FTL communication and time travel

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

peregrine_crow
Posts: 180
Joined: Mon Apr 07, 2014 7:20 am UTC

FTL communication and time travel

Postby peregrine_crow » Mon Jul 07, 2014 9:10 am UTC

So I understand from reading some threads on these forums that the whole "Traveling faster than light equals traveling back in time" thing occurs whenever information gets from one place to another faster than light would have if it traveled in a straight line even if nothing is actually moving faster than light, but I'm having serious problems wrapping my head around this. So I'm trying to translate it to a human sensible example and failing quite miserably, hence this thread.

Say you have a set of instantaneous communication devices, for instance, a set of magic scanners that can measure entangled particles without disentangling them. Split the devices between two planets A and B that are some light years apart, now what would happen if someone used these devices to communicate? would B be able to reply to A's questions before A sent them? How would that work if the response arrives before the device was even build?
Ignorance killed the cat, curiosity was framed.

User avatar
tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

Re: FTL communication and time travel

Postby tomandlu » Mon Jul 07, 2014 12:45 pm UTC

This looks like a pretty good resource, although it always makes my head ache when I try to get a handle on this one:

http://www.theculture.org/rich/sharpblue/archives/000089.html

I think the bottom line is that ansibles can work as long as they can only transmit between two points occupying the same frame (i.e. are motionless relative to each other). As soon as you can send an FTL communication to something moving at relativistic speeds relative to you, you're in trouble. Actually, scratch that - unless you forbid conventional messaging during a high-velocity fly-by, you can just pass the message conventionally I think, and just skip the ansible for that step.
Last edited by tomandlu on Mon Jul 07, 2014 12:50 pm UTC, edited 1 time in total.
How can I think my way out of the problem when the problem is the way I think?

User avatar
eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: FTL communication and time travel

Postby eSOANEM » Mon Jul 07, 2014 12:47 pm UTC

Information is not nothing. To send information, it needs to be encoded in something, be it written on a piece of paper or modulated on an EM wave or somesuch.

IIRC you only get answers arriving before questions if the two receivers are moving relative to each other (at which point "instantaneous" becomes an untenable concept), in general, the faster the message travels, the lower the relative speed needed to get the weirdness.

Edit: ninja'd. Tomandlu's link seems to suggest that I was right, if your receivers aren't moving relative to each other then everything's fine.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

p1t1o
Posts: 940
Joined: Wed Nov 10, 2010 4:32 pm UTC
Location: London, UK

Re: FTL communication and time travel

Postby p1t1o » Mon Jul 07, 2014 2:28 pm UTC

Is there any distinction drawn between instantaneous communication and "simply" FTL communications?

This image just jumped into my head:

Im on a rocket heading towards earth at 0.99c.
I observe earth, and due to time dilation, everyone is moving slower.
Somebody sends an FTL message to me, and it arrives before I observe them sending the messge.

Possibly the numbers might be such that in my rocket, I can then respond back to earth and I would observe people on earth receiving my answer before I observe them send me the question?

Is that the "causality weirdness"?

I feel like Im missing something.

Hypnosifl
Posts: 257
Joined: Wed Mar 02, 2011 10:05 am UTC

Re: FTL communication and time travel

Postby Hypnosifl » Mon Jul 07, 2014 3:19 pm UTC

p1t1o wrote:Is there any distinction drawn between instantaneous communication and "simply" FTL communications?

A crucial aspect of why FTL implies causality violation in relativity is the concept of the relativity of simultaneity--different inertial frames in special relativity actually disagree about which events happened simultaneously, so events that are assigned the same time-coordinate in one inertial frame are assigned different time-coordinates by other inertial frames, and relativity says that the laws of physics must work exactly the same in all these frames. And it works out that for any pair of events that have a "spacelike" separation--meaning that no signal traveling at or slower than the speed of light could cross through both events--there is always some inertial frame where the two events are simultaneous. So, if there's one frame where a signal is sent in 2015 and received 10 light years away in 2020 (FTL but not instantaneous), you can always find some other inertial frame where the signal is both sent and received in 2015 (instantaneous), and yet another frame where the signal was sent in 2015 but arrived at its destination in 2010 (backwards in time). And since the laws of physics must work the same in all frames, if it's possible in one frame for a signal to be received before it's sent, this must be possible in all frames.
p1t1o wrote:This image just jumped into my head:

Im on a rocket heading towards earth at 0.99c.
I observe earth, and due to time dilation, everyone is moving slower.
Somebody sends an FTL message to me, and it arrives before I observe them sending the messge.

Possibly the numbers might be such that in my rocket, I can then respond back to earth and I would observe people on earth receiving my answer before I observe them send me the question?

Is that the "causality weirdness"?

