"Heavier-than-air flying machines are impossible." -- Lord William Thomson Kelvin, British mathematician and physicist, president of the British Royal Society, 1895.

Edit: See https://news.ycombinator.com/item?id=27816225 for a discussion of the paper that launched this whole topic.

Edit 2: The actual paper you want is this one by the author of OP, the first one is referring back to this: https://arxiv.org/abs/2006.07125

Scientifically speaking, the fact that someone was wrong in the past, doesn't mean someone else is wrong now, even if they use a similar sentence structure.

PS Yes, I am fun at parties, if I ever get invited to one again!

You're technically correct but missing the point. Past hubris is a warning that we are capable of similar hubris.

What we can say is our current model of the universe does not allow for any practical travel beyond the speed of light.

We can also say that our current model of the universe is orders of magnitude more accurate, so it's more likely that we're correct about the speed of light - something which has been experimentally tested and verified by hundreds if not thousands of people - than the opinion of one particular Lord Kelvin regarding the viability of aircraft.

It's just as hubristic to assume that science is as ignorant now as it always has been.

Yes but we do have theories that suggest getting from point A to point B faster than light could be possible. We just have to do some things that appear impractical such as bend spacetime.

If FTL were possible, don’t you think there’d already be evidence of the time travel? First thing I’d do with FTL is travel back to the earliest habitable epoch in the Universe and start building galaxy-sized Dyson spheres.

Can you elaborate please.

Why does FTL produce time travel?

Why go back and build Dyson spheres?

To answer the second question, so that I can be the bad guys from the end of that Cixin Liu trilogy. Exponential growth is infinite and resources are not, so take any available advantage.

Short answer to the first, because the speed of light is the top speed at which information can propagate in the Universe.

I don't think a gold standard is a viable pyramid scheme for a multiplanetary species. There's too much of it in space. You need something that is rare even after traveling for hundreds of years. A lot of wars were simply fought over resources. It's even better if your society uses the resource as money. The only thing I can imagine is using habitable planets as currency, this would require your galactic empire to have a dozen planets though because a single planet does not make a currency and the debt trap of borrowing 1 planet at 3% interest would fall apart because colonizing the next planet may take too long to pay off the interest. I.e. you need some planets to lose before you take that loan.

Edit: Great filter hypothesis to explain Fermis paradox. The pyramid scheme needed to enslave the galaxy is also powerful enough to destroy a planet before it develops space faring technology.

Just to point out obvious, you are applying a human logic to an theoretical alien species of theoretically any type of behaviour/value/technological/political angle.

Who knows what they are like and what they value.

Thank you another book/trilogy to add to the reading list.

I suggest you also add The Wandering Earth by the same author to your list, which is a collection eleven of his short fiction stories. I'd even go as far as say it's better to read it first before the trilogy.

You’re welcome! Incidentally, there’s no FTL in the story.

In special relativity, faster-than-light travel or communication is equivalent to time travel.

But, it doesn't enable time travel. It just makes calculation easier, like «bending» of space-time array.

If you switch from relativity to regular physics with medium instead of pure vacuum (10 years passed since 2012 already, so relativity is busted at this point), then c is the speed of light in medium, so we don't need to bend anything or use negative t to calculate outcomes.

> so relativity is busted at this point

Excuse me?

Yes but that is only if you are traveling through space faster than light. If you simply skip some of the distance with something like an Alcubierre drive there is no time dilation to deal with.

> simply skip some of the distance

My understanding as a lay person of the physics that allow for things like this, is that they require starting conditions that wouldn't produce a universe anything like what we actually live in, or require energy levels around the output of galaxy superclusters to be focused into planck lengths or such. "Technically not impossible" is different for physicists than to normal people ;-)

No, it still results in time travel.

> To answer your question: any FTL drive can be used to create closed time-like loops. With these you can travel back in time, but you get stuck in a loop.

https://physics.stackexchange.com/a/247048

Okay, I think there is something I don't understand here. That link references a book I might have to read "Time Travel and Warp Drives".

Thanks!

Those theories would still result in time travel becoming possible and all the assorted paradoxes that come with it.

