Most acceptable FTL methods?

I can tell you my favorite, but I'm not sure how valid it is.

I've heard that an object contained in a strong enough magnetic field would be pushed into a dimension (not sure if that's the right word) where light moves faster.

The hypothesis is that everything is going to move faster, and that you could exceed the speed of light in our dimension, while still obeying the laws of physics in the "slip" dimension.

One of the cool aspects of this idea is that relativity might not take its toll on the travelers while they're doing this, since the slip dimension may have its own time.

The problem of 40,000 years passing for everything else while they reach their star in three days still exists, but because they're in a separate timeline, when they pop back into their normal dimension, it'll (hopefully, maybe) be around the same time as they left.

Creates another, somewhat smaller problem: FTLT astronauts would then age faster than other people, not slower.

Even if its completely bogus, it's my favorite.

Jump drive, at least a jump drive that requires some kind of "time interval" between "here" and "there", is probably the one that causes the least problems with causality as I understand it (you cannot be both "here" and "there" at the same time and time travel is essentially impossible).

Some kind of other-dimensional hyperdrive that actually requires some sort of time to travel between destinations also seems okay, but I wouldn't swear to it as it would seem to allow some sort of "forward" time travel so long as you do not exit hyerspace or whatever.

Warp drive, which allows travel both forward and backward in time, causes so many causality problems with the "here" and "there" and thus "might be there" and "but is here" problems that I absolutely would hesitate to try to use it anywhere anyhow.

It's a fantasy element, so it doesn't matter which type of magic you use save that you pick the one that best fits your story's themes. Only you as the writer can do that.

Causality, relativity, FTL. Pick any two.
What kind of stardrive you use doesn't change that at all, though the most realistic would probably be some kind of wormhole drive, since there's at least some sort of theoretical grounds for their existance.

FOTSGreg said:

Jump drive, at least a jump drive that requires some kind of "time interval" between "here" and "there", is probably the one that causes the least problems with causality as I understand it (you cannot be both "here" and "there" at the same time and time travel is essentially impossible).

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No doesn't really help. The problem is that relativity has no absolute concept of synchronicity, since there's no absolute time. Any kind of FTL movement, done right, will see you arrive before you left.

Some kind of other-dimensional hyperdrive that actually requires some sort of time to travel between destinations also seems okay, but I wouldn't swear to it as it would seem to allow some sort of "forward" time travel so long as you do not exit hyerspace or whatever.

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Forward-only time travel isn't a problem. It doesn't even really deserve to be called time travel.

All forward time travel really needs is a paper bag.

Dethklok said so, so it must be true.

Lhun wrote, Any kind of FTL movement, done right, will see you arrive before you left.

Forgive me for not understanding. Maybe it's my understanding of time and its relationship to space/time, but if time flows forward and I leave a Point A in space at 1:46pm and arrive at Point B in space at 1:47pm, for example, wherein falls the problem with causality and how could I possibly arrive at some point in time before I left Point A in space, especially assuming that time flows forward at the same rate at all points in space/time (I mean, I know if I leave Point A at 1:46pm local time I might arrive at Point B at 1:15pm local time for Point B, but my time in my frame of reference should remain the same)?

FOTSGreg said:

Forgive me for not understanding. Maybe it's my understanding of time and its relationship to space/time, but if time flows forward and I leave a Point A in space at 1:46pm and arrive at Point B in space at 1:47pm, for example, wherein falls the problem with causality and how could I possibly arrive at some point in time before I left Point A in space, especially assuming that time flows forward at the same rate at all points in space/time (I mean, I know if I leave Point A at 1:46pm local time I might arrive at Point B at 1:15pm local time for Point B, but my time in my frame of reference should remain the same)?

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Your use of "time flows" alone shows that you kind of still assume an absolute time. There really is no "time" at all, not in the sense that it "flows forward at the same rate at all points in space". Because of relativity, the rate at which time progresses depends on your relative speed. That means two event which happen simultaneously in one frame of reference will happen non-simultaneously in another. I.e. two balls dropped from the same height would arrive at the ground at different times, if observed from the right reference frame. And if you do those transformations for reference frames and allow FTL, the right reference frame makes it possible to see a ball hitting the ground, and then being dropped. That doesn't mean the ball itself would perceive it that way. It's important here to not confuse the FTL/time travel problem with time dilation at high speeds.
But i suspect that didn't really help in understanding, it's not really easy (maybe not possible) to explain that in a few sentences without graphs. (And i'm not very good at explaining physics anyway)
I very much recommend reading this: http://www.theculture.org/rich/sharpblue/archives/000089.html
It does a really good job at illustrating the problem, even if it does take some time to wrap one's head around the concept.

