Where are all the aliens?

I saw a UFO once. I was eight or nine, playing in the street with a friend who was a couple of years older, and we saw a featureless silver disc hovering over the houses. We watched it for a few seconds, and then it shot away incredibly quickly. Even as a kid, I got angry it was ignoring the laws of physics. We ran inside to tell the grown-ups, and they were skeptical -- you'd be skeptical too, right? I got my own back a few years later: one of those grown-ups told me, "Last night I saw a flying saucer. I was coming out of the pub after a few drinks." I stopped him there. I said, "I can explain that sighting."

  Psychologists have shown we can't trust our brains to tell the truth. It's easy to fool ourselves. I saw something, but what's more likely -- that I saw an alien spacecraft, or that my brain misinterpreted the data my eyes were giving it? Ever since though I've wondered: Why don't we see flying saucers flitting around? At the very least, why don't we see life out there in the cosmos? It's a puzzle, and I've discussed it with dozens of experts from different disciplines over the past three decades. And there's no consensus. Frank Drake began searching for alien signals back in 1960 -- so far, nothing. And with each passing year, this nonobservation, this lack of evidence for any alien activity gets more puzzling because we should see them, shouldn't we?

  The universe is 13.8 billion years old, give or take. If we represent the age of the universe by one year, then our species came into being about 12 minutes before midnight, 31st December. Western civilization has existed for a few seconds. Extraterrestrial civilizations could have started in the summer months. Imagine a summer civilization developing a level of technology more advanced than ours, but tech based on accepted physics though, I'm not talking wormholes or warp drives -- whatever -- just an extrapolation of the sort of tech that TED celebrates. That civilization could program self-replicating probes to visit every planetary system in the galaxy. If they launched the first probes just after midnight one August day, then before breakfast same day, they could have colonized the galaxy. Intergalactic colonization isn't much more difficult, it just takes longer. A civilization from any one of millions of galaxies could have colonized our galaxy.

  Seems far-fetched? Maybe it is, but wouldn't aliens engage in some recognizable activity -- put worldlets around a star to capture free sunlight, collaborate on a Wikipedia Galactica, or just shout out to the universe, "We're here"?

  So where is everybody? It's a puzzle because we do expect these civilizations to exist, don't we? After all, there could be a trillion planets in the galaxy -- maybe more.

  You don't need any special knowledge to consider this question, and I've explored it with lots of people over the years. And I've found they often frame their thinking in terms of the barriers that would need to be cleared if a planet is to host a communicative civilization. And they usually identify four key barriers.

  Habitability -- that's the first barrier. We need a terrestrial planet in that just right "Goldilocks zone," where water flows as a liquid. They're out there. In 2016, astronomers confirmed there's a planet in the habitable zone of the closest star, Proxima Centauri -- so close that Breakthrough Starshot project plans to send probes there. We'd become a starfaring species. But not all worlds are habitable. Some will be too close to a star and they'll fry, some will be too far away and they'll freeze.

  Abiogenesis -- the creation of life from nonlife -- that's the second barrier. The basic building blocks of life aren't unique to Earth: amino acids have been found in comets, complex organic molecules in interstellar dust clouds, water in exoplanetary systems. The ingredients are there, we just don't know how they combine to create life, and presumably there will be worlds on which life doesn't start.

  The development of technological civilization is a third barrier. Some say we already share our planet with alien intelligences. A 2011 study showed that elephants can cooperate to solve problems. A 2010 study showed that an octopus in captivity can recognize different humans. 2017 studies show that ravens can plan for future events -- wonderful, clever creatures -- but they can't contemplate the Breakthrough Starshot project, and if we vanished today, they wouldn't go on to implement Breakthrough Starshot -- why should they? Evolution doesn't have space travel as an end goal. There will be worlds where life doesn't give rise to advanced technology.

  Communication across space -- that's a fourth barrier. Maybe advanced civilizations choose to explore inner space rather than outer space, or engineer at small distances rather than large. Or maybe they just don't want to risk an encounter with a potentially more advanced and hostile neighbor. There'll be worlds where, for whatever reason, civilizations either stay silent or don't spend long trying to communicate.

  As for the height of the barriers, your guess is as good as anyone's. In my experience, when people sit down and do the math, they typically conclude there are thousands of civilizations in the galaxy. But then we're back to the puzzle: Where is everybody? By definition, UFOs -- including the one I saw -- are unidentified. We can't simply infer they're spacecraft. You can still have some fun playing with the idea aliens are here. Some say a summer civilization did colonize the galaxy and seeded Earth with life ... others, that we're living in a cosmic wilderness preserve -- a zoo. Yet others -- that we're living in a simulation. Programmers just haven't revealed the aliens yet. Most of my colleagues though argue that E.T. is out there, we just need to keep looking, and this makes sense. Space is vast. Identifying a signal is hard, and we haven't been looking that long. Without doubt, we should spend more on the search. It's about understanding our place in the universe. It's too important a question to ignore.

