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E Is for Energy


Thursday, December 21, 2006

An SBCC professor emeritus and former UCSB lecturer in physics, Professor Elwood Schapansky retired several years ago to be reborn as a glacier-hopping pilot, ferrying climbers and tourists to Mt. McKinley. But he is still a good and patient teacher.

Last Wednesday afternoon, by the heat of a wood-burning stove and the refracted light of numerous prisms, he sat and talked with Martha Sadler for almost two hours about Einstein, electrons, and why the sky is blue. And eventually, after some initial resistance, he gave up the secret of E=MC².

What follows is an edited transcript of that discussion. If energy is neither created nor destroyed, where does it come from?

I cannot answer that question. Either God created it, if you are a religious person, or — all I know is that it’s existed from time immemorial. If you want to talk about just on earth, the sun is bombarding earth with millions of joules of electromagnetic energy every moment. We cannot create it, we can only find a way to use what is there.

I’ve heard it said that the universe is winding down. What does that mean?

That’s the idea of entropy, which has to do with the conversion of energy from an ordered state into a disordered state. What that means to most scientists is that the usable energy is decreasing. It’s easier just to concentrate on what’s happening here on earth. Every time we go out and burn a gallon of gas, that gallon of gas was in a nice container and it was localized and readily usable, and once we’ve used it, where is it? It still exists. It’s in the molecules of carbon dioxide and other gases that are floating around our universe — and somebody can still use those, but they’re less usable to us. The gas is converted into kinetic energy which makes your engine hot, you spew out heat as you go down the road, and when you apply the brakes to stop the car the car all of its remaining kinetic energy is converted into heat.

I suppose that’s why brake pads are fire resistant.

Yes, you have to have something that can get very hot and can dissipate the energy without burning up.

What is energy? Does E=MC² mean energy equals mass times the speed of light squared?

No, you’re just taking one isolated little Einsteinian equation and making it significant, and that’s way, way, way, way beyond where we are. We can define energy very simply: It is the ability to do work.

Is it measured in Joules? Yes, Joules or ergs.

Does each thing have an intrinsic amount of energy? Like that pillow on that couch, does it have a specific amount of energy?

Sure, I could throw that into the fire and the chemical energy of the bonds that keep that together will be converted into heat. Most of it will go up the chimney but some of it will make you warm and so, yeah, there’s an intrinsic amount of energy there. Every atom in our universe has energy, some very difficult to get to and some easier to get to. In fact, there’s such an immense amount of energy that’s inaccessible because everything at the nuclear level — if we could somehow get the energy that’s stored in the nucleus out of there, then we’d have —

A nuclear bomb!

Well, a bomb or a nuclear reactor. But it’s not that easy, there are only certain atoms that have nuclei that we can easily get that energy out of there. It’s there by the barrels full, but we just can’t get it. You can’t get the carbon atoms that most of that pillow is made of to engage in a nuclear reaction. You can get that kind of energy from thing like uranium and plutonium. Those atoms are unstable enough that they will split and give up their energy.

Do two things that weigh the same have the same amount of energy, I mean, if they’re in the same position? No, they have the same amount of gravitational potential energy. They don’t have the same amount of chemical or nuclear energy, just gravitational energy.

Oh. That’s probably an important distinction.

There’s no relationship. It’s a completely different form of energy.

One hears talk about electrons changing energy levels. What does that mean?

That has to do with photons. A little bit earlier, my wife’s crystals created a lot of colors in here, each of those colors that you saw has to do with electrons changing energy levels. Let me ask you something: What is your concept of an electron?

That model of an atom that used to sit in classrooms that looks kind of like a solar system.

Okay, yes, the classical concept is that electrons whiz around a stationary nucleus. The reality is that you cannot isolate an individual electron, you can only talk about where it is quantum mechanically. What that means is that you can’t say where it is — you can say what the probability is of finding it in a certain place. Also, classically, an electron could have any degree of energy, and it could even fall into the nucleus like a satellite falling to earth. Well, they don’t behave like that.

Is there another visual model that you could suggest? Like a rainbow, or storm, or something?

No, it becomes very abstract. So for practical purposes the planetary model is useful in visualizing, even though we realize it’s not correct. It’s what we have to rely upon due to our human frailty.

Is an electron a particle of matter?

Yes, it’s has mass, so it’s a particle of matter.

Okay, is it true that the space between atomic particles is much, much, much, much, much, much larger than the particles themselves?

Right, just as the space between the earth and sun is larger than the objects themselves. The same is true at the atomic level.

And do the electrons orbit the nucleus?

It’s still okay to think of a nucleus with electrons in orbit at certain distances. The electrons have different energies, and we can represent those energies as a hierarchy of orbits. In this scheme, an electron can be in this energy, or that energy, but it can’t ever be in between. I’m kind of using a hybrid idea—I’m trying to simplify it—but let us imagine that there are certain allowed orbits, corresponding to certain energy levels. And that’s very important, because let’s say we have an electron that’s up here in a high orbit because it has a lot of energy. It can only switch from this high energy orbit to a specific lower energy orbit. How does it get rid of that extra energy?

