
#37
Feb208, 01:32 PM

Sci Advisor
P: 8,470





#38
Feb208, 04:32 PM

P: 3,540

See my thoughts about distinguishing objects moving in different directions in time: http://www.physicsforums.com/showthread.php?t=211332 



#39
Feb208, 05:02 PM

P: 869

A.T. is right. There may be no true distinction between "forwards" and "backwards" but the point is that everything is either moving in the same direction in time or not.
JesseM, you're clearly going to argue with me regardless of what I say, but my point has been made. 



#40
Feb208, 05:58 PM

Sci Advisor
P: 8,470





#41
Feb208, 07:55 PM

P: 869

And I am not claiming anything. I'm trying to have a conversation that's getting derailed because you are claiming ignorance of virtually every word in the English language. 



#42
Feb208, 08:20 PM

Sci Advisor
P: 8,470





#43
Feb208, 08:22 PM

P: 1,060

Good morning Peter0302.
Just to clarify things for me, i take it that for a particle to be at a certain spacetime coordinate (event) more than once it must "travel back in time". If we were to alllow the possibility of a particle's ability to "travel back in time" surely it would have to be one and the same particle to satisfy this requirement of being at the same spacetime coordinate more than once. If it were an anti particle "travelling backwards in time" it would not be the SAME particle at the same spacetime coordinate more than once. Matheinste. 



#44
Feb208, 08:33 PM

P: 869

Another way to tell if antiparticles move forward in time or not would be if they were gravitationally repelled by normal matter. Anyone know if this has been tested or can be tested? 



#45
Feb208, 08:42 PM

Sci Advisor
P: 8,470





#46
Feb608, 07:05 PM

P: 6

Time dilation and tachyon speed are two different things. That negative time may in reality just mean that tachyon gets from start to finish faster than information. And that is a general problem in theory of relativity i think. It's not well defined in terms when do we think about the problem as physical matter and when in terms of information that we receive. When they sort that out, everything will be much clearer.
I think someday in future it will come as a surprise to scientists that relativity can be explained with Mach's principle and Doppler effect. 



#47
Feb608, 07:18 PM

Sci Advisor
P: 8,470





#48
Feb608, 07:39 PM

P: 6

When you're approaching C you're making information so dense that it radiates in terms of Cerenkov radiation and traveling as fast as infromation about yourself relative to the observer towards which you're flying. So it would appear to the observer somehow like this: Let's assume that traveler and the observer are so far apart that relativistic delay of information is observed. Let's say that you slowly accelerate to 0.5c and then suddenly to 1c. What observer on the other side would see is a very small ship instantaniously followed by your physical self. It is very likely that he would also pick up on sensors a lot of Cerenkov radiation. Now let's assume that we developed technology that allows us to go FTL. You accelerate to 0.5c, then to 1c (a lot of gravitational disturbance occurs at this point) and then you engage FTL drive at that point you have negative time (as seen in the equation)  you're traveling faster than the information about yourself which means you will be at the finish faster than your information (that is not backwards in time). What the observer on the finish line would see is this: you suddenly appear from nowhere followed by your own image. 



#49
Feb608, 08:49 PM

Sci Advisor
P: 8,470





#50
Feb708, 05:47 AM

P: 6





#51
Feb708, 12:15 PM

Sci Advisor
P: 8,470

Do you understand what is meant by "the relativity of simultaneity"? Do you understand that if two events happen at different spatial coordinates but the same timecoordinate in one frame, then they happen at different timecoordinates in other frames? Do you understand that this means that for any exchange of FTL signals, different frames must disagree on whether the event of the signal being received happened at an earlier timecoordinate or a later timecoordinate than the event of it being sent? 



#52
Feb808, 01:15 PM

P: 1,545

Example; assume you can detect and use the information from a tachyon passing by at twice the speed of light in our reference frame and know that it came from a Gamma Burst Event some 10 light years away. That would signal us that 5 years ago a GBE occurred and we won’t see the light form it for 5 more years. Such an early warning signal, not only does not send information into the past, it does not mean “it would appear that event happened before it even started”. It would only signal us 5 years in advance of receiving a message with normal light tell us about a GBE that happened 10 years ago. Even with light we know the information is 10 years old, receiving the information 5 years early does not make it appear the event happened before it started. Also from standard Doppler effects already useful within SR we would know tackyons traveling at such a speed in our reference frame view would not be seen to travel at that same speed any other reference frame. Every frame would see a different speed, meaning no one speed as every frame should see a variety of different speeds for them, so measuring their speed would also be required to get usful info. So I don’t see where SR would require tachyons cause even the appearance of “backwards time”. However, understanding that does not improve the chance that such a thing traveling faster than light or gravity might exist anywhere but in our imaginations. 



