Starts With A Bang! » black holes Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 en The Most Energetic Mystery in the Universe Wed, 25 Mar 2009 20:05:43 +0000 ethan When we look out at galaxies, we find the most energetic particles we’ve ever found anywhere in the Universe coming from their centers. Why?

Because as far as we can tell, all galaxies, at their centers, have huge, supermassive black holes! When matter (like a star, globular cluster, intra-galactic gas, etc.) gets too close to one of these black holes, it gets ripped apart, and settles into a disk around the black hole. This disk is called an accretion disk:

Like everything in a strong gravitational field that moves, these particles radiate (give off high-energy photons), fall in towards the center of the black hole, and sometimes get accelerated and shot out of the galaxy!

For a galaxy like ours, with a black hole a few million times as massive as our Sun, we can get extremely energetic particles out: up to 1018 eV, which is 70,000 times more energetic than the LHC!

But our black hole is kind of a commoner — a weakling, even — when you look at other galaxies. There are huge galaxies out there, such as active galaxies and quasars, where instead of a few million times as massive as our Sun, their black holes are billions or even tens of billions times as massive as our Sun:

Well, in theory, the energies of these particles can be thousands or even tens of thousands of times higher than what our galaxy can produce! We’re talking about energies of 1022 eV, which is not only insane, it’s impossible!

Why? Because there’s a maximum energy that particles traveling through the Universe can have. There’s a bath of leftover light from the Big Bang permeating the entire Universe: the CMB. If you smash a particle with too much energy into one of these CMB photons (which is unavoidable as you travel millions of light years), it causes these high-energy particles to slow down until they’re below the “speed limit”. (Okay, it’s an energy limit, but it’s really close enough.)

And unlike cops that pull you over, the light that fills outer space slows you down until you’re below the cosmic energy limit: 5.7 x 1019 eV. Well, do we see a cutoff there?

Maybe. The AGASA experiment says no, there is no cutoff, but the Pierre Auger Observatory says yes, there is one. Who’s right? On one hand, we’ve definitely seen events where we’ve measured more energy than should be allowed. It may mean our theories need revising, or it may mean that there’s something super-energetic happening in our own backyard that we don’t know about. Or — on the other, more boring hand — perhaps we’ve just done a bad job of measuring things at very high energies. Whatever the case, explaining these events that exceed the cosmic energy limit of the Universe is, in fact, the most energetic mystery in the Universe!

Will the LHC destroy the Earth? Fri, 19 Dec 2008 20:42:22 +0000 ethan Despite multiple articles on the topic, I keep getting comments and emails telling me things like the following:

The LHC COULD be safe, nobody can honestly know.

Frank Wilczek once said that smashing quarks will create the earth into a supernova

Because there is a chance for basic failure of current theories, the threat is potentially a global one, the issue develops into an ethical, or political one. Here, all the physicists are simply ruthless.

Really? Now these arguments are ridiculous, but it isn’t obvious. Remember the first time you heard the argument that the twin towers couldn’t have collapsed without “help” because burning jet fuel isn’t hot enough to melt steel? It sounded like a reasonable point, didn’t it? But what really happened in that situation was that steel weakens at high temperatures, and at jet fuel temperatures (about 1400 °F or 760 °C) steel loses about 90% of its strength, even though it’s still a solid.

And so things make sense. Now what if I told you that biologically, Aleut eskimos from Atka, Alaska and Mbuti pygmies from the Democratic Republic of the Congo were so different that they couldn’t interbreed! Would you believe me? After all, there are significant genetic differences between them, interbreeding never occurs in nature, and they have very different physical traits from one another. But that isn’t what the science supports. In fact, not only can all known races of humans interbreed, but humans, in the past, interbred with neanderthals!

But I’m not an expert on the tensile strength of steel or on evolutionary biology. I am an expert in theoretical physics, however. Credentials? Let’s see:

So with all of this in mind, my question to everyone is WHY?! Why do you think the LHC has a chance of destroying the Earth?

Why do you think that physicists are covering up some huge conspiracy so that they can get away with secretly destroying the planet with black holes? Not only is this a ludicrous idea, it isn’t physically possible.

Let me recap the real science here in as simple a way as I can:

  1. Black holes that are the mass of stars (or greater) have strong gravitational fields, and can grow by sucking up more of the matter around it.
  2. It may be possible to create black holes at a powerful particle accelerator that are much smaller than stars if there are special types of extra dimensions.
  3. These black holes, if we can create them, will not suck up matter quickly and grow. They are also energetically unstable, and will decay due to the laws of quantum mechanics.

But somehow, this doesn’t make it in to people’s brains, even when I spell it out like this. People hear the words “black hole” and freak out and dream of horrific deaths. Then they ignore the laws of physics that we already know and understand, and make up apocalyptic ones in their place.

I will state it again, clearly and unambiguously:

There is no conceivable, realistic scenario where a black hole or anything else created at the LHC could destroy the world.

Even if black holes are created there, there is no way that it poses any danger to anyone. But I bet you I still get comments at the bottom of this post where people will write about Hawking Radiation, Walter Wagner, Plaga and Tankersley, and a whole host of other legitimate-sounding theories.

