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View Full Version : If I pick FTL and Relativity. . .



PsychicToaster
09-12-2010, 07:04 AM
So, this came up as a result of the whole "pick any two" concept. (Between Relativity, Causality, and FTL, you can choose any two)

I'm not here to talk about theoretical FTL travel though. (Insert sigh of relief) I'm more interested in the consequences of throwing causality out the window. Or, to be more specific, how causality is nominally preserved on the human scale (or human-experience scale, such as the observable universe), but doesn't hold on whichever other scale would make the most sense. Similar to the explanation of how experiential dimension depends upon scale rather than the pop culture "dimension."

When writing fiction, there's always that danger of polluting perfectly good magic with back-of-a-napkin physics, but this is all just thought exercise spawned by those numerous FTL discussions.

What would a universe without causality look like? Or is there a way to craft a universe that looks and feels and acts much like our own even though causality isn't absolute? For instance, 1G gravity feels absolute in the here-and-now on Earth's surface, but start messing with mass and distance and that changes rapidly. In other words, how would a universe "work" if what we perceive as causality is an illusion, or pareidolia? Could there be local-scale cause-and-effect relationships (I push a coffee cup off my desk and it spills) while on some other scale (quantum or cosmic) that no longer holds true? e.g. I push a galaxy-sized coffee cup off my universe, or a sub-lepton-sized coffee cup out of my energy state, and it turns into a potato because that's where it was when the wave function collapsed (even though it had some ungodly low probability)?

I'm playing a bit fast and loose with all that, but I'm curious where things break down in a universe without absolute causality.

Maxx
09-13-2010, 05:00 PM
So, this came up as a result of the whole "pick any two" concept. (Between Relativity, Causality, and FTL, you can choose any two)

I'm not here to talk about theoretical FTL travel though. (Insert sigh of relief) I'm more interested in the consequences of throwing causality out the window. Or, to be more specific, how causality is nominally preserved on the human scale (or human-experience scale, such as the observable universe), but doesn't hold on whichever other scale would make the most sense. Similar to the explanation of how experiential dimension depends upon scale rather than the pop culture "dimension."

When writing fiction, there's always that danger of polluting perfectly good magic with back-of-a-napkin physics, but this is all just thought exercise spawned by those numerous FTL discussions.

What would a universe without causality look like? Or is there a way to craft a universe that looks and feels and acts much like our own even though causality isn't absolute? For instance, 1G gravity feels absolute in the here-and-now on Earth's surface, but start messing with mass and distance and that changes rapidly. In other words, how would a universe "work" if what we perceive as causality is an illusion, or pareidolia? Could there be local-scale cause-and-effect relationships (I push a coffee cup off my desk and it spills) while on some other scale (quantum or cosmic) that no longer holds true? e.g. I push a galaxy-sized coffee cup off my universe, or a sub-lepton-sized coffee cup out of my energy state, and it turns into a potato because that's where it was when the wave function collapsed (even though it had some ungodly low probability)?

I'm playing a bit fast and loose with all that, but I'm curious where things break down in a universe without absolute causality.

You might enjoy getting a more current view of causality. For example, the collapse of wave functions is just quantum mechanics a la 1927, but causality works in a more refined and interrelated way in any field theory after 1947. Similarly there are quite elaborate discussions of causality in GR as it has been practiced after say Hermann Bondi quit being a Steady State man (early 1960s?).

Here's a little thing on Bondi:

http://www.answers.com/topic/hermann-bondi

Lhun
09-13-2010, 07:21 PM
Of the three options, relativity is the only one actually supported by physics. FTL is not ruled out by any principle, relativity just defines some ways of FTL that definitely don't work, there is nothing to say others couldn't. (Wormholes are so popular in fiction because they are theoretically possible)
And causality is mostly a matter of convenience. A world without causality would be extremely weird, we're just not wired to think that way. But there's nothing to say that's not actually the way the world works.

As for the scale, well the "pick any two" question already kind of explains how that'd work. If you don't go FTL you don't violate causality. If you do go FTL, you do violate causality. Simple as that. So, in a world where FTL travel is possible, causality will be preserved for any two events happening within the same lightcone.
The more interesting question here is what people would do with the capacity for time travel.

Maxx
09-13-2010, 08:02 PM
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And causality is mostly a matter of convenience.

Causality is one of those rules of thumb (like conservation of charge and parity as in TCP, I'm not sure what the T is off-hand) that a "good" theory would need very good reasons for violating.

Like apparently simple concepts such as a topological manifold, causality turns out to have more assumptions built in than you would think at first glance and these can be teased out in different ways.

For example: Caustics and foliations, or "Warps" of different kinds in gravitational fields:

http://th-www.if.uj.edu.pl/acta/vol17/pdf/v17p1017.pdf

PsychicToaster
09-13-2010, 08:16 PM
Of the three options, relativity is the only one actually supported by physics. FTL is not ruled out by any principle, relativity just defines some ways of FTL that definitely don't work, there is nothing to say others couldn't. (Wormholes are so popular in fiction because they are theoretically possible)

I thought the problem facing wormholes sending information backwards was that it burned itself out on energy feedback.



And causality is mostly a matter of convenience. A world without causality would be extremely weird, we're just not wired to think that way. But there's nothing to say that's not actually the way the world works.

Thus my question of pareidolia. Consider a universe where all cause-and-effect relationships are purely probabilistic. Physics works to model outcomes because the laws of nature usually work as expected, not because they always work as expected.


