A science-fiction question.

[theweaselking]

Charlie Stross has said that FTL inherently requires time travel - in fact, the direct quote I'm thinking of is "if you permit violations of special relativity, you're also implicitly permitting global causality violation — time travel. (Go read a physics textbook if you're not sure why.)" from here.

Thing is, I'm not 100% sure he's right on that, and while my degree involves a lot of math it was way more quantum mechanics than relativity. And so, since none of my convenient physics textbooks cover this, I'm asking The Tubes:

1. Imagine a universe where time is absolute. You can perceive time as running faster or slower, but there is, fundamentally, an Absolute Time(tm). Any given event Z can be precisely said to have occured at X seconds after Y event, all the way back to the Big Bang. In fact, all events can be timed relative to the Big Bang, assuming perfect knowledge.

(1.1 this does mean you can extend your life indefinitely by travelling at near-light-speed and never slowing down for long, Space Hitler-style. Still, you're causing time to flow slower for yourself, not reversing time.)

2. In this universe, FTL travel exists. Even near-instant FTL. But at the same time, your departure is at a specific, set, universal time, and your arrival is at some later, specific, set, universal time. Even if it's nanoseconds later.

(2.1 yes, you can perceive an event as occuring before its cause in a given frame, but this is a failure of your perception. The cause still happened before the effect.)


In that universe, either:
A) How can you use FTL to violate causality? You can perceive a lightspeed event and then arrive at its source faster than a lightspeed response, but that doesn't mean arriving before you leave. And you can learn of an event before a lightspeed signal can reach you, and respond, but not before the source event. Unless I've missed something.
or
B) How is this universe impossible? Basically, how does my hypothetical sci-fi setting with Absolute Time(tm) break things such that it is internally inconsistent?



I figure I must be missing something, but I can't think of what. And the local physics textbooks don't cover it. So: What am I missing, and/or which physics textbook should I be looking at?

(Possible thought: I've broken Special Relativity completely, granted. But what does that MEAN, beyond "gee, Special Relativity wasn't correct with regards to this universe?". Once I've assumed that a One, True, Really Really True frame of reference absolutely exists, in the context of whatever allows FTL? What have I broken, even if Special Relativity still holds for non-FTL-Drive uses?)


EDIT: Yes, the answer appears to be that I've broken Special Relativity completely, and the distinction between my hypothetical universe and the one Stross was talking about is that he meant "if Special Relativity holds, violating it via FTL allows time travel", whereas I've started from "Special Relativity does not hold and thus there's nothing to violate". And I'm inviting a Did Not Do The Math entry on TV Tropes for my universe when I add " Special Relativity still holds for non-FTL-Drive uses"

Comments

[duskwuff.livejournal.com]

If it's possible to transmit information instantaneously in some privileged reference frame, then I'm pretty sure that other reference frames will perceive that information transfer to violate causality. (And, as such, it will become possible to do impossible things in those frames.)

[fridgepunk.livejournal.com]

Well it's more that in a minkowski space time, FTL looks like someone who's experiencing time-like space and space-like time, and then they start violating thermodynamics by gaining velocity while losing energy or gaining energy as they lose velocity, and sonic rainbooms literally start to happen (http://en.wikipedia.org/wiki/File:Tachyon04s.gif), but only for some observers and it all gets a bit weird and you can never properly balance the energy budget for any systems you try to model, and then time paradoxes start to occur in your margins and beetles start to crawl out of the wallpaper and under your skin, just itching and itching they won't stop and why why why why why why whyw hy whyw hywhywhy wyhy wy why why why wyh wyhw yhw yhwy hwy hwy hwh ywh hywywh ywh yhwy hywh ywhy hyh wh wyhwyhyh yhw.

[theweaselking.livejournal.com]

That's actually a pretty good explanation.

[lafinjack.livejournal.com]

LIGHT CONES!!

*brain dissolves into a simpering pile of goo*

[herewiss13.livejournal.com]

It's late and I'm not sure, but think of it this way (maybe?)

