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Time Travel
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Dredwulf60
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PostPosted: Tue May 21, 2019 10:05 pm    Post subject: Reply with quote

Zarn wrote:
Any FTL drive violates causality. Therefore, time travel. That your own, local sequence of events is well established doesn't really matter, as far as I can tell.



Only if you don't want to break relativity.

Star Wars breaks relativity by having its FTL occur in the alternate dimension of 'Hyperspace' which is not subject to relativity and therefore won't impact causality.

Since there is no need to calibrate everything to the constant of lightspeed, the problems illustrated in the blog are moot and only meaningful in our apparent reality.

That's assuming I followed the example correctly.
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Whill
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PostPosted: Tue May 21, 2019 10:42 pm    Post subject: Reply with quote

Zarn wrote:
Any FTL drive violates causality. Therefore, time travel. That your own, local sequence of events is well established doesn't really matter, as far as I can tell.

This blog explains it better than I can:

http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel

The most important segment: "If we could just say that there was only one frame of reference where we needed to set up cause and effect, then we could have FTL without worrying about causality. However, there is no special frame of reference, there cannot be one if relativity is to be true. And relativity is true, because we all measure light to travel at the same speed (also, you need relativity for electromagnetism to work, which you probably do want)."

Thanks for sharing that blog. I have either read that before a similar example, but it had been a while. Having a degree in mathematics and an interest in physics, I enjoy stuff like that. I've only taken two physics courses but the degree involved doing the math used in a lot of applied physics, and I've studied a bit of physics on my own.

Quote:
Pick two:

Relativity
Causality
FTL

The Universe has picked relativity and causality, it seems. Thus, we cannot travel or communicate faster than light.

Agreed. My senior year in high school I did a report on special relativity. The formulas for calculating time dilation, length contraction, and mass increase were amazingly derived only use high school algebra and geometry - The real genius of Einstein was just in thinking of doing it. When entering velocities above the speed of light, some equations produce imaginary numbers (the square root of a negative number). In our universe, FTL is fantasy. However to address your statement about any FTL violating causality, I'll specifically use Star Wars to refute that.

Star Wars demonstrates that realspace and hyperspace experience causality. Relativity is something not dealt with in Star Wars much, but it has been touched on as something that realspace has. There have been a couple SW authors over the decades who refered to traveling FTL in realspace, but those are a tiny minority and I view those references as errors. (The WEG SW Sourcebook corrected one reference in the 2e, and why correct it if it wasn't an error in the 1e?). Which leaves us with the premise of Star Wars FTL travel taking place in hyperspace, another "dimension" which is a sci-fi convention intentionally hand-waived to not be subject to the same laws of physics as realspace. In the list of three things above, relativity is the one that seems removed from hyperspace to allow FTL.

There seems to be an absolute passage of time in hyperspace, one that is experienced by passengers as extremely close to what I referred to as "planetary time". Characters that travel through hyperspace do not seem to ever experience that time flowed differently than in the non-relativistic speeds and gravities on the surface of the human life supporting planets. Of course Star Wars still deals with going back and forth between these two dimensions, but there still seems to be an absolute rule of causality with hyperspace travel - You never come out of hyperspace before you went into it, from any frame of reference. So ultimately, the actual nature of time in hyperspace travel (and the transitions to and from it) are inconsequential. No matter what weird stuff is going on, it seems to humans on planets and in hyperspace that there is no noticeable difference when comparing time passage.

The blog example involved ships moving relativistic speeds, and that does make it more complicated. This is another reason that it seems to me that hyperspace travel makes the most sense if ships never travel relativistic speeds in realspace, which would mean that ships must enter and leave hyperspace at speeds significantly lower than the speed of light. Even brief instants of relativistic speeds would dilate time so there is no evidence of hyperspace travelers experiences any significant difference in time than non-travelers. Therefore, there is no significant time dilation in realspace or hyperspace. The violation of causality in the blog example also involved FTL communication which Star Wars also has, using hyperspace or some similar fictional physics-waiving dimension to send messages through. I'm pretty sure that no Star Wars authors have written a scenario that just happened to be like this where FTL communication resulted in receiving a message before it was sent (from someone's frame of reference), and I'd almost be willing to bet that a scenario like this will never just happen to happen.

To your point, since Star Wars FTL travel avoids relativity, causality is maintained and thus not all FTL involves time travel. To my greater point, the fictional "time travel" being discussed here doesn't need ships traveling relativistic speeds or FTL communication to violate causality only for certain observers. It is the sci-fi occurrence of someone leaving their time and entering the past. To speak to Star Wars specifically, the entire franchise of Star Wars has tons of FTL and almost no backwards time travel.
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MrNexx
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PostPosted: Wed May 22, 2019 10:23 am    Post subject: Reply with quote

I think hyperspace avoids violating relativity and causality by simply not being in a universe where those apply. Even if relativity applies in the Galaxy, it doesn't apply in hyperspace... basically, the Galaxy chose Relativity and Causality, and hyperspace chose causality and FTL.

(This suggests, through symmetry, that there is a universe that eschewed causality and chose relativity and FTL, which I think is a terribly interesting one to consider).

Now, if you get sufficient sublight speed going, can you engage relativity and time dialation? At what Space rating does time dialation become a noticeable factor?
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Whill
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PostPosted: Wed May 22, 2019 8:14 pm    Post subject: Reply with quote

MrNexx wrote:
Now, if you get sufficient sublight speed going, can you engage relativity and time dialation? At what Space rating does time dialation become a noticeable factor?

Even the ship with the fastest Space is still far under relativistic speeds. I think that any ship would run out of fuel trying to accelerate to relativistic speeds with sublight engines long before they got there. Plus, unless you are trying to take a one-way trip to the future, most travelers would not want time to go by faster in the universe. They would miss stuff. Hyperspace travel conveniently bypasses all that.
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