Carl Sagan REVEALS How Far We Can Really Go: The Limits of Humanity

Added: 2026-06-01

[00:00:00]There are two trillion galaxies in the observable universe.

[00:00:07]Humanity has, in 50,000 years of existence, managed to brush the edge of one.

[00:00:17]And yet, we are already too late for most of them.

[00:00:24]I want to sit with that for a moment because it took me years to fully feel the weight of it. Not just to know it, but to actually feel it.

[00:00:37]My name is Carl Sagan, and I have spent my life in love with the cosmos, its distances, its silences, its almost unbearable scale.

[00:00:53]I have written about it, taught it, dreamed of it under many skies. And the thing about falling in love with the universe is that it will eventually tell you the truth about itself, whether you are ready to hear it or not.

[00:01:14]So, let me tell you something true tonight.

[00:01:18]Think of the universe as an inheritance, a vast cosmic estate bequeathed to every civilization that will ever arise.

[00:01:32]I want to read you the terms of that document. What you have actually been given because the first clauses sound almost impossibly generous, a limitless estate measured in light and time.

[00:01:50]>> [music] >> And then you read on, and the document reveals that most of the property was already gone before the ink was dry.

[00:02:03]Begin with what we have physically reached. What we have actually touched.

[00:02:09]On September 5th, 1977, we launched a small spacecraft called Voyager 1.

[00:02:19]It weighed less than an automobile. It carried no crew, only instruments and a golden record, a message in a bottle thrown into the cosmic ocean.

[00:02:34]At Carl Sagan's urging, that record held greetings in 55 languages, music from cultures across our world, the sound of rain, wind, surf, and the cry of a newborn child.

[00:02:52]We placed it there because we hoped someday something might find it.

[00:02:59]Over 45 years later, Voyager 1 is the most distant human-made object in existence, traveling at roughly 17 km per second.

[00:03:13]As of today, it sits approximately 23 billion kilometers from Earth.

[00:03:21]That number doesn't mean very much on its own. So, let me translate it.

[00:03:28]If you compressed the distance from the Earth to the Sun, 150 million kilometers, down to a single meter, then Voyager 1 would be standing about 150 m away. A city block.

[00:03:48]That is the entire scope of our physical reach in the cosmos.

[00:03:55]One city block surrounded by an ocean that stretches billions of light-years in every direction.

[00:04:04]And that city block took 45 years of continuous travel to cover.

[00:04:11]At Voyager's current speed, it would need approximately 75,000 years to reach Proxima Centauri, the nearest star to our own sun, at 4.24 light-years away.

[00:04:30]75,000 years. When the first human beings were painting horses and bison on the cave walls at Lascaux, that journey would have had to have already begun. We would not be getting close yet.

[00:04:49]This is the opening clause of the will.

[00:04:53]This is what we have managed to touch.

[00:04:56]Now, let me read you the second clause, because this is where the inheritance first seems unimaginably rich.

[00:05:07]Our galaxy, the Milky Way, our home, contains somewhere between 100 billion and 400 billion stars. The sun is one of them, an ordinary middle-aged yellow dwarf, unremarkable by most cosmic standards, and yet the source of every photon that has ever warmed your face or cast your shadow on the ground.

[00:05:40]The light currently reaching your eyes from the far edge of our galaxy departed before our species existed.

[00:05:50]And our galaxy is not alone. The observable universe, the sphere of space from which light has had time to reach us since the Big Bang, some 13.8 billion years ago, contains approximately 2 trillion galaxies, 2,000 billion of them. Each one a vast city of hundreds of billions of suns with their own planets, their own histories, their own possible stories.

[00:06:21]The total number of stars in the observable universe is estimated at something around 200 sextillion, a two followed by 23 zeros.

[00:06:34]When I first sat honestly with that number, I felt something I can only describe as a kind of conceptual vertigo, as if the ground of ordinary human experience had dropped away beneath me. I sometimes think that the capacity to feel properly astonished by it is itself a rare and precious thing.

[00:06:56]The inheritance sounds at this point almost obscenely vast, but there is a third clause in this document, and you need to read it carefully because it changes everything that came before.

[00:07:10]In 1998, two independent teams of astronomers were using distant supernovae as reference points, those brilliant stellar deaths that burn briefly with the light of billions of suns to measure how quickly the universe was expanding. They expected to find that the expansion was slowing. Gravity pulls things together. The cosmic expansion from the Big Bang should, they reasoned, be decelerating, like a ball thrown upward that is gradually losing momentum to gravity's patient pull.

[00:07:46]What they found stopped them cold. The expansion was not decelerating.

[00:07:52]It was accelerating.

[00:07:55]The universe is not coasting towards some eventual equilibrium. It is rushing. And whatever is driving that acceleration, we call it dark energy, which is perhaps the most honest name in all of science since it openly confesses that we do not know what it is, constitutes approximately 68% of the total energy content of the universe.

[00:08:21]You cannot feel it. You cannot see it, but it is real. And it has been working quietly, continuously in every direction for billions of years.

