# If faster than light travel were possible, how badly would it break physics? **URL:** \ **Category:** Factual Questions\ **Created:** [September 9, 2015, 11:07pm UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475 "2015-09-09T23:07:51Z")\ **Posts on this page:** 9\ **Page:** 4
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [September 17, 2015, 7:09pm UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/61 "2015-09-17T19:09:35Z")
> [@Stranger\_On\_A\_Train](#): > > I’m not certain what you mean. If aliens originating from somewhere else in our universe were able to traverse a distance of space faster than light could travel along the geodesic path, then it would indeed be evidence for superluminal travel of some sort, and would suggest that our understanding of physics, and specifically relativity is incomplete but not invalid for all normally observed phenomena. If aliens came from someplace outside of our universe, or an alternative universe, or somesuch, it would suggest that there are physical phenomena that are outside of any established models of physics whatsoever, and would put the discussion into the range of exotic cosmology that we’re not equipped to address except by wildly speculative suppositions. > > Stranger That’s pretty much exactly what I meant. The aliens might be from our universe but able to travel through others. No matter - them showing up is not going to mean we trash relativity within our universe. And them being able to travel faster than light is going to by definition expand physics into new areas - but not trash physics as we now know it. And of course there is no good reason to speculate until we have some evidence that speculation needs to be done. ---
**Author:** ![Nelson\_Pike](https://avatars.discourse-cdn.com/v4/letter/n/c6cbf5/32.png) [@Nelson\_Pike](https://boards.straightdope.com/u/Nelson_Pike)\ **Post date:** [September 17, 2015, 8:26pm UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/62 "2015-09-17T20:26:19Z")
> [@Asympotically\_fat](#): > > I think the common misnomer that inflation was “superluminal expansion” is that in an inflationary Universe regions are causally connected that aren’t in the non-inflationary equivalent. I.e. compared to a similar non-inflationary model it “appears” that signals have been able to propagate FTL in an inflationary model. > > This is simply because, whilst we still call the region that would be the observable Universe in the non-inflationary equivalent “the observable Universe” when talking about the inflationary model; _using the strict definition of “observable Universe”_, the observable Universe in the inflationary model is in reality much, much larger. > > There is however no real sense I can think of in which it would be correct to say space expanded FTL in inflation. As has been pointed put spatial expansion is measured in units of frequency and superluminal recession velocities within the confines of the observable Universe occur outside the inflationary epoch too. Take it up with: [NASA](http://www.nasa.gov/vision/universe/starsgalaxies/wmap_pol.html) > [@NASA](#): > > Inflation poses that the universe expanded far faster than the speed of light and grew from a subatomic size to a golf-ball size almost instantaneously. This concept, however, was a mere product of calculations done with pencil and paper around 1980. The idea stands on much firmer ground today and with: [The University of Oregon](http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec25.html) > [@University of Oregon](#): > > inflation was effectively at more than the speed of light, but since the expansion was on the geometry of the Universe itself, and not the matter, then there is no violation of special relativity among others. I seem to recall that you are one of those who absolutely must have the last word in every exchange, as though doing so makes you right when you are wrong. I’m tired of this thread, so go for it. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [September 17, 2015, 9:31pm UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/63 "2015-09-17T21:31:05Z")
> [@Nelson\_Pike](#): > > Take it up with: > > [NASA](http://www.nasa.gov/vision/universe/starsgalaxies/wmap_pol.html) > > and with: > > [The University of Oregon](http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec25.html) > > among others. > > I seem to recall that you are one of those who absolutely must have the last word in every exchange, as though doing so makes you right when you are wrong. I’m tired of this thread, so go for it. Is the rudeness really necessary? Do you think it comes across well, especially when what you’re saying is not entirely correct? As I stated it is a common misnomer, so finding examples of the misnomer hardly disproves the point made. The two examples you use is an article by a journalist and a bit of hand-waving by an astronomer in a non-technical lecture. I have explained the reasoning behind what i am saying whereas you have not and the sources you quote do not either. If you won’t believe me, [this paper](http://arxiv.org/abs/astro-ph/0310808) (which I must admit I over-quote) is a very good paper on common misconceptions about cosmological expansion. Note: though they talk about “superluminal expansion”, it is clear from the earlier parts of the paper and the quote itself what is meant is superluminal recessional