There seems to be little recognition of the fact that FTL implies time travel, and just how unlikely that really is. I expect major advances in physics in the far future, but I’m not sure that bypassing causality and FTL will ever be one of them.
The "warp drive" they are discussing, and which was so hyped in 2010 or so, doesn't involve FTL, it involves contraction and expansion of spacetime as the ship moves in freefall. So no time travel.
It doesn’t matter, it still implies time travel. Use the wormhole acceleration-return thought experiment as an example. If you can leave point A and arrive at point B before light could, it doesn’t matter if you took a shortcut.
The thing that always bugged me about time travel fiction is that the reference frame is just assumed to be the surface of the earth. How fast are we wizzing through the universe? Why wouldn’t I reappear somewhere out past Saturn when I travel forward or backward in time?
Similarly, if wormholes are real, and you construct one in a gravity well, what happens to gravity? Especially if both ends are in a gravity well?
It seems that the usual thing to do is put the wormhole in orbit around your planet or whatever. That's based more on science fiction than physics, albeit scifi by smart knowledgeable people.
I'm not sure what to think about gravity "reaching through". Gravity and the wormhole are both just curved spacetime. Maybe the larger body distorts the wormhole throat in a way that looks like "reaching through"? Certainly, you need to have some effect that causes energy conservation when you make a perpetual water wheel by suspending one mouth over another and running water through it...
OK, touche - it's true that an Alcubierre drive could be used for time travel, but it's not necessarily implied, whereas it is for an object actually travelling beyond the speed of light. The discoverer of the method itself [0] (Miguel Alcubierre ) believed that chronology protection [1] would prevent time travel, as the spacetime bubble would destroy itself somehow.
I actually spent some time sketching out the math for an aborted science fiction world in which I had FTL that was prevented from time travel by a cosmic censor. You don't really have to "work it out" because the numbers are quite plain as day.
It turns out that if you use any remotely sane numbers on the speed of an FTL craft, it is extremely difficult to travel in time. To get time travel, you have to accelerate on the other end to very significant fractions of the speed of light, which itself takes so much time that it's hard to recover it via the FTL drive again. You have to put in absurd numbers for the speed of the drive and your ability to accelerate (and your ability to carry energy for conventional acceleration!) before you can get time travel out of most FTL drives, because bending your light cone enough to put the past into your FTL drive's path takes a lot of energy.
The only fictional drives that have serious time travel possibilities are the ones in universes where accelerating to significant fractions of the speed of light is easy-peasy, and the FTL drive is very, very fast (some are instantaneous) and can be arbitrarily steered. It's actually not that long of a list; a lot of FTL drives can hypothetically be used for time travel but the engineering in the universe still doesn't practically permit it. Just adding a cosmic censor post facto would preserve the story just fine. Many of the universes that meet those criteria are already so fantastical that they do in fact have time travel, so... those work out too.
(There's also several video universes where it's nominally possible, but the video wildly contracts the documentation. Both Star Wars and Star Trek have documentation that their ships can get to pretty significant fractions of the speed of light in small amounts of time, with ludicrous amounts of acceleration available to them, but on screen we only see them moving with absurd levels of acceleration like 50 or 100G, but which still takes a long time to get to light speed as a result, and with refueling being necessary often enough that it's hard to believe they could actually make it.)
> Both Star Wars and Star Trek have documentation that their ships can get to pretty significant fractions of the speed of light in small amounts of time, with ludicrous amounts of acceleration available to them, but on screen we only see them moving with absurd levels of acceleration like 50 or 100G, but which still takes a long time to get to light speed as a result, , and with refueling being necessary often enough that it's hard to believe they could actually make it.)
I don't know about Wars, but Trek strongly implies that impulse drive reaches significant fractions of light speed with near-zero acceleration time; there is certainly nothing to indicate that the on-screen non-warp acceleration is limited to 50-100G, and there is little on-screen indication that refueling (as opposed to drive system maintenance, and replacing non-fuel consumables used onboard) is regularly needed.
In both Star Trek and Star Wars, the ship accelerates very fast and then "blinks" away. Maybe it just needs a running start, like the turbocharger in car engine.
