> we have evolved to perceive everything that can possibly threaten us and that includes aliens

This argument is comical because it contains two components that are exactly the sort of thing that blunts perception: arrogance and absolutes.

Nothing new here. People always thought they already know "everything that's important" and "only a few details are missing". We, as a species, are quite arrogant and entitled to our perceived "large knowledge".

That's why real progress is so slow even "things are obvious"—at least afterwards.

Thanks to technology we can perceive many things that we have no evolutionary reason to be able to perceive. Our current physics also doesn't seem to be _so_ wrong that it allows much room for technological societies to hide from our instruments.

> Thanks to technology we can perceive many things that we have no evolutionary reason to be able to perceive.

We might be able to perceive, but that doesn't mean we understand what we see. Every time I engage with medicine, I learn how little humanity actually knows, despite all the technology. We still don't understand what causes many diseases, or how to heal many wounds. How would we find completely alien life when we can't even find the cause of a disease that plagues 23% of the adult population?

The same way we find the disease: by the effects the aliens have on their surroundings.

You are missing the point, in our arrogance to believe we understand things, we do not really understand them. The drive to understand more is powerful, and has lead to huge advances, but we still have a long way to go.

I agree that we still have a very long way to go. I disagree that the areas of the universe that are still mysterious to us are likely to be hiding places for aliens.

> We still don't understand what causes many diseases, or how to heal many wounds.

Right, but we still know the diseases and wounds exist.

Sometimes, you have to wait until you're dead or dying to know they exist. I'm not convinced that humans have mastered perception after mastering writing 5 thousand years ago.

You're partially right, but missing something important.

Where you're correct: Aliens that are only found on planets would be almost impossible to spot, and even if we look at them and see the consequences of their existence on their atmosphere we may interpret that as non biological in origin.

What you're missing: However, if they are expansionist and can build space habitats and automation of the sort we have reason to believe is possible, the question is "why are there visible stars? Why are they not all already surrounded in Dyson swarms?" which is hard to miss regardless of underlying chemistry or biology.

So far as we can currently understand, it seems that about 96% of the matter-energy of the universe is stuff that we cannot see, detect, or explain. Dark matter/energy.

There's more than enough room for fanciful explanations.

Except that dark matter doesn't even interact with itself. Particles of dark matter just pass right through each other rather than clumping together to form complex molecules, rocks, planets, etc.

A Dyson swarm emits approximately same energy as its host star, just in infrared. Dark Matter could, I guess, be Matryoshka brains organised carefully to not hit each other by their keepers, so long as they are also actively "gardening" what we can see so they never ever get Dyson-d.

Wrong isn't the same as incomplete and we have a weird tendency to overlook things, we find then "obvious" afterwards.

Things we do not easily perceive are such that are too fast, too small, too high energy generally and, most importantly, too unexpected.

Further, you can hide in plain sight: you only have to look "unbelievable" (for example), so society will not transmit the information of your sighting.

You can hide where (nearly) no one is looking as well: deep under the ocean, far below the surface, high in the sky, etc.

Most likely, this isn't exhaustive. Places we don't know about are perfect for hiding ,-)

Yeah, we are at the point that we are getting data on black holes colliding millions of light years away from us based on the deviation of a Lazer across miles over the width of a few atoms.

To suggest the universe is full of life and we simply can't see it? I don't buy it.

I think it's more likely that the signals just aren't there, because they are... Not hidden, but are too weak and crossing too grand a distance to be detected.

There are plenty of things that we are aware of that are currently impenetrable by our instruments, such as the inside of black holes or the past. And who knows what we aren't aware of. Is there any reason to believe that physics is a complete description of reality? Or even a significant slice? It could be just describing a very tiny human centric slice of all that exists.

Dark Matter and Dark Energy make up 95% of the Universe (the rest is all the normal matter and energy in all of the galaxies put together). Our understanding of physics is both very complete, and contains large unknowns. You might imagine that there are a lot of aliens hiding in the Dark Matter, but as far as we can tell Dark Matter particles pass right through each other, as well as normal matter. They cannot clump together to make stars or planets or life forms the way normal matter can. Thus all aliens would be made of normal matter like us.

