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There's a perfectly consistent explanation that doesn't really break Occam's razor unless you have huge assumptions about AI takeoff and the drake equation:

- These are really old things

- They are autonomous and are driven by some computing system/AI

- They have the ability to harvest, store, and leverage an immense amount of energy and convert it into propulsion (or more specifically, allows the repositioning of the object's atoms in spacetime) in a way we don't understand but doesn't break the laws of thermodynamics and allows travel at near light speed for long periods of time.

- Their purpose isn't to communicate back to the origin (since the speed of light) but to autonomously take interventional action at their destination upon arrival based upon observation (this is the part that should scare you and if I were to go out on a limb is the reason these things aren't disclosed)

That's it. Speculation about their original origin is irrelevant with regards to assuming it's an extremely unlikely thing for unmanned autonomous agents to exist with (a relatively incremental) post-human level of technological capability.

If anything, there are many variants of resolving the drake equation which would make it surprising to not see such things, since it would imply there was never AI takeoff anywhere or any mechanism to deploy long-lived, self sufficient, autonomous probes. I'd expect us to be able to construct primitive versions of this within 100 years.

4. The entire world is a simulation and UFOs are moving around in debug mode. The size of the simulation could be anywhere from the entire universe to the Solar System (or less, if you can come up with a plausible reason why the Pioneer probes appear to continue to operate despite being outside of a smaller simulation volume).

Occam's Razor would throw that one out too of course.

I used to be fascinated with the UFO reports and such as a kid.

However, I grew up, and learned physics. And then later I learned about things like Molecular Nanotechnology.

Aliens, as popularly conceived, have not been here, and will not be coming here.

Why? I'll explain.

Space is big. Like, really big. The distance to the nearest star system is 4 light-years away. Even for a small object, it takes an enormous amount of energy to get going at a "reasonable" speed, like at some fraction of the speed of light. For an example of a more reasonably-sized object, Voyager 1 would take 70k years to get to the closest star system (4LY away) if it was headed in the right direction.

But then you have to stop! If you accelerate up to 0.1c (very fast), you have to decelerate to arrive at your destination. This is where the rocket equation kills you. Even with super-mass efficient ion drives (which with our current technology provide very low thrust), accelerating your spaceship to a reasonable speed, and then have enough delta-V to stop at the destination requires an even more crazy amount of energy.

If any aliens are headed to our solar system, we'd see their drive flairs from a long way out.

The most efficient system is to use a light sail (really, a laser sail). This is so you don't have to carry all that reaction mass with you, thus bypassing the rocket equation. And then there's a trick where you detach most of your sail, allowing it to reflect light back towards the ship for the deceleration half of the trip.

But again, even for a small (like 1kg) payload, this requires a stupendous amount of energy. And we'd see them coming because they'd be shining a big, big laser in our direction.

There are also Bussard ramjet drives, and other speculated technology, but these would all likely be slower because of the mass required to build it.

Now simply flinging out a bunch of von Neumann replicators out in every direction would be much less obvious (though very slow), until the space dust starts building infrastructure in our solar system... but that's a talk for another day.

But maybe the UFO are really old machines (like millions of years old) in which case they can take all they time they need to travel and decelerate, such that wouldn't produce drive flairs.

Anything is possible, but that doesn't seem likely.

It is helpful to think of what would aliens want from us that they can't get at home.

Raw material, like in the mini-series V, Oblivion, Uplift War trilogy, etc? It is too expensive to ship matter anywhere. If you need a lot of it for a construction project, you'd be better off heading to the galactic core.

Maybe they want our cultural uniqueness? But they could easily simulate many virtual worlds with different starting conditions, and poke around those without leaving the comfort of home. That would be much easier, especially after they get bored with humans.

And if they have advanced molecular nanotechnology, it isn't just waiting millions of years for results. They are likely running at a much higher "clock rate" than we are now. You can run an uploaded mind much faster than a biological one. So these hypothetical aliens would instead be waiting billions of years of subjective time for their probes to find something interesting.

