Why Hasn’t the Universe Collapsed Into a Giant Black Hole? – Ask the Expert #13


Hi, I’m George Musser. I’m a contributing editor at Scientific American magazine and author of : The Complete Idiot’s Guide to String Theory, and I’m here to answer your questions. Okay. The first question is: Let’s say I’m an alien on a ship 65 million or more light-years away. Using a telescope, I look at Earth and I see dinosaurs living their daily lives. If my ship started travelling towards Earth near the speed of light, would I see the dinosaurs moving faster, fast forwarded? That’s a great question, a subtle question. It’s the kind of thing that comes up a lot when you deal with relativity theory, Einstein’s theory of relativity. And actually asked an astrophysics to make sure I’ll get the right answer for this. So, the bottom line is the dinosaurs are going to be moving a lot faster as you see them on the ship, and the reasoning goes something like this: When you set out, you’re seeing the dinosaurs 65 million years ago. When you arrive, you’re moving at something like the speed of light, you see them 65 million years in the future, so, 130 million years passed in all. According to the dinosaur’s time, things look a little different on your ship. On the ship, the distance seems to be so much shortened by the fact that you’re moving so close to the speed of light. If you’re moving at 99 % of the speed of light the distance looks something like only 10 million light years, so you need 10 million years to cross that distance. OK, 130 million years have passed on earth, 10 million years have passed on your ship; the dinosaurs seem sped up by the factor of thirteen. They’re moving and they’re stomping around and chomping on prey, thirteen times faster from your view on the ship than they are to them, to themselves. OK, question number two, another great question: If all movement is relative, how can there be a speed limit? In order to measure speed you have to measure against something, so, what is the speed limit measured against? So it’s really a great question because it gets to the heart of relativity theory, Einstein’s theory of relativity, and what that theory means and the weird predictions that it makes. The bottom line is that the speed of light is the same for everybody, it doesn’t matter what you’re measuring it against. You can measure it against a spaceship, train, car, boat, walking down the street, standing still, whatever, and you’ll see the same speed of light for everybody. And that’s completely different from other things that we observe in nature; from traveling in a car and I threw a ball off the car, I’d have to add or subtract, the speed of the car, to the speed of that ball. But that’s not true of light. if I shine a flashlight off that car it will be moving at the speed of light relative to me on the car, and the speed of light relative some walking down the sidewalk, the speed of light relative to someone in orbit looking on, an alien on a distant planet. Everyone will see the same speed of light for that light beam that I shine out the car. And that’s a weird fact that the speed of light is the same for everybody but it underlies everything you ever heard about in Einstein’s theory. It underlies the fact the time ticks differently for different observers, lengths look different to different observers. The space and time themselves are combined in something called “space-time” You can’t really treat space and time independently, they’re united in this thing called “space-time”. And it’s all consequence of this fact developed from the speed of light being the same for everybody The third question goes in a somewhat different direction, but I really like it as well. If a black holes event horizon gets bigger, every time its density gets bigger, ie: it absorbs mass. Why hasn’t the whole universe collapsed into a enormous black hole yet? Shouldn’t this kind of expanding black hole be getting bigger exponentially? And will this be the end of our universe and possibly the birth of a new one; or where ours began, a big bang? That’s a very perceptive question, because it’s true. As black holes take material in they get bigger, so you think there is an exponential growth to them, that as they take more stuff in they get bigger, they’re bigger target, they suck more stuff in , they get bigger, bigger, bigger at an exponential rate. And it’s true, black holes do grow at an exponential rate if you keep feeding stuff into them, if there is material for them to eat. In fact, there is something in astrophysics called the Salpeter time, which is something like a few tens of millions of years and that shows how fast they will double in size. So keep doubling and doubling in size every few ten’s of millions of years If stuff is being fed into them. Now that “if” is a big “if” Although black holes are monsters and they’re vacuum cleaners and they’re sucking everything in they’re still pretty small on a galactic scale, so they’re not a huge target. They are sitting at center of the galaxy in this case typically and it’s hard for them to get enough material to feed on, so they tend to choke off basically their own growth; not just because it’s hard to feed stuff in but because the black hole is actively changing its environment, it’s spraying stuff out in the space and actually pushing stuff back out, it’s heating stuff preventing it from falling in so they regulate their own growth. So it’s very hard for them to grow, to continue growing at an exponential rate. They probably did initially, but then they kind of level out and they achieve something like a few tens of a percent of the mass of their overall galaxy. They have a self regulatory mechanism. Thanks for the great questions, If you have other questions for the next expert. please leave them in the comments below. And challenge the next expert, give them hard questions like you gave me ♪ (music) ♪