Knocking on Heaven's Door - Lisa Randall [43]
But confusing our ability to picture phenomena with our confidence in their reality is a mistake that scientists cannot afford to make. Not seeing or even having a mental image doesn’t mean that we can’t deduce the physical elements or processes that are happening at these scales.
From our hypothetical vantage point on the scale of an atom, the world would appear incredible because the rules of physics are extremely different from those that apply to the scales we tick off on our measuring sticks at familiar lengths. The world of an atom looks nothing like what we think of when we visualize matter. (See Figure 15.)
Parts of the Atom
[ FIGURE 15 ] An atom consists of electrons orbiting a central nucleus, which consists of positively charged protons, each of charge one, and neutral neutrons, which have zero charge.
Perhaps the first and most striking observation one might make would be that the atom consists primarily of empty space.26 The nucleus, the center of an atom, is about 10,000 times smaller in radius than the electron orbits. An average nucleus is roughly 10-14 meters, 10 femto-meters, in size. A hydrogen nucleus is about 10 times smaller than that. The nucleus is as small compared to the radius of an atom as the radius of the Sun is when compared to the size of the solar system. An atom is mostly empty. The volume of a nucleus is a mere trillionth of the volume of an atom.
That’s not what we observe or touch when we pound our fist on a door or drink cool liquid through a straw. Our senses lead us to think of matter as continuous. Yet on atomic scales we find that matter is mostly devoid of anything substantial. It is only because our senses average over smaller sizes that matter appears to be solid and continuous. On atomic scales, it is not.
Near emptiness is not all that is surprising about matter on the scale of an atom. What took the physics world by storm and still mystifies physicists and nonphysicists alike is that even the most basic premises of Newtonian physics break down at this tiny distance. The wave nature of matter and the uncertainty principle—key elements of quantum mechanics—are critical to understanding atomic electrons. They don’t follow simple curves describing the definite paths that we often see drawn. According to quantum mechanics, no one can measure both the location and the momentum of a particle with infinite precision, a necessary prerequisite for following an object’s path through time. Heisenberg’s uncertainty principle, developed by Werner Heisenberg in 1926, tell us that the accuracy with which position is known limits the maximum precision with which one can measure momentum.27 If electrons were to follow classical trajectories, we would know at any given time exactly where the electron is and how fast and in what direction it is moving so that we could know where it will be at any later time, contradicting Heisenberg’s principle.
Quantum mechanics tells us that electrons don’t occupy fixed locations in the atoms as the classical picture would assert. Instead, probability distributions tell us how likely electrons are to be found in any particular point in space, and all we know are these probabilities. We can predict the average position of an electron as a function of time, but any particular measurement is subject to the uncertainty principle.
Bear in mind that these distributions are not arbitrary. The electrons can’t have just any old energy or probability distribution. There is no good classical way to describe an electron’s orbit—it can only be described in probabilistic terms. But the probability distributions are in fact precise functions. With quantum mechanics, we can write down an equation describing the wave solution for an electron, and this tells us the probability for it to be at any given point in space.
Another property of an atom that is remarkable from the perspective of a classical Newtonian physicist is