A while back, I asked if FTL drives mandated time travel, and I was referenced this previous question:

Are there any ways to allow some form of FTL travel without allowing time travel?

Where the top-rated answer provides what is apparently a very-well crafted explanation for why any given FTL system can be exploited to create time travel. Unfortunately, I don't fully understand it. Looking at the 'Time Travel Happens' section, though, where the scenario is illustrated with time cones and world lines, a thought occurred to me: Could we exploit this system by going backwards? That is, can be exploit the same mechanics that turn FTL travel into time travel in order to create FTL travel without time travel? I'll try to explain what I mean.

Here's the 'time cone' illustration from the question I'm referencing.

enter image description here

When traveling within the 90 degree cone 'above' A or B, you are traveling towards/away from either destination with the bounds of the speed of light, and all is well. A horizontal line between A or B represents instantaneous travel, and a line between the horizontal and 45 degrees above the horizontal is faster-than-light, while the 45 degree line above the horizontal is the speed of light. A line BELOW the x-axis is trouble, as that means you're going back in time.

IF I understand it, Derrel is traveling at a relativistic velocity in such a direction when he engages his FTL drive to go from A to B, the horizontal line representing instantaneous travel 'tilts' downwards, below the x-axis, creating time travel, which we don't want. This can happen with any speed between infinite speed and the speed of light, because depending on your direction and velocity, your 'travel line' can be tilted up to 45 degrees down. I'm not sure if I'm talking gibberish or not, but if you had a ship with the ability to reach, say, 90+% of the speed of light using conventional engines (somehow), AND had an Alcubierre drive that could take you up to exactly (or slightly below) the speed of light, couldn't you use relativity to 'tilt' your line of travel 40 degrees down, so that when you engage your speed-of-light Alcubierre drive, it's only 5% above the horizontal, and will therefore get you to your destination faster than light, without actually being faster than light?

I have no clue what I'm talking about, don't go easy on me.

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    $\begingroup$ If you have time travel you can convert this ability into travelling faster than light with no need of rockets or Alcubierre drives. You can walk faster than light. Light travels 1 kilometer in about 33 microseconds. A man can walk 1 kilometer in about 15 minutes, or 900 seconds, or 900,000,000 microseconds. Walk 1 kilometer than time travel back 899,999,997 microseconds: you have travelled that kilometer ten times faster than light. Repeat to cover any distance. $\endgroup$ – AlexP Sep 4 '20 at 21:44
  • $\begingroup$ I explained it badly. I don't have time travel, I'm trying to use the mechanics that turn FTL into time travel in reverse, starting WITHOUT time travel, so that my FTL method can't be used for time travel. I'll rewrite a bit to make it more clear. $\endgroup$ – FlyingLemmingSoup Sep 4 '20 at 21:56
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    $\begingroup$ I've been thinking about this, and what I'm going to conclude is this: This is too-story based. I've not VTC'd yet, because I'm still thinking about it, but we can't actually travel FTL, and the effects on time are only mathematical, which analysis is completely subject to empirical evidence when and if we ever can travel FTL. Conclusion: if you want this to work, declare it to be so and move on. If all you're really asking is, "does this sound suspension-of-disbelief-worthy?" my answer is "absolutely." $\endgroup$ – JBH Sep 4 '20 at 23:57
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    $\begingroup$ While those who know their textbook relativity believe FTL leads to time travel, some working scientists aren't so dogmatic. Though there are others who are. FTL is a common trope of science-fiction. They usually ignore the time travel possibilities & get on with the story. You should do likewise. No-one stops reading if there's FTL without time travel. $\endgroup$ – a4android Sep 5 '20 at 4:16
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    $\begingroup$ It always bothered me how the time travel explanation works. It uses deliberately changing points of view to achieve time travel with the inherent paradox that if you stick with a viewpoint this doesnt happen. Light already has the paradox that it travels the same distance compared to you no matter what your speed, direction or perspective is, which should create paradoxes compared to other people going different speeds. Since it doesnt, why should time travel suddenly appear if you exceed it? Its just another paradox that doesnt even need to happen. $\endgroup$ – Demigan Sep 5 '20 at 6:46

"Plausible" FTL

As mentioned, travelling and time are linked. Time and space are relative. If you move close to the speed of light, your time moves slower to a non-moving observer.

To resolve this, the ship isn't the only one that moves. If it moves 1km/h in one direction, it moves the universe 1km/h in the other direction. If both move faster than the speed of light, you'll experience the same time travel, so you'll arrive at the time the journey took. If you travel 4C distance at 2C speed, you'll arrive there in a year as the universe is moving 2C around you. As the whole universe moves, it'll be the exact same for all people in it, making no difference for them or physics (besides the part of travelling faster than C).

