How To Prove You're A Time Traveller

Summary

Podcast Summary – The Rest Is Science, "How To Prove You're A Time Traveller"

Date: April 22, 2026
Hosts: Professor Hannah Fry & Michael Stevens (Vsauce)

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Overview

In this engaging Field Notes episode of The Rest Is Science, Hannah Fry and Michael Stevens explore the conundrum of how a person could prove they are a time traveller, diving into both the science and thought experiments behind it. The hosts answer audience questions that take them from hypothetical time travel credibility tests, to Jurassic food dilemmas, Moon communication lags, and wormhole perils. The discussion is laced with humor, wonder, and memorable anecdotes, all while exposing deep, sometimes overlooked scientific and philosophical challenges.

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Key Discussion Points & Insights

1. The Time Traveller's Credibility Problem

Topic: How could you prove to 2007 that you’re from the future?

• Scenario Set-up (04:02–04:44):
  Michael proposes the challenge: If you were transported instantly with only what you have now to 2007, how would you prove you’re from the future, particularly as quickly as possible?

• Problems Identified:

  • Long-range predictions (like the 2020 pandemic) require years to verify (05:04).
  • Possession of specific future-knowledge (lottery numbers, #1 hits) can’t be immediately proven or might trigger butterfly effects (05:29–06:10).
  • Even repeat accurate predictions could be disbelieved; people might assume you’re a con-artist, not a time traveller (06:10–06:26).

• Michael’s Solution: The Slap Bracelet (06:27–09:53):

  • Michael’s invention is a slap bracelet displaying timelines and GPS coordinates for famous lost & found objects — e.g., Titanic's coordinates, Sutton Hoo, Archimedes' lost work.
  • This lets you tell people exactly where to unearth these objects and is verifiable in real time:

    “With multiples, especially different types of things... come on, you’re a time traveller.” — Michael (09:30)

  • However, unless you always wear it, the plan has a flaw (09:56–10:18).

• Limitations & Comic Relief:

  • Debates on wearing vs tattooing such evidence, with Michael worried it may make him look like a witch if taken to the deep past (12:08–12:49).
  • Anecdotes on functional tattoos (Adam Savage’s ruler tattoo), and measuring-tape slap bracelets with Pi intervals (10:57–11:50).

2. Could You Prove You’re From the Past?

Forwards Time Travel (16:32–18:56):

• Hannah flips the scenario: If you travel forward in time, how do you prove your origins?
• Only “evidence” is ignorance of current knowledge, or skills/objects now obsolete. Even skills (e.g., memorizing phone numbers) can be learned, so proof is tough.
• If something familiar is lost in the future, producing it might validate your story (“a very priceless relic that’s well known”—17:11). Or, “maybe all you need is to prove that you know how a slap bracelet works” (18:45).

3. Eating in the Age of Dinosaurs

Question from Alex (21:42–25:50):

• Would a human in the Jurassic (150 million years ago) find digestible food?
  • Virtually no fruit, grains, legumes, or even root vegetables: “...at the late Jurassic period, there’s no flowering plants. No fruit, no grasses.” — Hannah (22:33)
  • Meat (e.g., dinosaur) would be “chewing on an old goat,” plus highly carcinogenic due to plants dinosaurs ate.
  • Many edible plants today have been bred to be non-toxic; ancient versions were quite poisonous.
  • CO₂ levels were 5–10× modern levels, so you’d also be “perpetually out of breath” just trying to survive (25:21).
  • Takeaway: “You could survive, but you’ve basically got roasted dinosaur and pine nuts and that’s it.” — Hannah (24:53)

4. Communication Lag from the Moon and Beyond

Question from Adam (26:29–34:35):

• On FaceTime with someone on the Moon, the delay is ~1.357 seconds one-way — demonstrated with a role-play (30:12–30:51).
  • Mars: 3–21 minutes delay each way; communication becomes “voice note territory”— Hannah (31:51)
  • Jupiter: 33–53 minutes, Pluto: 5 hours, nearest star Proxima Centauri: 4+ years.
• Discussion:
  • The speed of light is the hard limit (“not something technology is gonna fix”).
  • Earthshine on the Moon provides enough light for distinct shadows at "night." (29:37–29:44)

5. Wormhole Pressure (and Disaster)

YouTube Question: Downsides of Pressure Differential Between Cities (34:35–38:18):

• Instant wormhole from sea-level London to high-altitude Denver would create a deafening, catastrophic pressure blast—air would rush through at 150 m/s (way worse than a hurricane).
• Other side effects:
  • Rapid cooling—“air would probably drop by tens of degrees instantly” (37:39).
  • Formation of clouds, and possibly destructive “Category 5, worse than Category 5 blast of freezing air” (37:58–38:02).
  • Michael: Air pressure in Denver is like “being inside a vacuum cleaner at sea level” (36:39).

6. “Omniscience” and Collective Human Knowledge

Question: When Was It Impossible to Know Everything...? (38:40–47:16):

• Books:
  • You could truthfully claim to have read every English book until 1529; after that too many were published per year to keep up (40:52–41:21).
• Broader Knowledge:
  • Even in the earliest organized societies, global collective knowledge was inaccessible to any one person; only in the Stone Age could all skills overlap (43:39–47:16).
  • Hannah posits: “There was such simplicity to our knowledge that there just wasn’t very much to learn at all.”
  • Once tool use and agriculture took off, knowledge grew “very, very quick... vast wealth of stuff” (47:16).

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Notable Quotes & Memorable Moments

• On the Need for Multiple Proofs:

  “With one [lost-and-found object], you’re just a good looker, but with multiples, especially different types of things... come on, you’re a time traveller.” — Michael (09:30)

• On Time Travel Paranoia:

  “I could tattoo them on my body. The one fear, though, is that the existence of the tattoo itself might make me look like a witch.” — Michael (12:18)

• Michael’s Realization:

  “I don’t anticipate accidentally time traveling at any point, but man, wouldn’t it be hilarious if that happened? And I was unprepared, and I’m like, man, I specifically made a tool for just this, and I don’t have it.” — Michael (10:18)

• On Eating in the Jurassic:

  “It would be a lot less like eating salad and more like eating furniture, essentially.” — Hannah (23:31)

• Space Communication Role-Play:
  – [Demonstration of 1.357 sec delay] (30:12–30:51).
  Hannah: “It does slightly get in the way of really having a good conversation.”

