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Showing content with the highest reputation on 08/03/20 in all areas

  1. See, told you a written source would be better. 😃 If it described it with great clarity, you wouldn't have needed to ask the question 🤨
    2 points
  2. Interesting how we all perceive space-time differently. Studiot in terms of intervals between events. Markus in terms of metrics and tensors I'm old fashioned. I see space-time diagrams in my head; and now I'm told that there is no actual need for my imaginary co-ordinate systems. I'm crushed !
    2 points
  3. this thread can be for stashing links to webpages with good explanations of astronomy stuff in Cosmology forum I just saw where aman asked about the slingshot effect (used a lot to save fuel on missions to the outer planets) and swansont gave this link: http://www.mathpages.com/home/kmath114.htm explaining clearly how the slingshot maneuver gains energy and angular momentum (taking away from the planet being used) and then Jenab confirmed having seen slingshotting in simulations he'd run http://www.scienceforums.net/forums/showthread.php?p=30823#post30823 I'm thinking of adding other good links i see to this thread, to have them handy. like link-answers to astronomy FAQ. Join in if you feel like it.
    1 point
  4. It is allowing us to rethink the idea of online learning and remote participation. Most local school districts in the US have been resistant to online learning as primary, and the stay at home process has made that much more acceptable and less feared by millions. The risk with this is the lack of broadband internet for so many hundreds of thousands of children, and also the challenges parents face working when kids are taught from home. It also increases the risk of hunger for kids whose only meals came from schools, and also the risk of abuse detection for kids at home with abusive parents (as the teacher would detect bruises and wounds in-person and get help for the child, but those opportunities to detect abuse are not happening with online learning). Another gap is socialization and the non-book / non-syllabus learning which comes with school... learning how to sort ourselves as social animals and all the informal playground learning and side conversations that help make us who we become. *her (and yes!)
    1 point
  5. I too am a lucid dreamer. I too have had dreams where people and or places in my dreams have established histories. So this topic is potentially interesting me. However I think it should be couched within understand the basic premise. It is too huge of a leap to connect it with evolution and what not in advance of understanding anything about it. Assuming you believe your own mind creates the entirety of your dreams how and or why do you think it creates backstories within dreams?
    1 point
  6. Thank you for the very useful animations I fully understand this machine now so progress can be made. +1 (I had to go look on a modern computer to view them, mine is Windows XP) I also see what you mean about a 'displacer', not a bad term although I would have chosen mixer or stirrer as that is what it does. To summarise; 1) The unit comprises an air chamber with a small offset piston, (small being the volume displaced as the piston moved is small compared to the total volume of the chamber) 2) If one end of the unit is heated ( or cooled) relative to ambient the heat flowing into the air in the chamber results in an increase (reducion) in pressure, which displaces the piston a small amount. 3) The piston drives an output shaft with a flywheel, via a conventional conrod and some gearing. 4) The output shaft drives a liftrod and crank which lifts and lowers the displacer within the chamber. 5) The mixed or stirred air away from the heat or cold source experiences a pressure reduction (increase), maintaining the cycle.
    1 point
  7. Michel in particular might like to note the anther point hidden in all this. By imposing a coordinate system something has been added to the subject, not taken away. That difference became clear to modern geometers as the important difference between classical Euclidian Geometry and the Geometry of that Frenchman. Eddington points this out (he was not the first to realise this) towards the end of the second paragraph on page 9 of his book. That which is added is inherent in the coordinate system, not the subject. And the principle of relativity says that there are features of the subject that are independent of the coordinate system (including even no coordinate system ) (How often do we forget zero when substituting values into some universal statement : What is the direction of the zero vector ?)
    1 point
  8. A closer look at the displacer. As can be seen, just a thin, lightweight, loose fitting disk of styrofoam.
    1 point
  9. What I mean is that CPT symmetry implies and requires Lorentz invariance, and vice versa. We know that Lorentz invariance is a purely local symmetry - it holds across small enough patches of spacetime that can be taken as Minkowskian. However, an ensemble of many such patches of spacetime taken together is generally not guaranteed to remain Minkowskian. At the very least, the same must hence be true for CPT symmetry as well. Just exactly what “local” encompasses will depend on the circumstances - every time you add another particle to the ensemble, the failure of spacetime to be Minkowskian becomes a little less negligible by degrees, and eventually there comes a point where one can’t ignore it anymore, at least not on the characteristic length scales of the interactions in question. Determining where that point is would be quite a difficult undertaking, since the gravitational effects of each particle do not add linearly. One might argue that a very large number of particles is needed, but I am not so sure, because it is also a matter of scale - what is negligible on our scales may already be very significant on QCD and electroweak scales. Perhaps - and I am just speculating here - the second law emerges precisely due to the failure of background spacetime in that region to be exactly Minkowskian. It would seem too strong a correlation to be a mere coincidence. Purely technically, if one was to be really strict, there is no such thing as a pure CPT symmetry in the real world, because the presence of even a single particle already makes the spacetime region non-Minkowskian - we just choose to ignore this, because the deviation is vanishingly small by all practical standards. So maybe it would be better to say that what is CPT symmetric isn’t the physical system as such, but rather its dynamics, i.e. its Lagrangian. Question to you, since you know much more about QFT than I do - is there some version of the CPT theorem that holds in curved spacetime QFTs as well? I don’t see how it could be possible, since in general curved spacetimes there won’t be any time-translation symmetry.
