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I am considering how to implement faster-than-light (FTL) travel in a sci-fi setting without the attendant problems with causality. One of the ways I've been thinking about for this is establishing a preferred frame of reference.

Per Wikipedia:

In theoretical physics, a preferred frame or privileged frame is usually a special hypothetical frame of reference in which the laws of physics might appear to be identifiably different (simpler) from those in other frames.

My idea is that going FTL involves a point-to-point jump where both ends of the trip must be at rest in the preferred frame, which itself is approximately at rest relative to the cosmic microwave background (CMB) (that is, in the preferred frame the observed CMB dipole is effectively eliminated). Thus to make an FTL jump from one star system to another, you must first decelerate your ship to rest relative to the CMB (which is approximately 370 km/s relative to Sol), and then after the jump you must accelerate to match velocities with your target star. Importantly, causality is always maintained relative to the preferred rest frame, though there may be other reference frames where it appears to have been violated due to Lorentz transformations. Crucially, even if you contrive a situation where someone observes an apparent causality violation (e.g. they are moving at a high fraction of lightspeed relative to the preferred frame and see a starship arrive at its destination before leaving its origin), they cannot send information about what they have observed in a way that violates causality (e.g. they cannot observe the arrival and then send a message to the starship's origin telling it not to depart in such a way that the message arrives before the departure occurs) because sending anything FTL requires being at rest in the preferred frame (e.g. they have to decelerate, rotating their space and time axes such that their message does not go backwards in time).

My question is: how would our understanding of physics need to be modified to account for such a preferred frame while also fitting the real-life experimental data we have regarding general relativity and special relativity? The theories of relativity specifically say that there is no preferred frame, so in my universe there must be something they're missing. I want it to be something that we could plausibly have missed, for instance something that is only apparent when you get close to being at rest within the preferred frame. I also do want it to be consistent with the experiments we've performed and observations we've made that have so far agreed with relativity.

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  • $\begingroup$ I would go another route: light itself already has a paradox where it moves at the speed of light regardless of who's frame of reference you use and the apparent speed that frame is moving. So if one person is moving left at 10% speed of light and another is moving left at 90% the speed of light, then they still measure the light overtaking them at lightspeed compared to themselves. FTL just has the same paradox: it doesnt care about switching reference frames to create time travel, there just is none as it only counts the individual reference frames regardless of their FTL/non-FTL speed. $\endgroup$
    – Demigan
    May 9, 2022 at 20:15
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    $\begingroup$ The paradox is resolved by time dilation between the different inertial reference frames. $\endgroup$
    – Slarty
    May 9, 2022 at 20:41
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    $\begingroup$ @Demigan The constancy of the speed of light is not considered a paradox, as far as I am aware. Though it certainly is mind-bending. The problem with being able to go FTL relative to different reference frames is that it allows for true paradoxes, like arriving at a location before you left that location. $\endgroup$
    – Lawton
    May 9, 2022 at 20:42
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    $\begingroup$ In the book Mote in Gods Eye, Jerry Pournelle, Larry Niven. Written for the CoDominium universe. He went into some detail about the the FTL drive used in this universe. And sounds much like what you propose. The drive (and by extension a force field) was discovered completely by accident and by all previously known means would have never been logicly perused. . Also outstanding SciFi if you interested. Here is a good place to start [Alderson drive][1], harder science behind it is elsewhere. [1]: en-academic.com/dic.nsf/enwiki/262368 $\endgroup$
    – Gillgamesh
    Jun 6, 2022 at 20:00

2 Answers 2

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There's really nothing needed to make a privileged frame consistent with what we currently know - you just have to make it so that the privileged frame is empirically undetectable with any experiment based on our current understanding of physics. (But it could, and probably should, be detectable via whatever new physics allows for the FTL jumps in your setting.)

In fact, there's a pretty strong argument from quantum mechanics that we should be considering the possibility of a privileged frame even from our current understanding of physics: Bell's theorem shows that our universe doesn't actually respect relativistic local causality.

(There is one thing you should be aware of, which is that in general relativistic contexts a preferred frame needs to be replaced by a preferred foliation of spacetime. You can think of a preferred frame in special relativity as a slicing of spacetime into parallel flat 3D hypersurfaces; a preferred foliation is the same thing but the hypersurfaces can be curved.)

