## FTL-com without breaking *too* many laws of physics

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### FTL-com without breaking *too* many laws of physics

So, I'm writing a piece of scifi fan-fiction. In the universe I'm using there exists a device which allows for instantaneous information transfer over any distance, at a very limited bandwidth. Since I want my fanfic to be as scientifically sound as possible, this presents me with a bit of a problem.
Firstly, "instantaneous" is not a well defined concept once one takes relativity into account. Since no further explanation of the meaning of instantaneous is given in the available canon, I'll have to make something up myself. The most obvious meaning, I think, would be that the information propagates along the simultaneity of the transmitter. This results in more problems however, since if implemented without restrictions such an interpretation would allow for information to be sent back in time. Consider the following thought-experiment:

Observers A and B each have one of these magical devices. According to A:s frame of reference, B is moving away from him at a velocity of [imath]\frac{2}{3}c[/imath]. A decides to send B a message by means of his magical device.
When B receives the signal, which has propagated along A:s simultaneity, it appears to her as if A has yet to send his message. She responds with a new message, which reaches A before he has a chance to send his first message.

Not OK. I very much doubt the original creator of the universe intended for this to be possible, so I don't want it to be possible in my fanfic. In order to avoid it, I have implemented a completely arbitrary limitation on the magical device: you can only use it to communicate with people who are moving towards you. The velocity at which they move towards you matters insofar that the lower the velocity, the lower the SNR of the signal.
Now, I think this removes any possibility of time-fuck, but since it's been awhile since I studied relativity I'd like another opinion.
Because phlogiston.

douglasm
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### Re: FTL-com without breaking *too* many laws of physics

defaultusername wrote:Not OK. I very much doubt the original creator of the universe intended for this to be possible, so I don't want it to be possible in my fanfic. In order to avoid it, I have implemented a completely arbitrary limitation on the magical device: you can only use it to communicate with people who are moving towards you. The velocity at which they move towards you matters insofar that the lower the velocity, the lower the SNR of the signal.
Now, I think this removes any possibility of time-fuck, but since it's been awhile since I studied relativity I'd like another opinion.

Sorry, you'll have to find another restriction. The standard example I use for explaining how and why instantaneous communication allows communication backwards through time involves the communicators moving towards each other.

Quick thought experiment to prove that any limitation scheme involving velocity relative to you must be translationally symmetric:
Suppose there are communicators A, B, and C. A and B are moving at the same speed in the same direction - they have the same reference frame, they're just in different positions. A and B can obviously talk to each other. A cannot talk to C, but B can talk to C. If A wants to send a message to C, all he has to do is tell B to pass it on.

The only "magic" restriction I can think of that might work is a "magic causality enforcer" - two people cannot open communications with each other unless they both agree on which communications they are aware of are in the past. If a particular conversation would send information back in time, the communications channel fails to open. This would tend to require the communicator's choice of strictly one-way communication, low relative velocities, or sufficiently long delays in replies to negate the point of it all, however. Allowing the choice of using either the sender's or receiver's simultaneity would fix that, though.

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### Re: FTL-com without breaking *too* many laws of physics

douglasm wrote:Quick thought experiment to prove that any limitation scheme involving velocity relative to you must be translationally symmetric:
Suppose there are communicators A, B, and C. A and B are moving at the same speed in the same direction - they have the same reference frame, they're just in different positions. A and B can obviously talk to each other. A cannot talk to C, but B can talk to C. If A wants to send a message to C, all he has to do is tell B to pass it on.
True. But at least this way C can't relay that information back to A before he can send it, right? Or am I missing something?
Also, A would actually not be able to talk to B, and vice versa, unless they were moving towards each other. Of course, they could do so very slowly, so it's not really important to the thought experiment.

Edit: No, C can send it to A, if he relays it through D, who is moving towards both A and B. Crap.
Spoiler:

Oh well, the universe I'm writing about only involves two star systems and ships moving back and forth between them. For that purpose, my limitation is enough, right?
Because phlogiston.

douglasm
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### Re: FTL-com without breaking *too* many laws of physics

defaultusername wrote:Edit: No, C can send it to A, if he relays it through D, who is moving toward both A and B. Crap.

