Are we alone? This course introduces core concepts in astronomy, biology, and planetary science that enable the student to speculate scientifically about this profound question and invent their own solar systems.

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From the course by Princeton University

Imagining Other Earths

243 ratings

Princeton University

243 ratings

Are we alone? This course introduces core concepts in astronomy, biology, and planetary science that enable the student to speculate scientifically about this profound question and invent their own solar systems.

From the lesson

Quantum Mechanics and Fingerprinting Planets

This lecture introduces the Pauli exclusion principle, which requires that only one electron can be in any state. We use this principle to understand the properties of materials and the atomic lines seen in planetary and stellar structure.

- David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair

Department of Astrophysics

Welcome back. Now we're going to talk about Quantum Mechanics, introduce some of its basic ideas, and then apply it to brown dwarfs, the height of mountains, and how we fingerprint planets.

Then, quantum mechanics and how it determines some of the structure of atoms, and then we'll apply it to Jupiters and brown dwarfs. So, quantum mechanics. Let's begin with two important ideas in quantum mechanics. The first is what's called the Heisenberg Uncertainty Principal. That you can't simultaneously know the position and momentum of a particle. That there's an intrinsic uncertainty in figuring out exactly where a particle is, and exactly what it's momentum is. And no matter what you do, you can't determine the position and momentum better than to a constant. Actually this constant h we've seen before, that's the Planck's constant involved in properties of radiation. Then Planck's constant divided by two pi

As the joke goes, when Heisenberg was ticketed for speeding, the cop asked him do you know how fast you were going, and he said no, but I know where I am. As Heisenberg knew you couldn't simultaneously know your position and your momentum.

The idea here is that you can only put a single electron in any given state. And by a state, we're going to mean a position, a given value of position and momentum.

It has an interesting effect when we think about properties of things like metals. In a metal, you should think about electrons. Some electrons tightly bound around each nucleus.

And then, a set of electrons that are are free, they're floating around in what we call a sea of electrons in the middle. These electrons are whizzing around, back and forth, through the metal. That's why metals are such good conductors, because electrons, there are some electrons that could move freely through a metal.

If we were to make a plot showing where the different states we can put the electrons in. You can imagine making a graph where you write down the momentum of each electron and its position. And each electron occupies a different position in momentum position space. And we can't put them closer together than that, because of the Heisenberg Uncertainty Principle. And we can't put two electrons in each block because of the Pauli Exclusion Principle. So if we add more and more electrons to the material, they take up, they have larger and larger momentum, and with more momentum, we have more energy.

We could quantify this, and that's going to determine, the properties of metals and as we'll see white dwarf stars.

Given the number of electrons I have in my metal. That's going to determine the average distance between them.

As they get more and more tightly bunched together. Their momentum goes up. So we can solve for the momentum in terms of the spacing and positions. The momentum goes here h bar is also the same as h over two pi. It's a notation we use a lot.

We can solve for the momentum. In terms of plancks constant and the number density of electrons. The more we pack the electrons in, the higher the density. The faster they have to move because of the Uncertainty Principle, so the momentum goes up and hence their velocity goes up. So the more we pack them in, the more momentum they have, the faster they move.

Eventually, as they move faster and faster, their pressure goes up. The pressure in an electron is going to depend on the number of electrons, times their mass, times the velocity squared. So that implies the pressure in a metal, depends on the electron number density to the five thirds power. This is the important result that we're going to need from Quantum Mechanics. This is different from the Ideal Gas Law. Remember, the Ideal Gas Law told us, in a gas, the pressure depends on the density and the temperature. The hotter it is, the higher the pressure.

That's not true of our solids. The pressure in the solid depends only on the electron number density You'll notice, it doesn't depend on the temperature. That's because the electron velocity is determined by the Heisenberg Uncertainty Principle. This is also called the generosity pressure. So the pressure goes as density to five thirds, doesn't care about temperature. That has a big impact on the behavior of materials and the structure of planets like Jupiter and low temperature stars called brown dwarfs. It's also the source of support for things like white dwarfs.

As the, the binding energy of that metal is associated with the pressure you put the metal on. If you put more and more pressure on the metal, make it denser and denser. As you make the squeezed material more and more tightly, eventually you squeeze it so much it's under so much pressure that the electrons move around so quickly, that they're no longer bound. So if you take a metal and squeeze it enough, it will heat up so much the electrons will gain so much energy, that, that pressure will overcome the atomic binding energy. Typically about electron volt for atoms and it will cause the metal to melt.

The fact that there's a maximum pressure that material can handle and this is kind of a rough explanation of it, means that there's a maximum height, for the tallest mountain.

In order to work out the height of the mountain, we want to balance the characteristic velocity of the material against the strength of gravity and the height of the mountain.

So we can equate the height of the mountain to the melting temperature divided by the electron mass, times the strength of gravity,

or equivalently, the sound speed of the material, which is about 600 meters per second, divided by the strength of gravity on earth. The strength of gravity is on the surface, is about ten meters per second squared.

Sticking in these numbers, that implies that the height of the tallest mountain you can make for the base of the mantel, the crust where it melts to the top, is around 30 kilometers and if you look at the Earth's crust, that's about right. You stack things higher than that, it's going to melt.

So, you can see that basic physics determines the height of mountains, and the height of mountains are set by the balance between the strength of materials.

The strength of materials are going to be the same everywhere. What's going to vary as we go from the Earth to Mars, to an asteroid, to an extrasolar planet, is the strength of gravity.

And you can see if gravity's stronger, the tallest mountains are shorter. If gravity's weaker, mountains could be quite high.

So, given this relationship, I'd like you to now go off and figure out, what's going to be the tallest mountain that you expect on Mars or on Pluto. Work out the strength of gravity on Mars's surface Right? Remember, that's going to go as GMmars divided by radius of Mars squared. That's the strength of gravity on Mars.

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