Faster-than-light and 'hard' scifi

A hyperspace where the speed of light is larger only partially helps, since (without a phlebotinum engine) you still have to get yourself up to that speed, and that's tremendously expensive and/or results in Chunky Salsa.

(This is besides the problem of the physical constants changing, which might result in everything getting quite a lot colder, heavier, and redder.)

@23, a "normal" extra dimension doesn't help either, in the same way that rising up from a piece of paper doesn't help you reach a point on the paper any faster, unless the paper is seriously curved. (However, warped extra dimensions, such as the 'time' component of spacetime, will help. Good luck explaining or visualising this, though.)

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Hmm, true, I fear.

Again, my physics is a bit rusty, so I'm not sure of how to proceed, other than a general suspicion that there's a solution somewhere in the concept of messing with fundamental constants. :/

Otherwise there's the old idea of a "hyperspace" having smaller distances between points than normal space - the mapping is still one-to-one and onto, in mathematical terms (if I recall them correctly), but moving x units in hyperspace is equivalent to moving x/y units in normal space, where y > 1.

(Would a smaller Planck length, perhaps arrived at by a higher speed of light or smaller Planck constant, do that?)

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There's a reason it's called "pick two" and not "pick one". All the alternate-space drives that dodge violating relativity by going to hyperspace or whatever they're called, still violate causality. There really is no way around the choice, because FTL+relativity means time travel it means causality goes out the window, and having FTL without time travel means relativity is wrong. The specific means of FTL don't matter for this, they just determine which of the two you're ditching.

On that note, if i'm understanding what Major Tom meant correctly, then yes, causality is the weakest link. There are VERY good reasons for relativity to be true (can't change it a lot without the whole universe working differently) there is nothing in physics that demands for causality to be preserved. There are plenty of ways a universe could function without preservation of causality, although most of them are not conductive to storytelling. (or human sanity)

The situation isn't quite as dire for causality as that's making it sound.

All observations can really tell us is that relativistic phenomena are quantitatively well-described by relativistic maths, the same way that non-relativistic phenomena are well-described by Newtonian maths. The former fact does no more imply that the theoretical framework of Relativity genuinely describes reality, any more than the latter fact is invalidated by the knowledge that the theoretical framework of Newtonian mechanics evidently doesn't describe reality at all.

That's why "relativistic effects, causality, FTL, pick all of them" is still within the bounds set by the original maxim, but - unfortunately, from our point of view - pulling that off without glaring flaws would take a physicist for an author, and even then would likely suffer from the circumstance that theory papers make for poor plot vehicles, in my experience.

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

"relativistic effects, causality, FTL, pick all of them" - is causality violation obligatory in wormholes, or just a possibility? I've seen several papers on the topic but I've never read that FTL==causality violation in all circumstances.

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Yes, it is obligatory. The problem is the crossing a distance faster than light; it doesn't matter how you do it.

Any theory which fills in the gaps that Relativity doesn't handle would have to reduce to relativity in the cases where we have tested it. Unfortunately, one of those is time dilation, hence the FTL = time travel thing.

edited 4th Sep '12 3:29:30 PM by Yej

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This. There's no hard evidence to suggest a tangible thing called causality exists. It's little more than a philosophy or correlation of cause and effect and the little niggling thing called time.

We've confirmed experiments that validate at least some of relativity (general relativity mostly but a few cases of special relativity). We've had no experiments that confirm causality, it's kinda just assumed to be true.

For all we know in the grand scheme of things, causality in physics is bunk.

Also, where's this notion that faster than light velocities automatically entails retrograde time travel? I've done a little math on the matterNote

and time dilation does not reverse if you extrapolate the equation onto the other side of the speed of light asymptote, at least in the sense that the faster you go from light the further back in time you go. On a 2-D plot of time dilation where Y is the time elapsed to the outside observer to pass a certain distance (say a light year) and X is the velocity it's a two-dimensional hyperbolic equation. On the other side of the asymptote the dilation line comes back down from the infinite time at c and starts using time units more resembling that close to reality. (At infinite velocity you have effectively the same thing going as if you were at absolute rest and since you can't be at absolute rest in the Universe this is a non-issue.)

Amusingly the equations for time dilation I've used differ from the relationship between mass(Y)/energy required to accelerate to Y and velocity(X). That behavior seems to be a traditional hyperbolic equation as in everything faster than light must have negative mass. (And there are theories that negative mass can exist.)

It's because of what happens when you combine a FTL object and the finite speed of light.

If you imagine an object heading towards you, emitting some sort of signal as in waves in water. However, as it approaches, it moves faster than its own signal. (By definition) Obviously, it reaches you before any of its emitted signals do, but something else odd happens: the signals reach you in reverse order of when they emitted. That is, the closest signal arrives first. Therefore, you see it as it arrives, then you see the signal it emitted when it was a small distance away, and then the signal when it was slightly further away...

