Janus

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Janus last won the day on March 26

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  1. While here, in the Pacific Northwest, we broke records last week with a couple of days with temps in the high 90s. The general trend I've seen in the last few years is towards a longer "dry" season, The spring rains tend to not be as much ( we just had one of our driest Mays) and the fall rains don't really start until later. The result is the increase in wildfires. I've lived in Portland for 30 yrs, And it is only during the last three that we've had to deal with smoke from wildfires (and with that I'm not including the Eagle Creek fire from a couple of years ago which was quite close, and could be considered a one-off event, but smoke from wildfires quite a distance away that is extensive enough to effect us here.)
  2. Janus

    Idea for a rocket engine

    I'll clarify for him. For the reaction mass to produce an upward force on the rocket, it has to be accelerated downward relative to the rocket. In order to return that mass to the top of the rocket, any downward velocity the mass has relative to the rocket has to be stopped and reversed. This is an acceleration just as much as the one producing the upwards force on the rocket (acceleration is either change in speed, direction or both). This action will exert a force on the rocket opposite to that caused by accelerating the fuel downward. The end result of this force will be counter any upward movement by the rocket. This ends up with the net movement of the rocket as being zero. There is no way around this. There is no "clever" way to "fool" the rocket into having net movement by recirculating the fuel/reaction mass.
  3. Janus

    How does light behave in antimatter?

    There is nothing "uncontrollable" about antimatter. It doesn't have any bizarre properties that you seem to imagine it has. We use antimatter in PET scans. Isotopes of some elements decay through the emission of positrons ( the antimatter counterpart to the electron). These positrons then mutually annihilate with the first electron they encounter, producing a couple of gamma ray photons. By giving a patient a small dose of a substance that contains one of these isotopes, they can use scanners to track it through the body by its positron emissions. What is difficult to do with antimatter is store it on Earth in any great quantities. When antimatter comes in contact with "normal" matter, they mutually annihilate each other. Since any antimatter we make is surrounded by regular matter, the trick is to keep the two apart. This is done by using "magnetic bottles" which hold the antimatter in a vacuum and use electric and magnetic fields to keep it from touching the material walls ( this in itself shows that antimatter is controllable with natural forces. Even then, we can't store it for too long. We can't produce a perfect vacuum, which means the antimatter is still going to encounter stray atoms over time and slowly be "eroded" away. The log term storage of antimatter is a technological issue and not one due to some "uncontrollable" nature of antimatter.
  4. Janus

    Landing on a black hole!!

    Just to add a bit to what Zapatos said. You wouldn't have to kill quite all of the Earth's orbital speed in order to drop something into the Sun. Using the Earth's mean circular velocity of 29.9 km/sec, You would need to drop that to ~2.9 km/sec, meaning you need to kill "just" 27 km/sec of the Earth's velocity. Essentially, what you are doing is putting a object into an elliptical orbit that just grazes the surface of the Sun at perihelion. The same works for a black hole. However, a black hole makes for a smaller "target". A solar mass black hole only has an event horizon radius of ~3 km ( vs. 695000 km for the Sun) This means you would have to shed even more of the Earth's orbital velocity in order to "drop" an object into it, because you need to drive the periapis lower. For all intents and purposes, you would have to kill all of the Earth's orbital velocity in order to score a hit. So it would be easier to hit the Sun from the Earth than it would to hit a solar mass BH from the same orbit.
  5. Janus

    Do we need to go light speed

    Also, that across the universe in 30 years ship time at 99.9% of c, is quite a bit too optimistic. At that speed the time dilation factor is only about 22.4. You would only be able to travel ~670 ly in 30 years ship time. That would barely get you about 0.67% across the Milky way galaxy, let alone across the universe. Besides, even getting a ship up to 10% of c is a herculean task. Even with the most efficient propulsion systems we are presently experimenting with, it would take more fuel than there is mass in the galaxy to reach 10% of c for even a Avery small ship. If we were able to increase that efficiency by 50 times, then you could do it for 74 kg of fuel for every kg of ship( that's just getting the ship up to speed, if you want to slow back down again at the end of the trip it jumps to 500 kg per kg) Increasing the efficiency of a rocket engine means increasing the exhaust velocity. A 50 times increase in efficiency equates to a 50 times in crease in exhaust velocity, which equates to a 2500 times increase in the energy needed. And 10% of c isn't going to gain you hardly any advantage in terms of time dilation; the factor is only 1.005 ( a 10 ly trip would take 99.5 yrs ship time rather than 100 yrs)
  6. It seems accurate to me, up to that point, the only problem that I see is that That the text after "Not only that..." should be clearer that the speed at which each of each train would measure that sphere of light as expanding at would be c, relative to itself. The red train measures both lights expanding away from itself at c, and the Green train measures both lights as expanding away from itself at c. (this also means that each train would measure the light's speed with respect to the other train as being something other than c. ) So for example, according to the Red train after 1 microsecond, the edge of the beam will be ~300 meters in front of him. Let's say that he is traveling at 0.5c relative to the Green train in the the same direction as the beam. Then the Green train, after 1 microsecond would measure the beam as being ~300 from him, but only ~150 meters ahead of the Red train.
  7. Janus