Exactly. Any restriction of that sort can be circumvented by routing through a third party.

As far as I can figure out, you pretty much have to come up with some equivalent of a universal reference frame for the magic communication device, or a Cosmic Ordering Principle or something similar. Also, two-way communication between people with very different velocities just plain won't work at all well unless at least one of them is using a simultaneity other than his own.

Hmm, I wonder how it would work if the device used the simultaneity of the reference frame in which the average velocity of the communicators is 0. I don't know the math well enough to actually work it out, but that might get an A->B->C message to reach C at the same time as an A->C message, which would prevent time travel (of information) abuse between inertial communicators. I'm pretty sure sufficient acceleration would still break things, though, but it would at the very least solve the two-way communication problem.

quantropy
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### Re: FTL-com without breaking *too* many laws of physics

Why not have preferred frame of reference depending on the general distribution of mass in the system. Then if you needed to explain how the device works you could bring in EPR and Quantum Gravity.

gmalivuk
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### Re: FTL-com without breaking *too* many laws of physics

douglasm wrote:Any restriction of that sort can be circumvented by routing through a third party.

Yeah, even if you require that there be zero relative velocity for it to work, you can pass on the message at lightspeed to someone who is moving, and then they can pass it "back" to someone else who's moving, who can then relay it to the originator at lightspeed before the originator sent it in the first place.

If there is relativity of simultaneity (for the purposes of this communicator), then FTL breaks causality.

And even if you pick a preferred frame for the device, you still have to account for the fact that someone moving relative to that can see what appears to be backward communication between nonmoving communicators.
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Tass
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### Re: FTL-com without breaking *too* many laws of physics

I think the simplest would be to just arbitrarily say that "instantaneous" is in one privileged frame not in any other, the obvious choice would be the comoving frame. This way you only violate the principle of relativity, and nothing else.

BlackSails
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### Re: FTL-com without breaking *too* many laws of physics

Just say that the ether exists. Bam, you have a universal frame.

ikrase
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### Re: FTL-com without breaking *too* many laws of physics

That'-s also my suggestion, maybe. Bizzare conditions in which the device works seem good to me. Alternatively you coudl have a wormhole so small taht it only takes radio waves. The wormhole end must be moved slower than light.

My preference for sci=fi is for simply saying that nanotech and genetic engineering end limit on human lifetime so aging stops at age 35 and the half life of a human (based on accident rate) is about 30,000 years, and it;s all slower than light but that doesn;t matter because human lives move really slowly. Like the old days when mail took months to go from one end of the country to the other, intercontinental transfer dangerous and very expensive.
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BlackSails
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### Re: FTL-com without breaking *too* many laws of physics

You could also send information by magically making the no-cloning theorem go away. Now you can use quantum entanglement to send data.

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### Re: FTL-com without breaking *too* many laws of physics

BlackSails wrote:You could also send information by magically making the no-cloning theorem go away. Now you can use quantum entanglement to send data.

Doesn't that require making certain assumptions about which quantum mechanics interpretation is correct? IIUC, even without the no-cloning theorem entanglement still can't carry any information under the MWI, because it's not a relationship between two particles, but rather just a particular state of the universe.
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Tass
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### Re: FTL-com without breaking *too* many laws of physics

BlackSails wrote:You could also send information by magically making the no-cloning theorem go away. Now you can use quantum entanglement to send data.

...and still have the problem of transmitting back in time.

idobox
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### Re: FTL-com without breaking *too* many laws of physics

ikrase wrote:Alternatively you coudl have a wormhole so small taht it only takes radio waves. The wormhole end must be moved slower than light.

This is the only way to have apparent FTL without all the time traveling paradoxes. The transmit time is quasi instantaneous because the different parts are actually near to each other.

An other way to counter the most obvious time paradoxes could be to include some kind of doppler-like effect : When the two stations move in regard to each other, the signal is somehow distorted and slowed down. Still, having a message traveling FTL means you can have the consequences to happen before the causes.
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Winter Man
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### Re: FTL-com without breaking *too* many laws of physics

You could add something about having to be in matched gravity wells so time dilation doesn't make the signals violate causality.
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### Re: FTL-com without breaking *too* many laws of physics

Time dilation is also generated by high speeds, so matching gravity wells wouldn't help much.
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### Re: FTL-com without breaking *too* many laws of physics

BlackSails wrote:Just say that the ether exists. Bam, you have a universal frame.

