Name: The Physics of the Multiverse
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What the Physics?!

The Gordon and Betty Moore Foundation

Sometimes the world seems suspiciously perfect, fine-tuned for life. This impossible coincidence has led some physicists to think there could be many—maybe infinite—universes. Find out why in this episode of <a href="https://www.youtube.com/channel/UCj1gfrsi8H8zTrmR0ft1Kjw/">What the Physics?!</a>

A multiverse could explain why our universe is fine-tuned for life.

The Physics of the Multiverse

Physics of the Multiverse

A multiverse could explain why our universe is fine-tuned for life.

Physics & Math

The Physics of the MultiversePublished March 16, 2018Greg Kestin: This is what our universe looks like. This is what our universe probably should look like.Orange fish: How did we get so lucky?Blue fish: What? Why?Orange fish: We don't sink down or float up. It's a perfect balance. Pretty lucky!Blue fish: Or is it...the multi-pond?Orange fish: Wha...?Greg Kestin: There seems to be a nearly perfect balance in the universe. Seriously. There's a force pulling everything together—gravity—and a kind of force pushing everything apart—dark energy. If, say, dark energy were even a little bit stronger, then there'd be no stars, there'd be no atoms, there'd be no life at all.The most baffling number in all of physics is the value of the cosmological constant, denoted Lambda. It's the measured density of dark energy, and it controls how fast the expansion of the universe speeds up or slows down.If Lambda were a positive number, like 1 or 56 or 576 trillion, then the universe's expansion would have sped up so fast that all the particles in the universe would've exploded away from each other in the first second of the universe.If Lambda were a negative number, like -0.2 or -7 or -576 billion, then its expansion would have slowed down and everything in the universe would have collapsed back in on itself.But if Lambda were zero, then the universe wouldn't have exploded outward, it wouldn't have collapsed in on itself; it would just expand at a constant rate and look very much like it does today.Now, here's the baffling thing: In 1998, astronomers discovered that Lambda isn't zero. It's actually 1.1x10^-52. So, why that number? It seems pretty random. Even to physicists it seems random. But, coincidentally, it's so close to zero that that explosion outward of the universe is slow enough for galaxies, stars, planets, and life to exist. And the probability of luckily getting a number like that, just by chance, well, let's just say, it's less likely than you winning the lottery.This isn't the only number in physics that seems a little too perfect. The strength of the strong nuclear force—tweak that a little bit and chemistry's just gone. You don't have different kinds of elements. There's no nuclear fusion in stars, there's no oxygen to breathe, there's no carbon, and so life wouldn't have existed in the first place.The masses of the fundamental particles—tweak that and we would all be so massive that we would just turn into black holes.So, why do the fundamental laws of nature seem perfectly tuned for stars, planets, atoms, and life?Blue fish: I'm just saying, this not-sinking thing, maybe it's not luck at all.Orange fish: How's that?Blue fish: What if there are parallel ponds? Like a pond without water, where we just sink to the bottom? Or a pond with water that's too stiff to swim in?Orange fish: Uh...so what?Blue fish: Well, if all these ponds exist, then naturally, we'd live in the one perfect for us.Greg Kestin: Consider for a second the possibility that we live in one of many—maybe infinite—universes. There could be a universe where everything stretched away from each other so quickly in the first second of the universe that there's no stars or galaxies. There could be a universe where everything is black holes. And we couldn't exist in those universes. But there's at least one universe that has fundamental constants perfect for stars, galaxies, planets, atoms, and life.So maybe we live in just one universe that can support us.Orange fish: Are you saying that we live in a multi-pond? So, what—you think there are ponds where aliens sink to the bottom and flop around? Or ponds where aliens float around without water?Blue fish: Well, what if we swim up? Maybe we could see what's past the edge.Greg Kestin: Some physicists are trying to "see past the edge." They're scanning the sky for imprints in the thin radiation that fills the entire universe. If they find them, these imprints, or "bruises," might be places where our universe bounced against another universe.So, how do you think our universe came to be so perfect? Let me know in the comments.

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