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    $\begingroup$ Can you explain the distinction between frame and foliation a bit more, specifically why it is important? I tried searching for an explanation but there doesn't seem to be much accessible information. The mathematical explanations of foliation are quite dense and very much beyond my knowledge base, to the point that I can't even tell if the mathematical sense of the word is the same as the general relativity sense. $\endgroup$
    – Lawton
    May 10, 2022 at 14:03
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    $\begingroup$ Spacetime is 4-dimensional. A foliation divides up spacetime into successive 3-dimensional spaces - effectively assigning a universal "time" coordinate. Locally (i.e. near any given spacetime point) it will just look like a preferred frame; the distinction probably isn't relevant unless you have very involved black hole shenanigans in your story. :) $\endgroup$
    – Matt Dickau
    May 10, 2022 at 19:10
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    $\begingroup$ I think I understand. Part of my idea is the somewhat-common-in-sci-fi restriction that the FTL drive doesn't work close to a gravity well because the curved spacetime throws it off, with bigger gravity wells having larger exclusion zones, so it sounds like the distinction between frame and foliation can be safely glossed over. $\endgroup$
    – Lawton
    May 11, 2022 at 13:23
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    $\begingroup$ I don't think it has to be undetectable with current understanding of physics. It has to be undetectable with current studies of physics, but that doesn't mean we have no way of detecting it, just that we didn't try the method that works. It could be somebody tried the method, got a "That's strange...." result and turned that into a trip to Stockholm. $\endgroup$
    – Loren Pechtel
    May 12, 2022 at 4:38
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There aren't any adjustments needed in order to implement a preferred reference frame. However, there is a problem. You see, our Milky Way is moving towards the Great Attractor (constellation Norma) at 627 km/s, relative to the CMB, and our Solar System is orbiting the center of our galaxy thus moving towards Cygnus at som 250 km/s. As Cygnus and Norma is almost 180 degrees apart in the sky, this gives us a net speed of 368 km/s relative to the CMB. The motion of the Earth around the sun doesn't matter as our orbit is almost perpendicular to the galactic plane. All you need to do is to accelerate towards the Cygnus constellation at 368 km/s until you cancel out your relative velocity with the CMB.

Problem is, you're never going to match the CMB perfectly. Everything is in motion. If the temperature inside your ship is 290 Kelvin, then the air molecules will be vibrating at 500 m/s. The molecules in your body will be vibrating even faster, and not to mention various things inside your body is constantly moving. As you can never match the CMB perfectly, there has to be some range of error allowed. This would cause various things inside the ship to arrive at the destination at different times. Even a tiny difference in time of arrival of different atoms of your ship (I'm talking nanoseconds) would cause your ship to arrive as a jumbled mess.

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  • $\begingroup$ A "preferred frame of reference" is not a capricious choice; it is the frame of reference where equations become simpler. The frame of reference of the CMB, even if there was such a thing, is not a preferred frame of reference for anything in physics-as-we-know-it. $\endgroup$
    – AlexP
    Jun 8, 2022 at 16:47
  • $\begingroup$ @AlexP I recently found the paper "Absolute Motion and Quantum Gravity" by Reginald T. Cahill, which states: "In Ref.[3] it was shown that [...] Michelson interferometers reveal absolute motion when operated in dielectric mode, as indeed experiments had indicated, and analysis [3] of the experimental data using the Múnera [5] review of that data showed that the measured speeds were consistent with each other and together also consistent with the Cosmic Microwave Background (CMB) dipole-fit speed of 369km/s. [...] So the CMB preferred frame is detectable in non-microwave laboratory experiments" $\endgroup$
    – Lawton
    Jun 10, 2022 at 14:09
  • $\begingroup$ @AlexP The citations are [3] R.T. Cahill and K. Kitto, Michelson-Morley Experiments Revisited and the Cosmic Background Radiation Preferred Frame, physics/0205070 (correcting physics/0205065) and [5] H.A. Munera, Aperion 5, No.1-2, 37-54(1998). $\endgroup$
    – Lawton
    Jun 10, 2022 at 14:10

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