• On the Impossibility of Omniscience:

  “We don’t even know everything we collectively know.” — Michael (43:11)

• Philosophical Aside:

  “Could there come a point where the past is so big and new discovery is so slow that our species is actually forgetting things faster than we're learning new things?” — Michael (48:42)

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Timestamps for Important Segments

• Time Traveler’s Credibility Problem: 04:02–10:18
• Lost-and-Found Objects Bracelet: 07:34–09:53
• Pi-Tape Measuring Bracelet Fun: 10:57–12:49
• Proving You’re from the Past: 16:32–18:56
• Jurassic Food Dilemma: 21:42–25:50
• Moon Communication Lag: 26:29–34:35
• Wormhole Pressure Dangers: 34:35–38:18
• Limits of Human Omniscience: 38:40–47:16

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Tone & Style

The episode is curious, geeky, playful, and occasionally self-deprecating. Both hosts use humor and friendly banter, relating wild, sometimes philosophical ideas back to familiar experiences and practical thought experiments.

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For Listeners Who Missed It

You’ll walk away with a real sense of the scientific and philosophical hurdles a real time traveller would face, from the deeply practical (how to prove your origin?) to the absurdly physical (150mph wormhole gusts!) and existential (can anyone ever “know it all”?). The slap bracelet, Pi-tape, and delightful Jurassic menu are just a few of many fun details. It's a rich, inspiring, and very human exploration of science’s “what ifs.”

Transcript

[Hannah Fry]

Welcome to the rest is Science. I'm Hannah Fry.

[Michael Stevens]

And I'm Michael Stevens.

[Hannah Fry]

And this is Field Notes, which is the part of the week where Michael and I are supposed to take a bit easier to turn up and just answer your questions and show each other an object and have a lovely old time. And frankly, you guys don't notice the amount of effort that we put in in the other one because you like this one just as much.

[Michael Stevens]

Well, you know what, it's the magic of that we have Hannah. We should try to do something really boring, like how few views can we get on a podcast episode.

[Hannah Fry]

Make that the challenge. Yeah. See what we can optimize for.

[Michael Stevens]

We could do just like a clip show, like not, not a best of clip show, but a worst of clip show. The moments where jokes fell flat, where we, we, we said something kind of wrong.

[Hannah Fry]

Unfortunately, it's not going to be today because it's absolutely, it's jam packed full of amazing stuff.

[Michael Stevens]

I know today's is actually good. Today I want to show off a tool I invented that helps solve a problem that will never happen.

[Hannah Fry]

We've done something a bit different today. We've done something a bit upside down. We've gone through the mailbag and we have found questions that are all to do with time travel or sort of unexpectedly popping out of existence in one place and popping up somewhere else.

[Michael Stevens]

So I'm going to begin with a question for you, Hannah, and this is also for all the listeners out there.

[Hannah Fry]

Okay.

[Michael Stevens]

If I were to time travel you right now at this instant with what you have on your person to today. Okay. This exact same day, but in 2007.

[Hannah Fry]

Right.

[Michael Stevens]

How would you prove that you were from the future?

[Hannah Fry]

This episode is brought to you by Cancer Research uk.

[Michael Stevens]

Here's something strange. Your DNA contains more ancient viral fragments than genes. The genes that build our cells make up only 2% of our DNA. And for years, that is what scientists focused on. They treated the rest, the ancient viruses and stuff, as junk.

[Hannah Fry]

But now we know that that hidden majority, sometimes called the dark genome, influences how our biology works and how diseases like cancer behave.

[Michael Stevens]

It's a remind progress rarely comes as a single breakthrough. It builds gradually. Cancer Research UK plays a central role in that progress, supporting decades of research into over 200 types of cancer. Work that's helped double survival in the UK over the past 50 years.

[Hannah Fry]

For more information about Cancer Research UK, their research breakthroughs and how you can support them, Visit Cancer Research uk.org Thereest is science.

[Michael Stevens]

I sold my car in Carvana last Night.

[Hannah Fry]

Well, that's cool.

[Michael Stevens]

No, you don't understand. It went perfectly. Real offer down to the penny. They're picking it up tomorrow. Nothing went wrong. So what's the problem? That is the problem. Nothing in my life goes as smoothly. I'm waiting for the catch.

[Hannah Fry]

Maybe there's no catch.

[Michael Stevens]

That's exactly what a catch would want me to think.

[Hannah Fry]

Wow.

[Michael Stevens]

You need to relax. I need to knock on wood. Do we have wood? Is this table wood? I think it's laminate. Okay. Yeah, that's good. That's close enough. Car selling without a catch. Sell your car today on car funnel. Pickup fees may apply. No one goes to Hank's for spreadsheets. They go for a darn good pizza. Lately, though, the shop's been quiet. So Hank decides to bring back the $1 slice. He asks copilot in Microsoft excel to look at his sales and costs and help him see if he can afford it. Copilot shows Hank where the money's going and which little extras make the dollar slice work. Now Hanks has a line out the door. Hank makes the pizza. Copilot handles the spreadsheets. Learn [e-mail] work.

[Hannah Fry]

2007.

[Michael Stevens]

Yeah. So you're in the same room that you're in right now. If you're listening and you're in the car or you're at home, boom. It's just suddenly 2007. What do you do? How do you get anyone to believe that you actually came from 2026? This whole challenge of a time traveler being able to prove that they're from somewhen else in time. I call it the time traveler's credibility problem.

[Hannah Fry]

Nice.

[Michael Stevens]

There's just too many easier explanations for their behavior and knowledge than the truth, which is that they actually traveled through time. How do we crack it?

[Hannah Fry]

Do I get advance notice that I'm going to go like, can I prep? Can I bring stuff with me?

[Michael Stevens]

No, no advance notice.

[Hannah Fry]

Just straight away there, instantly.

[Michael Stevens]

Or you could imagine that you accidentally time traveled. You had no preparation. You can't bring anything with you except what you have on your person.

[Hannah Fry]

Okay, so I'm trying to think 2007. Because the thing is, is that there's. You could say, oh, in the year 2020, there's going to be a big pandemic. But then people have got to wait 13 years for that to happen.

[Michael Stevens]

Exactly.

[Hannah Fry]

So that's not going to work. I don't have any lottery results memorized.

[Michael Stevens]

Nope.

