Abstract:
A method and apparatus for accelerating and a decelerating particles based on particle surface interactions. In particular, in some embodiments, particles traveling away from a surface are made to travel at either a higher or lower speed than that with which they approached the surface. This change in velocity is effected by prompting the particles to undergo atomic transitions during their interaction with the surface.

Description:
STATEMENT OF RELATED APPLICATIONS 
     This application claims priority of Provisional Application No. 60/135,868 filed May 25, 1999. Moreover, this application is related to applicant&#39;s co-pending patent application entitled “Method and Apparatus for Energy Extraction,” filed on May 25, 2000 as Ser. No. 09/578,638, now allowed, and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to accelerating or decelerating particles, such as, for example, atoms. 
     BACKGROUND OF THE INVENTION 
     It is a well-known result of non-relativistic quantum mechanics that two neutral atoms or molecules will attract each other because of their uncorrelated dipole-dipole interactions. In a related manner, a neutral atom will be attracted to a nearby perfectly conducting surface because of the interaction with its own image. The force between a neutral atom and a perfectly conducting parallel plane was first explained in the case of isomeric atoms by Lennard-Jones in non-relativistic perturbation theory and it is typically referred to as the van der Waals force. 
     When the distance between the atom and the surface becomes much larger than the typical wavelength of atomic transitions for that atom, Lennard-Jones&#39; non-relativistic treatment of the force fails (as a consequence of the fact that the speed of light is not infinite). In fact, relativistic quantum mechanics is required for analysis of such a case. The relativistic analysis was first preformed by Casimir and Polder. In this regime, which is referred to as “retarded,” the intensity of the force of attraction decreases more rapidly than in the van der Waals, or so-called “un-retarded” case. 
     Since all forces of this type depend on the optical (dielectric) properties of the materials involved, they are also referred to as “dispersion” forces. There is a connection between atom-surface forces and surface-surface forces. The former can be viewed as the limit of the latter when the second surface is an extremely rarified layer of neutral atoms. Consequently, surface-surface forces can be explained as simply a large-scale manifestation of the atom-surface force. 
     This understanding, which comprises the “source theory” of dispersion forces, is of relatively recent vintage. The first complete theory of surface-surface forces was given by Casimir in 1948 with the introduction of “zero point energy,” with no mention of the Lennard-Jones potential. 
     Zero point energy or “vacuum energy” or “ZPE” is energy that is associated with a non-thermal radiation that is believed to be present everywhere in the universe—even in regions that are otherwise devoid of matter and thermal radiation. This non-thermal radiation is believed to result from random fluctuations occurring at the quantum level that result in a continual creation and destruction of virtual particles. This radiation is often referred to as a “zero point field,” or by the acronym “ZPF,” and the energy that is associated with the field is the aforementioned zero point energy. It is now understood that all dispersion forces, whether they be Casimir, Casimir-Polder, van der Waals, etc., are closely related to one another and can be explained by means of mutually exclusive, although equally acceptable, theories. 
     Until recently, there has been relatively little experimentation in the area of dispersion forces. In the case of surface-surface forces, this is due to the fact that the interacting boundaries must be highly polished and must be closer than about 1 micron for the forces to even approach measurability. The technology required for such experimentation was not available until recently. 
     Experimentation as to atom-surface forces, however, has not been quite as problematic. In particular, to measure atom-surface interactions, an atomic beam is directed near a conducting cylindrical surface and the deflection of the atoms from the surface due to van der Waals forces is measured. This “atomic deflection” approach, which is referred to as the Raskin-Kusch experiment, has been practiced for the last forty years to investigate van der Waals forces. 
     Investigators continue to use the techniques of the Raskin-Kusch experiment to probe van der Waals and other forces. Of late, experimentation is being performed with slow moving atoms that are obtainable using the relatively recent techniques of atomic trapping and cooling. Slower moving atoms are much more readily detected thereby facilitating more sensitive measurements. 
     SUMMARY OF THE INVENTION 
     Illustrative embodiments of the present invention provide methods and apparat uses by which a particle is accelerated or decelerated based on particle-surface interactions. In particular, in some embodiments, particles traveling away from a surface are made to travel at either a higher or lower speed than that with which they approached the surface. This change in velocity is effected by prompting the particles to undergo atomic transitions during their interaction with the surface. 