"there is certainly nothing to indicate that the on-screen non-warp acceleration is limited to 50-100G,"
The indication I refer to is that when you see a ship moving on screen from the perspective of another ship, it never suddenly blinks to "near lightspeed", because what that would look like is that in one frame the ship is there, and the next it isn't. That is never what happens. What happens is that the ship lumbers off, and slowly turns over the course of a couple of seconds, and can be seen to visibly accelerate in a way that the ship doesn't even cover its own length in less than five or six seconds. If you work the math it comes out on the order of 50-100G. Accelerating to near-lightspeed in a couple of seconds is several orders of magnitude larger than that.
Of course the ships don't blink around at near lightspeed; it wouldn't be photogenic. So all ships on all shows in all screens move within roughly a single order of magnitude of screen speed, no matter what the tech manuals say. Similarly, no matter what the claimed ranges of the weapons are, all engagements take place within a range where the two ships can clearly see each other, meaning no more than a handful of kilometers at most. No matter how far the ships are nominally apart from each other, a bolt of energy fired from one ship to another will take on the order of half a second to arrive. Almost no matter how zoomed in we are or are not, the energy bolts will subtend roughly the same amount of our eyes. If the ship is, say, firing on a planet, the bolts will perspective shrink maybe by a factor of two or three, if they shrink at all, rather than doing what they should do which is disappear entirely (except for perhaps one lit pixel if the bolt is sufficiently emissive). There's this one ST:TNG episode where the Enterprise uses phasers to quickly mine holes in the ground and when you start to see this sort of thing it's hard not to see the Enterprise as firing on a roughly 5-mile-diameter ball sitting right next to it, because if you follow the perspective of the "beam" that's what it implies about the size of the target.
Space isn't photogenic. Even today on earth, real combat takes place at ranges where you don't have a direct visual on the target a lot of the time. Any even vaguely realistic space combat will take place across ranges where you can't see a mile-sized ship with the naked eye. But that's not what they want to show. Consequently there's a massive, massive mismatch between the numeric claims of performance and what we actually see on the screen. So you might say something like "oh, the screen is maybe being symbolic", but then they routinely do things that imply what's on the screen is the real truth, such as in the one TNG movie where Data leaps between two ships. That's just one example, there's plenty of other situations like that.
It’s a cool idea, and admittedly would be a sort of loophole, but it might just mean that such a drive would require an arbitrarily high amount to work. Maybe Stephen Hawking was right and as soon as you build a machine capable of time travel, turning it on blows it up, because of QM effects. It does seem a little neat if how we intend to use the machine is the deciding factor in how it works.
Back in his collection All the Myriad Ways, Larry Niven wrote a brief essay called "The Theory and Practice of Time Travel" that explored ideas like these. One excerpt:
> GIVEN: That the universe of discourse permits both time travel and the changing of the past.
> THEN: A time machine will not be invented in that universe.
> For, if a time machine is invented in that universe, somebody will change the past of that universe. There is just too much future subsequent to the invention of a time machine: too many people with too many good motives for meddling with too many events occurring in too much of the past.
thus
> Niven's Law: If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.
Any progress on that idea (Alcubierre drive)? Always seems the most interesting. I do a Google Scholar search, but I don't know the physics journals, so its hard for me to know legit from crank abuse, without laboriously checking each result for journal impact factors.
I suspect using GR to design a warp drive is like using a spoon to build a freeway. Without a theory of quantum gravity - which appears to mean not just a theory of quantised space time, but also of the non-locality and complex distributed relationships that haunt QM - you're probably not going to get very far.
Doesn’t the wormhole scenario require bending the thing back on itself? I always thought that just because something could violate causality doesn’t necessarily mean it would, but IANAP and maybe that distinction doesn’t matter.
If you go from A to B before the photons can, from B's reference frame you are arriving from the future with perfect knowledge of events that have not yet "happened".
That's always struck me as sort of inconsequential, even though light is the apparent "speed limit" of the universe.
