[Spaceships] Real Life Non-Super Science Reactionless Engines

[Anthony]

Originally Posted by malloyd

Doesn't help. Relativity is about the relationship between reference frames, there is no special "frame you are traveling in", you are traveling in all of them.

So? I'm talking about the observer frame. This is simply an assertion that there exists a frame from which all FTL is forward in time. This can actually be true without the special frame being privileged by anything but coincidence, the wormhole network with virtual particle collapse create a preferred frame that matches the wormhole network.

[Agemegos]

Originally Posted by scc

I'll give you an example based upon my understanding of the problem. You're on a satellite that due to the speed of it's orbit experiences some time-dilation. Now the satellite your on only communicates once per day with Earth, at precisely midnight, Greenwich Mean Time, with asynchronous computer transfers. One day you notice that the daily upload from the ground has started early, you realize that it's because of relativity, but no special frames of reference says that the clocks on Earth are running slow, not yours, and that causality has been broken.

I don't think you have that right. "No special frames of reference" is one of the postulates that you derive relativity from. Relativity is never going to contradict one of its own postulates.

In your example the satellite is constantly accelerating, and the ground server is supported in a gravitational field, both or which bring General Relativity into play. You're talking about accelerating frames of reference, and unlike straight-line motion acceleration is absolute. The statement that there are no privileged frames of reference is more strictly that no inertial frame of reference is privileged.

[malloyd]

Originally Posted by Anthony

So? I'm talking about the observer frame. This is simply an assertion that there exists a frame from which all FTL is forward in time. This can actually be true without the special frame being privileged by anything but coincidence, the wormhole network with virtual particle collapse create a preferred frame that matches the wormhole network.

There is no "observer" frame. The problem necessarily involves two observers in different frames - neither of which is necessarily stationary with respect to the ship.

FTL travel is by definition forward in time in the frames it is FTL in, that's inherent in the definition of the word "faster", which assumes you can define velocity sensibly somehow. The problem is that if some observer can see something move FTL, some other observer can see it arrive long enough before it departs to send a signal to abort the departure.

[scc]

Originally Posted by Agemegos

I don't think you have that right. "No special frames of reference" is one of the postulates that you derive relativity from. Relativity is never going to contradict one of its own postulates.

In your example the satellite is constantly accelerating, and the ground server is supported in a gravitational field, both or which bring General Relativity into play. You're talking about accelerating frames of reference, and unlike straight-line motion acceleration is absolute. The statement that there are no privileged frames of reference is more strictly that no inertial frame of reference is privileged.

Like I said, I'm pretty sure I don't understand how it works, but satellites being affected by Special Relativity in the form of time dilation is a problem in the real world.

[Agemegos]

Originally Posted by scc

Like I said, I'm pretty sure I don't understand how it works, but satellites being affected by Special Relativity in the form of time dilation is a problem in the real world.

Of course it is. And it is corrected for using General Relativity, without having to posit any privileged inertial frame of reference.

[Agemegos]

Originally Posted by malloyd

The problem is that if some observer can see something move FTL, some other observer can see it arrive long enough before it departs to send a signal to abort the departure.

Not a lightspeed signal. And if the FTL mechanism has properties that are not invariant between frames of reference it can work out that an FTL signal won't do it either. That's why you can choose "causality" and "FTL" out of the "causality, relativity, FTL: choose only two" trilemma.

[malloyd]

Originally Posted by Agemegos

Not a lightspeed signal. And if the FTL mechanism has properties that are not invariant between frames of reference it can work out that an FTL signal won't do it either. That's why you can choose "causality" and "FTL" out of the "causality, relativity, FTL: choose only two" trilemma.

Of course you can. You can select a preferred frame in which the physics are defined. That works fine, but you have to abandon the notion there is nothing but coincidence that's special about that frame.

[RyanW]

Originally Posted by scc

Like I said, I'm pretty sure I don't understand how it works, but satellites being affected by Special Relativity in the form of time dilation is a problem in the real world.

Time dilation and the frames of reference issue mean that you can observe something before the event that causes it occurs, from your point of view. However, the cause will always be outside the future light cone of any observer of the effect. Without FTL, such causality violations are cosmetic.

[Agemegos]

Originally Posted by malloyd

Of course you can. You can select a preferred frame in which the physics are defined. That works fine, but you have to abandon the notion there is nothing but coincidence that's special about that frame.

Agreed. I'm just saying that if you do (abandon the Postulate of Relativity) you can end up with a case in which travel occurs along a space-like interval (FTL in some inertial frames of reference, backward-in-time in others, instantaneous in boundary cases) but it is not possible for any signal from the arrival event to get back within the past-ward light-cone of the departure event.

[malloyd]

Originally Posted by Agemegos

Agreed. I'm just saying that if you do (abandon the Postulate of Relativity) you can end up with a case in which travel occurs along a space-like interval (FTL in some inertial frames of reference, backward-in-time in others, instantaneous in boundary cases) but it is not possible for any signal from the arrival event to get back within the past-ward light-cone of the departure event.

Can you? If you have a special frame (and hence can define an absolute clock, the one that runs in it) aren't space-like and time-like intervals cleanly separated?