Instantaneous Communications plus Time Dilation Equals ???

[Xavier Onassiss]

Warning: weirdness ahead.

 

I'm working on a campaign in which quantum entanglement is used for instantaneous communications over extremely long (interstellar) distances. (I know this assumes some major technological breakthroughs. I'm okay with that.) In most cases a pair of entangled communicators are created together and one of them is transported to another star system via FTL spacecraft. No problem -- the two star systems can now communicate in 'real time.' It's easy enough to simulate this in Hero using Mind Link with some limitations. Yes, this has some serious causal implications, but they're nothing compared to what's next....

 

What's keeping me up at night is the question of what happens if one of two entangled communicators is transported to another star system on a slower than light spacecraft, at relativistic speeds. Say its destination is a star 10 light years away. And due to time dilation, when it arrives, the 'away' communicator is effectively 5 years 'younger' than the one that stayed at home. What does this mean? When they communicate, are they really separated by 5 years? And does this mean that the 'younger' communicator is effectively receiving messages from 5 years in the future?

 

Or am I just missing something obvious here?

 

Any clarifications on this from our more physics-savvy board members would be much appreciated, 'cuz I'm seriously confused.

 

Don't look at me,

Xavier Onassiss

[ajackson]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Okay, the physics-savvy answer is that instantaneous communication is a nonsensical concept within the framework of general relativity, because simultaneity is not well-defined under GR, and in any case entangled particles cannot be used to transmit data. Thus, the answer cannot be clearly defined. If you have an option that actually allows data transport (for example, the two ends of a wormhole), the answer is yes, it sends messages to/from the future.

[Xavier Onassiss]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Thanks! I'm strongly leaning towards traversable wormholes as the special effect for FTL travel in this campaign, so perhaps I should also look at a variation of that for FTL communications as well. But that raises still more questions. Suppose you have a pair of communicators linked by a wormhole; is it even possible to transport one of them via an FTL wormhole drive without destroying the link in the process? I'm thinking No.

 

The resulting state of affairs reminds me of the Charles Stross novels Singularity Sky and Iron Sunrise. Both stories featured quantum-entangled communicators with interstellar range, and FTL travel, but the technologies weren't compatible. The entangled communicators had to be shipped at sub-light speeds, which made them prohibitively expensive.

 

Don't look at me,

Xavier Onassiss

[austenandrews]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Say time runs half as fast for the STL ship than for the planet. When the ship receives signals from the planet, they're twice as fast as normal signals. The ship hears chipmunks. The ship's signals are half as fast to the planet - they hear a record player running slow. When the ship slows down at its destination, the signal frequencies normalize out. No time travel required.

[ajackson]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Thanks! I'm strongly leaning towards traversable wormholes as the special effect for FTL travel in this campaign' date=' so perhaps I should also look at a variation of that for FTL communications as well[/quote']

Why bother? You can just use conventional communications methods through your transport wormholes.

[CptPatriot]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

I would say that quantum entangled communications (QEC), which has a data transmission rate of X, the fastest that can be achieved, are linked even on the FTL drive based trip, assuming the FTL trip doesn't break the entanglement or have a time dilation factor. So, it ends up meaning that on the trip to destination 1, the data is transmitted to them both is the same, but when in the STL trip to destination 2, the datastream by the non-time dilated party would have to be slowed down to be understood by the ship by a factor equal to the rate of time dilation. So, if traveling 5 lightyears at .9c, the relativistic change factor is approximately 2.294 and would only be able to receive 2.18 years worth of data.

[dmjalund]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

It is not so bad to receive comms from 5 years in the future, if you are more than 10 ly from the source, as long an you have no FTL and can only communicate back to that period.

[dmjalund]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

for these purposes, travel is a form of communication

[Xavier Onassiss]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Why bother? You can just use conventional communications methods through your transport wormholes.

 

The wormholes used by starships aren't permanent, so this wouldn't be practical. I chose wormholes for FTL travel because I didn't want 'all that tedious mucking about in hyperspace.' In practice, the wormhole drive in my setting ends up functioning pretty much like a jump drive with a lengthy travel time.