The same thing happens with sound regularly. A thing happens, and we receive light before we receive sound. Some third faster thing that gets to us before light would just be some other thing.
The distance of travel requires time relative to both reference frames.
If a star explodes at A and I travel to B at 2x the speed of light to tell folks at B, so what? Some time has elapsed and those two frames are still chugging along with causality.
Maybe there's some physics I've not learned that makes this obvious, but I've never grasped exactly why violating light speed prevents entropy from continuing. Even if at two different rates.
If we perfectly timestamped everything happening on two sides of the world and sent a telegram at twice lightspeed from A to B, it would still arrive after it is sent.
Let’s say you can travel at 10c. At Point A, the Arlington Photons win the Galaxy Cup 1-0. You zip to Point B and place a bet on that game, borrowing the money to place that bet from the Baltimore branch of the Arlington Legbreaker Gang. Wait for the photons, collect your winnings at Point B, then zip back to Point A ahead of your loan. The loan info arrives at Point A, and you pay it off a microsecond later, and pocket the difference.
Congrats, you have just traveled “back and forth” through time. To people in both the Point A and Point B reference frames, you have “seen the future” and acted on it.
Now, the Legbreakers aren’t stupid. Once they know someone is screwing around with FTL they’ll just hold bets and payoffs until light can travel. Yes, but… how do they synchronize their clocks between A and B? And how can they distinguish between a lucky person and a time traveler? Etc etc, ad regressum.
"Now" and "Here" as fundamental concepts only make sense inside a single reference frame. If you break the idea that information has a speed limit, then both of those ideas shatter into tiny little pieces.
I've always had the view that FTL travel necessitates time travel in certain frames of reference. Special frames of reference are currently "unpopular".
It sounds like you are referencing a special reference frame where FTL could prove such a special reference frame exists, and that such a frame would allow FTL to be possible without time travel.
If the math says so, or if some preliminary experiments say so, it's the task of a scientist to go there and check. Not doing so because you don't like the consequence is incredibly arrogant and anti-scientific.
That said, the usual disclaimers about confidence intervals apply. Also, not checking because you have better things to do is perfectly ok.
I've heard that many times now, but I'm not sure I fully understand it. So, could someone ELI5 how, given a starship equipped with a warp drive (or in range of a series of cleverly put stable wormholes), how can I give myself winning lottery numbers, and meet my great-great-grandfather?
I don’t think there is a true ELI5 answer, at least, I can’t think of one. The basic concept involves general relativity, and while there are ways to simplify it (i.e. no math) it’s still complicated. This does an ok job:
As you approach the speed of light, the amount of time you experience gets slower as compared to a stationary observer. This means that a photon, which travels at the speed of light, actually experiences no time at all.
I like to think of time and space as two different directions you can go i.e. forward/backward (time) or left/right (space). A stationary observer travels only forward, i.e. through time. So they experience lots of time but very little or no space. As you get closer and closer to the speed of light you're moving less in time and more in space. Your vector gets closer and closer to pointing right (using this analogy). An observer traveling at light speed experiences only space, i.e. no time at all.
Then, once you exceed the speed of light your arrow starts pointing a little bit backwards. You're still traveling mostly in space, very little in time but now the time you do travel in is actually negative.
I forget where I read this vector explanation but it's one that makes the concept tractable in my mind :)
I'd like to add that you're always traveling at the speed of light. If stationary you're traveling through time at exactly the speed of light, but as you move through space you start traveling through time more slowly. In other words, the speed at which you travel through time + space is always c.
> This means that a photon, which travels at the speed of light, actually experiences no time at all.
Yep. If a photon could talk, it might say "What is this 'time' thing you speak of?"
And since time has no meaning, neither does 'traveling'. A photon is always 'everywhere it's ever been or ever will be' so it might also say "What is this 'space' thing you speak of?"
The picture you're talking about is a Minkowski diagram and it's very useful for conceptualizing how time and space change in fast reference frames. [0]
The way I like to think of it, and I'm not sure how valid my thoughts are... I think about what you would experience as an observer in a ship traveling faster and faster. If the ship had a window to look out, you wouldn't really be noticing anything happen to time, you'd be seeing the universe get squished, with everything getting closer and closer to you.