     In accordance with some embodiments of the present teachings, to accelerate a particle, such as an atom that is in its ground state, the particle is “excited” (i.e., transitions to a higher energy level) on its way to a close approach with a surface. In one embodiment, this excitation is accomplished using a laser. When the particle is at or near its closest approach to the surface, it is then advantageously prompted to return to its ground state. 
     During the approach to the surface in the excited state, the van der Waals force between the particle and the surface is quite high. After returning to the ground state, the van der Waals force between the particle and the surface is substantially reduced. This reduced attraction results in a significant increase in particle velocity. 
     To decelerate a particle, such as an atom in its ground state, the particle is excited, but that excitation occurs just after closest approach to the surface. Thus, the outbound particle, now in an excited state, experiences a much higher van der Waals force with the surface than when it was inbound toward the surface at the round state. This increased attraction significantly decreases particle velocity. 
     In some embodiments, an apparatus for carrying out the present methods comprises a particle source, a particle collimator, a particle exciter, a surface, and a particle de-exciter. 
     Underlying the present invention is the discovery that by prompting a particle to undergo an atomic transition during its interaction with the surface, its outgoing speed (i.e., speed as it moves away from a surface) can be changed relative to its incoming speed. The method is consistent with the conservation of energy principle. 
     The methods and apparatuses described herein are an improvement on the Raskin-Kusch experiment. The apparatus used in Raskin-Kusch experiments is similar to the present apparatus, but it does not include a particle exciter (or de-exciter). As a consequence, particles, such as atoms, are not prompted to undergo an atomic transition during their interaction with the surface. 
     Since the particles in Raskin-Kusch do not undergo an atomic transition during their interaction with the surface, they emerge from the interaction with the surface with a changed direction but with no change in speed. The constant speed is due to the fact th at the van der Waals force behave s as a conservative force. As such, when a particular atom is at a large distance from the surface, its speed is the same as it was on the incoming leg of the trajectory. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an illustrative embodiment of a particle accelerator in accordance with the present teachings. 
     FIG. 2 depicts an illustrative embodiment of a particle decelerator in accordance with the present teachings. 
     FIG. 3 depicts a first illustrative method for accelerating/decelerating particles in accordance with the present teachings. 
     FIG. 4 depicts a second illustrative method for accelerating/decelerating particles in accordance with the present teachings. 
     FIG. 5 depicts a third illustrative method for accelerating/decelerating particles in accordance with the present teachings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Concepts such as the van der Waals force, the Casimir-Polder force, the Casimir force, the Bohr radius, the quantum number and the like will used herein without further definition or description due to their well-defined meanings and the familiarity of those skilled in the art with such concepts. 
     Illustrative embodiments of the present invention provide methods and apparatuses by which a particle is accelerated or decelerated. The illustrative methods and apparatuses utilize particle-surface interactions, but the invention is not limited thereto. 
     In most embodiments of the present invention, the particle that is accelerated will be quite small. Examples of small particles used in various embodiments, include, without limitation, neutral atoms, neutral molecules, and neutral elementary particles. But the present methods apply irrespective of size. For example, in some embodiments, micro-scale neutral specks of dust are accelerated/decelerated. And, on a much more macroscopic scale, a neutral ball one-foot in diameter is accelerated/decelerated in accordance with the present teachings. It will be understood, however, that the amount of acceleration/deceleration imparted to such macro-scale objects will be quite small compared to that imparted to atomic-sized particles. In general, the term “particle” includes all such sub-atomic-, atomic-, micro- and macro-scale objects. 
     The van der Waals and Casimir-Polder forces depend on the quantum state of the particle, for example, an atom, next to the surface of interest. In particular, van der Waals and Casimir-Polder forces are proportional to the second power of the Bohr radius of the atom. And, the Bohr radius is itself proportional to the second power of the principal quantum number, n, which describes in part the state of the atom. Therefore, van der Waals and Casimir-Polder forces are proportional to the fourth power of the principal quantum number: 
     
       
         force∝ n   4   [1] 
       
     
     This dependence on the fourth power of n is an important ramification in the context of the present invention. Consider, for example, a “cold” particle, such as a cold atom. When cold, the particle is usually in its ground state, which, for an atom, corresponds to a principal quantum number n=1. Consider that atom in an excited state having, for example, a principal quantum number n=10. In such a case, the van der Waals force attracting the excited particle to the surface is, in accordance with expression [1], 10 4  or 10,000 times greater than when the particle is in its ground state. 
