The idea that something in quantum physics exists only when observed is a commonly misunderstood concept related to the phenomenon known as the "observer effect" or "quantum collapse." However, it does not imply that an object only exists when observed in the way you might think.
In quantum physics, particles can exist in what is called a superposition, which means they can be in multiple states simultaneously. It's only when an observation or measurement is made that the superposition collapses into a definite state. This collapse is often described as the particle "choosing" one state out of the many possibilities.
The observer effect refers to the influence of the act of measurement or observation on the system being observed. When a measurement is made, the system interacts with the measurement apparatus, causing the wavefunction (a mathematical representation of the quantum state) to collapse into one of the possible outcomes. The observer effect does not imply that the object doesn't exist unless observed; rather, it means that the act of observation or measurement affects the state of the system being observed.
In the case of the chocolate cake, you observing or not observing it does not make it appear or disappear out of thin air. The cake would exist regardless of whether or not you observe it. The observer effect is typically observed at the quantum level, where the behavior of particles is governed by probabilistic laws. On macroscopic scales, objects like cakes follow classical physics and are not subject to the same quantum phenomena.
In summary, the observer effect in quantum physics is about the influence of observation or measurement on the state of a system, not about the existence of objects themselves. The concept can be counterintuitive and is often misinterpreted, leading to misconceptions about the nature of reality at the quantum level.