Which means, judging from the signals its emitted, the object travelled away from you, and arrived before it left! (Because it's arrival was the first thing you noticed, and logically, its departure will be the last.)

Now, in Relativity, these "signals" are any information at all, including merely looking at it. A FTL starship will look like some sort of reversed ghost!

edited 4th Sep '12 4:23:55 PM by Yej

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Just a possibility, I think, but a very strong one. Wormholes are connections between two points in spacetime, not merely two points in space. There is no qualitative difference between the type of wormhole that acts as a bridge across space and the type that acts like a time machine, and turning the one kind into the other kind is technologically comparatively trivial. A network of wormholes in which all are of the former and none are of the latter kind would be the cosmic equivalent to American city planning, with roads running only in cardinal directions.

FTL == theoretical potential for causality violation in all circumstances. FTL automatically permits closed timelike curves, but as long as nothing travels along those curves, causality isn't actually being violated. It's the difference between owning a car that can go faster than the speed limit, and the act of speeding.

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

Time dilation has been tested, but there are a number of untested theoretical concepts hiding under your "hence", I suspect.

So what? This is not yet time-travel, by any reasonable definition of the term. If I shout a sentence at you while running towards you with my (supersonic) seven-league boots on my feet, you'll similarly hear the last word first and the first word last. Am I time-travelling?

And, conversely, as long as you are talking about spatially separated points, non-comoving observers will already disagree on the order in which events occur at the one point relative to events occurring at the other point before FTL ever enters the picture.

Things get interesting, causality-wise, only when we are talking about the order and causal relationship of events occurring within each others' light cones - meaning, in the simplest case, at a single point.

edited 4th Sep '12 10:11:01 PM by kassyopeia

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

Re: Wormholes are connections between two points in spacetime

Would this mean that wormholes don't travel forward in time with the rest of us? So from our point of view, would a wormhole only stay open for Planck-time?

There are wormholes like that, with spatial and temporal extent at or below the Planck threshold, in the "quantum foam" interpretation of vacuum energy.

That's not what I meant, though, apologies for the lack of clarity.

Rather, I meant that one shouldn't expect the temporal coordinates of the two ends of the wormhole, relative to some common benchmark, to be the same, any more than one should expect their spatial coordinates to be the same. One of the traditional hypothetical processes by which wormholes form is that the spacetime distortions caused by two black holes (the "dents" in the stretchy fabric, in the usual illustration) somehow find each other and merge. This finding and merging happens outside our universe, or at least outside of what we usually consider to be the totality of our universe. Therefore, spatial and temporal distances are equally meaningless there, and there's no real reason to suppose that two stellar-mass black holes existing in the same region at the same time are any more likely to link up than two such black holes which are billions of light-years and billions of actual years apart.

We don't have the mental tools to make any sense of the creation or evolution of a wormhole in that "other realm", so the best we can do is suppose that it does begin at some point, or rather some two points, in time, from our perspective, and that it ends at some later point in time, either because the connection just goes away again, or because it destabilizes on the inside (which is part of our universe, more or less), or because one of the two terminals is disrupted somehow.

Maybe a better way of putting it would be to say that wormholes connect series of points in spacetime. The entrance has a certain size, and the inside has a certain size, presumably, and the exit has a certain size, and each of them exists for a certain duration, and the spacetime volume defined by all of those contributions is what we collectively refer to as "the wormhole". As seen from the outside, though, the entrance and exit exist in distinct places and/or at distinct times, and those places don't have to remain static, nor do those times have to pass at the same rate - one end could be time-dilated due to yet other relativistic effects, for instance. It's all very convoluted and mind-straining.

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

It is. This page here has a pretty good and easy to understand explanation. (as far as that's possible explaining relativity)

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

But to summarize, because of relativity, your point of view affects time, so depending on your speed and position, you might measure the time difference between two events differently. I.e. watching a clock tick, whether the "tick" comes every second or sooner or later depends not only on the clock, but also the observer. So, if someone throws a ball, it you can observe different times until the ball hits the wall. However as long as everything moves STL, the perspective will never shift enough that you see a negative time, i.e. observe the ball hitting the wall before you observe the ball being thrown. (note, this is not about the time light takes to arrive at the observer)

However, FTL changes that, and for certain observers, a ball moving at FTL will appear to arrive before being thrown, thus breaking causality. Likewise, if an observer is moving at FTL, he can observer other (STL) events in "reverse". The method of FTL makes absolutely no difference here, and there is no way around it short of ditching this aspect of relativity. (Thus: Relativity, Causality, FTL: pick any two)

Addendum: Note that this does not mean that any traveller going at FTL automatically arrives in his own past. You do not have to move in a CTC. What it means is that any FTL travel will look like time travel for at least some observers. But that is enough to break causality, there's no such thing as violating causality "a little".

