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Is Faster-Than-Light Travel or Communication Possible? (1997)
- pippy 12y agoOne way would be to fire an entangled photon down both ends of fibre optic cable in the middle. On the sending end you could observe/ ignore it, and on reviving end you'd observe it. We'd have to develop cables capable of preserving the entangled state of light particles, and a beam splitter that's more reliable. You could improve the bandwidth considerably by measuring the polarisation of the particle, even more if you could somehow encapsulate the entangled photon for a given period of time.
- tim333 12y agoStill wouldn't be much use for communicating. The observer would just see random photons.
- sgt101 12y agoPretty harsh to downvote this person for writing out an idea - ok, if you know the physics you might dismiss this, but dismiss with words not votes!
- Retra 12y agoThis is a very common idea. It is so common, in fact, that the only real way to not know why it is wrong is if you've never heard of entanglement or relativity before. I personally find it a bit insulting to propose something like this, as it assumes all those physicists working on quantum physics and relativity over the last hundred years were too dumb to think of this. Yet they all go around saying FTL communication is impossible? To suggest such a thing, you'd have to think they were all very stupid or you'd have to not be thinking at all.
- simias 12y agoTFA addresses this point directly so one can assume the parent didn't read the article before posting.
- gwern 12y ago> In 1935 Einstein, Podolsky, and Rosen published a thought experiment that seemed to produce a paradox in quantum mechanics, as well as demonstrating that it was incomplete. Their argument used the fact that there can be an apparent instantaneous interaction in the measurement of two separated particles that have been prepared in a certain "entangled" manner. Einstein called it "spooky action at a distance". It has been shown by Eberhard that no information can be passed using this effect; so there is no FTL communication, but the paradox is still very controversial. See the FAQ article The EPR Paradox and Bell's Inequality for more details.
- xg15 12y agoYou couldn't use this for communication as there is no way to "set" the spin of an entangled photon. You'd effectively have two receivers for the same random signal but no way to actually influence the signal. I think there was a HN link a while ago about ideas to use this as a perfectly secure key exchange mechanism for crypto. I don't know how serious this idea is, though.
- placeybordeaux 12y agoI believe OP was thinking that there was a way to tell if the photon had already collapsed it's state once before you collapse it by observing it. If that was the case you could convey information by collapsing the state or not. However as far as I know there is no information about if the photo is in a superposition just prior to observing or not.
- placeybordeaux 12y agoIf this worked you could cut down the travel time by doing this multiple times with the same distance between each node. However because when you observe the photon it collapses randomly and there is no way to tell if the photon has collapsed before you observed it or for the first time as you are observing it you don't actually have any information on if the other side observed it or not.
- placeybordeaux 12y agoIf this worked you could cut down the travel time by doing this multiple times with the same distance between each node. However because when you observe the photon it collapses randomly and there is no way to tell if the photon has collapsed before you observed it or for the first time as you are observing it you don't actually have any information on if the other side observed it or not.
- rwmj 12y agoIs even near (but slower than) light travel possible? Doesn't your spaceship get destroyed the first time you hit some interstellar dust?
- adrianN 12y agoYou want a big block of ice in front of your ship.
- rwmj 12y agoOut of interest, why ice and not (say) steel?
- adrianN 12y agoWater is really good at blocking proton radiation (those stray hydrogen atoms in the interstellar medium become really dangerous once you go fast enough). Plus you can drink it. You also don't want to stop particle radiation too quickly, or you get x-ray Bremsstrahlung. So something a little less dense than steel works better in practice.
- Lerc 12y agoBut it's full of dust. Yuck.
- imglorp 12y ago... yeah, and water your greenhouses (you're going to be in space a very long time), split it to generate O2 for breathing, etc. It's also probably a lot easier to find water to replenish your shield along the way, or at your destination, than steel or lead.
- foo2312 12y agoOne thing which might be of (tangential) interest here is the novel The Songs of Distant Earth, by Arthur C. Clarke, where the replenishment of this sort of ice buffer plays a prominent role in the plot.
- gabriel34 12y agoA negative to this is a proposed solution to Fermi's Paradox
- DennisP 12y agoOur galaxy is only 100K lightyears across. A civilization that attained only 10% lightspeed could populate the entire galaxy in a million years.
- Retra 12y agoThat's still ~500x faster than any man-made object has ever moved.
- spacehome 12y agoWell, as long as we're fantasizing, make the speed 0.00001c and a billion years.
- DennisP 12y agoI don't think there's any reason to think that we've reached any kind of serious limit at the moment, or that a technical civilization, say, 10,000 years older wouldn't be much more capable. There's certainly no fundamental physics in the way of that sort of velocity.
