how much this contributes to the blackness, relative to the other points raised already. But it seems worth a mention.

Thanks, Greg. I'm beginning to think 40kg is right. We were given (twice the radius, twice the mass) as you might find for Kepler-11b in http://kepler.nasa.gov/Mission/discoveries/, so it's a real question. OTOH, if you think about it, if a [earth-like] planet is twice the radius of earth it should have 8 times the mass of earth, leading to an attractive force of 8/4 what you would have on earth, so my 80kg on earth becomes 160kg on this other planet (not Kepler-11b). So the reason I weigh only 40kg on Kepler-11b is that it's about 1/4th the density of earth. It's sort of a "popcorn planet".

I weigh 80kg here on earth. Suppose I travel to a planet that is twice the size of earth, and twice the mass. Assuming an honest scale, what would I weigh on this planet? The knee-jerk response of 160kg is almost certainly wrong — that would make sense if the planet were earth-sized but still twice the mass. But if we start from there and use Newton's (or is it Hookes' ?) inverse square law, then for a planet twice the size, twice the mass of earth wouldn't the scale read (½)² = ¼th of 160kg, or 40kg? Intuitively, that doesn't make much sense. I understand the universe is under no obligation to satisfy my intuition, but am I even right in my calculations?

Thanks, Cap'n. I looked into the possibility of a balloon-assisted launch, where you piggyback a rocket on a balloon and when the balloon reaches max altitude (either by popping, or balancing with atmospheric pressure) then you launch the rocket. Long story short, it looks like the most optimistic altitude you can reach is 30km. While that's a lot higher than the 5km from the Chilean mountains, it doesn't seem to be enough, based upon reply #8. The CubeSat is starting to look like the most promising approach.

The Philippines are basically at sea level while there are several observatories at 5000+ meters in the Chilean mountains ( http://en.wikipedia.org/wiki/List_of_highest_astronomical_observatories ). Does it make sense to launch from such a higher altitude?