Or a "Bell continuum", like Greg Bear used for his "Noach" (No Channel) communications.

Turtlewing
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### Re: FTL-com without breaking *too* many laws of physics

If I were to approach this problem from a fiction-writing perspective (and i couldn't just say "a quantum wizard did it") than I'd probably try this bit of technobable:

" As it turns out, for every set of communicating reference fraims there exists a defenition if 'simultanious' that woun't break causality. because the universe is causal and time becomes highly subjective at relativistic velocity, this deffenition of simultinaety is always the observed one. So when two comunicators are moving in a manner where their comunication could risk a causality violation, one or both will begin to experience transmission lag, or simply dropped data, as the transmission medium becomes unstable. However fortunately it turns out that in practice this is at worst a minor nusance as it doesn't come up very often."

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### Re: FTL-com without breaking *too* many laws of physics

What's wrong with having reverse time-travel of information (or matter) be one of the side effects of having FTL? Does it break the story?

After all, you have weird enough forward time effects just considering relativity with slower than light travel. For example, there's no such thing as something globally simultaneous, unless it occurs at the same point.

Personally, I always thought dealing with the timetravel-effects would be a more physically realistic treatment of FTL. At least then you don't try to throw relativity under the bus.

Hydralisk
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### Re: FTL-com without breaking *too* many laws of physics

Futurama still has the best answer for this; increase the speed of light.

Aside from that, I think that having miniture, man-made, portable wormholes that are only big enough to send communications (i.e. Radiowaves) would be the best answer. Sorta like having a big red post box, except there's two wormholes in it; an 'out' wormhole (at the top, for sending 'letters') and an 'in' wormhole at the bottom for recieving. Any thoughts on this?

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### Re: FTL-com without breaking *too* many laws of physics

Is there a reason your wormhole can't be bi-directional?
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### Re: FTL-com without breaking *too* many laws of physics

MadRocketSci2 wrote:What's wrong with having reverse time-travel of information (or matter) be one of the side effects of having FTL? Does it break the story?

After all, you have weird enough forward time effects just considering relativity with slower than light travel. For example, there's no such thing as something globally simultaneous, unless it occurs at the same point.

Personally, I always thought dealing with the timetravel-effects would be a more physically realistic treatment of FTL. At least then you don't try to throw relativity under the bus.

Well, relativity has kind of already been thrown under the bus, what with the introduction of FTL and all that. And yes, time travel does break the story, since it introduces the possibility of radioing your old self from a couple of weeks ago, and saying "hey, don't do this and that, because bad things will happen". Which makes proper storytelling pretty hard to accomplish.
I'm thinking of bailing myself out with multiple time-lines. Sure, you can talk to your own past, with the caveat that it won't be yours any longer. Some other version of you will be super surprised at receiving the transmission, while you-you wont have any recollection of the event.
Because phlogiston.

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### Re: FTL-com without breaking *too* many laws of physics

Robert'); DROP TABLE *; wrote:Time dilation is also generated by high speeds, so matching gravity wells wouldn't help much.

As you approach the speed of light you get more massive, no? Either way, just match the time dilation.
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ikrase
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### Re: FTL-com without breaking *too* many laws of physics

Some people have FTL for the specific purpose of having the casualty mindf**k. One story had some condition which prevents time travel from really working, but you still are scanning and comming all over the place, screwing up all casualty after the invention of FTL and causing a long stalemate war of thousands of years due to retroactive battles and forsight.
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### Re: FTL-com without breaking *too* many laws of physics

2 things:
1) Why don't you set it up so that the communication doesn't necessarily travel along anyone's line of simultaneity, but that on that diagram, it would always travel in a horizontal line... a sortof speed-limit depending on the relative velocity. Basically B would send the information into A's future as B sees it. I don't know how you would go about explaining how or why this happens. That's the best I can do.

