## Do these FTL mechanics work?

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### Do these FTL mechanics work?

(This is for a sci-fi novel that I want to make as consistent and rigorous as possible. I know that we have absolutely no idea how the RL physics works, if it does at all, but just imagine Relativity et. al is 100% correct for this thread.)

Imagine there is a Portal-like wormhole, which can teleport objects as arbitrary distances instantaneously. The wormholes' size can be modified, and the teleportation works such that the world-lines of the portals match up. As an example, you keep one portal on Earth, and send the other to Alpha Centuari at such a speed that, from the ship's frame, it only took 1 year to make the journey. If the ship sends a signal through the wormhole saying "We've arrived!", Earth will receive it one year after launch. (An Earth-based telescope, however, will see the ship as less than 1/4 of the way there.) I know that this produces timey-wimey effects, but I've got a decent handwave for why that doesn't cause problems. (i.e. the self-consistency principle means you've already changed the past.)

Question: Assuming that non-paired portals can be sent through each other, (e.g. with A<->B, and C<->D, C can be sent through B to arrive at A.) is it possible to send an object arbitrarily far back in time, as opposed to only as far back as the wormholes themselves were created?
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zmatt
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### Re: Do these FTL mechanics work?

*personal opinion*
I don't think time travel back in time is possible with relativity. Relativity allows forward time travel because as the name states we are changing what is experienced "relative" to the observers. In practice it is less actual time travel than it is slowing down time for you and letting the world pass you by. The explanations I have heard and read for possible backwards time travel with relativity is very similar to forwards, it seems as though its a "hack" if you will on our perception of the universe. You travel to a distant place where the light from earth is very old and shows a younger one, then you travel back at relativistic speeds and viola you are with dinosaurs (over simplification). I think that sounds a little silly.
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### Re: Do these FTL mechanics work?

Well, the whole point of most of it is that only FTL travel lets you go backwards rather than forwards at varying speeds.

(And I worked out an answer: throw the portal gun through the portal.)
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### Re: Do these FTL mechanics work?

Sorry, you said "I worked out an answer" -- you are claiming that if you throw a portal gun through a portal, you can cause events to occur prior to the creation of the "first" wormhole pair? (I'm unclear what you meant)
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### Re: Do these FTL mechanics work?

Yes, I think. If you take a second relativistic vehicle and the portal gun with you through the first portal, you can use them to send yourself further back in time. If the time dilation is large enough, I think it's possible for you to arrive back on Earth before the first set of portals existed. (Though I haven't drawn a diagram.)
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### Re: Do these FTL mechanics work?

To simplify, what if you created pairs of portals at once, locally.

These portals can be separated, but clocks adjacent on each of these portals are "local" -- there is no time dilation.

I don't think this lets you go back before the first such pair of wormholes is created?

That, at least, is how I imagined what you are describing would work.
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### Re: Do these FTL mechanics work?

Yes, there is no time dilation across the portals, and the two ends always remain "in synch." However, would you agree that separating the portals and then moving them around at high speed would you let travel back in time relative to your starting point? (i.e. Going from A, through a moving portal, would allow you to leave A's lightcone.)
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### Re: Do these FTL mechanics work?

I don't see how. That is, if by A you mean "the point where the wormholes where created".
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### Re: Do these FTL mechanics work?

By A, I mean "where one wormhole is." The point is that you're moving the other end at relativistic speeds to take advantage of time dilation. (EDIT: I may need a diagram. I'll draw one up later.)
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### Re: Do these FTL mechanics work?

Yes, you can spawn a pair of such wormholes, move one fast, then create close time-like loops without a problem with wormholes.
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### Re: Do these FTL mechanics work?

Once you've got one timelike curve, can you then use the same method again to push yourself further back in time?
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Yakk
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### Re: Do these FTL mechanics work?

As far as I know, not always? (Ie, I don't think so in this case)

Of course, you do get some pretty powerful computation out of it.
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### Re: Do these FTL mechanics work?

Why wouldn't you be able to, in that case?

