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George Jones

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Everything posted by George Jones

  1. I, too, keep mine fully assembled. For the last 8 years I have stored my fully assembled in a shed. I carry it into the back garden and back when needed.
  2. Several years ago a colleague asked me to write a short homage to physics that emphasized fundamental curiosity-driven physics. Writing to order, I produced the following hyperbolic passage, which my colleague has used several times in presentations. "Why study and research fundamental physics? Why study curved spacetime and general relativity? Cosmology? Elementary particle physics? One possibly selfish reason for me and many other physicists is "Because it's fun!", but other reasons exist. Science, including non-applied fundamental science, is part of who we are as a species. Fundamental science is as much part of our culture as music, art, and literature. If we lose the desire and ability (possibly through politics) to ask fundamental “Why?” questions of our world, we have failed as humans."
  3. @kurdewiusz, I have largely stopped arguing with people with very non-standard physics views. I had too many bad experiences at Physic Forums, where I was a moderator for years. I will let others here carry the torch.
  4. Okay, I have much more important things to do, like watch the first Saturday of Premier League football.
  5. I really love this beautiful book, and I bought my copy shortly after it was published, but I don't think that this is quite what @cloudsweeper wants, given that he wrote and given that the YouTube lecture course in meant for folks who want a well-motivated but advanced course, i.e., "8.962 is MIT’s graduate course in general relativity, which covers the basic principles of Einstein’s general theory of relativity, differential geometry, experimental tests of general relativity, black holes, and cosmology." More appropriate might be a recent book by Sean Carroll, "The Biggest Ideas in the Universe: Space, Time, and Motion". This book has a lot prose, but does use a few equations. Carroll attempts to explain what equations mean without giving the reader facility to calculate with equations. Carroll explains this in more detail in the book's Introduction, which can be read using Amazon's Look Inside feature. Roger Penrose, who has a Ph.D. in pure maths, and who won a Nobel Prize for applying pure maths to black hole physics, recommends the following technique for dealing with lines of equations
  6. I am going to flip this around somewhat. If logic is a branch of mathematics, how is it possible to define "consistent" without mathematics? 😁
  7. Our (cosmological) universe does not have time symmetry, but it does have 6 spatial symmetries. At any instant of cosmological time, there is a 3-dimensional space that, roughly, is symmetrical for all spatial rotations and all spatial translations. This is true even when the 3-dimensional spaces are curved.
  8. I guess it's different strokes for different strokes for different folks. If when at university I had been forced to major in something other than physics, I would have chosen pure maths, not one of the more obvious choices, e.g., engineering or chemistry. Engineering and chemistry are both great programs, and I expect that many people, if put in my hypothetical situation, would have chosen one of them. My love affair with pure maths started when I was exposed to Euclidean geometry in high school, and has yet to end. I took longer than normal to do my physics B.Sc. so that I could take courses in pure math for which there was no room in a standard physics program. I currently am on the thesis-examination committee of an M.Sc. student in pure maths.
  9. The 4-year physics B.Sc. that I took in Canada 4 decades ago had many, many math courses as required courses. No math courses were required for the Ph.D., although I did take several. This was on the reading list for one of my undergrad courses. Interesting passage from its Preface "The course from which this text evolved was originally based on lectures By Professor R.P. Feynman at Cornell University."
  10. How is this different from electrons that strike a detector? Here is an interpretation of the HUP that I like. Prepare a large number, say 2N, of systems (e.g., electrons) that are in identical states. Measure the position of the electrons in half (i.e., N) of the systems. and measure the momenta of the electrons in the other N systems. Even though all the electrons are in identical states, the measured positions of the electrons will not all be same, i.e., there will be a statistical spread (standard deviation) of the measured positions. Similarly, there will be a statistical spread for the measured momenta. The product of these statistical spreads will satisfy the HUP. Note that on any single system only one measurement is made. In this interpretation there is no system on which both position and momentum measurements are made.
