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The Casimir effect is caused by the fact that space is filled with vacuum fluctuations, virtual particle-antiparticle pairs that continually form out of nothing and then vanish back into nothing an instant later. The gap between the two plates restricts the range of wavelengths possible for these virtual particles, and so fewer of them are present within this space. As a result, there is a lower energy density between the two plates than in open space; in essence, there is less "nothing" between the two plates, creating negative energy and pressure, which pulls the plates together. The narrower the gap, the more restricted the wavelength of the virtual particles, the more negative the energy and pressure between the plates, the more restricted the vacuum modes and the smaller the vacuum energy density, and thus the stronger the attractive force. Since the Casimir effect is small and falls off as the fourth power of distance, its effect is greatest on small objects that are close together. It can become an important consideration in studies of the interaction of molecules, together with other small scale effects, such as the fluctuations in the electronic structure of molecules causing transient dipoles which lead to Van der Waals forces.