(And the physics get into all sorts of knots if you use your time-loop computer to calculate how to build a general-purpose time machine.)
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### Re: Do these FTL mechanics work?

zmatt wrote:*personal opinion*
I don't think time travel back in time is possible with relativity. Relativity allows forward time travel because as the name states we are changing what is experienced "relative" to the observers. In practice it is less actual time travel than it is slowing down time for you and letting the world pass you by. The explanations I have heard and read for possible backwards time travel with relativity is very similar to forwards, it seems as though its a "hack" if you will on our perception of the universe. You travel to a distant place where the light from earth is very old and shows a younger one, then you travel back at relativistic speeds and viola you are with dinosaurs (over simplification). I think that sounds a little silly.

It might sound silly but it's the way it works. The whole "wait-a-minute this implies faster than light travel is equivalent to time-travel" was one of the early criticisms of GR. Turns out that for every experiment we can do, it upholds those laws. We can't go faster than the speed of light, but there exist solutions to the GR metric that allow for such weird time loops. The general expectation is that if we ever get a chance to experiment with one of these, it will explain a lot that we don't know. In short, it may not be possible, but as the theorem stands it is, and nothing has ever suggested that the theory of General Relativity is wrong.

Now, in regards to the op, as far as the question goes no. We can send something back to the wormholes creation from arbitrarily far in the future, but not any further. We also need two wormholes for this skullduggery, but there's no requirement for them to travel through eachother. This is going to be a monster post, so you may want to just go with the pat answer, and the edit at the end noting one possible other restriction, but if you want an explenation and some interesting results:

What we have created is a simple system. I'm not going to screw around with the dynamics of creating it, and use the far simpler situation of a stable created system. Therefore, we now have a portal from Alpha Centauri to 3-years-in-the-past-Earth, or conversely we have a portal from Earth to 3-years-in-the-future-AC. With this portal alone it is impossible to arrive somewhere before you left. Why? Simple, because to travel between either end NOT using the portal takes four years. Thus, we could do, 4 years Earth-AC, -3years AC-Earth. We could also make it take 7 years if we went +3(portal) years Earth-AC, then the slow classical way 4 years AC-Earth. We could also take 8 years by doing the classical trip without any portals.

Importantly, we could also take almost 0 time, by hopping throught the portal twice, +3 (portal) Earth-AC, -3 (portal) AC-Earth.

In Essence what we've done is allowed any transit time that can be made up of a combination of any two numbers of the set [3,-3, 4, 4], but we can choose each number only ONCE. In fact, what we're actually doing is choosing one of [3,4] for the Earth-AC leg, and one of [-3,4] for the AC-Earth Leg. Thus the choices outlined above can be represented as: (4, -3) = 1, (3, 4) = 7, (4,4) = 8, (3, -3) = 0.

Physically this is a simple scenario, Standing on earth I have two routes to Alpha Centauri, the classical way (4yrs) and the portal (3yrs), represented by our choice of on of the set [3,4]. On AC I have the classical way (4yrs) and the portal (-3yrs), giving a choice of the set [-3, 4]

However, this is reliant on the fact that there is ONLY one portal. With two we can quite easily go back arbitrarily far. By simply constructing a second portal (again not bothering with the dynamics of it's creation) we add to our list of possible ways to move. Let's suppose that we send a second ship with a wormhole in the exact same manner out from earth, with some time dialation the gives us a wormhole whose discrepency is "a" years, much like our first one had a discrepency of 3 years. Again skipping all the dynamics until the wormhole is settled.

So now I'm standing on Earth looking at the three possible ways to go to AC, I now have the classical route (4yrs), Wormhole 1 (3 yrs) and Wormhole 2: Electric Bugaloo (a yrs). On AC I have the choices of classical route (4yrs), Wormhole 1 (-3yrs) and Wormhole 2 (-a yrs).

So we chose one from each set of [4, 3, a] and [4, -3, -a]. From this we can choose (amoungst others) either (a, -3) = a-3 yrs, or (3, -a) = 3 - a yrs = -(a-3) yrs. As long as (a-3) is not zero (i.e., as long as our second wormhole isn't the exact same as our first wormhole) either (a-3) is negative, or (3-a) is negtive. Since either of those are valid choices I can go back in time with just the two wormholes. Note also that if we sent the wormhole from AC to Earth instead of Earth to AC, a would be negative, that is we could be going backwards (or forwards) on both legs of the trip.

