A Fresh Kilogram Emerges, Rooted in the Principles of Quantum Physics

Physicists have recently made a significant change regarding the kilogram. Instead of being represented by a physical object, it is now defined as an abstract concept related to light and energy. This shift is a remarkable accomplishment as it ensures that the kilogram remains constant and unaffected by any changes over time. Consequently, this new definition allows for the universal communication of mass, even with extraterrestrial beings, without the need for a physical cylinder. However, in practical terms, the weight of the new kilogram remains virtually unchanged from the previous definition. Therefore, for most everyday purposes, the alteration in definition will have minimal impact, with only specific scientific applications being affected.

The intriguing aspect here is not just the practical change in how we use the kilogram, but rather the astonishing difficulty in precisely defining a unit of mass. Unlike other fundamental forces that have been comprehended in terms of fundamental reality, mass remains elusive. For instance, a second of time was once defined by the swings of a pendulum clock, but now it is understood as the duration taken by an atom of cesium 133 to undergo 9,192,631,770 cycles of emitting microwave radiation. Similarly, a meter is now defined as the distance traveled by light in 1/299,792,458th of a second.

However, mass does not conform to such straightforward definitions. Typically, we measure kilograms based on weight – the force exerted by an object on a scale. Yet, this measurement varies depending on the location of the weighing. If the same cylinder were taken to the moon, it would weigh significantly less, and even slightly more or less in different parts of the Earth.

According to the National Institute of Standards and Technology (NIST), the new kilogram is founded on the fundamental relationship between mass and energy, as elucidated in Einstein’s famous equation E=mc^2. This equation signifies that energy is equivalent to mass multiplied by the speed of light squared. Mass can be converted into energy and vice versa. In comparison to mass, energy is more easily measurable and definable in discrete terms.

That is made possible by another equation, which is even older than E=mc^2. In 1900, physicist Max Planck demonstrated that E=hv, as stated by NIST. He revealed that on a sufficiently small scale, energy can only change in discrete steps and can either increase or decrease. The equation E=hv signifies that energy is equal to the product of “v” (the frequency of a particle, such as a photon) and “h” (Planck’s constant, which is 6.62607015×10^minus34).

In E=hv, “v” must always be an integer, such as 1, 2, 3, or 6,492. No fractions or decimals are allowed. Therefore, energy is inherently discrete and changes in increments of “h” (6.62607015×10^minus34).

The new kilogram unifies E=mc^2 and E=hv. This allows scientists to define mass in terms of Planck’s constant, which is a constant value in the universe. A collaboration of international scientific laboratories joined forces to make highly precise measurements of Planck’s constant, ensuring accuracy within several parts per billion. The mass of the new kilogram corresponds to the energy of 1.4755214 times 10^40 photons oscillating at the same frequencies as cesium 133 atoms used in atomic clocks.

Although it may not be the most convenient item to place on a scale, as a concept, it is much more portable than a cylinder made of platinum-iridium alloy.

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