[Hannah Fry]

I don't have like a, you know, a Catalog of like number one hits in my head. That would be like, oh, I know that this person's going to be a big deal. I mean, every bet that you could lay would take some time to prove. How quickly do I have to prove that I am. That I am from the future?

[Michael Stevens]

Well, as fast as possible, right? That's the goal here. And the more you think about it, the more you realize how difficult it is. Because even if I gave you some prep time and you brought an almanac with you or a list of lottery numbers, you run the butterfly effect risk where just by you being there and having this book and speaking the numbers, you cause, you know, a little bit of a wind current change, which changes how the balls are collected. And the number doesn't even match what it should have been if you hadn't been there. Therefore nullifying your attempt to prove you're from the future.

[Hannah Fry]

Like Biff's dad and Biff and his dad in Back to the Future. Where they swap roles.

[Michael Stevens]

Exactly. And similarly, if someone were able to predict the lotto numbers, like, I don't know, three days in a row, I still wouldn't think they were from the future. I would think that they were in cahoots with the lottery. That's a much simpler explanation. So what do you do?

[Hannah Fry]

What do you do, Michael?

[Michael Stevens]

Well, I'm not entirely sure, but I think I've gotten as close as I can with my own brain. I created last year some slap bracelets that look like this. And I've got them on wooden hands because, you know, you gotta. You gotta have em modeled.

[Hannah Fry]

They were pretty fancy in 2007, to be fair.

[Michael Stevens]

Yeah, exactly. Right. And this is good for any time travel. From 20, let's see, from 2022 all the way back to the year 600. Okay, so if you're gonna go before 600, you'll need a different slap bracelet. If you're gonna go back just like a year, you'll need a different slap bracelet. But this thing will stay on you at all times. And it is an emergency tool to prove that you've traveled through time to the people in the past. The way it works. And I wanna hear what your thoughts are. Cause I'm not saying this is perfect, but what this gives you is a timeline with some bars on it showing famous, widely known things that were lost and then later found. Oh, so.

[Hannah Fry]

So hold on. We know where they are now because we live in the future. But if you go back in time, people don't know where they're hidden.

[Michael Stevens]

Exactly. Because you can't just go back with, like a diagram of an iPhone to say, the 50s, and show people, see, this is a future technology. They'll just think that you're really smart and came up with it, you know.

[Hannah Fry]

But could you go back with the exact coordinates of the Titanic?

[Michael Stevens]

Yes, you could. And so this slap bracelet gives you the timeline and the GPS coordinates. So if you go back to, say, let's say you. You pull a Marty McFly, you go back to the 1950s, you can say, okay, guys, well, the Titanic has. Has been lost, but it hasn't been found yet. But it can be found at 41 degrees north, 49 west, and some change. And they'll go, wow, how did you know that? You must be a sea expert. And you're like, no, no, no, I can also tell you where. Let's see, what's another good one for the 80s. I can also tell you where the HMS Endurance, Ernest Shackleton ship is.

[Hannah Fry]

Oh, nice.

[Michael Stevens]

So they're stuck going, either this guy independently discovered these two sunken ships or traveled through time, in which case, I believe time travel is the better explanation. We've also got Archimedes Ostomachion, the method of mechanical theorems, which was lost in 1204 and not recovered until 1906. We've got the Baltimore Gold Hoard, which was lost in 1856 and found in 1934. The Sutton Hoo ship burial from the 7th century, lost until 1938.

[Hannah Fry]

I don't know what that one is. What's that one?

[Michael Stevens]

I can't tell you any more details, Hannah. I'm just helping time travelers. I'm not a history teacher. It was our science writer, Scott Frank. I'll give him a shout out. He's the one who found this. I'm like, they need to overlap. There must always be at least two things for every year, because you can't

[Hannah Fry]

just prove it with one. You could just have got lucky.

[Michael Stevens]

Can't prove it with one. With one, you're just a good. You're just a good looker, but with multiples, especially different types of things, like a shipwreck, a gold hoard, and an ancient manuscript. Come on, you're a time traveler. You're from a time when these things have been found. And unlike the result of a sporting event or a lottery, you can immediately have people go out and verify your claim.

[Hannah Fry]

Hang on. I want to know whether you're wearing on your person at all times.

[Michael Stevens]

No, I don't. I'll admit that this feels like a flaw in Your plan, Michael, I know, you know, I don't anticipate accidentally time traveling at any point, but, man, wouldn't it be hilarious if that happened? And I was, like, unprepared, and I'm like, man, I specifically made a tool for just this, and I don't have it.

[Hannah Fry]

Yeah, that would be the worst.

[Michael Stevens]

The dream is to eventually make an entire, like, pocketbook that has much more than just a few items on it. And that will allow you to travel through all of human history, you know, and that's gonna have to teach you how to speak the languages you might encounter. You need to be able to speak Middle English, perhaps, or who knows what, right? We'll try to put that together. We also made a slap bracelet. That's a ruler. It's got inches and centimeters on it. But since a slap brace wraps around.

[Hannah Fry]

You've never looked more cool, Michael.

[Michael Stevens]

I've never looked more cool. Imagine it looks. It looks more like a medical bracelet. Like, oh, this guy's got some kind of.

[Hannah Fry]

It does a bit. It just escaped from the hospital.

[Michael Stevens]

I just came from the hospital. But we also made one that takes advantage of this wrapping effect. And it is not a ruler. It's a pie tape. So instead of having a mark every centimeter and every inch, we put a mark every PI centimeters and every PI inches. So from here to here, that gap between my Two thumbs is 4 PI centimeters. Why would you want to know that? Well, if I wrap that around something, what I'm finding, when I look at 4 PI centimeters, that's the circumference, which means the diameter is 4 centimeters.

[Hannah Fry]

So you can instantly measure the diameter of something?

[Michael Stevens]

Yeah, you can instantly measure the diameter of something. Like, imagine you find a pipe, and the ends of the pipe are inaccessible. So you can't actually just go across the diameter of the pipe to tell. You have to go around it and divide by PI. Not with the pie tape. You just go around the number it reads is the diameter.

[Hannah Fry]

This is extremely useful. I do feel like making these tools and then not wearing them is your real downfall here, though. I think you need to commit. I think you need to start tattooing some of these onto your body.

