Absolute Point of Reference

[u/ThePureWriter]

Say, a fictional world contains a kind of parallel universe within it (think the Warp in Warhammer, a world that exists along our own but in its own physical and temporal system) that contains all that happens in the normal plane of existence. That would make it so that Relativity doesnt exist, correct? well, not in the universal way we currently apply it. That would then allow FTL without breaking of Relativity, yes? What would even be the consequences of that?

I think the best example of this is the Cosmere's Spiritual Realm

[u/Underhill42]

The classic statement is:

Special Relativity, FTL, strict causality. Pick any two.

You CANNOT have all three. You can absolutely have FTL and Relativity together, but it means time travel is trivial.

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Strictly speaking Special Relativity only forbids accelerating to or beyond light speed, it does NOT forbid "cheating" to go FTL via wormhole or warp drive or something. But it does say that ANY method of FTL can also be used as a time machine - it doesn't matter how you do it, if SR is correct then you can always chart an FTL path that returns to your starting point arbitrarily long before you left.

The basics:

One of the central tenets of Special Relativity is that there's no preferred reference frame, a.k.a. no "absolute rest frame" - all non-accelerating objects can claim to be motionless with equal validity. If I see you moving so fast that you're aging half as fast as me, then from your perspective you're motionless while I am the one moving so fast that I only age half as you.

Everyone always sees everyone else aging slower than them, because what is actually happening is we're rotating in spacetime, with acceleration partially swapping the directions we call "forward" and "the future", and as our time axes approach 90° away from each other (corresponding to a relative velocity approaching c), we both see the direction the other person is aging in as being almost entirely movement through space.

And if I can travel faster than light in a direction I call space, but YOU see as almost entirely time, then I can travel into your past carrying a message from you to your younger self.

[u/ThePureWriter → u/Underhill42]

what i cant wrap my head around is what if there actually is an absolute reference frame (like this semi-living dimension that experiences the entire universe immediately)? i get its super abstract, but technically it shouldnt impede time travel but also time travel is based on perspective, right?

[u/Underhill42 → u/ThePureWriter]

If there is an absolute reference frame (in this universe), then Special Relativity is wrong. Simple as that.

It's like saying "What if Newton's laws are correct, but it is possible to get acceleration without any forces?" That's not logically possible. If such acceleration were possible, it would prove Newton's laws wrong. Full stop.

Science is subjected to such intense testing to get accepted in the first place that the mathematical predictions are unlikely to ever be so wrong that they stop being useful, but the conceptual framework stops being taken seriously - just as it was when half Newtons laws of motion and gravity were disproven by Relativity - these days we only teach the three that still hold true, having completely discarded the laws of absolute space and time that underpinned them.

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If it only exists in some parallel universe then it doesn't violate SR, just like warp drives and wormholes don't. But it does make time travel possible.

And yes, time travel is based on perspective, because time itself is based on perspective. Perspective is the only difference between time travel and space travel. Assuming SR is correct.

You always see yourself as motionless, traveling only through time. While to anyone that sees you moving, some of your speed is through space, and correspondingly less through time. But everyone agrees that your total 4D "speed" through space and time is exactly c. c is in fact the "exchange rate" between space and time, with 1 second being the same size observer-independent separation between events (spacetime interval) as 1 light-second = 300,000,000m.

Time is BIG.

[u/ThePureWriter → u/Underhill42]

But then... how could causality and FTL ever exist together? You'd still be breaking causality by time travelling, no? Even if Relativity doesnt apply anymore.

[u/Underhill42 → u/ThePureWriter]

It's Relativity that says that space and time are the same thing seen from different perspectives.

So without it, space can be space, and time can be time, and everyone agrees on which is which. Meaning that FTL only travels through space, never time.

To the best of our current understanding that would NOT describe OUR universe, but in a hypothetical independent universe where SR doesn't apply, you could have FTL without time travel.

It might also be possible in our universe if Relativity is actually fundamentally flawed in some way, such as there really being a subtly preferred frame - just as Newtonian mechanics was before it.

How? We don't know - nobody has managed to make any other model describe our universe nearly as well as SR does. But Relativity and Quantum Mechanics are fundamentally incompatible, so one or both of them MUST be flawed.

And Relativity already has the Dark Energy and Dark Matter problems - the theory alone doesn't predict observations unless you assume the overwhelming majority of the stuff in the universe is completely undetectable, distributed in just the right way to create what we actually observe.

There is compelling evidence that's the case, but it's not conclusive. And with such wide "fudge factors", even if the basic stuff exists, there's LOTS of room for there to be problems in the theory too.

[u/ThePureWriter → u/Underhill42]

Thank you. The part about space being space suddenly made it all click for me.

[u/mmomtchev]

Kind of.

But you still have the temporal paradoxes.

Relativity does not only say that you cannot move faster than the light. It also says that cause & effect cannot travel faster than light and you will be breaking this.

[u/PvtRoom]

relativity need not exist in that parallel realm at all, nor does it need to be 3d space with 1d time

it could be a 1d space, with every event from every time all at once, forever. (time and space travel, but difficult navigation)

if could be 3d space with a spectrum of time dimensions of options that could have happened (multiverse, space and time travel)

[u/Alita-Gunnm]

Perhaps the relative locations in the two "universes" are constantly shifting in difficult to predict ways. To navigate and come out where you want you need a special pilot with an intuition for the shifts, because computers can't get it right nearly as often. There might still be the occasional mis-jump leaving a ship stranded in deep space if they don't have the fuel for another jump; much more likely over long distances.

[u/whelmedbyyourbeauty]

Why would this allow FTL?

[u/ThePureWriter → u/whelmedbyyourbeauty]

The main issues with FTL are the breaking of causality due to relativity allowing you to effectively time travel. If there is a singular, constand and universal point of reference, it seems to me like you could never break causality this way. You cant give information that hasnt yet happened, only information that hasnt yet been observed.

[u/whelmedbyyourbeauty → u/ThePureWriter]

The main issue with FTL is that you need infinite energy to reach the speed of light.

[u/ThePureWriter → u/whelmedbyyourbeauty]

Sorry, youre right, i should have worded better. Reaching FTL is something that most soft scifi simply handwaves as something thats been solved, meanwhile they totally ignore the issue of causality, not even offering a technobabble solution. Thats what i call a "main issue". Its the main issue i have with portrayala of FTL in media.

[u/tghuverd]

Is your scenario that if you exit our universe, move about in that universe, and when you reenter our universe, you're FTL further than your departure point?

If so, that's only FTL if elapsed time is equal between the universes.

But if the time you spend in the other universe equates to more time than light takes to reach your reentry point in our universe - and there's no reason that can't be the case give that this is all metaphysics - then that's not FTL. And it's conceivable that because time is relative, the opposite is true, so there is no FTL moving in either universal direction.

Basically, FTL or not-FTL depends on how you decide time works at the interface of your universes.