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Janus last won the day on March 26

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  1. 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.
  2. 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.
  3. 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.
  4. Right, which mean that the annihilation will produce an exhaust of these particles, traveling at moderate fractions of the speed of light. If we take "moderate" to mean ~50%, then we can work out the efficiency of the rocket in terms of mass-ratio. It would work out that for a craft to reach 70% of c and then reduce speed at the end of the trip, you would need ~31 kg of matter-antimatter "fuel" per kg of ship of which 15.5 kg would need to be anti-protons. The ship would have to be almost entirely fuel and antimatter containment systems.
  5. Janus

    near light-speed travel

    Okay let's start with the scenario where he leaves in a straight-line away from the Earth, turns around and returns along a straight-line. We'll pick 0.99c as his relative speed to the Earth. This gives a time dilation of ~ 1/7. So if he is gone for 1 year Earth time, he returns having aged 1/7 of a year. What would he measure in terms of radio/video from the Earth? While going away he will receive signal that are Doppler shifted by a ratio of sqrt((1-v/c)/(1+v/c)) where v is positive when he is receding from the Earth. Since by his clock it takes 1/14 of a year for him to reach the turn around, he gets ~0.005 y worth of Earth time transmissions (a bit under two days). It takes another 1/14 of a year to make the return trip. But now that he is approaching the Earth, we need to make v negative in our equation. This means he gets Earth transmissions at a much faster rate, and gets just a under a year's worth of transmission over the return leg ( if you plug the numbers I used above in you actually get a bit over a year, But this is due to my using rounded out numbers rather than more accurate ones.) The end result is that he will see a total of 1 yr worth of signals from the Earth during the 1/7 of a year he measures by his clock. From the Earth, you will also measure a Doppler shift in the signals coming from the ship while it is receding and approaching, and we use the same formula as above. The difference is that while the Ship sees a shift in the signals received immediately upon his turn-around, this is not the case for the Earth. For the Earth the ship is nearly 1/2 a light year away when it turns around. So while by the Earth clock, 6 mo pass before the ship turns around, nearly another 6 months must pass before the signal carrying that info can get to Earth. The upshot is that the Earth receives "slowed down" transmissions from the Ship for just a bit under a full year ( about 2 days short), and gets ~1/14 yr ship time worth of transmissions. Then for the rest of the year( ~2 days), measures sped up transmissions, where he gets 1/14 of a years worth of ship time transmissions), the end result is that he gets 1/7 of a years worth of ship transmissions upon its return. Now imagine the ship circling the Earth at some fixed distance (while ignoring gravitation) . In this case, the distance never changes. For the Earth, All we have to worry about is the relative speed of the Ship. This generates a time dilation of 1/7. This is also the rate at which the Earth would receive signals from the Ship. This is the "transverse Doppler effect" The Doppler effect you would measure from an object passing you, but at the moment it is neither approaching or receding. For the Ship, things are a bit different. For one, unlike the Earth, observations made from it are not being made from an inertial frame. Since it is circling the Earth, it is constantly changing its velocity. ( velocity combines both speed and direction) , which means it is constantly accelerating, and that acceleration is always towards the Earth. Observations made from within a non-inertial frame of reference follow different rules than those made from inertial ones. In an non-inertial frames, clocks placed in different positions relative to the acceleration vector run at different rates even if they are not moving relative to each other. So while the ship can consider the Earth as hovering directly "overhead" with no relative motion, it is also displaced with respect with the acceleration, and the clocks there would be expected to run fast. The end result will be that the ship, will measure sped up transmissions from the Earth, even though it is not approaching it and those signals will be ~7 times fast. So in this case, both the Ship and Earth will agree that Earth and ship time differ by a factor of 7 and that the ship clock is the slower of the two.
  6. The colors represent intensity. Increasing from red to white.
  7. Extremely unlikely. The core of galaxies tend to be Population I stars, which have lower metalicity than the Population II stars out in the disk. In addition, you need second generation stars (stars formed from remnants of Supernovae) to form stellar systems with the type of element diversity needed. All this takes time. Galaxy center black holes form early on during the Galaxy's formation. This formation could even start before the first stars came to life. ( just because it is made up of 6.5 billion solar masses worth of material doesn't mean that that material was in the form of stars to start with.) Also, during the early active part of its life, when it is gobbling up all the nearby material, collisions between the in-falling material produces a lot of energetic radiation. Until it settles down, this radiation would be too intense to allow life to take hold in the galaxy. Any civilizations would likely have risen long after the BH had finished growing for the most part.
  8. Janus