Okay, my father - who has a PHD in nuclear physics - and I have hashed out a kind of FTL that seems like it makes sense. Now, I am trying to remember all the fuzzy details that he explained, so I may be getting a lot wrong. I'll ask him to remind me the theoretical physics behind it tomorrow so I can re-explain.

The basic problem is that if you go somewhere before light gets there, then you're getting there before you left from the perspective of the people at the end of the voyage, because information travels at the speed of light (the sight of you leaving) so you appear, then they see you leave way back at your start place. That is a paradox, and is bad.

Right?

And this is because of the 2 theories of relativity, general and special.

NOW

The problem with the concept of "finding new physics that lets us go FTL" is that - while there is a lot of stuff that we have yet to figure out - every NEW kind of physics that we find has to simultaneously explain the new thing we are seeing AND all the OLD things we are seeing.

For example - this isn't physics, but it illustrates my point - we see planets in the sky. People theorize that planets circle around us. Then we see planets have moons. So we come up with a new theory that the planets orbit the sun, and only appear to orbit us, with moons around them. It explains the new thing we are seeing AND the OLD ones.

And right now, everything we see in the night sky PROVES that the theory of relativity is TRUE.


But my Dad and I thought of a bit of wriggle room...I...can't remember the actual explanation behind it, but the idea is that you...do...something...

Shit, I completely forgot the supposed physical mechanics behind the FTL and all I can remember is the "story" mechanics: You trade places with matter in a different solar system. Matter in the area you targeted reforms into your shape, and then your matter becomes the shape of the matter you targeted. So, you find a gas giant in another solar system, determine it's location, then "target" that area. In the atmosphere of gas giant, your ship assembles out of local matter. Your ship's atoms reshape into the gas that you took over. And then you just need to accelerate out of the gravity well in question.

I don't know how plausible that is - specially cause I can't remember the theoretical physics behind it - but I do know that it looks really really cool and results in a lot of nifty story based restrictions that promote conflict and character creativity.

Zoombie said:

The basic problem is that if you go somewhere before light gets there, then you're getting there before you left from the perspective of the people at the end of the voyage, because information travels at the speed of light (the sight of you leaving) so you appear, then they see you leave way back at your start place. That is a paradox, and is bad.

Right?

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No. Seeing someone arrive, before light arrives of the event is not in itself a causality violation.

And right now, everything we see in the night sky PROVES that the theory of relativity is TRUE.

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Well, i wouldn't say proof, but it is evidence. And there is of course the rule that observed facts always trump theory, so you're right that any new new theory still needs to explain all current evidence.

Shit, I completely forgot the supposed physical mechanics behind the FTL and all I can remember is the "story" mechanics: You trade places with matter in a different solar system. Matter in the area you targeted reforms into your shape, and then your matter becomes the shape of the matter you targeted. So, you find a gas giant in another solar system, determine it's location, then "target" that area. In the atmosphere of gas giant, your ship assembles out of local matter. Your ship's atoms reshape into the gas that you took over. And then you just need to accelerate out of the gravity well in question.

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Sorry, but that still doesn't resolve the problem. You're still able to transmit information at FTL speeds, and thus can create a closed timelike curve to transmit information into your own past.

The problem pretty much boils down to this: Outside of your lightcone, there is no causal ordering of events. This is a direct result of the invariancy of the speed of light. So, anything that can take you outside of your own lightcone (i.e. FTL travel) will take you to a "region" on the space/time coordinate grid from where your original lightcone is outside the current lightcone. So, the order of events in your original lightcone is now no longer fixed. So you can again move out of your current lightcone to your original lightcone and arrive at an event that happened before your original departure.

Edit: Whoops, i meant spacelike curve, not timelike curve. (Obviously )

My head hurts.

LOG said:

I thought the speed of light was supposed to be constant/the same to an observer in any frame of reference?

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Yes?

Doc Smith used what's probably my favorite FTL technique--he assumed that he could cancel the effects of inertia, so that by one cute "little" violation of physics as we know it, any little bit of thrust would make his ships travel at FTL speeds. One of his characters explained his FTL travel with a metaphor involving cars--your car can go about 60 to 90 miles per hour, but my space ship can go about 60 to 90 parsecs per hour. When the anti-inert device is turned off, you still have all your original momentum, BTW--in whatever direction it was before you left. Smith called the non-inertia state "free", and the normal state "inert."

Seems to me that Larry Niven's known universe series uses some very similar principle.

While it's as good an example of technobabble to "explain" FTL, it's not just one cute little violation of physics. It has still all the problems of FTL, and some more.

Lhun said:

Causality, relativity, FTL. Pick any two.

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As a physicist, I agree with Lhun completely. It's not enough to invent a drive that *can* go FTL, you also have to explain how it doesn't screw up either causality or relativity. So there's actually two aspects to this problem, from a writer's perspective.

