Suppose they/we discover a scifi trope to be real: FTL travel/communications.
You know how certain franchises make use of a “magic dimension”…subspace or hyperspace…to get around known, proven Relativistic barriers?
Suppose we make use of a dimensional “shortcut”: imagine…the square. To get from one corner to the opposite one, you either walk the 2 units of the perimeter…or the 1.414 unit diagonal in the area. Walk 3 units of a cube’s surface, or walk the 1.422 unit diagonal through the volume.
Let’s also suppose…to make “shortcut drive” feasible in the scale of human comfort time…a FTL formula through 4d space-time as (X^4)/(X^(1/4))=181.019c
Since gravity is 4d as per Einstein and electromagnetism is 5d as per Kaluza/Klein, FTL communications have a formula (X^5)/(X^(1/5))=2264.936c
A speed of half a lightyear per day, a round trip to our closest stellar neighbors is feasible in a month.
and the shortcut wouldn’t be visible, since travel through “space” isn’t happening.
Well I wouldn’t put it quite like that. Humans have only known about the c limit a little more than a century ago, and we left earth’s atmosphere for the first time even more recently.
If we’re talking species thousands or even millions of years ahead of us, I would be astonished if they weren’t warping space or making use of technology that we would be unable to comprehend.
HOWEVER, obviously for the Fermi paradox, we have to limit the discussion to existing technology with relatively modest improvements, to make the problem more precise and to show that there genuinely is an explanatory gap (i.e. that it doesn’t just tell us huge improvements in transport technology are impossible. Even if they are impossible, the explanatory gap remains).
(And FTR, when I talk about advanced transport technology, I’m under no illusion that Relativity will be shown to be “wrong” and that one day travelling to Alpha Cen will be just like taking a flight to Paris with no time dilation or whatever to worry about. Not what I’m saying at all.
I’m just disagreeing with the idea that, in the context of advanced alien species, any technology beyond more efficient rockets is crazy talk).
On a separate matter, did anyone ever explain why the use of a propellant to decelerate couldn't be avoided by harvesting energy from the target's magnetic field (or even that of intermediate systems en route)?
If you’re going slow enough, you can – and you can also also use gravity assists from planets or the star itself – you can use their gravity to slow down just like you can use their gravity to speed up. But if you’re moving at relativistic speeds, you are gonna have a hard time getting enough negative g’s to slow down without flying past the system.
That isn’t necessarily a problem; any number of self-replicating probes could have passed through the solar system (or even still be sitting in the solar system) undetected.
Yes, but the universe is so big that these self replicating drones could also have built a dyson swarm back home, for example. In that case, we could see them light years away. Or rather-- not see them, and only see blackbody radiation instead.
The fact that we don’t see anything like that tells us that either such a thing is not possible (we have no reason to believe this), that it is possible but nobody chooses to do it (and this is where you run into trouble-- because if life is common, and building a dyson swarm is possible, even if 99% of civilizations choose not to build one, there are so many stars that have been around for so long that the 1% who do go down that path should be easily visible), or nobody is out there to do it (Either because there’s no life out there, no advanced life, no intelligent life, no civilized life, or simply no advanced civilized life).
I think you mean 1.732 unit diagonal (unless I’m picturing what you mean incorrectly). The length of a cube’s diagonal is proportional to the square root of 3 not the cube root of three.
Yes, that is the assumption. And if the answer is that there are zero civilizations with interstellar travel capabilities in our galaxy then the answer to the equation in the Fermi Paradox is zero. And there is no paradox. Imaginings of civilizations far, far in advance of us, with technologies we cannot understand is just that, imaginary.
It is an interesting parlor topic of discussion and that is all. Without some indication of other technological life it will remain only that. It is boring and very disappointing to admit that the evidence is that we are the only planetary traveling civilization and there currently are no interstellar civilizations, but that is what the evidence say so far.
Even a really good idea needs some basis in reality.
Once again… We have NO WAY of detecting radio emissions from anyone more than maybe a few dozen light years from us. There could be a thousand civilizations in the Milky Way, or a million, and we would have no way of knowing it unless they are modifying their stars or beaming massively powerful signals directly at us.
So a perfectly valid answer to the Fermi Paradox is, ‘We have no idea what’s out there.’
The only argument really left is the self-replicating machines one, but that requires a whole host of assumptions we do not have answers to, and there are already some good reasons to call the scenario into question.
Think about the stupendous energy and resources you have to expend to get to another star system. So you dedicate your whole civilization to getting to another star system, only to find - another star system with a few uninhabited planets. How many more times do you repeat that until you decide you’ve seen enough? And why would you travel to that uninhabited system, when you could use the energy and material expenditure to colonize your own?