2) If anyone feels like they have the time and patience, would you care to explain why B's simultaneity extends backwards in time for A? I haven't taken any courses in relativity so this is fairly unfamiliar to me.
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### Re: FTL-com without breaking *too* many laws of physics

Mavrisa wrote:1) Why don't you set it up so that the communication doesn't necessarily travel along anyone's line of simultaneity, but that on that diagram, it would always travel in a horizontal line.

That's what fixing a universal reference frame would do. The problem is that to an observer moving relative to this frame, it would still appear as though some communications had gone back in time, and thus be very difficult to reconcile that with the "fact" that they hadn't actually done so for the people communicating.
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quantropy
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### Re: FTL-com without breaking *too* many laws of physics

gmalivuk wrote:That's what fixing a universal reference frame would do. The problem is that to an observer moving relative to this frame, it would still appear as though some communications had gone back in time, and thus be very difficult to reconcile that with the "fact" that they hadn't actually done so for the people communicating.

I think that if you accepted that there was a preferred frame of reference then it wouldn't be that difficult to accept that messages appeared to go backwards in time according to clocks moving with respect to that frame. In fact you'd probably make sure that your clocks matched the time of the preferred reference frame, and think of any other clocks as showing the wrong time.

selene
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### Re: FTL-com without breaking *too* many laws of physics

Does the in-universe communications device necessarily work on moving spaceships?
I would have it only work between stations that are not moving relative to each other (with some leeway for eg. planetary motion manifested as signal degradation). Or have the devices be too cumbersome to fit on a ship at all.
This way, a ship would at least have to change reference frames to pass communications through a third party.

Also, even if communications travel instantaneously, it still takes time to speak or read the message and for the recipient to form a response. And due to time dilation, while B takes five minutes to answer A could be waiting five years. It may be that practically, B could respond to the <i>beginning</i> of A's message before A sent it, but B could not send any <i>useful</i> information before a great deal of time has passed for A.

By any chance, is the fandom the Ender series? As I recall, in Speaker for Dead only Jane, the supercomputer that lived in the FTL device, was actually capable of making sense of time-dilated/compressed messages; everyone subject to relativity had to drop to lower speeds in order to communicate.

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### Re: FTL-com without breaking *too* many laws of physics

selene wrote:Does the in-universe communications device necessarily work on moving spaceships?
I would have it only work between stations that are not moving relative to each other (with some leeway for eg. planetary motion manifested as signal degradation). Or have the devices be too cumbersome to fit on a ship at all.
This way, a ship would at least have to change reference frames to pass communications through a third party.
Yes, but you'd still get back-in-time communication that way unless you also fix a universal reference frame, as discussed above when I explained why even requiring zero relative velocity wouldn't cut it.

It may be that practically, B could respond to the <i>beginning</i> of A's message before A sent it, but B could not send any <i>useful</i> information before a great deal of time has passed for A.

This is still hand-wavy, because you can make the times involved arbitrary by changing the distance and relative velocities, so at a certain speed and distance B's reply goes back in time a year or 10 years or a million years in A's frame.
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Hydralisk
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### Re: FTL-com without breaking *too* many laws of physics

Robert'); DROP TABLE *; wrote:Is there a reason your wormhole can't be bi-directional?

Well in case there's a huge pile up or letters get lost. You know how it is, be it via wormholes or postal van, letters will get messed up if you make things too complicated. Two letters in a wormhole collide with each other, and cancel their momentum exactly, you'll never get those guys back!

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### Re: FTL-com without breaking *too* many laws of physics

So let's say hypothetically there's this person, who isn't me, who has not yet taken college-level physics and has no gorram idea how FTL communication would break causality. What would you tell this hypothetical person who, and i can't emphasize this enough, isn't me?
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Turtlewing
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### Re: FTL-com without breaking *too* many laws of physics

selene wrote:By any chance, is the fandom the Ender series? As I recall, in Speaker for Dead only Jane, the supercomputer that lived in the FTL device, was actually capable of making sense of time-dilated/compressed messages; everyone subject to relativity had to drop to lower speeds in order to communicate.

