Who thinks Faster Than Light travel is possible?

[Swank]

They're usually especially offended by the more trivial examples because of course, in a scenario where you spin with your finger out, the direction you're pointing changes at a rate faster than the speed of light - that's not FTL movement, it's a trick - it's conflating angular motion with linear motion.

That sounds like my example of moving your gaze from star to star.

 

[Brian G Turner]

it's conflating angular motion with linear motion

I think this is what I was trying to reference when I mentioned synchrotrons.

 

[StilLearning]

That sounds like my example of moving your gaze from star to star.

That's the one - it's all the same issue of an observer being stationary in their own reference frame, whether it's a rotating reference frame, an accelerating reference frame, or an inertial (moving at constant speed/ stationary, which are treated as the same thing) reference frame.
If you guys have already beaten this into the ground I'll leave it alone - I've seen some names here that I know from experience are knowledgeable wrt general relativity.

 

[hitmouse]

Meanwhile, Neymar (a football player) transfer fee to Barcelona - 222 million euros!
What hope is there for the future of mankind with societal priorities like that?

View attachment 92437

Thats not a societal priority. Nor is it Neymar’s salary, astronomical as that may be. It is simply a commercial bet by one of the biggest clubs in the world. It may be ridiculous, but it has to looked at in terms of international sports business and marketing in the same sense as the Olympics, NFL, Fifa, F1, 20-20,etc.

 

[StilLearning]

It's probably worth mentioning that the use of near-Earth-space as a resource to do things here on Earth - the off-world portion of the global economy, so to speak - is worth between $400 billion and $150 billion annually, depending on where you draw its boundaries. That is almost entirely unmanned satellites, with a little smidge of space tourism and commercial space experiments tended to by humans on the ISS and the Chinese space stations (in more recent times). The combined manned spaceflight budgets of NASA, ESA and Roscosmos don't top $10 billion a year most years IIRC.

It seems likely that, without the need to demonstrate technological prowess (if our goals in space were simply it's use as a resource and for-knowledge-only exploration), humanity would probably have given up on manned spaceflight back in the 70's. The US military space program did exactly that.

That need of different nations and cultures to demonstrate technological prowess is, in a lot of ways, fuelled by the same need to demonstrate international physical and cultural prowess via sport. So, in a way, I suspect that Neymar's insane seeming worth might come as a parcel with the things that have driven us to have ongoing manned space programs at all. That has changed a bit with space tourism ( a welcome change IMHO) - it's worth noting that Blue Origin's next flight will be a purely science oriented one (Blue Origin to launch next space flight on Aug 31, no tourists this time ) - but not a lot. The field has just been opened to the merely wealthy, rather than the world-dominatingly-wealthy.

https://space-economy.esa.int/article/33/what-is-the-space-economy

The Space Economy in Figures: How Space Contributes to the Global Economy | en | OECD

The space economy is expanding and becoming increasingly global, driven by the development of ever-more governmental space programmes around the world, the multiplication of commercial actors in value chains, durable digitalisation trends, and new...

www.oecd.org

Biden proposes $24.7 billion NASA budget in 2022 to support moon exploration and more

It's a $1.5 billion increase over NASA's 2021 budget.

www.space.com

 

[alai]

So, in a way, I suspect that Neymar's insane seeming worth might come as a parcel with the things that have driven us to have ongoing manned space programs at all.

Neymar's personal wealth -- coincidentally reportedly in the same sort of range as his transfer value, in principle a very distinct concept -- isn't insane in the global scheme of things. The numbers of people with that sort of net worth is likely in the tens on thousands. Though I suspect it might be a resonant one on this forum, where a hectomillionaire sportsballer is more appalling than that of -- say! -- an author with that much loot. OTOH Neymar really is objectively one of the world's very best players. Are JK Rowling, Jeffrey Archer, Danielle Steel and Dan Brown the world's best writers? De gustibus!

But is it sane world? All indications point to 'no'.

This can be explained by the rate of time internal to the object having gone to zero and introduces all sorts of divide by zero issues. I assume that this means that time fails to pass for the internal observer.

It means "this theory has broken, bring me another one!" Or failing such, "please don't ask that..." But more usefully it predicts that for any finite amount of kinetic impulse, you're still going less than c, so you need some method of getting there other than 'just keep accelerating'.

