Can this version of the Alderson Drive be used to violate causality?

Question

I'm working on a simulator-type game which I want to be at least internally consistent, and which I'd like to work as close to reality as possible. That being said, it's set in space, which means to avoid player boredom there needs to be some kind of FTL travel.

I'm trying to work in functional analogs to the four general categories of FTL travel (warp, jump, gate, and hyperspace) and I think I've managed to work it down to only two core mechanics – the Alcubierre Drive and a modified version of Alderson Points. Originally I was going to use stabilized wormholes in place of Alderson Points, but after looking into it I found that that makes it easy to build a backwards time machine (which is annoying/impossible to reliably implement in something like an MMORPG), so after failing to figure out how to "break" all of the versions of the time machine as per the Chronology Protection Conjecture, I gave up and started working through other mechanics.

After expanding on the original Alderson Drive concept for story, time, and map reasons, I've come up with the following list of desired/assumed mechanics for the points.

• Suitable points exist naturally (~1-5 per star system) positioned based on rules which have yet to be determined, but preferably there's always at least one within 0.3 AU of the habitable zone.
• Points don't "orbit" the host star, and they probably don't exist in and of themselves; they are either stationary relative to the star or they revolve based on the position of other large bodies similar to the Lagrange points. (One or the other, not both, I just haven't decided yet in case physics decides for me.)
• Points are tied to networks; they can all connect to more than one point, but only within their network. Being in the same network involves always having the same inertial frame of reference, though that may just be a side effect of some underlying reason.
• Using the points involves "charging up" the points with a device on your ship for an amount of time (i.e. not instantaneous or measured in nanoseconds, but also not measured in hours or days unless the drive is broken or poorly tuned). The destination is selected by how much energy is "dumped in." (This will likely be proportional to the distance between the points, but may need to be related to the ship's mass as well.)
• While the point is being charged, both the source and the destination with the energy level closest to the current level give some kind of indication that they're being activated (e.g. "spontaneous" EM radiation, visible or otherwise, possibly similar to black body radiation but emitted from a field).
• When the drive is turned off, provided the input energy precisely matches what is required to connect to a destination, all of the the matter within a certain radius of the source and destination points is instantaneously swapped. (The definition of instantaneous, in this case, is "now" according to the point network's reference frame.)
• Artificial point networks can be constructed using devices ("stargates"), though this may not actually need to be explained because I could easily say that "the Ancients/Precursors/Forerunners built the devices and we don't know how they work" or something to that effect.

Now here are my questions:

• Most importantly, can this set of mechanics be used by itself to violate causality – and not just apparently from an extraneous reference frame, but literally be used to either tell yourself not to do something before you were going to do it or, in the more serious case, "take half a critical mass of Plutonium back to meet itself"? If so, what could I change to make it impossible?
• Can it be used in concert with something else to violate causality? If so, what would be necessary/what should I watch out for?
• Are there conditions related to causality violations that could dictate where I need to place the points inside of the star systems or the nodes in the point networks inside of different star systems?
• Is there any possible scientific or pseudo-scientific base I could latch onto to try to explain some/all of these mechanics? (The original version mentions "Equipotential Thermonuclear Flux" but I thought I'd try finding something that sounds a little more real...)
• What should happen near the "edges" of the effective area of the points? Or should I just say "nothing good happens" and make determining the radius a function of the drive construction so that individual ships shouldn't ever have to care?
• Is there a way (plausible or not) that I could pull this out into also enabling FTL communication? Because, like I said, my accidental time machine ruled out movable wormholes, and therefore my only lead on how to make an Ansible. If I have to, I could use something like the "space pony express" where ships download mail, pop through, and offload it, and for urgent/secret messages use unmanned probe "carrier pigeons," but I'd like to figure out a way to do it without physically sending matter to the destination (i.e. "subspace radio") if possible.

_{(Note: I would have asked this on the Physics StackExchange, but I'm pretty sure it crosses the fictional "line" so I decided to ask here first because of their definition of "on-topic.")}

Answer 1

FTL time paradoxes generally use one or both of the following circumstances:

1. A round trip between two objects that are moving (relative to each other) at a significant fraction of light speed.
2. A wormhole that allows physical contact between two separated locations simultaneously.