[00:08:34]And what it has been doing to our inheritance, to those two trillion galaxies, to all that seemingly boundless cosmic property, is something no protest, no physics, no appeal to any higher authority can reverse.

[00:08:50]It is taking most of it back.

[00:08:53]What this means is that space itself is expanding.

[00:08:57]It isn't that galaxies are flying through space away from us like cosmic shrapnel. It's that the very fabric of space-time between us and them is stretching, carrying them away.

[00:09:11]Imagine a long rubber cord with ants crawling on it. The ants are the galaxies. Now, you start stretching the cord. From the perspective of any single ant, all the other ants seem to be moving away. The farther away an ant is, the more rubber cord there is to stretch between them, so the faster it appears to recede.

[00:09:35]At a certain point, the is stretching so fast that no matter how quickly an ant crawls, it can never reach its distant neighbor. The space between them is being created faster than it can be crossed. This is our situation in the cosmos. And this is the revocation clause in our cosmic will.

[00:09:59]The speed of light, 299,792 km per second, is the absolute speed limit for any object, any information, any message we could ever send.

[00:10:14]But there is no speed limit for the expansion of space itself. Beyond a certain distance from us, right now, it's about 14 and 1/2 billion light-years away.

[00:10:28]The universe is expanding so quickly that the galaxies there are being carried away from us faster than the speed of light.

[00:10:39]Their light, the light they are emitting right now, will never reach us.

[00:10:45]Not tomorrow, not in a billion years, not ever.

[00:10:52]We are forever causally disconnected from them.

[00:10:57]Let me say that again because it is one of the most profound and least known facts about our universe.

[00:11:07]The light from those distant galaxies can travel towards us for a million years, and at the end of that million years, because of the expansion of space, it will be even farther away from us than when it started.

[00:11:25]So of those two trillion galaxies in our observable universe, the ones whose ancient light we can still see, 94% of them are already gone.

[00:11:39]They have been removed from the will.

[00:11:42]They are beyond our physical reach, permanently and irrevocably.

[00:11:49]We can see their past, a faint ghostly afterimage from a time when they were closer, but their present and their future is a country to which we will never have a passport.

[00:12:04]We have already lost 94% of the universe, and the loss continues.

[00:12:12]Every second, as space expands, more galaxies cross this threshold, slipping over a cosmic waterfall from which nothing returns.

[00:12:24]This defines a new, much smaller boundary, not the edge of what we can see, but the edge of what we could ever possibly reach.

[00:12:36]It's called the cosmic event horizon.

[00:12:40]It marks the absolute outer limit of our future.

[00:12:44]Even if we could build a starship that traveled at 99.999% of the speed of light, leaving today, this is the frontier we could never cross.

[00:12:58]Right now, that frontier is about 16 billion light-years away. Everything beyond that is lost.

[00:13:08]Everything inside that is, theoretically, on the table.

[00:13:14]So, let's do the accounting.

[00:13:17]Of the two trillion galaxies in the observable universe, how many lie within our reach? How many are of our true, final inheritance?

[00:13:29]The answer seems to be about 3%.

[00:13:34]Our inheritance was not two trillion galaxies. It is, at most, a few tens of billions.

[00:13:42]The rest was just a beautiful illusion, a glimpse of a cosmic fortune that had already been spent by physics before we were even born.

[00:13:53]All of human history, all of our art and science and dreams, has taken place inside a tiny, shrinking bubble of accessible space.

[00:14:06]I sometimes wonder if this is the answer to the great silence.

[00:14:12]The question, first asked by the physicist Enrico Fermi, of why in such a vast universe we have not heard from anyone else.

[00:14:23]We listen for radio signals, for signs of other technological minds, and we hear nothing.

[00:14:31]An unearning quiet.

[00:14:34]Perhaps the silence is not because life is rare.

[00:14:39]Perhaps it's because every civilization on every world in every galaxy eventually discovers this same terrible fact.

[00:14:50]They look up at their skies, they measure the expansion, and they realize they are quarantined, trapped not by walls of rock, but by the stretching fabric of space-time.

[00:15:05]The silence we hear might be the sound of a million voices, each crying out from its own isolated island, islands that are drifting apart so fast that no message can ever bridge the gap.

[00:15:22]The universe may be full of life, but it may also be fundamentally, physically, lonely.

[00:15:31]And this process, this quiet theft by dark energy, will not stop.

[00:15:37]It is relentless.

[00:15:40]Let us travel far, far into the future.

[00:15:45]Imagine a time, perhaps 150 billion years from now, if there are any beings left in our galaxy to look up at the night sky, what will they see?

[00:16:01]They will see the stars of our own galaxy, which by then will have merged with Andromeda and the other nearby galaxies into one enormous island of stars.

[00:16:15]But beyond that, beyond our own local shores, nothing.

[00:16:22]Absolute, perfect, unbroken blackness.

[00:16:28]Every other galaxy will have been pushed beyond our cosmic event horizon.

[00:16:35]Their light will no longer reach us.