    Nimitz UFO sighting.

    The problem that I have with this argument is that any civilization with the technology to across interstellar space surely has the technology to do surveillance of us completely unnoticed by us. There would be no reason to play "peek-a-boo". If it their intent to "do no harm", they are being very sloppy about it. Take the "abduction" stories for example. If they really needed a physical human subject, why not just pick up some lone hiker in the woods? I mean people disappear under these circumstances all the time. Someone walks into the woods and never comes out and it is just put down as another lost hiker. No need to return them and risk the story of their abduction being uncovered.
  8. Janus

    Gravity (split from Infinite gravity)

    You are making the mistake of confusing infinite in extent with infinite in energy. Gravitational force falls off by the square of the distance. Because of this, the energy of the gravitational field remains finite out to any distance, even infinity. If you integrate GMm/r2 in order to get gravitational potential energy, you get E= -GMm/r. To get the energy difference for a mass at different distances from a planet, you take the difference in GPE at those distances. For example, between the surface of the Earth and a point an infinitely far away. As r tends towards infinity, GPE tends towards 0. Thus you end up with 0 -(-GMm/re) or just GMm/re, where re is the radius for the Earth , M is the mass of the Earth, and m the mass of our object. This works out to be ~62511759 joules per kilogram for mass m. This also works out to being the energy it takes per kilogram to accelerate the object up to 11.18 km/sec or escape velocity from the Earth. Ergo, any mass, even infinitely far from the Earth, has a finite gravitational energy with respect to it, and an infinite in range gravitational field does not require infinite energy.
  9. Janus

    Tell me what you think about this.

    When dealing with rockets, its the speed of the exhaust relative to the rocket and the total fuel load that determines the final velocity of the rocket, and not how fast the rocket uses its fuel. The later, as pointed out just determines the thrust ( and g forces experienced by the occupants) of the rocket) The two generally run counter to each other. It comes down to the rocket equation: dV = Ve ln(MR) where dV is the final velocity change of the rocket. Ve is the exhaust velocity of the propellant. ln stands for the natural log MR is the mass ratio or fully fueled rocket mass divided by the "dry mass" of the rocket. The rate at which you use up the propellant has no effect on the final velocity, just how long it takes to reach it. Chemical rockets, such as those we use for launches have very good thrust, but fairly low exhaust velocities. Which means they are good at lifting rockets against the pull of gravity, but have practical limits on final velocity. (you can always increase the final velocity of any rocket by adding more fuel, but at diminishing returns. Doubling the fuel mass only increases the final velocity by 58%, and even that is assuming you don't have to add dry mass to the rocket in order to hold that extra fuel or more engine mass to lift the extra weight. Increasing your fuel load by a factor of 10 increases the final velocity by a factor of less than 3 1/2. When it comes to launching a rocket from Earth, you reach limits as how large a rocket you can build.) Even in space, where you don't have to worry about "weight", too high a thrust is not a good thing. The structure of your craft must be able to withstand the g forces that go with that thrust. Higher thrust requires a more robust superstructure, which, in turn increases the dry mass of the ship, reducing the mass ratio and ultimately, the final velocity. You can get higher final velocities by increasing the exhaust velocity, but there is a catch to that also. While doubling the exhaust velocity will double the final velocity, it take 4 times as much energy to double the exhaust velocity. There are limits to how much energy density you can get with chemical fuels, and thus limits to what kind of exhaust velocities you can achieve. Nuclear rockets, like the NERVA design are the next step up and are capable of an exhaust velocity twice that of a chemical rocket, however, a typical NERVA would have a thrust of only 75,000 lbf, compared to a like-sized chemical rocket with a thrust of over 1,000,000 lbf. Ion rockets can achieve much higher exhaust velocities, and thus attain much greater velocities for the same fuel cost, but have very low thrust. The Dawn mission to Ceres was the first deep space probe to make use of this type of propulsion. VASIMR (VAriable Specific Impulse Magneto-hydrodynamic Rocket) which is still under development, falls somewhere between, better thrust than Ion, but better exhaust velocity than chemical. The trick is finding a energy source-engine combination that provides both a high exhaust velocity, but can also generate a significant thrust. One possible candidate is nuclear pulse propulsion. The earliest version of this was project Orion. The basic idea is that you launch nuclear explosives behind you and use the explosions to propel you forward by absorbing the energy with a "pusher" plate. Modern versions would involve detonation of small nuclear fuel "pellets" rather than full-sized nuclear bombs. This type of propulsion is still in the theoretical stages. It really comes down to having a high energy-density fuel source, and being able deliver that energy at a fast rate to generate significant thrust.
  10. Janus

