Interesting; I had always taken Sagan's opinion as gospel, but I suppose there is room for questions. A bit of googling shows both Jimmy Carter and Dennis Kucinich have seen a UFO, and they're honest people. That certainly doesn't make me a believer, but it's nice to see there's at least a small possibility.
Keep in mind that UFO just means that it is unidentified and not necessarily from another galaxy. In my opinion Sagan and Hawking are right and that actual visitation from an alien species would be really unlikely given the size of the universe.
Not necessarily -- a fact alone that they could visit Earth implies that their technology is sophisticated enough to fight gravity, travel at speed of light (or faster) and do plenty of other things that may not even been covered by science-fiction just yet.
Now, before you say it doesnt matter whether you travel at the speed of light, faster or slower, the moment you catch up with space debris, you become a crashed and burnt debris yourself. If all physical objects vibrate at some rate, then as wild as my imagination can go, it may be possible to speed up this rotation of a physical object so much that would eventually become... a wave.
It's not sufficient for the aliens to be sophisticated. They would also have to exist in exactly the right time frame.
Consider the age of the universe: 14.6 billion years. Earth was formed about 10 billion years after the universe, and it took 5.4 billion years for the Earth to see the emergence of a civilized species, which has existed for an absolutely microscopic amount of time compared to those enormous intervals.
How long does a species last, realistically? Counting from its "space age", would a species survive a thousand years, ten thousand? Extinction from natural disasters, epidemics, war and the like could be averted by scattering oneself across a solar system or wider. How many civilizations could reach that stage?
I have absolutely no doubt that a myriad of alien life exists. But I think the chances that there exists alien species that:
* has evolved before us or roughly at the same time,
* has not driven themselves to extinction
* achieved the technological capability for crossing galaxies, and
* is willing to do so
…is exceedingly slim. Even if a species develops space-faring tech needed to visit Earth, I think it's extremely unlikely that their existence will coincidence with us. They are either all dead by now, or they will be dead before we/they develop the tech to visit, or they don't exist yet, and we will all be dead first.
Yes, I did leave time out of it for the purpose. Our civilization is so young, but we already can easily replace almost every body part we have, not to mention better ways of getting new ones [1].
In the near future, not only we will continue to stretch average life expectancy, but we will continue to mesh technology with living tissue. Today we can have metal bones, joints, metal plates in human brain, porcelain teeth, all sort of signal strengtheners (fake eye, hearing devices, etc), nothing tells we will stop developing those more in the future. No, we have a long way before we could live forever, but definitely mechanical devices that can least 500 years (fake heart, lever, lungs, etc etc) will stretch our lives X-fold. I would assume if some civilization flies UFO crafts, they most likely advanced on other fields as well.
