Hello, SFIA audio listeners. In this month's Nebula exclusive, Big Alien Theory,

we're asked at the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and ad-free, plus hours of bonus content,

check out go.nebula.tv slash Isaac Arthur and use my code, Isaac Arthur.

We often fly through thin air to get to places faster, but in the future we might travel

through no air at all, even here under the sky.

So today we'll be looking at the concept of the vacuum train, which is a very large category

of vehicles for use on and off planets, including one designed for getting off planets like a

mass driver, and more recently a strangely controversial subject. I'm assuming some of this

is because of Elon Musk's hyperloop, and all sorts of videos point you out real

or perceived engineering or economic issues with the design, or its planned implementation,

along with rebuttals.

I'm going to preface this video by noting that the Hyperloop is one type of vacuum train,

indeed one running on very low pressure rather than a full vacuum, and there are

many designs for many purposes, both of hyperlopes and of the wider category of vacuum

trains, along with pneumatic options like scaled-up versions of the message tube systems

a lot of office buildings or banks have. Some, like the gravity train, aimed to move objects around

a large body with virtually no energy expended. Some are akin to the hyperloop, while others

are meant to run around planetary-scale megastructures at near-orbital speeds. There are a lot of

points of overlap in basic function with the hyperloop, so we will

address some of the common concerns raised about that, where it is relevant. I should emphasize

that some of those are quite legitimate concerns, though others are less so, and none

...