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SR, LET, FTL & Causality Violation 
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#1
Nov2811, 10:26 AM

P: 275

In SR. Anything FTL or superluminal can affect causality because there would be some frame where things move backward in time. Since SR is equivalent to Lorentz Ether Theory. And LET is about additional dynamics that occurs in the backdrop of newtonian absolute space and time. Then how can you model FTL and LET and things moving back in time since the background is supposed to be absolute space and time (where things moving backward is in conflict with its main postulate)? I've been googling this but can't find the information. Thanks.



#2
Nov2811, 10:49 AM

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P: 8,332

In SR and LET (the form that is equivalent to SR), every inertial frame is a preferred coordinate system, and in that sense physics in an inertial frame is "as good" as Newtonian absolute space and time (Rindler, p43).
However, what fundamentally distinguishes SR/LET and Newtonian physics, since both have inertial frames or preferred coordinate systems, is the transformation between the inertial frames. In SR/LET, the transformation between inertial frames is given by the Lorentz transforms, whereas in Newtonian physics it is given by the Galilean transform. This is why forms of FTL that violate causality are still forbidden in LET. 


#3
Nov2811, 11:11 AM

P: 1,097

So then what does negative propertime mean? Seems meaningless. So I think that means FTL breaks the "all physics the same in all FoR", not causality. I don't know for sure but isn't a postulate of SR that c is the maximum whatever? Seems like this would have to be assumed for the rest of the theory to "work". Said differently SR doesn't (directly) address FTL. Is that right? http://www.physicsforums.com/showthread.php?t=522200 Picturing the light cone in 3D (as a sphere) was pretty enlighting, including seeing that speed of cause isn't as "important" as it preceeds effect. 


#4
Nov2811, 11:28 AM

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PF Gold
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SR, LET, FTL & Causality Violation



#5
Nov2811, 11:44 AM

P: 1,097

My post above though, is only suggesting that besides the point of whether FTL is possible, it would mean measurements done in that FoR aren't "transferable" / "applicable" to the reality. 


#6
Nov2811, 12:51 PM

P: 1,097

I read that post and I don't understand this part;
"If she doesn't recieve a message at (0,0), she sends 1 at (8,0). Bob receives that message at (8,10), and replies with 1 at (8,10). So Alice receives 1 at (0,0), and we still have a contradiction." That part I don't get. I haven't drawn the diagram (will at home). But Ima guess it's because it's a ST diagram where it's always implied that 1ct = 1x. and that slope seprates timelike from spacelike which also happens to illustrate the line between cause/effect. Any cause effect relationship (from a time perspective, I guess the ONLY way causality can appear broken, but is propertime that's "broken" ) that's on the spacelike side means causality isn't illustrated by the photon path anymore. It would be illustrated by the tachyeon things path. Said differently a ST diagram where 1ct=1x; 1ct=1x is the speed limit. Seen as the path of a photon (null line) on the diagram. Anything faster and wouldn't ct have to be redefined? I'd say clearly because of cause>effect. That's my understanding of FTL and causality. With all that being said, I see you qualified the statement with "...the standard argument for why it can't be possible to send instantaneous messages in a special relativistic universe." Which I take as meaning 1ct=1x in this context. 


#7
Nov2811, 02:47 PM

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This short reply is all I have time for today, but I can probably answer followup questions tomorrow.



#8
Nov2811, 04:22 PM

P: 275

http://sheol.org/throopw/tachyonpistols.html Now. How do you model the same thing using LET (just for sake of discussion because we knew there was no FTL). 


#9
Nov2811, 09:32 PM

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PF Gold
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That's an interesting question that I don't know the answer to, because I've never studied LET or spent any significant time thinking about it. Is there a way to see that my Alice/Bob scenario is logically inconsistent in LET too? If someone who reads this has thought about such things, I wouldn't mind being told the answer so that I don't have to think about it.



#10
Nov2811, 11:51 PM

PF Gold
P: 1,376

In LET only one inertial frame is absolute frame. You just don't know which one.
For that reason Lorentz transform is not symmetric  it only appears symmetric. And as there is only one "real" simultaneity in LET there is no paradox with FTL particles. 


#11
Nov2911, 12:32 AM

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#12
Nov2911, 12:53 AM

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#13
Nov2911, 02:32 AM

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#14
Nov2911, 05:21 AM

PF Gold
P: 1,376

Wordline of FTL particle propagating at constant speed is spacelike. That means that we can find inertial reference frame where it propagates from A to B, another inertial reference frame where it propagates from B to A and another one where it "happens" all at once along the path from A to B. Now to send some information you have to perform some modulation of FTL signal and you have to be able to "read" that modulation of signal. Now the question is in what direction this modulation will propagate? Form A to B or from B to A? I would say that SR does not give you answer about that. 


#15
Nov2911, 07:29 AM

Mentor
P: 16,947

Any scenario which violates causality in SR violates causality in LET. The only way around it is to have the aether measurably violate the principle of relativity (eg tachyonic signals go at 2c, but only in the aether frame) 


#16
Nov2911, 05:46 PM

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#17
Nov2911, 07:22 PM

P: 275

Anyway. You said there was no spacetime in LET. Are you saying the length contraction or time dilation are properties of objects that occur in newtonian absolute space and time? But PeterDonis wrote the following: "I'm not familiar enough with Lorentz's papers to know whether he thought at first that his results could be explained by just adding on length contraction to Newtonian space and time. But I don't think it really matters, because Einstein's 1905 relativity papers did make it clear that that wasn't possible; that to make kinematics consistent with the speed of light being constant for all observers, you *had* to give up Newtonian space and time." What do you think? 


#18
Nov2911, 08:43 PM

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