# Why is FTL travel impossible if the universe expands FTL?

If the universe is expanding spacetime faster than light (FTL), is FTL travel no longer completely impossible?

Do not care about energy requirements or needing new tech, just if it is NOT physically impossible given this natural, actual observation/phenomena.

• Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer.
– Community Bot
Jan 28 at 12:22
• Possible duplicates: physics.stackexchange.com/q/26549/2451 and links therein. Jan 28 at 12:57
• In the spoken languages I know, "travel" refers only to motion relative to other objects that are either stationary, or travelling at rates different from the rate at which the object in question is travelling. Expansion is inherently different from travelling, because it is different from relative motion, although the same object might, in some situations, do both simultaneously: For instance, I might stretch when waking up in the sleeping car of a moving train. Notions confusing them often involve confusion between Special Relativity & GR. See arxiv.org/abs/astro-ph/0310808. Jan 28 at 18:36
• I haven't posted my comment as an answer because of two other facts: First, time and simultaneity are, as pointed out by Vilenkin, "not well-defined in GR"; second, nothing is not in a gravitational field. As the errors described in the D&L piece are subtle, and as some of them vary with the cosmological model, I'd tend to go with the most complicated answer. Jan 28 at 19:26
• At whatever rate the Universe might expand, it does not travel through itself Feb 2 at 20:57

The expansion of the universe is not measured in units of speed, so it cannot really be compared to c in the first place. Saying that it is faster than the speed of light is “comparing apples and oranges”.

The expansion of the universe is currently about 70 (km/s)/Mpc. It was much larger in the inflationary epoch, but would still have the same units. So even then it does not make sense to compare the inflation rate to the speed of light. There is always a distance where the expansion between two points separated by that distance is less than c.

In contrast, the speed of light is an actual speed. Even on a local scale a light wave travels at c. This is important because in GR only local speeds are physically meaningful. Speeds of things that are not colocated are not even well defined in a curved spacetime.

The speed of a light wave is local, and therefore meaningful, and is c. The expansion of the universe is not a speed and cannot be converted into a local speed other than 0, so it is not meaningful and therefore cannot meaningfully be compared to c.

Now, you asked specifically about superluminal travel. In curved spacetime it is possible for there to be multiple paths through spacetime and for one of them to be shorter than the other such that matter (always traveling slower than light locally) taking the short path can arrive before light taking the long path. These can be arranged into a type of superluminal travel, and are often called wormholes or warp drives. Wormholes and warp drives are permissible according to general relativity, but they require matter with negative energy density. No such matter has ever been found and there is no reason to suppose it exists.

• There's a non negative-energy solution involving a naked singularity, which is just as problematic. Jan 30 at 20:03
• "The expansion of the universe is not measured in units of speed" The Hubble constant isn't measured in units of speed. The expansion between two points is. "They are permissible according to general relativity, but they require matter with negative energy density." You don't need negative energy densities to have two light-like paths between two points to be of different lengths. If you send a beam of light towards a black hole and it's bent back towards you, it's possible to "beat" the light to its destination. Jan 31 at 1:30
• The expansion between two points is not the expansion of the universe.
– Dale
Jan 31 at 1:49
• The expansion between two points is not the expansion of the universe.” That’s just nitpicking on the questioner’s wording, rather than addressing the actual question. The obvious point is, the expansion of the universe implies expansion between two points faster than c and that’s what the question is about. Everything before “This is important because in GR only local speeds are physically meaningful.” reads as if you try to misunderstand the questioner intentionally. Why not just start with “the expansion between two points can be faster than c but that’s no contradiction to GR”? Jan 31 at 14:08
• "That’s just nitpicking on the questioner’s wording, rather than addressing the actual question." No, it isn't about wording, it is about concepts. The expansion of the universe is not a speed and it simply cannot legitimately be compared to a speed. If more people realized that the expansion of the universe is not a speed then variations of this question would not arise so frequently.
– Dale
Jan 31 at 15:17

Compare this to an ant walking on a very stretchy rubber band. The ants walking speed is limited, it can never exceed a certain speed (let's call it $$c$$). Now if we continuously stretch the rubber band it is possible for two points on the rubber band to move apart faster than $$c$$. But any ant moving on the band can still never move faster than $$c$$ with respect to the rubber band.

