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#21
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could we shine a laser on this new solar system and detect something in 180 years?
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#22
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could we shine a laser on this new solar system and detect something in 180 years?
"Keith F. Lynch" writes:
Henry Spencer wrote: This *particular* thought experiment doesn't violate causality, but there are others (somewhat more complex, involving high sublight velocities as well as the magic rocket) which do. Any FTL communication system permits them. It's possible that FTL is possible without violating causality. There could, for instance, be a preferred frame of reference. Perhaps the one in which the cosmic microwave background is maximally isotropic. Or perhaps the one in which the Big Bang was the same amount of time ago. ....However, there is no evidence that the "preferred frame" represented by the microwave background exerts any _physical_ effect on an otherwise isolated observer (beyond a slight warming effect and a minute amount of viscous drag if you are ploughing through it); nor is there any evidence that the microwave background provides a "preferred clock" governing any other physical phenomena. Finally, there is no a priori reason why the "preferred frame" in which the microwave background is isotropic should be correlated with the "preferred frame" represented by a cosmologically "co-moving" observer in the sense of a General Relativistic cosmology, and there are perfectly physically valid solutions of GR in which the microwave background and co-moving observer frames can be "tilted" relative to each other (where "tilted" here means something different from what it means in "inflation theory" circles). Most would consider such a "tilted" cosmology to be quite un-aesthetically asymmetric and thermodynamically unlikely, but it is NOT RULED OUT by the physics of GR! Maybe signals can travel at any finite positive speed relative to that frame of reference. One would need to find a plausible physical mechanism why the "microwave background" or whatever should exert such an effect on temporal processes, yet otherwise produce no "aether drag' effects on microphysics. So far, no such physical mechanism even exists, let alone a physically plausible one. Furthermore, the Hughes, the Drever, and the Eotvos Experiments impose stringent experimental limits on =ANY= "preferred frame" effect that couples to "normal" matter (see below). Another way to get FTL without violating causality is if there's a preferred *direction* in space. Maybe it's possible to send signals at any finite positive speed in that direction, but no other. This too would allow FTL without violating causality. Most physicists find this notion of such a "tachyon corridor" even more un-aesthetic than a preferred reference-frame or a preferred time --- and again, one must ask why it apparently has _NO OTHER OBSERVABLE PHYSICAL EFFECT_. In particular, one must ask why such a spatial "preferred direction" does not show up in the Hughes and Drever Experiments, which establish spatial isotropy of inertia to the unprecedented accuracy of better than one part in 1e20. To see why the observed experimental isotropy of space is in serious conflict with the idea of a "tachyon corridor," consider that in order to be useful for communication purposes, the tachyon or whatever type of particle mediates the FTL communication must be capable of interacting with some form of "normal" matter. If the tachyon or whatever is capable of interacting with some form of "normal" matter, quantum mechanics demands that those "normal" matter particles must be surround by a cloud of virtual tachyons (or whatever) that contribute to its inertial mass --- and since by hypothesis, spatial isotropy has been broken, in general there is no reason why the virtual particle distribution should be the same for a particle moving parallel to the "tachyon corridor" versus perpendicular to it, and as Dicke has shown, this would lead to forces that violate the Equivalence principle. Yet the Hughes and Drever experiments and the Eotvos experiment both show that the inertia of all forms of matter tested are isotropic and the equivalence principle is satisfied to a truly astounding degree --- arguing strongly against the existence of a "tachyon corridor." Finally, since every particle is surrounded by a cloud of virtual particles mediating every force they couple to, and those virtual particles in turn interact with every particle that _they_ couple to, it would appear that the Hughes, the Drever, and the Eotvos Experiments rule out any sort of "preferred frame" coupling to =ANY= form of matter that couples to "ordinary" matter at all, to high precision. Hence, if the "tachyon corridor" exists, it must not couple to ordinary matter, rendering it useless for communications purposes, or it must couple to ordinary matter so weakly that it is utterly impractical for communications purposes. A more subtle version of this argument also argues strongly against the existence of a preferred reference frame, as the cloud of virtual tachyons or whatever would lead to an anomalous variation in particle energy with velocity, and this variation would in principle show up in particle physics experiments, allowing on to determine one's absolute