# Janus

Resident Experts

1670

7

1. ## Sports

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.
2. ## 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.)
3. ## 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.
4. ## 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.
5. ## 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.
6. ## 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.
7. ## Unregulated Wattage

A voltage regulator works by maintaining a set output voltage. This set voltage will be less than the nominal voltage available from the source, such as a battery. So you might have a 5 volt regulator being sourced by a 9 volt battery. A typical voltage regulator uses a voltage reference such as a Zener diode. A Zener diode has a specific break down voltage, which allows it to maintain a constant voltage drop regardless of the current flowing through it ( within limits). The regulator then compares this to its output. If the current load on the regulator increases and tries to draw down the output voltage, the regulator compensates by drawing more current from the battery. This of course draws down the voltage of the battery. This only becomes a problem if the battery voltage drops too close to the required regulated output voltage. As long as the battery is rated with a voltage greater than the regulated voltage by enough leeway, you would get a constant voltage output from the regulator for quite a while; right up to the point where the battery drops to the cutoff point. To maintain a constant wattage, the output voltage of the regulator would change to compensate. So for example, if you want to maintain a constant 1 mw of power, and the load is 25k ohm, the output voltage would be 5v, and a current draw of 0.2 ma. If the load changed to a resistance of 10 k ohm, the output voltage would need to drop to ~3.16 v as the current raises to ~3.16 ma. On the other hand, if these two resistors were connected directly to a 5v battery, you would still get the 1 mw power on the 25 k load, but with the 10k load, the power would jump to almost 2.5 mw. The regulator controls the output and draws on the battery to do so. The battery just needs needs to be large enough to meet these needs even after it's no longer operating at its fully rated voltage.
8. ## A moon coverall to simulate Earth gravity on the Moon

If you want to simulate a full 1g, the floor of your centrifuge would have to be tilted at ~80 degrees to the horizontal.
9. ## Why does the Christian God allow slavery in the Bible?

That definition of slavery or "indentured servitude" only applied to Hebrew males. Anyone else was considered property. There is even a part dealing with what occurs if a Hebrew man married one of his master's women servants while in his service. Once he served his term, he was free to leave, but his wife and any children remained with the master. If he refused to leave them, he would be taken to a doorpost, and have an awl driven through his ear to mark him as his master's slave forever.
10. ## Could you travel using nuclear bombs?

The only effect springs would have would be to lessen the "jerk", or the rate of change from no acceleration to full acceleration; they will not reduce the g forces due to the acceleration itself. The springs aren't even necessary; all you need to do is to ramp up the acceleration from 0 to full over a period of time rather than doing a sudden transition.
11. ## Could you travel using nuclear bombs?

It's not that simple. For one thing, there is such thing as a maximum theoretical speed that a spaceship could reach using a given propulsion method. The limits are the practical ones due to how much fuel per kg of rocket mass you need to reach a given speed with any given propulsion method. This is a function of the exhaust velocity your propulsion method generates. Modern day chemical rockets generate exhaust velocities in the order of 4.5 km/sec. There is an equation for determining final velocity with a given fuel/ rocket mass ratio and exhaust speed. It is known as the "rocket equation." Vf = Ve ln (MR) where ve is the exhaust ratio, ln means the natural log and MR is the mass ratio (fully fueled rocket mass/ empty rocket mass) Your 35,000 km/hr = ~9.7 km/s By rearranging the rocket equation, we can work out what mass ratio we would need to reach it with a modern chemical rocket. it works out to a mass ratio of ~8.6 , or 7.6 kg of fuel for every kg of rocket. In the late ''60s and early '70s, NASA worked on developing a fission-based rocket that used a fission reactor to generate its propulsion(NERVA). It could achieve exhaust velocities of ~ 8.1 km/sec. Thus NERVA, with the same mass ratio could reach 17.4 km/sec or just twice that of a chemical rocket. While this does not seem like much of an improvement, it would have made a manned Mars mission practical. Congress however, lost interest in manned space exploration and cut funding to the project. Even with further development, this type of fission reactor rocket design wouldn't have improved its performance that much to make even a dent in the speed of light. A newer approach is the fission fragmentation rocket, which actually uses the daughter products of the fission reaction as the exhaust medium. (NERVA just used the reactor to heat a different working fluid) Theoretically, a Fission fragmentation rocket could achieve exhaust velocities in the order of 3% of the speed of light. With the same 8.6 mass ratio, this gets you up to ~6.5% of c. not bad, but consider the fact that this means that your rocket has to carry 7.6 kg of fissionable material per ever kg of rocket. And the mass of that rocket would have to take into account all the shielding for this, the superstructure to support it, and a means of keeping your fissionable material from prematurely undergoing fission. (anything thing more than critical mass too closely packed will go BOOM, or at the very least generate enough energy to cripple your ship.) This drives the amount of the rocket that is used for useful payload down even further. It gets worse if you plan to slow down. Accelerating to 6.5 c and then decelerating back down to 0 requires you to carry 73 kg of fissionable material per kg of ship. The problem is that you are making linear extrapolations in instances where they don't apply.
12. ## A moon coverall to simulate Earth gravity on the Moon