    1 point
  10. It looks like our policy of, “Don’t worry bro, it’s just the flu” didn’t work. This is pretty funny, but does have some salty language.
    1 point
  11. So can we all analyse the system in my picture in relation to the laws of Thermodynamics ? Like other members I have already said that this machine in in perfect accord with these laws, including the second law. We have also asked why you think the second law is appropriate, when this is really a First Law matter.
    1 point
  12. Is this the type of engine you are referring to ? American Stirling Company Displacer Type Stirling Engine Gas expands when heated, and contracts when cooled. Stirling engines move the gas from the hot side of the engine, where it expands, to the cold side, where it contracts. DISPLACER PISTON When there is a temperature difference between upper displacer space and lower displacer space, the engine pressure is changed by the movement of the displacer. The pressure increases when the displacer is located in the upper part of the cylinder (and most of the air is on the hot lower side). The pressure decreases when the displacer is moved to the lower part of the cylinder. The displacer only moves the air back and forth from the hot side to the cold side. It does not operate the crankshaft and the engine. In other words, the connecting rod to the displacer could be a string in this engine and it would still work. POWER PISTON When the engine pressure reaches its maximum because of the motion of the displacer, a power piston is pushed by the expanding gas adding energy to the crankshaft. The power piston should ideally be 90 degrees out of phase with the displacer piston. The displacer type Stirling engine is operated by the power of the power piston.
    1 point
  13. Really, misread it, sorry. So you not expressed your belief here, not towards realism or towards idealism.
    1 point
  14. I'm in group H: H: The lazy group: not investing enough time to post anything of value, but having enough time to study and learn from the other members' interesting contributions. Note regarding language: That points at one reason why I find it tricky to contribute to this interesting discussion. I checked translation of "change" and its actual meaning* in Swedish; it seems to (implicitly) include time; If I say "change" in Swedish the definition, AFAIK, means change as time passes. Using the example from @studiot I get the point and I can follow the arguments but I can't express my views using the word "change" as a translation from Swedish. Maybe I could say that without time the colour still "switch", "shift", "alternate", "fluctuate" or "vary" in the circle? I'm not sure which word, if any, that is best. By the way, this is a version of the drawing where the colour "changes" (hopefully the animation works): This is not an attempt at finding flaws in studiots example. I just use it to highlight my view: the original question "What is time" is really hard for me to discuss due to the way various languages and words "works"; I end up trying to analyse the meaning of the words instead of thinking about an answer. Maybe others have the same experience? *) SAOB – a comprehensive historical dictionary published by the Swedish Academy, covering the entire modern Swedish period from 1521 onwards https://www.saob.se/in-english/
    1 point
  15. Sounds like censorship to me. It's a silly idea. There are already mechanisms to handle mis-categorized posts. Next, we will be judging posts on the basis of other criteria besides being "overly" argumentative like being contrary to conventional wisdom. Shall we ban unorthodox views? Editors commonly use their perception of the consensus view to decide whether to accept a paper or article. Shall we require that all posts conform to the consensus? Are we to be editors or moderators? Censors or just guardians of civility and good behavior? People do not have to read any given post. They can censor it for themselves. Like the song says "Paranoia strikes deep." But, megalomania strikes deeper.
    1 point
  16. Here is a space-time diagram I made by myself. I have some questions about it. In this kind of diagram, 3D Space is reduced into a single line, the horizontal white one. Time runs from down to up. The past is beneath, the present is upon the space line, and the future is above. The observator by convention sits on the center of the diagram, because he belongs to space at present time. The diagonal lines represent the Speed Of Light lines (the intersection of the light cone). As everybody knows, all the observable universe is in the past, in the lower part of the diagram. My question is quite simple: Which of the points A,B,C,D,E,F,G, are observable? As you may conceive, this is not really a question, because I have made an idea on this, but I am wondering maybe I got wrong somewhere. So I prefer asking the question instead of presenting false ideas.
    1 point
  17. You can be deliberately obtuse and try to drag this out but my sentence is perfectly clear. I was referring to THE DISPLACER!!!
    -1 points
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