@S1-A.ARPA,@MIT-MC:pduff%ti-eg.csnet@csnet-relay.arpa (05/14/85)
From: Patrick_Duff <pduff%ti-eg.csnet@csnet-relay.arpa> While space-digest is answering questions from people who have always been bothered by some aspect of the theory of relativity, I have a few which have been puzzling me for some time. Is it theoretically possible to create a device which, after being "locked" onto an object (e.g., the Earth), could always tell you your velocity relative to that object (even after a period of near-light speed travel, various maneuvers, etc.)? I'm not talking about a computer which would perform calculations based upon a history file of past accelerations, but rather a "device" which reacts to the accelerations it experiences. What about a "clock" which would always tell you the time & date on the other object? It seems to me that if you can make either one you can make the other one as well; they are *almost* the same device, aren't they? If these devices are possible, would they require lots of mass (as massive as a planet, perhaps?) to achieve reasonable accuracy over inter-stellar distances, or could they be something more like a wrist-watch? An early science fiction book (\Skylark/ by E. E. Doc Smith) had another interesting device which was something like a compass; where-ever you were in the universe, it would point at whatever you had "locked" it onto (the farther away you were from the object, the longer it took the needle to stop moving, or the more power you had to feed it, or something like that; at one point the characters in the story measured this to find out not only the direction, but also the approximate distance to the object). In science fiction stories, the ease with which ships travel through time without traveling through space has always bothered me. If you could exchange one of the three space axes for a time axis (such as inside the event horizon of a black hole?), travel along it, and then rotate back, then to move 1 second you would have to travel approximately 186,000 miles. Am I missing something here? Also, what difference would it make whether you traveled that distance at a slow speed (.001 c) or a fast speed (.999 c)? Does such travel avoid any cause/effect paradoxes? After all, you would be staying within the cause/effect light-cone, wouldn't you? Finally (for now anyway), I have heard it said that Einstein's theory of relativity could be replaced by a quantum theory of gravity. I'm unconvinced; it seems to me that they concern fundamentally different aspects of the universe. regards, Patrick Patrick S. Duff, ***CR 5621*** pduff.ti-eg@csnet-relay 5049 Walker Dr. #91103 214/480-1659 (work) The Colony, TX 75056-1120 214/370-5363 (home) (a suburb of Dallas, TX)
@S1-A.ARPA,@MIT-MC:gwyn@Brl.ARPA (05/14/85)
From: Doug Gwyn (VLD/VMB) <gwyn@Brl.ARPA> > Is it theoretically possible to create a device which, after being > "locked" onto an object (e.g., the Earth), could always tell you your > velocity relative to that object (even after a period of near-light speed > travel, various maneuvers, etc.)? Not really, because it would have to know all about the structure of the region of space(-time) it was operated in. If you happen to know what the structure is (e.g., essentially flat in intergalactic space), then a close approximation could be done by keeping track of perceived accelerations. If the "other object" were another spaceship, it would be even harder, since the device would also have to know how the other object was moving. > I'm not talking about a computer which > would perform calculations based upon a history file of past accelerations, > but rather a "device" which reacts to the accelerations it experiences. What's the difference? > What about a "clock" which would always tell you the time & date on the > other object? Similar situation. Both cases assume that there is a meaning to where the distant object "really is" and what its time "really is"; in general there is no single answer to these questions. > In science fiction stories, the ease with which ships travel through time > without traveling through space has always bothered me. Gee, I find it easy to move through time without moving through space. Rip van Winkle found it even easier.. > Finally (for now anyway), I have heard it said that Einstein's theory of > relativity could be replaced by a quantum theory of gravity. Funny how the people who say this haven't been able to do so. > I'm unconvinced; > it seems to me that they concern fundamentally different aspects of the > universe. Yes, indeed. More relevantly, their conceptual foundations are quite dissimilar. General relativity (more precisely, generalized field theory) is best expressed as a theory about an objective reality. Quantum theory (QED, QCD) fundamentally denies this. Both theories are claimed to work; no single theory has yet been able to unify these two. Most recent such attempts start from the quantum approach; Einstein started from the field theory approach. There are some striking similarities in some of the resulting technical details (e.g., non-Abelian gauge groups for "internal" symmetries) but there are still considerable differences in the concepts.
