Video Transcript: Black Holes

Seems like an insane idea, and it was first suggested not by Einstein, but by a minister, John Mitchell, who's shown on the right side of the first picture in our gallery. Mitchell was, a he's not a well-known scientist, but he was involved in many different science projects and made a number of discoveries and invented a number of experiments which then other people did and received credit for it naturally.  He was visited by prominent scientists in his day, you know, Benjamin Franklin came to visit him and meet and talk, so people certainly knew who he was. And he studied stars and had this notion that, you know, it's possible, you could have a star so massive, that not only it has so much gravity, that light can't escape from it. 

And this bared out when Einstein first came up with his theory of general relativity.  So this is a revised understanding of gravity, where space is warped by, by massive objects.  And in Einstein's equations, which are super complicated, but in his equations, the possibility exists that an object could have so much mass and so much gravity in such a small spot, that light could be completely trapped, and the object would be completely invisible.  

And so this is, in many ways, sort of consistent with this idea of a star that simply doesn't exist.  Its mass exists, its gravity exists, but it can't be seen in any other way.  Really strange. Really strange. 

Okay.  How does this work?  What's the idea here?  Well, we can go back, we don't really have to understand the details of general relativity.  We can use Newton's understanding of gravity to see how black holes would work.  And so here's a famous thought experiment, the second picture in our gallery, that Newton, Isaac Newton suggested when he was describing how gravity works. 

And that is, here we live on the planet, and imagine that you have a cannon shooting a ball.  Or maybe you're throwing a pitch or something like that, throwing a baseball.  Well, that ball is just going to go a little bit of a way and then it's going to hit the ground.  But if you could somehow launch that cannon faster, it might go further before it hits the ground.  And if you could launch it really fast, it could maybe go almost all the way around the earth before it hits the ground.  

Well, there is a speed where if you launch it that fast, it'll go all the way around the earth and stay at the exact same height above the ground.  And that speed is the orbital velocity. That's how fast you have to get something going before it will orbit all the way around.  And that's true. Like we do that with spacecraft.  That's how we set them in orbit. 

Well, if you extend this further then, if you want an object to escape the gravity of the earth, you would need to send it even faster, so fast that it never falls back to the earth and in fact, does not keep going around the earth but simply escapes.  And that is a speed that you can calculate; the escape speed. 

Now the idea here is, well, what if you have an object that is so massive, that the escape speed is so - you have to go so fast to escape that no one could possibly go that fast.  Now there are practical limitations, like, well, you can't build a rocket big enough.  But the universe has a built in speed limit.  And this is what Einstein discovered.  The built in speed limit of the whole universe is the speed of light. 

Now light travels, as we've said, incredibly fast.  No person could ever travel that fast.  But light can travel that fast.  And nothing can travel faster than that.  So the question is, how massive would something needs to be, so that the escape velocity is bigger than the speed of light.  That is that even light traveling at the ultimate speed limit is not going fast enough to escape the pull of gravity. 

And this is what Einstein kind of gave us a framework for, is that light can actually be manipulated by gravity. And so light, you might think, well, light doesn't have any mass, it doesn't have gravitational attraction, so it wouldn't be pulled back by gravity, but in fact, light can be distorted and moved and bent by gravity and if the gravity is strong enough you can reach this point where not even light can escape.  So imagine, instead of a cannon, in that drawing, you have a laser pointer, and the light itself cannot escape, it just gets pulled right back in and lands back on the surface. Well, that is what a black hole is.  It's an object that's so dense, that not even light can escape it.  

Now it's a real object, in principle.  It could be, it doesn't necessarily have to be like infinitesimally small when you can't see it.  So you can imagine standing on the surface of a black hole, and taking your laser pointer and shining it up and the light goes up, and comes right back down again, because gravity is so strong.  Of course, you wouldn't be able to do that, because gravity would be so strong.  The point is, the light can't get out.  So we can't see what's going on inside the black hole. 

In fact, there is a there's a boundary around this black hole, that's called the event horizon. And the event horizon is the line that once you cross that, there's no escaping, because to escape, you'd have to go faster than the speed of light. What that means is that any light that is on any particles of light that are on the inside of that event horizon cannot get out.  So literally, like right here could be on the inside of the event horizon and I could be standing on the outside, and someone could be shining a light right in my face, I would not be able to see it.  It's crazy.  So we have no way really of knowing what's happening inside the black hole, other than our own predictions. 

So how can we even say that these things exist?  You can't see them; you can't see inside.  How do we know that they're actually there? Well, this is what's so cool.  And like, to me, this is how God gives us an opportunity to make observations like it doesn't have to be that way.  It could be we just don't know.  But in reality, there are observations that can be made, that we can actually answer those questions.  So we can make observations to say, yes, black holes are there.  Let's look at a couple of those.  

So, in the case of… let’s put this way, many stars in our universe in our galaxy, many of those stars orbit in pairs, called binary stars.  And if one of those stars happened to have this tremendous explosion where it became a black hole, well, then those two stars now are orbiting each other, except one of them is invisible.  It's like it's a black hole now.  But the other stars, they're still orbiting each other, which raises a good point… 

You know, if something becomes a black hole, it's not like it just becomes like this cosmic vacuum cleaner sucking everything in.  If our sun became a black hole, like right now, we would lose the light, but the Earth would just keep orbiting like normal because its mass didn't change, the mass of the star didn't change.  So black holes are not just like cosmic vacuum cleaners.  They're just extremely dense objects, compact objects that have the gravity of a whole star. 

