The concept of the smallest entity in theoretical physics often leads us to the Planck scale, which sets fundamental limits on our ability to measure or observe quantities in the universe.

Here are some key features associated with the Planck scale:

1. Planck Length:
   • The Planck length is a fundamental length scale. At scales smaller than the Planck length, our current understanding of physics breaks down, and quantum gravitational effects become significant.
2. Planck Time:
   • The Planck time is the time it takes for light to travel one Planck length in a vacuum. Below the timescale, our current understanding of time becomes uncertain, and quantum gravity effects are expected to dominate.
3. Planck Mass:
   • The Planck mass is the mass corresponding to the Planck length and Planck time. Objects with masses on the order of the Planck mass would have gravitational self-energy comparable to their rest mass energy, leading to quantum gravitational effects.
4. Quantum Foam:
   • At the Planck scale, spacetime itself undergoes fluctuations, leading to a concept known as "quantum foam." This notion explains that at these extremely small scales, the structure of spacetime becomes inherently fluctuating and uncertain.

Moreover, there are structures and entities at scales smaller than the Planck length.