Temporal Mechanics as Nukes.

[SirNitram]

This is something I've mused on for a while. When you're wanting to write a story that doesn't try and acheive a non-casualty-violating FTL system, but does want to acknowledge that this stuff can happen, how should you treat it? Arbitrary failure of the drives if you would return before you started is iffy.

I've thought of having time travel that could cause meaningful alterations of the past as considered the same as launching a rather unreliable nuclear weapon: Sure, it might not blow the region to bits, but it's not wise.

Of course, since this involves time travel, this suggests interested parties(Say, those who will evolve on that planet in the future) will want to protect their own past. So from it evolves a setting of enigmatic groups who halt almost all casuality violations, with a small amount getting through due to sufficient power of the travels, the advancement of one of the major powers agendas, or pure sneakiness.

Is it unadulterated suck?

[Molyneux]

Doesn't sound bad to me, at least as a setting; what kind of stories are you going to base in this setting?

[Ford Prefect]

This is something that I've been wondering about, given that I'm trying to work in a reasonably 'hard' universe, yet allow for travel faster than light (no frills, you were there, now you're here sort of stuff). However, I have to have the question asked: why doesn't this thing allow us to travel through time? I don't particularly want to replicate the Eschaton decree (don't violate causality in my light cone, or else). Even making it dangerous, ala Nitram's 'almost like riding a nuke' probably isn't going to work, because someone out there is going to try.

[Ariphaos]

This is something that I've been wondering about, given that I'm trying to work in a reasonably 'hard' universe, yet allow for travel faster than light (no frills, you were there, now you're here sort of stuff). However, I have to have the question asked: why doesn't this thing allow us to travel through time? I don't particularly want to replicate the Eschaton decree (don't violate causality in my light cone, or else). Even making it dangerous, ala Nitram's 'almost like riding a nuke' probably isn't going to work, because someone out there is going to try.

You need to break some aspect of Relativity. The easiest is to establish a common frame of reference, which requires some funky things if you require relativity to still apply at sublight speeds.

[Ford Prefect]

You could answer it the same way I do in my setting: "Because it just doesn't." Scientists in-universe would surely know the answer (at least in more detail than that), but it isn't something they talk about often, so it never works its way into the dialog of any story.

'It just doesn't' is my answer and the in-universe answer, and while scientists have fathomed out the drive's workings and principles, they still don't understand why it can't violate causality (probably at the will of the intelligences that first developed them). There is at least one group which is attempting to break this mysterious inability, though as I don't even know what it is, they are doomed to ignomious failure. Which is fine, because I don't particularly want time travel in this setting either.

You need to break some aspect of Relativity. The easiest is to establish a common frame of reference, which requires some funky things if you require relativity to still apply at sublight speeds.

If it is possible, could you expand? I'm not so fussed about 'funky' because it would only ever be used for idle musings, or my own personal satisfaction.

[Ariphaos]

You need to break some aspect of Relativity. The easiest is to establish a common frame of reference, which requires some funky things if you require relativity to still apply at sublight speeds.

If it is possible, could you expand? I'm not so fussed about 'funky' because it would only ever be used for idle musings, or my own personal satisfaction.

There is going to be an absolute 'when' in any such Universe and thus it will be possible for some observers to see an effect precede a cause. That is, there will still be an apparent violation of causality for certain frames of reference when FTL motion occurs, even though, in 'absolute time', there is no such violation. In order to have such a Universe, our idea (and perception) of time itself must be incorrect, at least in any mental model I've been able to come up with.

This was the previous discussion - this was the last big discussion on the topic, I think.

[Darth Raptor]

Handwavium/Technobullshit Alert: My only setting to incorporate FTL travel and communication features a unique cosmology that allows for it. This uniquely-structured universe has a definite center which is at the same time parallel and adjacent to everything on the upper planes. Without first establishing a link with this Nexus, as its called, any FTL jump does result in time travel; permanently and irrevocably removing the traveler from his native timeline. Any body traveling faster than light must be in contact with the Nexus via a special type of extradimensional wormhole in order to protect causality and keep the traveler in his native universe. This is possible because while it exists on a separate "plane", the Nexus has a linear distance of zero from any point on the upper planes.

[Atlan]

This is something I've mused on for a while. When you're wanting to write a story that doesn't try and acheive a non-casualty-violating FTL system, but does want to acknowledge that this stuff can happen, how should you treat it? Arbitrary failure of the drives if you would return before you started is iffy.

I've thought of having time travel that could cause meaningful alterations of the past as considered the same as launching a rather unreliable nuclear weapon: Sure, it might not blow the region to bits, but it's not wise.

Of course, since this involves time travel, this suggests interested parties(Say, those who will evolve on that planet in the future) will want to protect their own past. So from it evolves a setting of enigmatic groups who halt almost all casuality violations, with a small amount getting through due to sufficient power of the travels, the advancement of one of the major powers agendas, or pure sneakiness.