Maybe they just want to study us. If we discovered an alien race it'd be pretty interesting to watch them evolve. It'd be just like the fun parents have seeing children grow up.

Maybe they just want to study us. If we discovered an alien race it'd be pretty interesting to watch them evolve. It'd be just like the fun parents have seeing children grow up.

Maybe, but I find that unlikely.

If they do want to pursue those sorts of activities, then can do much more, much faster while closer to home. Why settle for observing one naturally evolved species, when you can make dozens (or thousands) of your own?

Unknown unknowns mean you don't know if there's a way to make fast, long distance travel feasible.

From a Newtonian perspective quantum physics sounds like a complete joke. There are undoubtedly other discoveries yet to be made. I can certainly countenance that future scientific advancements could make what we think of today as being impossible as being possible without violating the laws we have discovered.

While there are definitely gaps in our understanding, the standard model of physics is one of the best scientific theories... ever.

So we don't yet have a grand unified field theory, but for what we do understand, at reasonably attainable energy levels, there just isn't room to jam in some sort of faster-than-light communication or faster-than-light travel.

If you look at how it all fits together, any sort of magic like that seems highly unlikely.

Nothing we've observed in the sky, dating back billions of years, suggests FTL either. Until I see something that can even hint at FTL, I'm completely discounting it.

4. Intelligent life has evolved on Earth, alongside (or before) humans, and has mostly managed to cloak itself from human discovery.

Occam's Razor still suggests #3, but #4 is more likely than #1 and #2.

> fundamentally misunderstand physics

Keep in mind that we think we know how to reach a destination faster than light (although not travel faster than light). We just don't know how [edit: to build the engine] or even if it's a practical approach. What it shows is that physics does allow us to reach a distant star and return in a single lifetime.

> inconceivably long lived

I'm not sure about this one. That species would have had to evolve somehow, unless they solved the mortality problem or unless Earth-like evolution is not universal. Even if they could, given the timeframes, why would they visit?

My point is that, yes, we don't have enough evidence to abandon possibility #3. We also don't have enough knowledge to abandon possibility #1 and #2.

That being said, I'd wager that if ET has visited Earth less than 1% of the sightings are authentic - we aren't that interesting.

Not sure I agree with the analysis, although I do agree with the conclusion.

We've only known there's a pervasive limit for a century. We've known for a few decades that the limit is quite subtle i.e. the violation of local realism. It's not at all difficult to imagine that there is more to know in this field. I mean, it took 200-odd years before Newtonian mechanics was supplanted. So there's form.

Also, you don't need to go faster than light as long as you don't go back. If alien explorers are going at a decent fraction of light speed, they can get a fairly long way in a sensible timespan for them. Just not for the folk back home.

My objection to the conspiracy theory idea is that it just doesn't make any sense. They drew huge pictures and built great monuments then got shy. They now play peekaboo with rednecks and the odd fighter pilot.

Governments could be covering up alien tech from a long gone race but one that's still here? A race that they collude with but let out to do a bit of probing because, presumably, the government can't find bodies for them.

So nah. They're not here but it's not technically inconceivable that they haven't been in the past or won't be in the future. It just doesn't seem very likely.

Yeah #3 is certainly the most likely and is what I always assumed. But over time I've come to think that they are here. So probably the next most likely is #1. Maybe they just know more science than we do. Look how far we've come in the last 200 years. What if they have a few millennia on us?

What if it's some sort of psychological phenomenon, but one which has a physical manifestation?

Yes, I know that that idea is pretty out there, but we're already talking about ideas that bend or break the known physical laws, so why not consider it? I honestly don't see it as being any less absurd than the idea that the speed of light is not a universal constant, an that is required for almost all hypotheticals involved extra-solar intelligence.

Isn't it much more likely that an Alien society developed an artificial intelligence capable of exploring the universe AND self replicating.

A satellite program like this could easily monitor distant parts of the galaxy without travelling faster than light, or require a generational ship.