If you managed to actually achieve the speed of light, the universe would be completely flat. Everything would be contained in a single plane perpendicular to your direction of travel.
So now, what would it mean if you went even faster? The universe would no longer be flat, but would be achieving a negative size. What that really means is not clear to me, but I suppose it's here where the traveling back in time starts to make sense mathematically.
>As you approach the speed of light, the amount of time you experience gets slower as compared to a stationary observer.
My impression was that the wormhole was used as a "shortcut" between two distant places to avoid having to travel very fast. Imagine a race where everybody else runs the whole course but you take shortcut. You haven't physically travelled fast but your path has been shorter.
I'd guess that Big Banking is blocking the development of this, it'd be a nightmare for inter-galactic ATM machines.
I love this explanation because when I started thinking of it this way it blew my mind in ways that no other physics concept has. It's something that truly sounds crazy except that as far as I know it is a valid interpretation. (Layman here.)
We think of time being this thing that just moves forward around us, and we can't stop it, time just goes.
But in this interpretation, it's us who are falling through time, literally.
I find that fascinating because it changes entirely how I think about the universe. I used to think of the "big bang" as a big explosion of little balls of energy (quarks, electrons, whatever) that eventually coalesced into matter, when enough time had passed and there was enough room for things to cool down. The part that I never understood before this interpretation is how "enough time could pass", when time itself was also supposedly expanding.
But in fact with this interpretation, which I guess is probably more correct (?), space and time simply exploded and began expanding.. which is what we're told but it's hard to picture, exactly. But if you think about energy always moving at the speed of light, and initially all of it flying off in the same direction (time), then matter becomes those particles which were somehow slightly diverted onto the spatial axes -- momentum is conserved, just redirected. And if you think about particles being ripples in fields of non-zero constant energy, an interactions between the fields causing ripples to bump away from each other in non-time directions which we describe as space, this whole relativity thing, and the fact that electric field particles (photons) do not have mass and move at c, starts to make a bit of sense.
It's crazy to think that what we perceive as time passing, the very possibility of things happening, is actually an emergent property of simple geometry and interactions of pure energy.
The way I visualize it is: start two marbles rolling towards each other down an inclined slope. They bump, and bounce off in opposite directions on the X axis. Now imagine there is no table. Would the marbles "feel" anything in the vertical direction? Or would they only feel something in the horizontal direction? Time, for them, emerges from the fact that they can get closer, bump, and move off in the other direction. The other axis, which they are both moving very quickly in, they have entirely in common and they might agree with each other that it is simply a stationary frame which allowed the "bump" to happen. Same for us falling, unrelentingly and unstoppably, through time. (Maybe?)
Except mass/energy must be really drawing a line across time dimension, instead of being points in it, right? Otherwise anything traveling in space - and thus slightly sideways - would immediately desync with the universe around it and interact only with emptiness?
I’m stumped by that as well. Traveling faster than the speed of sound doesn’t make you silent; it feels like time is being arbitrarily defined relative to light(?)
The speed of light is actually the speed of causality. It just happens that since photons don't have rest mass, they always move at the speed of causality.
Gluons are also massless like light, but don't exist on their own. If there's a particle that carries gravity, it would also be massless. You could say light travels at the speed of gravity.
From what I remember reading about relativity, I believe the difference is that speed of sound is just an arbitrary speed of mechanical waves in the air, while speed of light (in a vacuum) is literally what defines what it means for two events to occur before/after each other or simultaneously.
Unfortunately, I am confused about implications of that.
You've already got many good answers. But it's important to note that moving faster than light would break some fundamental rules of relativity, what means that we would have to basically throw the entire theory away.
I don't know who this group or person is, but their "knowledge" of what they're talking about is tenuous at best, seemingly obtained from mediocre pop physics books. This is essentially empty garbage.
Of course, this may not bе actualized until many years in
the future, but consider the many spectacular physical
phenomena that are believed to bе true at this early point
in the 21st century.