velocity, which is distance-dependent. > [@Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe](#): > > **3.2 Misconception #2: Inflation causes superluminal expansion of the universe but the normal expansion of the universe > does not** > > Inflation is sometimes described as “superluminal expansion” [App. B: 22–23]. This is misleading because any expansion described by Hubble’s law has superluminal expansion for sufficiently distant objects. Even during inflation, objects within the Hubble sphere (D \< c/H) recede at less than the speed of light, while objects beyond the Hubble sphere (D \> c/H) recede faster than the speed of light. This is identical to the situation during non-inflationary expansion, except the Hubble constant during inflation was much larger than subsequent values. They do go on to offer a sense in which they believe it would be correct to reserve the term “superluminal expansion” for inflation: > [@](#): > > It would be more appropriate to describe inflation as superluminal expansion if all distances down to the Planck length, l[sub]pl[/sub] ∼ 10[sup]−35[/sup]m, were receding faster than the speed of light. Solving D[sub]H[/sub] = c/H = l[sub]pl[/sub] gives H = 10[sup]43[/sup]s[sup]−1[/sup] (inverse Planck time) which is equivalent to H = 10[sup]62[/sup]kms[sup]−1[/sup]Mpc[sup]−1[/sup]. If Hubble’s constant during inflation exceeded this value it would justify describing inflation as “superluminal expansion”. AFAIK though there is no reason to assume that the value of the Hubble parameter during inflation had to reach this value. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [September 22, 2015, 12:45pm UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/64 "2015-09-22T12:45:12Z")
> [@Lemur866](#): > > Note of course that time travel itself lets you travel faster than light. You just get in your spaceship and head for Alpha Centauri at 50% of c. Eight years later you arrive. Then you set the time controls for 7 years and 364 days in the past, and now it took you only a day to travel 4 light years. > > Or you can set the clock for 8 years and 1 day, and now it took you -1 days to travel 4 light years. That’s not really the same thing. I take 4 years to get to Alpha Centauri, 4 years to get back, dial the clock back 8 years to the day after I left, and I’ve just arrived 1 day after I left and made the journey both ways. But I’m also 8 years older. I kinda want my FTL to keep me from spending 8 years for the trip. YMMV. > [@sbunny8](#): > > But you’re only thinking about causality _ **in your own frame of reference** _. > > If you were able to travel to VY Canis Majoris in only a week, then there would be at least one inertial reference frame in which you arrived before you left. According to relativity, all IRFs are equally valid. But you described a situation in where you violated causality in some IRFs but not others. So, either it’s NOT true that all IRFs are equally valid, or we have to live with the fact that an effect can precede its own cause and there goes causality. I think you’re swapping reference frames. > [@filmore](#): > > How fast would a change in the past ripple to the future? For example, today I send a poisoned lollipop 40 years in the past to my room where the kid version of me eats it and dies. Would that change take 40 years to reach today, by which time I’ve aged another 40 years? Or would it instantly affect me? > > Like if I’m traveling down the river in a boat and just as I approach some marker, I throw a bomb backwards which dams the river. It doesn’t yet affect me or the river at the marker. After some time there will be no water at the marker, but by that time I’m further down the river. If I don’t look back, I may never realize anything has changed about the river. I may only notice the change if I actually go upstream and see that there’s no water in the river. > > Or would the change be more instant, where as soon as I threw the bomb all the water in the river downstream from the dam was instantly gone and my boat would fall through the air to the riverbed? > [@Stranger\_On\_A\_Train](#): > > There is no sensible answer to that question which is why physicists like to assume that a rigorous causalilty exists and is enforced (the chronology protection conjecture) [snip]. > > Some (at least in fiction) like to point to the Everett-DeWitt Relative State/Many Worlds interpretation of quantum mechanics as offering the possibility of “branching” into a different universal quantum state, [snip]. Thanks for that. To dumb it down, one concept is that history is defined, so any attempt to change it will necessarily fail in some manner. You were alive to send the lollipop, so you managed to avoid dying from the lollipop in some manner - you never got it, you didn’t eat it, you got quick treatment, etc. The other concept is the “many worlds” interpretation that states that all possibilities that could occur did occur, so the “universe” in which you received the lollipop and died is a different “world” than the one that sent the lollipop. We have as much proof for both of these as we have proof of time travel. Fiction writers can propose whatever they want. Some make the change instantaneous unless the viewer is in a privileged position (inside a TARDIS, wearing a magic ring, whatever), others make it some murky slow process, e.g. “Back to the Future”. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [September 27, 2015, 10:40pm UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/65 "2015-09-27T22:40:01Z")