    light speed

    As already pointed out, we take the light travel time into account when we determine simultaneity. So judging the synchronization of another clock would involve not just what you see, but how far away the clock is. If we assume that the clock never moved between the light leaving it and the light arriving, it is one light year away, and you read a time 1 year behind your clock, you will say that they are synchronized. (This would not be true for someone moving with respect to the two clocks, but I'll get to that later.) There is a quote by Robert Heinlein that may be applicable here: "You can go wrong by being too skeptical as readily as by being too trusting." There are a lot of resources you can try, but no matter which one you choose, you are going to have to approach it with an open mind, and be aware that you will likely have to discard some notions you already have about how things work. I'll try to get you started. First off, we need to start with Special Relativity. This is basically General Relativity when you assume no gravitational fields. SR is based on two postulates: 1. The laws of Physics are the same in all inertial reference frames. 2. The speed of light in a vacuum is a constant in all inertial reference frames and is independent of the relative velocity of the source. An example of an inertial reference frame would be a spaceship coasting in space. If it fires its engines to change speed, while doing so it would be considered a non-inertial reference frame. Once it starts coasting again it will once more be a inertial frame, just a different one than it was earlier. What the first postulate mean is if you had a lab in your spaceship and used it to do any conceivable experiment, you would not get any different results between before you accelerated the ship and after. If you hadn't been aware of the acceleration, there would not be any way for you to tell that there had been a change in velocity in the lab between the two sets of experiments. The second postulate basically means that this also applies to any measurement of the speed of light. Measuring the speed of light always gives the same answer: c ( 299,792,458 m/s), and it doesn't matter if the source is moving relative to the lab or not. In other words, we would measure light as moving at 299,792,458 m/s relative to the lab, both before and after the acceleration and it would not matter if the source of that light was moving or at rest with respect to the lab. This also means that two different labs moving relative to each other would each measure the same light as moving at c relative to themselves. So for example, if you have two labs, A&B, in relative motion with respect to each other, and as they pass each other a flash of light is emitted from where they meet, Lab A, will measure events like this: With itself in the center of an expanding sphere of light as B chases after one edge of that expanding sphere. However, B would measure this as happening: B would remain at the center of the expanding sphere and A would be chasing after one edge. And as counter intuitive this may seem, countless experiments and measurement have confirmed this. So how does this effect how we measure simultaneity? Imagine you have a set of train tracks with an observer along side it, there is also a railway car on the tracks, moving relative to the tracks. the track observer is halfway between two light sources that emit flashes of light that meet at the track observer at the same moment the railway car passes him. For the track observer, events unfold like this: with the expanding circles representing the light flashes. Both flashes are emitted at the same time, and reach both observers at the same moment. For the railway car observer however, things occur differently. He agrees with the track observer that the flashes arrive at the same time as they pass each other, but not that they were emitted at the same time. Here are events as they unfold for him: Unlike the track observer, who remains a fixed distance between the two sources, the railway car is only halfway between them when he sees the flashes. When either of the two flashes is emitted, he was closer to the left source than the right source. Since each flash must travel at c relative to this observer in his frame, the only way for the two flashes to meet when he is half way between the sources is for the right flash to be emitted before the left flash. Thus according to the track observer, the emission of the two flashes are simultaneous events, but according to the railway car observer, they are not. If we were to put clocks at the sources and each flash carried the information of the time stamp for the clock reading when the light left, then if the track observer reads identical time stamps when the flashes arrive, he can conclude that the clocks at the sources are synchronized and read the same at all times. The railway car observer will see the same two identical time stamps, But for him, the light from one flash left earlier and took longer to reach him then the other, So the two clocks can not be synchronized with each other and don't read the same at the same time, and that one clock always reads ahead of the other. This is known as the "relativity of simultaneity". This is a fundamental concept in Relativity, and one you really need to come to grips with if you wish to understand it (many of the "contradictions" people think they have uncovered in Relativity stem from not grasping this concept.) Other concepts such as Time dilation and length contraction build from here. I'll leave it here for now, but first I want to make one thing clear. In these examples we use light. But Relativity isn't really about light itself. It's really is about the nature of time and space. Light behaves the way it does because of this nature. Light doesn't dictate the rules of Relativity, it adheres to them. Because of the nature of time and space, the speed c is special. Light travels at c in a vacuum, and since it is something we can detect and measure, it is a good tool for examining those rules that govern time and space. Light is convenient for use in these examples, but is not required for Relativity to hold true.
  9. Janus


    As measured from the frame that an object is moving with respect to, there would be an apparent density increase, merely because it would measure more mass in less volume. But this is not in any way connected to the apparent mass increase. That is simply due to the fact that The moving object has a kinetic energy as measured with respect to the measuring frame. That KE has a mass equivalence. One way to think of it is that energy imbues an object with properties ( such as inertia) that used to be just considered associated with "mass". In modern parlance, the "mass" of the object does not change as mass is generally restricted to mean "rest" or "invariant" mass. You have to be careful when dealing with changes in velocity of extended objects. If you have an object that measures itself so that it is x units long, and that object is moving at 0.866c relative to you, then you will measure it to be x/2 units long. If you now bring it to a rest with respect to yourself, you have to consider how this is done. Let's say that it's done so that the object never measures a change in its size during the process. This mean that you will measure its length to increase as it slows. This is due to the relativity of simultaneity . In the object's frame, all its parts "slow down at the same rate at the same time. But events that are simultaneous for the object will not be simultaneous according to you. You will measure the trailing end of the object start to slow down before the leading end does and as a result by the time the whole thing comes to rest, it will be x units long. I don't get where you would get the idea that an object would be more contracted at the leading edge. Photon don't have a "volume". While they have particle-like properties, you can't think of them as little spheres of something.
  10. Janus


