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11-14-2019, 11:46 AM   #41
shawnhcorey

Join Date: Jun 2006
Location: The Great White North
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by awesomenessofme1 Jump gates that work by bending space rather than actually going FTL shouldn't have any of those problems as far as I can tell.
They do. To understand why is to review Special Relativity.

If there are two events, A and B, such that B is in A's future light cone, A will be in B's past light cone. Every observer in the universe, regardless of their relative speed will always have A in B's past light cone and B in A's future light cone. The time and distance between A and B will change but they will always have this relationship.

If there is another event, C, such that C is outside of A's light cone, A will be outside of C's light cone. For every observer, this will always be true but for have of the observers, A will occur before C and for the other half, C before A. And to change the order they appear in, an observe simply has to change their velocity.

For a wormhole, A and C are the ends of the wormhole. To time travel, go through the wormhole; change your velocity so that your entrance to the wormhole is far in the future; than fly back through the wormhole. You are now in your past and have broken causality.
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11-14-2019, 11:47 AM   #42
Fred Brackin

Join Date: Aug 2007
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by AlexanderHowl That is an untestable proposition,
It's not really that hard to set up an FTL system where the practical uses are untestable regardless of what theory says they should be.

For a simple example you could have a Real World observer (not an Observer with a capital"O") who can't see both ends of a spaceship's FTL journey. Indeed, that capital "O" Observer upon being asked "Can you Observe God?" would probably have to say "Only when I look in a mirror.".
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11-14-2019, 12:03 PM   #43
ericthered
Hero of Democracy

Join Date: Mar 2012
Location: far from the ocean
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by shawnhcorey For a wormhole, A and C are the ends of the wormhole. To time travel, go through the wormhole; change your velocity so that your entrance to the wormhole is far in the future; than fly back through the wormhole. You are now in your past and have broken causality.
You are now in your past, but you have not broken causality yet. You must send something (probably a communication) to your past position before you go through the wormhole in order to violate causality, and you're still very much outside of the light cone.

The most common way I see people break causality with wormholes is via moving the wormhole around, creating an age difference between the two ends via the twin paradox and then bringing the two ends next to each other. Which is different than saying you can violate causality via FTL with a wormhole outside of each other's reference frames.

Of course, insisting that the locations of wormholes are "fixed" and can't be moved is imposing an absolute reference frame.
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11-14-2019, 12:06 PM   #44
shawnhcorey

Join Date: Jun 2006
Location: The Great White North
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by ericthered Of course, insisting that the locations of wormholes are "fixed" and can't be moved is imposing an absolute reference frame.
Fixed to what? There is no preferred frame of reference.
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11-14-2019, 12:30 PM   #45
Anthony

Join Date: Feb 2005
Location: Berkeley, CA
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by shawnhcorey They do. To understand why is to review Special Relativity.
You can have permanent wormholes as a spatial feature and they just mean space has a weird shape (though we'd probably see astronomical evidence); you aren't actually going faster than light, it's just that the shortest distance between two points isn't the obvious one. However, once you can create them and move them around, you've got an issue.

In general it's possible to have a network of wormholes in a region and there are quantum effects that can be justified as causing collapse of links that cause causality violations (basically, once a closed time-like loop becomes possible, an infinite number of virtual particles start trying to follow the loop, resulting in feedback that will do... something. Such as the wormholes collapsing or exploding).
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11-14-2019, 12:32 PM   #46
ericthered
Hero of Democracy

Join Date: Mar 2012
Location: far from the ocean
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by shawnhcorey Fixed to what? There is no preferred frame of reference.
That's what I just said. And you haven't explained how to violate causality without accelerating the wormholes (which you can't do by merely changing your reference frame)
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11-14-2019, 12:33 PM   #47
shawnhcorey

Join Date: Jun 2006
Location: The Great White North
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by Anthony You can have permanent wormholes as a spatial feature ...
Yes, if there is a god-frame, FTL can exist without break causality. But there is no god-frame; everything is Relative.
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11-14-2019, 12:43 PM   #48
Anthony

Join Date: Feb 2005
Location: Berkeley, CA
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by shawnhcorey Yes, if there is a god-frame, FTL can exist without break causality. But there is no god-frame; everything is Relative.
You don't need a god frame. You just need to not have wormholes exist in multiple frames that are both close enough and different enough to result in causality problems. The problem comes when you can construct wormholes at will, because then you can fairly easily force the above constraint.
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 11-14-2019, 12:53 PM #49 AlexanderHowl   Join Date: Feb 2016 Re: FTL 'Fuel' [Space] Well, the galaxy has as much of a frame as anything else, so the wormhole could be associated with the galactic frame.
11-14-2019, 02:34 PM   #50
Ulzgoroth

Join Date: Jul 2008
Re: FTL 'Fuel' [Space]

Quote:
 Originally Posted by shawnhcorey They do. To understand why is to review Special Relativity. If there are two events, A and B, such that B is in A's future light cone, A will be in B's past light cone. Every observer in the universe, regardless of their relative speed will always have A in B's past light cone and B in A's future light cone. The time and distance between A and B will change but they will always have this relationship. If there is another event, C, such that C is outside of A's light cone, A will be outside of C's light cone. For every observer, this will always be true but for have of the observers, A will occur before C and for the other half, C before A. And to change the order they appear in, an observe simply has to change their velocity. For a wormhole, A and C are the ends of the wormhole. To time travel, go through the wormhole; change your velocity so that your entrance to the wormhole is far in the future; than fly back through the wormhole. You are now in your past and have broken causality.
The ends of a wormhole are not events. If they were, you wouldn't be able to turn around and go back through the wormhole you just exited because it would be unreachable in your past.
Quote:
 Originally Posted by vicky_molokh Most traditional magical gates are FTL though . . .
To be fair, most traditional magical gates are in settings where nobody would be able to verify whether they get you from one end to the other faster than light. 'Instantaneous' for typical fantasy purposes could include enough lag for travel to be sub-light. The diameter of Earth is less than 43 light-milliseconds.
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Last edited by Ulzgoroth; 11-14-2019 at 02:40 PM.

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