Heat?

No, in this particular case it’s light. It gives off a burst of light called a photon, so every color that you see in a rainbow is really coming from electrons going from a high energy state to a lower energy state.

That is beautiful!

It is beautiful. And this is one of the fabulous things that Einstein did: One of Einstein’s relationships is the relationship between the energy of the photon and its wavelength. We can imagine — and don’t say that Schapansky said this is true, but we can imagine — that electrons are going to try to stay as close to the nucleus as possible, though they can’t fall into the nucleus. So along comes another atom, which bangs into it, and gives it some energy, and all of a sudden the electron is in another orbit. But it can’t be just any old orbit: If the amount of energy that came in isn’t exactly right, it wouldn’t go up to this higher level. Nor will it stay long in that high orbit. Just like if you let loose of a ball it’s going to fall to earth, that excited electron is going drop down to its lowest allowable energy state. And it sheds that extra energy as a photon or a packet of light.

Speaking of photons, I was looking at the moon the other night, and I was bothered that I could see the light coming from the moon but I couldn’t see the light going to the moon. Can you only see photons when they’re coming straight at you?

In essence, yes.

I didn’t know they were so directional. Yes, they are, they only go in a straight line. Except that, believe it or not, in relativity, Einstein was able to show that photons coming from the sun that don’t hit the earth actually curve around the earth, because the photon has a mass equivalence, and as such it is attracted to the earth.

That’s part of its crazy wave-or-particle riddle?

Exactly. So here’s an even better question for you since you’re talking about photons. Why is the sky blue?

Because it’s reflecting the ocean?

No!

Okay, okay! Because something’s releasing photons at the blue wavelength? Because some electrons are descending down from one energy level to another in blue packets?

That’s kind of right, but where is that energy coming from?

From the sun? But the sun should be giving energy, driving the energy levels up, not down?

Yes, but the oxygen in the atmosphere has exactly the right energy structure so that a blue photon coming from the sun will be absorbed. Then the blue photon will be instantly reradiated. It has nothing to do with the ocean.

Why is it reradiated?

Well, because—remember, we said earlier that when it absorbs it the electron goes to a higher energy state—

But it can’t stay there!

Right. It is excited but it wants to come back to its natural state. So it gives off this blue photon. But which way does it give it off? It gives it off any way it wants to. It’s completely random. So if I look over there I see blue, because countless billions of oxygen atoms are spitting out blue light every which way so that some of it comes to my eye.

So why is the sunset orange?

Because the yellow light goes straight on through. The long wavelength, lower energy light is not absorbed. If the electron could absorb it, it would move the electron up to one of these forbidden energy bands, and that’s not possible. So only the light that can move it from an allowable state to another allowable state is okay. In physic classes, we would call this scattering. The oxygen is scattering the blue light, and leaving the rest of it alone.

Do electrons ever die or disappear?

Yes, they can change from mass into pure energy. If an electron runs into a positron, they will be turned back into a photon.

Can you change pure energy into mass?

Say you have a very, very high-energy photon. It comes into contact with an atom and lo and behold, magically, two electrons are created.

And the gamma ray disappears? Yes. It’s the reverse of the nuclear reaction. In a nuclear reaction mass is converted into heat and other forms of pure energy—light, potential energy, kinetic energy, etc. It has to be reversible, though there are not many examples of this. The best example of converting pure energy into mass is the creation of an electron and its partner, called a positron.

You know this? Absolutely. You can spontaneously have a gamma ray, which is a very very energetic X ray, moving through space and it vanishes and—poof—an electron and a positron come out. Now, this is the first time your E=MC² becomes meaningful. E is equal to MC² is the formula I use to calculate how much energy this photon needs in order to create the mass of an electron and a positron. When you first brought that up, that was so far removed from everyday energy, because that literally has to do with relativity and the mass-energy equivalent. It means here’s energy, and there’s mass, and a constant—which is the speed of light squared—relates them. If I want to create some mass, I’m going to have to have this much energy—I can calculate how much energy I have to have. If we’re going to convert radiation energy, or light energy, into mass, then that’s the formula.

So that’s the energy the formula refers to, not just any old energy.

Right, it’s not a useful formula unless you’re trying to create mass. And that happens all the time. You go to any particle accelerator, and you’re taking neutrons or protons or alpha particles and you’re banging them into something, so all of this stuff comes out. And one of the things that comes out is high energy gamma radiation. You can see it happen. They put all these photographic films around and all at once, out in space, two electrons appear, one of them positive, one of them negative, and this is all in a magnetic field so one of them is circling one way, one of them is circling the other way, and you say, “My God! There are two electrons! Where did they come from?” Well, E is equal to MC squared. There was enough energy in that gamma ray to create exactly that amount of mass.

Thank you. I’ve wondered about that for a long time.

Keep up the conversation by leaving your comments below or emailing Martha Sadler.