Basic Idea and the Question
From the Quantum Field Theory, we are aware that empty space is actually not empty. A Quantum Field permeates all of space. This Quantum Field randomly fluctuates in energy at many points. We know that mass is the same as energy from Einstein’s energy-mass equivalence principle. Therefore, these energy fluctuations give rise to two particles, a pair. One is a matter particle and the second is an antimatter particle. These are known as virtual particles. These particles destroy each other as fast as they appear and dissipate energy back into the quantum field. Energy is conserved in this way. But, how do we know these particles actually exist?
The Casimir Effect
Hendrick Casimir proposed an experiment in 1948. In the experiment, he told to put two electrically neutral and conducting plates parallel to each other separated by a distance of 100 nanometers (1nm = 10-9m). It was known at the time that matter had a dual property– it is both a wave and a particle. Therefore, all virtual particles are also waves. When the two plates are put at 100nm apart, the types of waves that can be present between them are restricted. Only natural number wavelengths can fit in between the plates. For example a simple way to understand this is to imagine a wave whose number of crests and wave add up to 4 can fit, but, a wave whose crests and troughs add up to 4.5 or 3.8 can’t fit.
With the property of waves mentioned above, the number of virtual particles in between the plates is very less. Outside the plates, virtual particles of any wavelength can exist. Due to the energy difference outside the plates and in between the plates, the plates are pushed towards each other. This is exactly what was observed and is by far one of the strongest proofs for the existence of virtual particles.