First off: absolute time in a universe where light can only move at 'c' is definitely a no-no. You don't get _any_ relativity with an absolute frame of reference...so no time dilation, no Space Hitler. If light is limited to a single speed, there's no way to figure out how every clock should synch up...it depends on how fast any particular observer is moving.

We can, however, still talk about something that can violate causality in our normal Einsteinian universe. Consider the following thought-experiment:

You are 50 light years from earth. You pick the Beatles first hit "Love me Do" on your space-radio. The sound has been traveling outward to you since 1962. Since it has taken 50 years to get to you in your present moment, it is established that you are currently in the 2012 timeframe. You now FTL jump halfway to earth in an instant (it works at slower than instant-but faster-than-light speeds, but the math gets more complicated). You are now 25 light years from earth. You're now hearing the jazzy music of 1937. It's taken 25 years for the signal to reach your current position so you are now in the 1962 time frame. Rinse, repeat, hit Earth in the mid 1900s with your I-pod and change music history (paradox!) By moving faster than light, you can travel to any point in the light cone that you want. In other words, you can follow any photon back to the source that emitted it..._at_ the point in time it was emitted.

Want to go back to the dinosaurs? Jump out 65 million light years. Can you see the little teeny tiny flash of light at Chicxulub? Now jump back to earth. You are travelling upstream of the light wave faster than it moves outward, so that now, even though you initially started in 2012, you can travel back to the moment of impact of the asteroid that killed the (non-avian) dinosaurs. This is definitely arriving before you left.

I think this makes sense. I'm hoping it's not "it's almost midnight and you only _think_ it makes sense" sense.

I think actual physicists talk about "space-like" and "time-like" curves/loops/pathways in space but beyond hearing about them in SF literature, I don't have any good details at this (late) time.

Happy New Year!

[kowh.livejournal.com]

I think you almost made sense, but it's not clear why a jump in space ("jump halfway to earth") is also a jump in time. Why do you not end up hearing 1987 after the first jump?

[herewiss13.livejournal.com]

Yeah, I either biffed the motion or the math (such as it was). I think the _concept_ still holds, even if I didn't describe it well.

[theweaselking.livejournal.com]

You are now 25 light years from earth. You're now hearing the jazzy music of 1937.

Why are you not hearing the insipid pop of 1987 instead?

Light that left 50 years ago has reached 50 LY away. Therefore, at that same instant, light 25 years from the source is only 25 years old.

First off: absolute time in a universe where light can only move at 'c' is definitely a no-no.

In a universe where NOTHING can move faster than c, sure, but we're already postulating that FTL for things other than light is possible, via the handwaving of jump drives, warp engines, wormholes, something.

[herewiss13.livejournal.com]

Yeah, I either biffed the motion or the math (such as it was). I think the _concept_ still holds, even if I didn't describe it well.

[torrain.livejournal.com]

*blinks*

When you're fifty light-years away from earth, you'd be hearing music that's been travelling for fifty years; and if there's an absolute 2012, then you'd be hearing music from 1962. If you're twenty-five light-years away from earth, you'd be hearing music that's been travelling for twenty-five years; if it's absolutely 2012, wouldn't you be hearing music from 1987?

Unless...hrm. Are you saying that light matches reality--that, when you can see the light from the dinosaur extinction, it's happening somewhere, so you can jump to the source of it? I keep trying to understand that, and I keep running up against... well...

If a stone hits the water by your left hand at 8:00 a.m. and the ripple reaches your right hand at 8:00:02 a.m., it doesn't mean that your left hand can be there when the stone hits the water at 8:00:02 a.m. That already happened, in absolute time. It didn't reach your right hand until 8:00:02, but even if you can go from one place to another in an instant (or switch your focus from your right hand to your left hand in an instant), the event is still over by the time it reaches you, right?

[herewiss13.livejournal.com]

Yeah, looks like I biffed the description (it seemed so clear last night!). Travelling freely through the light-cone...and following it back to its source, though...I'm pretty sure that's still standard FTL/causality violation paradigm. I just don't have the right language. Maybe wormholes...