As for the scale, well the "pick any two" question already kind of explains how that'd work. If you don't go FTL you don't violate causality. If you do go FTL, you do violate causality.

By scale, I meant the notion that dimensions perceived disappear when relative scales get large enough or small enough. E.g. unaided, the moon is a two dimensional circle, not a three dimensional sphere. We need to travel there or do mathematical calculations to determine its third dimensional properties.

Combine that with the probabilistic universe, where by the time it reaches "human scale" the cause-effect chain has created "laws" that are too probable to not occur seemingly always within a human lifetime. For instance, it would be astronomically improbable to see an entire apple violate gravity, even if individual atoms do from time to time.

Maxx
09-13-2010, 08:29 PM
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Combine that with the probabilistic universe, where by the time it reaches "human scale" the cause-effect chain has created "laws" that are too probable to not occur seemingly always within a human lifetime. For instance, it would be astronomically improbable to see an entire apple violate gravity, even if individual atoms do from time to time.

Well, a naked singularity might annihilate whole galaxies in a couple of seconds and that would violate causality in a non-probabilistic (non-frequentist?) way as well as being on a scale larger than an apple, moreover nobody seems to have figured out how to exclude naked singularities by using basic GR.

RemusShepherd
09-14-2010, 06:26 PM
If you lose causality in the 'causality/relativity/FTL' choice, then all you have is easy time travel. You can use FTL to travel back in time and change the past. So there is no way to guarantee that events remain in place, because their causes can change without notice. In a story this usually leads to a Time War.

In my stories I usually drop the relativity part of the equation. Then there would be a preferred direction or pattern where the usual laws of physics can be broken.

zerospark
09-15-2010, 12:33 AM
The way I prefer to look at it is that ditching causality means you *can* alter the past, not that you *must* alter the past (i.e., real meaningful violation of causality is optional, not mandatory). The mechanics of it get wonky and difficult to calculate for a math dunce like myself, but as far as I understand it the problem with back-time travel comes when you move between two inertial frames with substantially different velocities.

This was written to explain the Star Wars hyperdrive (http://www.theforce.net/swtc/hyperspace.html#physics), but it is authored by a bona-fide astrophysicist so it's worth a read. The whole thing is interesting though you may find the section on causality and consistency (http://www.theforce.net/swtc/hyperspace.html#causality) to be most relevant.

The problem is that the FTL effect can't "know" what's happening at the other end of the jump; all it can "know" is what's happening in the local frame, so if you jump to an inertial frame that's moving fast relative to your initial frame, you risk winding up out of order with local events at the end-point.

This applies to both legs of the jump, so coming home would be the problem more so than the outbound trip - in other words, it's easy to go somewhere, but not necessarily easy to come back at or after the time you left, by the home-world's clock.

I find that using Minkowski diagrams (http://en.wikipedia.org/wiki/Minkowski_diagram) to map things out for BOTE calculations is useful.

The trick is to make FTL flight self-consistent between the ship and any causally-connected venues it may visit. There's lots of ways to solve that in-universe. Put clocks on board and slave the hyperdrive to them. Speculate on some kind of absolute reference frame that for some reason doesn't affect our measurements of relativity* the way Star Trek uses subspace or everybody else uses hyperspace. Have a vengeful time-traveling AI god that nukes your solar system if you venture outside your light-cone and risk altering the past. Etc. etc.

* This would technically be throwing out relativity, but meh.

FOTSGreg
09-15-2010, 12:43 AM
Zzerospark, how about having a race of (now departed) godlike aliens build a galaxy-wide network of jump points (which in turn themselves contain at least one other galaxy) and wire/program it to "keep time" with the rest of the "real" universe?

That's the prevailing theory in one of my WIPs (though certainly not te only one).

zerospark
09-15-2010, 01:21 AM
I've used a similar premise to that. ;) It's just as good as anything; if the network is causally linked through whatever mechanism and FTL travel is synced to that network, then it wouldn't wind up different from the kind of network in Carl Sagan's Contact, using Visser wormholes.

You're using a technological means to establish a "false" frame of reference to keep everybody on the same page, as it were.

Lhun
09-16-2010, 04:40 AM
By scale, I meant the notion that dimensions perceived disappear when relative scales get large enough or small enough. E.g. unaided, the moon is a two dimensional circle, not a three dimensional sphere. We need to travel there or do mathematical calculations to determine its third dimensional properties.

Combine that with the probabilistic universe, where by the time it reaches "human scale" the cause-effect chain has created "laws" that are too probable to not occur seemingly always within a human lifetime. For instance, it would be astronomically improbable to see an entire apple violate gravity, even if individual atoms do from time to time.I think i see what your question is, but it doesn't really make sense to talk about causality that way. (Not that it makes sense to talk about atoms violating gravity either)
Causality is not a physical law, i.e. mathematical relationship, which can change depending on scale, or be more or less precise, like Newton's laws of motion are pretty accurate at low speeds but less and less accurate the faster objects move.
Causality is just a kind of general rule, i.e. causes always occur before effects. There is no way for it be slightly off. Either it holds, and events can only influence things in their own future, or it doesn't, and events can influence things in their own past.
With FTL (and keeping relativity), causality will be violated on any scale. You could theoretically move on a space-like curve between any arbitrary chosen two points and create a paradox.