On a typical day at school, endless hours are spent learningthe answers to questions, but right now, we'll do the opposite. We're going to focus on questionswhere you can't learn the answers because they're unknown.
I used to puzzle about a lot of thingsas a boy, for example: What would it feel like to be a dog? Do fish feel pain? How about insects? Was the Big Bang just an accident? And is there a God? And if so, how are we so surethat it's a He and not a She? Why do so many innocent peopleand animals suffer terrible things? Is there really a plan for my life? Is the future yet to be written, or is it already writtenand we just can't see it? But then, do I have free will?I mean, who am I anyway? Am I just a biological machine? But then, why am I conscious?What is consciousness? Will robots become conscious one day? I mean, I kind of assumed that some day I would be told the answersto all these questions. Someone must know, right? Guess what? No one knows. Most of those questionspuzzle me more now than ever. But diving into them is exciting because it takes youto the edge of knowledge, and you never know what you'll find there.
So, two questions that no oneon Earth knows the answer to. (Music) [How many universes are there?] Sometimes when I'm on a long plane flight, I gaze out at all thosemountains and deserts and try to get my headaround how vast our Earth is. And then I rememberthat there's an object we see every day that would literally fitone million Earths inside it: the Sun. It seems impossibly big. But in the great schemeof things, it's a pinprick, one of about 400 billion starsin the Milky Way galaxy, which you can see on a clear night as a pale white miststretched across the sky. And it gets worse. There are maybe 100 billion galaxiesdetectable by our telescopes. So if each star was the sizeof a single grain of sand, just the Milky Way has enough stars to fill a 30-foot by 30-footstretch of beach three feet deep with sand. And the entire Earthdoesn't have enough beaches to represent the starsin the overall universe. Such a beach would continue for literallyhundreds of millions of miles.
Holy Stephen Hawking,that is a lot of stars. But he and other physicistsnow believe in a reality that is unimaginably bigger still. I mean, first of all,the 100 billion galaxies within range of our telescopes are probably a minusculefraction of the total. Space itself is expandingat an accelerating pace. The vast majority of the galaxies are separating from us so fastthat light from them may never reach us. Still, our physical reality here on Earth is intimately connectedto those distant, invisible galaxies. We can think of themas part of our universe. They make up a single, giant edifice obeying the same physical lawsand all made from the same types of atoms, electrons, protons, quarks, neutrinos,that make up you and me. However, recent theories in physics,including one called string theory, are now telling us there could becountless other universes built on different types of particles, with different properties,obeying different laws. Most of these universescould never support life, and might flash in and outof existence in a nanosecond. But nonetheless, combined,they make up a vast multiverse of possible universesin up to 11 dimensions, featuring wondersbeyond our wildest imagination. The leading version of string theorypredicts a multiverse made up of 10 to the 500 universes. That's a one followed by 500 zeros, a number so vast that if every atom in our observable universehad its own universe, and all of the atomsin all those universes each had their own universe, and you repeated that for two more cycles, you'd still be at a tinyfraction of the total, namely, one trillion trillion trilliontrillion trillion trillion trillion trillion trillion trillion trilliontrillion trillion trillion trillionth. (Laughter) But even that numberis minuscule compared to another number: infinity. Some physicists think the space-timecontinuum is literally infinite and that it contains an infinite numberof so-called pocket universes with varying properties.
How's your brain doing? Quantum theory adds a whole new wrinkle. I mean, the theory's been proventrue beyond all doubt, but interpreting it is baffling, and some physicists thinkyou can only un-baffle it if you imagine that huge numbersof parallel universes are being spawned every moment, and many of these universes would actuallybe very like the world we're in, would include multiple copies of you. In one such universe,you'd graduate with honors and marry the person of your dreams,and in another, not so much. Well, there are still some scientistswho would say, hogwash. The only meaningful answer to the questionof how many universes there are is one. Only one universe. And a few philosophersand mystics might argue that even our own universe is an illusion.