Almost, but to have a causality violation you'd actually have to be moving away from Earth, not towards it. If you and Earth are moving apart slower than light, and you send you a signal which moves FTL in your frame but backwards in time in Earth's frame, and after receiving the message Earth sends a reply which moves FTL in Earth's frame but backwards in time in you frame, you can actually receive the reply before you sent the original message, an obvious causality violation in all frames. This sort of setup is sometimes known as the "tachyonic antitelephone", there's a detailed numerical example on the wiki page:

http://en.wikipedia.org/wiki/Tachyonic_antitelephone

User avatar
tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

Re: FTL communication and time travel

Postby tomandlu » Mon Jul 07, 2014 4:00 pm UTC

This and the strange wheel shape in the main relativity thread just defeat me. It's a sad thing to admit that I have to take some science on faith... (but then don't we all)
How can I think my way out of the problem when the problem is the way I think?

User avatar
drachefly
Posts: 196
Joined: Thu Apr 23, 2009 3:25 pm UTC

Re: FTL communication and time travel

Postby drachefly » Mon Jul 07, 2014 9:02 pm UTC

tomandlu wrote:This looks like a pretty good resource, although it always makes my head ache when I try to get a handle on this one:

http://www.theculture.org/rich/sharpblue/archives/000089.html

I think the bottom line is that ansibles can work as long as they can only transmit between two points occupying the same frame (i.e. are motionless relative to each other).


No. Ansibles can't work. What you mean is, Ansibles don't permit time travel so long as there is exactly one frame of reference in which their message is instantaneous regardless of which way they are transmitting, and you consider them in that frame.

In some frames, they will send messages back in time going one way and forward in time going the opposite way. So, you still wouldn't be able to relay a message to yourself in the past, only to spacelike-separated events that are in the past in that inappropriately-chosen coordinate frame.

If the restriction were based on the ansibles' current rest frame, even if that had to be the same between any pair, then you could send a message back in time by oscillating them rapidly, forming a ladder - accelerate to change whether you're communicating with the other ansible in your (net-rest-frame) future or in your (net-rest-frame) past.

Hypnosifl
Posts: 257
Joined: Wed Mar 02, 2011 10:05 am UTC

Re: FTL communication and time travel

Postby Hypnosifl » Mon Jul 07, 2014 9:41 pm UTC

drachefly wrote:If the restriction were based on the ansibles' current rest frame, even if that had to be the same between any pair, then you could send a message back in time by oscillating them rapidly, forming a ladder - accelerate to change whether you're communicating with the other ansible in your (net-rest-frame) future or in your (net-rest-frame) past.

Also, if it was based on the ansibles' rest frame, you could just use two different transmitter/receiver pairs in motion relative to each other, i.e. transmitter 1 and receiver 1 are at rest relative to each other, but in motion relative to transmitter 2 and receiver 2. Then you could arrange things so that when receiver 1 gets an FTL signal from transmitter 1, it's passing close to transmitter 2 from the other pair, so the message can be relayed to the transmitter 2 via radio signal or some other ordinary non-FTL means, and then transmitter 2 can pass it on at FTL speed to receiver 2, which can be passing near transmitter 1 at that moment, at a time before transmitter 1 send the original signal.

The only way to have FTL messaging without causality violation is to have a universally preferred frame for FTL signals, one which doesn't depend on the rest frame of the transmitter and receiver. But in that case the laws of physics governing FTL would violate the fundamental postulates of special relativity. Somewhere I read a nice summary of the situation for sci-fi writers, "relativity, causality, and FTL: pick any two."

User avatar
drachefly
Posts: 196
Joined: Thu Apr 23, 2009 3:25 pm UTC

Re: FTL communication and time travel

Postby drachefly » Thu Jul 10, 2014 1:52 am UTC

That's what I meant, but looking back I see I didn't emphasize the universality of this reference frame adequately. Thanks.

User avatar
tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

Re: FTL communication and time travel

Postby tomandlu » Thu Jul 10, 2014 7:30 am UTC

Hypnosifl wrote:"relativity, causality, and FTL: pick any two."


relativity, causality, ftl, pliant sf: pick four... ;)

IMHO this aspect of relativity is obscure and abstract enough to be (respectfully) ignored without drifting into sci-fantasy...
How can I think my way out of the problem when the problem is the way I think?

peregrine_crow
Posts: 180
Joined: Mon Apr 07, 2014 7:20 am UTC

Re: FTL communication and time travel

Postby peregrine_crow » Thu Jul 10, 2014 9:12 am UTC

Thanks for answering and sorry for not responding earlier, I had a bit of a hard time understanding the answers. :D

Hypnosifl wrote:Also, if it was based on the ansibles' rest frame, you could just use two different transmitter/receiver pairs in motion relative to each other, i.e. transmitter 1 and receiver 1 are at rest relative to each other, but in motion relative to transmitter 2 and receiver 2. Then you could arrange things so that when receiver 1 gets an FTL signal from transmitter 1, it's passing close to transmitter 2 from the other pair, so the message can be relayed to the transmitter 2 via radio signal or some other ordinary non-FTL means, and then transmitter 2 can pass it on at FTL speed to receiver 2, which can be passing near transmitter 1 at that moment, at a time before transmitter 1 send the original signal.