We live in an era where the thought of exploring the stars seems vastly exciting and something every civilization would want to do. On the other hand, we thought the same thing about landing on the moon, but once we did it it only took one more mission before people would rather watch reruns of ‘I Love Lucy’ than another lunar landing. Imagine how fast we would have lost interest if each landing cost an entire year’s worth of global energy and resources?
Maybe there are lots of civilizations, but the average number of stars explored by starfaring cultures is approximately one. Or perhaps the logical pattern is that civilizations tend to explore telescopically first, and the vast majority never find anything interesting in their telescopic searches.
For example, if life is spread out enough, perhaps only one civilization in a thousand or a million is close enough to another to detect it with remote instrumentation, and all the rest just stay home because there is nothing near them worth the expense and effort to get to.
Rat Avatar, I think you are highly intelligent and knowledgeable, but reading your posts, I sense a huge and irrational cognitive bias. Here’s an example of this.
You name two problems and for both problems, accuse the authors of a paper of spherical cow assumptions and avoiding a possibly intractable issue.
One problem is something a 10 year old could think of the obvious solution to. Another is a quite subtle and thorny real problem.
Yet you proclaim both ones are just intractable and thus your overall thesis must be correct.
Notably, the problem with the pebble beam generating station producing thrust could be dealt with, well, the simplest way is to just go twice as large. Two stations, coupled to each other, firing in opposite directions. Double the material and energy consumption, which is actually not really a problem at these scales because, as you should know, a civilization building planet earth sized orbital solar collectors almost certainly has self replicating machinery and a license to tear down celestial bodies for materials like Earth’s Moon.
Obviously you can do a **lot **better than double by using counter-thrusters that are far lower velocity and thus lower energy consumption, and just periodically resupply them with fuel from the readily available mass in our solar system. Since you must know that ISP scales with the square root of kinetic energy, if you go for 10 times less ISP you need 1/100 the energy consumption, so the mass of your pebble launcher is increased by just 1% to solve the problem completely.
Hardly a spherical cow, this problem is barely a rounding error.
As for the Unruh effect heating the pebbles so their sensors are blinded and they possibly melt to vapor, this is a serious and real problem. Well, ok, you could use only mildly relativistic pebbles. 0.5 C instead means your pebbles are still half as good as the unachievable 1C, but only experience 15% of the effects of relativity. Negligible.
If you’re going to decide that other scientists are ignorant in their assumptions, and thus your hypothesis you have already pre-decided on must be correct, make sure the effect you decry isn’t negligible. (are you religious? Your attitude reminds me a lot of a religious medical student I met when I talked about whether or not life extension was viable. He insisted that God would never let humans understand biology well enough to accomplish this)
Again I think this is a misunderstanding. It is not claiming that we *should *see aliens, just saying we don’t know right now why we don’t.
There are various factors, which we can put in an equation, but we don’t know the values for most of those factors.
There are lots of things in cosmology that once looked like abstract mathematics or even philosophy but is now concrete science.
There is no reason why the Fermi paradox cannot take the same path, even if aliens never visit us and make the whole thing more simple.
For example, just now I said we don’t know the values of most of the values of the Fermi equation. But in fact it was not so long ago that we could have said none of the values, as there was still some debate over the approximately number of stars in the galaxy.
Gradually we are getting a better understanding.
That video seemed good and perhaps I should watch some more in that series. Of course I was already aware that, relative to any specific event, other events are either (a) in the “forward” portion of the light cone, (b) in the backward portion, or (c) outside the light cone. But, although he refers to ‘causality’ a few times, he just seems to assume that it is unidirectional. Am I wrong that the two halves of the light cone are, geometrically, just mirror images of each other?
Many great physicists including Hawking have challenged the notion of causality. Chronos recently posted something on this topic — I couldn’t find it because “‘cause’ is rejected from SDMB search as too common a word” but I did find this Chronos post that touches on the notion of causality.
Your own comment, connecting causality to entropy, might suggest that “causality” is related to thermodynamic statistics rather than an absolute arrow of time. (One theorist claims that the strangeness associated with quantum mechanics is just what we would expect if time is truly isotropic!)
[quote="Babale, post:105, topic:827614"]
If you're going slow enough, you can -- and you can also also use gravity assists from planets or the star itself -- you can use their gravity to slow down just like you can use their gravity to speed up. But if you're moving at relativistic speeds, you are gonna have a hard time getting enough negative g's to slow down without flying past the system.
[/QUOTE]
Any guesstimate at the numbers? If my ship is travelling at 0.010 c could I decelerate to 0.009 c when passing a star, and thus brake completely if I plotted a trajectory to pass ten stars? I need to know for my sci-fi epic!