[Michael Stevens]

I could tattoo them on my body. Yeah. The one fear, though, is that the existence of the tattoo itself might make me look like a witch. I mean, the knowledge of where these things are might make me look like a witch. If I went back to, like, the 1100s, and I'm like, hey, guys, I know where this Archimedes thing is, and blah, blah, Blah. They might say, okay, either you're from the future or you're. You're in a pact with the devil. I haven't actually solved that problem, have I?

[Hannah Fry]

I do like the idea of you going to the, the center in Paris where they have the, the definition of the meter, and you've been like, hang on a second. And just whopping out your leg and being like, look, this is. I've got the measurement on me. You know, my sister, who makes a lot of her own clothes, she's been discussing for a while now having an inch tattooed onto her thumb.

[Michael Stevens]

Ah, yes.

[Hannah Fry]

Just so that you can do an inch hem. Isn't that. I like that idea.

[Michael Stevens]

Yeah, it's a really good idea. So Adam Savage is someone I know in real life who has a ruler tattooed on his body.

[Hannah Fry]

Does he? Is it accurate?

[Michael Stevens]

Well, no, it's symbolic. You know, the problem is that your body changes, your skin changes, and so the inch changes. But again, as a sign of, like, I'm a maker and I'm a measurer, and I love quantities and quantification. It's a great tattoo to have. But we struggled with this at the Curiosity Box, trying to come up with a shirt that had rulers on it because inevitably the shirt will stretch. The shirt is a material that moves. It's just not useful as a measuring tape. It can be useful for, you know, formulas and conversion tables. That's great. We've done two shirts that have that property, but this is so far the best I've been able to come up with. And I'm just now realizing that this is the prototype. The real ones we made were adult sized. Look at that.

[Hannah Fry]

Ah, more space for more coordinates.

[Michael Stevens]

This is for your time traveling children. This is an adult slap bracelet. It is, like, really long, by the way.

[Hannah Fry]

You could wear it as a neckerchief.

[Michael Stevens]

You could. Yeah, let's see if I can. Oh, no, I can't. Look at that.

[Hannah Fry]

Any of you who are listening, rather than watching you, you just missed the absolute delight of watching Michael Stevens of Visual Spame slap himself in the neck with the slap bracelet.

[Michael Stevens]

I'm kind of a Gastoni guy. My neck is too thick. Even my PI tape won't tell me the diameter of my neck. It's just not long enough. Well, that'll be. That'll be for a future item. Look at that. See, doesn't make it so close. So close.

[Hannah Fry]

Careful now. You have to lift up your head quite far to get your beard out the way. I don't want you to get your Top jar vertigo.

[Michael Stevens]

I know. Isn't that funny? Like, my beard is so long now. It's changed. It's been long for so long. It's changed even my own perception of what my head looks like. I think of my head as being this huge thing, but actually my head is like a little bean. Look at this guy.

[Hannah Fry]

Yeah.

[Michael Stevens]

My daughter looked at our wedding photos the other day, and she was like, dad used to be so small. And it's because my head used to be half the length.

[Hannah Fry]

I'm slightly disappointed, I've gotta be honest with you, that when you pulled your beard back there, your head was small underneath. I really. I really. I really wanted you to have a jaw all the way down.

[Michael Stevens]

I don't. I don't have anything else to share about my object. I'm open to all kinds of questions and comments. I did just finish a time travel book bid Time Return by Richard Matheson. I love it. Because the method of time travel is literally just believing strong enough that you've traveled.

[Hannah Fry]

I mean, look, some forms of time travel are not necessarily completely impossible, right? It's just they're going backwards. That's gonna be difficult.

[Michael Stevens]

Exactly. I can travel through time forward at regular speed, effortlessly. That's just called living. Sleep is pretty good, too. With sleep, you can travel through time with your consciousness. Like, the travel seemed to not happen.

[Hannah Fry]

If you did jump forwards in time, though, rather than backwards, how could you prove you were from the past? Whoa, that's really tough, isn't it?

[Michael Stevens]

I don't know.

[Hannah Fry]

Where's the slap bracelet for that?

[Michael Stevens]

I know, because what would you do? Because your only evidence would be your own ignorance of things and that anyone can have that.

[Hannah Fry]

Unless there's something the other way around. Unless there's something that we have now that is lost to the future.

[Michael Stevens]

Okay. In that case, you need to carry around with you a very, very priceless relic that's well known, so that when you jump forward, people will say, oh, hey, that, you know, medieval chalice has been missing since you jumped.

[Hannah Fry]

And.

[Michael Stevens]

And you're like, I know. I've got it because it came with.

[Hannah Fry]

Here it is. Or what about a skill? You know, you could say, ask me a phone number. I know six different phone numbers. You know, and people in the future just won't. Won't know any phone numbers. Or, like, I can write with a fountain pen. I still. Because actually, I think if you got somebody from, you know, whatever, the 1700s, they would probably have skills that. That modern people. I'm sure they could weave a Basket. You know, make ice cream in the traditional way.

[Michael Stevens]

Yes. So skills, obviously, Although they're funny. The skills anyone could learn. You could learn to read cursive even though it's the year 2700. I don't know why I'm anticipating the demise of cursive. I've just always felt that it was confusing

[Hannah Fry]

and difficult to read. You're right. It's gonna be difficult to prove.

[Michael Stevens]

I have a friend who I was talking to about this time travel idea and I wanted to do it as a Vsauce episode. It became a product, but she said it should just be a book. It should be a science fiction novel about time travelers and how they have to, you know, have slap bracelets like this or almanac books they carry. We didn't think about going to the future. In which case, yes, there needs to be like a big collection of important relics that are always kept track of. And get to carry one with you when you travel through time to prove that you're you, you are the time traveler and that you're from the past.

[Hannah Fry]

Maybe we're overthinking this. May. Maybe it's just that all you need is to prove that you know how a slap bracelet works. You know, maybe that's the skill that's going to be lost to the future. And maybe you get a two in one.

[Michael Stevens]

Yeah, you could say, hey, guys, I remember the joy of a plastic straw. What's that? Tweet? It's like, look, I just want a straw that somewhere in between dissolves immediately and kills turtles and destroys the rainforest. Just somewhere in between would be nice.

[Hannah Fry]

Is there nothing in between? Bamboo. You know, maybe. Maybe bamboo. Should we have a quick break first? Let's do that. Foreign. This episode is brought to you by Cancer Research uk.

