Why don’t scientists tell the people that there obviously is no such thing as people on other planets or time travel instead of getting up our hopes? If there was time travel, people from the future who discovered it would have travelled back here to tell us about it, and they haven’t. And if there was anybody else in the universe, the waves from their radio and television programs would have reach us by now, and we would be having interesting sitcoms and more shows about angels and demons and young love and big business (because beings on other planets would be just like us, since if they weren’t there would be no communication anyway). And assuming that spacemen were more advanced than us, we would have their advanced signals by now. Everybody knows this, but they still put articles in magazines about life on other planets!
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In a Universe the size of ours, it would be extremely arrogant and ignorant to proclaim, when we have only looked at a tiny, minescule, extremely small part of it, that there can be no life elsewhere.
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The rules of physics as we know them do not entirely rule out time travel. Again, it would be stupid to say it’s impossible without proof, at least at a mathematical level. How do you know there aren’t time-travellers among us? How do you know I’m not one? (Cue Twilight Zone Music)
Oh, I forgot to mention, regarding the first question: check out the book Probability 1.
“Sometimes I think that the surest sign that there’s intelligent life out there is that they haven’t tried to contact us yet.”
I think the classic argument against time travel is basically: if I can go back in time, why can’t I go back to 1967 and strangle myself in my crib (not, admittedly, the first thing I’d really want to do)? But then I’d never grow up in order to go back in time, etc. It’s catch-22. Or, if time travel is possible, show me your time machine. You can’t say it hasn’t been invented yet, because time travel supersedes all yets. The inventor in, say, 3000 A.D. can take his machine back to 2000 (or 1776, 1492, or whenever) and we’d already be going back and forth and strangling ourselves and others in their cribs and doing general mischief).
As for extraterrestrial life, well, it could be just very rare. Or, maybe we humans are the most advanced race in the galaxy. (What? Someone has got to be.) But arrogant and ignorant, I don’t know. Some intelligent and thoughtful folks (I’m thinking of, like, Freeman Dyson) have said, yes, we are alone, or at least, that the universe is not nearly so populated as a lot of people tend to believe.
Anyway, I expect to see this thread in Great Debates when either manhattan or Chronos wakes up.
Uh-oh, Chronos is on the board now…
We’ve been listening for transmissions from space for the last - what? 20 years or so? Ever heard of the speed of light? That means that there are probably no advanced alien cultures within a 20-light-year radius (assuming, of course, that all aliens use radio and not, say, tight-beam antineutrino based communication systems). That’s like saying that because you can’t see anybody camping in your back yard, you’re alone on the planet.
Well, if this ends up turning into a debate, I’ll be glad to move it, but in the meantime, it’s no worse than any other speculative science question. Besides, I like time travel questions 
First things first: For extraterrestrial life, don’t consider it a given that we’d detect their signals. With the detection capabilities we have right now, there could be another race just like us around Alpha Centauri (the nearest star to the Sun), and we’d never know it. They might be able to just barely contact us, if they specifically tried and we were actively listening, but that’s about it. When folks talk about our television and radio signals being intercepted by aliens, they’re assuming that the aliens have detector technology many orders of magnitude more advanced than our own, and that they’re looking for us.
Now, as to time travel, there are a few possible explanations for how it might be possible, and yet have us not see any travellers. First of all, some proposed methods would only allow you to travel as far back as when the time machine was invented. Maybe as soon as someone invents a time machine, there’ll be a flood of time tourists, but they can’t get back this far. Secondly, maybe it’s possible, but extremely difficult. It might take, for instance, the entire rest energy of Jupiter to send a person back, in which case, it’s not likely to be used just to say “Hi”. Thirdly, as friedo mentioned, maybe they’re already among us. Finally, maybe they just don’t give a darn about the late 20th century.
No, that just means that we’d only be able to detect cultures that have achieved radio technology at the same time as us, or previous to us within a 20-light year radius. If, for instance, radio technology arose on Antares six hundred years ago, we would have seen it by now (since Antares is about 540 light years away).
…I should have said, previous to us or after us. Sorry! 
…no, No, NO! I mean: at the same time as us or after us! Sheesh! It’s time for me to go…
Oops. Yeah, you’re right. Still, my point is still valid - the universe is so damn big that none of us has the right to claim that something can’t exist.
Geez Don, this is a really good question that I would have been afraid to ask … but you did! You are right … the odds of intellegent life of any kind, carbon, silicon, etc., based, in the universe is as close to zero as lets say Plank’s contant permits. Just as a tiny snow flake or grain of sand on the beaches of the world would never have the same exact shape. I know I will take a lot of heat on this … but the combinations of shapes in a snow flake is lets say conservately 10^20!(yes this is a factorial). A number which is impossible comprehend. The probability of one person hitting all of the lotteries in the world in the same day is a piker compared to this.
Considering any reasonable estimate of the number of potential planets and all previous planets and lets be generous, multiply it by a 10^3. Still a piker! Why should it then be that there is a planet, or any other life based planet, with the almost impossible set of physical conditions (distance from sun, size of both, water concentration, CO2 capture, ozone, free O2, etc…) of this Earth have any probability to exist elsewhere just as there are were two identical natural snow flakes … you really do have a much better chance of hitting every lottery in the world in a single day.
The formula used currently to calculate the probabilty of life is a multiplication of several “opinion” factors … it does not rigorously assess the probabablity … like the phrase “billions and billions”. Geez, a whole 10^18 and it is not factorialed (sp)!
We will keep the alien and UFO thing, considering life span, travel energy requirements, generational space ships, reason for coming, level of intellect to want to come here, how they could crash after coming all that way (FDL), etc., for another day.