Could you elaborate on that, because I'm having trouble figuring out how the message arrives back at transmitter 1 before it was send. Supposes transmitter sends the first message at T01 Receiver 1 receives this messages and transmits it at T11. It arrives at transmitter 2 at T12. Transmitter 2 sends it on to receiver 2 which passes it on to transmitter 1 at T22. The message arrives back at transmitter 1 at time T21.

Information exchange between the reference frames occurs at non-FTL speeds so we know that T11 < T12 and T22 < T21. We also know that T01 <= T11 and T12 <= T22 because transmits through the device takes non-negative amounts of time. So we have T0 <= T11 < T12 <= T22 < T21 and by transitivity of < we have T01 < T21.

tomandlu wrote:
Hypnosifl wrote:"relativity, causality, and FTL: pick any two."


relativity, causality, ftl, pliant sf: pick four... ;)

IMHO this aspect of relativity is obscure and abstract enough to be (respectfully) ignored without drifting into sci-fantasy...


Isn't that the same as picking causality and FTL or are there more serious consequences of dropping relativity than just making the physics involved less like real world physics?
Ignorance killed the cat, curiosity was framed.

User avatar
tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

Re: FTL communication and time travel

Postby tomandlu » Thu Jul 10, 2014 12:16 pm UTC

peregrine_crow wrote:
tomandlu wrote:
Hypnosifl wrote:"relativity, causality, and FTL: pick any two."


relativity, causality, ftl, pliant sf: pick four... ;)

IMHO this aspect of relativity is obscure and abstract enough to be (respectfully) ignored without drifting into sci-fantasy...


Isn't that the same as picking causality and FTL or are there more serious consequences of dropping relativity than just making the physics involved less like real world physics?


Well you lose all the stuff that most people are familiar with, and suddenly you're into full-blown fantasy imho. Take a book like The Martian - pretty much 100% hard sf - but it completely ignores solar radiation. Bottom line, most people are, I think, familiar with the basic principles of relativity, but not Minkowski space...
How can I think my way out of the problem when the problem is the way I think?

Hypnosifl
Posts: 257
Joined: Wed Mar 02, 2011 10:05 am UTC

Re: FTL communication and time travel

Postby Hypnosifl » Thu Jul 10, 2014 12:27 pm UTC

peregrine_crow wrote:Thanks for answering and sorry for not responding earlier, I had a bit of a hard time understanding the answers. :D

Hypnosifl wrote:Also, if it was based on the ansibles' rest frame, you could just use two different transmitter/receiver pairs in motion relative to each other, i.e. transmitter 1 and receiver 1 are at rest relative to each other, but in motion relative to transmitter 2 and receiver 2. Then you could arrange things so that when receiver 1 gets an FTL signal from transmitter 1, it's passing close to transmitter 2 from the other pair, so the message can be relayed to the transmitter 2 via radio signal or some other ordinary non-FTL means, and then transmitter 2 can pass it on at FTL speed to receiver 2, which can be passing near transmitter 1 at that moment, at a time before transmitter 1 send the original signal.


Could you elaborate on that, because I'm having trouble figuring out how the message arrives back at transmitter 1 before it was send. Supposes transmitter sends the first message at T01 Receiver 1 receives this messages and transmits it at T11. It arrives at transmitter 2 at T12. Transmitter 2 sends it on to receiver 2 which passes it on to transmitter 1 at T22. The message arrives back at transmitter 1 at time T21.

Information exchange between the reference frames occurs at non-FTL speeds so we know that T11 < T12 and T22 < T21. We also know that T01 <= T11 and T12 <= T22 because transmits through the device takes non-negative amounts of time. So we have T0 <= T11 < T12 <= T22 < T21 and by transitivity of < we have T01 < T21.

Which frame are these times meant to be expressed in? In this example, each FTL message travels forward in time in the rest frame of the transmitter/receiver it's sent between, but backwards in time in the rest frame of the other transmitter/receiver pair which is moving at some slower-than-light speed relative to the first pair (because of the relativity of simultaneity, these two inertial frames disagree about the order of the events "message sent from transmitter" and "message arrives at receiver"...if you put some specific distances and velocities in to get a numerical example, you can verify it would work this way using the Lorentz transformation to switch between the coordinates of any given event in one frame to the coordinates of the same event in the other frame). So for example, T11 > T01 in the rest frame of pair 1, but T01 > T11 in the rest frame of pair 2.

User avatar
Quizatzhaderac
Posts: 1784
Joined: Sun Oct 19, 2008 5:28 pm UTC
Location: Space Florida

Re: FTL communication and time travel

Postby Quizatzhaderac » Wed Aug 27, 2014 8:34 pm UTC

peregrine_crow wrote:
tomandlu wrote:... IMHO this aspect of relativity is obscure and abstract enough to be (respectfully) ignored without drifting into sci-fantasy...


