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Tiki

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Everything posted by Tiki

  1. For sure. I've just finished my copy this morning. I feel it's also worth mentioning Zwicky's determination. He spent thousands of hours using relatively poor equipment hunting for the first 'looked for' supernovae. It took several years.
  2. From the same thread, another mentor 'Nugatory' says: Gravity is indeed a force in classical physics, but to avoid the criticisms from @Dale and @PeterDonis above you will have to be a bit more precise about what that means: Newton’s first law defines an inertial frame. Newton’s second law defines force, not just as acceleration but as acceleration in an inertial frame. Thus the Newtonian definition is based on coordinate acceleration. The distinction between proper and coordinate acceleration is irrelevant to this definition; what matters is that there is coordinate acceleration in an inertial frame. Gravity as a real force (a falling object has coordinate acceleration in an inertial frame) but centrifugal force is not (produces coordinate acceleration only in the non-inertial rotating frame). General relativity (more cleanly, IMO) treats all coordinate acceleration as a mere convention and defines force in terms of proper acceleration. That definition doesn’t change the interpretation of the classical fictitious forces, but it does exclude gravity as a force. Source https://www.physicsforums.com/threads/is-gravity-a-force.975552/#post-6214766 So we are both right. It depends upon how you define force. I never realized that GR used such a definition of force. One thing for certain, no matter which definition of force that you use, when you fall over it will always hurt!
  3. Perhaps not. The em force that you refer to is a reactionary force to the gravitational force. Io's interior heats up because of work done by Jupiter's tidal forces. Also, if you had a frame of reference in which ALL of the gravitational forces disappeared then then there would be no curvature at all (in that frame the geodesics would remain parallel). In reality, the curvature of spacetime describes the tidal forces, therefore we can justifiably say 'The curvature of spacetime is gravitation'. Tidal forces are at the heart of GR. Really.
  4. Yes. This tidal force is the 'real' force of gravity.
  5. Thanks Andrew. I meant spacetime. Interestingly though, Newtonian gravity can be formulated as the curvature of time. I am unsure of what you mean by '..it is not a force.' Aren't tidal forces the part of gravity that can't be removed by invoking a free fall observer? Can't tidal forces be used to calculate the curvature of spacetime? (More complicated than test particle trajectories I admit but equivalent)
  6. Tidal forces are in fact very real, as they cannot be made to vanish even for a free-fall observer. Gravity is a force. In GR, gravity can be modelled as the curvature of spacetime. It is still a force. In the case of Io, the disparate gravitational pulls on its near and far sides cause the whole moon to change shape. This deformation causes internal friction within Io which consequently heats up. Alternatively, you could say that owing to the curvature of spacetime, different parts of Io try and follow slightly different paths through spacetime. Since Io is a bound lump, there must therefore be internal forces which hold all the constituent particles together on an approximate parallel journey through spacetime.
  7. No-one really knows what gravity is. Gravity is modelled in different ways and as such it is useful sometimes to think of gravity as a force and sometimes as the curvature of spacetime. Newtonian gravity makes much use of the concept of force and is an almost complete description of how us earth dwellers perceive the effects of gravity. All Newton 'knew' was force. Einstein, using the notion of a free-fall observer (no force other than tidal forces) was able to extend Newtonian theory by accounting for the very few anomalies it contained. However, Einstein still needed force (tidal forces in fact) to account for the curvature of his spacetime. The curvature of spacetime and the path of free-fall objects are one.
  8. I was wondering if anyone here has read the John von Neumann biography by Norman Macrae. A friend of mine (who passed the book on) described it as being very dull and un-technical. I'd be pleased to hear to hear the opinions of any members who have read the book. Thanks.
  9. Thanks Olly. Compulsive is good.
  10. 'Universe' by Freedman and Kaufmann. A bit pricey but well worth it. Packed full of all sorts of info and some basic formulas.
  11. Somewhat astonishingly, for a Universe roughly 10 billion years old with a value of Omega not wildly different from 1, the value of Omega when the Universe is just 1 second old could not differ from unity by more than one part in 10^15. ( From 'Just Six Numbers')
  12. I suppose it could be argued that it is not surprising since we are here talking about it at all. A slightly less flat Universe would have collapsed long ago or would have disallowed the formation of galaxies and stars. What is truly surprising though is that there are a whole bunch of constants at critical values (Omega being just one of them) that conspire to make a habitable Universe. (cf. 'Just Six Numbers' by Martin Rees or 'The Road to Reality' by Roger Penrose)
  13. A little bit different to what you have in mind but you could use a piece of spray painted (matt black) aluminium foil and monitor its temperature as it is heated by the sun with an infrared thermometer. You could then shade the foil and measure the temperature drop. You could combine the two sets of results to obtain an approximation of the intensity of the sunlight.