> Our understanding of physics is both very complete

Citation please.

I’m sure I could find some, but I haven’t bothered.

We know that our current understanding of physics is incomplete (various niggling little problems with the Standard Model, the whole Dark Matter question etc), but these are surprisingly small problems. We only know about these problems because of the extreme sensitivity of our instruments. The new physics that will eventually fill these gaps will answer fascinating questions, but won’t change anything about our understanding of life.

For example, petaquarks were first observed in the early 2000’s, and the Large Hadron Collider provided additional evidence for their existence. But they’re so unstable that they can never stick around long enough to participate in any life processes, so as far as our understanding of aliens goes they are irrelevant. Maybe they exist naturally inside the cores of neutron stars, but that’s not really a very habitable environment.

Similarly, however we resolve the apparent paradoxes in our understanding of gravity and quantum mechanics won’t matter at all to the study of life. Gravity is so weak that it is essentially irrelevant in any sort of chemical system, even when quantum mechanics is necessary to understand the fine details.

What I mean is, "complete" according to what criteria? It seems trivially obvious that our human model of physics could only be complete in the sense of accounting for all things we are capable of observing, but that isn't the same as being able to account for all things. There is nothing I am aware of in the model of physics that claims that it accounts for all aspects of reality, observable or not. We don't even know if there are multiple universes, and to what degree they interact, and if there are universes that mostly don't interact with ours at all. There is nothing in physics that precludes another universe that normally doesn't interact with ours, but advanced conscious beings within it being able to bridge that gap if they so choose, similar to how we could go to some island to visit an ant hill if we so choose.

On that last point you are incorrect. The known laws of physics rule out all forms of Einstein–Rosen bridge (aka “wormholes”) that you might imagine, except possibly inside of black holes (and those wouldn’t be traversable). That includes wormholes between points in our own universe, and between universes. Einstein and Rosen studied them because they are technically a valid solution to the equations of General Relativity, but not all solutions to those equations are realizable. In this case, opening a wormhole requires a considerable quantity of negative mass, and there is exactly zero negative mass available. Mass (or energy density), is simply never negative.

As for completeness, each revision we make to the known laws of physics is always smaller than the ones that came before. Einstein’s Relativity trumped Newton’s laws of gravity and motion, but only at high speeds or energies. At high speeds you start to notice weird effects like time dilation, but at low speeds the time dilation is so small that you would never notice it unless you knew to look for it.

Likewise, in most situations the difference between classical mechanics and quantum mechanics is unobservable. There were some edge cases where classical mechanics just couldn’t explain observable phenomena, like the photo–electric effect or phosphorescence, and all quantum mechanics does is fix those edge cases while still being able to explain all the rest of chemistry and such that we already knew about. The changes introduced by Quantum Electro–Dynamics and later Quantum Chromo–Dynamics were even subtler than that. The only easily–measured thing that QCD explains that QED couldn’t is radioactive decay, which is basically irrelevant to life. QED is much more useful to biologists because it explains how chemical reactions can release light (fireflies!), how chemicals can be sensitive to particular parts of the light spectrum (think rhodopsin in the cone cells in your eye), etc.

Because every new addition to our understanding of physics must not contradict anything we already understand (if it did then we would know it was wrong), we are asymptotically approaching a limit (as it were). That limit is either the full laws of physics, or I suppose you would say some limitation of our own minds. But why should our minds be unable to comprehend the full laws of physics? Even if those get harder and harder to learn about and understand, we have all the time in the universe to come to grips with them. Any individual can maybe hit a limit of how clever they can be, but the human race as a whole? I doubt it; we’ll crack that nut eventually. Even if humanity gives up at some point, nothing that we fail to learn could contradict what we have already figured out.

I wasn't talking about Einstein-Rosen bridges. In fact I was wary of using that analogy because I suspected, correctly, that it would be misunderstood.

It sounds like you are saying that any possible reality could only interacti with another one using Einstein-Rosen bridges. This is far too narrow a limiting of what is possible. If it's possible at all, regardless of likelihood, that reality could be a simulation, then anything you say about the Einstein-Rosen bridge is a property of the simulation, and not a limit on reality as a whole.