Isn't that the same as picking causality and FTL or are there more serious consequences of dropping relativity than just making the physics involved less like real world physics?
If we want to analyze it thoroughly, I'd say it's dropping Occam's razor.

Restrictions on the ansible's relative motions could prevent FTL, but the restrictions would be complicated and no mechanism is known. Arbitrary FTL + relativity breaks causality; FTL restricted in exactly the way needed to protect causality breaks Occam's razor.

Literature has it's own virtues of simplicity, but one can meet those by simply ignoring the corner cases your physics must have.
The thing about recursion problems is that they tend to contain other recursion problems.

User avatar
eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: FTL communication and time travel

Postby eSOANEM » Wed Aug 27, 2014 9:45 pm UTC

Even if the ansibles have to be comoving, having rapidly moving ships communicating subluminally with the ansibles you can still send stuff FTL.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26739
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: FTL communication and time travel

Postby gmalivuk » Thu Aug 28, 2014 3:36 am UTC

I believe that if *all* ansibles have to be comoving, that essentially sets up a privileged rest frame and maintains causality.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

lgw
Posts: 437
Joined: Mon Apr 12, 2010 10:52 pm UTC

Re: FTL communication and time travel

Postby lgw » Thu Aug 28, 2014 4:00 am UTC

gmalivuk wrote:I believe that if *all* ansibles have to be comoving, that essentially sets up a privileged rest frame and maintains causality.


And it's worth noting that while relativity eliminates any requirement for a privileged rest frame, you have the freedom to pick one if needed for some other theory. As a SF premise goes, that would be a nifty one. The reference frame in which the CMBR comes as close to being the same temperature in all directions as possible is one that people can "synchronize" on without light speed delays (the CMBR also gives a synchronized universal common clock, if only to a useless number of significant figures).

I believe that you can do a bit better than all ansibles co-moving, however. Two ansibles can move "slowly" relative to one another, where IIRC the farther apart they are the more they must match velocities. And I think you can have distinct co-moving groups of ansible endpoints, if you keep them distant from one another, proportional to relative velocity. At least, if I were writing a SF story, I might go with that set of rules, as it makes two ansible-equipped fleets encountering one another fun.
"In no set of physics laws do you get two cats." - doogly

Hypnosifl
Posts: 257
Joined: Wed Mar 02, 2011 10:05 am UTC

Re: FTL communication and time travel

Postby Hypnosifl » Thu Aug 28, 2014 4:50 am UTC

lgw wrote:And it's worth noting that while relativity eliminates any requirement for a privileged rest frame, you have the freedom to pick one if needed for some other theory.

Not if by "privileged" you mean that the basic laws of physics work differently in this frame than in others, so that even an experimenter in a totally sealed room who couldn't see any external landmarks (like the CMBR) would be able to determine whether she was moving or at rest relative to this frame. One of the two fundamental postulates of special relativity is that all laws of physics work the same in all inertial frames, so if that's violated, relativity is falsified.

User avatar
eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: FTL communication and time travel

Postby eSOANEM » Thu Aug 28, 2014 8:43 am UTC

gmalivuk wrote:I believe that if *all* ansibles have to be comoving, that essentially sets up a privileged rest frame and maintains causality.


Yeah, you're right. I was misremembering stuff about having moving observers communicating with the ansibles.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

User avatar
Quizatzhaderac
Posts: 1784
Joined: Sun Oct 19, 2008 5:28 pm UTC
Location: Space Florida

Re: FTL communication and time travel

Postby Quizatzhaderac » Thu Aug 28, 2014 2:47 pm UTC

eSOANEM wrote:Even if the ansibles have to be comoving, having rapidly moving ships communicating subluminally with the ansibles you can still send stuff FTL.
I wasn't really positing one restriction on motion (co-moving ansibles) or even two (co-moving and limited acceleration affecting use) . I'm positing as many restrictions as it takes; which is why said it'd be violating Occam's razor.
The thing about recursion problems is that they tend to contain other recursion problems.

User avatar
drachefly
Posts: 196
Joined: Thu Apr 23, 2009 3:25 pm UTC

Re: FTL communication and time travel

Postby drachefly » Thu Aug 28, 2014 5:14 pm UTC

Hypnosifl wrote:
lgw wrote:And it's worth noting that while relativity eliminates any requirement for a privileged rest frame, you have the freedom to pick one if needed for some other theory.

Not if by "privileged" you mean that the basic laws of physics work differently in this frame than in others, so that even an experimenter in a totally sealed room who couldn't see any external landmarks (like the CMBR) would be able to determine whether she was moving or at rest relative to this frame. One of the two fundamental postulates of special relativity is that all laws of physics work the same in all inertial frames, so if that's violated, relativity is falsified.


Relativity is a property of a theory. If a theory covers forces and things that obey relativity, that is a relativistic theory. If it does not, it is not.

