How can radioactive decay just happen with nothing triggering it?

Category: Physics
Published: August 11, 2022

Although a radioactive decay event seems spontaneous and is unpredictable, it is indeed triggered by a physical agent. That physical agent is a vacuum fluctuation. Due to the quantum nature of the universe, a vacuum always contains vacuum fluctuations. Vacuum fluctuations are also called vacuum energy and zero-point energy. You can think of vacuum fluctuations as a sea of particles and antiparticles briefly popping into and out of existence. These particles originate from the vacuum itself due to intrinsic quantum uncertainty. Vacuum fluctuations are very short-lived (short-lived enough that they do not violate any conservation laws, within the level of quantum uncertainty). However, vacuum fluctuations are physically real and cause real effects. For instance, vacuum fluctuations tend to weaken, or screen, electromagnetic fields. Vacuum fluctuations also give rise to the Casimir effect as well as the Lamb shift in hydrogen energy levels.

Public Domain Image, source: Christopher S. Baird.

Every spontaneous quantum transition is actually triggered by a vacuum fluctuation. Lasers crucially depend on vacuum fluctuations. The light emitted from a laser is generated by a chain reaction of coherent photon emissions. This chain reaction is triggered at the beginning by a vacuum fluctuation. When an electron is put in an excited atomic state and left alone, it will eventually, naturally transition back down to its original state. When exactly this happens seems spontaneous, but it is actually triggered by a vacuum fluctuation. Similarly, vacuum fluctuations are what trigger a radioactive decay event. As part of the background quantum fluctuations that are intrinsic to the vacuum, a particle pops into existence just long enough to collide with the nucleus and trigger radioactive decay. The exact moment that radioactive decay happens is random and unpredictable because vacuum fluctuations are random and unpredictable.

All of this just leads to the question, what triggers vacuum fluctuations? The answer is that nothing triggers vacuum fluctuations. They are constantly happening due to the quantum nature of the universe. Vacuum fluctuations are a well-established principle of mainstream physics. Note that there are a lot of incorrect, unscientific notions online about vacuum energy. Even though vacuum energy is real, it cannot be harnessed as a free source of energy. Quantum physics is strange, but it still must obey the physical conservation laws of the universe, including the law of conservation of energy. A vacuum fluctuation cannot permanently give its energy to another object. That would violate the law of conservation of energy. In fact, conservation of energy is what prevents a vacuum fluctuation from continuing to exist beyond its short life. However, if conditions are right, a vacuum fluctuation can be given enough energy from another object for it to be promoted to a stable particle that continues to exist. For instance, in an effect called spontaneous emission, an electron transitions spontaneously from an excited quantum state to a lower quantum state and emits a bit of light called a photon in the process. It looks like the photon is just created out of nowhere. However, the more accurate description is that a photon vacuum fluctuation collides with the electron, triggering the electron to transition. In the process, the electron gives some of its energy to the vacuum fluctuation, thereby promoting it to a regular photon that can continue to exist forever.

Christopher S. Baird is a physics professor at West Texas A&M University and author of the book, The Top 50 Science Questions with Surprising Answers.

Topics: atom, electron, radioactive, radioactive decay, radioactivity, vacuum fluctuations