Exploring Time, Inhabiting Space: Carl Sagan's Cosmic Odyssey

“ The Cosmos is all that is, all that has been, and all that will be.”

There are ideas that, on their own, can profoundly alter our perception of the world. One of them is the possibility of time travel. Not as a literary fantasy, or riding in a Delorean DMC-12, but as a real implication of the laws of modern physics.

I wish it were enough to reach 140 km/h.

In Cosmos episode 8, Travels Through Space and Time , Carl Sagan examines this idea from a scientific perspective, walking us through key concepts like the speed of light, relativity, and the passage of time under extreme conditions.

The episode opens with an important reminder: the sky we observe is not fixed. Stars move through the galaxy on individual paths that can last millions of years. From our vantage point, they appear fixed, but if we could observe the sky by accelerating time, we would notice how the constellations change, warp, and disappear. This silent transformation is a natural way of traveling through time: looking back, through the light that reaches us from ever more distant places.

It's a great start to this magnificent episode.

Personally, I've always felt that astrophotography is a way to experience that journey. Every photograph of the sky I take is a journey into the past: a light that has taken years, centuries, or even millennia to reach us. It's a way to directly contemplate the history of the universe, and perhaps that's why I'm so passionate about it. With every image I take, it's possible to connect with that time scale that transcends our everyday lives and reminds us that everything is in motion.

M63 the sunflower galaxy, this photograph captured photons of light that had traveled 37 million years through space, because of this, this image is a reflection of the galaxy's past.

Sagan poses a question that has recurred throughout the history of thought: what would happen if we could escape our own time? Could we see what the Earth was like in the distant past, or anticipate its future? The episode explains how, under certain physical conditions, especially when traveling at speeds close to the speed of light, time passes differently for the traveler compared to those at rest. This is the time dilation predicted by the theory of relativity.

Using simple, visual examples, Sagan describes how an astronaut traveling at a significant fraction of the speed of light could experience a few years of travel, while on Earth it would have taken centuries. This phenomenon is not just a theoretical curiosity: it is a direct and proven consequence of how the universe works.

One of the most interesting concepts presented is the possibility that a sufficiently advanced civilization could build structures capable of supporting very long-term interstellar travel: generation ships. These would be authentic, self-sufficient miniature worlds, with entire populations living and reproducing for thousands of years, on a journey to another star system. It's an idea that forces us to consider the future of humanity from a very long-term perspective and raises technical, ethical, and cultural questions.

Rather than speculating about unlikely scenarios, the episode shows how these possibilities are supported by real physical laws, even though current technological challenges mean they remain out of reach for now.

This is what future interstellar spacecraft could look like according to AI.

Interesting curiosities

• Long-term stellar motion : Although stars appear fixed in the night sky, they move through the galaxy over millions of years. If time could be accelerated, the constellations we recognize today would completely change shape or disappear.

• Time dilation is a real phenomenon : Experiments with atomic clocks on airplanes and satellites have shown that time does indeed slow down for fast-moving objects or those in strong gravitational fields, just as Einstein predicted.

• Generation ships : The idea of a ship in which several generations of humans live and die during an interstellar voyage, although still purely theoretical, has been seriously considered in discussions about future space colonization.

• The Farthest Journey We Could Take : If we wanted to visit even the nearest stars, like Alpha Centauri, with current technology, it would take tens of thousands of years. The episode underscores the scale of the challenge posed by interstellar travel.

• Symmetry in the laws of physics : One of the deepest principles of modern physics is that the laws of the universe appear to be the same in all places and times. This supports the idea that we could make predictions about the past and future of the cosmos based on current observations.

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Key themes of the episode

• The relativity of time : How speed and gravity affect the passage of time and what implications this has for space travel.

• Movement of stars and evolution of the night sky : A reminder that even the sky changes over time, although these changes are not perceptible on a human scale.

• Long-term interstellar travel : The possibility that technologically advanced civilizations might attempt to colonize other solar systems using self-sufficient ships.

• Cosmic Time Scale : Understanding the universe involves shifting our intuition about time to scales of millions or billions of years.

• The future of humanity beyond Earth : A reflection on what a civilization would need to ensure its long-term survival.

In the end, Journeys Through Space and Time isn't just an episode about advanced science. It's a love letter to the scientific imagination. To our capacity to ask ourselves: what if…?

Because, as Sagan reminds us, perhaps the longest journeys are not those we measure in kilometers, but those that force us to imagine other possible futures.

See you next week. In the next episode, The Lives of Stars , Carl Sagan will tell us about the birth, evolution, and death of those suns that light up the night. Because the universe doesn't just move: it also lives.