This is not going to work. Stars aren’t that close together that this will make any kind of sense to do. On average the nearest star to a given star system is about 5 lightyears. But that’s only in one direction. In the other directions, the nearest star may be 10, 15, or more lightyears away. You don’t want to be travelling lightyears out of your way just to slow down. It’ll double or more the the trip time.
As a side note, if you look into this more seriously - assume very small starships (if you needed to send humans you would send them later as data files of their neural maps + DNA + a little bit of data on their body) - you realize that acceleration is the easy part.
In principle you could build an accelerator large enough to accelerate the small starship to near the speed of light right here in our solar system. No different than a linear particle accelerator, just scaled up many orders of magnitude.
The problem is deceleration. Accelerating using an external system cuts the rocket equation problem down quite a bit (much more than 2 times, remember, logarithm) but you still need an engine to decelerate.
As far as I know, antimatter is very much viable. Now I’m handwaving slightly - the thing you need to make antimatter work is anti-elements. It is possible to contain antimatter without ever touching it using electrostatics and magnets and light. And if you could shove the antimatter into an energy negative fusor of some type (doesn’t need to be a full fusion reactor) and in a series of steps, get it to a heavier element, you could then laser cool it and abuse the fact that Type 1 superconductors reject magnetic fields.
If you can do that, you would have solid pieces of antimatter that, as long as they stay cold enough (a few kelvin), will levitate away from magnets, even simple static permanent magnets.
Anyways this would require a vast low gravity solar orbit manufacturing plant. And most of the energy would be lost in the process of creating antimatter from scratch. But in principle, once you have solid and stable antimatter fuel, starships are completely feasible, at least for non-human, very light weight crew.
I love how the manufacturing of anti-matter, which we do already, is where you flag handwaving and not in the reconstruction of specific humans through a neural map, DNA and a souvenir. :rolleyes:
The thermodynamic arrow of time is the the core asymmetry. The second law of thermodynamics is not time-reversible and entropy has far more to do with free energy than the typical description of “disorder” that is used for Entropy.
Equating “disorder” with Entropy is a viewed by some as being destructive to learning by some.
Under the math of General Relativity is is possible that one’s timeline could loop around to itself but that is not possible under big bang models.
While we don’t know why we experience time in a temporal form has humans and our intuitions don’t really work for the non-euclidean nature of our universe as described under GR you can think of it as thermodynamics telling you the direction and GR telling you how it is connected.
Light cones are a visualization tool and are not complete descriptions of how the underlying math works but the forward light cone is what you can interact with and the backwards lite cones are events that can interact with you.
Chronos’ post you linked to was most pointing out the limitations of human perception and especially with our mistaken belief that there is a fundamental basis in our temporal perception of time.
Hawking’s argument was mostly that our human perceived arrow of time is dependent on the thermodynamic arrow too.
Under GR you are your “world line” and all of those events exist already and Chronos was clear to point out issues with the local thermodynamic arrow with our mental arrow of time. All of these analogies break down at some point, especially with our mistaken belief that “now” can even be an objective concept.
Causality when defined as the relationship between causes and effects, which is what the geometry of spacetime describes, was not seriously questioned by Chronos’ post or Hawking’s argument.
Here is a link to Hawking’s chronology protection conjecture.
The past light cone and future light cone are not mirror images of each other, as each point in space time will have it’s own light cone.
As an example that will try to keep a euclidean geometry is that we can view distant galaxies Hubble horizon, which due to expansion are receding away from us faster than the speed of light but we will never be able to interact with those galaxies because they are moving away from us faster than the speed of light due to expansion.
Here is another way of thinking about it:
[ul]
[li]Arrow under GR: what you can interact with[/li][li]Arrow under thermodynamics: Free energy is energy that is available to do work depends on differences in energy, this energy tends to equalize over time and thus has less ability to do useful work.[/li][/ul]
Those two different arrows both influence our perceptual arrow of time and causality in complex ways. But as Hawking pointed out the dream of time travel or even faster than light travel for macroscopic objects is probably going to be limited more by Thermodynamics than GR.
The 2nd law of Thermodynamics is may very well be the reason why we even have matter at all. But the concepts between connectives and reversing the 2nd law are really separate outside of the fact that you can’t assume that two locations in spacetime are causally connected, were always connected, and will always be connected.
Basically you have to do all the hard math for every observer unless they are co-moving and close enough for the problem to reduce to the Newtonian model.
We have produced about ~0.000000001 grams of anti-matter at a cost of about ~$100 Trillion per gram and we would need 100-1000 metric tons to get to the nearest star.
All of these thought experiments require a lot of hand-waving.
Like the Dyson swarm would require dismantling the entire planet of mercury yet we haven’t even found a way to drill to the mantle on earth with huge human crews, but making self repairing, self replicating robots is somehow obvious and inevitable.
It is valuable for people to consider the possibilities but all of these ideas depend on a lot of unknowns.