- jljljl 12y agoWhile true, Isn't this more an "engineering problem" barrier (like the sound barrier) instead of a "theoretically impossible" barrier?
- Retra 12y agoThere may be an engineering barrier below c when you account for fuel efficiency. Once you start requiring large volumes of nearly impossibly efficient (thrust-weight) fuels, then you will be stuck, even if you aren't anywhere near the speed of light. It depends how much material you are transporting. If you are sending a few photons, the speed of light it no barrier at all. But if you are sending a crew of humans...
- cygx 12y agoPoint 19 seems to conflate quantum fields with imaginary mass and superluminal particles. As far as I know, tachyonic quantum fields and tachyonic 'particles' are unrelated except for an unfortunate naming convention (historically, tachyonic fields were thought to result in tachyonic particles; the name was kept around even after the mistake had been realized). Note that tachyonic particles are not very particle-like at all, so I prefer the name 'tachyonic interaction': Tachyons cannot be properly localized, there's a critical frame where they have no energy, their 'life-time' is space-like, both endpoints of the interaction may have the same (intrinsic) type (absorption-absorption, emission-emission), to name just some of the peculiar properties. Recami argues (imo convincingly) that you cannot use such tachyons to violate causality. Point 13 is also somewhat questionable, but this is more of a matter of semantics. While each of the galaxies are at rest (or in free fall, if you prefer), they are not at rest relative to one another according to parallel transport along the light ray (otherwise, there would be no red-shift). This is imo the correct way to handle relative velocities in a general-relativistic setting; the fact that their comoving distance is constant doesn't mean they are at rest in any physically meaningful way, but one can at least argue this point both ways.
- scobar 12y agoA thought I've had about this is that perhaps the speed limit within a universe (299,792,458 m/s in ours) is relative to a constant at its Big Bang moment such as total energy contained or maximum rate of expansion of space-time. If the smallest particles of our universe are smaller universes and our universe is a tiny particle within a larger universe, then the speed limit within each could be different. This theory would allow FTL travel in a larger universe relative to the speed limit of our universe, but not FTL communication. FTL travel within one universe may be the escape velocity required to enter the next larger universe outside of the origin universe's boundary. But a transmission, originally traveling FTL, sent into a smaller universe may be slowed by its speed limit (similar to light propagating through a non-vacuum medium). In a universe expanding like ours, the transmission would never reach the destination.
- spacehome 12y ago> But multiverses are entirely out of keeping with the Ockham's Razor approach to doing science, and constitute more of a popular interpretation of quantum mechanics than a serious physical theory. The multiverse is not a theory - it's a logical consequence of a simple theory. This claim is logically akin to claiming that the universe is identical to the observable universe, because to propose that matter exists outside of direct observation is a violation of Ockam's Razor.
- dnautics 12y agoremember, though that ockham's razor is not an axiom, it's a heuristic.
- spacehome 12y agoYea, but I feel it's a good heuristic, and I'd like to hold onto it. It seems like a crude, non-mathematical formulation of Solomonoff theory. Also, Ockham's razor is the best argument in support of the Multiverse view.
- tjradcliffe 12y agoOckham's razor is a terrible heuristic, and one of the reasons the Multiverse view gets so much skepticism is that Ockham's razor is the best argument for it. Consider: Ockham himself believed that "not multiplying entities beyond necessity" was an argument for the existence of God and an argument against the existence of everything else. That is, once you had an omnipotent god, everything else was just a product of that god's imagination. This idea wasn't necessarily original to Ockham, but he was one of it's more notable proponents. Furthermore, "simplicity" is an extremely wobbly concept. To a circularist, elliptical orbits are horribly complex, as they have to be approximated by an infinite series of epicycles. Acknowledge ellipses as primary and they become simple: adding just one parameter over circles. There are of course fairly objective measures of complexity (Shannon entropy and Kolmogorov complexity) but neither of them apply very nicely to the kind of conceptual arguments where Ockham's razor is typically introduce. Finally, we know that there is exactly one consistent way to update our beliefs in the face of new evidence: Bayes' rule. Since it is very simple, we don't really need to invoke any additional heuristics like Ockham's razor. And we find, in fact, that in the few cases where Ockham's razor makes sense it is nothing but a consequence of Bayes' rule, and can be expressed as follows: In cases where two causes would give rise to the same evidence with roughly equal probability, and one cause has a much higher prior plausibility, the presence of the evidence means that cause is the more likely of the two. That is, "When you hear hoofbeats think horses not zebras" is a good idea in most cases because the prior plausibility of "There are horses around" is a whole lot higher in most cases than that of "There are zebras around". The presence of hoofbeats raises the plausibility of both propositions by exactly the same amount, but the ratio between them remains the same, so the evidence is most likely evidence of horses, not zebras.
