Does making a quantum measurement truly obliterate information? The act of observation does not merely fix a previously undefined state; it can also annihilate information.

Consider a scientist aiming to decode the essence of reality. The approach would involve dissecting matter into minuscule, comprehensible components, designing experiments to scrutinize the attributes of these subatomic particles under various conditions, and cleverly utilizing the outcomes to decipher the governing laws of the Universe.

In theory, one might presume that with sufficient measurements or experiments, extensive knowledge about any particle or collection of particles could be attained. This was the belief held by many at the beginning of the 20th century. However, the quantum realm had different plans. Certain measurements can entirely invalidate the information previously gathered. Indeed, the act of measuring can genuinely obliterate information. Here’s how this realization unfolded.

The narrative begins with a foundational concept in mathematics: commutativity. This term implies that rearranging elements doesn't alter the outcome. For instance, addition is commutative: 2 + 3 equals 3 + 2. The same applies to multiplication: 2 × 3 equals 3 × 2. However, subtraction and division are not commutative, as altering their order impacts the result.

In physics, the principle of commutativity transcends mathematical functions and extends to physical operations or measurements. A straightforward example involves rotations. If you hold a three-dimensional object, like a cellphone, and execute two rotations—first rotating 90 degrees counterclockwise about the axis facing you and then rotating it 90 degrees clockwise about the vertical axis—the sequence of these rotations significantly influences the outcome.

This non-commutativity is evident even in classical physics, but its most prominent application is within the quantum domain, particularly illustrated by the Heisenberg uncertainty principle. In the classical realm, various properties of an object can be measured simultaneously. You can determine mass on a scale, momentum with a motion sensor, position using lasers, energy in a calorimeter, and time with a stopwatch.

In the quantum realm, while many of these measurements are valid at the precise moment of observation, they do not hold indefinitely. Certain quantities, known as conjugate variables, are inherently linked. If you measure momentum with high precision, your ability to ascertain position is constrained, irrespective of any previous precise measurements.

The concept of Heisenberg uncertainty was hard for many to accept, yet the Universe mandates it. This principle extends to various pairs of conjugate variables, including position and momentum, energy and time, electric potential and charge, as well as angular momentum and orientation.

To establish the physical necessity of these concepts, experimental validation is crucial. Simply claiming uncertainty in measurements isn't sufficient; one must demonstrate that information previously acquired has been obliterated by subsequent measurements.

In 1921, physicist Otto Stern devised a clever experiment to test this notion.

Imagine a quantum particle, such as an electron or a proton. This particle possesses inherent quantum properties, including mass and electric charge, and also exhibits a form of angular momentum known as spin, analogous to a top spinning about its axis.

With a spinning top, you can visualize two possible spinning directions: clockwise or counterclockwise. In a gravity-free environment, the top could spin in any direction across three dimensions. This setup underscores the existence of intrinsic angular momentum for quantum particles.

How could one measure the spin of such particles? Could spin be a continuous property, as classical physics suggests, or is it inherently quantized with discrete values?

Stern proposed that if a charged, spinning particle passed through a magnetic field oriented perpendicularly to its movement, the field would deflect the particle based on its magnetic moment, related to its spin. A particle without spin wouldn't deflect, while one with spin would be directed by the magnetic field.

If spin is quantized, specific locations would receive these particles, while continuous spin would yield random placements.

In 1922, physicist Walther Gerlach tested Stern's ideas, leading to the creation of the Stern-Gerlach experiment. Gerlach arranged an electromagnet around a beam of silver atoms, which could be accelerated uniformly. With the electromagnet turned off, all silver atoms landed at a single point on a detector. Upon activating the magnet, the beam split into two: half deflected in the magnetic field's direction and half in the opposite direction, corresponding to spins of +½ and -½.

This experiment validated the existence of quantized spin. However, the subsequent findings revealed quantum mechanics' ability to obliterate previously known information. When silver atoms passed through the Stern-Gerlach apparatus, they split, aligning with their respective spins.

What occurs if one of those halves is subjected to another Stern-Gerlach experiment?

Surprisingly, the outcome hinges on the orientation of the magnet. If the initial Stern-Gerlach apparatus is aligned in the x-direction, the particles will split into +x and -x orientations. If only the +x particles are retained and passed through another x-oriented magnet, they won't split; they will remain aligned in the +x direction.

However, if the second magnetic field is oriented in the y-direction, those +x particles will split again into +y and -y directions. Now, if one retains the +y particles and reintroduces them to a magnetic field aligned in the x-direction, they will once again split into +x and -x orientations.

This reveals a crucial aspect: the new measurement re-randomizes the previously determined information. In essence, the three spin directions—x, y, and z—do not commute, meaning measuring one type of variable obliterates any prior information about its conjugate variables.

The implications of the Stern-Gerlach experiment are profound. By 1927, it demonstrated that this splitting occurs even in hydrogen atoms, showing a non-zero magnetic moment. Atomic nuclei also possess quantized angular momentum and exhibit similar splitting behavior. Researchers learned to manipulate magnetic fields over time to induce state transitions, leading to innovations like magnetic resonance, which remains vital in modern MRI technology.

The notion that measurement and observation should not affect the outcome appears paradoxical. Yet, in the quantum realm, this phenomenon is not only real but was demonstrated even before its theoretical foundations were fully established. Measuring a particle's spin along one axis obliterates any previously acquired information about the other axes. The act of measurement fundamentally randomizes earlier knowledge.

For many physicists, when they recall Einstein's remark about how “God doesn’t play dice with the Universe,” the Stern-Gerlach experiment should serve as a counterexample. Regardless of one's understanding of reality or the precision of measurements, any new measurement will inherently randomize previously established information. Thus, making a measurement obliterates old knowledge, and all it takes is a magnet and some particles to illustrate this truth.

Starts With A Bang is authored by Ethan Siegel, Ph.D., who has written Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.