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Tachyons travel backward in time? 
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#55
Feb908, 02:56 PM

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#56
Feb908, 03:59 PM

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You set the experiment to for 10 units of time, with your other frame moving 6 units of distance, light moving 10 units of distance and your tachyon moving 100 units in those ten units of time, Is that a trivial part of your own thought experiment you did not see? You’re the one declaring for a preferred frame for x’ otherwise exactly how do you establish simultaneity of anything anywhere with event x’=117.5 t’=62.5. In your example SR can only define the causality relationship of that event with one and only one other event t=10 x=100. SR cannot and does not define those two events as simultaneous with any other events includes those with t=10 or t’=62.5 wherever they may be. The rest is just not worth commenting on except to say that if there is anything that is FTL such as a Graviton, Higgs Particle or Tachyon it should be obvious they would have to follow rules of physics beyond what know now. And nothing so far shows that such rules exist. But if they do they do not need to cause “backwards time”. 


#57
Feb908, 05:24 PM

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If two events happen at the same time they are simultaneous. That is the definition of simultaneity. The relativity of simultaneity just means that two events which happen at the same time in one inertial frame will not happen at the same time in another. The relativity of simultaneity is not the absence of simultaneity, it just means that you have to specify the reference frame in which they are simultaneous. 


#58
Feb908, 08:13 PM

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#59
Feb1008, 03:12 PM

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Any one frame can ignore all other frames and show separated simultaneous events and get usable answers as long as it never uses a value defined in some other frame. Simultaneity says that the simultaneous space like separated results you find using such a “preferred frame” cannot be understood as actually being simultaneous. Sure even Einstein uses the word simultaneous within individual frames for SR speed problems such as trains and embankments. That is how he illustrated getting to the conclusion that simultaneous events in one frame were not simultaneous in the other frame. Likewise simultaneous events in the other frame were shown to not be simultaneous in the first. From that Einstein established the principle of simultaneously, in order to maintain a consistent set of physics laws usable in all frames of reference. The simple rule of simultaneity is that simultaneous events within a single reference frame cannot be considered as actually being simultaneous  That's It! If not that rule, then you must establish a single “preferred frame” and use a different set of SR physics laws in all others or at least recognize simultaneous observations in those other frames as wrong. Now you and Jesse seem to think that simultaneity can be used to identify the correct time for tachyon in absolute values well enough to when it reached a distant point to know realtive to local time. Balderdash. If either of you could do that, then I have simple challenge one of you should be able to solve – no tachyon needed. If you can do it for tachyons, this should be easy. Use Jesse’s problem where the other frame moves at 0.6c with observers at x= 0 & 10 and x’ = 0 & 5. Do the math to be sure we all agree on the fallowing three sets of truly simultaneous events based on SR simultaneity rules: A: x = 0 correctly observes a flash of light (following the tachyon) start off at t=0 simultaneously x’ = 0 sees the same flash start at t’=0 B: x =5 meets x’ =0 at t=8 1/3 simultaneously x’= 0 meets x = 5 at t’ = 6 2/3 C: x = 10 sees the flash of light arrive at t = 10 simultaneously x’ = 5 sees the same flash arrive at t’ = 5 When did the simultaneous C events occur before or after simultaneous B events ? Observer x = 5 claims before as do all other x observers while x’=0 claims after as do all other x’ observers! If either of you can solve this using SR and simultaneity, without a preferred reference frame. I will issue an apology and a retraction. But I expect an unambiguous definitive answer for one or the other before or after! If not you have no business claiming to do any better with a hypothetical tachyon at even greater distance. 


#60
Feb1008, 05:46 PM

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Now, events C and B have spacelike separation. In the unprimed frame B occurs before C (BC=(1.67,5)), while in the primed frame B' occurs after C' (B'C'=(1.67',5')). In the (double primed) reference frame moving at 1/3 c relative to the unprimed frame the two events are simultaneous (B''=(7.07'',2.36''), C''=(7.07'',7.07''), B''C''=(0'',4.71'')). For ANY arbitrary pair of events with spacelike separation you can always find a reference frame where they are simultaneous, another where one occurs before the other, and a third where the order of occurence is reversed. 


#61
Feb1008, 06:00 PM

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What do you mean by "local time" anyway? Do you mean something different than "the time in a particular frame"? After all, frames of reference in SR are not "local" to a particular region, they are coordinate systems that describe events throughout the entirety of spacetime. x = gamma*(x' + vt') = 1.25*(0 + 0.6*6 2/3) = 1.25*4 = 5. And it's also true that t = gamma*(t' + vx'/c^2) = 1.25*(6 2/3 + 0.6*0) = 8 1/3. So, all you're really saying here is that the event with coordinates (x=5, t=8 1/3) in the unprimed frame has coordinates (x'=0, t'=6 2/3) in the primed frame. These aren't two simultaneous events, it's just a single event that is assigned different coordinates in the two coordinate systems. 