    The feather, the hammer, and the moon?

    It really matters what frame you are measuring the rate of "fall" by. We tend to want to measure it as relative to the Large body ( in this case, the Moon). So in that case, if you drop the feather and hammer separately, then the hammer will " fall" just the tiniest bit faster than the feather. However, if switch to the frame for the barycenter of the Moon and object being dropped, then what is happening is that while the object dropped "falls" towards the barycenter, the Moon also "falls" toward the barycenter. The object's acceleration is determined by the mass of the Moon, and the Moon's acceleration by the mass of the object. For example a 1 kg object will accelerate at ~1.62 m/sec2 towards the Moon, while the Moon accelerates at ~2.21 m/sec2 towards the 1kg object. In this view, both hammer and feather fall at the same rate, while the Moon would fall faster towards the hammer than the feather. This means that the "closing acceleration" will be different for the hammer and feather when they are dropped singly. However, if you drop them together, side by side, then it will be the same for both. With three object, the barycenter is determined by all three, and the acceleration of the Moon is determined by the combined mass of both feather and hammer. When ever we teach a subject, it is generally better to build up from simple concepts and then add the complicating factors later. When we first start learning to subtract, we are taught that you can't subtract a larger number from a smaller one, then later we are introduced to the concept of negative numbers. Later, we are taught that you can't take the square root of a negative number, and then along comes complex numbers. If you tried to teach all at once, it would just be information overload. Before you start dealing with GR and space-time curvature, you should already be thoroughly familiar with how this situation is dealt with under Newtonian rules.
  11. Janus

    Fire in Notre Dame in Paris

    Or in some cases, when a building is taken out of service and then returned to service. This occurred in a local school district. Due to a decline in student numbers, it was decided to close an older outlying elementary school. However, against the chance that they might, in the future, need to reopen it, they still used a couple of classrooms for Alternative School. As long as some part of the building was being used as a school, they could, if needed, just reopen the entire school. But if they stopped using it as a school entirely, they would have had to bring the building up to modern code before they could reopen it again. ( As it was, student numbers continued to fall, and they eventually sold the building and property).
  12. The LMC is ~ 14,000 ly across, so even if we were viewing it along its longest dimension ( which I don't think is the case), 14,000 years is the most time separation we would see. Planetary Nebula last for 10's of thousands of years. That being said, I don't think those rings are planetary nebula.
  13. Janus

    Fire in Notre Dame in Paris

    The problem, as I understand it, with that suggestion is that they are afraid that the force from the water dropped from tankers could collapse the already weakened structure.
  14. One of the bizarre things that occurs after crossing the the event horizon is that time and space switch roles. So describing what you would "see" is a bit difficult. For example, outside of a black hole, if we are looking at a point 1 light hr away, we can only see, at any given moment, events that occurred 1 hr ago. Inside the event horizon, If you are looking at a point further out from the center than you are, at the same 1 light hr away, you would see everything that occurs at that point between 1 hr in the past to 1 hr in the future, all at once.
  15. Can objects orbit a common center of gravity? Yes. Can this explain or effectively reduce the predicted mass of the central black hole? No. It's not just the orbital speeds involved, but the shapes of the orbits. When you look at the plot of the orbits of those stars you will note that since they are very elliptical, sometimes a given star will be closer to the center than other stars and sometimes further away. Any star that is further from the center than you are will not contribute to the force you feel pulling you towards the center.* By plotting the shape of the orbit as well as its speed at different points, you can calculate just how much of the total mass of the entire system has to be actually be located at the center. This is what gives you the mass of the BH. * An extreme example of this would be a spherical cloud of stars with a hollow at it center. For any star in that cloud its orbit is determined by the stars as close or close to the center than it is, the other stars have no effect. An object that wandered into the central hollow, would be behave as if there were no stars surrounding the hollow at all.