I assume by "coverall", you mean something worn by a person. in that case: 1. It wouldn't make you fall any faster in response to the Moon's gravity, so you would still have to adjust your gait for walking on the Moon. 2. It would add to your overall mass. Your inertia would be that much greater when starting, stopping or going around a curve, for which you would have to compensate. 3. in the end, you are just adding extra work for your muscles to do. It would give you no advantage, while having many disadvantages.
13. ## What is your favorite tv series at the moment?

For right now, I'd have to say "The Orville". There are other shows that I watch regularly, but it seems to be the one I look forward to the most.
14. ## At what age did you move out of your mother's house?

It also depends on the circumstances. At 19, I had a job that would have allowed me to move out on my own. However, my dad was having health problems and was in and out of the VA hospital. We had a small farm, and with dad being away so much, I hung around to pick up the slack. (Even with my helping out, since I was pulling in money from a job, I paid my parents a small amount for room and board.) After dad passed, mom put the place up for sale and moved into an apartment in town. The place still needed to be looked after, so I stayed to care take it until it sold. I was about 23 by the time I moved out into my own apartment.
15. ## Infinite monkeys and Shakespeare

By a rough estimate, let's make it 101565 bits or flips. So after this many flips, there would be a 1 in 101565 chance that I reproduced those lines in ASCII. If I continue for another 101565 flips, I increase the chances that one of those two series is a prefect match. With each successive series of flips I further increase the chances of one of those series will be perfect match. If I am allowed an unlimited number of series of flips, I am guaranteed to hit the perfect match eventually. Both of these statements are examples of "not seeing the forest for the trees". Nobody is suggesting that either of these methods of generating random sequences of infinite length are practical, and that you could actually assemble enough monkeys and have them bang away at typewriters long enough to produce said results, or flip a coin enough times to do so (The coin itself would disintegrate first). These examples are just used as ways to visualize the generation of infinite random series. It's like if I say that it would take just under 143 days to drive to the Moon at 70 mph. This is just meant to illustrate the distance to the Moon in terms of everyday speeds. To argue that you can't "drive" to the Moon, or that traveling to the Moon at a fixed speed of 70 mph is impractical, is missing the point.
16. ## Infinite monkeys and Shakespeare

Sure, why not. A seven bit binary number has 128 possible different combinations of 1s and 0s, this is enough for all the upper case, lower case letter, all the numbers, and all the punctuation marks, with a few codes left over for special functions such as carriage return, end of page, start of text and end of text, etc. we are not talking about stopping randomly after some finite number of flips, we are talking about an infinite number of flips. Because the entire works of Shakespeare has a finite number of characters, the particular sequence of 1s and 0's that would be a perfect ASCII copy of them has to appear somewhere along an infinite series of coin flips. And your point is? Sure random mistakes or long strings of complete gibberish will be more likely. But that does not preclude every possible string of characters appearing somewhere in the infinitely long string, including the perfect copy and ones that are just slightly off. The odds of flipping 1,000,000 heads in a row is pretty low, The overwhelming number of consecutive 1,000,000 coin flips will contain both 1s and 0s, But if you are allowed to keep flipping the coin long enough, eventually you will hit a stretch of 1,000,000 heads in a row.
17. ## Infinite monkeys and Shakespeare

As Strange has already pointed out, it won't. But it doesn't have to in order for the basic premise to be true. Let's replace the monkeys with a single coin that is tossed over and over again. If it comes up heads, we'll consider this a 1, and tails is a 0. starting with the first flip, each succession of 7 flips (8 if you want to include a parity bit.) represents the ASCII code for a character. In an infinite series of coin flips, you will find any finite series of "1"s and "0" somewhere, Including a perfect ASCII version of the works of Shakespeare (along with partially correct versions, including every possible variation that is off by just a single 1 or 0, and a version where every 1 and 0 are inverted) The fact that a huge number of incomplete or incorrect versions will be produced (like the version where every instance of the name "Hamlet" is replaced with "custard"), does not prevent the perfect version from eventually occurring.
18. ## Sunrise to sunset lapse... on the moon.