[Michael Stevens]

We often think of beating cancer as treatment, but imagine stopping it before it begins. After years of work, Cancer Research UK scientists are launching a clinical trial of lungvax, the first vaccine designed to prevent lung cancer.

[Hannah Fry]

It builds on Tracer X, the world's largest cancer evolution study, which tracked lung cancer cells over many to uncover the disease's earliest warning signs. Lungvax is designed to train the immune system to spot these signs early on, destroying faulty cells before cancer develops.

[Michael Stevens]

So it's not treatment, but preventative, with the potential to stop lung cancer before it starts. The first stage of the trial starts this year. Focusing on people at higher risk, it

[Hannah Fry]

shows what long term research makes possible.

[Michael Stevens]

For more information about Cancer Research uk, their research breakthroughs and how you can support them, visit cancerresearchuk.org thereest ISS ready to soundtrack your summer with Red Bull Summer All Day Play. You choose a playlist that fits your summer vibe the best. Are you a festival fanatic, a deep end dj, a road dog, or a trail mixer? Just add a song to your chosen playlist and put your summer on track. Red Bull Summer All Day Play. Red Bull gives you wings. Visit red bull.com brightsummerahead to learn more. See you this summer.

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[Michael Stevens]

All right, and welcome back. We are having the time of our lives answering and reading your questions about time. Here's one for Hannah. It comes from Alex. Watching a documentary about the dinosaurs got me thinking. If somehow a human ended up on Earth 150 million years ago, would they be able to find food they can digest? What is the earliest time when we would be able to find food?

[Hannah Fry]

This is such a good question. This has never occurred to me. I like how your mind works, Alex, because until you mentioned this, I just honestly hadn't even thought about it. But you are totally right, because 150 million years ago, this. The late Jurassic period, there's no flowering plants. I mean, you can go back and listen to our episode on what's the most vegetable vegetable to discover that you're just.

[Michael Stevens]

Wait, wait, wait, wait. I didn't know this. So there was no fruit.

[Hannah Fry]

No fruit, no grasses. There's no legumes, there's no root vegetables.

[Michael Stevens]

Surely I could eat a dinosaur.

[Hannah Fry]

I mean, sure. Meat, Meat. Well, you can. Except that. Well, okay, two things. First off, it would be like chewing on an old goat, right? Like, the meat would be so stuff. It would be really horrible.

[Michael Stevens]

One of my favorite foods. Go on.

[Hannah Fry]

But also loads of the stuff. Loads of the. I mean, you're. You're basically in fern territory. There's. There's like conifers. There's loads. It's like big green stuff, you know, like you got conifers, you've got ginkgos, and you got cycads, I think they're called. And the problem is, is that all of the dinosaurs are eating those fine, but they are totally loaded with carcinogens. So it's almost certain that the, the dinosaurs themselves would be carcinogenic.

[Michael Stevens]

No kidding.

[Hannah Fry]

So I mean, your options are basically eat nothing or cancer. So enjoy. Basically, none of the aisles of the supermarket exist. You've got, I mean, you've sort of got fiddleheads, which are like the coiled fronds of young ferns. And you've got something that's like kind of old school pine nuts, like prehistoric pine nuts, like conifer seeds. But the thing is, is that, you know, even if you're trying to eat the vegetation and it's not carcinogenic, even if you manage to find some stuff like that. Modern humans, we've, we've basically lost the ability to ferment really high fiber, kind of woody plant material efficiently. Yeah, and Jurassic plants, they are way tougher, way more fibrous than, than the soft vegetables that we get today. So it would be a lot less like eating salad and more like eating furniture, essentially. It's going to be, it's not going to be very fun. And then also loads of the foods that we eat now, we've like bred the toxins out of them. You know, things like almonds, potatoes. We've like bred them over the years. They don't kill your, your kidneys or your liver. So. So I mean, you could survive, but you've basically got roasted dinosaur and pine nuts and that's it. That's all you got. You got, you can't even flavor them. You know, chilies don't even exist for another hundred million years. It's, it's going to be boring.

[Michael Stevens]

Wow. I had never thought of that before, but Yeah, I mean, 150 million years ago, you can go further than that before they're even early mammals and then you're really in trouble because the mammalian diet needed to be there for mammals, which we are to exist now. I feel like I could maybe eat some fish. But you're saying that the food chain is probably pretty contaminated with stuff that we're not ready for.

[Hannah Fry]

Absolutely, absolutely. I mean, you probably would be able to. You probably would be able to, but it's not. I mean, you're really gonna struggle. Also, by the way, it's not just the food, because if you're going back 150 million years ago, the CO2 levels are 5 to 10 times higher than you've got today. So you are gonna be perpetually out of breath. Basically. You might be trying to hunt down a dinosaur to eat them, but you're gonna. You're gonna be suffering from CO2 toxicity. Just. Just trying to get dino steak.

[Michael Stevens]

That's wild. How have I never thought of that before? Yeah. Of course, it'd be easier to go to Mars than to go live in the, like, early Jurassic. But you're on Earth, and yet it's just not for you. Yet the air isn't right. If you're a vegan. Sorry. If you're willing to hunt living creatures, you don't have any chicken. You've basically got fish, sharks, reptiles. Other than that, you've got a bunch of, like, tree bark.

[Hannah Fry]

Yep, bit of tree bark. That's it. Not even any grass. Can't even chew on any grass.

[Michael Stevens]

So no wheat?

[Hannah Fry]

No wheat, no grains, nothing. Yeah, it's gonna be. It's gonna be. You're gonna have a terrible time. And no one around to tell about the bracelet. Okay, here's a question from Adam. I really like this question because, of course, everything that's going on with Artemis at the moment, that amazing expedition around the dark side of the moon. Adam asked, what would the lag on a FaceTime call be like from space or from the moon?

