Point 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.
> 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.
A better response is that it's completely in keeping with Ockham's Razor, just in terms of number of rules instead of number of things.
For example, in an expanding universe we expect lots of galaxies that we see now to end up beyond the observable horizon in the future. Should we assume they stop existing when they cross that horizon (minimizing # of things) or should we avoid adding an extra rule that removes things that cross out of our horizon?
Many worlds is just quantum physics without collapse. It replaces a rule with a whole lot of things. It is both simpler and more complex than Copenhagen, depending on how you weigh simplicity.
No, it's a necessary consequence of the fact that the observable macrostates we care about can be composed of many, many ontologically-basic microstates in any reductionist ontology. If you live in a world where large things are composed of small things that run according to fixed rules, with causality being more fundamental the further into the small you go, then any observable "question" will have low Kolmogorov complexity compared to the total information necessary to write down the exact micro-state of the system you're asking the question about, which means simpler "questions" or "concepts" will capture larger portions of the fundamental particles' configuration-space.
Ockham'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.
I agree with some of what you said, but Bayes' Rule by itself will never give you priors. Regardless of it's unfortunate origins, the way that Ockham's razor is used now in common language is a way to give approximate qualitative priors.
Also, if you don't have enough mathematics to understand ellipses, then ellipses are quite complicated. I'm comfortable with ellipse-based theories needing additional evidence in this case.
> Is even near (but slower than) light travel possible?
Sure; send yourself as data. ;) Just don't use comcast.
On a less flippant note, in addition to the interstellar dust issue, another fun engineering problem is providing the energy necessary. If you want to send a 50 kilogram person to .99c, you'll have to provide about 4 months worth of the United States current energy production (7 quadrillion btu per month or about 7 exajoules)[1].
Now, if you are wiling to increase your flight time by a factor of ten (reducing yourself to .1 c), you can cut this down to merely 15 days worth of California's total consumption[2]. Mind you, that's just the poor 50 kilogram astronaut. The space shuttle weighed in at 75,000 kilograms (empty). So plan on using about 9 months of the United States energy production capacity for your dinky .1 c.
Given that you'll probably want to STOP once you get where you're going, antimatter engines are probably a prerequisite for relativistic space travel.
Water 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.
... 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.
One 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.
One 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.
> 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.
This 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.
You 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.
I 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.
If 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.
A 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.
The 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.
We in the west are a bit too obsessed with moving fast.
What do we call a common trick in fantasy and SF that allows someone or something to move large distances in a short time? Teleportation. Spacetime stays the same, you just move through it very fast, whether physically or electronically.
By contrast, fantasy tales in China, Japan, and Korea, both modern and traditional, often show the protagonists practicing shukuchi [1], a trick that literally means "reduced earth" or "compressed land". Space is shrunk, and then you move through the shrunk space at a normal speed. It's basically warp drive, but imagined at least a thousand years before Star Trek. The guru's eyelashes don't even blow in the wind because, after all, he's just walking at his usual pace.
A change of paradigm indeed opens the door to a lot more possibilities, at least in theory if not in practice.
Quite a lot of our Sci-Fi's "FTL drives" are actually "Jump Drives". Take, for instance, Battlestar Glactica. They don't move through space, they jump to a new place within it.
> The consensus interpretation is that FTL is not possible in normal space
I don't think this really makes sense, because "normal space" is not a well-defined term.
What our current relativistic theories say is that spacetime contains a causal structure marked out by light cones, and nothing can move outside the light cones. In certain spacetimes (like the one that describes our expanding universe), it is possible to assign coordinates to events so that it looks like objects are moving apart "faster than light", but when you look at the light cones in those same coordinates, you can confirm that the objects are still moving inside the light cones.
> new space in between these points is being created
This isn't really part of the physics of the expanding universe; it's an interpretation that some people put on it. There's nothing in the actual math that corresponds to "new space being created".
> space can "flow" faster than the speed of light on the far side of event horizons
This isn't really part of the physics either; it's an interpretation (part of what's often called the "river model" of black holes) that can help with visualization, but again there's nothing in the actual math that corresponds to "space flowing faster than light".