#53
Feb808, 03:22 PM

Sci Advisor
P: 8,470

[tex]x' = \gamma * (x  vt)[/tex] [tex]t' = \gamma * (t  vx/c^2)[/tex] [tex]\gamma = \frac{1}{\sqrt{1  v^2/c^2}}[/tex] And here with v=0.6c, [tex]\gamma[/tex] is 1.25. If we plug in the coordinates x=0, t=0 in my frame (the event of the tachyon emission), the Lorentz transformation tells us that in the other observer's frame this event happened at x'=0, t'=0. But if we plug in coordinates x=100, t=10 (the event of the tachyon being received), in this observer's coordinate system this happens at the coordinates: x' = 1.25 * (100  0.6*10) = 1.25 * 94 = 117.5 t' = 1.25 * (10  0.6*100) = 1.25 * 50 = 62.5 So you can see that in this coordinate system, the tachyon was received a full 62.5 seconds before it was emitted. It's just part of the way the Lorentz transform works that if you pick two events with a spacelike separation (like two events on the worldline of a tachyon), you'll always be able to find a pair of frames which disagree about which of the two events happened earlier than the other, as well as some frame where the events happened simultaneously. If you agree that it's possible for a tachyon to be received before it's emitted in some frame, then if tachyons obey the postulate of relativity that says they should follow the same laws in every inertial frame, then it must be possible for ever frame to see a tachyon received before it's emitted. This means that if you are moving away from me, and I send you a tachyon signal such that it moves FTL but forward in time (you receive the signal at a later time than I emitted it) in my frame but it's moving backwards in time (you receive it at an earlier time than I emitted it) in your frame, and then you send a reply which moves FTL but forward in time but backwards in time in my frame, then it can work out that I'll receive your reply before I sent the original signal, a violation of causality. Again, check out the page with the minkowski diagrams illustrating such a situation that I linked to near the beginning of this thread. 



#54
Feb908, 02:32 PM

P: 1,545

That is a gross misunderstanding of SR simultaneity! First you establish a preferred frame with t=10 at x= 6, 10, 100 all happening simultaneously. When you calc t’= 62.5 at x=100(x’= 117.5) for one of those events; no problem with that. But then you assume that event is simultaneous with the x’=0 starting point at t’= 62.5 (near x= 47) only because the t’ times are the same! Not only does “relativity of simultaneity” not say those events are simultaneous, SR specifically states you cannot consider them as simultaneous events. Orthodox SR does not even acknowledge the t= 0 times at x = 6, 10 and 100 as being simultaneous! And also says a preferred frame should not be assumed. I understand Astrophysics does use a preferred reference frame based on CMBR. I have no real problem with that. But what will never work is using two different frames as preferred at the same time as you are doing here. Once you define one preferred frame all other frames must be considered inaccurate for establishing causality, you cannot have two preferred frames. Your thought experiment will need to reroute the tachyon back into local proximity with the starting point in one or both frames, and attempt to show it returning before it started. And that is not going to happen in any thought experiment that follows the SR rules rationally. “ …superluminal transmission. Alice transmits from event P and the signal is instantaneously received by Bob at event Q. Alice and Bob are at rest relative to each other. ” FTL and instantaneous are not the same thing, NO where in SR does it expect FTL to mean instantaneous. It is a bad starting assumption that renders the entire example pointless and false. 


Register to reply 
Related Discussions  
Is backwardintime causality a requirement in Cramer's transactional interpretation?  Quantum Physics  3  
Looking Backward In Time  Cosmology  6  
Tachyons and Time  General Physics  6  
Backward or forward in time?  Special & General Relativity  8  
Why the tread on tires appears to travel backward . .  General Physics  20 