[Michael Stevens]

Oh, yeah. What would it be like? So, you know, the lag that we get in FaceTime calls is caused by all kinds of factors, but ultimately, at the very end, even if your technology is perfect, you're always bound by the speed of light, the speed at which an electromagnetic signal travels. It's finite and it's really fast. But when you talk about really long distances, it becomes a big problem. So for the astronauts on Artemis 2, at the. their furthest point. Well, I mean, at their furthest point, they couldn't contact Earth at all. Right. For 40 minutes, they were out of contact with Earth because the moon was in the way. They couldn't radio. And just as a side note, on the Apollo missions, when people actually left and walked around on the moon's surface and someone stayed up in the command module, that one person in the module was alone on the far side of the moon by themselves with no radio contact with anybody. And the first guy to do that was Michael Collins, who also, I think, went the furthest away, but he did it alone. Artemis 2 crew, they have each other. Michael Collins was alone, and he was the furthest any human has ever been from any other human. Even Buzz and Neil on the moon's surface were further away from him than you can be from another human here on Earth on the surface.

[Hannah Fry]

Yeah, because he was orbiting around the dark side.

[Michael Stevens]

Well, the far side, let's call it, because the dark side is always changing. But, yeah, he was. He was on the far side of the moon by himself in the command module. And the closest people were the two other guys on the surface of the Moon on the other side. No humans have ever been on. On. On the Moon at night, by the way.

[Hannah Fry]

Is that true?

[Michael Stevens]

Yeah.

[Hannah Fry]

Wow.

[Michael Stevens]

Yeah. So, I mean, it would kind of make sense. Every crewed mission to the Moon has happened during the lunar day, and it makes sense because at night, it would be harder to see things. Right.

[Hannah Fry]

You want to be able to see where you're going.

[Michael Stevens]

But what that means is that no human has ever experienced Earthshine alone at night on the Moon, it's actually quite bright because the Earth is. You know, when there's a full Earth on the moon, it's like a full moon, but time's like 20, because the Earth is bigger and it's brighter. The Moon is actually pretty dusty gray, but Earth is very white because of the clouds and the ice caps. So night on the Moon is just not nearly as dark as what we get here on Earth.

[Hannah Fry]

Oh, that's so beautiful.

[Michael Stevens]

Yeah, right? I mean, night on the Moon with a full Earth is going to give you, like, pretty distinct shadows at night on the Moon because of Earth. But look, I'm actually going to answer your question. The light speed delay from Earth to Artemis 2 is at its maximum was 1.357 seconds. That's how long it took a light speed message to get from mission control on Earth to the Artemis spacecraft. And that's just one way.

[Hannah Fry]

Just because of how far away it is. Incredible.

[Michael Stevens]

That's how far away it is. And so, like, just to give you guys a sense of the radio delay, because of how far Artemis 2 is, Hannah and I are going to have a conversation with that 1.357 second delay. Thanks to our editors. Okay. Hannah, how are you?

[Hannah Fry]

I'm good. Michael, how are you? How's the Earthshine?

[Michael Stevens]

The Earthshine is fantastic, but I'm not loving this 1.357 second delay.

[Hannah Fry]

It does slightly get in the way of really having a good conversation.

[Michael Stevens]

And so there you go. That is the delay between the Earth and the Moon.

[Hannah Fry]

Hey, Mom, I'm on the other side of the Moon.

[Michael Stevens]

Yeah. Did we fool you? Thanks, editors, for recreating, for demonstrating that fact. But when you get further than the Moon, things get really messy. I mean, Mars. Okay, we're gonna. Yeah, let's go to Mars. All right. Mars is so much further Away. And it's also much more variable. The Moon is sometimes a little bit closer, sometimes a little bit further away. But I think that the difference in its distance literally creates a light speed delay difference of only about a tenth of a second between when it's at its closest and furthest from Earth. But for Mars, that radio message delay is going to be anywhere from three minutes at its closest to 21 minutes. Three minutes. We're not talking about seconds anymore.

[Hannah Fry]

That's not a conversation anymore, is it? That's, that's, that's voice notes. You're, you're in voice note territory.

[Michael Stevens]

That's right. It's voice note territory. It's like, okay, even when they're, I mean, three minutes is actually pretty long to wait to be like, hey, hello, are you okay? And then you wait three minutes just for them to get the message, then they have to reply. And then once they send, you have to wait three more minutes. So it's at least six minutes minimum.

[Hannah Fry]

Or up to 40.

[Michael Stevens]

Up to 40. And once you get to Jupiter, that's 33 to 53 minutes one way.

[Hannah Fry]

Wow.

[Michael Stevens]

Pluto, five hours, is it? Goodness me, five hours.

[Hannah Fry]

Do you know what, though? That also really demonstrates how much closer we are to the Sun. I mean, the sun is nine minutes light speed. Right. From us.

[Michael Stevens]

Yeah.

[Hannah Fry]

And then if we are five hours to Pluto, I mean, that's a really good way to think of the scale of the solar system and just how close we are to the sun in comparison to the other planets. Wait, how long was Jupiter? An hour, basically.

[Michael Stevens]

Jupiter, yeah. Between half an hour to an hour, depending on where each planet is in its orbit. Right. And then the nearest star that isn't our sun, that gets you out of the hours into the years. Right. Four years to Proxima Centauri, the nearest star to Earth. And what's so interesting to me about Proxima Centauri is that it's the nearest star. It's about 4.25 light years away, and yet it's so dense, dim, we can't see it with the naked eye. It's like, guys, I'm right here. I'm the closest to you. And yet we cannot see it unless we have a tool.

[Hannah Fry]

Four years really doesn't feel like very long time. I mean, if you were sort of, if you were giving it an update on what was going on on Earth and be like, hey, we've got a new president. Oh, no, wait, strike that. We've got a new president, it would be.

[Michael Stevens]

That's a Good point. Yeah. You would tell them the results of the election and by the time they got the message, there would have been another election. And then by the time they responded and you got their response, more than eight and a half years would have passed. And so you would have had, you would have been having the third election already by that point.

[Hannah Fry]

It's just not really worth keeping them in the loop, is it?

[Michael Stevens]

No, you wouldn't. And this isn't something that technology is gonna fix. Well, I mean, it could, I guess, but this is the limit of the speed of light. We would have to discover something very brand new about space time. For example, create wormholes for instantaneous communication across these light year distances. Otherwise the separation is essentially total.