An Alderson style system can avoid trouble if:

1. The traveling object is (briefly) removed from communication with the universe, then brought back in at the other end.
2. The endpoints are reasonably stationary and cannot be artificially moved.
3. You cannot return to your starting point "quickly" (aka before you left).

Here's some math which may or may not make physical sense (IANAP). The Sun and Wolf 424 have a relative motion of about 555 km/sec (0.0018 c), with a relative time dilation of 0.0000017. At 14 light year distance, that's a discrepancy of about 13 minutes. As long as your charge up delay is that long, causality should be safe. That should be the worst case; other nearby stars have less relative motion, allowing for shorter delays.

Answer 2

If you can do FTL travel, you're going back in time according to some potential observers. If you can do FTL travel in different reference frames, you've got time travel. If you say FTL only works in one reference frame, you're ditching Special Relativity, and I'd rather not do that. It is the solution of choice for fiction, since it isn't completely intuitive and audiences usually don't notice it being broken.

The idea of putting the Alderson points all in one reference frame seems brittle to me, at least if you like relativity. It's extremely unlikely that a universal reference frame will match ours reasonably well, and so it works better if the reference frame can change gradually over space.

Given that you have a fixed number of discrete Alderson points, you can fake an ansible pretty easily. You have information-carrying drones that do endless round trips between Alderson points. They pop out of the communication station near point A, hop to B, transmit and receive information with the communication station near point B, then hop back. Repeat as desired. Lightspeed delays over 0.3 AU could be more significant than drone transmission delays.

In my story, I just had an FTL drive that I slapped a name on. It could propel things at up to a certain speed. There were no theoretical limitations, just "this is how fast we can go with current technology". FTL was relative to the mass in a large volume around the ship, which means the reference frames can't be very far within, say, a thousand light-years of Earth. With FTL but not instantaneous movement, and a limited choice of reference frames, there's no time travel going on right now. Increase possible speeds too much and it becomes possible. Go sufficiently far away to get a different reference frame and it becomes possible. With what we've got now? Not a chance.

Answer 3

Disclaimer: I'm not a physicist.

That begin said:

Short Version: From what I understand from the various expeditions in the world of Physics.SE FTL-questions, I'd say any kind of FTL travel or communication will allow causality violation.

Longer version:

There are a few potential problems I see with your description: First of all, how can points belonging to the same network share the same inertial reference frame, and still move relative to their respective stars? Check with your trusted physicist, but as far as I understand, inertial reference frame means no relative motion between each other. Also, I don't know what being in the same inertial reference frame, but far apart, would do, and if it matches your expectations of having a well-defined "now"/"simultaneous"/"instantaneous".

Typically, violating causality goes like this(crude explanation, look at the many answers on Physics.SE for a better one): Move from point A to point B at superluminar speed. Now move back. You end up in a reference frame at position A but before you left.

Remember that "simultaneous" is something very unintuitive in Relativity (not sure whether that's General or Special Relativity), and depends on the observer (=is relative).

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But this is fiction, you could just say: We found these things, they don't allow for time travel, so all this Relativity stuff wasn't 100% right after all. What's the problem with that?

Answer 4

The key requirement for FTL time travel is making two separate FTL jumps in two different reference frames. Thus, to prevent paradoxes, you simply need to pick one reference frame (e.g. that of the cosmic microwave background, which the closest thing to a universal stationary reference frame in the universe) and declare it as the only frame in which FTL travel can take place.

With your Alderson-based system, this is quite simple: Make all swaps instantaneous in the CMB frame. And you're done.

The Alderson points themselves don't have to be stationary relative to anything. If one point is moving rapidly or deep in a gravity well (e.g. orbiting close to a black hole), it will experience time more slowly than a point nearly at rest relative to the CMB in interstellar space on the outskirts of the galaxy. However, this just means that clocks attached to these points will disagree on how much time passes between consecutive swaps. There's no way to use this sort of system to create any kind of truly reality-breaking paradox. No meeting yourself, no murdering your grandfather; nothing stranger than the classic "twin paradox" from special relativity.

