Casimir Effect - What causes this force?

Transcript

the Casimir force is widely viewed as a force that originates from what is called vacuum energy it is often invoked as decisive evidence that the Zero Point Energy of the quantum field is real but there is an alternative way of examining the force which could be considered far more fundamental in 1948 Dutch theoretical physicist Henrik Casimir was working at Philips Research Laboratories in Eindhoven he was investigating the properties of viscous materials that contain microsized particles in a liquid Matrix which are referred to as colloids their properties are determined by the van der waals forces these are long-range attractive forces that exist between neutral atoms and molecules as a side note in 1873 JD Van de vaal's introduced the idea of intermolecular forces but did not theoretically explain them then in 1930 Fritz London was the first to provide an explanation for the intermolecular forces in terms of non-relativistic quantum mechanics Dale overbake also worked alongside Casimir and realized that the theory currently used to explain Van Der Val's forces did not properly explain the experimental measurements they had obtained on colloids Casimir then worked with Dick polder and found a simple expression for the intermolecular forces with relativistic effects included their result is a generalization of the London Thunder valve's Force and includes retardation due to the finite speed of light intrigued by the Simplicity of the results Casimir search for a simpler explanation after a conversation with Niels Bohr who suggested that it might somehow be related to the vacuum energy Casimir found that calculations based on vacuum energy further simplified when molecules are replaced by perfectly conducting plates this is the calculation you will most often find in textbooks the idea is that when two uncharged conductive plates are placed in a vacuum a few nanometers apart an attractive Force arises in a simplified classical sense there is no external field present except for Gravity which should be extremely small in this case so there should be no force measured in a Quantum vacuum electromagnetic fluctuations appear and disappear as intermittent electromagnetic modes that span an infinite range of wavelength in free space in between the two plates the larger wavelength do not fit the difference between the waves that can exist on the outside compared to those on the inside creates an effective net force that pushes them inward the optical cavity formed by the gap between the two plates restricts the number of modes that can exist inside the cavity to those with wavelengths that are half integral divisors of the separation due to the fact that this Force falls off rapidly with distance it can only be measurable when the distance between the objects is extremely small on a submicron scale this Force becomes so strong that it becomes the dominant force between uncharged conductors when the separation is about a hundred times the typical size of an atom about 10 nanometers the Casimir effect produces the equivalent of about one atmosphere of pressure Casimir was able to calculate the force by summing over all the cavity modes the problem is that this quantity diverges but it is possible to obtain a finite result by taking the differences in energy between the plates at different separations although this was kasimi's approach it is often expressed in terms of Zero Point Energy of a quantized field in the intervening space between the objects treatment of the boundary condition in these calculations has led to some controversy casimir's original goal was to compute the van der waals force between polarizable molecules it is possible to calculate the force without any reference to the vacuum energy of quantum fields in 1956 yevgeny Lipschitz found a general theory for computing Van Der Val's forces between plates which are non-perfect conductors and was able to show that the Casimir Force emerges as a special case in 1975 Julian schwinger found yet another way of computing the Casimir Force without referring to vacuum energy then in 1997 Steve lamoreaux quantitatively measured the force to within five percent of the value predicted by the theory it is often quoted as the most famous mechanical fact of vacuum fluctuations generally high energy physicists view the Casimir Force as a force that originates from the vacuum energy but amongst the condensed matter Community the more popular view is that it has the same physical origin as the van der valve's Force which does not depend on the energy of the vacuum the vacuum energy approach focuses on the macroscopic origin whereas the van der valve's approach focuses on the microscopic origin in specialized literature the two approaches are usually considered as two complementary methods but which of the two approaches is more fundamental after all the conceptual picture of the world in which the vacuum energy has a direct physical role is very different from the picture in which it does not more recently Robert Jaffe argued that the physically correct approach is one based on the van der Val's Force while the approach based on vacuum energy is merely a heuristic shortcut valid only as an approximation in the limit of the infinite fine structure constant building on this if you order a nikolich presented a simple General proof that the Casimir Force cannot originate from the vacuum energy of the electromagnetic field in his paper he starts by examining the quantum vacuum approach and points out that one of the main advantages is that it is relatively simple to calculate but he also points out that the electromagnetic forces are forces between charges but where are those charges the force originates from the boundary conditions but the microscopic origin of the boundary condition is not taken into account the vacuum energy explanation is not a full microscopic explanation he then outlines how the van der valve's explanation works the Casimir force is explained by the polarization of the medium the polarization can be traced down to the microscopic polarizability of atoms classically there is no spontaneous polarization as the two molecules cannot simply decide which of the two types of polarization to adopt from a quantum mechanical perspective we could consider the two polarizations as a superposition Van Der valve's Force could be considered a Quantum effect the vacuum energy explanation originates from boundary conditions these originate from the fact that there is no electric field inside a perfect conductor due to a rearrangement of charges rearrangement of charges is polarization the interaction energy originates from the correlation between the polarization and the electric field this is Van Der valve's energy which is fundamental because it does not depend on the phenomological macroscopic parameter the dielectric constant and a macroscopic dielectric constant dependent level it can be interpreted as either the energy of polarization fluctuations or the energy of electric field fluctuations so although the vacuum energy approach is an effective macroscopic description the van der valve's approach gives a fundamental microscopic description as always be brave be curious the truth is waiting for us until next time [Music]