 

OTOH, permanent wormholes for communications wouldn't necessarily need to be big enough to traverse -- just wide enough to pass a signal.

[Curufea]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Okay' date=' the physics-savvy answer is that instantaneous communication is a nonsensical concept within the framework of general relativity, because simultaneity is not well-defined under GR, and in any case entangled particles cannot be used to transmit data. Thus, the answer cannot be clearly defined. If you have an option that actually allows data transport (for example, the two ends of a wormhole), the answer is yes, it sends messages to/from the future.[/quote']

 

I should point out that String Theory is the new combination of GR and Quantum Theory. But even so - travelling at relatavistic speeds falls under GR not QT, whereas Entanglement is definitely QT. So the question is best answered in String Theory which covers both. I have no idea as it's entirely unproveable and theoretical at this point.

 

However, be that is it may - travelling ships in GR use subjective time. You can choose to either have the entangled particle move at the subjective time, or at the same time as its partner - whichever is most interesting

 

Also, I should point out that FTL ships aren't instantaneous either and whether the particle moves at sub or supra light speeds, the same problem should occur, unless the rules change in your version of FTL.

[ajackson]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

I should point out that String Theory is the new combination of GR and Quantum Theory. But even so - travelling at relatavistic speeds falls under GR not QT' date=' whereas Entanglement is definitely QT. So the question is best answered in String Theory which covers both.[/quote']

Doesn't matter; standard QM says that entanglement cannot carry data. Incidentally, there's no theory that lets you have GR, FTL travel, and causality (i.e. no time travel); the least disruptive solution is usually to discard GR by adding a privileged reference frame, in which case your FTL comms will not result in messages from the future.

[austenandrews]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Incidentally' date=' there's no theory that lets you have GR, FTL travel, and causality (i.e. no time travel); the least disruptive solution is usually to discard GR by adding a privileged reference frame, in which case your FTL comms will not result in messages from the future.[/quote']

That's what most SF settings do de facto anyway. It would be intuitive to everybody.

 

Of course you could build a pretty interesting setting based on the idea that messages from the future are possible. What would a battlefield look like?

[Xavier Onassiss]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Doesn't matter; standard QM says that entanglement cannot carry data. Incidentally' date=' there's no theory that lets you have GR, FTL travel, and causality (i.e. no time travel); the least disruptive solution is usually to discard GR by adding a privileged reference frame, in which case your FTL comms will not result in messages from the future.[/quote']

 

The 'privileged reference frame' sounds workable with the FTL drive I'd like to use, and would prevent a lot of headaches. (Although it doesn't really address the sub-light travel situation in the OP.)

 

Another possibility is the Novikov self-consistency principle (hat tip to Nyrath's massively useful website for this) which would prevent FTL from creating any unsolvable paradoxes. This would mean that yes, it might be possible to receive information from the future / send information to the past, but this would not allow history to be changed in any way. The past is deterministic; the future inevitable.

 

Don't look at me,

Xavier Onassiss

[Kristopher]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

My solution is that the method of FTL travel and comms in my SF setting has what the natives have somewhat misnamed a "causality barrier". For example, if you send an FTL ship on a set of "otherspace transits" that would have it arrive back at the origin before it left, when it makes that final exit from otherspace, it will never appear before it left. There's no "time lag" for the crew, they're not stuck in the moment of exit for however long, they simply exit into a later time than it would seem they should.

[Xavier Onassiss]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

My solution is that the method of FTL travel and comms in my SF setting has what the natives have somewhat misnamed a "causality barrier". For example' date=' if you send an FTL ship on a set of "otherspace transits" that would have it arrive back at the origin before it left, when it makes that final exit from otherspace, it will never appear before it left. There's no "time lag" for the crew, they're not stuck in the moment of exit for however long, they simply exit into a later time than it would seem they should.[/quote']

 

I think in some interpretations, this would be an excellent example of the Novikov Principle in action. The probability of anything which violates causality is exactly zero, so the probability of events which prevent such violations increase correspondingly. Thus, FTL drives 'malfunction' and ships arrive at their destinations 'too late' to create a paradox.

