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Janus last won the day on August 17

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  1. And in a great deal of cases, things in nature don't "look designed". Compare the human brain to the aforementioned lap-top. With the lap-top, everything in it has a purpose aimed towards the proper operation of the device. It doesn't have buttons or switches that were needed for earlier computers and no longer have any function now, for example. The human brain however, is a series of more complex structures built on top of earlier more primitive brain structures. There is no indication that its final form arose from any design process. It would be like designing the lap top by starting with a ENIAC base structure, and tacking a IBM 360 architecture on top on that, then adding a 6502, then a 16 bit microprocessor... No sane designer would go about it that way.
  2. For a reference frame with a relative velocity with respect to the guns equal to that which the bullets have relative to the gun after being fired, the bullets start with a non-zero kinetic energy and momentum due to the velocity they have relative to the above reference frame before the guns are fired. The guns firing produces forces that, as seen from this frame act as a "brake" on the bullets, slowing their velocity to zero, and lowering their relative KE and momentum to zero. From the bullet's point of view, they were making measurements from one inertial frame before firing, and a different one after. They spent a brief period in an accelerated non-inertial frame. So while they can infer that they changed momentum and KE in the transition between the two inertial frames, all they can really say is that the KE and Momentum is different after firing than before firing. They cannot say anything about their absolute KE or momentum.
  3. And while we are now just seeing this, it occurred 27,000 years ago; during the Upper Paleolithic, and while we were in the Last Glacial Period. Woolly mammoth still roamed the mainland.
  4. "Dark" in reference to "dark matter" simply means undetectable by electromagnetic radiation, either consisting of MACHOs (MAssive Compact Halo objects, e.g, Black holes, neutron stars etc. ) or WIMPs (Weakly Interacting Massive Particles; basically a type of matter that does not participate via the electromagnetic interaction; something similar to the neutrino)* "Dark" if reference to dark energy is just a placeholder label chosen because we had already coined the term "dark matter" and Science isn't always overly creative when when comes to naming stuff. In other words, don't make too much of the usage of the word here. * Observations tend to limit how much dark matter can be made up of MACHOs. Basically, an large amount of mass consisting of MACHOS today would have had effects on how the universe evolved during its earlier stages, and would had led to a universe that would look different than what we see.
  5. I'd Have to go back and shift through the book to be sure, but I seem to remember that whatever led to the extinction of the previous civilization was not something that could have been prevented by colonizing other worlds. Whether is was some genetic fault that occurred naturally, or an unintended result of their own attempts at genetic engineering ( I do know that the "snouts" evolved from work animals that were modified to fit their purpose), or some other cause, I'm not 100% sure. ( it may be that the novel never actually answers the question for certain).
  6. I always enjoyed the Niven-Pournelle novel Footfall. It addressed the issue as to why a civilization with the resources to cross interstellar space would even bother to invade the Earth. They also "leveled the playing field" a bit by handicapping the invaders in terms of psychology and actual intelligence (while supplying a rationale behind both.)
  7. In one day, as measured by the traveler, he will have traveled about 22.3 Light days as measured by the Earth. This is about 1/70 the distance to the nearest star or 3863 times the distance of the Earth from the Sun. Length contraction does not directly alter this result, As far as the Earth is concerned, the length contraction of the ship does not alter either the distance traveled by the ship of the time it takes to travel the distance. From the perspective of the space traveler, length contraction shrinks that 22.3 light days down to just under 2 light days, and he doesn't measure any time dilation. Thus according to him he traveled traveled just short of 2 light days in 2 days, traveling at 99.9% of c. * * though to be technically correct, from his perspective, it is the Earth that traveled that 2 light days in two days.
  8. Here's the problem: The equation for time dilation is T = t/sqrt(1-v2/c2) with c being the speed of light In your example t = 2 days, v=c, and T would be the time measured on Earth. But if v=c, the above equation reduces to T = 2/0 days. 2/0 has no answer, it is "undefined". Now in reality, we can know how long it takes light as measured by the Earth to travel some distance. For example, we know that it takes roughly 8 min for the light from the Sun to reach the Earth. So really what you are asking is how much time would the light measure? That in equation form reduces to 8 min = t/0 But we have run into the same problem, there is no time period that, when divided by zero, gives an answer of 8 min. The upshot is that it is meaningless to talk about how much time is measured by the light. In physics parlance, you would say that light, or anything else that travels at c, does not have a valid inertial frame from which to make measurements.
  9. As already mentioned, a "negative " pressure reading just means that you are reading the pressure relative to a chosen value, like one standard atmosphere. In absolute measurement, 1 atm is a out 14 psi. It is like measuring altitude, we measure it relative to mean sea level, and can get below sea-level values, but even sea level is thousands of miles higher than the center of the Earth. So no, vacuums do not create any information ward force.
  10. One cubic ft of perfect vacuum would just not produce any gravity field. The cubic ft of matter would produce a gravity field with a strength depending on just how much matter there was. There is no negative pressure involved in making a vacuum.