If we allow a third wormhole we can get some very strange effects, for instance, suppose this new third Wormhole has a time difference of "b" years (i.e., travelling from earth you arrive at AC b years into the future). If (3-a) and (3-b) and (a-b) are not factors of eachother (e.g., it isn't a=1 and b=5) We can go back fractions of the time (3-a) rather than ONLY having the choice of incrementing back by (3-a). So if for example the loop between wormhole 1 and wormhole 2 allowed you to travel back 2 years, you could only travel back 2yrs, 4yrs, 6yrs, etc. but if there is another loop with wormhole 2 and 3 that takes 3 years off, you could also go back 3 years (loop 2 and 3) and then forwards 2 years (reverse loop 1 and 2) to arrive one year in the past. If you set your times up right you can reach some pretty arbitrary times in the past, an irrational time backwards would allow you to reach ANY point in the past (after the wormhole loops were setup).

If we allow ourselves to shove one wormhole through another we can of course shove it through a series of these loops, which results in it having the same effect as if we just went through the loops ourselves. Still, if people are commonly wrapping through the same 19 loops or whatever, shortening that will at the least alleviate some congestion (which is going to be a problem, more on that later). The important point though is that this loop shove through a loop is exactly equivalent to if I'd just taken the loops that our "loop in a loop" took.

Alright, now so far I've explained a lot of what it CAN do, which is important to show what it CAN'T do, specifically it CAN'T send us further back than the wormhole creation. Why? Because all the above relied on the idea that the wormholes were already there. If I go through my magical take me to the past loop far enough I will come to a point in the past where, standing on earth, the wormhole has not yet reached AC. At that point there is no loop, and I can't ride it back any further. I must wait for it to be setup, and that's the very first (or last if we're thinking of it in terms of the subjective time of someone travelling backwards) leg I can take. And since each leg is, by the necessity we've already pointed out, less time that it took to setup, that leg won't take you back to before the wormhole existed.

"but aha!" you say "I have a plan! I'll use my loop in a loop!". Sorry, but we've already established that that is exactly the same as just running the loops yourself. the loop in a loop only goes back as far as you. And it you can't set it up to go back further, because to set it up the original loop needs to exist! So the "loop in a loop" has an effective setup time that is always equal or greater (it could be greate if you took some time moving it between loops) than going through it removes, just like the original loops.

"But aha again!" You say, being the annoyingly-eternally-optimistic sort, "I have another, even more fiendish, scheme! I'll bring the portal gun with me!". Alright, so now you have a portal gun with you, great. What can you do with that? Well, you could setup another series of loops, but since this series won't take you back before it's creation, you can't use it to go back further than the new series is created (This new series could also have been created as part of the old series using the loop-in-a-loop method, so it's actually equivalent to the previous ill-thought-up-plan of yours you hypothetical dunce). "Okay, but what I do is setup a portal now, and then wait for the first wormhole to arrive, then I jump into it and arrive on earth one year after the original series (not of Yugioh) was created. Then I fire that end and climb through which gets me...back to the creation of the second series".

Yes. Yes it does. A similar trick (of portalling to the ship as it moves arcross the 4 light-year just from earth to AC setting up the first portal) would allow you to go back even further, to the moment of creation of the first wormhole, back on earth. Which is great, but it relied on that wormhole existing to do it, which was the original problem. You can never go back further than the creation of the first wormhole, so you can never setup a prior wormhole to connect to.

So, in short, you can travel between any two arbitrary spacetime points where the wormhole network exists (note that it has to exist at both those spaces and times), but you can never travel to before the network was setup.

Of course, this is all because the original setup involved a ship that moved slower than the speed of light. Because of that our network can only ever connect to points within the light cone of the creation of the original wormhole. If we could get outside that with some other form of FTL, then we could easily go back arbitrarily in time. Of course, if we have that kind of FTL, what the hell are we using wormholes for?

EDIT: Oh right, forgot to mention the reason congestion could be an issue. Specifically, if I can go through a loop an arbitrary number of times (because I can go back in time, piss around, and then take the loop back again, piss around, and take the loop back again alongside the two previous iterations of me etc.) so can anything. Because, in quantum mechanics, any path and particle that IS possible must be considered (gross oversimplifcation), we get an infinite number of these particles taking these loops an infinite number of times. This is a bad thing, since for one it will have an infinite mass (or mass-energy for photons) which will warp spacetime into a singularity, which will destroy the wormhole. So it's arguably possible that the first wormhole will be fine (since no "going back in time" loops are possible) but any subsequent ones (which, as mentioned before will allow for such messing around) will end up taking out the entire series of wormholes. You could created a series of wormholes that goes outwards, but it would have to be all expanding out radially (or at least mostly radially) from a single hub. Any crossing of two sets of outwardly moving wormholes would screw the entire thing over. Don't cross the streams.
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### Re: Do these FTL mechanics work?

