Mindscape Ask Me Anything, Sean Carroll | July 2026

**Sean Carroll** (0:00)
Awkward time to ask this, but hey, did you download the trail map?

**SPEAKER_2** (0:04)
Yeah, no, I don't need to.

**SPEAKER_3** (0:05)
I don't understand, you're trusting your signal out here?

**SPEAKER_2** (0:08)
I'm trusting T-Mobile.

**Sean Carroll** (0:09)
They have the best network, and if we end up in bum-tots nowhere, well, we've got T-Satellite for backup. Well, I don't trust my carrier that much.

**SPEAKER_3** (0:17)
We'll just use your phone as a flashlight.

**SPEAKER_2** (0:20)
With America's best network and T-Satellite, we're keeping you connected in places you never thought possible. And if you switch today, you get free phones for zero down, and only 25 bucks a month per line for four lines. Find out more at tmobile.com or visit your local store.

**SPEAKER_3** (0:37)
Best mobile network based on analysis by Euclid Speedtest Intelligence Data 2Age 2025 with 24 Lengthened Bill Credits and four eligible port-ins on Essentials. For well-qualified customers with auto-pay plus taxes, fees, and $35 connection charge per line. Credits and imbalance too. If you pay off earlier, cancel. Contact us. Finance Agreement Example 29999 Moto Edge 5G Required. T-Satellite available with compatible device in most outdoor areas in the US where you can see the sky. Included with experience beyond $10 a month. For news monthly, cancel anytime, visit tmobile.com.

**Sean Carroll** (1:00)
Hello everyone. Welcome to the July 2026 Ask Me Anything edition of the Mindscape Podcast. I'm your host Sean Carroll. I'd kind of planned out a intro here because I just wrote a thread on Blue Sky, where I talk about Boltzmann brains. I went on a little Boltzmann brain rant and I thought that would be a perfect lead in to the AMA. But then I realized that we have an AMA question about exactly this topic, so it just makes more sense to do it in the body of the podcast, which leaves me nothing to say. For the intro, and you know what, that's okay. We don't need anything special for the intro. You're here for the AMA, not for the intro. I will do the usual thing of thanking Patreon supporters of Mindscape, who are the people who make these AMAs possible. Patreon supporters of Mindscape, well-known worldwide to be the most attractive, intelligent, and charming people that anyone has ever met in their lives. And it's kind of amazing that there's such a coincidence or correlation between being a Mindscape supporter and those other good qualities. No real idea why that's true, but it turns out to be true empirically. And amazingly, you could be a Patreon supporter of Mindscape. It's not that hard. No initiation, hazing rituals, or anything like that. Just go to patreon.com/seanm.
Carroll and sign up. Nominal fee gets you membership in this elite community, as well as ad-free versions of the podcast and the right to ask AMA questions. So your name could appear on a future Mindscape episode. It's just all part of the bargain. And with that, let's go.
Eric Olav Chen asks, Could you give an overview of the current state of debate on Boltzmann brains? I'm specifically curious about, number one, how long the current evidence is, how strong the current evidence is, from the Swamp Land program, the long-lived desider space is not possible at all in quantum gravity. And number two, how widely accepted is the argument you put forward with Body and Pollock, that eternal desider space does not support the kinds of fluctuations that give rise to Boltzmann brains at all.
Right, so as I mentioned in the intro, Boltzmann brains have been on my mind for a while. So I've been thinking about them and I did a little thread on Blue Sky. It really belongs like a blog post. But right now, my blog is kind of down. It's not 100 percent down. The website was down for a day or two. The blog is just messed up. The theme is messed up. I need to get around to do that. But as I will complain about later in the podcast, I've been too busy to do things like that.
So the reason why I was thinking about Boltzmann brains is because there's a question.
How big is this problem? Well, actually, let me start with my own perspective on the problem.
And I'll tell you, and then I'll back up and explain what the problem is. But my perspective on it is the prospect of Boltzmann brains dominating the universe is an absolute killer for models that predict that. And among the models that plausibly predict that is lambda CDM, the best, most popular, default cosmological model among modern cosmological thinkers. And so this is a potential disaster. We're saying that everyone's favorite cosmological model is potentially incoherent, potentially inconsistent internally. It can't possibly be the right theory of the world. Now, of course, you can say, well, okay, we'll tweak it to make it more compatible. Great. Tell me exactly how you're going to do that. So you would think that given this situation where you have a very popular theory and it's potentially self-undermining and completely incoherent, that a huge amount of work would be done in the theoretical cosmology and physics community trying to come to terms with this problem. And the fact is that there isn't.