Actually I think the limitation was that you had to use redicilous amounts of transmission time (you had to corrdinate the data comming in at different rates due to time dilation) and that was prohibitivly expensive for civilians (it might have been completely disalowed by the government i don't recall). However the military regularly used the ansibles to relay orders to ships in flight in that series even though they didn't know of Jane's existance until she made an attack fleet go dark to prevent them recieveing a kill order.

Sizik
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### Re: FTL-com without breaking *too* many laws of physics

Spambot5546 wrote:So let's say hypothetically there's this person, who isn't me, who has not yet taken college-level physics and has no gorram idea how FTL communication would break causality. What would you tell this hypothetical person who, and i can't emphasize this enough, isn't me?

The key lies in understanding the OP's graph.

The vertical axis is time (lets say in seconds). The horizontal axis is distance divided by the speed of light (which would be meters/299,792,458). This means something moving along a line with a slope of 1 is moving at the speed of light (since one unit in the x direction is 299,792,458 meters, so the speed is 299,792,458 m/s). Anything moving along a line (hereafter just called line) with a slope >1 is moving less than the speed of light, and a slope <1 is moving faster. A vertical line would represent not moving at all, whereas a horizontal line would contain all things happening at the same time (e.g. "simultaneously"), which is why it's shown by a dotted line.

The thing about relativity, though, is that it says that the faster you move, the more time dilates, so you that you observe the speed of light to remain constant. For example, line B (the path traveled by person B at a constant velocity) on the graph has a velocity of, say, 0.5c. Since B is not accelerating, it would be perfectly valid for B to conclude that he's standing perfectly still, and everything else is moving past him, so he sees the angle between himself and c to be 45 degrees. Since B observes that the angle between himself and c is 45 degrees, and B observes that he is standing perfectly still, he also observes that the angle between c and the "line of simultaneity" to also be 45 degrees., and therefore horizontal to B.

However, A sees the angle between B and c to be less than 45 degrees, since B is moving relative to A. This also means A sees B's simultaneity line to be non-horizontal (it makes the same angle with c as B does). Lets say A sends a message to B that reaches him instantaneously (i.e. travels along the simultaneity line). Upon receiving the message, B sends it back, also instantaneously. However, since
B's simultaneity line doesn't match up with A's, the message will reach A before A sent the original. The same would happen if B sent A a message, then A sent it back. Thus, causality is a casualty.

Note: this example works for any FTL communication, not just instantaneous. Since the speed of light is the same in both reference frames, any message moving equal or slower will undoubtedly arrive in the future, which was probably it's intended destination.

Please correct me if I'm wrong.
she/they
gmalivuk wrote:
King Author wrote:If space (rather, distance) is an illusion, it'd be possible for one meta-me to experience both body's sensory inputs.
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idobox
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### Re: FTL-com without breaking *too* many laws of physics

About the wormhole idea, you don't exactly need a wormhole. You need a process that causally links two places of the universe, able to transmit information, but matter and energy are not needed.
Some bullshit like
"-a kleinmann-schass pair is causally connected over large distances, and any any change in the metaspin of one of the clusters is instantaneously repercuted on the other.
-but that should a lot of time paradoxes, doctor
-mwaha, no! young student, because the distance between the two clusters is actually null
-oh! and why can't we use this link to transmit extensive quantum data and teleport?
-are you nuts? it takes at least 5µs to change the metaspin, and a single pair needs a megafuck watt to maintain."

You might want to change the style a little bit, but I think two german sounding names with a hyphen make a very sciency name.
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Technical Ben
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### Re: FTL-com without breaking *too* many laws of physics

As mentioned by Jmorgan3 in another thread
Sound waves are isentropic: changes in pressure happen in a gradual, reversible manner. They travel at the speed of sound. Shock waves are not isentropic: changes in pressure happen suddenly and irreversibly. They travel faster than the speed of sound. Stronger shock waves (higher pressure ratios across the shock) travel faster than weaker ones. Visible shock waves can be caused by water condensation or by diffraction.

