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(Music)

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Sometimes when I'm on a long plane flight,

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I gaze out at all those mountains and deserts

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and try to get my head around how vast our Earth is.

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And then I remember that there's an object we see every day

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that would literally fit one million Earths inside it.

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The sun seems impossibly big,

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but in the great scheme of things, it's a pinprick,

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one of about 400 billion stars in the Milky Way galaxy,

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which you can see on a clear night as a pale, white mist stretched across the sky.

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And it gets worse.

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There are maybe 100 billion galaxies detectable by our telescopes,

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so if each star was the size of a single grain of sand,

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just the Milky Way has enough stars to fill

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a 30 foot by 30 foot stretch of beach three feet deep with sand.

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And the entire Earth doesn't have enough beaches

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to represent the stars in the overall universe.

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Such a beach would continue for literally hundreds of millions of miles.

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Holy Stephen Hawking, that is a lot of stars.

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But he and other physicists now believe in

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a reality that is unimaginably bigger still.

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I mean, first of all, the 100 billion galaxies within range of our telescopes

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are probably a minuscule fraction of the total.

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Space itself is expanding

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at an accelerating pace. The vast majority of the galaxies

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are separating from us so fast

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that light from them may never reach us.

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Still, our physical reality here on Earth

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is intimately connected to those distant, invisible galaxies.

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We can think of them as part of our universe.

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They make up a single, giant edifice,

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obeying the same physical laws and all made from the same types of atoms, electrons,

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protons, quarks, neutrinos that make up you and me.

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However, recent theories in physics,

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including one called string theory,

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are now telling us there could be countless other universes,

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built on different types of particles,

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with different properties, obeying different laws.

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Most of these universes could never support life,

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and might flash in and out of existence in a nanosecond,

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but nonetheless, combined

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they make up a vast multiverse of possible universes.

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in up to 11 dimensions, featuring wonders beyond our wildest imagination.

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And the leading version of string theory

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predicts a multiverse made of up to 10 to the 500 universes.

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That's a one followed by 500 zeroes,

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a number so vast that if every atom in our observable universe

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had its own universe

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and all of the atoms in all of those universes

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each had their own universe,

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and you repeated that for two more cycles,

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you'd still be at a tiny fraction of the total --

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namely, one trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillionth.

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But even that number is minuscule compared to

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another number: infinity.

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Some physicists think the space-time continuum

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is literally infinite, and that it contains an infinite number

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of so-called pocket universes with varying properties.

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How's your brain doing?

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But quantum theory adds a whole new wrinkle.

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I mean, the theory's been proven true beyond all doubt,

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but interpreting it is baffling.

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And some physicists think you can only un-baffle it

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if you imagine that huge numbers of parallel universes

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are being spawned every moment,

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and many of these universes would actually be very like the world we're in,

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would include multiple copies of you.

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In one such universe, you'd graduate with honors and marry the person of your dreams.

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In another, not so much.

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There are still some scientists who would say, hogwash.

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The only meaningful answer to the question of how many universes there are is one,

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only one universe.

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And a few philosophers and mystics

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might argue that even our own universe is an illusion.

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So, as you can see,

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right now there is no agreement on this question,

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not even close.

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All we know is, the answer is somewhere between zero and infinity.

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Well, I guess we know one other thing:

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This is a pretty cool time to be studying physics.

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We just might be undergoing

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the biggest paradigm shift in knowledge that humanity has ever seen.