[curious music] (host) Hey!

Cool?

Wow is a special presentation of It's Okay to be Smart.

The history of burrito transport.

It used to be if you wanted to get one of these from here to here, you had to use one of these.

That will all be a thing of the past when this opens.

It's the transcontinental burrito hypertunnel.

[fanfare] (host) A tunnel, dug through the Earth, zooming burritos from coast to coast in just minutes, powered by nothing but gravity?

It almost sounds like something that's completely made up for making an elaborate and overly theatrical scientific point.

For centuries, subterranean gravity transport like this was the stuff of dreams, but great minds of physics made it possible.

How does it work, and what does its future hold?

We'll explore!

One of the most monumental undertakings humankind has ever attempted, the fall and rise of the burrito tunnel, is a story for the ages.

As civilization has advanced, we've looked for quicker and quicker ways to transport people, information, and everything else from there to here.

And not just faster ways of traveling but also faster routes between where we are and where we want to go.

Columbus was just looking for a shortcut, too, but what he found was the Americas, a land full of rich and vibrant cultures, like the Aztecs, a people who had invented quite a few things of their own, including an ingenious way of making a meal easy to carry.

The tortilla!

Fast-forward four centuries or so, and that wrapped-up meat has evolved into this, the burrito, a self-contained meal in a flawless geometric shape.

And the most perfect of these perfect foods are made in the Bordeaux of burritos, San Francisco's Mission District.

Problem is, if you're in, say, New York City, how do you get one of these in just minutes?

Forget about it.

Science changed all of it.

Not too long ago, you could get a hot, fresh burrito from around 3,000 miles away delivered to you in around 40 minutes?

You may have heard of it.

Mr. Roberts and The Burrito Tunnel: An Origin Story.

In 1956, Duane Roberts had found success selling frozen burger patties to McDonald's, so he turned his attention south of the border for his next invention, the frozen burrito.

Maybe you've heard of it.

He realized there was money to be made, as burrito makers on the West Coast could flash freeze their creations and get them to hungry mouths on the East Coast by using refrigerated trucks, trains, or planes or, say, using a burrito-sized tunnel through the Earth.

Wouldn't that be something?

Roberts' idea was to use gravity to propel burritos through a deep transcontinental tunnel in a nearly perfect vacuum wrapped in special foil to enable frictionless magnetic levitation, allowing coast-to-coast burrito travel at speeds once imagined only for space vehicles.

Sounds too good to be true, and it was until Mr. Roberts' eventually built it at a cost of $420 thousand billion.

First, some science.

And to figure out how it worked, we have to go back to 17th-century England.

A guy named Robert Hooke wrote a letter to a guy named Isaac Newton.

Maybe you've heard of him.

They were trying to figure out what would happen if you dropped a ball through a hole that reached through the Earth.

But they might as well have dropped some rice, beans, meat, cheese, and avocado wrapped in a tortilla, because what Newton realized about gravity inside the Earth is the reason Duane Roberts' burrito tunnel ever worked.

There's a gravitational force acting on you right now from the Earth, and the same force is acting on all the things around you.

We call it gravitational potential because it means something has the potential to move.

How much potential?

Well, that depends on how far you are from the center of the planet.

So if you dig a tunnel down through the center of the Earth like Newton and Hooke imagined all the way to the other side and dropped a burrito in, what would happen?

Whew!

That would be one hot trip.

For a moment, let's imagine the Earth is a perfect sphere and the same density the whole way through.

In reality, neither of those things are true, but let's pretend.

The pull of gravity from the whole Earth's sphere is the same as if all its mass was concentrated at a point right in the center because the stronger pull from whatever side is closest to you and the weaker pull from the side farthest away, they just cancel each other out.

But what about inside the Earth?

As the burrito falls and the distance between it and that center mass shrinks, it feels the same gravitational potential as it would on a smaller, lighter Earth, as if we were just peeling layers off the Earth.

As the burrito falls and the closer it gets to the center of all that gravity, it'll keep speeding up, but it's being pulled less and less because there's less of Earth's mass under it.

Once it's past the center and hurtling towards the other side, now it's being slowed down by the gravity of the rapidly enlarging Earth.

Now, what about this path?

It works an awful lot like a pendulum, only instead of gravity staying the same and the path changing, it's gravity that changes, and the path stays straight.