Valid warnings also don't mean anything.

Which ignored the obvious and prevalent heavier-than-air flying birds. We have no such examples to point at with FTL travel.

"Inertia is a property of matter" - fundamental unchangeable property of the universe

...so far.

What about perpetual motion machines though? :)

> Faster than light travel and time travel are both fundamentally impossible in our universe and can't be achieved.

That may very well be, and true in our lifetime, but given how little we know about a lot of things, and that in many cases we don't even know what we don't know, I would hesitate in making an absolute statement.

It's been proven rather thoroughly though. Look at Kip Thorne's body of work. His book: Black Holes and Time Warps is accessible.

c0 is the speed of light only incidentally. It is the speed of causality first. You can warp spacetime but you can't change the speed limit.

You might think there would be a hypothetical way to cheat it from some unknown phenomenon, but then any occurrence of that phenomenon would destroy itself in zero time.

Plenty of current physics allows for the possibility - even special relativity allows for it.

Personally, I think that the “speed of light” is a misleading misnomer. It’s the speed of information propagation in flat space - and even that’s a misnomer, as speed is a very human concept, as is time, as is space.

For all we know we’re waddling around in flatland, thinking that what we perceive is what is. More likely than not, most everything we know isn’t even wrong.

To borrow Haldane - “Nature is not only queerer than we suppose, it is queerer than we can suppose.”

Doesn't the concept of Wormholes, folding space or Warp engines bypass FTL blocks?

You're essentially relocating from one location to another by bypassing the distance... Leaving this universe long enough to travel a comparatively smaller distance/time in another then pop back into our universe a long long way but short amount of local time later.

Sure you dont travel FTL but you do arrive in the location Faster than light would take to travel in mundane space.

I believe all of those violate causality, at least if you have more than one of them (more than one pair in the case of wormholes).

Causality might very well be locked to this universe/space.

I mean its all just sci-fi at the moment. But lots of things we take advantage of today was at one time in the past as well.

I prefer to be optimistic about potential breakthroughs.

I am not familiar with this field but extraordinary claims require extraordinary evidence and I am not aware of any evidence absolutely ruling out the possibility. From my understanding we are still unsure about very fundamental aspects of physics and the universe so there are certainly unknown unknowns left which could dramatically change our understanding of everything.

> extraordinary claims require extraordinary evidence

I think you've got the sign on your skepticism reversed. The burden of proof is very much on those who claim that FTL travel is possible.

From Wikipedia:

    The special theory of relativity implies that only particles with zero rest mass (i.e., photons) may travel at the speed of light, and that nothing may travel faster.

    As of the 21st century, according to current scientific theories, matter is required to travel at slower-than-light (also STL or subluminal) speed with respect to the locally distorted spacetime region. Apparent FTL is not excluded by general relativity; however, any apparent FTL physical plausibility is currently speculative.

See https://en.wikipedia.org/wiki/Faster-than-light

The burden of proof for people making that claim is on the person making that claim. The burden of proof for making the claim that it’s “fundamentally impossible” is likewise on the person making that claim.

For all we know, our understanding of the universe could be at a stage of analogous to “there are four elements: earth, air, fire, and water”. Many things that we enjoy daily were once “fundamentally impossible”.

> Many things that we enjoy daily were once “fundamentally impossible”.

Not so. They may have been conceptually impossible but did not run into the "fundamental" laws of physics. FTL travel breaks physics. A lot abiut our observable universe would be different if it were possible.

The evidence that the universe obeys these laws is significant and, currently, not counter indicated.

> but did not run into the "fundamental" laws of physics

Those "fundamental" laws of physics didn't exist several centuries ago.

Yes they did, we just hadn't figured them out yet. If we were very wrong in them, we'd have to be wrong in a way that doesn't affect all the technology using the wrong theory - which is part of why we refined them, because shit didn't work.

That's what it means to be contradicting fundamental physics. It implies all of our other knowledge is wrong, but we are just lucky enough that our machines work anyway. Which, unless we see a phenomenon that we can't fit into our existing theories, seems unlikely

Let's invent new laws of physics and hope the universe adopts them.

