Aliens: Yes Or No?

[Clint_L]

I'm very late to this party, but I voted no. Maybe the universe is infinite, but short of a "basically magic" breakthorugh, we are limited to the observable universe. Anything outside is unobservable, unknowable and impossible to interact with, so it doesn't matter if there are countless civilizations in a galaxy far away many universes over, it is the same as they not being there. And well, the observable universe is finite, and within it, I think we are the only ones. Even if we aren't, short of "basically magic" advancements, we'll never meet another sentient species. And that is assuming there is anybody there. I'd bet there are none. Evolution is aimless and never guaranteed to create complex life, let alone intelligent life. So TLDR: I think the Drake Equation actually amounts to 0 and we are the rounding error.

And well, belief in "we are being visited" and stuff, well it is the same belief in witches, imps and succubi, just with another coat of paint. One that is dressed in a sci-fi coat of paint, but not fundamentally different. Heck, a lot of contactees claim aliens "are extradimensional visitors" which muddies the waters.

Evolution doesn't create life at all. Evolution by natural selection is the algorithm that describes what happens when you have a combination of replicators, competition for scarce resources, mutations, and time. But evolution is a very strong optimizer. The strongest. Though what it optimizes for is replication, not complexity. However, given the one example we have, history of life on earth, it seems that complexity is a powerful strategy for replication.

We don't know how life on earth began, though there are a number of hypotheses. However, there is nothing to indicate that earth is particularly special - even if we limit our speculation to life as we know it, the building blocks are common throughout the observable universe, and it turns out that planets are, too. And there's been plenty of time for life and subsequent evolution to happen. So it's not like the idea that life, including intelligent life, could happen somewhere else is a goofy hypothesis; to the contrary it seems like a very reasonable one. The challenge is in testing it, but again, there are already various methods to do so, with more becoming available as our technology improves.

So it makes sense to keep looking. Right now, we have recognized zero direct evidence of life elsewhere, so it would be wrong to express certainty on the issue. But expressing confidence that it exists and will likely be discovered in the future is not unscientific any more than expressing confidence, as Darwin did, that some means of passing inherited information to offspring had to exist, though he had no observations of such a thing. Still, the only rational response you can give to the question right now is an agnostic one.

At the same time, I am sympathetic to your claim that "short of "basically magic" advancements" we'll never meet another sentient species (that we didn't help create, anyway). Space is vast and hostile, and I think interstellar travel is far more daunting than we can easily comprehend. Heck, I think interplanetary travel is far more daunting than is popularly supposed - I am extremely doubtful that we see humans on Mars anytime in the foreseeable future, for example. And I concur that all the "visitation" mumbo jumbo has far more in common with Medieval superstition than with science.

 

[briggart]

Current evidence is that the rate of expansion is increasing.

Sure, but that is not relevant to my original point. I was trying to clarify why it makes sense to consider different bubbles within an eternal inflation as forming a multiverse, even though they are part of the same continuum. The space between different bubbles is still inflating so, barring FTL, it is not possible to travel from one bubble to another. On the other hand, under some conditions, within a bubble it can be possible to travel to an arbitrarily large distance given enough times. Those conditions just happen to not be true in our Universe, but I'm not aware of any fundamental reasons while they could not be true, which to me is enough to differentiate travel within a bubble from travel between bubbles. But I feel you do not agree?

The rest in spoiler because I think it will be interesting only to the two of us.

Spoiler

I’m intrigued by two follow up questions: For a time-like curve that is continued indefinitely in to the past and future, for every other such curve, is there always at least one light-like path from a point of the first curve to a point of the second curve? What if the curves are continued only until the end of the early inflationary period? And, can there be points which remain un-observable even if the rate of expansion is asymptotically 0?

TomB

Technically, those are 3 questions.

Let me start from the last one. Assuming a flat or open Universe, the comoving distance a photon can travel is the integral between the emission time and infinity of

c dt /a(t)

where c is the speed of light, t is time and a is the cosmological scale factor. Assuming that asymptotically

a(t) ~ t ^ n

with n > 0, then the requirement that asymptotically the expansion rate goes to 0 implies n < 1 and the comoving distance diverges, which is indeed the case for matter or radiation dominated universes. For a curvature dominated universe n = 1, the expansion rate does not go asymptotically to 0, but the comoving distance still diverges, albeit logarithmically. The future travel comoving distance diverging to me implies there will be no un-observable zones given enough time, but maybe I'm missing something?

Second one, I'd say no in general. If there is a cosmological constant, then the comovig distance integral does not diverge, even allowing for infinite future time.

For the first one, I'm not sure I understand what you mean by "continued indefinitely in to the past": something like cyclic cosmologies?
I know people have been working on that and figured out a way to have stable cosmological solutions that continue through the bounce, but I've not been following that line of work, but if you have an answer, I'd be happy to hear it.

 

[Starfox]

There are no aliens within our light cone that have made gross changes to their surroundings, such as creating a dyson swarm. We'd see that. If there are intelligent aliens, they are very subtle or very far away. Likely several galaxies away.