Is it unadulterated suck?

Well, you can always do something similar to FTL in Alistair Reynolds "Revelation Space", in which FTL isn't exactly impossible. It's merely so horrendously dangerous that no slightly sane person will attempt it. The trick in his state transition drive (e.g. going from tardyonic to tachyonic) seems to be that if you don't get it just right on the very first (and every other) attempt the universe responds by retroactively erasing you from existence: You will never have existed to attempt breaking the lightspeed barrier in the first place.
Although usually the attempts erase only those directly responsible for the attempted FTL even entire species have been removed from existence in this way, although a very few have apparently succeeded in FTL attempts...
In Revelation Space causuality is a BITCH. Cross her at your own risk.
You could use something like that.

[Junghalli]

This is something that I've been wondering about, given that I'm trying to work in a reasonably 'hard' universe, yet allow for travel faster than light (no frills, you were there, now you're here sort of stuff). However, I have to have the question asked: why doesn't this thing allow us to travel through time? I don't particularly want to replicate the Eschaton decree (don't violate causality in my light cone, or else). Even making it dangerous, ala Nitram's 'almost like riding a nuke' probably isn't going to work, because someone out there is going to try.

For my own universe I simply use a warp drive which works by contracting space in front of the ship so it only needs to cover a small fraction of the distance it otherwise would have. The ship never experiences any relativistic effects (unless it was already moving at a high fraction of c before you turned the warp drive on, but then it experiences the same relativistic effects as any ship moving at that speed), so I can't see how it could possibly be used for time travel. Though I don't know much about relativity, so I could be overlooking something.

Well, you can always do something similar to FTL in Alistair Reynolds "Revelation Space", in which FTL isn't exactly impossible. It's merely so horrendously dangerous that no slightly sane person will attempt it. The trick in his state transition drive (e.g. going from tardyonic to tachyonic) seems to be that if you don't get it just right on the very first (and every other) attempt the universe responds by retroactively erasing you from existence: You will never have existed to attempt breaking the lightspeed barrier in the first place.

Of course the irony of this situation is that nobody in-universe will realize the dangers. By definition, if individuals that mess up never existed nobody could ever realize they messed up. People would only ever know about the successes. Although if physicists in-universe have worked this out as a theoretical possibility it's rather awesomely scary. "You know, theoretically, for every successful invention of an FTL drive there are probably dozens or hundreds that result in the inventors being removed from existence ... of course, we wouldn't know about them, because they never existed..."

[Atlan]

Well, you can always do something similar to FTL in Alistair Reynolds "Revelation Space", in which FTL isn't exactly impossible. It's merely so horrendously dangerous that no slightly sane person will attempt it. The trick in his state transition drive (e.g. going from tardyonic to tachyonic) seems to be that if you don't get it just right on the very first (and every other) attempt the universe responds by retroactively erasing you from existence: You will never have existed to attempt breaking the lightspeed barrier in the first place.

Of course the irony of this situation is that nobody in-universe will realize the dangers. By definition, if individuals that mess up never existed nobody could ever realize they messed up. People would only ever know about the successes. Although if physicists in-universe have worked this out as a theoretical possibility it's rather awesomely scary. "You know, theoretically, for every successful invention of an FTL drive there are probably dozens or hundreds that result in the inventors being removed from existence ... of course, we wouldn't know about them, because they never existed..."

well, in the Revelation Space universe the deletion usually happens with the persons attempting the FTL transfer, e.g. the crew of the ship. But the effects tend to be... erratic, as witnessed by the disappearance of entire species in the case of serious causuality backlash...

[Ford Prefect]

For my own universe I simply use a warp drive which works by contracting space in front of the ship so it only needs to cover a small fraction of the distance it otherwise would have. The ship never experiences any relativistic effects (unless it was already moving at a high fraction of c before you turned the warp drive on, but then it experiences the same relativistic effects as any ship moving at that speed), so I can't see how it could possibly be used for time travel. Though I don't know much about relativity, so I could be overlooking something.

I've chosen a jump drive with mysterious but probably wormhole related mechanics, because I like saying 'Einstein-Rosenberg bridge'. I could change it to something else (or indeed remove it entirely), but I like the mechanics that the drive applies to the universe as it is.

Of course the irony of this situation is that nobody in-universe will realize the dangers. By definition, if individuals that mess up never existed nobody could ever realize they messed up. People would only ever know about the successes. Although if physicists in-universe have worked this out as a theoretical possibility it's rather awesomely scary. "You know, theoretically, for every successful invention of an FTL drive there are probably dozens or hundreds that result in the inventors being removed from existence ... of course, we wouldn't know about them, because they never existed..."

