The theory that we’re either the first form of intelligent life or the last

Not really a theory but being the oldest person alive. There’s just something so chilling about the fact that literally every human that existed at the time of her birth is now dead except for her. She’s the last of the human race in her snapshot of time.

A potential answer to the Fermi Paradox: we've never seen evidence of an alien civilization because no intelligent life survives long after the discovery of nuclear weapons.

Cold Death of the universe. That the speed at which the universe is expanding is to the point where gravity will never pull it back. The matter will spread out and cool down and decay until everything, everywhere, just stops. Forever.

The dual consciousness theory disturbed me when I read about it.

Basically (in my limited, layman understanding), a corpus callosotomy is a procedure that is sometimes used to treat conditions like epilepsy, where the connection between the two halves of the brain is severed. People can survive this and continue to live semi-normally, but usually with some cognitive impairments. Both halves of the brain continue to be 'alive', but there is little to no communication between the two sides.

People who undergo the procedure can also exhibit some strange phenomena like alien hand syndrome, where the patient feels like some of their limbs will move on their own accord. They can also have difficulty controlling their non-dominant hand, trouble speaking, and issues with certain parts of their vision.

The dual consciousness theory posits that both halves of the brain retain consciousness after this procedure. Essentially, you become two people sharing the same body, completely unaware of each other. One variation of the theory is that one half of the brain is dominant, controlling most bodily functions, while the other half is basically cut off from most external stimuli, unable to communicate with the outside world, for the rest of their life.

Just the concept of infinity/eternity. Regardless of who’s correct about what happens when you die, it’s terrifying. Either the lights go out for good and your consciousness is lost forever, or the afterlife is a never-ending groundhogs day scenario. Gives me anxiety sometimes if I think too deep about it.

I had a breakdown in Sunday school some 20 odd years ago about it, and the deacon told me that heaven was so amazing and perfect that eternity was a blessing. I didn’t buy it. Having it all end is immensely frightening, having it never end seems worse.

The fact that it would take 8 minutes for the Earth to know that the sun exploded and it could have exploded 7 minutes ago

That our consciousness doesn't actually direct our actions, but just makes up stories to explain our actions to itself. Consciousness might not even be useful to our evolution, it could just be a weird phenomenon that came about as our brains became more complex and developed more self-monitoring systems.

The theory of proton decay, where every single bit of baryonic matter will eventually just spontaneously dissolve into radiation and fundamental particles, guaranteeing that there will be no survivors at the end of the universe is... yeah terrible

Solipsism. Basically, there's no way to prove that anyone other than yourself actually exists. You can dive deeper if you like it’s actually pretty interesting.

That a more stable form of vacuum exists and if it did, we would collapse into it. It could be headed towards us now and we literally would never know it's coming.

That at any moment at any given time some unknown cosmic thing could come from outer space and wipe us out in a blink (ie. vacuum decay or Gamma ray burst). Even our Sun could just produce a huge enough solar storm and we are basically doomed

Once we develop virtual reality that is indistinguishable from our own, which it seems we are on well on way to achieving, the question becomes: how do we know that it didn't happen already?

The theory that messes with me the most is that space is infinite. I can’t wrap my head around the idea of something having no boundary or end and it creeps me out that it’s possible

The great filter. The idea that we see no other civilizations out in space because they hit some wall that made them go extinct. Whatever it may be, it could be the reason we see no intelligent life out there, the question is, are we past the filter, or is the worst still to come?

That humans may not have the ability to comprehend everything in the universe. Like literally physically impossible for our brains to grasp it. A gorilla is only 1% behind us on the evolutionary scale and what we see as amazing such as them painting is what our infants are doing, if there is an intelligent species even 1% further than us, their infants are doing Astrophysics and their adults are doing things every day that our brains couldn’t comprehend.

Quantum immortality. If correct, it guarantees that you will never die. It guarantees nothing else.

What if our understandings of physics and science, what we base our whole lives around, is just wrong. A man born without eyes cannot see, what if we are missing a huge chunk of reality, simply due to an inability to detect it?

The "universal common ancestor" theory, that all life evolved from a single cell 3.5 billion years ago. Think about the infinity of generations of beings fighting to survive only for you to read this comment that we're all just one big family killing each other for 3.5 billion years and counting.

The inevitable heat death of the universe is a harrowing topic. Essentially, the energy of the known universe is finite and will eventually deplete completely, leaving space as we know it dark, cold, and inert forever.

It's difficult to grapple with.

The idea that the Uncanny Valley is because of the ancient hominins we used to interact with during the Pleistocene. Sure, there were interbreeding events, but imagine if you had no idea what a Neanderthal or a Denisovan was and saw one for the first time.

You and your family have been traveling far from home and seek refuge in a cave for the night. You get a fire going, everyone starts to relax. Then out of nowhere, this "almost human" looking being walks up. You all panic and run.

Reacting with fear was probably the easiest way to make it out of an encounter like that alive.

Eternal Recurrence suggests that this universe will be reborn again over and over endlessly, and so we will also be forced to relive the same life just as many times.

Take a moment to consider your odds of being here, now, with infinite time before and after you.