Ah, yes, I think I understand. Does the problem basically come down to the catch-22 of it being impossible to arrive further back then you started without a wormhole already being there? (And obviously you can't get the wormhole there without arriving there.)

And since my original scheme didn't work, is there any combination of wormholes, or similar devices, that can arrive before the device was originally built? Obviously, I could use a magical teleporter, but I'd prefer a device that has limits built-in, like the wormholes. (I would prefer not having to hand-wave an entire TARDIS into the story's universe.)

Antimony-120 wrote:EDIT: Oh right, forgot to mention the reason congestion could be an issue. Specifically, if I can go through a loop an arbitrary number of times (because I can go back in time, piss around, and then take the loop back again, piss around, and take the loop back again alongside the two previous iterations of me etc.) so can anything. Because, in quantum mechanics, any path and particle that IS possible must be considered (gross oversimplifcation), we get an infinite number of these particles taking these loops an infinite number of times. This is a bad thing, since for one it will have an infinite mass (or mass-energy for photons) which will warp spacetime into a singularity, which will destroy the wormhole. So it's arguably possible that the first wormhole will be fine (since no "going back in time" loops are possible) but any subsequent ones (which, as mentioned before will allow for such messing around) will end up taking out the entire series of wormholes. You could created a series of wormholes that goes outwards, but it would have to be all expanding out radially (or at least mostly radially) from a single hub. Any crossing of two sets of outwardly moving wormholes would screw the entire thing over. Don't cross the streams.

I understand how you'd end up with perpetual motion like devices since as you say, you could send the same particle through the wormhole many times. However, I'm not sure where the infinities come from, since only a finite amount of energy can actually cross the wormhole at any one time, and the hole exists for a finite time. Though I'm guessing that only a finite amount of energy can cross the wormhole because the past-lightcone of the hole contains finite spacetime, and there isn't such a thing as infinite energy density.
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### Re: Do these FTL mechanics work?

If the wormholes already exist, then you just have to travel through them right?
So your "fictional" universe is populated by wormholes. Billions of billions of the things. These are the explanation of dark energy and dark matter. There is no extra matter or energy, it's just effecting a larger region of space than it should, because it's forces travel through the worm holes.
Your FTL device opens up or groups these wormholes into something large enough for a ship to travel through. To travel faster, you need to collect a larger number of wormholes, thus making more hops in a shorter period of time. Or stretch the existing wormhole into a much larger one.

If you want to prevent travel from the planets surface, gravity could effect the travel or wormholes in some way. No need to explain it, just the "it's bad" if someone tries it!
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### Re: Do these FTL mechanics work?

The issue isn't so much the narrative justification of FTL/time travel devices, since that's been done pretty much every conceivable way, but trying to get away with them while leaving the smallest "hole" I can both in my plot and in general physics. For instance, the wormholes I was originally conceiving of were only large enough to fit photons, and later a nano-machine, through. To get the time dilation, STL vehicles are still used.

The problem with using pre-existing wormholes or some other solution is that, once you've got a FTL drive, there's no logical reason why you can't use it to zap yourself almost anywhe{n/re} in the universe, even if it's limited by gravity wells and such. (Unlike the "logical" reason that with the wormholes, you need to wait for a very long time and expend a large amount of resources, i.e. a whole relativistic starship, to travel even quite a small distance.)
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### Re: Do these FTL mechanics work?

Create two paired wormholes with a time dilation. Somehow stop the universe from collapsing due to quantum effects (sure it is infinite energy: so subtract it from your equations!)

Use them to build closed time-like loops. The power of the computation you can do with wormholes of offset X and bandwidth B is a pretty fast growing complexity class.