So if you do, please consider the following as you write: is there anything I could state to you, any fact or any evidence I could show you, that would convince you that the world is safe?

LHC Black Holes: Worst Case Scenario Wed, 10 Sep 2008 17:48:00 +0000 ethan So, the LHC has turned on, and has circulated protons both clockwise and counterclockwise inside of its main ring. These particles, at their fastest, will get up to 7 TeVs of energy apiece, which means they’ll be moving at 99.99999898% the speed of light! To give you a feel for how close that is, the speed of light is exactly 299,792,458 meters per second. In the LHC, these protons move at 299,792,454.9 meters per second. Pretty damned impressive!

The world hasn’t been destroyed yet, obviously, but what happens if one of those collisions makes a black hole? What is the worst case scenario for the Earth here?

Well, the most massive black hole one can possibly make is if all of the energy from the collision goes into making that black hole. Let’s assume that one million of these collisions occur, and all of them make black holes, which can then merge together (again, this is incredibly, unrealistically optimistic, but let’s go for it). For the maximum collision energy at CERN (14 TeV), E = mc2 tells us that the end black hole would have a mass of 2.5 x 10-14 grams. That’s 25 femtograms, which means this black hole would have an event horizon trillions upon trillions of times smaller than the size of a proton.

Now, maybe you think it’s reasonable that this black hole, if it’s created at rest, would simply fall into the Earth, consuming all the particles in its path. Let’s assume it could do this, in fact, and let’s find out how much mass it would eat.

As it falls into the Earth, it starts running into protons, and let’s assume whenever it runs into one, it gobbles it up. By time it gets to the center of the Earth, it will have eaten about 10-16 grams of matter, which means it can grow by about 0.4% in the 30 minutes or so it takes to get to the center of the Earth. It will then head towards the other side, gobbling up that matter until it stops in the upper mantle, and then heads back towards the center of the Earth. It should do this over and over, each time gobbling up more matter (at a constant rate of about 4 x 10-16 grams per hour), each time getting farther and farther away from the Earth’s surface, never to quite reach it again.

At this rate, it would still take three billion years for the black hole to suck in even one gram of matter! So the chances of this happening?

Zero. Not one in 50 million, like Martin Rees says, but zero. This is the worst case scenario, and it still takes billions of years to even eat one gram’s worth!

And if we come back to reality, we learn one more thing. Black holes decay, and the smaller ones decay the fastest. Even if you managed to make this 25 femtogram black hole, it would decay into normal matter incredibly fast. How fast? According to Hawking radiation, this black hole will be gone in 10-66 seconds, which means, unless there is some incredible new physics (like extra dimensions), we can’t even make a black hole! Why not? Because anything that happens in a time less than the Planck time (10-43 seconds) cannot physically happen with our current understanding of physics.

So there you have it: the worst case scenario. Still scared? Back off, man. I’m a scientist!

Ours is pretty big… Wed, 11 Jun 2008 17:08:09 +0000 ethan When was the first time you ever heard of a black hole? Did you have any idea what one looked like? Did you have any idea how one was made? And maybe most importantly, did you know where to go and look for them?

Well, back in 1969, while astronauts were taking care of business, two astrophysicists, Donald Lynden-Bell and Martin Rees, proposed that the very center of the Milky Way galaxy not only had a black hole, but had one that was maybe a million times as massive as our Sun, or even more!

For decades, people tried to find it. They would find a strong radio source near the galactic center and say, “Oooh, maybe it’s that supermassive black hole?” But nothing concrete ever came from that. Then, in the 90’s, someone had a great idea that I’m a huge advocate of: use gravity to look for it! (Those of you who are regular readers know that my research is often about using gravity to look for dark matter.)

So this guy, Reinhard Genzel, decided to look at stars that were very close to the galactic center, and try to measure their orbits. This works the same way it works in our solar system: you measure the orbits of some bodies, and you infer the mass of the thing it orbits.

In 2002, they published their result: they had discovered the exact location of a mass that had the following three properties:

  1. It emitted no visible light. (So, it’s black.)
  2. It was a strong radio source (like quasars, like Cygnus X-1, like we expect black holes to be).
  3. It is a point-like source with a mass equal to over two million solar masses.

So this was definitely really strong evidence that we’ve got a black hole millions of times the mass of our Sun sitting at the center of our galaxy! And the guy who found it, Reinhard Genzel, was just awarded the million-dollar Shaw Prize for this discovery!

But I’m not going to argue; it is pretty awesome to see a star zipping around the center of the galaxy with a speed of 30,000,000 kilometers/hour, or something like 3% the speed of light! (To compare, the Earth goes around the Sun at about 30 kilometers a second.) And a million dollars for getting to do it seems not too shabby. So here’s to gravity!