If you have some subsystem which is relativistic, that does not preclude the existence of other subsystems which are not. That would, as you say, invalidate relativity's applicability to the universe as a whole. However, you could still apply it to the relativistic subsystem.

For instance, graphene is often modeled with the electrons in it obeying relativity... with a characteristic speed under a percent of actual light speed. If this were not an approximation, that would be an instance of what lgw was talking about.

User avatar
eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: FTL communication and time travel

Postby eSOANEM » Thu Aug 28, 2014 8:37 pm UTC

Quizatzhaderac wrote:
eSOANEM wrote:Even if the ansibles have to be comoving, having rapidly moving ships communicating subluminally with the ansibles you can still send stuff FTL.
I wasn't really positing one restriction on motion (co-moving ansibles) or even two (co-moving and limited acceleration affecting use) . I'm positing as many restrictions as it takes; which is why said it'd be violating Occam's razor.


I'd be intending to go for the most restrictive set possible (although I didn't explicitly say I was assuming they were inertial) but, as gmalivuk pointed out, I was wrong anyway and misremembering stuff.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26739
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: FTL communication and time travel

Postby gmalivuk » Thu Aug 28, 2014 9:02 pm UTC

lgw wrote:I believe that you can do a bit better than all ansibles co-moving, however. Two ansibles can move "slowly" relative to one another, where IIRC the farther apart they are the more they must match velocities. And I think you can have distinct co-moving groups of ansible endpoints, if you keep them distant from one another, proportional to relative velocity. At least, if I were writing a SF story, I might go with that set of rules, as it makes two ansible-equipped fleets encountering one another fun.
That seems way unduly complicated, though, if you're going to explain all of it, and if you're not then just handwave away causality violation altogether and have them work as you like.

If you want ansibles on ships, I see two simple ways of going:
1) However the ansible devices are themselves moving, the transmission of information is instantaneous only in some universal rest frame. Sometimes it would look like it's going slightly backwards in time to a moving ship, but not in a way that allows anyone to violate causality. This is probably the simplest way to deal with it and allows you to get on with your story.
2) Ansible devices are portable, but don't work as ansibles unless they're comoving with the rest of the network. So a fleet of ships would include one ship for communication, which would have to match velocity with the network any time instantaneous updates are required.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

PsiSquared
Posts: 126
Joined: Wed May 09, 2012 6:02 pm UTC

Re: FTL communication and time travel

Postby PsiSquared » Fri Aug 29, 2014 5:44 am UTC

peregrine_crow wrote:Isn't that the same as picking causality and FTL or are there more serious consequences of dropping relativity than just making the physics involved less like real world physics?


Actually, you don't need to "drop relativity" altogether. It isn't an all or nothing proposition. The principle of relativity may still hold for everything other than ansibles, for some reason. And you could use Gmalivuk's option #1, which ties every FTL signal to some universal frame of reference. Not only it the simplest solution, but it is also the most logical one.

And this isn't just a sensible approach from the literary perspective. It is also plausibly scientific. For all we know, the real world might just work that way. There is, of-course, no shred of evidence for this in the real world, but that's why we call it "science fiction".

A word of caution, though:

You definitely do not want to drop relativity when dealing with ordinary interactions between ordinary physical objects. Relativity is the cornerstone of modern quantum theory, which would simply make no sense without it. Without some form of relativity, our entire understanding of the physical world crumbles to dust. It is better to treat the ansibles as some form of exception. Say that it operates on completely different principles. Or even better: Make the inventors of the device stumble upon it by accident, with nobody - even within your story - being sure how it works.

speising
Posts: 2350
Joined: Mon Sep 03, 2012 4:54 pm UTC
Location: wien

Re: FTL communication and time travel

Postby speising » Fri Aug 29, 2014 8:32 am UTC

a "universal rest frame" is pretty much the opposite of relativity, so, yes, you drop relativity if that exists.

User avatar
eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: FTL communication and time travel

Postby eSOANEM » Fri Aug 29, 2014 12:02 pm UTC

No, you can recover all of the results of SR with a privileged reference frame given certain assumptions about how it works, this is what Lorentz Ether Theory does.

IIRC, in its normal formulation it's impossible to detect what this frame is; something which would not be the case in the comoving ansible network situation. I'm not sure if this is a the-theory-doesn't-work-now issue or a it-needs-a-bit-of-tweaking one though.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

PsiSquared
Posts: 126
Joined: Wed May 09, 2012 6:02 pm UTC

Re: FTL communication and time travel

Postby PsiSquared » Fri Aug 29, 2014 1:40 pm UTC

eSOANEM wrote:No, you can recover all of the results of SR with a privileged reference frame given certain assumptions about how it works, this is what Lorentz Ether Theory does.

IIRC, in its normal formulation it's impossible to detect what this frame is.


This is true by definition, given the principle of relativity.

eSOANEM wrote:[This is] something which would not be the case in the comoving ansible network situation.