Now if you want to have fun, the energy caused by a wormhole time-loop might manifest as an increasing amount of leakage as they approach a closed time-like loop that actually pushes them away from each other. The potential well you have to climb to create a closed time-like loop using wormholes could be infinite?
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### Re: Do these FTL mechanics work?

Wouldn't having the pair of wormhole sets simply far enough away from each other deal with the problem of them collapsing? A random walk in more than 1 dimension does not reach each point with probability one, so the feedback between the two should be dampened geometrically based on the distance between them should it not?

Oh, and tangentially, is there any theory as to what happens if you go through a wormhole at relativistic speed?
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### Re: Do these FTL mechanics work?

QM doesn't do a random walk. QM seems to follow all paths. In both directions (time-wise).

And the paths when there is a wormhole closed time-like loop route diverge. Kaboom.

The thing I was curious about is, as you approach a closed time-like loop (in some sense), would there be a potential gradient that occurred "forcing" the wormholes apart? Or is that divergence discontinuous? (it being continuous seems ... more pretty ... to me for some odd reason)

Would the effect itself provide a means of FTL communication? Not necessarily if the effect is basically determined by how close, in your past light-cone, the far entrance of the wormhole is, in some sense?

This is getting really vague.
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WarDaft
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### Re: Do these FTL mechanics work?

But each path is essentially a random walk is it not?

I mean, we have to have some way of valuing clusters of unlikely paths as less massive due to their unlikeliness... otherwise a single particle would have infinite mass due to an infinite number of diverging possible paths.

Basically, unless the wormhole paths plus the non-wormhole paths add up to a probability of more than one, a particle near the wormhole shouldn't be any more massive than one anywhere else. It's just less likely that the particle is actually very far away (which is pretty unlikely to begin with)

Or does a particle's path actually sum up to an infinite mass spread out over an infinitely large area so that it appears in a local context to be finite? That seems like total nonsense, but I must admit I don't know much at all about the nitty-gritty of QM.
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### Re: Do these FTL mechanics work?

Infinities abound! There is a mixture of paths "canceling out" with each other, and normalization of some kind (which gets rid of infinities).
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### Re: Do these FTL mechanics work?

That would require that "two" superimposed paths are more likely to cancel out than double in magnitude. That would be quite interesting. But really, whatever mathemagics that make it more likely for a particle to be "here" rather than "anywhere else", there's no reason that that should not still apply near (or far from) a wormhole - you're just wrapping some of the "anywhere else" into a smaller area. It should still be as likely to stay "here" as before.
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### Re: Do these FTL mechanics work?

http://en.wikipedia.org/wiki/Wormhole#Time_travel
It is thought that it may not be possible to convert a wormhole into a time machine in this manner; the predictions are made in the context of general relativity, but general relativity does not include quantum effects. Some analyses using the semiclassical approach to incorporating quantum effects into general relativity indicate that a feedback loop of virtual particles would circulate through the wormhole with ever-increasing intensity, destroying it before any information could be passed through it, in keeping with the chronology protection conjecture. This has been called into question by the suggestion that radiation would disperse after traveling through the wormhole, therefore preventing infinite accumulation. The debate on this matter is described by Kip S. Thorne in the book Black Holes and Time Warps, and a more technical discussion can be found in The quantum physics of chronology protection by Matt Visser. There is also the Roman ring, which is a configuration of more than one wormhole. This ring seems to allow a closed time loop with stable wormholes when analyzed using semiclassical gravity, although without a full theory of quantum gravity it is uncertain whether the semiclassical approach is reliable in this case.

And, in far more detail: (a different issue maybe?)
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### Re: Do these FTL mechanics work?

Damn, beat me to it. Read Traversable Wormholes, a fairly thorough exploration of how FTL travel via wormholes would work (given certain physical assumptions). In particular, if a wormhole pair is separated by too little distance given their time separation, you'll get an infinite virtual particle flux through it. In practice, the flux will close the wormhole sometime before that happens as you asymptotically approach the limit.
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### Re: Do these FTL mechanics work?

Did a search and found that this thread was closest to what I wanted to post about so I raised it from the dead by means of a dark arcane rite known only to myself and a few Honey Badgers!

Anyway, back on topic. I had a thought...
1. Assuming that the Higgs Boson is found and verified, we have a mechanism by which stuff is "given mass".
2. As an object approaches the speed of light its mass approaches infinity.