If you want to use this system to take a few good close-up photos of Sagittarius A* (the supermassive black hole at the center of the Milky Way galaxy), you might arrive back home a few weeks (or months, or years, or centuries, depending on how close up your your photos are) after you left, despite only experiencing a few minutes yourself, but that's about the worst that might happen.

For FTL communications, the simplest solution given the existing Alderson system would indeed be Alderson couriers- your "space pony express". If you want to make it less of a hassle, you'll need some additional mechanic. Maybe the energy required to power an Alderson swap increases with the size and mass of the object being transported. Flash drives are small and don't weigh much, so sending them through would be pretty cheap. Alternatively, you might be able to rig the Alderson points to swap extremely rapidly, with a period approximately equal to the time it takes for light to traverse its area of effect. Light beamed at the point will enter the point's area of effect, get swapped to a point in the target system, then exit the area of effect before being swapped again. This makes the pair of points act like a two-way wormhole that only light can cross- and since light has no mass, these swaps may be cheap enough to make the rapid cycling feasible.

For more info on FTL time travel, check out this video from PBS Space Time, which explains how a ship equipped with an Alcubierre drive with a maximum speed of only twice the speed of light could fly off into space and return home before it was built. Note that to pull it off, the ship must burn its Alcubierre drive twice: once in one reference frame (the Earth's, in this case), and once in a reference frame moving very close to the speed of light relative to Earth. This change of reference frame is the key, and an Alcubierre-equipped ship can probably pull it off by using conventional sub-lightspeed engines to accelerate itself to a substantial fraction of the speed of light between the Alcubierre burns.

If, however, the only available FTL tech is locked to a single frame of reference, all these paradoxical time-traveling shenanigans become impossible. This means no Alcubierre drives that don't cook themselves in infinitely-hot Hawking radiation (or just collapse into black holes the moment you turn them on), no stable wormholes that don't explode with virtual particle feedback (or, once again, collapse into black holes) the moment they're time-dilated and brought close enough together to permit backward time travel, and definitely no tachyonic antitelephones. But your Alderson system, with the restriction that all instantaneous FTL swaps occur in the same inertial reference frame, works just fine. It doesn't necessarily have to be the CMB frame; the CMB is just a nice example of a frame that anyone can measure their velocity relative to no matter where they are in the entire universe, as long as they have the right kind of telescope.

Answer 5

The short answer to the title question is no. I don't even need to know what you've built or how it works, there's a thing called Time's Arrow, or macro-system temporal asymmetry; basically it says 3-Dimensional objects move forward through time, period. No travel back in time is possible if you're a 3-Dimensional being. It's the reason Entropy and a couple of physical constants like Planck and Boltzmman exist.

As far as I can tell while we can see how Time's Arrow effects the universe we don't know why it exists (it does seem to allow us to see the universe in the first place). It means that causality violating "space-time geographies" are inaccessible so all FTL has to be locked into the same Inertial Frame of Reference so you can't get out of you "Light Cone" to violate causality.

Answer 6

Any kind of FTL at any "velocity" of FTL can potentially create a temporal paradox, so it doesn't matter what type you use so long as you set up some rule to prevent it from causing one. Any kind of warp drive is probably the worst example because at first blush it seems hard to make sure you can't do it due to the relative freedom of travel. Wormholes and any sort of jump points make it easier: you simply state that the second (or 3rd, 4th, whatever) trip that would cause a causality violation--a one-way trip really won't do it--prevents the jump/wormhole from allowing passage until such time as it would not violate causality.

Answer 7

If you limit your Warp Points (WPs) to occur only around starts and can't be moved then you only have to deal with causality if you can move the star.

Also, give the WPs a non-zero transit time.

The non-mobility of the WPs should do away with any meaningful time shenanigans since you can't accelerate one WP much more than the other one is being accelerated. The transit time can be manipulated to cancel out any distortions due to acceleration (the greater the difference in acceleration between two WPs, the greater the transit time). This should negate the effect even if they can some how move the WPs.