> ways of doing these manipulations may well exist
According to our current theories, any such manipulations would require what is called "exotic matter". There are good physical reasons to think that it is not possible to create exotic matter, especially in the quantities we would need to "hack spacetime" (nice phrase!) on the scales we would want to do it. Given that, I don't think it's fair to say that "FTL is most likely possible in theory"; I would say it's most likely not possible on our best current theories.
> I don't think this really makes sense, because "normal space" is not a well-defined term.
You are of course technically correct.
> There's nothing in the actual math that corresponds to "new space being created".
True, and this might end up being a horrible crutch, but right now it helps me to conceptualize what's (possibly) going on. I'm sorry for having exported this hypothesis into a context where it shouldn't have been said without additional qualifiers, but it's too late now to edit it out.
> This isn't really part of the physics either; it's an interpretation
That's actually one I stand by. In the end, this seems really more about how you choose to express ideas in a language that doesn't have any exact words for things going on in an object that lies far outside of our normal experience. You might (and I suspect you do) choose to not say anything at all and let equations speak for themselves, but that doesn't really work for most people. So you're stuck with choosing lenses that render this foreign reality on a canvas you can actually picture. I also think there is nothing wrong in principle about choosing this particular representation.
> I would say it's most likely not possible on our best current theories.
Well, we know spacetime is not rigid. We know that interesting dynamics exist at the extreme limits of physical values. We also know that there is a large chunk yet to be discovered about the nature of the substrate on which reality rests. You are completely within your rights to look at everything we know and have ruled out so far and proclaim a high likelihood of impossibility, but along the same line of thought I reserve the right to declare the glass half full ;)
The truth is that neither of us are likely to live to see a definite answer play out.
If you mean, you like using it as an interpretation, no problem; a lot of people do.
If you mean you think it's more than an interpretation, then you're going to have to show me explicitly where in the math the "flow of space" is represented.
Yes, I'm using it as a mental representation of a process which I can't picture directly. How do you deal with such things? I mean aren't you ever tempted to use a tool like that? I don't think there is anything wrong with the idea.
I'm having some trouble picturing how it could be more than an interpretation.
Whenever two objects are in relative motion to one another, they each perceive a time dilation taking place at the other object. The flow of time is directly linked to the relative speed of an object. On an object that moves at the speed of light, time stands still. For more information on the mathematics, I refer you to the Lorentz transformation (http://en.wikipedia.org/wiki/Lorentz_transformation), but it's actually enough if you imagine that increases in speed are causing a decrease in the flow of time, the limit of that process has to be the velocity at which the flow of time is zero.
Ok. When you say 'how fast an object moves through time', you mean 'how fast time passes for a moving object, as seen from the inertial reference frame of another object'.
I regard objects as moving at the same speed through time - 1 second per second.
It's of course a matter of taste what expression sounds more fitting to you, precisely because time is such a fluid concept.
Personally, when I think about a neutrino for example, I'd say that neutrinos go crazy fast, and they experience time unimaginably slowly, so it feels more natural for me to think that velocity and time are tradeoff values of each other.
> Whenever two objects are in relative motion to one another, they each perceive a time dilation taking place at the other object.
This is true.
>The flow of time is directly linked to the relative speed of an object.
I'm not sure this is even a coherent statement. "Flow of time" along what?
Consider for comparison: what would the "Flow of latitude" refer to on a planet's surface? Any coherent statement about the "Flow of time" should also make sense when applied to the "Flow of latitude", and I can't think of any way of conceptualizing the "flow of latitude" that makes sense.
Maybe I'm not understanding what you mean by "flow", but in relativity dimensions do not "flow" in any conventional sense, and the notion that "increases in speed are causing a decrease in the flow of time" makes no sense: to an observer on an object clocks continue to run at a constant rate, and the whole point of relativity is that observers on objects are just as good as observers anywhere else.
The laws of physics are the same for everyone (this is the principle of relativity) so since an observer on an object does not observe co-moving clocks slowing down, any talk of "a decrease in the flow of time" requires a violation of the principle of relativity.
> but in relativity dimensions do not "flow" in any conventional sense
That is not really the point. You are concentrating on the word flow, and maybe I chose that foolishly, so bear in mind that I'm of course not a native speaker.