[00:16:38]The sky will be empty of them.

[00:16:41]All evidence that there ever was a universe of two trillion galaxies will have been erased.

[00:16:50]An astronomer in that distant future would look out with their telescopes and see only the stars of their own galaxy.

[00:17:00]They would have no data, no hint of a Big Bang, no inkling of an expanding cosmos.

[00:17:08]They would conclude, based on all available evidence, that their galaxy was the entire universe.

[00:17:17]A single, solitary island of light in an infinite, eternal void.

[00:17:24]And the heartbreaking thing is they would be right.

[00:17:32]From their perspective, they would be utterly and completely alone.

[00:17:39]We are, it seems, living in a brief, privileged moment.

[00:17:45]A narrow window of time after the universe has cooled enough for us to exist, but before it has expanded so much that all evidence of its true scale is lost to us forever.

[00:18:00]We are the one generation that can know.

[00:18:06]I have, at times, found this knowledge almost unbearable.

[00:18:12]A kind of cosmic grief.

[00:18:15]To be shown a treasure map of two trillion islands, only to be told that all but a handful were swept away by a tide that rose before you were born.

[00:18:29]It is a profound lesson in humility.

[00:18:33]It reminds me, in a strange way, of my own mortality.

[00:18:39]I sometimes think about my own death, not in a morbid or fearful way, but simply as a fact, as a boundary, a limit that gives the days I do have their shape, their urgency, their meaning.

[00:19:00]A life without limits would be, in a sense, a life without shape.

[00:19:06]And a universe without limits might be a universe without a home.

[00:19:18]But, I've held something back.

[00:19:22]The reading of the will is not quite finished.

[00:19:26]There is one final clause, a codicil written not in the faint ink of distant starlight, but in the indelible force of gravity itself.

[00:19:49]And it says this, not everything is receding.

[00:19:57]There is a small cluster of galaxies, a local neighborhood bound together so tightly by our mutual gravity that the expansion of the universe cannot tear us apart.

[00:20:11]We are falling together, not flying apart.

[00:20:16]This is our true permanent home.

[00:20:20]We call it, with a wonderful and appropriate modesty, the local group.

[00:20:26]It contains our own Milky Way, and it contains the great Andromeda galaxy, our larger twin.

[00:20:35]It contains the Triangulum galaxy, and the large and small Magellanic Clouds, those ghostly fingerprints of light visible from the southern hemisphere.

[00:20:47]In total, there are more than 50 galaxies in this gravitationally bound family, and we are still discovering new, smaller members hiding in the darkness.

[00:21:00]These galaxies cannot be taken from us.

[00:21:04]They are not of the cosmic foreclosure.

[00:21:08]The space between us and Andromeda is not expanding.

[00:21:13]In fact, it is shrinking.

[00:21:16]Our own Milky Way with its hundreds of billions of suns and the Andromeda galaxy with its trillion stars are currently hurtling towards each other at a speed of about 110 km per second.

[00:21:32]There is nothing in the universe that can stop this.

[00:21:37]>> It is a slow, inexorable gravitational embrace that will culminate in about 4 and 1/2 billion years in a magnificent collision.

[00:21:50]The two galaxies will pass through one another, their stars mostly missing each other in the vastness of interstellar space.

[00:22:00]But their gravitational fields will tangle, drawing them back for a second and then a third pass until over hundreds of millions of years they merge into one single gigantic elliptical galaxy.

[00:22:19]A new home.

[00:22:21]Some have already given it a name, Milkomeda.

[00:22:26]This is not an eviction notice from the cosmos.

[00:22:30]This is a homecoming.

[00:22:33]So you see, the limit is not a cage. It is a clarification.

[00:22:40]It is the universe telling us in the plainest language of physics where our story is meant to unfold.

[00:22:49]The loss of those two trillion galaxies is not a tragedy.

[00:22:55]It is the act of pruning that allows the branch that is left to flourish.

[00:23:01]It defines our world. It defines our neighborhood. It tells us what is and what is not ours to care for.

[00:23:13]Everything we have lost makes what remains infinitely more precious. So tonight, if you can, go outside. And if you are far from the city lights in the northern hemisphere, you may be able to see it. A faint, fuzzy patch of light in the constellation Andromeda.

[00:23:37]It looks like a tiny, forgotten cloud.

[00:23:41]That is the Andromeda galaxy.

[00:23:45]Two and a half million light years away, it is the farthest object you can see with your unaided eye. And every second you are looking at it, it is 110 km closer than it was the second before.

[00:24:05]It is not just another galaxy. It is the other half of our future. It is the farthest shore of our true, final, and magnificent inheritance.

[00:24:20]It is coming to us.

[00:24:24]We look out upon the universe, and we see an unimaginable inheritance being swept away on the silent, irresistible tide of dark energy.

[00:24:37]We are the one generation that will ever know the full extent of what we have lost. And that knowledge, that heartbreaking, world-defining knowledge, is a form of grace. It is what tells us, finally, where home is.