biep@cs.vu.nl (J. A. "Biep" Durieux) (06/03/87)
>> So it turns out that signals (information transfer) faster than light are >> NOT possible by exploiting this "effect" (whatever it is). ... >> (If it had turned out differently, I would have been very annoyed, since >> it would utterly destroy the credibility of quantum theory in my view.) > >What is so bad about FTL communication? Granted, it would mean there >is either an absolute frame of reference somewhere or causality can be >violated. Most likely the latter. > ...Keith Not even necessary. Causality only would show not to be bound by light speed, as was thought until now. Before Einstein many people thought that, and didn't seem to have any problems with it. Another question: is there any inherent reason why our velocities are so *much* lower than light's? There is a story 'if light went with 55 mph', which made me wonder. i n e w s f o o d . -- Biep. (biep@cs.vu.nl via mcvax) Some mazes (especially small ones) have no solutions. -- man 6 maze
ogil@sphinx.UUCP (06/04/87)
[I've directed followups to sci.philosophy.tech, in an attempt to keep this discussion in one group]. In article <785@klipper.cs.vu.nl> biep@cs.vu.nl (J. A. "Biep" Durieux) writes: >>What is so bad about FTL communication? Granted, it would mean there >>is either an absolute frame of reference somewhere or causality can be >>violated. Most likely the latter. >> ...Keith > >Not even necessary. Causality only would show not to be bound by >light speed, as was thought until now. Before Einstein many people >thought that, and didn't seem to have any problems with it. >-- > Biep. (biep@cs.vu.nl via mcvax) Causality would not be violated for observers in the same inertial frame of reference, but it would be for observers moving relative to one another. Consider the following example: ILLUSTRATION 1: Space-time Diagram ----------------------------------------------------------------------------- t t t t t t s . t s . t ***********B . y A********** ###### s . y t ###### s . y C###### s . y t s . y t s . y t s. y t s y t .+ t .ys *xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx t ----------------------------------------------------------------------------- Let two observers be moving at a high velocity relative to each other (say .99 c). Observer 1's world line is represented by the t's, and his spatial axis by the x's. Observer 2's world line is the line of s's, and his spatial axis is the line of y's. The .'s represent the world line of a photon. Now, assume that each observer has an FTL gun (it shoots bullets at FTL; don't worry about the energy problems). 1 shoots his gun at 2 (the event marked A), and the bullet's world line is represented by the *'s. The bullet intercepts 2's world-line at event B, but 1's aim was off and he misses 2. 2, being upset by this hostile action, fires his gun at 1. The bullet's world line is represented by the #'s, and it is moving in the positive direction with regard to 2's time axis (world line). It intercepts 1's world line at C, killing him. Unfortunately, 2 has killed 1 before 1 fires the offending bullet. This presents something of a problem :-). By examining the space-time diagrams on paper, where precise drawings are easy to make (compared to a terminal, at any rate), one can see that causality is violated when information is transferred at a velocity greater than the reciprocal of the observers' relative velocity. Since the observers can move as close to c as we want, c is imposed as the limit of information transfer between the two. In order to preserve causality, one has to throw out special relativity, which has been confirmed too many times to be false. Thus, FTL information transfer is impossible (unless you want to go all the way and eliminate causality! :-). -- __________________________________ "Display me Aeolus above Brian W. Ogilvie | Reviewing the insurgent gales ...{uwvax,hao}!oddjob!sphinx!ogil| Which tangle Ariadne's hair ogil%sphinx@uchicago.BITNET | And swell with haste the perjured sails."
biep@cs.vu.nl (J. A. "Biep" Durieux) (06/05/87)
Things started like this: >What is so bad about FTL communication? Granted, it would mean there >is either an absolute frame of reference somewhere or causality can be >violated. Most likely the latter. ...Keith In article <785@klipper.cs.vu.nl> biep@cs.vu.nl (that's me) writes: >Not even necessary. Causality only would show not to be bound by >light speed, as was thought until now. Before Einstein many people >thought that, and didn't seem to have any problems with it. In article <1904@sphinx.uchicago.edu> ogil@sphinx.UUCP (Lord Julius) gives a longish detailed example of an FTL paradox, ending with: >In order to preserve causality, one has to throw out special >relativity, which has been confirmed too many times to be false. >Thus, FTL information transfer is impossible (unless you want to go >all the way and eliminate causality! :-). Well, what I meant to say was that, given FTL information transfer, I would be more inclined to drop relativity theory than to drop causality (at least on our macroscopic scale). But that doesn't necessarily mean choosing a fixed reference frame, does it? There might be other possibilities. Or again, there might be an absolute reference frame, and another explanation for the observations in favour of Einstein. Anyway, thanks for the paradox description. In this case it wasn't necessary for me, but it's things like that which make it a pleasure to read the net. Please, people, don't just ask questions about details of theories and give answers supposing intimate knowledge with the field. Lots of people prefer *much* reading a short intro to a specific problem. An example (nothing personal): there was some discussion about epistemic logic, where people talked about S4, S5 and T as appropriate bases. It would be very nice (and not too much work, I suppose) to type in the axioms in the different sets, so that everybody could follow and under- stand the problem, and learn something from it. -- Biep. (biep@cs.vu.nl via mcvax) My F-key has autorepeat
gustav@swanee.UUCP (06/14/87)
When you look closely at such events, you'll see that you can still preserve the causality, but you have to reinterepret what you observe depending on the system of reference. If you have two particles A and B moving with subliminal speeds and one of them, say, A emits tachyon which is then absorbed by B (fully equivalent to your argument with shooting the bullets at one another), from another system of reference, this will look like B emitting an anti-tachyon which is then absorbed by A. If the B dies upon receiving a tachyon from A in one system of reference, in the other one it will die because of the energy released in the emission process. There is nothing new about particles which move back in time being seen as antiparticles. The only novelty is that the same tachyon will be perceived either as a particle or as an antiparticle depending on the system of reference it is observed from. Another thing is that if you look at events of this kind, you start realising that the whole concept of causality is somewhat in doubt. We can observe here some events which are connected with one another through tachyon links, but the interpretation as to what is the cause and what is the effect depends on system of reference. What does not depend on system of reference is the physics of these events itself: i.e. the fact that they are CONNECTED through tachyon links. This shouldn't be anything new to those who studied EPR paradox enshrined in Bell inequalities and in recent experiments which seem to confirm that the causality as perceived by Einstein is indeed violated in the domain of quantum mechanics. Whether it is due to tachyon links or to something completely different still remains to be seen.
dhw@itivax.iti.org (David H. West) (12/21/89)
In article <5610@rice-chex.ai.mit.edu> miken@rice-chex.WISC.EDU (Michael N. Nitabach) writes: >metaphysical tradition, beginning with the British Empiricists, e.g. Locke >and Hume. This is the view that causation is not an aspect of the world >which our mentality can recognize, but rather a schema which our mind imposes ^^^^^^^^^ >on events with appropriate spatiotemporal relations. A conceptually >opposite--Realist--stance would be that causation exists as an actual >attribute of certain pairs of physical events. What, in this view, is "recognition"? Is it fallible? If so, how is it different from imposing (perhaps unconsciously) a schema which may need to be revised to accommodate later sense-data? If it is not fallible, why are we still doing science? [hint: this is a rhetorical question :-) ] -David West dhw@iti.org