 

Either that, or it's a really fun and interesting way to mess with the PC's collective heads.

 

Don't blame me,

Xavier Onassiss

[austenandrews]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

The 'privileged reference frame' sounds workable with the FTL drive I'd like to use' date=' and would prevent a lot of headaches. (Although it doesn't really address the sub-light travel situation in the OP.)[/quote']

If you use a privileged reference frame but keep time dilation as a general effect, dilation only affects signal frequency. An entanglement communicator wouldn't magically sync the time flows on both ends. It connects a slower flow of time to a faster flow of time.

[Clonus]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

There a number of impossibilities postulated in that question, so I prefer to go with what is gameable. Quantum entanglement is a handwave, it does create a special frame of reference, as does FTL, and that's all there is to that story.

[Kristopher]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

I think in some interpretations' date=' this would be an excellent example of the Novikov Principle in action. The probability of [i']anything[/i] which violates causality is exactly zero, so the probability of events which prevent such violations increase correspondingly. Thus, FTL drives 'malfunction' and ships arrive at their destinations 'too late' to create a paradox.

 

Either that, or it's a really fun and interesting way to mess with the PC's collective heads.

 

The concept of the "causality barrier" isn't one of coincidence or happenstance, no mysterious drive malfunctions. The crossover from otherspace to realspace simply cannot link up at a time and place that would cause a direct violation of causality.

[Xavier Onassiss]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

The concept of the "causality barrier" isn't one of coincidence or happenstance' date=' no mysterious drive malfunctions. The crossover from otherspace to realspace simply cannot link up at a time and place that would cause a direct violation of causality.[/quote']

 

I understand what you're saying here. I was just pointing out that both types of restrictions could potentially yield the same result. If the PC's didn't already know which one was in effect in a given campaign, they'd have a hard time figuring out which one was to blame for their difficulties.

 

 

Don't look at me,

Xavier Onassiss

[Nyrath]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Say time runs half as fast for the STL ship than for the planet. When the ship receives signals from the planet' date=' they're twice as fast as normal signals. The ship hears chipmunks. The ship's signals are half as fast to the planet - they hear a record player running slow. When the ship slows down at its destination, the signal frequencies normalize out. No time travel required.[/quote']

 

Say it is the year 2100. Say you have a wormhole with both ends in orbit around Earth. Take one of the wormhole opening, and put it on a slower than light relativistic starship. Take the starship on a round trip that lasts for ten years ship time but 1,000 years Earth time.

The end result is that wormhole mouth A is around Earth at 2100, and wormhole mouth B is around Earth at 3100. In other words, it is a time machine.

http://en.wikipedia.org/wiki/Wormhole#Time_travel

 

The same thing would happen with a quantum entangled communication device.

[Kristopher]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Say it is the year 2100. Say you have a wormhole with both ends in orbit around Earth. Take one of the wormhole opening, and put it on a slower than light relativistic starship. Take the starship on a round trip that lasts for ten years ship time but 1,000 years Earth time.

The end result is that wormhole mouth A is around Earth at 2100, and wormhole mouth B is around Earth at 3100. In other words, it is a time machine.

http://en.wikipedia.org/wiki/Wormhole#Time_travel

 

The same thing would happen with a quantum entangled communication device.

 

Wait.

 

1000 years passes at Earth, right?

 

When the ship gets back, it's 1000 years later at Earth, no matter how much time passed on the ship during its trip.

 

So how is either end still at the year 2100?

[BobGreenwade]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

First, going on the original question, I believe that using "quantum entanglement" for communication would be different from using a communications wormhole. The latter would be affected by time dilation, but when two particles are entangled anything that affects one also affects the other. Thus, if time is dilated at one end to 50%, then communication goes at half-time from one perspective, or at double-time from the other. Any given moment would be synchronized to the same objective moment (inasmuch as such a thing exists, which most physicists currently doubt).

 

An alternate take (and, to my mind, the more likely) would be that any time dilation runs a risk of breaking the entanglement; the greater the dilation, the more likely the disentanglement.