Hm... so if something is travelling faster then the speed of light relative to the observer, then they would go right through the observer without the observer feeling a thing?

It's possible -- who knows! Tachyons are an entirely hypothetical class of particles, so their properties are likewise entirely (or doubly?) hypothetical. And certainly some conventional particles with high (but not certain) probability behave like this this: neutrinos, notoriously.

One idea is that if they did exist, it wouldn't be possible to interact with them. Partly because they would be a wholly distinct class of particle -- you couldn't 'flip' states between the two in any model that allows them -- and partly by way of a causality cope. Physicists, always opening cans of worms, and then complaining that they need bigger cans to cram them back into!

After all, our bodies are subjected all the time to things we aren’t aware of (at least without measuring equipment). Example… Radon gas daughter product particles that often fill our homes without us realising that they are there. They can produce alpha particles that enter our lungs and potentially irradiate soft tissues but we just don’t experience any of that on a conscious level.

Sure, but those are still interactions -- and even 'observations', with the usual 'not really properly defined as a concept' caveats -- in the physics sense. If there are interactions that are occurring that aren't detectable with current experimental apparatus, or even there are particles that exist that don't interact even in principle with normal matter... than that's very much another day's work. @.@

We could go to Mars in 2 years if we broke the nuclear weapon space treaty, or sunk a couple trillion into it. It isn't really a technical problem as much as a political one.

"Make bricks without straw" is a sort of technical problem. Politically, there's been by my count 5.5 manned missions to Mars that have been politically announced already. Now, you might point out that none of them have been planned to happen within the current terms of the administrations of any of the polities involved (insofar as some (soon to be all?) of them really have 'administrations' and 'terms' in any politically meaningful way). And the funding and the planning may be inadequate to the task as set out...

That's the old science fiction vs fantasy discussion which never has a definite solution. If you want to get technical about it, 99 percent of what passes for science fiction is science fantasy. Might as well drag Speculative Fiction back out into the spot light.

You're giving me distinctly Atwoodesque twitches here!

Anything with space travel past the asteroid belt is pure fantasy, within the belt that's science fiction so long as it is done in tiny ships and not giant massive dreadnaught size ocean liners.

Perhaps life can never be accelerated past the speed of light without becoming a monument to its previous existence.

Wow, that escalated quickly! There's a whole middle you're excluding there between "current engineering" and "current physics". (Crewed -- clearly uncrewed exploration has already occurred) space travel beyond the 'Belt is perfectly possible, would just need time and (a lot of!) money. Don't see how that's remotely in the realm of fantasy, unless it's described in a gratuitously Buck-Rogers manner.

Never mind a return trip to Mars, there's no deep physics reason, in any "there's a strong basis to believe this is literally impossible" sense, you couldn't have a return trip to α-Cent by 2042. Never mind this mere Mars bagatelle! Indeed there's no deep reason I couldn't be there this time tomorrow, subjectively speaking, if we handwave away a few more mere technical and squishy biology problems.

OTOH FTL travel is vastly more problematic. I don't think it automatically qualifies as works including it as fantasy, but very often it might as well be. Or it's "soft" SF, or "overwise putatively hard SF but we're giving ourselves the standard pass on that". Very occasionally it's some sort of actually-making-it-a-meaningful-part-of-the-SF (like addressing the causality implications, say).

Sometimes [Vikings] dragged the ships over land to get to the other side.

Indeed so; witness all the places in Scotland and Ireland with names like "Tarbe(r)t"!

 

[StilLearning]

OTOH FTL travel is vastly more problematic. I don't think it automatically qualifies as works including it as fantasy, but very often it might as well be. Or it's "soft" SF, or "overwise putatively hard SF but we're giving ourselves the standard pass on that". Very occasionally it's some sort of actually-making-it-a-meaningful-part-of-the-SF (like addressing the causality implications, say).

I'll point out, in an openly self serving fashion because it's the subject of what I'm currently writing , that you can have star systems where the planets (including habitable ones) are comparable distances apart to the Earth and Moon, and that double, triple and quadruple star systems are well documented - where each star has room for it's own system of planets and habitable zone, and the systems are each a matter of light hours or days apart. (Alpha Centauri is such a system). Although they are rare, even septuple star systems have been found. Lastly there are parts of the galaxy, e.g. older open clusters, where the average separation between stars is less than a light year.