What Spins the Fastest in Space? Wed, 28 May 2008 22:43:04 +0000 ethan Almost every object in space doesn’t just move, but also rotates about its own axis. As those of you who know my Jewish heritage can attest, we are fascinated with spinning things. How else to explain this:

But seriously, what objects spin the fastest in space? Well, most people know that we spin once per day, and that’s a good place to start, as we’re a lot faster than our Sun, which takes about 25 days to spin around once. But we’re not even the fastest spinner out of the planets: Jupiter and Saturn both beat us, as despite their huge size, they each rotate once in just about 10 hours, with Jupiter a little bit faster and Saturn a little bit slower.

Well, amazingly, there are things in our solar system that rotate even faster, and the new record holder for our solar system was just discovered… by an amateur astronomer! Congratulations to Richard Miles of Dorset, in the UK, who discovered that the near-Earth asteroid 2008 HJ rotates once every 42.7 seconds, making it the first natural object in the solar system to rotate with a period under a minute! Despite being only 12 meters by 24 meters (about the size of a tennis court), it weighs over 10 million pounds (or about 5,000 tonnes, for the metric kids). For comparison, this is teeny-tiny, as the larger asteroids measure tens or even hundreds of kilometers across:

But what if we go outside of our solar system? Are there other things that rotate faster? Sure there are: collapsed stars!

Those of you who took physics might remember that a few things are always conserved: Energy, Momentum, and Angular Momentum among them. Well, what angular momentum means is that if I have a massive rotating body and collapse it, it rotates faster and faster. It’s the same principle behind this:

Well, let’s take our Sun; it has an angular momentum that’s proportional to its radius2 times its angular velocity. But the Sun is big. It has a radius of 700,000 kilometers. But someday the Sun will become a white dwarf, shrinking to roughly 1% of its current size. If it conserves angular momentum, that means instead of taking 25 days to rotate, it will spin around once in just 3.5 minutes. And when we look at other white dwarf stars, we find that this is just the right rotation speed for them, with the fastest one clocking in at 33 seconds!

Meh, you may say. 33 seconds is barely faster than our asteroid in our solar system. Well, I’ll show you. I’ll show all of you! Wait, no, sorry, that was my villain-voice. What I meant was, not all stars collapse to white dwarfs, with radii of thousands of kilometers. Some of them collapse even further, to form neutron stars, with radii that are only a few kilometers! Well, again, conserve angular momentum, and what would we expect for our Sun? A period of about 20 milliseconds. Crazy talk? Let’s see what we find when we tabulate observed neutron star periods:

They range from the slowest at 8.3 seconds to the fastest at only 1.4 milliseconds! If you do a little math, and ask how quickly is a star with, let’s say, a 5 km radius spinning with a period of 1.4 milliseconds, and you’ll find it’s about 10% the speed of light. You want to spin faster than that in space, and you’ll have to turn into a black hole. The only problem is, if you’re a black hole, I can’t measure how quickly you’re spinning, because you aren’t letting any light out for me to see. But I can measure the stuff orbiting you, and that’s moving about 10% the speed of light; I can only imagine the internal rpms you’ve got going on! Any ideas for how to measure that?

Big News about Black Holes! Fri, 16 May 2008 17:56:25 +0000 ethan This has been all over my inbox since the press release came out yesterday; it’s been on slashdot (thanks Brian), it’s been at, and there’s a mediocre writeup on Universe Today. What’s the big news? Black Holes don’t destroy information after all!

What is this whole information thing, anyway? Take a look at all the normal stuff in the Universe: photons, protons, neutrons, and electrons, for example. They have lots of different properties each. They move around one another, they get bound and unbound from one another, they exert forces on one another, etc. They’re aware of one another, and they interact with one another, and they have many properties, both intrinsic and extrinsic. And when they come together, they make complex things, like atoms, which contain even more information than the individual particles do on their own:

But when you put all of this stuff into a black hole, all of this information seems to disappear. Black holes have only three properties: mass, charge, and spin. Everything else, all the information you put into it, seems to disappear. Doesn’t matter whether you put a bunch of neutrons in, or a bunch of human beings in: to a black hole, they’re the same things. Black holes don’t remember quantum numbers, they don’t remember how many quarks or electrons you put into it. All they care about is mass, charge, and spin.

Or so the story goes. That’s what Stephen Hawking has been saying for years, leading to his famous information paradox about black holes.

Well, it’s a paradox no more! A team at Penn State, led by Abhay Ashtekar (who’s pretty famous for his work on trying to formulate a quantum theory of gravity), has shown how all of that “lost information” can be recovered. The catch?

It means that space-time is not a continuum.

Instead, on the smallest scales, it has to actually be discrete, like little “chunks” of space-time strung together to create a giant mosaic.

The reason for this is that without a continuum, there can’t be any such thing as a singularity. Here’s what Ashtekar had to say about it:

Information only appears to be lost because we have been looking at a restricted part of the true quantum-mechanical space-time. Once you consider quantum gravity, then space-time becomes much larger and there is room for information to reappear in the distant future on the other side of what was first thought to be the end of space-time.

Wow. Now this hasn’t been verified or even published yet (May 20th, in Physical Review Letters), but this is really ground-breaking! Have a great weekend, folks, and don’t forget to swing on over to this week’s Carnival of Space at Altair VI, where you can read about lots of interesting stuff, including how to avoid Armageddon from incoming asteroids!