So if a shockwave travels faster than sound... could we make a photon shock wave travel faster than light? In a Sci-fi way, or a theoretically possible way? (I'm guessing no for the possibility, but it might sound good for sci-fi)
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Sizik
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### Re: FTL-com without breaking *too* many laws of physics

Technical Ben wrote:As mentioned by Jmorgan3 in another thread
Sound waves are isentropic: changes in pressure happen in a gradual, reversible manner. They travel at the speed of sound. Shock waves are not isentropic: changes in pressure happen suddenly and irreversibly. They travel faster than the speed of sound. Stronger shock waves (higher pressure ratios across the shock) travel faster than weaker ones. Visible shock waves can be caused by water condensation or by diffraction.

So if a shockwave travels faster than sound... could we make a photon shock wave travel faster than light? In a Sci-fi way, or a theoretically possible way? (I'm guessing no for the possibility, but it might sound good for sci-fi)

she/they
gmalivuk wrote:
King Author wrote:If space (rather, distance) is an illusion, it'd be possible for one meta-me to experience both body's sensory inputs.
Yes. And if wishes were horses, wishing wells would fill up very quickly with drowned horses.

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### Re: FTL-com without breaking *too* many laws of physics

Cool, cool. I remember reading that page a while back. The wave is moving faster than light (in a substance). That's still cheating compared to the original question. We can travel faster than light [in a dense substance]. But not faster than light in a vacuum.
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### Re: FTL-com without breaking *too* many laws of physics

I propose that FTL is possible with no restrictions on motion or anything like that.
However, in the event that a pair of linked FTL transmitters is brought into a position that would allow time travel, they shut down. The relevant technobabble is: a virtual particle which travels along the time loop pops into existence, loops itself enough times to overload the transmitter, then joins up with itself in the past (causing its own existence). From the perspective of the people operating the transmitters: as soon as time travel becomes possible, a beam of energy shoots out of the "past" transmitter, flies at the speed of light across space, hits the "future" transmitter, and travels back in time to the "past" transmitter, destroying both before any useful information can be sent back in time. The energy of this beam is the minimum necessary to instantaneously shut down the transmitters.

This is not too far from our universe; it is theorised to happen if the two ends of a wormhole are brought close enough to permit time-travel. However, in our universe, the Roman Ring can circumvent this restriction (so time travel might be possible with two or more wormholes). Just say that Roman Rings don't actually work, and you have FTL without time travel.

Sound good?
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### Re: FTL-com without breaking *too* many laws of physics

snowyowl wrote:it is theorised to happen if the two ends of a wormhole are brought close enough to permit time-travel.
But they don't have to be close to permit time travel...
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### Re: FTL-com without breaking *too* many laws of physics

selene wrote:Does the in-universe communications device necessarily work on moving spaceships?
I would have it only work between stations that are not moving relative to each other (with some leeway for eg. planetary motion manifested as signal degradation). Or have the devices be too cumbersome to fit on a ship at all.
This way, a ship would at least have to change reference frames to pass communications through a third party.

This is my preferred scenario, although it may not be consistent with the existing fanfic world.

A similar scenario involves a network of wormholes that can be used to pass information, but not mass-energy in appreciable quantities. But you have to remain within range of a wormhole to send or receive a message and that's hard to do if you're going very fast. This doesn't stop FTL comms exactly, but it does throttle them back quite a bit.

selene wrote:Also, even if communications travel instantaneously, it still takes time to speak or read the message and for the recipient to form a response. And due to time dilation, while B takes five minutes to answer A could be waiting five years. It may be that practically, B could respond to the <i>beginning</i> of A's message before A sent it, but B could not send any <i>useful</i> information before a great deal of time has passed for A..

When this thread was first posted, I thought this kind of scenario might be a way out, but then I realized that it only takes one bit of information traveling into the past to break causality.

Ruling out FTL comms by saying that its too hard to compress / decompress the time dilated signal is a bit silly, IMHO. The computation is straightforward, and easier than encoding / decoding a DVD. Problems would only arise if the signal was degraded through the process of FTL transmission, but error correcting codes can deal with that issue if the noise level isn't too severe.

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### Re: FTL-com without breaking *too* many laws of physics

You could always just do what EVE Online did, and take place in a universe where accelerating to the speed of light does not increase your mass, thus creating no restriction whatsoever on rate of travel.
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