Given the two points above, if some way was found to inhibit the effect of the Higgs Boson, would FTL travel then be possible?
I imagine some kind of field being generated which almost completely isolates a spacecraft from whatever the Higgs boson actually is. The reason I say "almost" is that the desired effect is to reduce mass not eliminate it...I think that would be bad!!! Therefore, a craft under the effects of this field would have a near zero mass and could potentially travel faster than light.

Is my understanding, given the sci-fi context, flawed in any way?
Mostly kind of almost...ish.

idobox
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### Re: Do these FTL mechanics work?

It would not allow to travel faster than light. TO do that, you would need a negative mass, whatever that means.
By reducing your mass, you would need less energy to reach the same speed, and by reaching a perfectly null mass, you would get to light speed, but not above.

Also, higgs bosons will be produced by your craft. You don't need to protect yourself from the ones out there, but rather to find a way to stop the ones you have inside.
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### Re: Do these FTL mechanics work?

Ah I see...I've got it all frack to bunt!!!
Mostly kind of almost...ish.

idobox
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### Re: Do these FTL mechanics work?

Sanjuricus wrote:Ah I see...I've got it all frack to bunt!!!

I'm not what it means.

But the higgs thingie would be nice to lower space flights energy requirements
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### Re: Do these FTL mechanics work?

Apologies, just noticed you are French and probably wouldn't get an English colloquialism!
Frack to bunt = back to front.
Mostly kind of almost...ish.

ikrase
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### Re: Do these FTL mechanics work?

I still don't see why the particle flux has to go through the wormhole instead of missing it most of the time. Seems like it might make the wormholes perishable, and/or usable for perpetual motion though...
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### Re: Do these FTL mechanics work?

Most of the time, it *will* miss the wormhole. But when you approach the critical distance, there's more "chances" for a virtual particle to circle all the way through.

I'm not sure about the math/science here, but I suppose to be rigorous you probably want to track the energy fields directly, rather than the virtual particles spawned by the field. Then it's a (simpler?) matter of doing an infinite sum and seeing if it converges or diverges. The wormhole would collapse before the field diverged.

You couldn't use this for perpetual energy, because it's not creating the energy from nowhere, it's just layering existing energy fields. You'd be extracting energy from existing sources, and they would lose energy as normal as you extracted it.
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### Re: Do these FTL mechanics work?

Wouldn't a wormhole mess up with Maxwell's equations? The field would go trough the hole, too...
Or for a closed surface including the wormhole would a double sphere (one for each portal) be chosen, to avoid this?
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### Re: Do these FTL mechanics work?

No, they apply just like normal. They just do so over a space that is more warped than you realize.
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Giallo
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### Re: Do these FTL mechanics work?

And a closed surface in the 'classical' space would be also closed if containing a portal?
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### Re: Do these FTL mechanics work?

Xanthir wrote:Most of the time, it *will* miss the wormhole. But when you approach the critical distance, there's more "chances" for a virtual particle to circle all the way through.

I'm not sure about the math/science here, but I suppose to be rigorous you probably want to track the energy fields directly, rather than the virtual particles spawned by the field. Then it's a (simpler?) matter of doing an infinite sum and seeing if it converges or diverges. The wormhole would collapse before the field diverged.

You couldn't use this for perpetual energy, because it's not creating the energy from nowhere, it's just layering existing energy fields. You'd be extracting energy from existing sources, and they would lose energy as normal as you extracted it.

Wouldn't this mean that you could stabilise any wormhole, so long as you had somewhere to put the accumulated energy? (Or am I misunderstanding?)
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Xanthir
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### Re: Do these FTL mechanics work?

I'm not a wormhole physicist, but I think the problem is the flux *through* the wormhole. Even if you could dispose of the energy on either side, there's still a bunch "in" the wormhole.
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### Re: Do these FTL mechanics work?

New question: If I arbitararily say that maintaining a wormhole requires a certain amount of negative energy, and this energy gets larger the further apart in spacetime the two ends are, does this do anything bad like establish a preferred reference frame? (i.e. If the two ends are sitting in different gravity wells, you've got to continually pump in negative energy to counteract the time dilation, otherwise the hole collapses. If this difference is because they're sitting on different planets, you've got to continually put in and take out energy to balance out their relative spatial velocities.)
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