If we discard for a moment things that move at the speed of light, that leaves us with objects for which time does not stand still. Whether or not you agree with the word "flow", is it really that objectionable to characterize the experience of time as a movement through a fourth dimension?
My entire point in this regard was really that relative velocity and time dilation are linked. Which they are. It's right there in the math. Maybe my mistake was to use "flow" conversationally, but if you could see past that it would probably a more productive discussion for both of us.
> "a decrease in the flow of time" requires a violation of the principle of relativity
You misunderstood what I was trying to say, in a colossal way. I apologize for not being clearer. At no point was I suggesting a global "flow of time". In fact, I made a special effort to talk about individual reference frames.
It is mentioned in the linked article. Since speed is always relative (to a frame of reference), Special relativity by definition proposes that, nothing can go faster than C. The article gives an example, if a spaceship is traveling at 0.6c, and another spaceship is traveling at 0.6c - away from first first ship. Their relative velocities are not 1.2c, but 0.8c (http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity...)
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.
Interesting. Especially if the AI can learn and evolve along the way.
>Another observation worth considering: cosmic travel times seem utterly problematic right now because we individually live for extremely short periods of time.
Doesn't relativity "solve" this this problem if we can reach near lightspeed, and don't mind it being a one way trip?
Sort of. While being on the move at extreme speeds you would take advantage of the local slow-down you're experiencing. Of course, a lot of time will still pass in the meantime both at the origin and the destination locations. I'd argue for the traveler it's subjectively not much different from being in a state of stasis while going at sub-relativistic speeds for thousands of years.
No, there is still an lower limit of 4.37 years to get to alpha-centauri within any frame of reference no matter how much you accelerate within conventional space.
What relativity does, is that if you travel close to speed of light time taken for the trip as measured on ship versus on earth could be different by many orders of magnitude.
edit: Was wrong, as per [0] "This is something many readers don't understand well, so it bears repeating: The journey times as experienced by those on the ship are not limited by the speed of light. Instead what they experience is the planetary reference frame getting relativistic."
That's not true. From the perspective of the travelers space will appear to contract dramatically - though the speed of light remains constant, the distance traveled is reduced.
Your equation is correct in a Newtonian universe, but does not consider relativistic effects.
Generally when we talk about an object's acceleration, we are talking about its acceleration in its own reference frame. By definition, an object's velocity in its own reference frame is always 0, so acceleration does not become more difficult (ignoring the fact that the surrounding matter will be impacting you at higher speeds). From the perspective of an outside observer who is not accelerating, time would appear to slow down on the accelerating object, and the outside observer would record a decreasing acceleration.
2) Even though you don't observe yourself moving at increasing speeds, you do observe the rest of the universe moving at increasing speeds. This means that small particles impacting your ship will hit it with increasing energy.
I 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.
It's in the same order of magnitude as the Project Orion speed estimates, based on present-day technology - it ought to be possible if the political will was there.
There 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...
re: 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'.
Well, 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.
> 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.
My 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.
"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.
Well, the overall point is:
If FTL communication is possible, then you get grandfather type paradoxes, unless you have an absolute time (or it is resolved some other way). And SR does not rule out absolute time, it just discards it.
If absolute time has physical effects, which it must if it can prevent FTL grandfather paradoxes, then yes, SR does rule it out. SR is Lorentz invariant, and absolute time preventing FTL grandfather paradoxes would break Lorentz invariance.
That doesn't mean you couldn't construct a theory that included absolute time and in which FTL travel was allowed but grandfather paradoxes were prevented. But such a theory would not be consistent with SR. To be viable, such a theory would have to match the predictions of SR in the domain in which those predictions have been verified, but it would necessarily make different predictions from SR about FTL travel. (No such theory exists that I'm aware of; I've seen hand-waving about it in discussions of LET, but never any concrete theory.)
My personal prediction would be that such a theory would end up being falsified (either that or rendered meaningless by experiments showing that no type of FTL travel is possible at all). But that's a separate question.
You're right, it would violate Lorentz invariance, but only for FTL travel. In that sense it is consistent with SR for speeds <= c.
I think it's quite easy to make such a theory, I'll get around to writing a blog post on it some day :)
The main result is that for communication (or travel) faster than c, invariance is broken, and it becomes possible to distinguish the absolute reference frame.
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.