To make the leap from that premise, to practical utilization of even the most grounded phenomenon from the paper's title, Dark Energy, is essentially "magical thinking" at best, and at worst, a deliberate waste of time.
Consider that dark energy isn't even an assuredly verifiable phenomenon. Dark energy is a place holder in equations with a theoretical gap in understanding for the fundamental mechanisms driving the behavior of observable systems. We can't explain certain noticeable details, but, the difference between conflicting concepts that do explain disparate observations, is where the assumption of a hypothetical dark energy to bridge the difference, as an idea to help resolve conflicting observations, comes into play. On paper.
A thousand years ago, we did not know that microbial life was responsible for fermenting food, but we weren't ready to tackle that gap in awareness, until the microscope was invented. Prior to the invention of the microscope, ideas like spontaneous generation were still plausible. Without the proper equipment to declare dark energy or extra dimensions or worm holes as tangible, grounded facts founded in direct evidence, these ideas themselves are simply grasping at straw to reconcile known truths.
Given that these concepts deal in cosmological theory, there isn't even a lab to test or prove facts in. Such ideas are based on connecting the dots between imagery gathered from telescopes, for the purpose of studying astronomy. We probably wouldn't be able to construct a laboratory, until we graduate to a space-faring civilization and succeed at it for a couple of generations.
After further review I believe they found that for the EmDrive, the thrust was being generated by interaction of the electricity through the cables with Earth's magnetic field. Someone tested it by dampening the actual microwaves and still getting the same thrust result, which eliminates the microwave cavity as a source for the thrust. However the follow up test was with very low power, so they wanted to run it with magnetic shielding and higher power to fully rule it out.
"I believe they found that for the EmDrive, the thrust was being generated by interaction of the electricity through the cables with Earth's magnetic field." isnt that still thrust without and propellant or am I missing something?
I think what you’re missing is that the entire Earth is the reaction mass.
I’ve had a bit of fun doing the maths on space launch systems that function like this — if I remember right, you might even be able to do it… if you can make a monocrystalline superconductor, of whatever the best critical current density was about five years ago, about ten times the combined mass of your payload and cooling equipment.
Unfortunately there are limits on which direction this force can act in that make it even less useful than I’ve just made it seem.
You are correct in that this does not use propellant, however the Earth would need to be nearby for this to be viable for any sort of space travel. Perhaps maybe keeping a satellite in orbit or doing orbital maneuvers, but even then the amount of thrust is so small that it would likely not be usable. So unless the review team comes back and says they've eliminated magnetic interference then we're probably back to square one.
I understand the feasibility of such an endeavor as faster than light travel is not high, but does anyone else get kind of existential depression for not being able to live long enough to see these kind of advancements humanity might make?
The short answer is "yes". The long answer is: the longer you live, the more your concerns lay elsewhere (career, health, kids, etc.) and you stop worrying so much about what you are going to miss, and start focusing more on what you might be missing right now.
From a philosophical aspect, it also helps to remind yourself that - for all we know - we might be living in the "Golden Age" of humanity and it's all downhill from here.
Enjoy the present, maybe experiment with psychedelics. It gets better :)
Concentrate on experiencing as much as you can on this awesome planet we were all lucky enough to be a part of. There is more than enough here for a dozen lifetimes to explore.
Even at “warp speeds” you’ll likely live a life simmilar to this one on a single planet where ever it may be.
Space is so big that it’s nearly pointless to travel for place A to place B because place B likely won’t have anything place A won’t, especially if we are talking on a solar system scale, so traveling to another solar system is nearly pointless.
While we might spread to the stars were likely going to end up with solar system scale separate civilizations rather than some huge inter planetary alliance unless there is a clear need for e.g. if we would have discovered an enemy.
>Space is so big that it’s nearly pointless to travel for place A to place B because place B likely won’t have anything place A won’t, especially if we are talking on a solar system scale, so traveling to another solar system is nearly pointless.
What the heck are you talking about? We're not talking about humans making nearly-identical colonies with the same chain stores like Starbucks; different planets are going to look radically different, especially if we find planets with life, as it's quite possible life will look radically different on other planets where it evolved differently.