velocity relative to the "cosmic frame" --- yet no such effects have been observed. Since such anomalous direction or velocity dependences of mass have =NOT= been observed, one must conclude that either "physically preferred frames" allowing FTL communication do not exist, or that they couple to "normal" matter so weakly that they MIGHT AS WELL not exist. Another way to get FTL without violating causality is if there's some way of increasing the speed of light over a finite volume of space. If there's a limit to how quickly this can be done, and if the speed of light falls off gradually back toward its usual speed as you leave the region rather than there being any sharp edges, this would also allow FTL without violating causality. Actually, this commonly proposed notion doesn't work, since the "light cone" is what _DEFINES_ the "speed of light." People who have attempted to work on the so-called "Variable Speed of Light" (VSL) cosmologies run into this problem all the time: Careful analysis shows that, since the speed of light is a "dimensionful" quantity, it can only be compared to other quantities that have the _SAME UNITS_ . Hence, it is physically meaningless to talk about the speed of light "varying" unless physics provide a "preferred system of units" --- and there is no evidence that such a "preferred system of units" exists. In a sense, gravitational waves do exactly this, albeit to too small and too temporary a degree to be useful. There is =NO= sense in which gravitational waves do this in GR !!! In General Relativity, the propagation of gravitational waves is governed by the VERY SAME LIGHT-CONES that determine the "speed of light" for every other force, and for every other form of matter !!! (This is at the heart of the controversy over Kopeikin's "speed of gravity" claim, and why Clifford Will maintains that all Kopeikin has done is measured the "speed of light" by a perverse and inaccurate method.) Gravitational waves can only "change the speed of light" in NON-GR THEORIES, e.g., Rosen's "bimetric" theory (which is STRONGLY ruled out by experiment). -- Gordon D. Pusch perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;' |
#23
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could we shine a laser on this new solar system and detect something in 180 years?
Roger Stokes wrote:
Thus current theories should not be defended as if they were inviolable dogma - maybe most people who propose revisions and augmentations are ignored by the mainstream, but that doesn't NECESSARILY prove they are wrong, or are crackpots. The final point of the OP was to point out that 19th century physicists almost certainly viewed the Newtonian model as being "truth", yet there were clues (Michelson-Morley etc) that modifications were needed. My question was what clues exist pointing to areas of current theories where modifications might be needed? Quantum gravity theory, or the inability of such to be found? By present trends it would appear that we will arrive at the result that the folded up extra dimensions needed by string theory do not exist. There have been suggestions that a more complex and sophisticated notion of causality is eventually going to be needed. General relativity seems to be full of ways to build time machines. (Although people have rather less confidence in GR than in SR, not least because it appears to be fundamentally inconsistent with quantum mechanics.) This is obviously one of the clues I referred to Conversly, the real mechanisms of quatum mechanics might be different to what they are thought to be. Similarily, macro-level causality might be something the human mind invented, and the 'paradoxes' being the lack of understanding of reality. ...However, there is a theorem by Yorke that any such General Relativist "time machine" (or "space warp" FTL drive) must NECESSARILY contain "exotic" matter. While the apparent observational evidence for exotic "Dark Energy" has caused physicists to become somewhat less skeptical about such possibilities, it should be noted that =ANY= form of "exotic" matter NECESSARILY implies that the Second law of Thermodynamics can be violated --- and even the few remaining "Newtonian Recidivists" are (mostly) given pause by such a deep and disturbing revision to the Laws of Physics... Is this a clue, and if so in which direction is it pointing? BTW I have read that some VSL theories violate conservation of mass-energy, yet still are achieving a consistent description of "reality" - there were no details however. Temporary violations for tiny periods of time are possible, and have in fact been demonstrated. So the need to violate it is not in and itself a barrier to FTL, but a potential barrier to how long any given episode of FTL can last. So the usefulness would depend on what you can do say during the 10ns spacewarp - but you need high multiplies of c, and travel (and not say torpedoes or similar) needs you to be able to be able to produce lots of such warps regularily. But once you can produce a warp, even for a fleeting fraction of a second, and push for even a just slightly faster than c, it all becomes engineering. --Roger "Crackpots of the world unite, you have nothing to lose but your...er....um..." Pots, of course 8-P -- Sander +++ Out of cheese error +++ |
#24
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could we shine a laser on this new solar system and detect something in 180 years?