> [@Asympotically\_fat](#): > > Is the rudeness really necessary? Do you think it comes across well, especially when what you’re saying is not entirely correct? I doubt coming off good is his goal. At this point, it’s clear he feels that you guys have been being rude to him, and that he’s too frustrated to continue trying to continue to discussion. And I can see what he’s talking about. Rather than try to find the way in which he’s right or actually figure out where he’s went wrong, I see a lot of “the question is nonsense.” And then there’s your explanation. You say you explained things. I’m sure that, to a scientist, it seems that way. But, to a layperson like me, it looks like you said “superliminal expansion soon after the big bang isn’t real because it also occurs in the current universe (at large enough distances)” That comes off to me like saying “non-placental mammals never existed because we have the platypus.” The fact that something exists now is more evidence that it existed back then. It can be very frustrating talking with people who are knowledgeable about a subject and thus assume what they are saying makes perfect sense. You ask more questions, and you get told that your questions don’t even make sense. * * * I’m not as frustrated, so I will try to ask more questions. The NASA article mentions that the universe expanded from a very small size to about the size of a golf ball nearly instantaneously. Is this also incorrect? Or did it happen, but somehow this doesn’t require faster than light expansion? And if it is incorrect, why did NASA allow it to be published on their site? Is there a way that it can be seen as true or at least as an approximation of the truth? One thing I’ve noticed about advanced science is that there’s often more than one way to look at things. For example, some would say centrifugal force doesn’t exist, but it does if you view the system in a certain way. Do realize that the only real way most laypeople understand universal expansion is imagining an expanding balloon, where you cannot leave the surface of the balloon. And, in that context, superliminal expansion makes perfect sense to me. You need a high enough expansion rate so that most distant points on the balloon increase that distance faster than the speed of light. Only if the balloon is too small or the expansion rate too slow, it won’t happen. (Or if there is some relativistic effect that prevents it–but everyone seems to act like there isn’t.) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 27, 2015, 11:31pm UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/66 "2015-09-27T23:31:44Z")
What do you mean by “most distant points on the balloon”? What if there _is_ no most distant point? Under the simplest models consistent with current best evidence, the Universe is (and always has been) infinite. And even aside from that, it still doesn’t make inflation special, because even in our current boring humdrum era of the Universe, there are _still_ points sufficiently distant that their recession from us is at a rate greater than c. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [September 28, 2015, 12:18am UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/67 "2015-09-28T00:18:34Z")
> [@Blue\_Blistering\_Barnacle](#): > > The press would report that the radio signals were arriving at Earth about half a day after being transmitted at Pluto. By this reckoning, the probe had already sailed past Pluto before we got the news. > > I wonder if the more correct relativistic viewpoint is that (in our frame of reference) the Pluto flyby AND our receipt of signals of the flyby are simultaneous. Astronomy proposes weird situations for us to find a way to express. When we watch a distant stellar event, like a supernova, we can talk about when we observed the supernova. However, light speed transmission means the actual event occurred long ago, but we couldn’t see it until now. From the perspective of all of us stuck on Earth and never going to be any where else, it makes sense to talk about when we observed it as when it occurred. It’s only when you start having to talk about other observers who aren’t in our location that you need to consider when it occurred versus when we observed it. The Bad Astronomer actually wrote something on this, but I’m not going to go track that down. Anyway, with regards to the Pluto flyby (or the Mars Curiosity Rover landing), you have the folks in Mission Control wanting to know the status and what’s happening, and they get a telemetry feed that plays out in a 1:1 speed ratio of when the events occurred, but it’s tape delayed. Just like recording, say, a football game on the DVR, and then watching it an hour later. You watch at real-time speed, but the game is long over before you get to the end result. (Substitute the news or American Idol or any other live broadcast if football isn’t your thing.) > [@Irishman](#): > > I think you’re swapping reference frames. I will elaborate. If you pick any one reference frame, then you can describe the situation and be consistent, it doesn’t matter which reference frame you use. That is how reference frames are interchangeable. But you can’t go swapping reference frames in the middle