- Ono-Sendai 12y agore: point 18: It is quite possible to get around this paradox, if you have absolute time. Absolute time means you can't send a message back in absolute time, which means you can't get 'causal loops'.
- pdonis 12y ago> It is quite possible to get around this paradox, if you have absolute time Which, according to our best current theories, we don't.
- Ono-Sendai 12y agoWell, it's important to note that special relativity doesn't say there is no absolute time, but rather that if there is, it is undetectable in any inertial reference frames.
- pdonis 12y ago> special relativity doesn't say there is no absolute time, but rather that if there is, it is undetectable in any inertial reference frames. No, SR says there is no absolute time, because such a thing is undetectable in any inertial reference frame. The theory that makes all the same predictions as SR, but also says that there is such a thing as "absolute time", but it's undetectable in any inertial reference frame (so it doesn't change any experimental predictions), is Lorentz Ether Theory (LET), not SR.
- Ono-Sendai 12y agoMy understanding is that SR doesn't go quite as far as saying 'there is no absolute time', rather it dismisses the concept as undetectable and unnecessary (and useless). In other words, it doesn't disprove absolute time, it just dispenses with it.
- pdonis 12y ago"Absolute time" is supposed to be a physical concept, right? If it is, then showing that it is undetectable and unnecessary is "disproving" it. And the post of yours that I originally responded to certainly seemed to indicate that it was a physical concept--you were saying that if there is absolute time, it can restrict what kinds of FTL travel are possible, which is a physical restriction. If "absolute time" is just a philosophical concept with no physical consequences, then no physical theory, SR or otherwise, is going to have anything to say about it anyway. But then, it can't be used to resolve the "paradox" you were claiming it could resolve, because it can't restrict what kinds of FTL travel are possible.
- Udo 12y agoThe consensus interpretation is that FTL is not possible in normal space, since the speed of something is just another way of looking at how "fast" it travels through time - FTL becomes impossible by definition. However, there is reason to assume a lot of things may be going on that are not strictly speaking taking place in normal flat space. The article mentions the expansion of the universe as an example, and that's actually a good point to start. By virtue of the universe expanding, the distance between points grows faster than the speed of light as long as those points are sufficiently far apart. This becomes more interesting when you realize that new space in between these points is being created. Sure, the galaxies traveling on this expanding medium are not actually speeding apart powered by classical movement, but they are nevertheless changing position relative to one another in an interesting way. In fact, what we're interested in when we talk about FTL is not classical movement at all - what we want is to change positions instead. There are places in nature where we know this is happening right now, for example we do know space can "flow" faster than the speed of light on the far side of event horizons. So if we abandon the idea that we need to accelerate towards a destination, FTL becomes a broader idea of manipulating spacetime in order to be somewhere in less time than if we had actually flown there at the speed of light. There exist proposals for propulsion systems that go in this direction, for example the "warp drive" ideas that cheat by simply shortening the amount of space a vessel has to travel through. Some of these concepts seem to work in theory, for some definition of work. The trouble with all these ideas is they need impractical amounts of energy. Which is wasteful, since we only need to bend spacetime for a short while and then return it to its original state behind us, something that works analogous to a catalyst would be much better. At the most fundamental level though, what we are missing is a way of hacking spacetime. Huge masses and energies are how nature bends it, but ideally we would need some other way of manipulating the fabric of existence directly. This is the reason why drive systems in science fiction all have some kind of yet-unknown mystery substance critical to their function. Given how little we actually know about the basic fabric of the universe, ways of doing these manipulations may well exist. They might not. We most likely won't find out for centuries, if ever. Until then, we must answer the question so that yes, FTL is most likely possible in theory, but it's not even remotely within our grasp right now. It might never work in practice. Another observation worth considering: cosmic travel times seem utterly problematic right now because we individually live for extremely short periods of time. Spending a few thousand years in transit is much less worrisome if you're, say, an autonomous machine carrying an AI through the galaxy.
- lcfcjs 12y agoReading Item 20. Worm Holes, after watching Interstellar, is really intriguing and thought-provoking
- aaron695 12y ago> Is Faster-Than-Light Travel or Communication Possible? No. Pretty simple stuff. No, it is not. No matter how many sci fi movies tell you other wise. It is not. Every experiment that comes out saying different is either lying/incompetent or the news reporting on it is lying/incompetent. Just because you want it to be true does not make it either true or possible.
- fixedd 12y agoIt's also impossible to talk to someone miles away... Oh, electricity! It's also impossible to travel 30mph... Oh, Petroleum! It's also impossible to go to the moon... Oh, RP-1/LOX! -- Stop pretending we know everything about the universe when we clearly do not. The best you can claim is "No... with current understanding".