So, for the purpose of writing fiction, FTL isn't needed to create a setting where spacecraft travel to many different planets, between star systems, in relatively short periods of time. George Lucas didn't ever actually need hyperdrive for Star Wars. FTL only really becomes a needed plot device if you're wedded to the idea of Earth as a main location in said setting. And even then, there are still other approaches to make the transit times effectively shorter, such as the technology for functional immortality being discovered (this may bother biologists as much as FTL bother physicists, but it's just an example).

As a little bit of an example, here's a not-to-scale map of the septuple star system Nu Scopii:

​

If you took that as your starting point, gave each star 4 rocky planets (one naturally habitable orb apiece) and 4 giant planets with an average of 4 large & varied moons each (as Jupiter has - including 2 ice-covered ocean moons and a volcano moon), you have a setting with homes for 7 space faring civilisations, encompassing a total of 140 large solid-surfaced worlds and 28 gas-giants (and as many dwarf planets and large asteroids as you want). All within weeks or at most months of travel of each other, if your ships can hit 50% of lightspeed.

All that said FTL is a fascinating thought experiment, it's just not as vital to building a workable sci-fi setting as its prevalence would have it seem.

 

[alai]

I'll point out, in an openly self serving fashion because it's the subject of what I'm currently writing , that you can have star systems where the planets (including habitable ones) are comparable distances apart to the Earth and Moon, and that double, triple and quadruple star systems are well documented - where each star has room for it's own system of planets and habitable zone, and the systems are each a matter of light hours or days apart.

I wouldn't worry too much about serving yourself. It's not as if this is a salaryman drinking night-out!

I'd add the small cautionary note that there's maybe a question mark about whether such system are as likely to have habitable planets, much less having one each. OTOH current thinking seems to be leaning in the direction of "dunno, but seems no reason why not". Extreme cases of this, like the Fireflyverse, are highly unlikely... but if we're only going to limit ourselves to what's possible, then we could even have Klemperer rosettes of planets all within an especially easy commute of each other!

So, for the purpose of writing fiction, FTL isn't needed to create a setting where spacecraft travel to many different planets, between star systems, in relatively short periods of time. George Lucas didn't ever actually need hyperdrive for Star Wars. FTL only really becomes a needed plot device if you're wedded to the idea of Earth as a main location in said setting.

George Lucas very much did need it, as the style of exposition was very much, we're in one star system, clock wipe, we're in another. It's almost an anti-plot-device: we're brushing it under the carpet so much it might as well be "a little later in another town down the road after a brisk horse-ride by the posse." If you were rewriting the story to keep the same plot structure, but to tweak the "tech" elements, it'd be a darn sight easier to just say "actually not happening in space at all" rather anything else.

And even then, there are still other approaches to make the transit times effectively shorter, such as the technology for functional immortality being discovered (this may bother biologists as much as FTL bother physicists, but it's just an example).

I'd doubt it, as we already know of organisms that are much more longer-lived than humans, others that can survive cryogenic freezing, etc. So unless we're quibbling on "how many tech levels higher", or have some vitalist or essentialist belief that humans are some special unique case in some way, it's hard to imagine why anyone would think this is impossible, rather than "we dunno how to do that yet".

Also recall that relativity itself has exactly this effect, from the PoV of the crew. Which really is a SF plot point in any number of cases. (Forever War and Revelation Space spring immediately to mind.)

If you took that as your starting point, gave each star 4 rocky planets (one naturally habitable orb apiece) and 4 giant planets with an average of 4 large & varied moons each (as Jupiter has - including 2 ice-covered ocean moons and a volcano moon), you have a setting with homes for 7 space faring civilisations, encompassing a total of 140 large solid-surfaced worlds and 28 gas-giants (and as many dwarf planets and large asteroids as you want). All within weeks or at most months of travel of each other, if your ships can hit 50% of lightspeed.

That's a fair-sized "if", mind you! I know it seems very modest in the context of this thread -- "let's compromise between what's currently technologically feasible, ~0.001c, and relativistic velocities of ~0.999c, and say 0.5c!" -- attaining such speeds are pretty challenging. If we imagine "lighthugger"-type ships that can pull 1g at their discretion and at length (already fairly maguffinesque), then it takes about six months just to accelerate to such speeds -- you'd never actually get that fast even in systems as large as Nu Scopii. And thus far it's only Newton that's spoiling our fun, not Einstein. You could of course merrily make that on the order of a 10g acceleration if you're thinking of the crew being in not so much cruise-liner grade accommodation as jet-fighter cockpit. Much beyond that we'd be assuming reactionless drives or artificial gravity, or an extremely post-human sort of crew.