It depends on your latitude and the time of year. The Moon has a small axial tilt, so like the Earth, you can have shorter or longer periods of daylight vs night as you move towards the poles. ~354 hrs is what you would get at the equator or on equinoxes.
19. ## Interplanetary/Interstellar Travel

I remember a story by A.C. Clarke in which the interstellar craft used water as the reaction mass, which they carried as ice in front of the ship and this acted as their shielding during the acceleration phase of the trip. During the deceleration phase the ship has exhaust spewing out ahead of it offering protection (If you are going to reach any respectable fraction of c with any reason able ship to fuel mass ratio, your exhaust velocity is going to be a good fraction of c itself.)
20. ## Planets made of gases

Escape velocity vs. average molecular velocity. Escape velocity is the speed something would have to be moving to prevent gravity from pulling it back to the planet. Escape velocity for Jupiter is ~ 59.5 km/sec. Compare this to the average speed of air molecules near the surface of the Earth at 0 degrees C. This is under 0.5 km/sec. The average temp for Jupiter is -145 degrees C. Lower temperature means lower molecular velocities. Even allowing for the fact that Jupiter's atmosphere is composed of lighter gases than the Earth's, which would allow for higher molecular speeds at any given temp, they still don't ( on average) achieve escape velocity. As Sensei pointed out, this doesn't stop the loss of atmosphere completely, as random molecules can escape if their velocity is enough over the average, but this is a slow process which will not reduce the size of the gas giants significantly up until the Sun expands.
21. ## MUST SEE THIS THEORY!! IT MAY SEEM INSANE BUT IT IS VERY POSSIBLE

Here's the problem: On one hand, you say there are lots of sightings, yet on the other you say that they are not exposing themselves to us. Any race capable of crossing interstellar space, and creating an entire species would be able to observe that species' development with out exposing itself in any way. This means no bizarre objects in the sky for people to see that would even hint at their existence. You simply can't have it both ways. They either would have no reservations of revealing themselves to us, or they would give us no evidence of their existence at all.
22. ## Light clock - basic explanation needed

The whole underlying issue with this is that you are assuming that there is such a thing as "absolute" motion. That there is some way to say which of the light clocks is "really stationary" and which one is "really moving". This is not the case. There is absolute frame of rest against which all motion can be measured. Even the so-called "fixed stars" only provide a convenient frame of reference and not an absolute one. So when we talk about the "stationary" light clock, we mean the light clock that is at rest relative to the frame from which we are considering at the time. Thus in my animations above, A is the "stationary" clock in the animation where it doesn't move in the animation frame and B is the "stationary" clock in the animation where it doesn't move in the animation frame. This does not mean that we are looking at two different situations, just the same situation from different reference frames. On point 2. Once you give up on the idea of absolute motion, this doesn't become a issue. Each light clock is equally allowed to treat itself as being "stationary", and thus having its light traveling straight up and down relative to its own frame. Any other frame has to agree that the light stays between the mirrors of the light clock, and by default has to see the light pulse travel at a diagonal relative to their frame. Don't get too hung up on how you think light "should" behave. We have to deal with the universe on its own terms. Light does behave as described, and this behavior has been verified by real experiments. We need to accept this, and use it to help us understand the universe around. To address point 3 concerning what happens if you bring the Clocks back together, you have to consider more than just time dilation. Length contraction and the Relativity of simultaneity Also are involved. I won't go into the details here, but will say that what happens when the clocks are brought back together depends on how the clocks are brought back together. To do this, one or both of the clocks will have to accelerate, and acceleration opens a whole new can of worms. What I was saying is that the behavior of the the Light in the light clock experiment is not the cause of the time dilation. It is a symptom of time dilation. The behavior of the light in the experiment is revealing something fundamental about the very nature of time and space. The experiment could be done with a bouncing ball, and would still give the same results. The point of the matter is that it isn't the light itself that is important, but the speed, c, that which it travels, and what the fact that such an invariant speed exists tells us about the universe we live in. Again, you are going to have to provide more details as the the particulars of the experiment. For instance, you say that both clocks are stopped when the "stationary" clock reads 60, but you don't say which reference frame this determination is being made from. If it is from the "stationary" clock frame, then it has marked off 6 marks and the "moving"* machine will have marked off 3. However, if being judged from the "moving" machine frame, then the "stationary" clock doesn't read 60 until the "moving" clock read 120, and thus the "stationary" machine will have 6 marks and the "moving" machine 12. This is an example of the "relativity of simultaneity" I mentioned earlier. The "stationary" clock and "moving" clock will not agree as to what events are simultaneous. For example, the "stationary" clock will say that it reading 60 and the "moving" clock reading 30 are simultaneous events. However, the " moving" clock will say that when it reads 30, the "stationary" clock only reads 15. Thus if you insist that both the "moving" and "stationary" machines stop running when the "stationary " clock read 60 according to the "stationary" clock's frame, Then in the "moving" clock's frame, the two machines do not stop simultaneously. The "moving" machine stops working at 3 marks when its clock reads 30, but the "stationary" machine (whose clock reads 15 at this time), keeps running until it reads 60, and the "moving " clock reads 120, and then stops (at 6 marks). So in one frame, the machines stop simultaneously, while in the other they don't**. They'll never disagree as how many marks each machine made before stopping, but they will disagree as to whether or not the machines stopped at the same time. The standard light clock experiment is purely Special Relativity and assumes flat space-time. Gravity only comes into play if you have curved space-time and is the realm of General Relativity. *(I really hate using the terms "stationary" and "moving", as they imply a absolute nature that doesn't exist. It is so much better to use more generic labels like "machine A" and "machine B") ** Not knowing about,or failing to take the relativity of simultaneity into account accounts to ~99% of the problems people run into when dealing with Relativity. It really should be the first thing they tackle. A good grasp of it will prevent a good deal of headaches later.
23. ## The theory of space /time