In the first time we hear about this, and the actual time it happens, the erasure actually left someone around who did remember the person whom was retroactively killed. Also, the people in charge of the projects were able to discover that these did actually exist, but they didn't remember them because they apparently died some time before they met; yet inquiry into their backgrounds showed that had these people lived, then they would very likely be the sort of person who would be involved in the projects(s). It leaves an eerie sense of 'this is not meant to be' - which the inertial manipulation technology did anyway.

[Junghalli]

I've chosen a jump drive with mysterious but probably wormhole related mechanics, because I like saying 'Einstein-Rosenberg bridge'. I could change it to something else (or indeed remove it entirely), but I like the mechanics that the drive applies to the universe as it is.

I think with teleportation (jump drive) you also experience no relativistic effects, so you don't get the kind of casualty violation possibilities you get with wormholes and Krasnikov tubes. It shouldn't be an issue. Again, though, not an expert on relativity, it's possible I've overlooked something. One thing I can think of is with a jump drive time may be just another coordinate like the three dimension in space is, but the hard physics of jump drive are nonexistant so it shouldn't be hard to fudge up a reason why this isn't so.

If you're using wormholes as a jump drive though you have to consider that opening a portal in another star system apparently involves sending something FTL, otherwise it'd take years for the other end of the portal to pop into being.

[Sikon]

When you're wanting to write a story that doesn't try and acheive a non-casualty-violating FTL system, but does want to acknowledge that this stuff can happen, how should you treat it? Arbitrary failure of the drives if you would return before you started is iffy.

I've thought of having time travel that could cause meaningful alterations of the past as considered the same as launching a rather unreliable nuclear weapon: Sure, it might not blow the region to bits, but it's not wise.

Of course, since this involves time travel, this suggests interested parties(Say, those who will evolve on that planet in the future) will want to protect their own past. So from it evolves a setting of enigmatic groups who halt almost all casuality violations, with a small amount getting through due to sufficient power of the travels, the advancement of one of the major powers agendas, or pure sneakiness.

That's good if but only if such is best for the story's plot, because it doesn't really add realism.

Part of the problem is that all time travel alters the past. From an anthropogenic perspective, altering the past may superficially appear to be just directly taking an action like killing someone's grandfather. Yet, really, affect even one speck of dust or one photon even by one's very presence, and the past is still changed. Every trip will change the past, period. There won't be that much of a line between meaningful and non-significant alterations or a practical way to tell in advance. Over centuries to billions of years of future existence of a civilization with time travel, it could easily become sure enough that one of the many time travel trips would cause an escalating series of changes disrupting its past, like the "butterfly effect."

That's no reason against time travel use if trying to change the past, since greater average benefit than cost may be sufficient reason.

However, any time travel does lead to problems with causality, and any time travel is a terrible mess if the goal is to minimize unrealism.

But must the story absolutely 100% assume that FTL means going backwards in time?

Nothing can be much worse than time travel for apparent realism. Regular physics theory is plausible because there is no FTL in the real world, and any conclusion that FTL would mean going backwards in time causes no problems, since it never actually occurs. But if there actually was FTL, such would be a serious problem.

Predictions of relativity are good reason to believe that FTL is impossible but not something that goes well in combination with FTL being possible. I don't particularly doubt physics interpretations in the real world, but I sure would if FTL was actually possible, like the saying that extraordinary claims require extraordinary evidence ... needing vast, direct experimental evidence incapable of possibly being interpreted any other way before concluding that traveling backwards in time is obtainable.

A story with time travel plus FTL is almost as implausible as pure magic fantasy to me because I wouldn't believe going backwards in time was possible short of seeing it with my own eyes and even then would suspect an illusion. Of course, the average reader is more relevant to this discussion than me. But does even the average reader really view a setting with FTL + time travel as more plausible than one with FTL and no time travel? Unlikely.

FTL is probably impossible, but if one absolutely had to have it: Although this is only an unlikely imaginary scenario, maybe time itself doesn't slow down as lightspeed is approached, and maybe going faster than light doesn't mean going backwards in time, so maybe FTL doesn't violate causality.

On rare occasions over decades to centuries, the assumptions of scientific theories interpreting empirical results may be revised, but the past great body of experimental evidence is never disproven. Experiments have shown that particles accelerated to near-lightspeed by electromagnetic fields decay more slowly, and the resonance frequency of atoms oscillating in an atomic clock is slower at higher velocity (e.g. Hafele and Keating).

That supports the straightforward standard conclusion that not just those processes but time itself slows down as lightspeed is approached. Yet maybe the slower speed of oscillation and decay is somehow instead an effect of the observed mass increase which occurs at the same rate or some different cause not understood by 20th/21st-century physics. Such a suggestion may be utterly, absolutely wrong without a 0.0000001% chance of being close to right, for reasons a physicist could describe.