I don't think FTL exists, I just don't think it should be ruled out at this stage. Especially by SF writers. Along with all the other unlikely but not actually disprovable stuff that they might dote upon.

I don't think physicists (or indeed SF 'purists') are organising boycotts of woolly space-fantasy movies or book -- or if they are, I've missed the pickets, and so on. But it's about as "disproved" a possibility as anything really gets in physics. And way more than any (for example) legal standard of proof.

Physicists don't assume dark matter exists, they posit as a potential solution to the evidence of inflation. There's a high probability they're wrong.

That's more to do with dark energy, as I understand, which is a waaaay more vague and speculative a concept. If it even qualifies as a concept! Dark matter there's quite a lot of evidence for, all basically to do with the medium-scale gravitational structure of the universe. But it's also possible that (say) a whole series of observations are spectacularly wrong, or that gravity behaves in a non-uniform manner. Those just seem less likely, and less satisfactory as explanations.

 

[StilLearning]

EDIT: Fair enough about Star Wars too - it is openly taken right from Samurai films and westerns. I suppose it'd be more accurate to say you could do that kind of multi-world, multi civilization worldbuilding without hyperdrive. END EDIT

I'd add the small cautionary note that there's maybe a question mark about whether such system are as likely to have habitable planets, much less having one each. OTOH current thinking seems to be leaning in the direction of "dunno, but seems no reason why not". Extreme cases of this, like the Fireflyverse, are highly unlikely... but if we're only going to limit ourselves to what's possible, then we could even have Klemperer rosettes of planets all within an especially easy commute of each other!

A fair point, but as the object of the exercise is SF worldbuilding I figure we can tolerate a some divergence from what the most likely scenarios are, as long as we stay well within the possible. It's also worth considering the possibilities of terraforming, and that for around a billion years our own solar system boasted both Earth and Mars as potentially life supporting planets (https://www.science.org/doi/10.1126/sciadv.abl7497), and possibly Venus as well (Venus May Once Have Been Habitable). So... if we're building SF settings... there's some room to maneuver, for that purpose.

EDIT: The underlying thing is that I want to be offering people a fairly realistic setting in which to set their story, but I absolutely don't want to be dictating to anyone what that story should be like, even by the default of setting too tight a bounds on what I'll consider 'realistic'. So the setting guide should be equally useful tool to someone wanting to write outright science fantasy but with a realistic backdrop, someone writing about a human colony with hyper-tech limited only by fundamental physical laws, someone writing about an alternate 1970's Earth with a nearby stellar neighbor, and all points between. END EDIT

IIRC in Firefly the majority of the inhabited worlds were terraformed by colonists from Earth (possibly all - it's not clear from the lore). Something similar is true for the 12 colonies of Kobol from Battlestar Galactica, which were located in quadruple star system.
The most extreme version of the constructed rosette of planets - the ultimate solar system (The Black Hole Ultimate Solar System: a Supermassive Black Hole, 9 Stars and 550 Planets) - is going to feature in a future book - you wouldn't even need a spaceship to commute between planets in the same orbital ring, you could build space elevators between them! I also plan to cheekily shoe horn in an 800km diameter artificial world made of Osmium, which would have close to Earth-like gravity .

Also recall that relativity itself has exactly this effect, from the PoV of the crew. Which really is a SF plot point in any number of cases. (Forever War and Revelation Space spring immediately to mind.)

You really do have to snuggle up to lightspeed to get the effect - 85% of lightspeed for a 50% change in the perceived passage of time. Just for me, I'd prefer the option of extended lifespan over time dilation, especially since you stand a better chance of coming home to find some people who remember you.