  • $\begingroup$ The fun begins when there are two ships travelling faster than light in different directions. $\endgroup$ – AlexP Sep 5 '20 at 9:11
  • $\begingroup$ @AlexP I've been thinking all day about it. Would it? Each would move themselves and the universe, including the other, around it. I think it won't be like 2 kids trying to move a ball in opposite directions, although I had the same feeling as you when I wrote it. $\endgroup$ – Trioxidane Sep 5 '20 at 16:49
  • $\begingroup$ @Tridoxane: And isn't that fun? Anyway, the real problem with the proposal is that "moving the entire universe" is exactly what ordinary travel does -- when I walk towards the park, the park moves towards me. There is no such thing as a fixed frame of reference with respect the universe could move "absolutely". And if there was such a fixed referential, then all the paradoxes of FTL travel would disappear. This is one of the most common fictional "solutions" to FTL paradoxes: assume that all FTL travel takes place with respect to a fixed referential. $\endgroup$ – AlexP Sep 5 '20 at 17:10
  • $\begingroup$ If there's 2 doing it, wouldnt they "push off" each other using the universe? That way relative to them the universe moves at their FTL speed while the other ship relative to them moves at 4X the FTL speeds. $\endgroup$ – Demigan Sep 5 '20 at 17:10

OK, anyone with a basic knowledge of physics will shoot what follows down in flames. But;

Your basic problem is that you are trying to separate one vector (time) from 3 others (space). Its called space/time for a reason and trying to separate them is like trying separate one of the 3 physical dimension from the other two. So for the purposes of your story don't try. Assume by default that you cant travel in time without traveling in space and vice versa.

So using the old "killing Hitler trope" I am a brilliant scientist who invents a time machine and hates Hitler (cause like who doesn't?) So I grab my revolver and set the machine for 100 exactly years ago because obviously I want to kill him before he has a chance to start his murderous reign. It works and I'm magically transported back in time 100 years.

My problem I'm now also exactly 100 light years from Earth and I cant interfere with Earths time line no matter how hard I try because light relaying any message I send or action I take back to Earth won't get there for 100 years. Right after I left.

Now lets say by chance I landed on a Earth like Planet with friendly aliens who are happy to help me build a new 'time' machine and send me home. They also set it exactly for 100 years in the past. If it takes 3 weeks local time to build I get back to Earth 3 weeks after I left. From the Aliens perspective I have again traveled back in time (into their past) but again nothing I can do can change that. Importantly traveling either way I've had no interaction with the physical universe.

The thing is there is no such thing as FTL. All the hypothetical methods of beating the speed of light I read about like wormholes or the Alcubierre drive' don't 'work' by allowing someone to exceed the speed of light they work by warping space in such a way that the traveler does not physically interact with the 'outside' universe while traveling. Like my time traveler if they work at all they have to do so in a way that cannot interfere with causality.

In the case of wormholes there are configurations that can in theory allow you to beat causality (travel back in time and change reality) and there are ones that won't. If wormholes are real the overwhelming likelihood is that the the former cannot exist (they collapses if they are attempted) while the latter do work. Importantly they do so by shortening the distance between 2 points not accelerating the traveler past the speed of light. Likewise while in the wormhole there is no way to interact with the outside universe.

I can hear the forehead slapping from trained physicists already.

  • $\begingroup$ I travel 50 lightyears away from earth for 50 years back in time (taking into account the earth has moves through space). I travel 50 lightyears back to earth for a total of 100 years back into the past, and I can kill Hitler. The thing about the Hitler paradox is that it assumes a universe that cares: why should it care about Hitler, if just your presence and every particle you disturb and gravity you create will change how the universe will react forever? $\endgroup$ – Demigan Sep 5 '20 at 6:53
  • $\begingroup$ It won't work. As far as I can understand the problem, reversing the trip doesn't take you further back in your own time line. You can never end up back at your destination any earlier than the time you leave. In the example I gave you go back 100 years into 'our' past which is the alien's 'present'. Reversing the trip sends you into their 'past' but that's also our 'present'. You end up back where you started so causality is not violated. Your 50 year trip puts you in 1970. Reversing the trip puts you back in 2020. The 2nd trip is only time travel from the aliens viewpoint, not Earths. $\endgroup$ – Mon Sep 5 '20 at 8:09
  • $\begingroup$ if one of the aliens were to travel with me from 1970, he would suddenly arrive in 2020, which is a time travel method that cant even be created using the penrose diagram. So either both you and the alien arrive in 2020 which is impossible even mathematically, both of you end up in 1920 which would allow the travel back in time, or one of you ends up in the future and one in the present, which begs the question at what point the alien watches you and the ship disappear as your time paths diverge. $\endgroup$ – Demigan Sep 5 '20 at 11:34
  • $\begingroup$ You would also have the problem: what if the aliens have their own ship? You tell them to kill Hitler, and the aliens who are unable to travel to the future in their own ship would be able to kill him? And if construction of the FTL ship took more than 50 years and the aliens had seen its construction, they could also go there the moment you arrive and stop its construction? $\endgroup$ – Demigan Sep 5 '20 at 11:37
  • $\begingroup$ No you missing the point. Its perspective. The alien who traveled 'back' to the present with you would from its perspective/time line be traveling into his /her/its past. Only you would be traveling back to your 'present'. As for having their own time/space machine.? Hitler hasn't done anything from their perspective. At the time you arrive light from the 1920's is arriving at their planet, not the 1940's as far as they can tell he's still innocent. And even if you convinced them he had to die they wont arrive until 2020 'our' time. $\endgroup$ – Mon Sep 5 '20 at 12:08

Your referenced solution proposes like four or five ways why or how you might have ftl travel without time travel. I suggest you go back and read them... the most known trope is that you have to leave the gravity well in order to avoid those tidal effects of relativity on space time, so you avoid the observer problem to a degree.

He also proposes that, for your ftl drive to work, you first have to match your target's reference frame, which first involves much use of beautiful torch drives you can render in high quality, and then only then allows for a jump.


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