They create a mathematical model for Von Neuman self-replicating probes expansion in the galaxy
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We now have a bacteria/nanobot like growth rate doubling the numbers every 15 minutes!
What we are all forgetting here is that self-replicating machines (especially in swarms) cannot replicate exponentially (or even at the speed a lot of people think they can). There is a hard limit to how fast you can do something without causing so much friction that you just explode. Complexity as you might find in such nano-swarms will require a fair bit of energy to replicate. Complexity also will cause the machine to take longer to self-replicate, unless they replicate faster, which would then make them explode again. But the idea of self-replicating swarms is very complex and more then we are asking for here. Suffice to say it is not possible to colonize the entire universe (I know it was an exaggeration An'shur
) with them. There are extremely intricate engineering hurdles to overcome even if they are tasked with just traveling around a few light-years and terra-forming and/or exploring and passively replicating. But we were talking about FTL here, so to continue:
For the sake of argument, FTL is
possible. You just need to be inventive about it and read between the lines of your physics textbooks. The Alcubierre drive is proof of this. Its dynamics do not violate general relativity. Yes there are those 'minor' complications like requiring negative mass and so on, but certain designs can minimize the requirement
. My point here is that the laws of physics do not say you cannot travel faster then light. The term itself is a misnomer anyway. It is the maximum speed at which massless particles (including photons, but also gluons and theoretical gravitons) can travel to maintain electromagnetic casualty between points in the universe as determined by the Lorentz Transformation
in relation to the famous Maxwell equations of field electromagnetism
. I would suggest to anyone wanting to understand this type of things to learn more about the work of Lorentz, Maxwell and other pre-Einsteinian physicists (maybe not you guys on this forum, you seem to know a lot about this science
Naturally, to go faster then other matter, you must have negative matter or negative mass. No Mass (or neutral mass) = limit of casualty, and (positive) mass = impediment to motion. It stands to reason that negative mass is the opposite of positive mass and makes things go faster then mass-less matter, which acts as a speedometer in this case. Essentially the only thing standing in our way of building a Alcubierre warp-ship is this negative matter. For the moment we will ignore the other complications that will arise as a result of FTL travel like paradoxical phenomena. We can`t find negative matter, don`t know how nature could make it now and we cannot manufacture it like we can heavy elements.
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2) FTL civilizations don't exist. Less advanced civilizations don't get past the great filter, succumbing to the internal struggles we face on Earth today. If life in basic principles works the same as on our piece of rock (survival of the fittest), then I am afraid that extraterrestrial life would be territorial, power-mongering and domineering, both internally and externally to other aliens. If FTL was as easy as to allow aliens to spread faster than evolution could rid them of the aforementioned animal traits, they would just bring the problems of their home world out into space with them.
3) Which brings us to another possible solution. There may be countless FTL-capable civilizations, which wage wars of hardly imaginable proportions among each other, throwing death stars at one another as if they were merely cannon fodder. This would definitely slow their expansion rate, since it would not be safe to spread and no aliens would be able to reign across the universe.
5) Universe may be full of life and chaotic, but we don't matter, so they ignore us, which goes hand in hand with point 4. We are unable to communicate with them and vice versa.
These are probably the most likely, just based off of what we know about our universe. I omitted point 4 because it is only partially correct. I said earlier that we would ignore the actual effects of FTL travel. Now I would like to address it here in relation to your fourth point.
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4) The universe is radio-silent because if civilizations were capable of FTL travel, they would most certainly be capable of FTL communication. EM spectrum would be ridiculously ineffective and slow as means of communication.
That's how it is in Star Trek, they use extra-dimensional space to communicate and if you have FTL travel you can probably crack FTL communication.
There was a paper I read before about sending compressed FTL messages in pulses through distorted space, will have to see if I can locate that.
As mentioned earlier, the speed of light is actually the maximum speed at which points in the universe can `communicate` (i.e. maintain casualty) to each other, and be seen communicating to one another by any other point in the same universe. As soon as one area of the universe (say, a space ship) transcends that speed (going faster then mass-less matter), it starts to `communicate` backwards in time relative to it`s origin and it`s relative speed. If the space-ship travels twice the speed of light (2c
), to a destination 2 light years away from its origin, but then travel an extra year beyond that point from its origin`s point of view, beaming out homeward-bound messages, at some point these start arriving at its origin before the ship was even launched! This would be.. interesting to say the least. You could plan out entire expeditions via listening to the reports of your future self aboard the ship. Cool. All right maybe not, because this would cause paradoxes and all those bad things that you see happen in time travel movies will happen to you. The very fact that mass-less particles exist as the limit to casualty in the universe suggests that the laws of physics convene to prevent such disastrous things from breaking spacetime. The weird thing is that the messages sent from the traveling space ship in the example are not being sent FTL, they would be relativistic, at the speed of c. If you want to know how terrible FTL communication would be, check out the Tachyonic anti-telephone thought-experiment
devised by Einstein and Arnold Sommerfield
and later explored by Richard Tolman
in his Tolman`s Paradox.