[glaurung-quena.livejournal.com]

I am not a physicist of any kind, and trying to read about physics in detail generally causes my brain to shut down. However, from reading non-detailed summaries about the causality problems in FTL, I gather that what is deemed impossible is travelling faster than light through normal space -- because you can outrace the photons you emit (go faster than the sight of your going), you can violate casuality (because reasons my brain refuses to understand).

However, again from my reading, if you have to enter hyperspace to exceed light speed (eg, Babylon 5), or if you are using some method of FTL "Jump" (eg, rebooted Battlestar Galactica), then causality is not violated and you are allowed to do that. The only problem being that in order for hyperspace or warp jumping to be possible, you have to invent brand new branches of physics out of whole cloth.

[frith]

Right off the bat, a problem: You cannot observe time as running faster or slower. In your reference, time is constant. You can observe the effect of time dialation that has occured to objects that enter your reference after they have travelled at a speed that is different from what you have experienced. You will always experience time at the same rate, regardless of how fast you move or how close you approach a singularity (black hole).

Consider this: photons exist as borrowed energy. Because they travel at the speed of light, time for a photon = zero between the time it borrows the energy and the time it gives it back. Furthermore, for a photon, the size of the universe is a dimensionless dot. So, to a photon, relative to its frame of reference, it never existed and it never moved. There was no time.

There are photons bopping around that were part of the Big Bang: "background radiation". So timing back to the Big Bang in the reference relative to the photons that were formed then? Zero time has passed. Or will pass. Or whatever.

If you travel near the speed of light, time does not change for you, you won't live any longer. You'll just find that you missed a lot of stuff that happened at your slow-moving departure point while you were away. If you travel at the speed of light, you won't live forever, you will, in no time at all, meet the fate of the universe. That could be putting back all that borrowed energy, which means, in your reference frame, you won't even be a memory.

If you want to travel faster than light, you'd better skip over the as fast as light part to get there. 8^D

[theweaselking.livejournal.com]

You cannot observe time as running faster or slower. In your reference, time is constant

Well, yes, but while you spend a few weeks close to the speed of light, the rest of the world spends a few years. When you come out, your clock is wrong. That's not really the point I'm missing, though.

If you want to travel faster than light, you'd better skip over the as fast as light part to get there.

Granted. And yet, take Star Trek, for example. They routinely go faster than light and have FTL communications, but nothing ever goes backwards in time because of the everyday stuff, only the Particle Of The Week or the Weirdass Time Gate Plot Device Du Jour.

[frith]

If FTL travel were at all possible, it would probably be via something banal such as a 5th dimensional shortcut between folds in 4-D spacetime, and not true FTL movement. Meanwhile, to live to see the far flung future, suspended animation (with safeguards against radiation poisoning) would be more feasable. Just expect to wake up lonely and vulnerable.

[herewiss13.livejournal.com]

in re: Star Trek

A) in most episodes all objects are travelling in the same reference frame (either in orbit around something or on the Enterprise itself). I'm pretty sure the tech manuals talk about a galactic GPS system that keeps all the various ships and starbases, etc. synched to a single basic time (or Stardate).

(also: they travel through sub-space, so normal relativistic effects would only happen at impulse speeds).

B) The "Picard Maneuver." Super-quick warp jump means arriving at a target before you left. Enemy target fires at the "image" of your previous position while you get in close (they don't _hit_ you in the past because they're shooting at the light shell your former position emitted). Sloppy technobabble, but still causality violation.

[kafziel.livejournal.com]

Picard Maneuver doesn't mean you arrive at the target before you left, it means you arrive at a new location while sensor data is still coming in showing you at your old location. The ship then gets confused, seeing two of you at once, and shoots at the wrong one.

Like, okay. You are one light-second away from an object. You jump to .25 light-seconds away from the object. For a period of .75 seconds, their sensors will read two of you, assuming information is coming in at the speed of light. There's no causality violation, there's just tricking a computer.

[kowh.livejournal.com]

Not directly an answer, but related and useful: http://www.theculture.org/rich/sharpblue/archives/000089.html explains why of causality, FTL and special relativity, we have to pick two.