What we _can_ observe rules out a lot of things that we can not observe. For example we know to very high confidence that there is nothing that we can't perceive that influences the way nuclear reactions happen.

Do we know with "high confidence" though? What if we are in a simulation?

Then it is a very coherent simulation, with extremely deep and elegant rules that are always followed.

> Our current physics also doesn't seem to be _so_ wrong that it allows much room for technological societies to hide from our instruments.

Perhaps.

An idea I first heard from my brother a decade or two ago was that the Dark Energy expansion of the universe might be the waste product of all the aliens who figured out some way to trick the laws of physics into giving them free energy.

(Obviously that's more of a sci-fi plot device than science at this point, take it with the due level of seriousness).

That's pretty much the plot of Isaac Asimov's novel "The Gods Themselves".

https://en.wikipedia.org/wiki/The_Gods_Themselves

He probably read it, though that novel predates the discovery of Dark Energy.

I understand #2. I hope you understand the people refuting it.

Humans haven’t evolved additional perception, we created it. We evolved to see electromagnetic radiation in the “visible light” spectrum but we created tools to see every other part of it.

We’ve even created tools to accelerate matter to just under the speed of light, smashed atoms together and then detected the particles created from the collision.

We’ve explored our entire world, launched telescopes into space, sent probes outside of the solar system and so much more.

Can we see everything? Not even close. But our vision today is incomparably far compared to even just 150 years ago. Cave fish today see as well as cave fish 100,000 years ago.

Only, having the possibility to use such sensors isn't the same as actually using them.

Most advanced sensors are exceedingly rare and consequently locally far too restricted to be useful in this context. Social hurdles apply.

The most comprehensive sensor network on earth belongs to the US military. In spite of being provided extraorinary amounts of taxpayers' money for this, the resulting data isn't shared for common interests like science at all.

We barely see neutrinos, we don't even know what dark-matter and -energy are, we can't reconcile micro and macro physics, we've lived for an eyeblink, we really don't have much clue.

That's why I wrote, "Can we see everything? Not even close."

But the cave fish comparison is not accurate.

Not referring to 300bps, as they seem to get it, but unfortunately many of the most unimaginative people limited by dogma also seem to be the most confident and condescending.

You aren't taking the analogy far enough. That fish doesn't have a prefrontal cortex, and can't even comprehend what we are capable of or capable of perceiving. Whatever more advanced being that us would have perception and experience that we aren't capable of comprehending. They would do whatever they do that we are incapable of just as the fish is incapable of human-level creation.

I think the problem is that 2 relies on kind of woowoo logic. Where exactly do these aliens exist in this scenario? WE can't perceive everything directly but with LIGO we're pretty close to being able to measure most phenomena that exist in the Universe. Is there a mirror universe they exist in because even if not directly we should be able to detect their influence on Solar Systems, Stars, Black Holes, etc.

Are you familiar with quantum gravity? If humans are so advanced in our technology and understanding of physics why can't we explain gravity at the quantum level? https://en.wikipedia.org/wiki/Quantum_gravity#Problem_of_tim...

And if we don't understand something as basic as gravity, how can we possibly say that we can detect everything in the universe?

Actually we even don't understand time. Therefore space-time. Therefore the "stage" on which everything happens.

Also we don't understand mater fully. So we don't understand "the thing" that happens on said stage.

At the fundamental level we know shit. There's still plenty to figure out. Funny enough most of the fundamental things…

By finding out about those fundamental things it's quite possible that we discover completely unexpected aspects of our universe.

Not long ago we even didn't have the slightest clue how our sun works. A phenomenon that is central to our existence since inception.

We should be really more humble in what we assume to know about how the things around us work.

How do you know that we're able to measure most phenomena that exist in the universe?

Even if there is plenty of life in some form we cannot perceive, I think the problem remains:

Why isn't there more "cave fish" out there? The universe we do perceive is ultra gigantic, and seemingly empty.

The pool of water the cave fish lives in may seem incomprehensibly gigantic to its tiny, lonely brain.