Nor would it be the case given any form of working ansible which does not violate causality. FTL+Causality implies a privileged reference frame which can be detected. There's simply no way around that (hence that popular "pick two" statement quoted earlier).

speising wrote:a "universal rest frame" is pretty much the opposite of relativity, so, yes, you drop relativity if that exists.


That depends on what you mean by "dropping relativity". If you insist on a binary decision of "yes" or "no", than you are correct.

But the real universe (as well as any really interesting fictional one) is more complicated than that. You could definitely have a universe with ansibles, in which the Principle of Relativity is an excellent approximation of reality in most situations. Just like you can have a universe with transistors and atomic bombs, without completely throwing out Newtonian Mechanics as a useful approximation.

As long as your fictional physics behave like actual physics in the limit of what is known today, you're perfectly fine.

User avatar
Quizatzhaderac
Posts: 1784
Joined: Sun Oct 19, 2008 5:28 pm UTC
Location: Space Florida

Re: FTL communication and time travel

Postby Quizatzhaderac » Fri Aug 29, 2014 3:41 pm UTC

The preferred ansible frame could be preexisting, or it could be a creation of the ansibles: The frame is created once the ansibles turn on, determined by their motion, and eventually disappears.

Two ansible networks would become entangled whenever they causally connect. Once entangled, the maximum communication speed is c2/ difference in networks' velocities. The same pair of ansibles after acceleration would be a different network for this purpose. The entanglement would dissipate, being entirely gone once all FTL events are in all ansible's past light cones.

The only impact relative to the story is that somebody could deliberately sabotage an ansible network by creating another, rapidly moving network.
The thing about recursion problems is that they tend to contain other recursion problems.

speising
Posts: 2350
Joined: Mon Sep 03, 2012 4:54 pm UTC
Location: wien

Re: FTL communication and time travel

Postby speising » Fri Aug 29, 2014 4:44 pm UTC

how does this prevent relativistically moving spaceships communicating with the ansibles per radio, and through them, with each other, thus again causing causality problems?

Hypnosifl
Posts: 257
Joined: Wed Mar 02, 2011 10:05 am UTC

Re: FTL communication and time travel

Postby Hypnosifl » Fri Aug 29, 2014 5:47 pm UTC

eSOANEM wrote:No, you can recover all of the results of SR with a privileged reference frame given certain assumptions about how it works, this is what Lorentz Ether Theory does.

The first postulate of SR is really only about the all frames being experimentally indistinguishable in terms of the observable laws of physics, and Lorentz Ether Theory can only reproduce all the results of SR if the ether frame is only "privileged" in some sense that's completely hidden from experiment, analogous to hidden-variable interpretations of quantum mechanics which make no predictions different from ordinary QM (that's why they're "interpretations" and not "theories"). But FTL with a privileged frame would allow you to experimentally determine which frame was the privileged one--it'd be the unique frame where it's impossible to send a signal in such a way that it arrives at an earlier time-coordinate than it was sent.

Hypnosifl
Posts: 257
Joined: Wed Mar 02, 2011 10:05 am UTC

Re: FTL communication and time travel

Postby Hypnosifl » Fri Aug 29, 2014 5:59 pm UTC

Quizatzhaderac wrote:The preferred ansible frame could be preexisting, or it could be a creation of the ansibles: The frame is created once the ansibles turn on, determined by their motion, and eventually disappears.

Two ansible networks would become entangled whenever they causally connect. Once entangled, the maximum communication speed is c2/ difference in networks' velocities. The same pair of ansibles after acceleration would be a different network for this purpose. The entanglement would dissipate, being entirely gone once all FTL events are in all ansible's past light cones.

The only impact relative to the story is that somebody could deliberately sabotage an ansible network by creating another, rapidly moving network.

Something along these lines has actually been proposed for what would happen if you had one or more traversable wormholes--the chronology protection conjecture argues that it might be possible to have wormholes that allow for a shortcut between regions in spacetime with a space-like separation (relative to paths that take the "long way" outside the wormhole), allowing for a form of "FTL" travel, but that as soon as you try to move around the wormhole mouths so there is a time-like separation between them (which would theoretically allow for time travel in general relativity), quantum effects intervene in the form of a feedback loop of virtual particles which causes the energy density in the space around the wormhole to approach infinity, or at least the Planck scale (meaning that quantum gravity would be needed to figure out what happens, and it could plausibly prevent time travel by either destroying the wormholes or creating an uncrossable barrier between them and the outside universe). This page has a nice writeup thinking through the implications of this for a sci-fi civilization using a network of wormholes for interstellar travel.

If this is how it would actually work (as opposed to wormholes and any type of FTL just being impossible in quantum gravity), then I'd imagine something similar would happen if you used Alcubierre 'warp' bubbles to create an ansible. Alcubierre himself commented that the chronology protection conjecture might prevent you from using a system of more than one bubble to violate causality, writing here that "The conjecture has not been proven (it wouldn’t be a conjecture if it had), but there are good arguments in its favor based on quantum field theory. Notice that the conjecture does not prohibit faster than light travel. It just states that if a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole."