Okay, so if you've got one of these, like a black hole orbiting a nearby star, well then some of the material, and we see this with binary stars, some of the material from one star is pulled off, because it kind of brushes up against what we call an accretion disk.  So you might, this happens with ordinary stars too, but material bumps up against it, and it's kind of like there's a lot of friction. And so as the material is kind of like giving off heat because of the friction, its orbit kind of slows down.  And eventually, some of this material, eventually will, will bump its way and make it into the event horizon.  And because of all that friction that's happening as these materials rub up against each other, there's a lot of heat, and there's a lot of light being given off. 

Okay, so these accretion disks are places where material is eventually making its way into the black hole, not because it's being sucked in, but because there's so much friction of all the material around.  But there's so much heat that's there, that it's really hot, and it's glowing in the X-rays. It’s glowing brightly in the X-rays. 

Another weird thing happens, and there's physics to explain this, but as all this material is spiraling around and accreting, falling in, okay, what ends up having a some of that material gets launched outward as jets.  There's good reason for that.  I don't fully understand it.  But the point is, is that some of this material is falling in, and a small portion of it is being beamed outward as these jets into space.  And we can observe these things.  And we can not only observe these bright X-ray sources from these accretion disks because they're outside the event horizon, you'll notice right inside there is the black hole, that's the event horizon, we can't see light coming from there.  But all the stuff outside, we can see, and we can tell how much energy, how much gravity must be in that tiny little location.  

So here's a real observation. This is a picture of an elliptical galaxy. Okay, so this is a whole galaxy. And coming out of the middle of that galaxy, is this jet of material, this blue jet.  It's amazing.  So we can actually see some of these jets.  And the way that you can explain these is that there's a black hole with all this material around it collapsing down. 

Probably the best evidence ... so there's different kinds of black holes. You can imagine an individual star that's a really big star, collapses down, goes past white dwarf, past neutron star, becomes a black hole.  But there's also… what if you get a whole bunch of stars, or what if a couple black holes collide with each other, and you get this really big black hole?  And so there's evidence to suggest that at the center of galaxies… so like our galaxy’s the Milky Way; we have like 100 billion stars in this galaxy all orbiting around, but in the center, there's evidence to suggest that there's what we call a super massive black hole.  And when we say supermassive, we're not kidding.  Like, a black hole is an infinitesimally small spot that has the gravity of like, millions, you heard me right; millions of stars.  

So here's the evidence for that.  That's the last picture in the gallery.  If you want to impress your friends and family, you have them come look at this picture.  Like, look what I'm learning about. This was cool.  Okay, so this is an awesome, awesome data set.  Let's try to make some sense out of what this graph is showing. 

First, it's a picture. So it's a picture of the sky, of a small part of the sky, and it's a picture pointing right towards the center of our galaxy, the Milky Way.  The blue, what are they?  They're blue, red, yellow - they're little glowing circles.  Those blue, yellow, orange things, those are stars; stars that are near the center of our galaxy. 

All right.  And now you'll see there's a yellow star, okay?  That's what's believed to be the very center of our galaxy.  And now the other thing I want you to notice is that on the bottom right of this picture, it says 1995-2004.  

So what astronomers did is they came back over the course of a decade, and they took a picture of this part of the sky.  And what you'll notice is, so look in the bottom left.  You see there is a star and it has a yellow circle, and they mark the location of that star at several different points. So each of those yellow circles on that star represent its location as they measured over the course of those 10 years.  

Or at the very bottom, there's a purple one; maybe to the right of that, you see there's a pink one.  It's showing you the different location.  It's like it's trying to show in a single picture, a movie, of how these stars moved over those 10 years. 

Now, based on all that motion, you can make a little orbit.  You say, okay, these are all obeying the laws of physics, the laws of gravity, so these stars must be orbiting something.  And so that's what all those dashed lines are.  Each of those dashed lines represents the measured orbit, based on that data set, to calculate what the rest of the orbit would be like.  So what is that orbit for each of these stars?  

And now for each of those orbits they can find what point is this star orbiting?  Where is it orbiting?  It must be orbiting something.  Something has enough gravity to pull this star into an orbit.  And what they find is that these stars are orbiting that spot where the little yellow star is. And at that spot, they can calculate how much gravity would there need to be right here, how much gravity would there need to be, so that these stars could have these orbits.  

And when they do that, they calculate how many stars are there.  And there's like, maybe ten, maybe ten stars, they've done this for, but they've calculated that you would have to have a mass equivalent of 4.1 million stars in that tiny little space to be able to maintain those orbits. 

So we have here this amazing evidence…  because millions of stars in that space where I don’t see any light, and that tiny little spot?  I don't see any stars there.  So we see this evidence of what we call a super massive black hole.  

There's all kinds of books and videos about what are black holes like?  The current theories of black holes.  What happens if you fall into a black hole?  How would you die if you did fall into a black hole?  All kinds of crazy stuff.  And it's definitely worth exploring some of it because it's really cool.  But what's amazing is that these… you know what's most amazing to me about it all is?  That these things are beyond imagination.  Really, who could have just like said, “Oh yeah, invisible objects that are… you have millions of stars’ mass,” and they're just insane.  But we have observations that are showing us how amazing this universe is.  We can prove it in astounding ways. 

So it's amazing to me that we can come to these conclusions, and just how unreal how unimaginable our universe is.  So people you know who walk around thinking, you know, life's pretty ordinary, we've got everything explained.  It all make sense.  You know?  It does make sense, but it's crazy.  It's crazy. 

So to act like oh, yeah, it's all reasonable and perfectly ordinary, and makes sense to me is missing kind of the wonder that even though you understand this, or you can explain these graphs it’s still wonderful, and amazing, and awesome. 

Cool.  All right.  Well, we'll see you next time.