[theweaselking.livejournal.com]

That actually explains the problem nicely - and also explains the original objection, which is less "FTL involves time travel" and is more "FTL involves time travel unless special relativity is incorrect". And my postulated universe means that special relativity doesn't hold.

[sucrelefey.livejournal.com]

Maybe instead of calling it faster than light call it shorter than space.

[fridgepunk.livejournal.com]

The starting point of both relativities is that C is constant, therefore if you go really close to the speed of light and do a spectroscopy equation you can get a sensible answer on a CRT that isn't producing an infinite amount of energy for finite amounts of energy going in thanks to Lorentz transformations and Planck's constant.

(Remember that Special Relativity doesn't rule out an absolute frame of reference like a luminiferous aether, but it was just that the aether got ruled out because all the experiments that should have shown an aether didn't and SR made it unneccesary, amusingly enough the positive proof for an aether would have also served as a potential proof that the universe we're in is a simulation according to some physicist/philosophers who New Scientist interrogated a while back)

Which is why you have to rule out SR if you allow FTL and reformulate all of modern physics from the ground up, getting rid of C and instead forcing every equation to take into account the observer's velocity relative to the fixed and absolute frame of reference (and best of wishes for you to do that on a rotating planet orbiting a star that orbits a galactic center which is itself moving at phenomenal speeds through the intergalactic void).

(there's more about how the conclusion that follow from SR are so blatantly correct I'm pretty sure reality itself would be a very odd place if SR wasn't true, much as they would be if the laws of thermodynamics weren't, but that's a different and more complex issue)

[theweaselking.livejournal.com]

(Remember that Special Relativity doesn't rule out an absolute frame of reference like a luminiferous aether, but it was just that the aether got ruled out because all the experiments that should have shown an aether didn't and SR made it unneccesary, amusingly enough the positive proof for an aether would have also served as a potential proof that the universe we're in is a simulation according to some physicist/philosophers who New Scientist interrogated a while back)

Actually, I'd forgotten that SR doesn't rule out an absolute frame of reference. Of course, at half past midnight on New Year's Eve, when I posted, I'd also lost the thread of the explanation about how FTL *allows* time travel, so all I remembered was that I thought I had a hypothetical workable universe where it didn't allow it, without falling apart.

Okay, then, stupid question time: is there a way to have both an absolute frame of reference, a grid and stopwatch "outside the game" as it were, while also leaving C constant for all subjective frames of reference? And in that universe, can you still send information into your own past, or does something about the existence of the absolute break the effects of the relative inability to tell if I'm moving very fast, or you are?


(Nerd moment: I'm reminded of needing to explain to people that in D&D, "when you enter a zone" is VERY DIFFERENT from "when the zone moves to cover you", in part for game balance reasons and in part because the game does have an absolute frame of reference, the map grid.)

[fridgepunk.livejournal.com]

is there a way to have both an absolute frame of reference, a grid and stopwatch "outside the game" as it were, while also leaving C constant for all subjective frames of reference?

No, I actually mispoke when I called the luminiferous aether an absolute frame of reference: an absolute frame of reference, as far as physics is concerned, is any frame of reference that overrules all others, the reason why Special Relativity is called "relativity", despite it actually centering around an absolute that does not differ relative to an observer is because it takes Gallilean relativity ("if I am in a boat at sea and see the land move up and down, how do I know that I am moving and not the land?" Newton hated the notion) and accepts it as a fundamental fact of nature – there cannot be any way to tell whose perspective is correct because that in turn means preferencing and imposing someone's measurement for the speed of C upon all observers, and in turn means that C is not an absolute but variable for all observers.

What i was thinking of was some early attempts by people to reconcile the Aether with SR, but from what Google seems to be telling me their approach involved "local aethers" for all observers, which is a bit dull really.

TL;DR: Any universe where you have a constant C for all observers and FTL will also allow time travel being possible, No exceptions.

[maelorin]

a non-maths brain is why i took a left turn after first year into biology and biochem, rather than geology and geophysics.