  14. Thanks for calling me up on this. 'Pattern' is too loose a word to use in this context.
  15. Having not yet read any of the links, there is one thought that springs to mind. If 'random' is defined as an absence of pattern then a random sequence (of random numbers for example) will be patternless when looked at from left to right, right to left or any other arbitrary ordering. If there is any pattern encountered then the sequence cannot be random. I am not sure whether it is relevant here but a random sequence can also be thought of as incompressible. Mind bending stuff but my hunch is that the arrow of time points in one direction only.
  16. Straight from Wikipedia: Physical magic numbers and odd and even proton and neutron count[edit] Stability of isotopes is affected by the ratio of protons to neutrons, and also by presence of certain magic numbers of neutrons or protons which represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. As in the case of tin, a magic number for Z, the atomic number, tends to increase the number of stable isotopes for the element. Just as in the case of electrons, which have the lowest energy state when they occur in pairs in a given orbital, nucleons (both protons and neutrons) exhibit a lower energy state when their number is even, rather than odd. This stability tends to prevent beta decay (in two steps) of many even-even nuclides into another even-even nuclide of the same mass number but lower energy (and of course with two more protons and two fewer neutrons), because decay proceeding one step at a time would have to pass through an odd-odd nuclide of higher energy. This makes for a larger number of stable even-even nuclides, which account for 152 of the 253 total. Even-even nuclides number as many as three isobar (nuclide)s for some mass numbers, and up to seven isotopes for some atomic (proton) numbers. Conversely, of the 253 known stable nuclides, only five have both an odd number of protons and odd number of neutrons: hydrogen-2 (deuterium), lithium-6, boron-10, nitrogen-14, and tantalum-180m. Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: potassium-40, vanadium-50, lanthanum-138, and lutetium-176. Odd-odd primordial nuclides are rare because most odd-odd nuclei are highly unstable with respect to beta decay, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects.[2] Yet another effect of the instability of an odd number of either type of nucleons is that odd-numbered elements tend to have fewer stable isotopes. Of the 26 monoisotopic elements (those with only a single stable isotope), all but one have an odd atomic number, and all but one has an even number of neutrons — the single exception to both rules being beryllium. The end of the stable elements in the periodic table occurs after lead, largely due to the fact that nuclei with 128 neutrons are extraordinarily unstable and almost immediately shed alpha particles. This also contributes to the very short half-lives of astatine, radon, and francium relative to heavier elements. This may also be seen to a much lesser extent with 84 neutrons, which exhibits as a certain number of isotopes in the lanthanide series which exhibit alpha decay. https://en.wikipedia.org/wiki/Stable_nuclide
  17. Not very high. SN 1987A let out a burst of about 10^58 neutinos (which in terms of energy is more than 100 times the amount of energy emitted by the sun in its lifetime). At 160,000 ly distance, the IMB and Kamiokande neutrino detectors combined, encountered a 12 second burst of about 10^16 (only) neutrinos from the SN. There were 20 detections (flashes of Cerenkov radiation). Cerenkov radiation is pretty neat. https://en.wikipedia.org/wiki/Cherenkov_radiation
  18. Unfortunately not only wrong but also actively misleading. Perhaps a correction will be published. After all, a scientific magazine has a responsibility to the public understanding of science. The consideration of objects falling into black holes are a great way of looking at GR, it is a shame that it has gone so badly awry in this most public case.
  19. As Captain Magenta says, potential energy. The objects initial potential energy ( this PE being a consequence of it's initial position relative to the magnet) is converted to kinetic energy as the object 'falls' towards the magnet.
  20. Whilst it is impossible to visualize objects in greater than 3 dimensions it is simple enough to think about them. Objects with a fractional dimension (ie. non-integer) take a bit more effort to get your head round though. Dimension is a tricky subject. For example, it is possible to specify any point inside a square with two numbers. Two numbers therefore two dimensions, easy. However, if you were to fill a square with a space-filling curve ( eg. Peano curve, Hilbert curve etc.) then it is possible to specify any point within the square with just one number; the length along the curve until the desired point is reached. Plenty to think about with dimensions.
  21. Penrose is referring to a state of maximal entropy, ie. before the clock begins to tick. Time being necessary to define distance and hence 'size' is also rendered meaningless. Looking into the distant future Penrose talks of a state of maximal entropy as the last black-hole evaporates. Don't hold your breath for this one.
  22. Thanks for the reminder, this one is due for a re-read. An interesting book that I'm sure you'll enjoy.
  23. Thanks for all the replies. For reference, this was a very easy job. Everything unscrewed easily. The tube to lens cell thread was a bit tricky to start straight but other than that there were no difficulties whatsoever. It is now just a matter of waiting for the clouds to part......
  24. Thanks Stu, that sounds like some good practical advice. I just have to summon the necessary courage!
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