 

Regarding causality, remember that any trip backward through time, even by a single particle, automatically changes the past and creates a new quantum universe via the many-worlds interpretation. If a radio signal is being sent through a wormhole, that's electromagnetic energy that's altering the past, and actions taken in response to the message can do the same -- energy flows, gravity waves, air currents, and all sorts of miniscule things can cause changes.

[Nyrath]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Wait.

 

1000 years passes at Earth, right?

 

When the ship gets back, it's 1000 years later at Earth, no matter how much time passed on the ship during its trip.

 

So how is either end still at the year 2100?

Try thinking it through, step by step. Wormhole mouth A is still at Earth because nobody moved it.

 

Year 2100: Wormhole mouth A and wormhole mouth B are in Earth orbit.

Wormhole mouth B is loaded into a relativistic starship. The starship moves out of the solar system accelerating to 0.995 c.

 

2101: Wormhole mouth A is in Earth orbit. Time traveling ships from the year 3101 emerge from Wormhole mouth A, and any present year ship that enter A will be transported to 3101.

Wormhole mouth B is in the starship, almost one light year away from Earth.

 

2150: Wormhole mouth A is in Earth orbit. Wormhole mouth B is in the starship, about 50 light years from Earth.

Starship turns around and starts travelling back to Earth.

 

3100: Starship arrives at Earth and unloads. Now both wormhole mouth A and wormhole mouth B are in Earth orbit. Ships that enter wormhole mouth B will emerge from wormhole mouth A in the year 2100. Time travelers from 2100 will emerge from wormhole mouth B in the present.

 

Wormhole mouth A is still at Earth because nobody moved it.

 

From the Wikipedia article:

A wormhole could allow time travel.[4] This could be accomplished by accelerating one end of the wormhole to a high velocity relative to the other, and then sometime later bringing it back; relativistic time dilation would result in the accelerated wormhole mouth aging less than the stationary one as seen by an external observer, similar to what is seen in the twin paradox. However, time connects differently through the wormhole than outside it, so that synchronized clocks at each mouth will remain synchronized to someone traveling through the wormhole itself, no matter how the mouths move around. This means that anything which entered the accelerated wormhole mouth would exit the stationary one at a point in time prior to its entry.

 

For example, consider two clocks at both mouths both showing the date as 2000. After being taken on a trip at relativistic velocities, the accelerated mouth is brought back to the same region as the stationary mouth with the accelerated mouth's clock reading 2005 while the stationary mouth's clock read 2010. A traveller who entered the accelerated mouth at this moment would exit the stationary mouth when its clock also read 2005, in the same region but now five years in the past. Such a configuration of wormholes would allow for a particle's world line to form a closed loop in spacetime, known as a closed timelike curve.

[austenandrews]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Say it is the year 2100. Say you have a wormhole with both ends in orbit around Earth. Take one of the wormhole opening, and put it on a slower than light relativistic starship. Take the starship on a round trip that lasts for ten years ship time but 1,000 years Earth time.

The end result is that wormhole mouth A is around Earth at 2100, and wormhole mouth B is around Earth at 3100. In other words, it is a time machine.

Or, during transit, it's a wormhole where time runs twice as fast on one end as it does on the other. On the ship side it'd be brighter and hotter and you'd probably want to put it in an airlock, because I suspect the faster time flow will simulate higher air pressure and cause a strong wind to rush into the ship. It's ugly to contemplate what would happen to a human body that straddled the two ends, so you'd best have some length in the wormhole with a graduated time difference.

 

Move the shipborne end fast enough and it's practically a stasis machine from the Earthly perspective. Bring it back to Earth and the time flows match again, with no time differential, so no time machine.

[Nyrath]

Re: Instantaneous Communications plus Time Dilation Equals ???

 

Move the shipborne end fast enough and it's practically a stasis machine from the Earthly perspective. Bring it back to Earth and the time flows match again' date=' with no time differential, so no time machine.[/quote']

Did you not read the link?

http://en.wikipedia.org/wiki/Wormhole#Time_travel