John: This is our last discussion about Stephen Hawking.

Maggie: What topics are we covering?

**A Unified Theory**

John: Hawking’s big dream was to come up with a unified theory that combined Einstein’s theory of general relativity and quantum theory. I thought we would discuss this.

Maggie: Sure, no problem. I know all about that stuff.

John: Okay.

Maggie: Hello! I am joking, you know. Sarcasm. You are going to explain what you mean by a unified theory and quantum theory, right?

John: I didn’t quite pick up on the sarcasm this time, but I will work on that. And yes, my young friend, I intend to do my best to explain these terms.

Maggie: I think I know what quantum theory is from high school physics, so let’s start there.

John: Okay, and good connection to your high school class.

We can think of physics as dealing with two separate types of objects: big objects and little objects.

Maggie: Are you being sarcastic now?

John: Not at all. This is actually a nice way to categorize physics. Let me explain.

Some physicists focus on large objects, like stars and galaxies. This is what Einstein did when he came up with the theory of relativity. And this has led to ideas about gravity and the orbits of planets and the expanding universe.

Other physicists focus on really tiny objects, like atoms and particles. These physicists are called quantum physicists and their subject matter is sometimes called quantum mechanics.

Maggie: Yes, those are the objects we learned about in high school. So boring…

John: Yes, this is what most high schools teach in physics classes. But let’s be fair to quantum physics: there are no boring subjects, only boring teachers. Quantum physics can be just as exciting as cosmology.

Maggie: Fair enough.

John: So we have these two competing types of physics. And they often have contradictory and competing ideas. For example, to a cosmologist, space is like a smooth sheet. We saw that when we discussed Einstein earlier. But for a quantum physicist, space is grainy and filled with holes and voids. It’s actually kind of gross looking at objects on this scale. Because these two theories often produce contradictory results, the holy grail of physics is a theory that combines both quantum theory and relativity; and this is called a “unified theory.” So, just like other physicists, Hawking sought this unified theory.

Maggie: I get it now.

**Shrinking Black Holes and Hawking Radiation**

John: Which brings us to Hawking radiation.

Maggie: This is black hole stuff, right?

John: Right.

When Hawking first started thinking about black holes, he said that black holes can never shrink in size.

Maggie: Why? That seems weird.

John: Remember that a black hole is a gravitationally collapsed star with a super intense field of gravity. In fact, the gravity is so great, Hawking thought, that nothing can ever escape the gravity once it passes the event horizon.

Maggie: What is that?

John: Let’s use a picture to help us understand this term.

Maggie: Okay, I have looked at the picture.

John: Do you see the edge around the black hole, the edge of the circle?

Maggie: Yes.

John: Well, that edge is called the event horizon. It’s a weird thing to call it, I know. But it is basically the edge of the hole that differentiates the black hole from the rest of space. It is kind of like your lips, which surround your mouth. Your lips are the event horizon, surrounding the black hole that is your mouth.

You know what, I like that analogy.

Maggie: It is kind of funny.

John: The event horizon is the point beyond which nothing can escape the gravity of a black hole. So matter goes into the black hole, but matter never escapes. As a black hole swallows more matter, it gains mass. In other words, it grows. But because it can never lose matter, its mass never shrinks. Does that make sense?

Maggie: Yes, I get it. Because of its gravity, a black hole can never lose matter, so it can never shrink.

John: Right, and nice summary.

But then Hawking had a revolutionary and unifying idea.

Maggie: Exciting!

John: I know. Hawking borrowed from quantum theory the idea of virtual particles. Think of it like this. In the universe, pairs of particles are always coming into existence, with one particle having positive energy and one particle having negative energy. Because they are opposites, the two particles annihilate each other immediately, so they are not even detected. That is why we call them virtual particles.

Now remember our maxim: we don’t need to understand this perfectly; we only need to understand it sufficiently for our purposes. So let’s not freak out about open questions.

Maggie: I remember and I won’t.

John: So here is Hawking’s unifying idea. He suggested that these virtual particles could become real particles if they were created right at the event horizon of a black hole.

When a virtual particle is created at an event horizon, it wouldn’t necessarily annihilate itself. It is possible that the strong gravity of the black hole would suck the negative particle into the hole, and that the positive particle would go spinning off into space.

Two things would then result from this event.

First, since the negative particle was sucked into the black hole, the total mass of the black hole would in fact decrease, causing the black hole to shrink. This is a reversal of Hawking’s earlier position that black holes could not shrink.

Maggie: Whoa, whoa, whoa. I don’t get that.

John: Remember, when a negative particle interacts with a positive particle, they annihilate each other. In simple terms, both cease to exist. So when a black hole sucks a negative particle into it, that particle contacts a positive particle and annihilates it. Because a positive particle with positive energy and positive mass is annihilated, the black hole loses mass and shrinks in size.

Maggie: Let me see if I get this. If we threw ten negative particles into a black hole, it would shrink by ten positive particles, thereby decreasing is size.

John: Good enough.

Maggie: Okay, what is the second result.

John: Second, the positive particle that wasn’t sucked into the black hole would go spinning off into space, while radiating energy. That energy—which we will be able to measure someday—is now known as Hawking radiation.

Maggie: That is so cool.

John: I know! Hawking has some other discoveries, but these two are Hawking’s greatest contributions to cosmology, I believe. And, here is what is really cool. He started cosmology down a path towards a unified theory.

Maggie: Wait, wait, wait. Is this where the phrase “a theory of everything” comes from?

John: Yes, exactly. If we can combine relativity and quantum mechanics, we can explain the entire universe in one theory, and Hawking has started us down that path.

Maggie: He really energizes my interest in science. (And see what I did there?)

John: You are so funny.

**Summary**

Maggie: Since this is our last discussion, can you summarize what we have learned about Hawking?

John: Sure. In our past four discussions, we have discussed the following topics: Hawking’s biography; how scientific ideas grow and change; Newton’s theory of gravity; Einstein’s theories of space and gravity; Hoyle and the steady-state model of the universe; Penrose’s idea about singularities; and finally Hawking’s ideas about how the universe began as a singularity, the expanding universe, Hawking radiation, and the idea that black holes can shrink in size.

Maggie: Wow, we have covered a lot of territory in four discussions.

John: Yes, and some really interesting territory.

Maggie: Don’t forget that we also discussed how Neil deGrasse Tyson killed Pluto. Are we still going to send him a letter?

John: We should, shouldn’t we?

Maggie: That would be loads of fun. So what is our next topic?

John: What do you want to discuss?

Maggie: Since we are on the topic, how about a brief history of astronomy?

John: Yeah, right. I will just pull that off the top of my head.

Maggie: Seriously, let’s do it. It will be loads of fun.

John: Okay, if that is what you really want to do.

Maggie: I have really enjoyed these nerdy discussions.

John: Yes, me too.