Correct. As with any claim, the claimant is responsible for providing evidence. The complete lack of substantiated evidence is particularly glaring considering the number of people who take this seriously and spend their lives trying to prove it.

Also correct. Furthermore, given the vastness of our universe, the extremely precise calculations necessary to get anywhere in space, and the amount of radiation bombarding any spacecraft travelling near to a solar system, it is safe to say that there are not Star-Trek like ships out there just wandering around, hoping to run into something. Since humans have only been giving off detectable signals of our existence for around 100 years, any aliens visiting us would have had to find us within the past 100 years, and immediately set off on a journey to earth, meaning of course, that they are less than 100 light years from here. We've surveyed our immediate astronomical neighborhood pretty thoroughly, and seen nothing to suggest any intelligence near us.

Yep. You're exactly right.

A much better speculation is that they don't exist, or that they did exist, but were a fraud. Or, that the man who claimed this after his death simply had a really good sense of humor.

Given no outside corroboration for time travel, no scientifically verified evidence of such time travellers, and the track record of other extraordinary claims of the same sort (I'd guess zero out of fifty thousand or so, off the top of my head), there's no reason to treat this theory as anything other than wild, unsubstantiated speculation.

The first atomic bomb test happened around the same time that the genre of science fiction was being born. Science fiction was born right around the time we learned that the universe is actually immense, and that there are potentially tens of thousands of worlds that could sustain life, in theory. Furthermore, the virtually limitless energy potential inside the atom led to wild speculation about the possibility of interstellar travel.

Putting all these facts together, we can easily surmise that people with wild imaginations invented the idea of aliens visiting earth, and people with equally wild imaginations began to imagine that they had seen UFOs when they had actually seen aircraft, balloons, meteors, hallucinations, or optical illusions.

I think it's so unlikely as to be disregarded completely. Are you familiar with Occam's Razor? Basically, the idea is that whenever you have multiple possible explanations for something, the answer that fits the facts with the simplest explanation, and the least extraneous information, is usually true.

So, which is simpler?

1) Humans from the future, who manage to create time travel despite having obliterated their world, have come back in time to kidnap people, then put them back with no way for them to verify the experience, and to fly around in strange spacecrafts, doing nothing that can be scientifically verified.

2) When people learned how big the universe really is, and that the power of the atom is enormous, they imagined that we might one day be able to travel in space. They further imagined that if we could do it, other races could, too. Once that idea was planted in the public's head, someone thought they saw a UFO, but it was really something terrestrial. Once that hit the news, the public ran with it, and it's snowballed over the years, so that now, lots of people who haven't thought critically about the subject, think there are aliens visiting us, despite the lack of any evidence whatsoever.  Later on, people, believing firmly in the idea of UFOs, imagined that if they had spaceships here, they might land on earth sometimes.  When they imagined what aliens might look like, they created them in the image of man, since they'd never actually seen an alien.

For my money, I'll take Door #2, Bob.

Ok, then let's add an imaginative story.  You are sitting in your living room one day and the show you're watching gets interrupted with the following report.

"Scientists have discovered an unidentified body that is baffling!  Looking almost identical to the "Grays", the little men that are associated with UFOs, it stands at approximately 4 foot tall, with four limbs, appears bipedal, has an extremely large head, and large black eyes.  Biological test and examinations are being conducted.  More news as it arrives."

 What would you think that it could be?  Alien or descendent of Homo Sapiens or what?

The most important aspect of this, imho, is that it has no distinguishable effect on my life. Therefor I don't give it much thought.

To believe without evidence is absurd. To dismiss without cause is pernicious. To worry without relevance is mad.

You are much better off spending your time pondering solutions to real problems, like our beloved president.

hehe.  I agree Dan.  We can't be serious all the time can we?  How boring.  I think the conjecture on this is entertaining and a somewhat decent exercise in rational thinking.

I agree.  The arrogance of theistic humanity assuming that the universe is merely a backdrop to our existence is dumbfounding in the extreme. 

The entire universe was created only so our single species on this planet filled with thousands if not millions of species could admire and study it only by us.  How arrogant is that?

But international flights don't go against the laws of physics.  According to Einstein's theory, it isn't possible to travel faster than the speed of light.  That's where all the wierd stuff happens, like energy being converted into mass and vice versa.  Correct me if I'm wrong, I'm no scientist, but from what I understand the above is true.

Realistically I'd think its a fake. We have gotten them before so it wouldn't be a shocker.

Now if someone is claiming to find a dead one I have case study you should look at.

Its not pretty is it...

Now people didn't know what it was, but they did have something to go on.

Fokelore! However it was just one of these guys.

Just not as dead or half eaten...

I'm talking about the Stronsay Beast btw, its basically a sea monster.