[Hannah Fry]

I'll tell you what, actually talking about wormholes, we had in another question about wormholes, the downsides of wormholes. Let's say this was anderstx on YouTube. This is one of the comments on YouTube. Great question. If we had wormholes connecting to each other's homes, would there also be a rush of air through the wormhole due to pressure differentials? Like if you were connected from somewhere at sea level to another location at high elevation like Denver or Mexico City. And the answer, absolutely, yes, they would. It would be a complete nightmare. You guys are really good at thinking about the real caveats of instituting this, you know, time travel wormhole. What are the downsides? Okay, so of course air moves from, from high pressure to low pressure at sea level. Air pressure is 14.7 psi. Right. In Denver, which is at 5,000 odd feet, not far from you, Michael. Right. It's 12.1 psi. Mexico City, by the way, it's about 11.1 psi. So if you open a wormhole in London, right where I am to where you are near Denver, you've got a difference about, you know, 2.6 psi, which doesn't sound that small. But in fluid plumbing's terms, it's going to be an absolute monster because you've got this Bernoulli principle, you've got this pressure drop of like 17,000 pa. Essentially the air would rush through the wormhole at roughly 150 meters per second. I mean, you're talking it's way worse than a Category 5 hurricane. Essentially. It was not just like a little rush of air. It would be this absolute deafen structural failure inducing blast. It would just strip the wallpaper off your walls. It would sort of suck you through the wall before anything. So yeah, I mean, I would say careful, careful with what you wish for. It would Rip the snap bracelet right off your arm. Put it that way.

[Michael Stevens]

And, you know, it's another way to think of it, too, is that you're pretty near sea level right now, Hannah. More or less.

[Hannah Fry]

Yeah, pretty much.

[Michael Stevens]

I'm in Colorado, high enough up that the air pressure where I am is about the same as the air pressure in a Hoover that you would use today. And by Hoover, I mean vacuum, for those of you who are Americans. And that's kind of like, blew my mind when I learned that, to be like, wow, so this is what it's like to be inside a vacuum cleaner at sea level. That's literally true. And, yeah, that pressure difference is what it takes to hoover up all the debris.

[Hannah Fry]

See, the other thing is, though, you'd also have this temperature issue, because if you have. Oh, yeah, you know, the air rushing around. Right. It will go under this. This expansion. Because when you get an air that sort of expands rapidly Right. Into a much lower pressure environment without losing heat, then its temperature drops really significantly. So. So, so the air, as it blew out into Denver, because it would go from. It would go from London to Denver. Right. The air. That would be the direction of the airflow, or to Colorado more generally. You know, it would probably drop by tens of degrees instantly. So you would have this. This category 5, worse than category 5 blast of freezing air shooting. Shooting into your room.

[Michael Stevens]

Yeah. And you would have condensation. So clouds could form.

[Hannah Fry]

Mm. Or.

[Michael Stevens]

Or disappear, depending on. Yeah. The. The humidity in the areas. They would have their own weather. Yeah, I hadn't thought of that.

[Hannah Fry]

I mean, you would also have me, Michael. I would be there. I probably wouldn't be in one piece. I would probably be frozen and.

[Michael Stevens]

And.

[Hannah Fry]

And much mutilated by the journey. But nonetheless, I would be there. So there's upsides and downsides. That's what I'm saying. There's one more question that came in from Jacob Fletcher, and I think this is quite a nice one. This rounds it all off very neatly, actually. Going back to your snap bracelet containing the information, at what point did it become impossible for a single person to learn everything that humans had collectively learned?

[Michael Stevens]

Michael, I love this question, and of course, it's difficult to actually answer, but the first thing I thought about was a smaller question, which is one that XKCD tackled in what if? And many other people have taken their own approaches to. And it's the question of at what point could a person have actually truthfully said that they had read every single book in English ever written? Because today that's impossible. And that in and of itself is almost one of the saddest facts that you and I will never read every book ever written, even if we read really quickly. And that's all we ever did as a full time job. If we never slept, we still couldn't catch up. But if you look at the past, there was a time when there were fewer books that had been written. Now there's a. There's a distinction we need to make between written and printed. Printed becomes much easier to answer because we know a lot more about what was printed on a printing press. And the first book in English that was printed on a printing press was printed in 1473 and it was called the Requie of the Histories of Troy. That book printed in 1473, great, go ahead and read it. And you've read every book ever printed in English.

[Hannah Fry]

So yeah, you're right at that point, you read that, you're done.

[Michael Stevens]

You read that and you're done. But books get published. And there's this great, there's this great book content creator named Tom Ayling who I follow on all the different platforms. And he went through this like year by year because we know about how many books were published every year in English. So he's like, you can actually keep up with this process. I'm not going to get into the hairy details of it all, but Tom Ayling actually did something really interesting. He said, don't even start reading with the first English book printed. Just don't even start reading until the year 1500, at which point there were 294 books in, in English that had ever been printed. Different titles.

[Hannah Fry]

I could do that. I could do that.

[Michael Stevens]

You could do that. And every year there's like, you know, it looks like maybe 20 new ones printed. And he's like, you could actually catch up and have read every English book that had ever been printed until 1530. In that year, there weren't 15 or 18 books printed. New books, there were 96.

[Hannah Fry]

Ooh, that's going to be tough. That's one every three days. That's tough.

[Michael Stevens]

You can't catch up. So 1529 was his estimated year at which was the last year a person could have ever claimed that they had read every single book ever printed in English. 1529. So unfortunately we're not there. But that's, that's an answer to a question that's much more specific. We're talking about reading a book in English that has been printed, reading in any language or reading anything in English that's been written. That's where it starts to become basically impossible for this to have ever even existed. To have read everything ever written, you would have to talk about some time period just after the invention of writing. Because once people were writing, they were writing, okay? Just the accumulation of tax records alone would have meant that you would have had to have lived in the Stone Age to have actually read everything. You wouldn't have been able to get access to everything because there were people writing in China, in Mesoamerica, in Europe. You'd have to somehow get all this stuff. And it depends on how we define writing. Does this also require that you have read all the notches put on bones to represent numbers of oil jugs shared? But the question that is being asked here by Jacob Fletcher is when did it become impossible for a single person to learn everything that humans had collectively learned? And that. I think my answer is never to have learned everything that humans collectively knew. I mean, we don't even know everything we collectively know. And what I mean by that is that we've got knowledge, but then there's also the meta memory of it all. Do we know what we know? And we don't always. We've got to refer back to written records. We've kind of externalized our memories. And so I think to know everything that humanity has known collectively becomes basically, how do you define collective knowledge? But I'm gonna say it's never been possible because of the definitions of these words, which is an unhappy answer. But that's also gave you the like, when could you have read every book? Kind of an answer.