The rubber band is spacetime. Let's translate this to real life. At any point in spacetime we can assign a light cone, the cone that would be traced out if we send a light pulse in all directions. The future of any observer is always constrained to be inside this light cone. But two points in spacetime that are far apart can in principle move at any speed with respect to each other because space can stretch.

• At first, "stretch" had seemed inaccurate, because it usually means an exchange of some of an object's thickness for an increase in its length &/or width. However, in at least one Nobel winner's view of spatial expansion as "expansion of the universe into itself", such an exchange might be required, and I guess the "scale invariance" of GR is there to accomodate it. Jan 29 at 2:47
• @Edouard I don't know that "stretch" necessarily implies exchanging thickness for length or width. It's perfectly reasonable (as in this answer) to talk about a 1-d object stretching along its dimension. From my experience in engineering education, the idea that stretching an object causes it to contract in other dimensions is actually not common knowledge for students coming into an undergraduate program, and must be explicitly taught.
– RLH
Jan 29 at 18:47
• Thanks for the clarification: I'm surprised that its explicit teaching is part of an engineering program. I'd already indicated my approval of your answer, per the usual PSE policy of allowing the approval of multiple answers (subject to some daily limit) by any one participant. Jan 29 at 18:53
• @RHL from a practical point of view, if I stretch a rubber band and it gets "bigger", it also must get "thinner", otherwise I'm creating matter out of nothing. This shouldn't be that hard to grasp, just something one doesn't really think about until confronted with it, or do I miss something? Jan 31 at 11:56
• @D.Kovács I guess I've been doing too much physics because often I think of stretching in the abstract sense, for example $f(x/3)$ is stretched by a factor 3 with respect to $f(x)$. Jan 31 at 12:29

The limit on speed, the speed of light, affects objects and information within spacetime. It doesn't apply to changes to spacetime itself, which is what causes the universe's own expansion.

The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric (which governs the size and geometry of spacetime itself) that changes in scale. As the spatial part of the universe's spacetime metric increases in scale, objects become more distant from one another at ever-increasing speeds. To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer.

While objects within space cannot travel faster than light, this limitation does not apply to the effects of changes in the metric itself. Therefore objects existing at a great enough distance from a potential observer are receding at a "speed" (in terms of distance/time, not motion) which exceeds even the speed of light, and they cannot be observed (due to the impossibility of a signal ever being able to traverse the ever-increasing distance between), limiting the size of our observable universe.

As an effect of general relativity, the expansion of the universe is different from the expansions and explosions seen in daily life. It is a property of the universe as a whole and occurs throughout the universe, rather than happening just to one part of the universe. Therefore, unlike other expansions and explosions, it cannot be observed from "outside" of it; it is believed that there is no "outside" to observe from.