So I would think that communications at interstellar distances would need to be delayed or sped-up en route after being sent to their destination relative to their origin`s position and velocity in timespace to arrive instantly. I don`t know how this could be done.
After all that is said and done, EM might be the only medium in which to communicate effectively - unless the aliens use hypothetical means like neutrinos, quark entanglements or something even more exotic and fantastic that does not lead to paradoxes. But even if the majority of them would use such strange and to us, undetectable ways of talking, there would be those who inevitably do not for whatever reason. We should be able to pick those up at some point. And is my point: we have not been looking for long enough or hard enough to really make dent in this field. That is the attitude of SETI, and although it implores you to ``keep faith``, its really all you can do. Unless you want to be more pro-active and send out messages yourself. We did that a few times and none of them repeated, so I guess those are just another WOW! signal for somebody else.
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There has been some objections to this crazy expansion rate (which I think could be appliet also for your FTL-travel scenarios). It has been shown that only a society that has economic (and ecological) sustainability, with a strict birth control policy and a coherent cautios long term expansion strategy can really accomplish the colonisation of the galaxy. It has also been shown that a society that don't make control policies on its growth would exponentially consume more materials that those available in the increasingly large territory they expand on due to the limits of light speed travel. This would generate in a few millenia an interstellar societal collapse that would limit the "expansion bubble" to a few tenths of light years in what has been called the "ligth cage". So the reality is that no colonisation would occur in 5 million years (at those speeds they would fall in a Malthusian catastrophe quickly and would be limited to a tiny region of the galaxy). Only a very slow and gradual attempt would be victorious in invading the entire galaxy.
Yes, exponentiation tends to be blunted by other overlapping internal systems that arrange for self-regulation. Just take the population crises for example. A few decades ago scientists believed that the population boom of the industrial revolution and exponential growth thereof in the West and in Asia would doom us all to die from over-population difficulties by the 2050s, when humans would number over twenty billion. Now it has been learned that because of the development of first world countries, birth-rates in those countries has dropped. Population growth stabilized. Now the only human growth is from 2nd and 3rd world countries - but the reason why it is from there is because they are making the same industrial-era transition that the first-world countries made. After they pull through and provided we can clean up this planet and curb our excessive consumerist mindset, the population should stabilize overall at around 12 billion. Still a lot people, but technological and sociological innovation that results from the the prevalence of education in developed countries (which will be the ONLY countries by then mind you) should make that sustainable.
Anyway, to tie into what you said here about gradual colonization, I remember reading in Carl Sagan`s book Cosmos
that based on the expansion dynamic`s of civilizations here on Earth, if a alien civilization 200 light-years away from Earth using sub-light travel started colonizing the stars around them, only targeting those stars that are suitable for them (Sagan used yellow dwarf stars as an example) in a roughly spherical volume, they would take 2 million years to reach our solar-system with their scouts, assuming they did not make Sol an objective in their missions. A million years per hundred light years. 2 billion years for the entire galaxy (which, as of new research
, is around 170,000 light-years across). Bear in mind this is if they only colonize type G yellow dwarf stars.
The take away from all of this is that interstellar expansion is fiendishly difficult even with FTL. It is not a game changer, despite getting so much attention.
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There's one possible problem to interstellar travel. Dust. Interstellar dust is extremely sparse, but given the distances and the great travel speeds, could it pose a problem? Has anyone tried to calculate the risks? A small pebble, even just a gramme, will utterly destroy your spacecraft hits it at 10% the speed of light. That's nearly half a gigajoule of energy, or the equivalent of more than 100 tonnes of TNT.
At subrelative speeds, even the hydrogen gas is deadly. It depends what your space craft is made out of. You would obviously have a curved shield of some strapped on the front of your ship (which some of the SE ships have, if you are curious). The shields would have to be hard but also rather absorptive and flexible, possibly made out of a aerogel, metamaterial
or something similar. As they get bombarded, the shocks of kinetic energy are absorbed and redistributed evenly - maybe even powering internal mechanisms as a opportunistic generator. We must also understand that things in space do not blow up in the same way they do on Earth. Most if not all of the force an explosion makes up there is kinetic and hence easily manageable. Something bigger heading your way would need to be detected beforehand and vaporized by lasers into smaller chunks.
That being said however, At superluminal speeds, nothing will save you (unless you have FTL lasers
). The Alcubierre drive works around this problem because it does not travel through space, space carries it in a bubble of negative matter. Hence gas, dust and bigger chunks might be deflected by the negative matter or even annihilated. I`m not entirely sure what effect a bubble of spacetime would have on a asteroid.
Ahhh, nothing like a FTL alien discussion get those synapses firing