But I fail to see a truly great realism advantage in a sci-fi story having both time travel and FTL, versus a story that just ignored the assumption that FTL means going backwards in time. Something like changes in gravity propagating as FTL communication or some application of quantum entanglement allowing long-distance instant telepresence and communication is probably impossible (as most papers have concluded). But does such hurt the story by being even more implausible than time travel, more directly contradictory to the body of past experimental evidence?

If the story must have FTL, just have it without time travel, and then there are no casuality concerns, which makes the sci-fi setting pretty simple to understand. Of course, such is very implausible, but the same can be said for any mechanism of sending objects to FTL velocity.

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The only currently implemented method of sending particles to large fractions of lightspeed, particle accelerators, obviously has no potential for such. Reaching superliminal velocities by acceleration with electromagnetic fields propagating at c would be absurd, like sending a cart to 100 mph by throwing 10 mph mudballs at its rear end.

For a self-propelled craft, FTL isn't even the issue when only a limited fraction of lightspeed can be obtained in faster than years by a rocket, even an antimatter rocket. Materials made from any elements in the real world lead to limited acceleration when the engine can't withstood without melting an arbitrarily high number of megatons of energy release per second per ton of its mass.

A good illustration of the implausibilities of some far-out methods proposed is in a previous thread. One paper imagines a Alcubierre drive method involving about the mass-energy equivalence of the sun to theoretically work, ~ 2E47 joules. That's millions of times more than the energy released by the entire Milky Way galaxy in a hour. Some equipment would have to somehow concentrate that much into a region far smaller than a quark, and it is negative energy. Another paper imagines generating and controlling a negative mass-energy density on the order of 5E106 J/m^3, around ninety orders of magnitude greater energy density than the detonation of a nuclear bomb. A more detailed discussion is in my post there.

For ideas like these and wormholes, looking at what would theoretically sufficiently manipulate spacetime is not enough if there is no method of getting within many orders of magnitude of almost arbitrarily high densities of negative energy/matter, if such is little more than imaginary magic tech for something built from the elements in the periodic table.

There's also the issue of getting a macroscopic ship of meters in length into a region of spacetime separate from the rest of the universe through a neck connecting the bubble of subatomic diameter, without shredding it in the process. In general, FTL is more likely to be possible on the exotic subatomic scale, rather than conveniently practical with macroscopic objects like human-crewed ships. FTL communication between colonies and starships in an otherwise STL civilization is more conceivable. But even FTL communication is probably still impossible.

That's aside from the question of whether methods like the Alcubierre warp drive manipulation of spacetime or wormholes are really possible even if imagined densities of negative energy/matter could be obtained, if negative matter really exists outside of mathematical description. Such are extreme extrapolation of theories without a shred of direct experimental evidence. Long chains of assumptions stacked on assumptions are questionable, even when the starting assumptions are based on regular interpretations of experimental evidence, as the end result is so utterly outside the realm of anything tested.

At least Heim theory leads to a hypothetical mechanism for FTL that might be implemented in the doubtful event of it being valid, with equipment made from real-world materials perhaps handling the relatively reasonable number of Teslas magnetic field strength involved, etc. A ship jumping into another dimension with different physical constants (and conveniently with the crew and itself still functioning under those new physics) is still terribly implausible, though.

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Let the story have FTL without any time travel effect, rather than thinking that adding time travel adds realism. Most readers aren't going to expect FTL to mean going backwards in time anyway.

The whole idea of having FTL ships is already so implausible in the first place. Having FTL without pretending it corresponds to real-world physics like causality concerns avoids giving such the false pretense of realism, not that realism and time travel belong in the same sentence anyway. As the saying goes, if you don't really know how it works, don't describe it in detail.

Personally I enjoy reading soft sci-fi as much as hard sci-fi, often more so, since frequently the characterization and the rest of the story is better written. When reading such or a fantasy story, I just imagine that it is happening in an alternate universe where that is the nature of reality.

That isn't a problem for entertainment since such helps escapism from the mundane, even if it could not really exist outside of imagination or virtual reality. Something exceptionally unrealistic coming by surprise can distract from immersion in the story, but a lot is acceptable if it is the basic, consistent background of the setting.

There are some special aspects of hard sci-fi aside from direct entertainment, like the potential to provide a vision of the future. For example, it is regretful that even relatively hard sci-fi usually focuses on colonization of imagined habitable planets and either magic-tech drives or space elevators, too rarely giving the public consciousness a model that the real world could emulate now. (In contrast, space habitats, mass drivers, etc. require only the application of existing physics and existing materials, rather than waiting indefinite generations or forever). But none of that is helped by adding time travel.

Time travel can be part of an entertaining story when a well-written adventure but only in the same manner as magic, with realism being never the reason to add it. For example, Star Trek 4 is great but from characterization, humor, suspense, etc.