First of all, there is time travel. We all do it every second of every day of our lives. But of course, what you mean is backward (relative to what we’re used to) time travel. With all the inherent contradictions, i suspect that reverse time travel would be limited to observation of, rather than interaction with, the past. So I suppose that there always have been “observers” from the future, though they can’t interact with us, including letting us see or feel them. Of course, occasionally one of them might be accidentally perceived, hence stories about “supernatural” beings (don’t assume they’ll look like us).
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Ummm…where do your number come from? How do you know how likely or unlikely life is? The simple truth is that we really don’t know. Your assertion that life is so unlikely that even one planet in the universe with life is statistically near impossible is based on so many assumptions I don’t think I can count them all. Suffice it so say that life could be common or uncommon, but since we don’t really understand the origin of life yet, your statement is nothing more than an opinion.
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Now this is simply wrong. We know for a fact that many stars have planets, it seems pretty likely that most stars do. We can assume that we need liquid water for life, although this is just an assumption. But even so, many stellar systems could have two or three planets that are within the liquid water range. It doesn’t require a lottery to be in the water zone, it simply requires the presence of a lot of planets…one or two will fall in the zone just like some will fall in the liquid ammonia zone, some will fall in the liquid methane zone, etc. And why do we need a planet with our exact size? Nope.
Water concentration? Well, water is one of the most common compounds in the universe. (Take Hydrogen, Helium, Oxygen, Carbon and Nitrogen and stir. You get H2, helium, water, methane and ammonia. 99.x% of the universe is made of these things.) Lots of planets and moons in our solar system have water, it’s just that the water is frozen. So any planet in the liquid water zone is likely to have lots of water. Maybe it will have 1/10th or 10 times the water that earth does, but it is likely to have water because ALL planets are likely to have water unless they are too hot. And the other things…CO2, O2, ozone, etc…these are the RESULT of life, not things that are neccesary for life.
Anyway, it is true that anyone can plug whatever numbers they like into the Drake equation and get whatever number they like out of it. But to pretend that YOU know the answer and that the number must be some ultramicroscopic number is simply hubris. Hmmmm…you wouldn’t by any chance be a theist of some sort…?
Sorry to sound cynical, but part of the reason behind the hype about extra-terrestrial intelligence is money / funding. In other words, if they can get people excited about extra-terrestrial intelligence, it’s easier to get funding for lots of stuff.
There is very good reason to believe that there’s no need for those radio-telescopes and stuff. Why? Because it would probably be more economical and efficient (and almost as fast) for hypothetical extraterrestrials to send messages out with von neuman probes.
Of course, non-intelligent extraterrestrial life wouldn’t send out probes, but wouldn’t send out radio transmissions either.
I think a technological civilization, i.e. any civilization with the ability to contact us, implies the ability to alter their environment. Maybe there have been many extraterrestrial civilizations, but shortly after reaching the technological stage, they messed up their environments enough to kill themselves, or at least knock their civilizations back to their equivelant of the stone age, so they didn’t have much time to contact us.
Don’t think an intelligent species would do something to screw up their planet? Then you haven’t been paying attention.
Or maybe I’m tired and should shut up and go to bed.
The folks at SETI, who scan radio waves for evidence of extraterrestrial technological civilizations, note that they don’t have the technology to pick up signals of “I Love Lucy” strength from the nearest stars. Some of the stronger signals put out by military radar might be picked up.
Signals from the Arecebo radio telescope could be detected as far as 10,000 light years away, BUT they are typically broadcast in a very tight beam: it’s thought unlikely that advanced civilizations would be within those narrow spans.
See:
http://setiathome.ssl.berkeley.edu
You can download a screen saver at the above website that analyzes part of the radio spectrum for evidence of extraterrestrial civilizations.
Now what about this to be thrown into the hopper: I read some time ago that if there is intelligent life out there it looks just like us and not like any of the beings made up by science fiction (except for small deviations like Spock or even people with bony foreheads like Worf and his kind or even the Romulans). This is because evolution proceeds by degrees developing things necessary for the ultimate development of intelligence. The article said that one of the steps or degrees is the opposible thumb that enables us to grasp and the hand in general that makes us see as we do. The parts of the brain develop and one might develop for reasons having nothing to do with intelligence, but can be used for that or to support it when some other part of the brain develops. The eye of the octopus is an example of parallel development of the eye, but other things developed or didn’t and that is why the octopus doesn’t create anything or do anything using its eyes, but humans do. The octopus (and admittedly a lot of humans) uses its eyes just to see food and direct other boring practical matters. The octopus and the dophin, the whale and the elephant and other highly intelligent creatures and monkeys didn’t invent the Internet, and Whatsisname did and he is (almost, sort of)
human. Thus no outer space beings look like the lovable E.T., for instance, because wrinkled up skin and a wide skull just can’t evolve with other things that will add up to intelligence. And the enormous eyes and puny bodies of the classic silvery-gray beings from outer space also can’t add up to anything intelligent.
Maybe there’s a planet out there covered with water and inhabited by intelligent aquatic creatures. They may not be able to build radio telescopes, but intelligence and technological development don’t necessarily go hand-in-hand.
As for a species that develops advanced (at least to us) technology, some abilty to grasp tools seems to be required, but that doesn’t mean that they are going to be bipedal mammals with opposable thumbs. Who knows, if they hadn’t died off 65 million years ago, there might be extremely intelligent dinosaurs roaming the planet now, building spacecraft and watching cable television. I don’t think you can look at what evolution has produced on this planet and assume it is the only possible outcome.