If I were in the neighbourhood, I'd want to take a closer
look at a planet that seemed to be largely but not entirely
liquid water, even if there were only bare geology to see;
but as we've seen, based on our one data point, liquid water
also probably means some pretty fascinatingly complex carbon
chemistry, which would also be worth checking out, even for
purely aesthetic value.

The idea that we would detect aliens at any point before they entered orbit, or even landed, is a fallacy.

We don't scan the sky for crap. Unless their ships were the size of cities (not really likely, more likely to have lots of smaller ones), we won't know until they're ringing the bell.



The Jedi Master

The real reason aliens would bother invading us, is that we taste like chicken and is yummy with ketchup on.

I disagree, unless the intention was a direct attack a single mothership would be most logical, if they were just int the area and decided to check this planet out, then probably a single scout ship or probe.

This entire discussion is a fallacy then- the idea that we discuss plausible reasons aliens might invade is not based on anything other than suspicion and nothing more.

If intelligent life does exist, and if they are constrained in any way the way we think we are constrained from reaching far enough distances, then that would suggest why we aren't seeing this now- it's impossible, or they don't have the will to do it. In that case, I would bet all of us would hit the lottery twice tomorrow before aliens visit us.

What does that have to do with the fact that we don't have "sensors" sweeping the sky regularly that are at all likely of detecting anything? Well, I guess we might as well all stop using electricity then since this entire argument makes no sense! Makes as much sense as you just did.

We're regularly notified "oh, we just found a rock the size of Manhattan that will pass within 10,000 miles of Earth tomorrow". So if something THAT big escapes notice until it's that close, why would anyone see a ship that wasn't at least that size if not larger out beyond the orbit of Jupiter?


The Jedi Master

It has everything to do with the fact that we don't know what we're talking about at all. May as well shut the topic down.

I'm only presenting the idea as a theoretical exercise, not as a matter of fact or anything. But then that's what this entire discussion is predicated on- simply what we believe theoretical aliens might do by analogy as to how we understand it.

In other words, if you've seen a single alien behave in a way to discuss its behavior, then you might have some basis to proceed. If not, we're not basing this discussion on anything real, we don't even know what is plausible or not.

Uh, no.

Our ability (or lack thereof) to detect approaching spaceships in any likely size (ie smaller than an asteroid) is not hypothetical or theoretical. It IS a matter of fact.

The rest perhaps, but that part is not. It's like saying "in case of alien invasion, there's no WAY to say how many rounds an M-16 carried by defending US forces could have, it could be 2 or 200." We DO know how many rounds an M-16 can carry, whether it's shooting at people, animals, or aliens. Likewise we DO know the sorry state our "planetary sensor grid" or however you'd like to name it is in. It's pretty much a bunch of amateurs with telescopes looking for moving dots.

We scan Earth orbit regularly for the activities of others from Earth, we don't go looking beyond that.



The Jedi Master

It's a matter of fact we don't know anything about alien lifeforms. So, in that instance, anything is speculative. Nothing is known. Nothing said here has any qualification in fact or knowledge, other than analogies we draw from ourselves and anthropomorphize aliens, attribute abilities, technologies, and motives to them we have no basis for saying. It's not even a scientific discussion, it's science fiction, or sci fi fantasy.

As I have said before though, I'm not presenting the scenario as a matter of fact. Not as a matter of fact, you'd have to accept this supposition in order to understand our own psychological response, which is the reason I raise it, not to prove the scientific or historical fact of it, that's not the point. In other words, I say this for the reason to show by analogy what we understand about ourselves if we think aliens might be similar and invade us for the same reasons as we could only comprehend. That we detected them is merely the way to introduce that- otherwise, anything they do will be without our knowledge, in which case the whole discussion is self defeating anyway. I set up the scenario not to prove the likelihood of detection, that's not the point, but to understand why aliens might invade us, which is the point. There's no other way to get there without doing this.

But again, nothing about this is fact, and I will bet none of us will ever see an alien anyway to know what we're talking about.

If any one of us knew what we were talking about, then we'd certainly be more of authority than anyone else. But we all sound like any UFO following quack, talking about stuff we don't know.