Gordon D. Pusch wrote:
Another way to get FTL without violating causality is if there's a preferred *direction* in space. Maybe it's possible to send signals at any finite positive speed in that direction, but no other. This too would allow FTL without violating causality. Most physicists find this notion of such a "tachyon corridor" even more un-aesthetic than a preferred reference-frame or a preferred time --- and again, one must ask why it apparently has _NO OTHER OBSERVABLE PHYSICAL EFFECT_. In particular, one must ask why such a spatial "preferred direction" does not show up in the Hughes and Drever Experiments, which establish spatial isotropy of inertia to the unprecedented accuracy of better than one part in 1e20. But this would not rule out something that was as weak as the weak interaction - or would it? After all, localy, there are more neutrinos moving away from Sun compared to any other direction. -- Gordon D. Pusch perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;' -- Sander +++ Out of cheese error +++ |
#25
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FTL (was could we shine a laser on this new solar system and detect something in 180 years?)
[Posted and mailed.]
Gordon D. Pusch wrote: ...However, there is no evidence that the "preferred frame" represented by the microwave background exerts any _physical_ effect on an otherwise isolated observer (beyond a slight warming effect and a minute amount of viscous drag if you are ploughing through it); nor is there any evidence that the microwave background provides a "preferred clock" governing any other physical phenomena. I know. I was just pointing out that it's *possible* that FTL could exist without violating causality. Personally, I think it more likely that FTL doesn't exist. And that if it does, that it *does* violate causality, perhaps by building bridges between "multiple worlds". Finally, there is no a priori reason why the "preferred frame" in which the microwave background is isotropic should be correlated with the "preferred frame" represented by a cosmologically "co-moving" observer in the sense of a General Relativistic cosmology, ... Which is why I listed that possibility separately. Furthermore, the Hughes, the Drever, and the Eotvos Experiments impose stringent experimental limits on =ANY= "preferred frame" effect that couples to "normal" matter (see below). But the limit isn't zero. For instance, since gravitational waves have not yet been detected, it's possible that they travel slightly faster than light. Most physicists find this notion of such a "tachyon corridor" even more un-aesthetic than a preferred reference-frame or a preferred time ... Yes. It's ugly, and GR isn't. But who says the universe has to have beautiful or symmetrical laws just because we prefer them? Actually, this commonly proposed notion doesn't work, since the "light cone" is what _DEFINES_ the "speed of light." People who have attempted to work on the so-called "Variable Speed of Light" (VSL) cosmologies run into this problem all the time: ... Yes, the new (1982) definition of the meter implicitly *defines* the speed of light as constant. So if there's some process which could have been interpreted as speeding up light, it would now be interpreted as simply moving objects closer together. So, rephrasing, perhaps there's some way to move Alpha Centauri closer to our solar system, and to do so without making it look any brighter or have a greater gravitational effect on us. I have been puzzled by exactly what these VSL theories are trying to claim. Sure, maybe the fine structure constant was different in the past. But it seems to me completely arbitrary whether to attribute such a change to a change in the speed of light, a change in Planck's constant, a change in the charge of an electron, or some combination of these. Are VSL theories really variable FSC theories? There is =NO= sense in which gravitational waves do this in GR !!! They make the distance between objects vary, without either object experiencing acceleration. This could just as validly be interpreted as a constant distance and varying speed of light as vice versa, if you ignore the arbitrary choice in how the meter is defined. It's a very small and temporary effect in either case. -- Keith F. Lynch - - http://keithlynch.net/ I always welcome replies to my e-mail, postings, and web pages, but unsolicited bulk e-mail (spam) is not acceptable. Please do not send me HTML, "rich text," or attachments, as all such email is discarded unread. |
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