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Severian

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Everything posted by Severian

  1. I think my objection would be more in the sense of what you actually define as 'physics'. Engineering is just as worthy (perhaps more worthy?) as physics, but it is not physics. Even if you were the best tenor in the world, you would not be offended if I claimed you could not sing soprano.
  2. I think it is most likely to be just a cosmological constant.
  3. Magnetic fields are elctromagnetism too. The photon is invisible to all forces except electromagnetism and gravity.
  4. Well, I can't speak for anyone else, but I certainly could not understand Quantum Field Theory without the mathematics. (And let me be blunt - if you don't understand QFT, you don't understand much about physics.)
  5. Volume is frame dependent because of length contraction.
  6. That is outdated. Since neutrinos have mass they must also have a right-handed component. This is for exactly the reason you suggest. Whether a particle is 'left' or 'right' handed is a statement about whether its spin vector is aligned ('right-handed') or in the opposite direction ('left-handed') with respect the the direction of motion. It a particle has mass, one can move to a frame of reference where the particle is going in the opposite direction (with respect to the observer, ie. the observer overtakes the particle), and if the direction changes, then the handedness changes. So there is no denying that right handed neutrinos exist. The controversy is whether or not they are any different from the left handed ones. If the neutino is a Majorana particle, then the right handed component is the same as the left handed one. (Often there is some confusion of notation in this regard. The SU(2) symmetry which gives rise to the weak interaction, if unbroken, would allow its bosons to act only on left-handed states. So often people say 'left-handed' to mean particles with an SU(2) coupling. But since the SU(2) symmetry is broken in nature, one can have 'right-handed' particles (in the sense of spin) which have a coupling to the SU(2) gauge bosons. When people talk about 'right-handed nuetrinos' they usually mean a neutrino with no SU(2) coupling. Since there is no symmetry (not even a broken one) to keep its mass small, it is naturally extremely heavy.)
  7. No. The conversion of photons to electron-positron pairs is purely QED. Admittedly the photon is a superposition of the one of the SU(2) gauge bosons and the U(1) hypercharge boson, but this is not a unification.
  8. Let the solar constant be s (1.37 kWm-2), the albedo be a (0.3) and the radius of the Earth R (6400km) Energy input per second [math]= \pi R^2 (1-a) s = 1.23 \times 10^{17} W[/math] (roughly)
  9. Severian

    mass

    No - I was joking. (It is the sort of thing he would say though....)
  10. For the LHC you have to remember that 7TeV is the energy of the entire proton though. Not all of the protons collide, only the constituents. At these energies the proton is mainly made of gluons, so the LHC is mainly a gluon collider. Typically the gluons which collide have about 600GeV energy, but there are some which have much higher energies. For example, the LHC should be show the existance of squarks and gluinos (supersymmetric particles) as long as they are lighter than about 2.5TeV.
  11. The important thing is the centre-of-mass energy. This is the energy that the electron-positron pair have in the frame where their centre-of-mass is stationary. In this frame' date=' the photons are always colliding head on, and the centre-of-mass energy must be more than 2m[sub']e[/sub]/c2. So when thinking about your angles, think about making a Lorentz transformation to the frame where the photons hit head on, and see what this energy is. For (near) 0 degrees, this will be a strong boost almost in the original direction of the photons, so in the centre-of-mass frame the photons would have very little energy and could not convert to an electron-positron pair. At 180 degrees we don't have to boost at all, so the energy will be much higher and we will have a much higher production rate. So your supposition was correct! This sort of thing has been tested indirectly by colliding photons which are emitted from accelerating charged particles. The opposite reaction [math]e^+e^- \to \gamma \gamma[/math] has been exhaustively examined at many colliders, at many energies, the highest energy being the Large Electron-Positron (LEP) Collider at a little over 200GeV.
  12. I hate these sort of tests, because they don't test intelligence - they test conventionality of thinking. Usually the solution is determined purely by the setter's taste in problems, and there are many other possible solutions which are arbitrarily defined as wrong because the setter wasn't clever enough to come up with them.
  13. Yes. E=mc2 so the minimum energy you need to produces and electron positron pair is 2mec2. The mass of an electron (or positron) is 0.511MeV/c2. In practice, you won't be able to observe the electron positron pair unless they are moving. 2mec2 would produce them at rest, so you really need a bit more than this do do anything useful.
  14. Severian

    mass

    Frank says you should get back to work and not spend your time researching gravity....
  15. No, it has been observed. It is fairly easy to make photons energetic enough to make, say, an electron-positron pair. All you need is a centre-of-mass energy of 1.022MeV. But to do interesting physics, producing new exotic matter (like the Higgs boson, or supersymmetric particles which are much heavier than the electron) you need very high energy photons (hundreds of GeV in energy). That is what the proposed photon collider would look at.
  16. Severian