As already mentioned, this type of experiment has already been done. The result was just an Relativity predicted: The time dilation for the object in the centrifuge only depended on the speed it was moving and was independent of how many gs it was undergoing.
24. ## Light clock - basic explanation needed

Okay, now lets take the above and apply it to a light clock. we will use two light clocks, again labeled A and B. We will start the first "tick" of each clock while they are in the same spot, and they will then separate ( at 0.5c in this example) as we examine what happens with the light pulses for each light clocks. The pulses represented by the Larger yellow dots, while the expanding circles represent how far the pulses could travel at c in any direction. To keep things from getting too cluttered, I will delete these expanding circles once they are no longer needed to reference the motion of either pulse. First we will consider events as measured by someone at rest with respect to A. Both pulses start off at the same point. A's pulse continues straight up, while B's pulse sets off at an angle in order to stay between B's mirrors. B's flash at any moment cannot be any further from its initial emission point Than A's flash is as they climb. Thus A's pulse hit's it mirror first and begins it return journey before B's pulse does. From A's frame, B ticks more slowly. If we examine things from B's frame, keeping in mind what we covered in the previous post, we see this: Again each pulse travels away from the emission point at c, but in this frame, it is the bottom mirror of B that is the center of the expanding circle of light, and it is Light clock B that ticks slower.* So the reason that each light clock sees the other light clock's pulse travel at an angle is that each light clock must measure its own pulse as traveling straight up and down between it own mirrors, and that relationship must be consistent between the two frames. The next question might be, " But why does the behavior of light time?" The answer is: It doesn't. It isn't that light effects time, it that the very nature of time and space determines how light behaves. The behavior of light is not the cause but the consequence. We use light in these examples because it reveals the nature of space-time. *You may note that in this animation A's light pulse doesn't quite maintain its alignment with Clock A. This is due to an error in how the the motion of the Light clock was rendered. It doesn't move at a constant speed but accelerates up to speed and then slows down. This is the default for the program, and something I forget to correct before doing the final animation. It's a small thing, and I didn't think it was worth the trouble to go all the way back to fix.
25. ## Light clock - basic explanation needed

Just in case the OP is still monitoring this thread (he hasn't responded to any of the answers given so far), I rigged up some animations that might help clarify things: Assume you have two objects traveling in opposite directions and passing each other. At the moment they pass each other, a flash of light is produced from that point, like this: A and B is our object and the expanding circle of dots is the leading edge of the light flash This view is from a frame in which A and B are measured as moving in from the sides. The OP seems to assume that for someone traveling with A or B, this same flash of light would behave like this for A: And like this for B However, this is not what Relativity predicts (Nor what any experiment to date measures). The key is in the first postulate. When it claims that the speed of light is the same for every observer, it means relative to that observer as measured by that observer. If a flash of light is emitted from the same point as the observer, the flash will expand outward at c from that observer in all directions equally. So instead, for someone at rest with respect to A, that same flash of light behaves like this: And for someone at rest with respect to B like this. Keep in mind that these last to images are of the same flash of light as shown in the firast animation, just veiwed from different reference frames. In the following post, I'll deal with how this effects the light clock scenario.