User avatar
Quizatzhaderac
Posts: 1784
Joined: Sun Oct 19, 2008 5:28 pm UTC
Location: Space Florida

Re: FTL communication and time travel

Postby Quizatzhaderac » Fri Aug 29, 2014 6:08 pm UTC

speising wrote:how does this prevent relativistically moving spaceships communicating with the ansibles per radio, and through them, with each other, thus again causing causality problems?
Well, using the visuals here as reference: Imagine the Alice and Bob lines are the spaceships you suggested and the Carol and Dave lines are the ansibles.

Alice and Bob can send a message fro Q to R, which sure does look like it goes back in time. However, the PQ line is closed because they're in the aftershock of the first ansible event. A radio communication from R to Bob is possible, but that would be a 45 degree angle, which would reach Bob way after event Q, longer than the RP line.

I picked the limit c2/Δv for the second ansible event, because that would cause an ansible communication from Alice to Bob to be at the exact angle of the RQ line.

So here's how an attempted casual violation would play out:

At t=3, s= 4 Bob and Carol are very close. Bob sends a message to Carol, to relay to Alice. Bob sends a backup message directly to Alice by radio.
At t=3.01, s=4 Carol passes Bob's message to Dave
At t=0.01, s=0 Alice and Dave are very close. Dave relay's Bob's message to Alice
At t=0.02, s=0 Alice receives the message and replies to Bob; thinking ti's silly to use the public ansible network when both of them have private ansibles, she replies to Bob with her personal ansible. She sends a back up copy of the message to Bob by radio.

The first ansible pair was traveling at 3c/4 and the message traveled at 4c/3 (negative time over negative distance, -4c/3 with |Δs|/Δt). The velocity of the ansible message (from Alice and Bob's frame) is determined by c2/Δvframes. Limiting Alice's response the the Same slope as Bob's message means Alice's reply is the opposite direction must have the same speed.

At t=3.02, s=4 Bob gets Alice's ansible message; Seems like a pretty quick response to him.
At t=4.02, s=4 Bob gets Alice's radio message.
At t=7, s=0 Alice gets Bob's radio message.

This would all be simpler in Carol and Dave's frame, of course. But simplification is still quasi-local (the universe isn't affected, just a limited area). Alice and Bob's frame could just as easily been the semi-preferred frame if Bob sent his message directly.
Hypnosifl wrote:Something along these lines has actually been proposed...
Yep, that's where I stole the idea from alright.
The thing about recursion problems is that they tend to contain other recursion problems.

lgw
Posts: 437
Joined: Mon Apr 12, 2010 10:52 pm UTC

Re: FTL communication and time travel

Postby lgw » Fri Aug 29, 2014 8:03 pm UTC

Hypnosifl wrote:
Quizatzhaderac wrote:The preferred ansible frame could be preexisting, or it could be a creation of the ansibles: The frame is created once the ansibles turn on, determined by their motion, and eventually disappears.

Two ansible networks would become entangled whenever they causally connect. Once entangled, the maximum communication speed is c2/ difference in networks' velocities. The same pair of ansibles after acceleration would be a different network for this purpose. The entanglement would dissipate, being entirely gone once all FTL events are in all ansible's past light cones.

The only impact relative to the story is that somebody could deliberately sabotage an ansible network by creating another, rapidly moving network.

Something along these lines has actually been proposed...


Nicely put Hypnosifl. My proposal for how ansibles might work was in fact based on just that work (to the best of my limited understanding of it). It makes for a nice story, IMO. If humanity has spread through the stars and established a co-moving ansible network, that's all great until we meet an alien race with it's own separate co-moving ansible network, but it's moving briskly relative to ours, they shut down when they get too close, and of course the networks are moving towards each other. Clearly, a problem to be resolved through laser cannon fire and space fighters that maneuver like they were in atmosphere!
"In no set of physics laws do you get two cats." - doogly

User avatar
eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: FTL communication and time travel

Postby eSOANEM » Sat Aug 30, 2014 12:10 am UTC

PsiSquared wrote:
eSOANEM wrote:No, you can recover all of the results of SR with a privileged reference frame given certain assumptions about how it works, this is what Lorentz Ether Theory does.

IIRC, in its normal formulation it's impossible to detect what this frame is.


This is true by definition, given the principle of relativity.


LET doesn't assume the principle of relativity though so it being true by definition of it is, more or less, irrelevant. The principle of relativity is simply something cool which drops out of the maths in LET and not a principle in and of itself.