#62
Feb1108, 07:59 PM

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DaleSpam & JesseM you guys got some es’planen’ to doo
The C event is a set of observations made by a single observer located at x=10 including 1.getting nose to nose with another observer located at x’=5 with a clock on display 2. Receiving enough photons from two clocks in the local vicinity to make note of t=10 and t’=5 on those two clocks. 2. observing enough photons from a flash of light to indentify it as the flash starting at simultaneous events A & A’. The C’ event is another set of observations made by a completely different observer in another reference frame. Collecting a differ group of sampling photons, confirming the same information about getting nose to nose, times on a couple clocks, and a flash of light that has just appeared. Likewise I take B & B’ to be a pair of events as are A & A’, each pair being "actually simultaneous” paired events with no spacelike separation between the two. [I have no problem with using a four variable description of these pairs as (x,t,x’,t’) but the two variable (x,t) or (x’t’) for each separate event is no less a complete description for where both events occurred.] JesseM asks “What does "actually simultaneous" even mean?” That means for the above three pairs of events is that not only does each pair appear simultaneous in the x & x’ frames; but as they are not space like separated they will be “actually simultaneous” as measured by any and every other possible reference frame you might come up with. (Try if you like). It also means, Einstein simultaneity tells me that apparent same time events with spacelike separation such as t=0 for x= 0, 5, & 10 within a frame, CANNOT be considered to be "actually simultaneous" even if I live on that frame no matter how weird in might seem to me. Obviously you and JesseM disagree with me which being us to your second sentence: [b]“The fact that two given spacelike separated events happen to be simultaneous …”[\b] Clearly you and JesseM feel that it is possible find a case where spacelike separated events in a common reference frame are “actually simultaneous”. Such as when JesseM concludes that the x' = 117.5 t' = 62.5 is “actually simultaneous” with x’=0 t’= 62.5 and therefore the tachyon would have ended at x’ = 117.5 before it started at x’=0 Neither of you need to convince me of the tachyon example, just one of you show us you have the math to demonstrate any two space like separated in a reference frame such as t = 10 for x = 5 and 10 “actually simultaneous”. Meaning that any and all other reference frames will either agree they are simultaneous OR if any alternate frame disagrees you have math convincing to observers in such other frames the their observations are wrong and your evaluation of the two events as simultaneous is correct. If you can do that in the nonFTL case I will accept your applying whatever math you are using in the FTL case. But if your appoch cannot be confirmed in the nonFTL case I’ll never accept in the FTL example. 


#63
Feb1108, 10:32 PM

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But we can do it your wayin this case we can imagine an observer at rest in the C frame at position x=10 in that frame, and we can imagine another observer at rest in the C' frame at position x'=5 in that frame, and the two will meet at exactly the moment that the light reaches them. So we really have three events herethe event of the first observer's clock ticking t=10, the event of the second observer's clock ticking t'=5, and the event of the light being detected at that point in space. But note that it's not as if specific events require specific coordinate systems to describe themas long as these events have zero spatial and temporal separation between them, we would say that the event of the of the first observer's clock reading t=10 has coordinates t'=5 in the C' frame, and likewise the event of the second observer's clock reading t'=5 has coordinates t=10 in the C frame. Would you agree? So, the event of Bob receiving the tachyon signal happens at x=100 l.s., t=166 2/3 s in the C frame. What about Bob's own rest frame C'? In this frame the event has coordinates: x' = 1.25*(100  166 2/3*0.6) = 0 t' = 1.25*(166 2/3  100*0.6) = 1.25*(106 2/3) = 133 1/3 Also, in Bob's frame C' Alice has been moving in the x' direction at 0.6c since their positions coincided at t'=0, so at t'=133 1/3 her position in this frame is x' = 0.6*133 1/3 = 80 l.s. Now, suppose at this same moment of t' = 133 1/3 when Bob receives the tachyon signal from Alice, Bob immediately sends a tachyon reply which moves at 10c towards Alice in his frame (this is where the first postulate of relativity comes inif it's possible to send a signal at 10c in the coordinates of one frame, it must be possible to send a signal which moves at 10c in another frame). After about 8.5106383 seconds in the C' frame, at time t' = 133 1/3 + 8.5106383 = 141.84397, the tachyon signal catches up to Alice, since it is now at position x' = 10*8.5106383 = 85.106383, and meanwhile she has moved 0.6*8.5106383 = 5.106383 so her new position is also x' = 80  5.106383 = 85.106383. So, the coordinates in the C' frame of Alice receiving the reply are x' = 85.106383, t'=141.84397. Now go back to Alice's frame and see when she receives the reply. Again, this can be done with the Lorentz transform: t = 1.25*(141.84397 + 0.6*85.106383) = 1.25*(90.78014) = 113.4752 seconds. And of course, since Alice is always at the origin in her coordinate system, the position of this event is: x = 1.25*(85.106383 + 0.6*141.84397) = 0 Anyway, the thing to note is that the event of Alice receiving Bob's reply actually happened earlier in Alice's frame than the event of her sending the original tachyon signal at t=156 2/3 seconds! She actually received Bob's reply before she sent her first message, so if Bob had just replied by sending her message back to her, she'd know what message she was going to send before she sent it. And the event of Alice receiving Bob's reply has a timelike separation from the event of her sending the message, so all inertial frames agree on the order of these two events. 