[Hannah Fry]

Let me think about this, though, because I wonder whether there was a point, I'm just wondering about, I don't know, cats, right, where cats have different lived experiences. Of course they do. Each one is like their own individual entity. But if you think about the sort of spectrum of what cats know, right, It's a pretty tightly, tightly packed distribution, right? There's not like if you took an average cat, right? Sort of imagine the full distribution of everything that all cats know. Take an average cat, it's pretty much going to be overlapping, isn't it, really? There's like, there's maybe a couple of little things that some clever cats have learned here and there, but they're just quite a simple species. So I do wonder whether there was a point where actually the only things that we knew as humans were foraging for food, finding mates, taking care of our young, looking for. For shelter, you know, where actually there was such simplicity to our Knowledge that there just wasn't very much to learn at all.

[Michael Stevens]

Yeah, you're right. I'm not really happy with the answer that I gave. I think you're right that if we include sort of instinctive knowledge, then it could be possible to know, oh, it gets cold at night, or those kind of things.

[Hannah Fry]

Once you get tool use, I think you're right. As soon as you start getting tool use.

[Michael Stevens]

Yeah. Once you get into technology, tools, things that we aren't born already knowing, things that we have discovered and invented, then I guess it's tough. Cause you could say, well, yeah, do you know how to nap a stone? To create like a stone blade, how to make paper? Do you know how to interact with a dog? Like these things people invented. Oh, man. I think there really is an answer to his question, at least a hand, wavy one where you're like, well, you

[Hannah Fry]

know,

[Michael Stevens]

the fermentation of liquids into alcohol happened around this point. And that could be a skill that you learned and you could also learn. But it's. Hold on. It's gotta be things that we collectively know. And that's what I'm getting stuck on. Because to know how to use a musical instrument that was invented in China and Egypt and in the Americas, that's not collective knowledge. Only certain groups have that knowledge. So what does it mean the collective knowledge of humanity? I guess if someone were to write a book of everything that humans, like, invented and discovered, could you know everything in that book? And at what point was there just too much in it for someone to even be familiar with everything in it?

[Hannah Fry]

You know, I also think that actually human knowledge and technology, I mean, one of my guilty. It's actually not a guilty pleasure at all. It's a. What's the opposite of guilty proud pleasure is watching videos about primitive technology. Do you remember the guy who was in Australia who would be there just in his shorts and like every single tool he would. But he had a knife and that was it. That was all he was allowed to have. He would build the most unimaginably sophisticated, I mean, practically entire villages. Right. Pottery, you know, roof tiles, fires, I mean, weapons, all sorts of stuff. I think that the sort of collective knowledge of things that humans invented and created, it's very, very quick. I think that, that, that it becomes this vast wealth of stuff that you would have to potentially learn.

[Michael Stevens]

Yeah. Again, I think that you're going to need to look at some like Stone Age time period to even be able to wrap your head around all of the skills, techniques and observations that humans Collected and weren't just simply born with, like, knowing to drink when you're thirsty. You know, if that kind of stuff counts, then. Then you can go back pretty far. And we all had that knowledge. But, you know, I met the primitive technology guy.

[Hannah Fry]

Did you?

[Michael Stevens]

Yeah. When. When Adam and I were doing Brain Candy Live in Australia. He came to the show and that was. That was awesome.

[Hannah Fry]

Did he have to announce himself or did you know immediately who he was? Cause you don't really see his face that much on camera, do you?

[Michael Stevens]

No, no, no, no. But he had been in contact with Adam Savage, and so that's how it, like, happened. And Adam met him and I just kind of tagged along and. And was like, hello, me too. Hello.

[Hannah Fry]

Imagine Vsauce tagging along and being like, me too, rather than the central character.

[Michael Stevens]

I was like, sorry to butt in, but I just wanted to shake your hand. Yeah, that's what it was. It was really cool.

[Hannah Fry]

The other thing that I've become quite obsessed with is, like, ancient chemistry, you know?

[Michael Stevens]

Yeah.

[Hannah Fry]

My sasse mornings, that's. That's. That's the treat that I give myself. A little bit of a lion and watching videos about ancient chemistry techniques, ancient technology. I'll tell you what, we'll link some of my favorite channels in the description below.

[Michael Stevens]

Now, could there come a point where the past is so big and new discovery is so slow that our species is actually forgetting things faster than we're learning new things? That'd be a good, like, science fiction future premise for a book.

[Hannah Fry]

Yeah.

[Michael Stevens]

Because there's certainly a lot of things people used to know that we literally

[Hannah Fry]

no longer know, that we've outsourced, basically, to technology instead. Really?

[Michael Stevens]

Yeah. That we've outsourced or we literally have lost. Like, we don't know what they did. We don't know what they put in this concrete to make it so strong.

[Hannah Fry]

Well, that was a absolutely delightful field notes. We went backwards in time, we went forwards in time. We went through wormholes and around the other side of the. The other side of the. The moon. I hope you enjoyed it, too. Told you it wasn't going to be boring. Told you it was going to be. This is not going to be the one where we've got zero views on YouTube, Michael. We'll have to save that one for another time.

[Michael Stevens]

That one is coming. We're going to crack that nut. We're going to figure it out. Everyone's trying to beat the algorithm and we are going to try to be beaten by the algorithm.

[Hannah Fry]

Do you know that is that there is. That is quite a nice game theory idea, isn't it? Of like whether we could actually do it, whether we could get everybody to collectively agree to not watch one video.

[Michael Stevens]

Yeah. How would you? Oh, but see, there's going to be the reverse psychology of it, the psychological reactance of it where if you tell people not to watch it, there's going to make them want to watch it more honestly. Would have to be like an advertisement that we desperately want everyone to watch. Like this is an ad for a really boring product and we were paid to do this and please watch it. And people are like no. Literally no.

[Hannah Fry]

We'll think on that. We'll ponder on that. If you have any suggestions in the comments of of how we can get zero views then then let us know. I'd be well up for that. Be great. But anyway, anything else that you have, let us know in the comments or send them to us at. The rest is scienceoalhanger.com we love hearing

[Michael Stevens]

from you and we'll see you next time.

[Hannah Fry]

Bye bye. Some Follow the Noise. Bloomberg Follows the Money Whether it's the funds fueling AI or crypto's trillion dollar swings, there's a money side to every story.

[Michael Stevens]

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