So, as you can see, right now there is no agreementon this question, not even close. All we know is the answer is somewherebetween zero and infinity. Well, I guess we know one other thing. This is a pretty cool timeto be studying physics. We just might be undergoingthe biggest paradigm shift in knowledge that humanity has ever seen. (Music) [Why can't we see evidence of alien life?] Somewhere out there in that vast universe there must surely be countlessother planets teeming with life. But why don't we see any evidence of it? Well, this is the famous questionasked by Enrico Fermi in 1950: Where is everybody? Conspiracy theorists claim that UFOsare visiting all the time and the reports are just being covered up, but honestly, they aren't very convincing. But that leaves a real riddle. In the past year,the Kepler space observatory has found hundreds of planetsjust around nearby stars.
And if you extrapolate that data, it looks like there couldbe half a trillion planets just in our own galaxy. If any one in 10,000 has conditions that might support a form of life, that's still 50 million possiblelife-harboring planets right here in the Milky Way. So here's the riddle: our Earth didn't form until about nine billion yearsafter the Big Bang. Countless other planets in our galaxyshould have formed earlier, and given life a chance to get underway billions, or certainly many millionsof years earlier than happened on Earth. If just a few of themhad spawned intelligent life and started creating technologies, those technologies would havehad millions of years to grow in complexity and power. On Earth, we've seen how dramaticallytechnology can accelerate in just 100 years. In millions of years,an intelligent alien civilization could easily have spread outacross the galaxy, perhaps creating giantenergy-harvesting artifacts or fleets of colonizing spaceships or glorious works of artthat fill the night sky. At the very least, you'd thinkthey'd be revealing their presence, deliberately or otherwise, through electromagnetic signalsof one kind or another. And yet we see no convincingevidence of any of it. Why? Well, there are numerous possible answers,some of them quite dark. Maybe a single,superintelligent civilization has indeed taken over the galaxy and has imposed strict radio silence because it's paranoidof any potential competitors. It's just sitting thereready to obliterate anything that becomes a threat. Or maybe they're not that intelligent, or perhaps the evolutionof an intelligence capable of creatingsophisticated technology is far rarer than we've assumed. After all, it's only happened onceon Earth in four billion years.
Maybe even that was incredibly lucky. Maybe we are the firstsuch civilization in our galaxy. Or, perhaps civilization carries with itthe seeds of its own destruction through the inability to controlthe technologies it creates. But there are numerousmore hopeful answers. For a start, we're not looking that hard, and we're spendinga pitiful amount of money on it. Only a tiny fractionof the stars in our galaxy have really been looked at closelyfor signs of interesting signals. And perhaps we're not lookingthe right way. Maybe as civilizations develop, they quickly discovercommunication technologies far more sophisticated and usefulthan electromagnetic waves. Maybe all the action takes placeinside the mysterious recently discovered dark matter, or dark energy, that appear to accountfor most of the universe's mass. Or, maybe we're lookingat the wrong scale.
Perhaps intelligentcivilizations come to realize that life is ultimatelyjust complex patterns of information interacting with each otherin a beautiful way, and that that can happen moreefficiently at a small scale. So, just as on Earth,clunky stereo systems have shrunk to beautiful, tiny iPods,maybe intelligent life itself, in order to reduce its footprinton the environment, has turned itself microscopic. So the Solar Systemmight be teeming with aliens, and we're just not noticing them. Maybe the very ideas in our headsare a form of alien life. Well, okay, that's a crazy thought. The aliens made me say it. But it is cool that ideas do seemto have a life all of their own and that they outlive their creators. Maybe biological lifeis just a passing phase.
Well, within the next 15 years, we could start seeingreal spectroscopic information from promising nearby planets that will reveal justhow life-friendly they might be. And meanwhile, SETI, the Searchfor Extraterrestrial Intelligence, is now releasing its data to the public so that millions of citizen scientists,maybe including you, can bring the power of the crowdto join the search. And here on Earth, amazing experiments are being done to tryto create life from scratch, life that might be very differentfrom the DNA forms we know. All of this will help us understandwhether the universe is teeming with life or whether, indeed, it's just us. Either answer, in its own way, is awe-inspiring, because even if we are alone, the fact that we think and dreamand ask these questions might yet turn out to be one of the most important factsabout the universe. And I have one more pieceof good news for you. The quest for knowledgeand understanding never gets dull. It doesn't. It's actually the opposite. The more you know,the more amazing the world seems. And it's the crazy possibilities,the unanswered questions, that pull us forward. So stay curious.
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Thanks guys