**“Q**uantum mechanics is very impressive,” Albert Einstein wrote in 1926. “But an inner voice tells me that it is not yet the real thing.” As quantum theory matured over the years, that voice has gotten quieter—but it has not been silenced. There is a relentless murmur of confusion underneath the chorus of praise for quantum theory.

Quantum theory was born at the very end of the 19th century and soon became one of the pillars of modern physics. It describes, with incredible precision, the bizarre and counterintuitive behavior of the very small: atoms and electrons and other wee beasties of the submicroscopic world. But that success came with the price of discomfort. The equations of quantum mechanics work very well; they just don't seem to make sense.

No matter how you look at the equations of quantum theory, they allow a tiny object to behave in ways that defy intuition. For example, such an object can be in “superposition”: It can have two mutually exclusive properties at the same time. The mathematics of quantum theory says that an atom, for example, can be on the left side of a box and the right side of the box at the very same instant, as long as the atom is undisturbed and unobserved. But as soon as an observer opens the box and tries to spot where the atom is, the superposition collapses and the atom instantly “chooses” whether to be on the right or the left.

This idea is almost as unsettling today as it was 80 years ago, when Erwin Schrödinger ridiculed superposition by describing a half living, half-dead cat. That is because quantum theory changes what the meaning of “is” is. In the classical world, an object has a solid reality: Even a cloud of gas is well described by hard little billiard ball-like pieces, each of which has a well-defined position and velocity. Quantum theory seems to undermine that solid reality. Indeed, the famous Uncertainty Principle, which arises directly from the mathematics of quantum theory, says that objects' positions and moment a are smeary and ill defined, and gaining knowledge about one implies losing knowledge about the other.

The early quantum physicists dealt with this unreality by saying that the “is”—the fundamental objects handled by the equations of quantum theory—were not actually particles that had an extrinsic reality but “probability waves” that merely had the capability of becoming “real” when an observer makes a measurement. This so-called Copenhagen Interpretation makes sense, if you're willing to accept that reality is probability waves and not solid objects. Even so, it still doesn't sufficiently explain another weirdness of quantum theory: nonlocality.

In 1935, Einstein came up with a scenario that still defies common sense. In his thought experiment, two particles fly away from each other and wind up at opposite ends of the galaxy. But the two particles happen to be “entangled”—linked in a quantum-mechanical sense—so that one particle instantly “feels” what happens to its twin. Measure one, and the other is instantly transformed by that measurement as well; it's as if the twins mystically communicate, instantly, over vast regions of space. This “nonlocality” is a mathematical consequence of quantum theory and has been measured in the lab. The spooky action apparently ignores distance and the flow of time; in theory, particles can be entangled *after* their entanglement has already been measured.

On one level, the weirdness of quantum theory isn't a problem at all. The mathematical framework is sound and describes all these bizarre phenomena well. If we humans can't imagine a physical reality that corresponds to our equations, so what? That attitude has been called the “shut up and calculate” interpretation of quantum mechanics. But to others, our difficulties in wrapping our heads around quantum theory hint at greater truths yet to be understood.

Some physicists in the second group are busy trying to design experiments that can get to the heart of the weirdness of quantum theory. They are slowly testing what causes quantum superpositions to “collapse”—research that may gain insight into the role of measurement in quantum theory as well as into why big objects behave so differently from small ones. Others are looking for ways to test various explanations for the weirdnesses of quantum theory, such as the “many worlds” interpretation, which explains superposition, entanglement, and other quantum phenomena by positing the existence of parallel universes. Through such efforts, scientists might hope to get beyond the discomfort that led Einstein to declare that “[God] does not play dice.”