The Quantum Universe_ Everything That Can Happen Does Happen - Brian Cox [53]
In Chapter 10 we’ll see that we need to supplement the quantum rules we’ve articulated so far with some new rules dealing with particle interactions. At the moment, we have very simple rules: particles hop around, carrying imaginary clocks which wind back by clearly specified amounts depending on the size of the hop. All hops are allowed, and so a particle can hop from A to B via an infinity of different routes. Each route delivers its own quantum clock to B and we must add up the clocks to determine a single resultant clock. That clock then tells us the chance of actually finding the particle at B. Adding interactions into the game turns out to be surprisingly simple. We supplement the hopping rules with a new rule, stating that a particle can emit or absorb another particle. If there was one particle before the interaction, then there can be two particles afterwards; if there were two particles before the interaction, then there can be one particle afterwards. Of course, if we are going to work out the maths then we need to be more precise about which particles can fuse together or split apart, and we need to say what happens to the clock that each particle carries when it interacts. This is the subject of Chapter 10, but the implications for atoms should be clear. If there is a rule saying that an electron can interact by emitting a photon, then we have the possibility that the electron in a hydrogen atom can spit out a photon, lose energy and drop down to a lower energy level. It could also absorb a photon, gain energy and leap up to a higher energy level.
The existence of spectral lines indicates that this is what is happening, and this process is ordinarily heavily biased one way. In particular, the electron can spit out a photon and lose energy at any time, but the only way it can gain energy and jump up to a higher energy level is if there is a photon (or some other source of energy) available to collide with it. In a gas of hydrogen, such photons are typically few and far between, and an atom in an excited state is much more likely to emit a photon than absorb one. The net effect is that hydrogen atoms tend to de-excite, by which we mean that emission wins over absorption and, given time, the atom will make its way down to the n = 1 ground state. This is not always the case, because it is possible to arrange to continually excite atoms by feeding them energy in a controlled way. This is the basis of a technology that has become ubiquitous: the laser. The basic idea of a laser is to pump energy into atoms, excite them, and collect the photons that are produced when the electrons drop down in energy. Those photons are very useful for reading data with high precision from the surface of a CD or DVD: quantum mechanics affects our lives in myriad ways.
In this chapter, we have succeeded in explaining the origin of spectral lines using the simple idea of quantized energy levels. It would seem we have a way of thinking about atoms that works. But something is not quite right. We are missing one final piece of the jigsaw, without which we have no chance of explaining the structure of atoms heavier than hydrogen. More prosaically, we will also be unable to explain why we don’t fall through the floor, and that is problematic for our best theory of Nature. The insight we are looking for comes from the work of Austrian physicist Wolfgang Pauli.
7. The Universe in a Pin-head
(and Why We Don’t Fall Through the Floor)
That we do not fall through the floor is something of a mystery. To say the floor is ‘solid’ is not very helpful, not least because Rutherford discovered that atoms are almost entirely empty space. The situation is made even more puzzling because, as far as we