And yet you STILL miss the point.

I pointed to one small part that I said could be known with accuracy and all you want to do is talk about how NOTHING can be known, INCLUDING the part that I specifically proved COULD be. You're just throwing your hands up and saying "who can really knows anything about anything??! Do we even EXIST?! Why are we talking about this at ALL?!?!"

It's like you're making an argument about nothing for the sake of nothing because you're really bored.




The Jedi Master

Ok again, I'm not focusing on whether they are detectable or under what circumstances, that's not the point, but that's where you were saying it's a fallacy. If that's the case, then the whole discussion is a fallacy, it's based on nothing. I merely use that as a theoretical basis to get into the meat of what humans might conclude the aliens are here for, not to get focused on the means so much, but the why. You're focusing on a thing that really doesn't matter.

Don't get hung up on the idea that we know they're coming, otherwise, then we don't have time to think about what their motives might be (again, which is all theoretical). If we only know about an invasion after the fact, then analysis is after the fact- but we could only know if and when this would happen (which I would bet the farm it's not going to happen so that we could see it anyway).

Well I can think of a couple of methods by which we might very well detect incoming alien ships.

The ships themselves if travelling at high sub-light velocities will have a drive plume as bright as the sun itself, and by impacting directly or indirectly with dust and gas at that velocity would have to emit all sorts of detectable radiation.

Also, if travelling at these speeds, there will be a small but detectible distortion of space-time around the ships, similar to a gravity well. This will bend starlight passing through and will be visible to astronomers here. Remember that they will be on their way for at least decades, maybe centuries. Plenty of time to spot them.

I'd just like to point out that SOME possible spacecraft engines may very well be detected even outside of the solar systems, like a fusion engine pointing straight at us in the hypothetical case of deceleration from 10% of the speed of light. That requires a prolonged burning at G forces within the physiological limits of aliens who would have a similar enough physiology to find our planet appealing for colonization, and if the ship is really large (say, the size and mass of a hollowed out major asteroid) the deceleration may be way less than .1g, and thrust must be applied for YEARS to come to a stop.

At the same time we have lots of amateur astronomers that are hunting for comets or other deep sky objects. The chances of detection will dramatically rise if the approach vector is close to the galactic plane, which is also rather likely. A fusion drive is trillions of times brighter than a comet and would immediately attract attention, especially if it is moving. Spectral analysis will quickly reveal its nature, AND its velocity, AND, over time, that it is decelerating. This isn't rocket science. As soon as it is detected, a whole battery of instruments from professional astronomers worldwide will start observing it. An approaching fusion flame that changes course and velocity can only mean one thing, and every astronomer and science fiction reader will immediately recognize its significance.

A light sail powered by some monster laser would also attract attention - not the least because in order to generate a meaningful deceleration it will have to unfold a mirror of planetary dimensions with an albedo of .99999, directed straight at the inner solar system. We WOULD notice.


There aren't many other options left that aren't fantasy. As long as the spaceship has Newtonian/Einsteinian characteristics it must emit a colossal amount of energy in our immediate astronomical neighborhood.

We are totally in agreement Ssnake.

This means that rather than two years or so we will literally have decades of warning that they are incoming. If we can't do something constructive (destructive) with that information we don't deserve to win.

Another thing, if they are decelerating they will be effectively blind in the direction of the Earth, and relativity being what it is, if we throw a comet-sized mass in their path, even at low velocity, we can destroy them. This should be in our power to do within 20-30 years, particularly when there is a bright fusion source bearing down on us at a significant fraction of C. Talk about incentive!

Then of course the decision would be ours...do we kill them in the decel phase or wait for them to arrive and start their agenda, whatever it is?

Of course the incoming might merely be a probe expedition....if we destroy it in flight there is a Relativistic mass two years behind travelling at 0.99C. If the probe survives we prove certain things to them about our capabilities and the R-Bomb gets diverted. It can't be detected except by its mass disturbing starlight, as it is in free ballistic flight and will not decelerate.

