What if you could travel to another star and return to find that far more time had passed on Earth than you had experienced yourself?

It sounds like science fiction, but it's a real consequence of Einstein's theory of relativity. Time does not pass at the same rate for everyone. And the faster you travel, the more extraordinary that difference becomes.

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😴 Also from the Sleepy Family: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Biologist⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ | ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Geographer⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ | ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Investigator⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠

In this fourth episode of the series, we explore one of the strangest consequences of travelling through space: time itself begins to behave differently for the traveller.

We begin with particles arriving from the upper atmosphere, whose brief lifetimes are stretched by their extraordinary speeds. From there, we follow the mathematics of relativity into the possibilities of interstellar travel, where journeys lasting thousands of years on Earth might take only decades for those aboard the spacecraft.

But travelling fast enough is only part of the problem. We explore the challenge of slowing down, the extraordinary energy required to reach distant stars, and the human cost of arriving somewhere that may have changed beyond recognition.

Along the way, we encounter the famous twin paradox, astronauts whose journeys have left them fractionally younger than people on Earth, and spacecraft pushing the limits of speed within our own solar system.

We also look at the technologies that might one day carry us between the stars, from laser-driven sails to fusion and antimatter propulsion.

And finally, we return to a discovery made in the seventeenth century, when astronomer Ole Rømer realised that even light takes time to cross the immense distances of space.

Perhaps the most remarkable thing about time dilation is that it isn't a way around the speed of light.

It's something the universe gives us because of it.

So settle in, turn down the lights, and come with me on a journey through the strange relationship between speed, distance and time.

What happens when time itself becomes the traveller?

What if you could cross the vast distances between stars without experiencing the journey at all?

Interstellar travel may take centuries, millennia, or longer. Even if we eventually develop spacecraft capable of reaching other star systems, the journey itself presents another extraordinary problem: what happens to the people who have to make it?

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😴 Also from the Sleepy Family: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Biologist⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ | ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Geographer⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ | ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Investigator⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠

In this third episode of the Impossible series, we explore the idea of putting the human body into a state of suspended animation, allowing travellers to sleep through the years or centuries between worlds.

We begin with the remarkable ways life on Earth already slows its metabolism, from hibernating bears to Arctic ground squirrels, before exploring what medicine has learned about deliberately cooling the human body. From there, we venture into the possibilities of therapeutic torpor, spacecraft designed around sleeping passengers, and the enormous challenge of preserving biological tissue without destroying it.

But even if we could put someone to sleep for decades, another question waits at the other end.

What happens when they wake up?

The world they left behind may be gone. The people they knew may have disappeared into history. Civilisations may have changed beyond recognition.

And yet, for the traveller, the journey itself might feel like nothing more than closing their eyes on one world…

…and opening them on another.

So, settle in, turn down the lights, and let's imagine a very strange possibility:

Can we skip the journey?

What if humanity never had to send a living thing between the stars?

A spacecraft carrying humans would need to survive an extraordinary journey through interstellar space. But what if we removed the most fragile part of the equation entirely — the traveller?

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😴 Also from the Sleepy Family: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Biologist⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ | ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Geographer⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ | ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠The Sleepy Investigator⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠

In this second episode of the series, we explore one of the strangest possibilities in interstellar exploration: self-replicating probes. Machines that could travel to another star, land on an asteroid or moon, extract raw materials, build copies of themselves, and send those copies onward into the darkness.

The idea begins with a deceptively simple question: if a machine can build another machine, how far could that process eventually go?

We follow the concept from John von Neumann's original thinking about self-reproducing machines to the engineering challenges of building an autonomous probe capable of mining, manufacturing and repairing itself far from Earth. Then we follow the mathematics outward, where a single machine becomes two, two become four, and an apparently slow process begins to spread across the galaxy.

But exponential growth brings an uncomfortable question with it.

If we ever built a machine capable of reproducing itself among the stars, could we be certain we could control what happened next?

And if such machines are possible, and a civilisation had millions of years to develop them, what might the galaxy look like today?

Perhaps the strangest part of the idea isn't how quickly these machines could travel.

It's how patiently they could wait.

So settle in, turn down the lights, and come with me into the darkness between the stars.

Can machines build themselves?