@S1-A.ARPA,@MIT-MC:pduff%ti-eg.csnet@csnet-relay.arpa (05/15/85)
From: Patrick_Duff <pduff%ti-eg.csnet@csnet-relay.arpa> By now, most readers should have had an opportunity to reach their own opinions concerning the questions I posed. Now that 24 hours has passed since I mailed my last message, let me throw in a few of my opinions. > Is it theoretically possible to create a device which, after being >"locked" onto an object (e.g., the Earth), could always tell you your >velocity relative to that object (even after a period of near-light speed >travel, various maneuvers, etc.)? I'm not talking about a computer which >would perform calculations based upon a history file of past accelerations, >but rather a "device" which reacts to the accelerations it experiences. >What about a "clock" which would always tell you the time & date on the >other object? What's missing here is the unspoken assumption that the object does not accellerate after it is "locked" onto (I suppose one could compensate for predictable accellerations, such as those due to the object's orbit (e.g., the Earth's orbit around Sol)). An alternative (though much less useful) is to have a device which would be "reset" (perhaps while on the ground before takeoff) and would then give the velocity or time & date at that point in space (which will soon be empty as the planet moves on) relative to you regardless of subsequent manuevers. My opinion is yes, they are theoretically possible. Practical complications abound however. For instance, you would have to consider the accelerations experienced while moving in a gravitational field (such as when passing near a black hole). In the case of the clock, since the rate at which it would register passing time would change over a wide range, a purely mechanical solution is difficult. In some situations it would need to move so slowly that vibration, friction and random molecular motions (heat) would become overriding influences. In other situations the various parts of the mechanism would need to move extremely rapidly. These problems could be solved if the device were able to automatically change scales (e.g., one revolution of an indicator used to mean one week passing on the object, but now it means one hour passing) whenever things started going too slowly (or too quickly). I don't know whether you could do these operations without keeping a history of past accellerations. What I'd prefer is a device which simply changes its current operation in direct response to an accelleration it is currently experiencing. >An early science fiction book (\Skylark/ by E. E. Doc Smith) had another >interesting device which was something like a compass; where-ever you were >in the universe, it would point at whatever you had "locked" it onto (the >farther away you were from the object, the longer it took the needle to >stop moving, or the more power you had to feed it, or something like that; >at one point the characters in the story measured this to find out not only >the direction, but also the approximate distance to the object). In \Skylark/ the power to the compass was turned on only when a reading was needed (at least, that's the way I remember it happening). It seems to me that you would need to power such a device continuously unless it used a history file. What I'm discussing in the above paragraphs are some of the instruments which would be needed on a ship capable of traveling at relativistic velocities. When such a ship needs to know where to aim its communication laser, what frequency or bit-rate to use, when to start sending it, the distance to another object, how to rendezvous with another ship, etc., the instruments which are used today on sea-going ships and orbital vehicles are completely inadequate. We know enough right now to write programs for all of the necessary calculations; can someone who has experience as a navigator, communicator, etc. suggest a list of what calculations would be needed to answer all of the questions which would arise in the operation of such a ship? > In science fiction stories, the ease with which ships travel through time >without traveling through space has always bothered me. If you could >exchange one of the three space axes for a time axis (such as inside the >event horizon of a black hole?), travel along it, and then rotate back, then >to move 1 second you would have to travel approximately 186,000 miles. Am >I missing something here? Also, what difference would it make whether you >traveled that distance at a slow speed (.001 c) or a fast speed (.999 c)? >Does such travel avoid any cause/effect paradoxes? After all, you would be >staying within the cause/effect light-cone, wouldn't you? Since travel along the time-axis while it is exchanged with one of the space axes is space-like and hence limited by the speed of light, this implies that the rate at which one can travel through time (at least, by this method) is also limited. The question about the velocity of travel along the time axis really opens up a can of worms. Relativistic effects are tied to gravitational phenomena; what would a gravitational field look like while travelling along a rotated time axis? It seems to me that any velocity-related effects would operate in a time-like manner on the space-like time-axis; in other words, some kind of "meta-time" (I don't know what it is, but it was fun to include it!). As for cause/effect paradoxes, upon reflection I now realize that when you exchange a time-axis with a space-axis, you will actually be operating outside the cause/effect light-cone, not inside it as I had originally imagined. regards, Patrick Patrick S. Duff, ***CR 5621*** pduff.ti-eg@csnet-relay 5049 Walker Dr. #91103 214/480-1659 (work) The Colony, TX 75056-1120 214/370-5363 (home) (a suburb of Dallas, TX)