• This answer seems consistent with the views (that I'd quoted yesterday), regarding the nature of spatial "expansion", that are held by Nobel laureate (in Physics) John C. Mather. They seem like an inversion of the common use of "expansion", but, among languages evolved by creatures living on the surface of a ball, there may be no other way to describe it linguistically, and it does provide an explanation, in the case of any universe or multiverse whose mass/energy might be infinite and/or "eternal to the past", for whatever keeps the night sky from being a sheet of fire. Jan 29 at 19:12
• Coal miners might grasp the concept more easily than I do, but I guess "expansion", like "travel", doesn't necessarily imply any particular direction. Jan 29 at 19:14
• @Edouard - the complication is that from our perspective, we automatically think of the wrong thing expanding. Literally every time something physically expands in everyday experience, we mean the object occupies more space, and it shows that by taking up volume that previously it didn't occupy. Before it was 1m diameter, now its 2m diameter, so it's expanded, and space remained the same - that's our experience of the word....... Jan 29 at 19:44
• I suppose it would be a bit like this: you drive your car from A to B,along a conveniently straight road. Before you set off, you check satnav, A to B is 1000km, car does 100km/h, easy! After an hours drive you recheck satnav. Strangely, A is now 200km behind you, B is now 3000km ahead of you, but the trip computer and speedometer both say you've been doing a steady 100km/h on cruise control the whole time. You drive another 10 hours, convinced you must be there by now, only to find satnav says you've driven 50,000km, got another 1,200,000km to go, and been driving at 100 km/h all the way..... Jan 30 at 8:12
• In fact you drive and drive and drive (magic fuel!) and at the latest check, you've been driving 16 years without a break, travelled 4 billion miles from A, got 29 billion miles to reach B, and yet satnav is still adamant you've done exactly 100km/h all the time, and looking out of the window it certainly looks like you're doing 100km/h now. Everything looks.... ordinary. And yet. You never actually reach B. And that's light and the size of the observable universe......... Jan 30 at 8:12

There's a standard explanation for all "how can this be FTL" questions. Consider a laser beam that you point at a wall a couple light-years away. Ignore diffraction and all that for this Gedanken experiment . Then rotate the laser source and the spot moves along the wall. The rate at which the illuminated position moves is easily much greater than $$c$$. However, no energy and no information is actually travelling at the spot's speed.
Situations can change faster than $$c$$ but not information or energy (or mass, should that need to be said).

• Wouldn't some slight, faint (and perhaps unformulated) uncertainty, as to whether there had been light cast upon the wall or not, have conveyed itself to any spectator observing the wall from a position very near it? Please correct me if I'm wrong, but I had thought the quantum mechanical position on light, as a form of heat, had been that it always conveys information (regardless of how inaccurately that information might be interpreted). Jan 28 at 20:17
• When I'm saying "perhaps unformulated uncertainty", I'm saying that an observer might be left with a vague, misinterpreted impression that their vision had briefly improved. Jan 28 at 20:21
• @Edouard Consider two observers A and B on the wall separated by some distance. Although you can make the dot move from A to B faster than $c$, no information moves from A to B, it moves from you to A and from you to B. There is no way for A to let B know that he saw the dot and have the information arrive faster than $c$. The dot carries no information from A to B. Jan 28 at 21:12
• Yes, that's more exactly what would happen. To help the OP see why it isn't a harbinger that FTL travel's just around the corner, a quainter example would be an old theater's marquee, with rows of bulbs spelling out each letter in its name sequentially: Malfunctions in the timer could result in the illumination extending itself from one letter to the next with a delay coincidentally taking only as long as light itself would've taken to travel between them, but the flying saucer waiting to take us to Andromeda would still remain over the horizon. The information conveyed: A timer's broken. Jan 29 at 3:48
• "Alcubierre drive" is probably the great hope for interstellar travel that might be accomplished within individual lifespans, although the blog ("Backreaction") by Sabine Hossenfelder (a serious physicist, who has collaborated with Lee Smolin) has also had a recent piece about it. Jan 29 at 4:06

According to standard physics, it's not possible to move through space faster than c. With the expansion of space, space isn't moving through space, it's just expanding. In relativity, no matter what local reference frame you're using, the velocity of an object next to you can't be greater than c. But it is possible for an object far away to have its distance from you to be increasing at a rate that is "faster than c". One way of thinking about it is to imagine a bunch of yardsticks between you and another object. It's not possible for the number of meter sticks you pass divided by the time you pass them to be greater than c, but it is possible to cram more meter sticks in between the objects, which means that it is possible for the number of meter sticks between you and your starting point divided by the elapsed time to be greater than c (because more meter sticks were crammed into the space after you passed through it).