Is Faster-Than-Light Travel or Communication Possible? (1997)
math.ucr.edu
math.ucr.edu
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.
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.
Also, Ockham's razor is the best argument in support of the Multiverse view.
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.
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.
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.
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.
On a side note, I was surprised at his attempt at a romance novel. I was pretty good, but too long.
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.
I think I'd prefer micro-blackholes as a source of energy. Seems um..."safer" than matter-antimatter, and (probably) a lot cheaper, too.
http://io9.com/5391989/a-black-hole-engine-that-could-power-...
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.
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.
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.
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.
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.
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.
In what sense do you mean that? Can you elaborate?
I regard objects as moving at the same speed through time - 1 second per second.
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.
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.
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.
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.
Doesn't relativity "solve" this this problem if we can reach near lightspeed, and don't mind it being a one way trip?
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."
[0] http://en.wikipedia.org/wiki/Space_travel_using_constant_acc...
For example, if you have a ship capable of continuous acceleration at 1g, you can reach any point in the universe on the order of ~50 years.
it would take a year to get to the speed of light at 1g (if my equation was right.) Then you can't speed up anymore.
It isn't. Wolfram Alpha apparently doesn't know about the relativistic rocket equation. Try here:
http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.h...
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.
Jokes aside, there's no fuel source we know of that would allow a ship to accelerate continuously for 50 years at 1g.
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.
After two years: 89.9%
After four years: 97.2%
After eight years: 99.3%
[1] https://www.google.com/search?q=distance+to+alpha+centauri
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.
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.
I'm having some trouble picturing how it could be more than an interpretation.
Fun read:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2011001...
Interesting. Especially if the AI can learn and evolve along the way.
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...
Which, according to our best current theories, we don't.
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.
In other words, it doesn't disprove absolute time, it just dispenses with it.
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.
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.
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.
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.
It's also impossible to travel 30mph... Oh, Petroleum!
It's also impossible to go to the moon... Oh, RP-1/LOX!
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Stop pretending we know everything about the universe when we clearly do not. The best you can claim is "No... with current understanding".