#64
Feb1208, 06:15 PM

P: 1,545

Don’t have time to review your entire post but here is the important part you need to address
If you did not make this assumption (a preferred frame assumption by the way) explain exactly how you decided the x’ = 117.5 arrival happened before the x’=0 start? Obviously those results give the appearance of “backwards” time to x’ observers. BUT IT DOES NOT give that appearance to them if they apply the rules / understanding of SR simultaneity! You guys seem to think “simultaneity” says something like “you may consider same time events within a common frame to be simultaneous”! Or as you put it “that simultaneity is relative to your choice of reference frame”! That could not be more wrong, SR says nothing of the sort nor is it a part of “the way the Lorentz transform works”. Simultaneity applies uniformly the same in any frame of reference as any physics rule should. It says that those observers in order to truly understand the reality of their own reference frame must recognize that spacelike separated common time events cannot automatically be considered simultaneous! Additional, SR cannot and is not the tool to establish any two spacelike separated as being simultaneous. Dang, that makes it hard to use, how can astrophysicists get any work done! They make an assumption! A non SR, lets ignore Simultaneity for a special case inside a few thousands of lightyears around ( more if we can get by with it). You cannot call such a departure from SR “relativity” and they don’t – it’s called Astrophysics as they presume to establish a preferred reference for at least a local region of space (but often much larger) based on the CMBR. (Particale Physics doesn’t need or use such a device AFAIK) .........And by the way SO DID YOU! When you started the problem in post # 53 you made it clear that the tachyon was ten times faster than light, and it was clear you intended that to mean after t=10 light and the tachyon would simultaneously reach x=10 and x= 100. Unless you make it clear that those two events may not be simultaneous, and how you proceed with that fact included you simply cannot assert that you did not start without a preferred frame assumption. Frankly IMO you cannot even state the problem without making that assumption. And once you do it is exclusive, no other frame can be considered preferred and all time ordering issues must be converted the one preferred frame. That means you cannot draw conclusions from the t’ observations as you did. So unless you can state clearly how you avoided using preferred frames and still defined which space like separated events happened when relative to a single observer like at x’=5 or x=10 your example is pointless. But by sticking with your original preferred frame I can state the following. Tachyon and Light starting at t=t’=x=x’=0 arrived simultaneously at t=10 at locations x=100 and x=10 respectively in the preferred frame. Additionally the t’=5 at x’=5 is simultaneous with t’ = 62.5 at x’ = 117.5 However if you were using some level of science beyond what Astrophysics uses, now is the time to spell it out 


#65
Feb1208, 06:19 PM

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Please spend the time and effort to learn the basic concepts (spacetime, event, Lorentz transform, simultaneous, Minkowski diagram, spacetime interval, timelike, and spacelike) for yourself, and then come back and see if these comments make more sense. 


#66
Feb1208, 07:50 PM

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#67
Feb1308, 11:58 AM

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Conversely, if antimatter and normal matter gravitationally repel one another, and antiparticles move backwards in time, then antiparticles would appear to be gravitationally attracted to normal matter in normal time. 


#68
Feb1308, 12:32 PM

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#69
Feb1308, 01:40 PM

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Why not start your explaining with the one you left out, simultaneity. I put up mine: “… Einstein simultaneity means that apparent same time events with spacelike separation within a frame, CANNOT be considered to be "actually simultaneous" even if I live on that frame no matter how weird in might seem to me.” To be applied as is, in any reference frame you find. JesseM put up his: “ … each frame uses its own distinct definition of simultaneity, and all frames are equally valid as far as physics goes.” Thus each frame has a difference set of physics laws as each has a “distinct definition of simultaneity”. A concept my definition of simultaneity does not support. We both read the same Einstein but reach different opinions of what “simultaneity” means. So where is your definition and of the above which is correct. What do you use to support it, other than repeating whatever JesseM says? In other words Sir Spam, put up or shut up. 


#70
Feb1308, 01:41 PM

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And more than simultaneous it also includes declaring you can tell from inside a single reference frame if an event happened before or after some other space like separated event Which is exactly what you did! And I are you really so dense as to not recognize you used two different preferred frames to do it! There might be some cases where you can make the assumption of using a preferred frame outside the SR rules, but no one can justify using two different preferred frames in the same problem as you do. In any case this is my last post here I am unsubscribing from this thread, I’ll not participate in your nonsense any longer. If you two are able to convince others that your “backward time” assumptions can be supported by “Einstein Simultaneity” so much the worse for the credibility of these forums. 


#71
Feb1308, 03:12 PM

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#72
Feb1308, 04:20 PM

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Simultaneity: two events are called simultaneous if they share the same time coordinate. Relativity of simultaneity: as a consequence of the Lorentz transform, two events which are simultaneous in one inertial reference frame may not be simultaneous in another inertial reference frame. 


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