I know it seems very modest in the context of this thread -- "let's compromise between what's currently technologically feasible, ~0.001c, and relativistic velocities of ~0.999c, and say 0.5c!" -- attaining such speeds are pretty challenging. If we imagine "lighthugger"-type ships that can pull 1g at their discretion and at length (already fairly maguffinesque), then it takes about six months just to accelerate to such speeds -- you'd never actually get that fast even in systems as large as Nu Scopii. And thus far it's only Newton that's spoiling our fun, not Einstein. You could of course merrily make that on the order of a 10g acceleration if you're thinking of the crew being in not so much cruise-liner grade accommodation as jet-fighter cockpit. Much beyond that we'd be assuming reactionless drives or artificial gravity, or an extremely post-human sort of crew.

Actually, in this thread, we're specifically talking about faster than light travel, and people are throwing around ideas like Alcubierre metrics and wormholes, so it's a big compromise (almost going off topic!) just by insisting we stay below lightspeed ! But, again, that's a fair point, and obviously there are massive engineering problems beyond the engine and acceleration, such as even tiny particles of space dust striking a vehicle's leading edge and going BANG like an anti-tank mine.

While I picked Nu-Scorpii and 50% of C arbitrarily as examples, my main criteria when writing have been speed-of-light travel time between potentially habitable planets, and travel times by chemical rockets: Speed of light time between worlds, since that sets the time limit for sending any kind of message, and without being able to at least send and receive information in a fairly timely fashion any civilization, however post-human and advanced, is arguably not a civilization at all. And chemical rockets because that's probably the absolute lower bound for getting into space from an Earth-like planet and moving between the hypothetical planetary systems of two binary stars - using the more likely example of Alpha Centauri (if the 'A' and 'B' stars had planets): The average separation between the 'A' and 'B' stars is similar to the distance between Earth and Saturn, so you could imagine the inhabitants of star system A sending probes into the B system with cold-war technology.

There's a helluva lot to talk about regarding what might actually be possible in terms of star drives, but going into the details of stardrives for hard SF is another separate booklet in the series. Probably two. However it's a great idea (so thank you @alai ) to include a few paragraphs on how the survivable accelerations for a human being will limit the effective speed for any vehicle crewed by conventional humans, as well as what kinds of accelerations unmanned craft might hit.

Some moderately serious studies have suggested speeds of 20% of C, and accelerations of 1000's of g, might be achievable for an ultra-lightweight robot probe using fairly near-term technology (Breakthrough Starshot: A voyage to the stars within our lifetimes), so Nu Scorpii civilizations a bit further ahead than us might trade big chunks of information regularly enough, via vehicles that are like space-faring flash drives, but still find sending actual biological citizens from one end of their 7-star system to the other utterly impractical

 

[LordOfWizards]

Hi All, Sorry just now catching up. Been gone for a spell.

Wow! Those were some serious tangents taken from "Who thinks Faster Than Light travel is possible?"
It could have just been a series of posts: Me | Not Me | Not Me ... instead, 9 pages later I'm not sure we have an answer.

Off the top, I'll mention a book by Michi Okaku called "The Physics of the Impossible" with an entire chapter on FTL. Some knowledge of physics is required.

I'd love to address some of the ideas put forth here, but can't do all at once.

Here's one that caught my eye early on.

Lets say I have a shinny new FTL ship and I go for lunch at Alpha Centarie. Say I get there in just a few hours (by my watch).... when I am at Alpha Centarie, the light I see from the Sun left 4 years before I did. The light being emitted by the Sun is still another 4 years further behind that.
After lunch I decide to go home right away. Again lets say this takes just a few hours.
When I get back to Earth it will be 8 years before I left.

If I'm not mistaken (don't shoot!) it will be 8 years later on Earth, but for you only about 8 hours have passed. That is usually the depiction made in SF novels and film. Like someone mentioned earlier this is known as time dilation. Two types of time dilation exist; time dilation onset by a near light speed relative velocity, and time dilation brought about by the effect of gravity. Interstellar depicted an example of the second type (gravity).

I am not a physicist per se, but it is among my very top favorite subjects. I have done the maths of special relativity (which are easier than general relativity). So give me a percent of the speed of light and I'll tell you the difference in the rate of time between "stationary" and the speeding object's reference frame.

I think that one day we may be shocked to look back and see how primitive we still were in the twenty first century. Yes one day FTL will be possible but we have a looooooong way to go.

 

[Swank]

If I'm not mistaken (don't shoot!) it will be 8 years later on Earth, but for you only about 8 hours have passed. That is usually the depiction made in SF novels and film.