  11. Where were you taught this? This isn't correct; the universe is, in some sense, expanding faster than the speed of light 😁. The expansion of the universe is governed by general relativity, and general relativity can be munch more non-intuitive than special relativity! We are "over here", while distant galaxies are way "over there". Because of spacetime curvature between "over there" and "over here", it is difficult to define the speed of an object "over there" with respect to us "over here" in a way that respects all of our everyday experiences with speed. This leads to a first explanation for the possibility of recessional speeds greater than the speed of light. Special relativity prohibits speeds greater than the speed of light. Cosmology, however, is governed by the curved spacetime of general relativity, to which special relativity is a good *local* approximation. Consequently, we will never see anything moving faster than the speed of light in our local neighbourhood, where special relativity is a good approximation. Stuff at the edge of the universe is not in our local neighbourhood, and thus is not governed by the laws of special relativity. Alternate (more technical) explanation for recessional speeds greater than the speed of light. speed = distance/time, so if different definitions of distance and time are available, we can have have differing definitions of speed. The definitions of distance and time used in cosmology lead to cosmological recessional velocities that correspond not to velocity in special relativity, but to something different called rapidity (sometimes called the "velocity parameter"). In special relativity, there is a relationship between velocity and rapidity, which, for some reason is not used in cosmology. If this relationship were used in cosmology than a recession rapidity of 3.4 corresponds to a recessional speed of 0.998 times the speed of light.
  12. Nice. Interestingly for such a fundamental important result, more than 100 years passed before the original 1922 German publication (1:45 of video) was translated into English. https://arxiv.org/abs/2301.11343 (Full pdf at upper right.) I too am old enough. As a student I used chart recorders in physics labs.
  13. Welcome to SGL. Sorry, but I am not sure what you mean by this.
  14. I have had a chance to look at this in more detail. Looks good to me; you have clearly laid out your steps.. I worked the example independently, and I get the same result. I got a symbolic expression for Delta M, and then plugged in all the numbers at the end. Not sure if this is of any interest, but 0.4343 is 1/(ln10), and is involved in the conversion from log base 10 to nature logs. The error in a ln is easy to approximate using a derivative.
  15. What are you given the error in? Apparent magnitude? Absolute magnitude? Distance modulus? Something else? What are you trying to calculate the error in? Apparent magnitude? Absolute magnitude? Distance modulus? Something else? In this case, did you forget to take the final square root?
  16. Yes, the rest mass of a photon is zeoro This is (approx) true only for non-relativistic physics. Another way to @vlaiv's final equation. The world's most well-known equation, E =mc^2, is a special case of a more general equation (my favourite physics equation), E^2 - (cp)^2 = (mc^2)^2, where E is the total energy of the "particle", m is the rest mass of he particle, and p is the momentum of the particle. Suppose that a particle with non-zero rest mass m is at rest, so that its momentum p is zero. The equation then is E^2 = (mc^2)^2, which, after taking square roots on both sides, gives the famous E =mc^2. E =mc^2 is true for particles that have non-zero m, and that are at rest. E =mc^2 is not true for any moving particles. Now consider the case of a particle that has zero energy E, and has zero rest mass m, e.g., a photon. Then E^2 - (cp)^2 = (mc^2)^2 becomes E = cp.
  17. When I substitute the exact numbers given in the original post into the formula, I get 21510.57.
  18. Silly me. What I wrote is equivalent to edit sqrt( [15*v_RA * cos(Dec) ]^2 +v_Dec^2 ) It seems that @Gfamily's result is exact. RA speed is v_RA = -7909.5669 Dec speed is v_Dec = 5325.382 Dec = 79 + 52/60 + 58.5/3600 =79.8829167
  19. In general, try sqrt( (15*v_RA)^2 * [1 + COS(2*Dec) ]/2 +v_Dec^2 ). I haven't had time to check my result.
  20. No hypothesis in physics is provable. Edit. For example, how can Newton's second law (for constant m), F = ma, be proved?
  21. After more than a dozen years, an independent check to a formula I posted. Great! https://stargazerslounge.com/topic/72948-formula/ The formula I posted and the formula at astronomycafe are equivalent, as shown by cos(ra1 - ra2) = cos(ra1)cos(ra2) + sin(ra1)sin(ra2). I also attached an Excel spreadsheet in the above thread.
  22. An n + m spacetime has n timelike dimensions and m spacelike dimensions. The spacetimes of standard special and general relativity are 1 + 3, i.e., each observer has 1 dimension of time and 3 dimensions of space. To simplify the maths, often 1 +1 spacetimes are considered (only 1 spatial dimension). Standard string theory operates in a 1 + 9 dimensional spacetime for which 6 of the 9 spatial dimensions are curled up so tightly that we perceive spacetime as being 1 + 3. This work uses a 6-dimensional spacetime but instead of using a 1 + 5 spacetime with 1 timelike dimension, it uses 3 + 3 spacetime, i.e., 3 dimensions for time, and 3 dimensions for space. The superluminal observers seem to occupy a 3 + 1 dimensional subspaces..
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