Have you ever read "The relativity of wrong" by Asimov? I recommend it, it's a very good read, and makes an important point about the nature of truth in general and science in particular. Anyway, the point is that newtonian mechanics don't describe reality "not at all" but are simply inaccurate. And while it is correct that relativity might be disproven, the same way newtonian mechanics have been disproven, that does not mean they can just be wrong. Newtonian mechanics gives you perfectly correct results when you try to calculate accelerations, speeds, and distances for something like cars, with an accuracy not higher than a few decimal places. Not only does relativity not get different results in those ranges, it is impossible that any theory replacing Newtonian mechanics could have done so. Those are predictions made about reality that are demonstrably true, and no future theory can get different results in those cases, or it would plainly be wrong. A better theory can only get results that are beyond the scope of the current one, either offering accuracy higher than previously possible, or offering predictions in areas where the previous one just couldn't, or offering an easier line of reasoning for arriving at basically the same result.

Anyway, my main point being: yes, the To R is not some kind of metaphysical law, it is only descriptive of reality, not prescriptive. But that doesn't mean you could break it without dire consequences because reality really does follow it's predictions. Experimental results that fall in line with predictions of relativity cannot just vanish overnight, and having any fundamental change in the To R (the likes that would allow FTL without causality violations) and still having reality conform to it would mean major changes to reality. Unfortunately, that could easily be the sort of major change that wouldn't allow, for example, atoms to exist.

edited 5th Sep '12 4:35:55 AM by McKitten

So, say you have a person standing a km away or so with a high-powered rifle. He shoots a bell next to me. I'm going to hear the bell before I hear the rifle firing. If the rifle fired FTL rounds, I'd also see the bell be hit by the bullet before I see the muzzle flash^note well, assuming I can look at both at the same time and notice the very insignificant time difference. Why would that be time travel? It's only from my perspective things happen in the wrong order, but it's not what actually happened. To me that doesn't seem any more strange than seeing a spinning wheel seemingly shift to spin the other way just because it spins so fast.

edited 5th Sep '12 7:14:11 AM by AnotherDuck

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I'm going to be using entropy here instead of time to avoid some confusion. Tell me if this makes some sense.

The idea is that as you approach the speed of light you experience entropy at a slower rate. At light speed, you shouldn't experience entropy at all. If you move faster than light, and if the theory holds, you should experience reverse entropy. There is also the problem of how to accelerate when experiencing no entropy.

I am not sure what entropy here means. Backwards flowing time simply is decreasing entropy.

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman

Entropy refers to the progress of chemical reactions. Specifficly, towards the more chaotic but I'm using it to refer to an object's progress through time. According to special relativity at light speed time stops. The traveling object is essentially flash frozen and no longer experiencing time. This leads to an interesting concept of if you're accelerating to light speed time will slow for you so much as to prevent you from ever reaching that speed.

edited 5th Sep '12 10:01:01 AM by Belisaurius

@Another Duck: from what perspective do things happen in the right order?

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@yej

The entire causality issues stem from how both viewpoints are entirely correct.

@Another Duck You are talking about simple detection delay due to speed of light or sound, but that doesn't matter. As was pointed out before, that's got nothing to do with time travel. If a bullet goes FTL however, there will be real time travel observable by some observer. Imagine the gun at point a) and the target at point b). An observer being at some third point that is equally distant from both a) and b) can still observe the bullet first hitting the target and then being fired, if it goes FTL (also depends on the observers speed etc. but that's not important).

It is simply related to the CTC issue. You can use FTL travel to move in a circle, that will see you arrive at the point you started from, but at a time earlier than you began moving. But an outside observer would notice that for every part of the circle you arrive before you left, even though you yourself would not be able to notice that until moving in a complete CTC.

Well, yeah. That's what I'm trying to highlight.

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The issue I have is that i don't think this interpretation of relativity and causality are correct. The entire premise of paradox causing relativity falls apart if you assume there can be a third party.

Anyhow, I don't think linear acceleration to and beyond light speed is actually viable. To an outside observer your rate of acceleration would slow to almost nothingness. From the perspective of the pilot, the dial would move constantly toward light speed but just as it reaches the light barrier, one of three things will happen. First, you'd hit something so far away you didn't know it existed. Second, millions of years of accumulated light would erode you're ship to nothingness in an instant almost instantaneously, third, the universe would end.

What's the pilot measuring his speed in relation to?

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My conclusion is that this thread holds no explanation for why FTL isn't possible, it just claims "this is what happens or would happen" without actually saying why. (I'm the kid who asks why until I get an explanation that in itseld doesn't need further explanations.)

Anyone know a good place to read about why the speed of light is constant?

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Does modern science even know why? I know they've observed the speed of light under countless circumstances to measure effects like gravity (which actually slows light down a semi-negligible amount depending on how much gravitational force is used) and exact velocities but I've never heard of an experiment which even yields a theory as to why lightspeed is approximately 300,000 km/s.