  But there's an obvious answer: we're alone. It's just us. There could be a trillion planets in the galaxy. Is it plausible we're the only creatures capable of contemplating this question? Well, yes, because in this context, we don't know whether a trillion is a big number. In 2000, Peter Ward and Don Brownlee proposed the Rare Earth idea. Remember those four barriers that people use to estimate the number of civilizations? Ward and Brownlee said there might be more.

  Let's look at one possible barrier. It's a recent suggestion by David Waltham, a geophysicist. This is my very simplified version of Dave's much more sophisticated argument. We are able to be here now because Earth's previous inhabitants enjoyed four billion years of good weather -- ups and downs but more or less clement. But long-term climate stability is strange, if only because astronomical influences can push a planet towards freezing or frying. There's a hint our moon has helped, and that's interesting because the prevailing theory is that the moon came into being when Theia, a body the size of Mars, crashed into a newly formed Earth. The outcome of that crash could have been a quite different Earth-Moon system. We ended up with a large moon and that permitted Earth to have both a stable axial tilt and a slow rotation rate. Both factors influence climate and the suggestion is that they've helped moderate climate change. Great for us, right? But Waltham showed that if the moon were just a few miles bigger, things would be different. Earth's spin axis would now wander chaotically. There'd be episodes of rapid climate change -- not good for complex life. The moon is just the right size: big but not too big. A "Goldilocks" moon around a "Goldilocks" planet -- a barrier perhaps.

  You can imagine more barriers. For instance, simple cells came into being billions of years ago ... but perhaps the development of complex life needed a series of unlikely events. Once life on Earth had access to multicellularity and sophisticated genetic structures, and sex, new opportunities opened up: animals became possible. But maybe it's the fate of many planets for life to settle at the level of simple cells.

  Purely for the purposes of illustration, let me suggest four more barriers to add to the four that people said blocked the path to communicative civilization. Again, purely for the purposes of illustration, suppose there's a one-in-a-thousand chance of making it across each of the barriers. Of course there might be different ways of navigating the barriers, and some chances will be better than one in a thousand. Equally, there might be more barriers and some chances might be one in a million. Let's just see what happens in this picture.

  If the galaxy contains a trillion planets, how many will host a civilization capable of contemplating like us projects such as Breakthrough Starshot? Habitability -- right sort of planet around the right sort of star -- the trillion becomes a billion. Stability -- a climate that stays benign for eons -- the billion becomes a million. Life must start -- the million becomes a thousand. Complex life forms must arise -- the thousand becomes one. Sophisticated tool use must develop -- that's one planet in a thousand galaxies. To understand the universe, they'll have to develop the techniques of science and mathematics -- that's one planet in a million galaxies. To reach the stars, they'll have to be social creatures, capable of discussing abstract concepts with each other using complex grammar -- one planet in a billion galaxies. And they have to avoid disaster -- not just self-inflicted but from the skies, too. That planet around Proxima Centauri, last year it got blasted by a flare. One planet in a trillion galaxies, just as in the visible universe.

  I think we're alone. Those colleagues of mine who agree we're alone often see a barrier ahead -- bioterror, global warming, war. A universe that's silent because technology itself forms the barrier to the development of a truly advanced civilization. Depressing, right?

  I'm arguing the exact opposite. I grew up watching "Star Trek" and "Forbidden Planet," and I saw a UFO once, so this idea of cosmic loneliness I certainly find slightly wistful. But for me, the silence of the universe is shouting, "We're the creatures who got lucky." All barriers are behind us. We're the only species that's cleared them -- the only species capable of determining its own destiny. And if we learn to appreciate how special our planet is, how important it is to look after our home and to find others, how incredibly fortunate we all are simply to be aware of the universe, humanity might survive for a while. And all those amazing things we dreamed aliens might have done in the past, that could be our future.

  Thank you very much.

  How we could teach our bodies to heal faster

  What if you could take a pill or a vaccine and, just like getting over a cold, you could heal your wounds faster? Today, if we have an operation or an accident, we're in the hospital for weeks, and often left with scars and painful side effects of our inability to regenerate or regrow healthy, uninjured organs. I work to create materials that instruct our immune system to give us the signals to grow new tissues. Just like vaccines instruct our body to fight disease, we could instead instruct our immune system to build tissues and more quickly heal wounds.

