James Clerk Maxwell

The predictions of Newtonian theory matches the view of reality that we experience.

But individual atoms and molecules operate in a manner profoundly different from that of our everyday experience. Quantum physics is a new model of reality that gives us a picture of the universe. It is a picture in which many concepts fundamental to our intuitive understanding of reality no longer have meaning.

The double-slit experiment was first carried out in 1927 by Clinton Davisson and Lester Germer. They were experimental physicists at Bell Labs who were studying how a beam of electrons interacts with a crystal made of nickel.

Matter particles such as electrons behave like water waves was the type of startling experiment that inspired quantum physics.

Since this behavior is not observed on a macroscopic scale, scientists have long wondered just how large and complex something could be and still exhibit such wavelike properties.

It would cause quite a stir if the effect could be demonstrated using people or a hippopotamus, but as we’ve said, in general, the larger the object the less apparent and robust are the quantum effects. So it is unlikely that any zoo animals will be passing wavelike through the bars of their cages.

Still, experimental physicists have observed the wave phenomenon with particles of ever-increasing size. Scientists hope to replicate the buckyball experiment someday using a virus, which is not only far bigger but also considered by some to be a living thing. There are only a few aspects of quantum physics needed to understand the arguments we will make in later chapters.

One of the key features is wave/particle duality. That matter particles behave like a wave surprised everyone. That light behaves like a wave no longer surprises anyone.

The wavelike behavior of light seems natural to us and has been considered an accepted fact for almost two centuries. If you shine a beam of light on the two slits in the above experiment, two waves will emerge and meet on the screen. At some points their crests or troughs will coincide and form a bright spot; at others the crests of one beam will meet the troughs of the other, canceling them, and leaving a dark area.

The English physicist Thomas Young performed this experiment in the early nineteenth century, convincing people that light was a wave and not, as Newton had believed, composed of particles.

Though one might conclude that Newton was wrong to say that light was not a wave, he was right when he said that light can act as if it is composed of particles. Today we call them photons. Just as we are composed of a large number of atoms, the light we see in everyday life is composite in the sense that it is made of a great many photons—even a 1-watt night-light emits a billion billion each second.

Single photons are not usually evident, but in the laboratory we can produce a beam of light so faint that it consists of a stream of single photons, which we can detect as individuals just as we can detect individual electrons or buckyballs. And we can repeat Young’s experiment employing a beam sufficiently sparse that the photons reach the barrier one at a time, with a few seconds between each arrival.

If we do that, and then add up all the individual impacts recorded by the screen on the far side of the barrier, we find that together they build up the same interference pattern that would be built up if we performed the Davisson-Germer experiment but fired the electrons (or buckyballs) at the screen one at a time.

To physicists, that was a startling revelation:

If individual particles interfere with themselves, then the wave nature of light is the property not just of a beam or of a large collection of photons but of the individual particles.

Another of the main tenets of quantum physics is the uncertainty principle, formulated by Werner Heisenberg in 1926. The uncertainty principle tells us that there are limits to our ability to simultaneously measure certain data, such as the position and velocity of a particle.

According to the uncertainty principle, for example, if you multiply the uncertainty in the position of a particle by the uncertainty in its momentum (its mass times its velocity) the result can never be smaller than a certain fixed quantity, called Planck’s constant. That’s a tongue-twister, but its gist can be stated simply:

The more precisely you measure speed, the less precisely you can measure position, and vice versa. For instance, if you halve the uncertainty in position, you have to double the uncertainty in velocity.

It is also important to note that, compared with everyday units of measurement such as meters, kilograms, and seconds, Planck’s constant is very small. In fact, if reported in those units, it has the value of about 6/10,000,000,000,000,000,000,000,000,000,000,000.

As a result, if you pinpoint a macroscopic object such as a soccer ball, with a mass of 1/3 kilogram, to within 1 millimeter in any direction, we can still measure its velocity with a precision far greater than even a billionth of a billionth of a billionth of a kilometer per hour.

That’s because, measured in these units, the soccer ball has a mass of 1/3, and the uncertainty in position is 1/1,000. Neither is enough to account for all those zeroes in Planck’s constant, and so that role falls to the uncertainty in velocity. But in the same units an electron has a mass of .000000000000000000000000000001, so for electrons the situation is quite different. If we measure the position of an electron to a precision corresponding to roughly the size of an atom, the uncertainty principle dictates that we cannot know the electron’s speed more precisely than about plus or minus 1,000 kilometers per second, which is not very precise at all.