What kind of drive you use does not matter, in my opinion. People will believe ridiculous delta-V reaction drives as readily as they will hyperspace bubbles or wormhole generators.

The causality/relativity question, however, takes some thought. My favorite way of handwaving this is to mention in the story that Special Relativity is wrong -- there *is* a preferred reference frame, and that's the only frame in which you can go FTL. We 21st century humans just haven't discovered the right frame yet. With this, causality is preserved and we keep relativity for everything except a spacecraft in flight. This works particularly well for spacecraft travel that's near-instantaneous, like wormholes.

(I haven't had the guts to write a relativistic space battle, yet. In this odd space-time I've constructed, it might drive me -- the writer -- mad.)

Of course, all of this comes into play if you're going for realism. Which, alas, most readers won't care about unless it's important to the driving narrative.

RemusShepherd said:

As a physicist, I agree with Lhun completely. It's not enough to invent a drive that *can* go FTL, you also have to explain how it doesn't screw up either causality or relativity. So there's actually two aspects to this problem, from a writer's perspective.

What kind of drive you use does not matter, in my opinion. People will believe ridiculous delta-V reaction drives as readily as they will hyperspace bubbles or wormhole generators.

The causality/relativity question, however, takes some thought. My favorite way of handwaving this is to mention in the story that Special Relativity is wrong -- there *is* a preferred reference frame, and that's the only frame in which you can go FTL. We 21st century humans just haven't discovered the right frame yet. With this, causality is preserved and we keep relativity for everything except a spacecraft in flight. This works particularly well for spacecraft travel that's near-instantaneous, like wormholes.

(I haven't had the guts to write a relativistic space battle, yet. In this odd space-time I've constructed, it might drive me -- the writer -- mad.)

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These causality issues are largely what Charlie Stross is playing with in IRON SUNRISE - a pretty good book that excels in relativistic shenanigans, including those space battles.

Oh yeah, if anyone is interesting, my dad said that you'd preserve causality if relativity is a local phenomenon.

"local" meaning everything within our light cone.

So you can FLT jump beyond your light cone.

The only problem is targeting where you can jump...

Zoombie said:

Oh yeah, if anyone is interesting, my dad said that you'd preserve causality if relativity is a local phenomenon.

"local" meaning everything within our light cone.

So you can FLT jump beyond your light cone.

The only problem is targeting where you can jump...

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Do you have any more details about that idea? Because usually, exiting the lightcone is what causes problems. (Well, if you don't exit your lightcone, you're not going FTL)

Is the lightcone you speak of an imaginary sphere expanding at the speed of light starting at the time and point in space you iniated travel?

Lhun said:

Do you have any more details about that idea? Because usually, exiting the lightcone is what causes problems. (Well, if you don't exit your lightcone, you're not going FTL)

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The lightcone of an event is the cone-shaped graph you get when you draw a diagram of the other events that could be influence by it with time and space as the two coordinates. See the link above for an image.

http://www.newscientist.com/article/mg18925331.200-take-a-leap-into-hyperspace.html

I actually found the source of the hypothesis I mentioned in post 2.

Ah, yes. The Heim-Droescher Space drive concept.

Oddly enough HD is not, according to what I've read about it, a reactionless drive. It actually requires the spacecraft to continue to obey all the laws of physics, but somehow is supposed to be able to "translate" (and I use the word very loosely) the spacecraft into some other type of space where higher speeds and eventually FTL are physically possible.

Unfortunately, HD physics is nearly incomprehensible to the average physicist and completely so for the layman as it relies on Heim's own version, as I recall, of mathematics and has been, also unfortunately, largely discredited in the mainstream scientific community because of this incomprehensibility.

My understanding is that it has shown some promise in particle physics in predicting particle masses, but predicts things that modern particle physics has shown to be untrue. It also relies, as I recall, on certain properties of a graviton "particle" that has yet to be proven.

In addition, the counter-rotating magnetic fields required to generate HD "thrust" or "translation" are simply beyond our present day capabilities and would likely require true room temperature superconducting magnets (the low end is in the region of 25-30 Teslas, again as I recall).

BTW, There's apparently 1 machine on the planet, I repeat ONE, that can even come close to generating the magnetic fields required to even attempt to test HD theory - that's the Z Machine which I believe is at Fermi Lab and which is tied up for at least the next 5 years on other projects. It might, repeat might, be able to generate up to 29 Teslas, but the funding for such an experimental usage has not been forthcoming even from the DOD. At last report, even the scientist mentioned in the New Scientist article as being willing to conduct such an experiment had withdrawn his support.

It will be a long, long time before someone tests Heim-Droescher Theory.