So the destruction of the first probe proves firstly that we are not suckers, and secondly that we are now aware of them and will be busting arse to attain a true interstellar military capability. We will come looking for them.

This guarantees that the R-Bomb will not be diverted.

If the probe makes it to our star, then the crew will know that we have no significant military capability and can be enslaved, domesticated or exterminated at their leisure.

The R-Bomb is diverted.

Decisions, decisions.

You guys do realize that a freakin vacuum tube would be Alien technology to 19th century humans right?

And Newton and Einstein might just qualify for special remedial education to their standards.

Yeah, let's just nuke em!

What the hell do we have that an advanced alien civilization couldn't find on any unpopulated world(s)?

Bit coins?

Luxury hotels in Sochi?

Crappy humans that don't taste like chicken?

Uh, there are actually quite a few professionals doing this work with
automated banks of scopes, not to mention orbiting probes.

This is a nice summary of the discoveries of small objects, from
1980 to July 2012, with all orbits cumulatively displayed:
http://www.youtube.com/watch?v=HYp_WFZSBSo

the increased frequency of discovery reflects the steady increase
in the number of devices coming online. You can also see the amount of
detection going on, with various current running totals, here:
http://www.minorplanetcenter.net/
New probes are launched regularly, Canada put one up last summer.

You might find this topical to this thread, a nice article on the
MPC website:
http://minorplanetcenter.net/blog/lets-start-2014-with-a-bang-hello-and-goodbye-to-asteroid-2014-aa/

I don't think so.


The longer the projected arrival date, the more difficult it will be to motivate society to actually prepare for it. Humans don't handle abstract risks too well. Also, we don't know what kind of drive they are using. Suppose that magnetic monopoles do exist, you could use them to drive along the magnetic fields of all the stars in a galaxy, and I don't think that we could detect THAT. Of course we don't know yet if magnetic monopoles actually exist and if they can be retrieved/created in meaningful amounts, so it's still in the realm of fantasy. Likewise, if it was possible to create wormholes in the space-time fabric, we could probably not detect them either, or maybe only in the form of gravity waves which seem to be very hard to detect in the first place, let alone being able to poinpoint their proximity and direction.

Your argument of aliens arriving at relativistic speeds goes both ways, like tracer ammo. While they are vulnerable, our reaction window is very small. We will receive information about their presence only shortly before they are there (they are, after all, traveling an near-light speeds (which I personally find rather unlikely; .1 ... .15c is already very ambitious). If they are slower, they are much more maneuverable and therefore less prone to collisions with a few million tons of cobblestones that we left drifting towards them.

We barely managed to get Pioneer X out of the solar gravity well, and that was a few hundred pounds, and it took us decades and a particularly advantageous planetary constellation for a serial swing-by. A comet means millions of tons, and we have to move it upwards. I'm sorry, but no. What you really need are prepositioned rocks near the Pluto orbit , ideally in 20 to 50 locations to cover all directions, and there's just no way how we could do this in the foreseeable future.

We have no defense against a relativistic planetary killer. We don't have a defense against a slow boat throwing rocks from the Kuiper belt. We don't have a mentality to preempt incoming aliens, or to deal with abstract threats. For all practical matters we are defenseless, and will remain so during our lifetime.

We might calculate their intention is not to come all this way merely to destroy the Earth from a distance- although no one really knows for sure, the energy expended roaming around star systems just to destroy good looking planets seems impractical, unless these guys are the d-bag jocks of the galaxy who just go around doing that sort of thing, I think I would bet that they wish to make contact- that might not be so good either, though. Maybe they'll get close enough to cripple the Earth with focused EMP bursts in order to pacify us before snack time.

@Ssnake.

I meant we were in agreement that detection of the alien ship was very likely, not the other stuff.

I dunno, but given several decades of a united world (bloodily so if necessary), I reckon we could put up a good show against an expedition coming in at say 0.1c. The true R-Bomb though cannot be defended against, granted.

@K5.

I believe that it would be a grave error to allow any aliens to reach us. Whatever their motives might be, they would not be in our interests, but solely theirs.

They would just nuke us from orbit its the only way to be sure