> FTL travel breaks physics

Like quantum mechanics broke physics?

Quantum mechanics _fixed_ physics. Newtonian physics predicts that an electron will spiral into the nucleus of an atom within a few nanoseconds of the atom’s creation.

We observed quantum effects before the theory. We haven't seen any phenomena so far which require physics that could lead to FTL

By this standard, there is literally nothing that can be called impossible. Perpetual motion machines can't be said to be impossible because, who knows, maybe there is a God and tomorrow He could flip the switch for free energy from off to on.

It's simply the difference between claiming something is "impossible based on our current understanding/model of physics" and "impossible regardless of what we don't understand". Science gives us the first one, but the original commentator unambiguously claimed the second. Our models of the universe being wrong happens all the time, sometimes in significant ways, and even now there are plenty of unexplained things in our universe that have the potential to change our understanding. It seems pretty optimistic to assume our current models are guaranteed correct in every circumstance we haven't yet observed even when they can't explain everything we have actually observed.

So yes, perpetual motion could start working tomorrow, or could already exist for all we know. That doesn't mean it's wrong to say physics _currently_ says it's impossible and any claim better have some pretty good evidence to dispute it (which none currently do). But that also doesn't mean that if we're around centuries/millennia from now we won't have found a special case where our current understanding breaks down.

I don’t think that anyone made the claim that FTL travel is possible; they merely said that there’s still so much we don’t know about the universe, so definitely ruling something out is a bit rash.

With our current understanding of the universe, yes, indeed, FTL travel is not possible. But it’s naïve to think that our understanding of universe won’t change.

After developing some intuition for relativity you realize that asking whether FTL is possible is more akin to asking whether you can remain inside a circle after traveling radius + 1 from the center, than it is to asking whether a car can be modded to exceed its top speed. It is nonsensical, not logically coherent within all currently known physics and it goes to the fundamental nature of motion itself.

It can't be ruled out, but neither can Russell's Teapot.

"FTL is impossible" doesn't seem like an extraordinary claim at all, but actually quite an ordinary one. The natural world seems to obey a light-speed limit every time it's been tested.

Sure, it might be wrong, but as far as claims go it has a lot going for it, both experimentally and theoretically.

Well, technically time travel forward is easy.

And to say that backwards time travel isn't possible at this point doesn't make much sense. We don't even have a clear understanding of what time is, or why it goes forwards.

And the singularity should be around the corner, and that might present a computerized solution to emulate reverse time travel.

Not sure if it matters too much from an individualistic perspective. Near light-speed travel will allow a person to go anywhere instantaneously, they'd just have to contend with leaving the time and space they know behind them.

Only for sufficiently small values of anywhere. Even near-light-speed communication in fiber optics has detectable and significant delay just in crossing a continent. A trip made physically by humans every day.

Near-light-speed travel to the Moon would take a few seconds. Near-light-speed travel to Mars would take longer than my drive to the grocery store. These are not "instantaneously."

I'm talking about time dilation. From the perspective of the pilot of a near light speed ship a trip across the galaxy would be instantaneous.

It's instantaneous at 100% of light speed (impossible for massive objects). At "near" light speed, you need seventeen nines (0.99999999999999999c) to bring a trip across the galaxy down to a subjective day. I'm not sure how near your idea of "near" is. You also need instantaneous acceleration at both ends, which is quite a requirement.

OK. Only Pluto in a few hours, Mars in a few minutes, and the moon in less time it would take for me to reach the toilet from my living room.

Not literally instantaneous, but still practically instantaneous at the scale of the solar system.

> Near light-speed travel will allow a person to go anywhere instantaneously

Only if you ignore the enormous acceleration/deceleration forces needed for this.

Surprisingly it's not that much. At a constant 1G acceleration you could travel across the galaxy (100k light years) in 12 years (from your reference frame)[0].

I definitely agree that such travel would require enormous amounts of energy.

[0]: https://en.wikipedia.org/wiki/Space_travel_under_constant_ac...

also, "instantaneously" is a stretch on a galactic scale. 299792000 m/s isn't as fast as it sounds if you need to cross the entire galaxy.

Time dilation doesn't matter here.