Having read some more though, it is, like I said, a the-theory-doesn't-work-now issue to remove the principle of relativity from LET. Anyway, my point was mostly just that saying things like "a "universal rest frame" is pretty much the opposite of relativity" is patently wrong. LET definitely has a universal rest frame and reproduces all of SR; it just so happens that, because of the way the maths works, you can't detect what that universal rest frame is.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

speising
Posts: 2350
Joined: Mon Sep 03, 2012 4:54 pm UTC
Location: wien

Re: FTL communication and time travel

Postby speising » Sat Aug 30, 2014 12:14 am UTC

LET may reproduce he results of SR, but that is irrelevant to the claim. relativity presupposes that no universal rest frame exists.

lightvector
Posts: 224
Joined: Tue Jun 17, 2008 11:04 pm UTC

Re: FTL communication and time travel

Postby lightvector » Sat Aug 30, 2014 2:27 am UTC

I don't understand why this argument is still happening. It sounds like people are just talking past each other because they're using the word "relativity" differently, and not actually disagreeing on anything of actual substance.

1) If by "relativity", you mean "the principle of relativity", then yes, a universal rest frame or preferred frame is in some sense the direct opposite of relativity, because the principle of relativity is the precisely the statement that there is no preferred frame, that the laws of physics are identical in any reference frame.

2) If by "relativity", you mean "the theory of relativity and the various predictions and experimental phenomena that arise from it", then no, they aren't direct opposites, because, as we've seen in this thread, you can have consistent alternative theories or systems of physics that have a universal rest frame that nonetheless reproduce most of the phenomena that arise from the theory of relativity, or even all of them.

Edit: One could argue that something like LET shows that (1) is false, because its formulation contains a preferred frame and yet the laws of physics that result are still observationally identical in all reference frames. But again this is purely a disagreement about a vocabulary definition and not anything of substance. Whether this is a counterexample or not depends on what one means "preferred frame", such as "reference frame in which the laws of physics are experimentally distinguishable and/or special compared to other reference frames" versus "reference frame that is distinguished in the mathematical formalism behind the theory".

Hypnosifl
Posts: 257
Joined: Wed Mar 02, 2011 10:05 am UTC

Re: FTL communication and time travel

Postby Hypnosifl » Sat Aug 30, 2014 5:26 pm UTC

eSOANEM wrote:Anyway, my point was mostly just that saying things like "a "universal rest frame" is pretty much the opposite of relativity" is patently wrong. LET definitely has a universal rest frame and reproduces all of SR; it just so happens that, because of the way the maths works, you can't detect what that universal rest frame is.

But SR doesn't comment on the existence or nonexistence of a quasi-metaphysical "universal rest frame" that is impossible to detect experimentally, the first postulate of relativity only deals with experimentally-determinable laws of physics. As lightvector says this is basically just a matter of word-definitions, but I think it's important to understand what definition physicists actually have in mind.

PsiSquared
Posts: 126
Joined: Wed May 09, 2012 6:02 pm UTC

Re: FTL communication and time travel

Postby PsiSquared » Sat Aug 30, 2014 7:22 pm UTC

Hypnosifl wrote:
eSOANEM wrote:No, you can recover all of the results of SR with a privileged reference frame given certain assumptions about how it works, this is what Lorentz Ether Theory does.

The first postulate of SR is really only about the all frames being experimentally indistinguishable in terms of the observable laws of physics, and Lorentz Ether Theory can only reproduce all the results of SR if the ether frame is only "privileged" in some sense that's completely hidden from experiment, analogous to hidden-variable interpretations of quantum mechanics which make no predictions different from ordinary QM (that's why they're "interpretations" and not "theories"). But FTL with a privileged frame would allow you to experimentally determine which frame was the privileged one--it'd be the unique frame where it's impossible to send a signal in such a way that it arrives at an earlier time-coordinate than it was sent.


I've just thought of something:

If the privileged frame of reference is determined by the specific physical conditions currently existing (say, the distribution of all ansibles) than it doesn't violate the principle of relativity at all.

Let's take a simple example. Suppose the frame-of-reference in which ansible signals move, is tied to the motion of the center-of-mass of the universe. So if I'm sitting in a closed cabin inside a ship moving at 0.9c, I can use two ansibles to determine that my speed relative to the universe is 0.9c. But is it really me that is moving? Or is it the universe which is moving, dragging the "universal frame of reference" along with it? There's no way to tell. The principle of relativity holds.

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5530
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: FTL communication and time travel

Postby doogly » Wed Sep 03, 2014 3:08 am UTC

It's time to go to the fictional science subforum. Like, the Ender's Game fanfic nadir of the fictional science subforum.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

Hypnosifl
Posts: 257
Joined: Wed Mar 02, 2011 10:05 am UTC

Re: FTL communication and time travel

Postby Hypnosifl » Wed Sep 03, 2014 5:41 pm UTC

PsiSquared wrote:Let's take a simple example. Suppose the frame-of-reference in which ansible signals move, is tied to the motion of the center-of-mass of the universe.

Current models of cosmology generally use the assumption that the universe doesn't have any center of mass, see Where is the centre of the universe? from the Usenet Physics FAQ.


Return to “Science”

Who is online

Users browsing this forum: No registered users and 11 guests