    The sports I tend to watch are those I have some history with. I played football( American) in high school, so I do watch it. (Though with the NFL, I really only watch the two teams I follow; It just doesn't keep my interest otherwise. With College ball, I can watch games when I have nothing in stake with either team. My daughter played youth soccer, with which I became more and more involved until I was helping coach. Thus I watch MSL soccer ( though again mainly my team), and European Football if its on. I wrestled in high school also, but don't enjoy watching wrestling at the Olympics as much, mainly because the rules differ from the scholastic wrestling I'm used to. I threw discus and shot put also in high school, but these aren't exactly what I'd call spectator sports. Along those same lines, I've played a few rounds of golf in my life, and while I found it fun enough, I just don't get the appeal of watching someone else play it. The same goes for fishing. I've done it and enjoyed it, but watching other people do it on TV would bore me to tears.
  11. Janus

    Why do rockets work in space?

    Yes, you leave a trail of gases behind; You are trading the gases going in one direction for the rocket going in the other. If you took the mass of the gas trail and rocket together, you would find that the center of mass of the entire system doesn't move. The difference between the gases escaping from a rocket in a vacuum and the exhaust from a jet is the that the jet exhaust slows down after leaving the jet due to interaction with the surrounding air, while the exhaust gasses for the rocket don't. (unless they encounter something or are acted on by outside forces like gravity. But what happens to them after they leave the rocket has no effect on the rocket.)
  12. Janus

    Why do rockets work in space?

    Another way of looking at it is like this: Consider a completely enclosed sphere like the top diagram below. Hot gases in side try to expand and push outward against the walls. But since they push equally in all directions, their is no net movement of the Sphere. Now cut a hole in one side like in the bottom diagram. The gases escaping through the hole are not pushing against the wall of the sphere in that direction, but the gas is still pushing against the opposite wall, There is an imbalance in the forces acting on the sphere and you get movement of the sphere to the left. To answer your second question: Imagine that the there is an atmosphere outside the sphere and that we have pumped it up to the point where its pressure is equal to the pressure of the hot gases. Air pushing in at the opening pushes just as hard as the hot gases trying to get out. The result is that the gases are held in the sphere just like is there were no hole. If the gases can't escape, you can get no movement of the sphere. If the outside pressure is less, they can escape, but not as much as they could of there was no air outside. In other words, the atmosphere outside of the rocket reduces the effect of the exhaust gases and thus reduces the efficiency of the rocket. Rockets perform better in a vacuum than they do inside an atmosphere.
  13. Janus

    Propellant less space engine

    With the "imbalanced" wheel, all you've done is shift the center of mass away from the geometrical center of the wheel. The wheel isn't oscillating in terms of any shift in its center of mass. Even though it might "look" like it's oscillating. If I had two weights of different masse on the ends of a rod like this: o-------------------------O and spun it end over end, it is obvious that it will spin around a point closer to the larger mass as marked here: o---------------^--------O But you wouldn't say that the rod is oscillating back and forth. Is is rotating around a fixed center of mass. You could visually hide the fact that one end is more massive than the other, so that it would look like the rod is moving back and forth, but you you aren't actually changing the fact that it is still rotating around a fixed center of mass.
  14. Janus

    Reasons to not implement gravity on ISS ?

    With a 10 m diameter, your module would have to spin at ~1.4 radians/sec to get 1g at the floor. However, at head level, it will have dropped to ~2/3g, so you would have a 1/3 g difference between head and feet while standing. There is also the Coriolis effect to account for. If you are seated, the center of mass of your body is moving at a certain speed relative to the axis. When you are standing, in order to keep the same rotational rate, it has a smaller speed. If you go from a seated to standing position, your center of mass is going to want to keep moving at the same speed. The result is that you will feel a "force" that is trying to tip you over. Also, if you drop something, it will fall in a curve. This, and a changing g value, Would likely play havoc with your eye-hand coordination, especially if you are going back and forth between the spinning and zero g parts of the station.
  15. Janus

    Reasons to not implement gravity on ISS ?

    Here's a graphic showing how big the ISS is when compared to some other things, including the space station from 2001. If you look at the ISS, most of its size is due to the solar panels, and only a small part of it is the station proper. Spinning it while maintaining the proper orientation of the panels would present a problem. Also keep in mind that the difference in size isn't the only issue. If you spin the station in order to give it gravity, you also would have to make it strong enough to withstand the stresses, making it bulkier and more massive.