  11. Why? I'd think it get boring after a while. You'd be stuck in the airship and the view would be monotonous (you couldn't see the ground, just the surrounding clouds.) At least with a Mars base you could get out a take a walk once in a while, even if it is in a pressure suit.
  12. Mars has a bit over 1/3 the surface gravity as the Earth. We have no information as to whether that is too little for the long term health of humans, as we have only been able to study the effects of actual microgravity conditions.
  13. While Venus is closer in terms of travel time, it is actually slightly harder to get to in terms of the delta v needed for a ship to get there, and that's not counting the extra needed to enter and leave orbit around a planet significantly more massive than Mars. In addition, solar radiation is ~90% stronger at Venus than is at Earth. One of the issues spacecraft have to deal with is how to get rid of excess heat. You may have heard that space is cold, but it is not, it has no temp at all. What it is, is a good insulator, as the only way to lose heat is through radiation. A spacecraft sent to Venus would have to be able to shed a lot more heat. Landing would be out of the question, and you would be stuck orbiting above the clouds of a world that it a good description of Hell on the surface. I'm not to sure how good psychologically speaking that would be for the astronauts, nor any safer. Maybe if Venus had turned out to be like some of our earlier imaginings, basically a sister planet to Earth, but just warmer, it would have made a better target for manned Missions, but as it is, it's not as alluring.
  14. As already pointed out, the energy required is dependent on the speed you want to get there and so there is no real lower limit other than that needed to reach Solar escape velocity. But let's, so the sake of argument, assume that we have learned how to make and store a enough antimatter to get to another star at 10% of c. ( How much this would be depends on what kind of exhaust velocity our rocket can generate.) This could be a fair fraction of the mass of the ship.* The empty STS orbiter masses around 75,000 kg, so if we assume a 1 to 1 mass ratio, this, annihilated with an equal amount of matter would created an explosion with 1.35e22 joules of energy, or over 3 million megatons. A fairly significantly sized explosion to be sure, at least on a terrestrial scale. But still just 1/28592 the amount of energy the sun produces every second. Of course, at half the distance to the Moon, it would be a lot closer than the Sun (~1/800 the distance) and thus roughly 22 times as much energy would hit the Earth per square meter than does from the Sun every second, much of it as hard radiation. However, there is no reason why such a launch would be made from from the Earth, or anywhere near it. By the time we got anywhere near making a journey to the stars, we will have conquered our own solar system. The manufacture of the large stores of antimatter needed would be made and stored at some distant point of the system, maybe on a moon of one of the outer gas giants. At such a distance that an accident would pose little risk to Earth. There are a lot of hurdles to be crossed before we could ever even think of such a venture. (At present, we are only capable of making very small amounts of antimatter and store it for short times, at a cost of billions of dollars per gram), And is quite possible that the difficulties and costs involved would make it impractical. * if we assume a standard rocket, we have to carry not only the antimatter, but also matter with which to combine it with, This extra mass requires more antimatter, which requires more matter for the reaction... The total adds up pretty fast. There is is a possible way to drive this down. A few decades ago, a idea was introduced known as the Bussard ramjet. The design was based on the idea of "scooping up" interstellar hydrogen as you traveled and using it in a fusion drive to propel the craft. initially is was argued that such a ramjet could reach near light speed because they didn't need to carry their own fuel. Later studies showed this to be overly optimistic, and the whole concept might not be practical. However, a modification of this idea might work with an antimatter drive. With hydrogen fusion, only a small percentage of the mass is converted to energy, with antimatter-matter conversion, up to 100% of the mass is converted to energy. So maybe you would only need to carry the antimatter, and scoop up the matter for the reaction as you go along. Not needing to carry the reaction mass would drive down the amount of antimatter you would initially need. Still an immense undertaking, but maybe just not quite as immense.
  15. No. Space-time should not be comprised of anything like dough. It is not substance-like at all. The concept of space-time is just the recognition that the measurements of space and time are frame dependent and not absolute. The analogy is that in Newtonian physics, space and time are treated like North/South vs. East/West. In such a situation everyone, no matter what direction they are facing, all agree on these directions. Everyone, for example, agrees that town A is 40 miles North and 30 miles East of town B. However the Space-time manner of treating this is that each person uses his own sense of Left/Right and Front/Back. Thus one person facing one direction will say that town A is 30 miles to the left of and 40 miles in front of town B, while someone facing in another direction would say that town A is 50 miles directly to the right of town B. It makes no more sense to think of space-time as being "substance-like" than it would to think of Left/Right-Front/Back as being "substance like". Now I also realize that in GR, it is said that Space-time "curves" in the presence of mass. And to many people this implies space-time being a "structure or substance". This is not what this means. "Curvature" of space-time really just means that the geometry rules governing it are non-Euclidean. In other words, the rules of plane, Euclidean geometry just don't hold.