of the description. What I am saying is that your explanation that is providing wonky causality is swapping reference frames within the description. > [@BigT](#): > > I doubt coming off good is his goal. At this point, it’s clear he feels that you guys have been being rude to him, and that he’s too frustrated to continue trying to continue to discussion. > > And I can see what he’s talking about. Rather than try to find the way in which he’s right or actually figure out where he’s went wrong, I see a lot of “the question is nonsense.” I think you’re reading a different conversation. And conflating people in the conversation. **Nelson Pike** entered the conversation with declarations, not questions. When told he was wrong, he got defensive and snarky. In fact, **Asympotically fat** tried to give him credit for how his statement wasn’t wrong, but he interpreted it as snark. The “question is nonsense” stuff is all addressed at other posters, and it is not meant as “shut up, you’re stupid”, but rather a statement that the question cannot be expressed in a way that corresponds to reality at all. And, in fact, several posters did try to explain why those statements were nonsense, including **Stranger On A Train** (and me). > [@](#): > > And then there’s your explanation. You say you explained things. I’m sure that, to a scientist, it seems that way. But, to a layperson like me, it looks like you said “superliminal expansion soon after the big bang isn’t real because it also occurs in the current universe (at large enough distances)” That comes off to me like saying “non-placental mammals never existed because we have the platypus.” The fact that something exists now is more evidence that it existed back then. This is a fair criticism. I had to read some of those explanations through multiple times, and would have asked similar followup questions. > [@](#): > > It can be very frustrating talking with people who are knowledgeable about a subject and thus assume what they are saying makes perfect sense. You ask more questions, and you get told that your questions don’t even make sense. That’s not what happened. If fact, Quercus even said > [@](#): > > I’m not sure if this is non-specialist enough, without doing half of Physics 101. If there’s anything here confusing, feel free to ask. **Nelson Pike** ’s frustration is created a bit by specialist technical answers not dumbed down enough, but mostly by his attitude that he is right and the ones giving the technical answers must be wrong. I’m not posting this to tear apart **Nelson Pike** , I’m doing it to explain how you are misreading the conversation. * * * > [@](#): > > I’m not as frustrated, so I will try to ask more questions. The NASA article mentions that the universe expanded from a very small size to about the size of a golf ball nearly instantaneously. > > Is this also incorrect? Or did it happen, but somehow this doesn’t require faster than light expansion? It did happen, but the expansion wasn’t actually faster than the speed of light. Let me try a different explanation. Take two points. Send light from point A to point B. If the space expanded point B away from point A faster than light could travel from A to B, then that is being called “superluminal expansion”. What the technical description is saying is that the Hubble parameter talks about expansion, and the further away you look, the faster that expansion seems to be going. Look far enough, the expansion will appear superluminal. That is what happened during inflation. The universe was sufficiently small that the distance for the Hubble expansion to appear superluminal was a lot smaller than now. Tiny, in fact. A “true superluminal expansion” would be expansion greater than light down to Planck lengths. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [September 28, 2015, 12:21am UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/68 "2015-09-28T00:21:47Z")
> [@Chronos](#): > > What do you mean by “most distant points on the balloon”? What if there _is_ no most distant point? Under the simplest models consistent with current best evidence, the Universe is (and always has been) infinite. I think he just means really far points versus near points, not some objective end. But he could be mislead by the balloon analogy, because it wraps in 3D space, and many of us get stuck with the 3D wrap. ---
**Author:** ![Itself](https://avatars.discourse-cdn.com/v4/letter/i/d07c76/32.png) [@Itself](https://boards.straightdope.com/u/Itself)\ **Post date:** [September 28, 2015, 1:04pm UTC](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475/69 "2015-09-28T13:04:39Z")
> [@eschereal](#): > > Ultimately, Special Relativity is based on observation. The _only_ means we have to observe _anything_ comes down to EM, which propagates at c (in a vacuum). Aside from astronomical events, we also have the more indirect evidence of cosmic rays. The mean lifetime of a muon, for example, is on the order of 10[sup]-6[/sup] s. Yet we readily detect them on earth because of relativistic time dilation (or, equivalently, the length contraction in the frame of the muon) . (I’ll also mention here that special relativity is quite compatible with, and is indeed obligatory, in particle physics.) [Previous page](https://boards.straightdope.com/t/if-faster-than-light-travel-were-possible-how-badly-would-it-break-physics/730475.md?page=3)