  Now, regrowing body parts out of nowhere might seem like magic, but there are several organisms that can achieve this feat. Some lizards can regrow their tails, the humble salamander can completely regenerate their arm, and even us mere humans can regrow our liver after losing more than half of its original mass.

  To make this magic a bit closer to reality, I'm investigating how our body can heal wounds and build tissue through instructions from the immune system. From a scrape on your knee to that annoying sinus infection, our immune system defends our body from danger. I'm an immunologist, and by using what I know about our body's defense system, I was able to identify key players in our fight to build back our cuts and bruises.

  When looking at materials that are currently being tested for their abilities to help regrow muscle, our team noticed that after treating an injured muscle with these materials, there was a large number of immune cells in that material and the surrounding muscle. So in this case, instead of the immune cells rushing off towards infection to fight bacteria, they're rushing toward an injury. I discovered a specific type of immune cell, the helper T cell, was present inside that material that I implanted and absolutely critical for wound healing.

  Now, just like when you were a kid and you'd break your pencil and try and tape it back together again, we can heal, but it might not be in the most functional way, and we'll get a scar. So if we don't have these helper T cells, instead of healthy muscle, our muscle develops fat cells inside of it, and if there's fat in our muscle, it isn't as strong. Now, using our immune system, our body could grow back without these scars and look like what it was before we were even injured.

  I'm working to create materials that give us the signals to build new tissue by changing the immune response. We know that any time a material is implanted in our body, the immune system will respond to it. This ranges from pacemakers to insulin pumps to the materials that engineers are using to try and build new tissue. So when I place that material, or scaffold, in the body, the immune system creates a small environment of cells and proteins that can change the way that our stem cells behave. Now, just like the weather affects our daily activities, like going for a run or staying inside and binge-watching an entire TV show on Netflix, the immune environment of a scaffold affects the way that our stem cells grow and develop. If we have the wrong signals, say the Netflix signals, we get fat cells instead of muscle. These scaffolds are made of a variety of different things, from plastics to naturally derived materials, nanofibers of varying thicknesses, sponges that are more or less porous, gels of different stiffnesses. And researchers can even make the materials release different signals over time. So in other words, we can orchestrate this Broadway show of cells by giving them the correct stage, cues and props that can be changed for different tissues, just like a producer would change the set for "Les Mis" versus "Little Shop of Horrors." I'm combining specific types of signals that mimic how our body responds to injury to help us regenerate. In the future, we could see a scar-proof band-aid, a moldable muscle filler or even a wound-healing vaccine.

  Now, we aren't going to wake up tomorrow and be able to heal like Wolverine. Probably not next Tuesday, either. But with these advances, and working with our immune system to help build tissue and heal wounds, we could begin seeing products on the market that work with our body's defense system to help us regenerate, and maybe one day be able to keep pace with a salamander.

  Thank you.

  You aren't at the mercy of your emotions -- your brain creates them

  My research lab sits about a mile from where several bombs exploded during the Boston Marathon in 2013. The surviving bomber, Dzhokhar Tsarnaev of Chechnya, was tried, convicted and sentenced to death.

  Now, when a jury has to make the decision between life in prison and the death penalty, they base their decision largely on whether or not the defendant feels remorseful for his actions. Tsarnaev spoke words of apology, but when jurors looked at his face, all they saw was a stone-faced stare. Now, Tsarnaev is guilty, there's no doubt about that. He murdered and maimed innocent people, and I'm not here to debate that. My heart goes out to all the people who suffered. But as a scientist, I have to tell you that jurors do not and cannot detect remorse or any other emotion in anybody ever. Neither can I, and neither can you, and that's because emotions are not what we think they are. They are not universally expressed and recognized. They are not hardwired brain reactions that are uncontrollable. We have misunderstood the nature of emotion for a very long time, and understanding what emotions really are has important consequences for all of us.

  I have studied emotions as a scientist for the past 25 years, and in my lab, we have probed human faces by measuring electrical signals that cause your facial muscles to contract to make facial expressions. We have scrutinized the human body in emotion. We have analyzed hundreds of physiology studies involving thousands of test subjects. We've scanned hundreds of brains, and examined every brain imaging study on emotion that has been published in the past 20 years. And the results of all of this research are overwhelmingly consistent. It may feel to you like your emotions are hardwired and they just trigger and happen to you, but they don't. You might believe that your brain is prewired with emotion circuits, that you're born with emotion circuits, but you're not. In fact, none of us in this room have emotion circuits in our brain. In fact, no brain on this planet contains emotion circuits.