Hello, SFIA audio listeners. In this month's Nebula exclusive, big alien theory,

we're asking 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.

Good afternoon, everyone, and welcome to our monthly live stream Q&A.

This is for the day before Halloween, but as you can see, hopefully we are not actually dressed

up for anything, just as a regular way in the pie tie tie, and joined by my beautiful wife,

Sarah Fowler, author, who is our co-host today, and we'll be asking all of your questions as you get them into the chat.

We have a couple that came out early that we're going to go ahead and start with them. We'll get right to them.

I think I'm crooked. I was looking at that, and I was like for this once a month event, it always requires a bit of fine tuning.

Okay, is that better?

Yeah.

Okay.

Thank you.

I just didn't think I could ask a question on my side, which might actually be an interesting question for space.

To put you upside down on the screen, too.

Okay.

So let's start off with a question here from CR Smith, and he says,

Isaac Arthur, you often talk about light sails, but how can light impart a force?

Force equals mass times acceleration, and light is massless, so zero.

Anything times zero is zero, so there shouldn't be any force.

Please help.

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