The burrito goes downhill, then uphill, then in a straight line.

Whoa, that's weird.

Newton and Hooke also realized something pretty weird.

Turns out, it takes the same amount of time to make the trip no matter what two points on Earth you connect.

How long?

The closer the tunnel is to the center, you get going faster, but you've got a longer distance to travel.

It would be exactly 42 minutes for the Earth with uniform density, but because the Earth's density isn't the same way all the way through, the time could range anywhere from 42 to 38 minutes depending on how deep the tunnel is and the density of the Earth at different depths and complicated physics things kind of like that.

The demise of Mr. Roberts' burrito tunnel.

For a few glorious years at a maximum depth of 315 kilometers below the earth, Duane Roberts' original transcontinental burrito tunnel delivered hot, fresh Mission burritos coast-to-coast in the same time it takes to get a pizza.

But after the excitement of the Apollo moon landing had worn off, and thanks to an influx of convenient dining options ranging from gas station sushi to whatever Arby's is, the transcontinental burrito market collapsed.

Forty-two minutes was just too slow for the modern food consumer.

The original burrito tunnel was closed, and the remains of its burrito processing plant are all that's left.

Duane Roberts was better at making cold, hard burritos than cold, hard cash.

Oh.

Whoopsie-doops.

This-- If there's just one obstacle he could never overcome in a tunnel through the Earth, there's a speed limit.

That is unless you dig a different sort of tunnel.

Bernoulli & the Bratist-- Brachistochrone.

A few years ago, a young inventor named Egon Shmusk had an idea.

Maybe you've heard of him.

He loved California burritos, too, and he knew he could ship them coast-to-coast faster than Robert's straight-line tunnel ever could.

Remember how this all got started in the 17th century with Newton and Hooke thinking about holes in the Earth?

Well, a couple decades after them, a fella named Johann Bernoulli publicly challenged the world's mathematicians to solve this problem.

Maybe you've heard of him.

Powered only by gravity, what's the path that connects two points in the shortest amount of time?

Bernoulli thought he was hot stuff.

It had taken him about two weeks to come up with the answer, so he was pretty surprised when he got an anonymous letter from someone who claimed to have gotten the answer in one night.

It turns out, that letter came from-- you guessed it--Isaac Newton, who was very smart.

Maybe you've heard of him.

Newton and Bernoulli figured out the shortest path, a straight line, isn't the shortest time.

Inside our circle, let's trace a path using the edge of another circle.

There, that's called a hypocycloid.

Travel down that path, and you'll beat any other path between those two points every time, whether you're talking burritos or a subterranean passenger train.

They called it the brachistochrone curve.

It's Greek for "shortest time."

It's the perfect balance between picking up speed early and keeping the total path length short.

And when Egon Shmusk learned about that, he took the fortune that he made selling electric scooters and started digging.

His spicy innovation?

A brachistochrone hypertunnel.

A Bright Future of the Transcontinental Burrito Hypertunnel.

(host) In two years' time, the transcontinental burrito hypertunnel is scheduled to begin completion, and when it does, customers in New York City will be able to get burritos in just over 25 minutes' time all the way from San Francisco, hot and fresh, warm in their wrappers, by the geothermal convection currents of our planet's mantle, and 17 minutes faster than Duane Roberts' burrito tunnel ever could, and fast enough for a lunch break in today's fast-paced, virtual-reality, Internet, web-connected, algorithm, computer, semiconductor, quantum, social-media society.

But it's not without its challenges.

The brachistochrone tunnel's greater depth than its predecessor, running nearly 1,300 kilometers below Nebraska at its lowest point, means enormous pressures and extreme temperatures, nearing 2,000 degrees Celsius.

It required Shmusk's engineers to build their tunnel walls, their magnetic rails, and even their robotic mining drill out of a newly discovered, nearly indestructible metal alloy called unobtainium.

Invest in unobtainium.

You'll be a hundredaire.

So what's next?

Are brachistochrone burritos just another pipe dream, or are taco tunnels on Titan next, or maybe sushi on Europa?

Well, that's the funny thing about technology.

Often the right path isn't the easiest one.

But with enough creativity and innovation and a big shovel, there's always light at the end of the tunnel.

And also burritos.

Stay curious.

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