I believe you're mistaken. FTL doesn't exist (as far as we know), but the point of it is that it doesn't take 8 years but much less. Like the same 8 hours.

What you're describing is travel at speeds approaching the speed of light.

 

[Vertigo]

I believe you're mistaken. FTL doesn't exist (as far as we know), but the point of it is that it doesn't take 8 years but much less. Like the same 8 hours.

What you're describing is travel at speeds approaching the speed of light.

Which brings you to the whole (unanswerable*) question of whether an FTL drive will involve time dilation at all. If it doesn't then the previous post is still wrong as it will be 8 hours objective and subjective each way so no your won't have travelled back in time. Although you will get back before the light of your departure has arrived.

Unanswerable because we still don't have a working FTL mechanism. I think the Alcubierre drive avoids time dilation as you aren't technically moving but space is moving around you.

 

[LordOfWizards]

I believe you're mistaken. FTL doesn't exist (as far as we know), but the point of it is that it doesn't take 8 years but much less. Like the same 8 hours.

What you're describing is travel at speeds approaching the speed of light.

I was responding to the post I quoted. I never said FTL exists. I am trying to correct the perception that AllanR seemed to have. But you are correct. I was conflating FTL with near light speed effects. I was pretty tired when I wrote that. I had just read 9 pages of conjecture!

 

[Swank]

Which brings you to the whole (unanswerable*) question of whether an FTL drive will involve time dilation at all. If it doesn't then the previous post is still wrong as it will be 8 hours objective and subjective each way so no your won't have travelled back in time. Although you will get back before the light of your departure has arrived.

Unanswerable because we still don't have a working FTL mechanism. I think the Alcubierre drive avoids time dilation as you aren't technically moving but space is moving around you.

There is no reason to believe there is FTL, but if it is possible, the rules it would operate under are as unknowable as its mechanism.

There could be extreme time dilation - making the trip much faster than the scant external time. Or the lack of acceleration (the seeming mechanism of dilation, given gravity dilation) might mean no dilation at all.

Causality is really the big question. Will it prevent FTL? Is causality only a factor if something is going to cause a noticeable paradox? Will travel in pro-paradox directions be prevented, but everywhere else is okay? The universe seems to function on the basis of information - like Heisenberg's Principle. With FTL the prevention of paradox might be a fairly interactive process with travelers.

 

[LordOfWizards]

With FTL the prevention of paradox might be a fairly interactive process with travelers

Could you please explain what this sentence means?

 

[Swank]

Could you please explain what this sentence means?

They might find that the calculation of causality conflict is important to successfully travel to certain locations at FTL speeds. Do it wrong and the ship doesn't move, disappears, ends up somewhere else or takes as long as light to arrive. Understand and avoid conflicts and the crossing might be instantaneous.

 

[Swank]

Another factor in FTL (and dear to SF writers) is the manner that the ship moves. Off the top of my head, we have warp (where space changes shape to make normal velocities cover larger distances), hyperspace (a realm in our universe that has different laws), wormholes or other interconnected places in the universe, quantum coupling allowing matter to turn into information and return to matter elsewhere, moving between near identical universes (might be the same as the previous example), translation into particles that only exist above light speed (the hypothetical tachyon), and teleportation.

The move between similar universes is my idea, and avoids paradox.

 

[LordOfWizards]

When I get back to Earth it will be 8 years before I left.

This is the statement that messed me up. I don't think FTL introduces this kind of paradox. In fact the statement violates the law of Entropy. I suspect the jump in logic he took was that the light took 8 years (4 LY each way) so if he got back in 8 hours he would back in our system before the light got very far. It's an interesting observation, but I don't think FTL would mess with your time like that.

Every depiction I've seen of FTL (and wormholes) sends you to the destination without any distortion of time on either end. It seems to me the whole point of FTL is to have a shortcut through space. FTL should allow us to go back and forth light year distances without any effect on local time.

 

[StilLearning]

According to this Scientific American article ( Star Trek ’s Warp Drive Leads to New Physics ) , because powerful space warps mean/are powerful gravitational fields, you could expect time on a ship being 'propelled' by an Alcubierre-type arrangement of space time to, in effect, run slower for the occupants, who could exert some control over the flow of time by adjusting the exact geometry of their 'warp bubble'.

 

[AllanR]

here are a couple good videos on why FTL means time travel.
explanation starts at 8:30

6:53 on this one