    mass

    Yes' date=' that is right. I meant it is easier to make the light when you need it than it is to store it! Yes. In fact, if you had a perfect mirrored room the light would get brighter and brighter while you left the lightbulb on because the lightbulb would be making more and more photons which bounce around without escaping. When you switch the lightbulb off you would not be making any more photons and the level of light would stay contant. (Of course, you would have to be wearing a perfect mirror-suit too otherwise you would absorb the photons yourself.)
  17. Severian

    mass

    That's right. It can (and does) scatter off interstellar particles, but the space between stars is a pretty good vacuum, so this isn't a huge effect. In fact, the light from the CMBR has been travelling for 13.7 billion years! Light travels very fast, so it is very difficult to slow it down to our sort of speeds, but in principle it can be done. Bear in mind though, that this isn't really "slowing light" down. Between molecules it will still travel at c, but it is being absorbed and re-emitted by the moelcules/atoms in the medium; it is this absorption/re-emission which takes time and makes the light appear to be travelling slowly. Imagine you had a perfect mirror which reflected 100% of light. Just make the bottle out of that and you have your light store. No need even to slow it down. A lightbulb seems a bit more practical though.
  18. Imagine building a huge spherical solar panel around the sun. It will have a surface area of [math]4 \pi R^2[/math] where [math]R[/math] is its radius. Every photon coming out of the sun will hit the solar panel so all of the Sun's energy will be caught. Let's call that energy [math]E[/math]. So we have spread the energy [math]E[/math] over an area [math]4 \pi R^2[/math], so there will be [math]\frac{E}{4 \pi R^2}[/math] of energy for every unit area on the panel. Now, if we half the radius, bringing the sphere closer to the sun, the E remains the same (since we haven't changed the sun!), but we need to take our formula and replace [math]R \rightarrow R/2[/math]. This gives [math] \frac{4E}{4 \pi R^2}[/math] so the energy per unit area has increased by a factor of 4! This is why the sun appears hotter as you get closer.
  19. Severian

    mass

    To travel backwards in time the particle needs a negative mass-squared, which corresponds to an imaginary mass - not a negative one.
  20. Yes it is! In fact this is a proposal for a high energy particle physics collider. Collide two photons together and you can create particle-antiparticle pairs to study (and lots more!).
  21. This is an interesting question because it if a 'fine-tuning' problem. The angular size of the sun or moon is approximately given by [math]\theta = \frac{2R}{d}[/math] where [math]\theta[/math] is the angular size in radians, [math]d[/math] is the distance from the Earth to the sun or moon, and [math]R[/math] is the sun or moon's radius. The fact that they are nearly equal tells us (in an obvious notation) [math]\frac{d_s}{R_s} - \frac{d_m}{R_m}\approx 0[/math] The actual values are: [math]R_s = 6.961 \times 10^8 m[/math] [math]d_s = 1.496 \times 10^11 m[/math] [math]R_m=1.738 \times 10^6 m[/math] [math]d_m = 3.844 \times 10^8 m[/math] So the previous equation becomes: [math]\frac{d_s}{R_s} - \frac{d_m}{R_m} =214.9 -221.2 = -6.3 [/math] Now, in principle, a theory is fine tuned if changing the fundamental parameters (in this case, the sun/moon distance and radii) by x% changes a physical observable (in this case the difference in the inverse angular sizes) by y% where y>>x. Increasing [math]d_s[/math] by 10% gives [math]\frac{d_s}{R_s}=236.4[/math] so the difference becomes 15.2 which is a change of roughly 300%. So the theory is fine-tuned and many modern theoretical physicists seeing the model of the solar system for the first time would have discarded it on the grounds that it is fine-tuned. I think this illustrates the dangers of discarding theories on aesthetic reasons....
  22. You seem to have a complete misconception of the nature of science and the scientific method. It really doesn't matter how one comes up with a physical law; what matters is whether or not the physical law makes correct predictions. so the important thing is whether or not the second law of thermodynamics has been tested experimentally. It has, and it has proven to be correct. Even if one wants to put that aside, the second law of thermodynamics can be proven from other principles using statistical mechanics. It is on very firm footing. If you think it is wrong, then I suggest that you prove it by devising an experiment where the second law is violated. Edit: And why is this is the 'relativity' forum?
  23. If you are interested in more info, take a look at http://www.physics.gla.ac.uk/igr/ I work in the same building (but on a different topic) so I know Jim Hough and his team pretty well. (How does Jim manage to get so much publicity....?)
  24. But the non-locality of QM means that they DO communicate within a time T. The collapse of the wavefunction is instantaneous and global, so both electrons have access to the function information at time T.
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