In the
late 1890s, German theoretical physicist Max Planck proposed a set of
units to simplify the expression of physics laws. Using just five
constants in nature (including the speed of light and the
gravitational constant), you, me and even aliens from Xylanthia could
arrive at these same Planck units.
The basic Planck units are length, mass, temperature, time and
charge.
So, let’s consider the unit of Planck length for a moment. The
proton is about 100 million trillion times larger than the Planck
length. To put this into perspective, if we scaled the proton up to
the size of the observable universe, the Planck length would be a
mere trip from Tokyo to Chicago. The 14-hour flight may seem long to
you, but to the universe, it would go completely unnoticed.
The Planck scale was invented as a set of universal units, so it
was a shock when those limits also turned out to be the limits where
the known laws of physics applied. For example, a distance smaller
than the Planck length just doesn’t make sense—the physics breaks
down.
Physicists don’t know what actually goes on at the Planck scale,
but they can speculate. Some theoretical particle physicists predict
all four fundamental forces—gravity, the weak force,
electromagnetism and the strong force—finally merge into one force
at this energy. Quantum gravity and superstrings are also possible
phenomena that might dominate at the Planck energy scale.
The Planck scale is the universal limit, beyond which the
currently known laws of physics break.
Where Material Reality Emerges
Below the Plank limit, it has been postulated that quantum foam (or spacetime foam, or spacetime bubble) is a theoretical quantum fluctuation of spacetime on very small scales due to quantum mechanics. The theory predicts that at this small scale, particles of matter and antimatter are constantly created and destroyed. These subatomic objects are called virtual particles. The idea was devised by John Wheeler in 1955.
With an incomplete theory of quantum gravity, it is impossible to be certain what spacetime looks like at small scales. However, there is no definitive reason that spacetime needs to be fundamentally smooth. It is possible that instead, in a quantum theory of gravity, spacetime would consist of many small, ever-changing regions in which space and time are not definite, but fluctuate in a foam-like manner.
John Wheeler suggested that the uncertainty principle might imply that over sufficiently small distances and sufficiently brief intervals of time, the "very geometry of spacetime fluctuates". These fluctuations could be large enough to cause significant departures from the smooth spacetime seen at macroscopic scales, giving spacetime a "foamy" character.
