00:00Have you ever looked up at the night sky and felt... small?
00:04It's a feeling we've all had, that sense of awe and wonder.
00:07But have you ever truly tried to grasp the scale of what you're looking at?
00:18Let's start with something we can almost understand.
00:21Light speed.
00:22Light travels at an incredible 186,000 miles per second.
00:26That's fast enough to circle the Earth seven and a half times in a single second.
00:31Now, imagine a single particle of light, a photon, leaving a distant star.
00:37For that photon to travel for an entire year, that's what we call a light year.
00:41It's a measure of distance, not time, and it's about six trillion miles long.
00:46A number so large, our brains struggle to visualize it.
00:50Now hold that thought.
00:51The observable universe, the part we can theoretically see from Earth,
00:56is estimated to be about 93 billion light-years in diameter.
01:00That's 93 billion times six trillion miles.
01:04If you try to write that number out, you need a lot of zeros.
01:08It's a distance so immense that it's fundamentally beyond human comprehension,
01:13and that's just the part we can see.
01:15The universe beyond that could be hundreds, thousands, or even infinitely larger.
01:20We simply don't know.
01:21Inside this observable bubble, astronomers estimate there are at least two trillion galaxies.
01:27Two trillion.
01:29Our own galaxy, the Milky Way, is just one of them.
01:33And within our Milky Way, there are an estimated 100 to 400 billion stars.
01:38That is more stars in our galaxy alone than there are grains of sand on all the beaches of Earth.
01:44And remember, our galaxy is just one of two trillion.
01:47So let's do some quick mind-bending math.
01:50If we take a conservative estimate of 100 billion stars per galaxy and multiply that by two trillion galaxies,
01:57we get a number of stars in the observable universe, that is.
02:02Well, it's a one followed by about 24 zeros, a septillion.
02:06A truly astronomical number.
02:08Now for the really interesting part.
02:10We're learning that planets are not rare.
02:13In fact, they seem to be the rule, not the exception.
02:15Most stars we look at have planets orbiting them.
02:19The Kepler Space Telescope mission showed us that there are likely more planets than there are stars.
02:26Many of these are rocky planets like Earth, and a significant portion of them are located in what we call the habitable zone or the Goldilocks zone.
02:35This is the sweet spot around the star where the temperature is just right for liquid water to exist on a planet's surface.
02:41Not too hot, not too cold, but just right.
02:45So trillions upon trillions of planets.
02:48Billions of them are likely Earth-sized and in the habitable zones of their stars.
02:53And here's another crucial piece of the puzzle.
02:55The universe is old.
02:57Really old.
02:58About 13.8 billion years old.
03:01Our solar system and the Earth are only about 4.5 billion years old.
03:05If life could arise here on Earth, what's stopping it from arising on any of those other billions of potentially habitable worlds?
03:23Worlds that have had billions of extra years for life to emerge, to evolve, to become intelligent, and maybe even to develop technology far beyond our own.
03:31This is the heart of the Fermi Paradox.
03:35If the universe is so vast and so old, and the conditions for life seem to be so common, then where is everybody?
03:42Why haven't we found any evidence of other intelligent civilizations?
03:47Think about our own progress.
03:49In just a few hundred years, we've gone from horse-drawn carriages to sending probes past the edge of our solar system.
03:55Our radio signals have been leaking out into space for about a century, traveling outwards at the speed of light.
04:02They've already washed over thousands of nearby star systems.
04:06If a civilization just a few thousand years more advanced than us exists, wouldn't their technology be like magic to us?
04:14Wouldn't they have been able to explore the galaxy, or at least send signals that are impossible to miss?
04:20Yet we hear...
04:22Silence.
04:22The search for extraterrestrial intelligence, or SETI, has been scanning the skies for decades, listening for that one signal that tells us we are not alone.
04:32And so far, nothing.
04:34This is what some call...
04:36The Great Silence.
04:38There are many possible explanations for this silence, ranging from the hopeful to the terrifying.
04:44Maybe interstellar travel is just too difficult, a fundamental barrier that no civilization can overcome.
04:50Or maybe intelligent life is incredibly rare.
04:54Perhaps the jump from simple life to complex intelligent life is a hurdle so high that almost no planet ever makes it.
05:01Another idea is the Great Filter.
05:03This theory suggests that there is some obstacle, some filter that prevents life from reaching an advanced stage.
05:10The scary part is, we don't know if we've already passed this filter, or if it's still ahead of us.
05:16Are we one of the very first, or are we simply doomed to hit a wall that has stopped all others before us?
05:23Or maybe they are out there, but they're deliberately hiding from us.
05:27This is the zoo hypothesis.
05:29Perhaps they see us as a protected species in a cosmic nature preserve, to be observed but not interfered with.
05:36Or maybe the galaxy is already colonized and we're just in a quiet, rural backwater, and the galactic city dwellers have no reason to visit.
05:45And then there's the simplest and perhaps most profound possibility of all.
05:49We are truly, utterly alone.
05:51In this incomprehensibly vast ocean of stars and galaxies, our little blue marble is the only place where consciousness has ever flickered into existence.
06:02That would make our existence here, right now, the most precious and improbable thing in the entire universe.
06:09It would mean that the future of all consciousness rests on our shoulders.
06:13A truly heavy and humbling thought.
06:16So we come back to that initial feeling looking up at the sky.
06:19We are a civilization that has only just begun to dip its toes into the cosmic ocean.
06:25We've explored a tiny fraction of our own moon.
06:28We've sent robotic rovers to scratch the surface of Mars.
06:31In the grand scheme of things, we've barely left our front porch.
06:36We are like a single drop of water in a vast planet-sized ocean, trying to comprehend the whole world from our tiny perspective.
06:49The question of whether we are alone is perhaps the most important question we can ask.
07:16If you enjoyed this and want to explore more big questions, make sure to like this video and subscribe to the channel.
07:40Let me know your thoughts in the comments below.
07:43Do you think we're alone?
07:45Why or why not?
07:46I'd love to hear your theories.
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