I suggest you try to find a show called "Is it real?" its kinda like those In Search Of shows on scifi or the history channel except they try to show the scientific theories against it.

Dude no one needs anything coming into their house and attacking them. Btw I'd have a spas 15 AND AK if I thought someone was going to attack me.

The AK kinda depends on if I can aford it... Class 3 makes the wallet skinny. 

Spas 15 FTW!!! (oops, double post)

That's exactly what I'm talking about. You are assuming that what we know about physics right now is the absolute truth and will never be proven wrong or incomplete. History has shown that we are constantly changing and refining our understanding of the rules, and there is no reason to assume that something that seems impossible today will remain impossible forever.

I am absolutely not suggesting that everything we know is wrong, only incomplete and unrefined. I am suggesting that what you assume to be an absolute limit is only one way of looking at the problem, and we have only begun to examine other possibilities. To assert that we will never find a way over the next hurdle is pessimistic and ignores a long history of human ingenuity.

If you care to re-read what I have posted before, you will find that my critical thinking leads me to the conclusion that current alien visitation is highly unlikely, considering the evidence we have so far. I would ask that you not accuse me of making "wild, unsuported speculation", as my optimisim for the future is solidly based on the track record of human achievements thus far.

Yeah, I hear what you're saying.  The wild unsupported speculation I was referring to was the OP's speculation that humans from the future have come back, etc...   The flaw in your posts is that the claim that we could be wrong about quantum physics is being used as a supporting piece of evidence, and that's not critical thinking.  That's wild speculation.

Adding "we could be wrong, so this wild, unsupported speculation could be right" isn't adding any knowledge to the argument, but it sounds as if it is a supporting piece of information.  Any argument could be wrong, and any scientific knowledge could be mistaken.  This is a given before any discussion.  The thing is, until there's some evidence that what we know might be mistaken, "mistaken science" can't be used as a support for a conclusion.

So, you see, while you are technically correct, all that statement does is encourage bad thinking while contributing nothing of substance to the thought process.

Nah, you're misunderstanding what I'm saying.

Critical thinking is about getting to the answer that is either demonstrably correct, or most likely to be correct.

The question is, "Is it credible to say that what people believe are aliens are actually descendents of humans who have come back in time?"

The answer to that is "no" even though it's possible to think of a scenario in which it could happen.   Here's why:

1) Using available information, we can form a parsimonious explanation for the appearance of aliens in the public imagination.  This explanation is supported by the total lack of verifiable evidence for the presence of aliens.

2) Given the reliability of Occam's razor, we could discard the future man theory out of hand, but there is more reason to discard it.

3) For the future man theory to be correct, we have to postulate not only that our current theories in physics are wrong, we also have to postulate the way in which they are wrong (they are wrong in such a way that the truth allows time travel).  Given the complete lack of evidence for this postulation, we should discard it as wild speculation.

4) I am not arguing for limitations.  There are limits to everything, so nobody needs to argue for them.  I am saying that speculation about the limits of physics only have validity when there's evidence to suggest the speculation.  There is no evidence I'm aware of that suggests that physics may be wrong in the way you suggest.  Therefore, the speculation contributes nothing positive, and gives the false sense that the future man theory has more credibility than it does.

 Don't get me wrong.  I think it's a neat idea, and it would probably make a good show on the Sci-Fi channel.  But, it's not good critical thinking to entertain the idea without some backing, and there simply is none.

To sum up:

Is it possible that future man is coming back in time and we are perceiving them as aliens?

Yes.

Is it likely?

It is so unlikely given our current knowledge, that it's not worth considering.

With you so far.

Here's where I didn't follow you.

If we had a body, and it was that closely related to human, particularly if it had many of the same or similar organs, I don't know that the time travel theory would be any more attractive, simply because at this time it appears that time travel is impossible.

It would depend on the circumstances of finding the body what the best theory would be, but my best guess is that it would be from our time, and that it would be related to homo sapien.

Right.  The way I understand it, travelling forward isn't all that difficult, theoretically, but physically it's an incredibly daunting task.  At speeds far, far below those that would accomplish worthwhile time travel, a tiny speck of space dust would cause immense damage to sensitive machinery.  Imagine what thousands of such particles would do!

As far as backwards goes, I just can't see any way around the following logic:

1) Relativity and quantum physics, while not complete, have proven incredibly reliable, and must be viewed as accurate until there is evidence that they are not.

2) Time travel backwards contradicts everything we know about these sciences.

3) Therefore, any such creature discovered would have to be concluded to come from our own time, or, as an extreme stretch of possibility, from the past.

Note: This is all contingent on me being correct about physics' take on backwards time travel.  As always, the logic is only as good as the data, and I'm not a theoretical physicist.