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Janus last won the day on February 10

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About Janus

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  1. Janus

    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.
  2. 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.
  3. Janus

    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.
  4. Janus

    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.
  5. Janus

    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.
  6. Janus

    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.
  7. Janus

    Light clock - basic explanation needed

    Flip it around. Rather then thinking of the light clock moving with respect to the Observer, think of it as the observer moving with respect to the light clock. The light clock is stationary and our observer is flying by it at some fraction of. Of course, someone sitting next to the light clock will see the light go straight up and down. The observer flying past, would see the light travel at an angle relative to himself. The other point is that both observers will measure the light as traveling a c relative to themselves. So while the observer with the light clock measures the light as going up and down at c. the one moving relative to it measures it as traveling at a angle, at c, and thus taking longer to make the trip between the mirrors. You seem be hung up on the idea that there is some absolute meaning to the the word "moving". There isn't, all motion is relative. There is no way to say who is or is not "really moving". If you have two light clocks in motion with respect to each other, A and B, each with their own observer, then observer A would measure light clock A as ticking normally and light clock B as ticking slow, while observer B will measure light clock B as ticking normally and light clock A as ticking slow.
  8. Unless you set up an actual physical experiment with light clocks and recorded real results that conflicted with Relativity, you proved nothing.
  9. Janus

    The Fifth Force

    Let's use a concrete example: In Pisces, there are are a pair of stars, one 106 ly from Earth and the Other 492 ly distant. Thus these stars can be no closer than 386 ly apart. In Virgo, ~180 degrees away in the sky, We have another pair of stars, one 38 ly from the Earth and the othr 250 ly away. These stars can not be any closer than 212 ly apart. The star in Pisces that is 106 ly away, and the star in Virgo that is 38 ly away can be no more than 144 ly apart. Thus these two stars, on opposite sides the sky, are closer to each other than either of the same constellation star pairs are.
  10. Janus

    The Fifth Force

    You do realize that a constellation only looks like a group of stars because of where we are viewing them from, don't you? Some stars in a constellation can be close to us while others very far away, they just look close to each other by line of sight. There are stars in totally different constellations of the Zodiac that are physically closer to each other than they are to some stars in their own constellation.
  11. Janus

    The Fifth Force

    But it behaves just like one if something tries to change its axis of rotation.
  12. Janus

    The Fifth Force

    It all comes down to the fact that the Earth is not a perfect sphere and its axis of rotation is not perpendicular to its orbit around the Sun or the Moon's orbit. Because of the Earth's slight oblate shape, the differential in force acting across it due to the gravity of these two bodies, applies a torque to the Axis in an attempt to align the axis to being perpendicular. But the mass of the Earth has a set angular momentum that it tries to conserve. the end result is the This torque ends up being "deflected" into producing the "wobble" known as precession. If the Earth were a perfect sphere, there would be nothing for the gravity differential to get a "handle on" and we would get no precession.
  13. Janus

    "Microwave five minutes"...

    There two classes of unit systems, absolute and gravitational. So for example, there are two MKS systems.* With absolute MKS units, Mass is measured in kg and force in newtons, and in gravitational MKS units, force is measured in kg and there is no named unit for mass. So when we weigh something in kilograms, we are measuring kgf . Now using a standard g, 1 kgm has a weight of 9.80665 newtons in the absolute system. Converting between the absolute and gravitational MKS systems, 9.80665 newtons = 1kgf. So in a way, 1kgm weighs 1 kgf on the Earth, but this is actually using two different dimensional units from two different systems, just like when you say that 1kg weighs 2.20462 lbs The subscripts are implicit even when not included. * there are 3 different FPS systems. With the absolute, force is measured in pounds and mass in slugs. With the gravitational, force is measured in poundals and mass in pounds. Then there is the American engineering system where pounds is used for both force and mass; lbf and lbm. Something else that distinguishes this system from the others is that both lbf and lbm are considered fundamental dimensions, while in the other systems one or the other are "derived" dimensions ( for example, in the absolute MKS system, kgm is the fundamental unit and newtons are derived as kg-m/s2.)
  14. Janus


    The large market is due to a lot a of people don't really know why gluten is just a problem for intolerant people. Somehow awareness of this intolerance found its way into the public conscience. And, people being people, started to get the idea that gluten is bad for you in general and a fad of eating gluten-free started. People who had no real need to started avoiding gluten. As the fad grew, the market for gluten free products grew, and companies saw that market and obliged. Of course, as more gluten free products started showing up, more people s joined the fad... (Like you, they thought "Well if there wasn't anything to this gluten-free idea, there wouldn't be so many products.) The upshot of this is that those people who really do need to avoid gluten have suddenly had their choices expanded. The sad part for them will be when the gluten free fad ends ( and it will eventually end, as all fads do), companies will start to drop their gluten free selections as the market shrinks again.
  15. Even if were possible to point the Hubble telescope at the Moon, it could not resolve anything smaller than ~100m across. The energy of this impact would not make a crater anywhere near that size. The Lunar Reconnaissance orbiter could likely detect it, but it is on a prescribed polar orbit, which may have already mapped that region. If not, we would have to wait until it map the impact point and hope to recognize what would be a fresh crater.