Zero - Charles Seife [64]
Einstein’s idea explained the photoelectric effect brilliantly. Light is quantized into photons, directly contradicting the wave theory of light that had not been questioned for more than a century. Indeed, it turns out that light has both a wave nature and a particle nature. Though light acts like a particle sometimes, it acts like a wave at other times. In truth, light is neither particle nor wave, but a strange combination of the two. It’s a hard concept to grasp. However, this idea is at the heart of the quantum theory.
According to quantum theory, everything—light, electrons, protons, small dogs—have both wavelike and particle-like properties. But if objects are particles and waves at the same time, what on earth could they be? Mathematicians know how to describe them: they are wave functions, solutions to a differential equation called the Schrödinger equation. Unfortunately, this mathematical description has no intuitive meaning; it is all but impossible to visualize what these wave functions are.* Worse yet, as physicists discovered the intricacies of quantum mechanics, stranger and stranger things began to appear. Perhaps the weirdest of all is caused by a zero in the equations of quantum mechanics: the zero-point energy.
This strange force is woven into the mathematical equations of the quantum universe. In the mid-1920s a German physicist, Werner Heisenberg, saw that these equations had a shocking consequence: uncertainty. The force of nothing is caused by the Heisenberg uncertainty principle.
The concept of uncertainty pertains to scientists’ ability to describe the properties of a particle. For instance, if we want to find a particular particle, we need to determine the particle’s position and velocity—where it is and how fast it is going. Heisenberg’s uncertainty principle tells us that we can’t do even this simple act. No matter how hard we try, we cannot measure a particle’s position and its velocity with perfect accuracy at the same time. This is because the very act of measuring destroys some of the information we are trying to gather.
To measure something, you need to prod it. For instance, imagine that you are measuring the length of a pencil. You could run your fingers along it and measure how long it is; however, you’ll probably give the pencil a nudge, disturbing the pencil’s velocity slightly. A better way would be to place a ruler gently next to the pencil, but in fact, comparing the lengths of the two objects also changes the pencil’s speed a tiny bit. You can only look at the pencil when light is bouncing off it; though the disturbance is very slight, the photons that carom off the pencil nudge it ever so gently, changing the pencil’s velocity a tiny bit. No matter what way you think of to measure the pencil, you will give it a tiny nudge in the process. Heisenberg’s uncertainty principle shows that there is no possible way to measure the pencil’s length—or an electron’s position—and its velocity with perfect accuracy at the same time. In fact, the better you know a particle’s position, the less you know about its velocity, and vice versa. If you measure an electron’s position with zero error—you know exactly where it is at a given moment—you must have zero information about how fast it is going. And if you know a particle’s velocity with infinite precision—zero error—you have infinite error when you measure its position; you know nothing at all about where it is.* You can never know both at the same time, and if you have some information about one, you must have some uncertainty about the other. It’s another unbreakable law.
Heisenberg’s uncertainty principle applies to more than just measurements performed