     As used herein, the term “excite” or “excited” refers to a particle that, due to the introduction of energy, is at a higher energy level than its ground state. In the particular case of an atom, an excited atom will have a principal quantum number that is greater than 1. 
     FIG. 1 depicts an embodiment of a particle accelerator  100  in accordance with the illustrative embodiment of the present invention. Particle accelerator  100  comprises particle source  102 , collimator  106 , particle exciter  114 , surface  116 , and optional particle de-exciter  120 , interrelated as shown. 
     Particle source  102  contains the particles that will be accelerated by accelerator  100 . Particle source  102  can be embodied as any of a variety of well-known devices/systems, such as, without limitation, an oven containing a hot gas or an apparatus for atom trapping and cooling. In the illustrative embodiment, particle source  102  includes orifice  104  that allows at least some of the particles in particle source  102  to escape. In some embodiments, orifice  104  is a hole that is about 1 millimeter in diameter. 
     Escaped particles  101  encounter collimator  106 . Collimator  106  provides a highly directional beam of particles  103 , the direction of which is toward surface  116 . Collimator  106  is realized, in the illustrated embodiment, as two plates  108  and  110  that are spaced from one another such that slit  112  is defined therebetween. Other arrangements that provide a slit or other orifice operable to collimate the escaped particles  101  may suitably be used in place of plates  108  and  110 . 
     In the illustrative embodiment, at least some of collimated particles  103  are excited by particle exciter  114 . Upon leaving particle source  102  and passing collimator  106 , particles  101 / 103  are at their ground state, |A&gt;. Particle exciter  114  excites at least some of particles  103  to a higher energy state |B&gt;. In some embodiments, particle exciter  114  is a source of electromagnetic radiation. And, in one specific embodiment, particle exciter  114  is a tunable laser that is advantageously tuned to the transition energy of the particles  103 . 
     Excited particles  105  (and unexcited particles) approach surface  116 . The surface is advantageously a material capable of being polished to a high degree of smoothness, such as, for example, gold. In various embodiments, surface  116  comprises a dielectric, a semiconductor and a conductor. 
     In the illustrative embodiments, surface  116  has a cylindrical shape, but in other embodiments, surfaces having arbitrary shapes, including irregular shapes, are suitable used. Regardless of its specific geometry, surface  116  advantageously allows a particle to make a “minimum distance” or “closest” approach (to the surface) and then allows the particle to escape without contact. For example, a spherical surface or cylindrical surface facilitates such a minimum distance approach or closest approach since there will be one location on the surface that protrudes further than others from the perspective of the particle&#39;s trajectory. That point is the location of the minimum distance or closest approach. Since a flat surface (e.g., a plate) does not readily allow for a minimum distance approach and escape, it should not be used. 
     Recalling expression [1], excited particles  105  will experience a substantially larger van der Waals attraction to surface  116  than unexcited particles. At or near closest approach  118 , excited particles  105  decay back to their ground state with the emission of photon γ. Decay to the ground state is caused by spontaneous decay, or via stimulated decay using optional particle de-exciter  120 . The time for spontaneous decay can be predicted by those skilled in the art, so that spontaneous decay can be relied on to cause the required transition at or near closest approach. 
     In some embodiments, particle de-exciter stimulates decay by injecting additional energy into excited particles  105 . In some embodiments, particle de-exciter  120  is a tunable laser. 
     After decay to the ground state, the van der Waals attraction between outbound decayed particles  107  and surface  116  is much less the van der Waals attraction between inbound excited particles  105  and surface  116 . As a consequence, particles  107  will be traveling much faster after the interaction with surface  116  than they were when they first left particle source  102 . The trajectory of particles  107  can be calculated using classical mechanics (i.e., scattering theory). 
     In the context of the present invention, the decay must occur “near closest approach” to the surface since van der Waals forces fall off rapidly with distance: 
     
       
         force=− K/R   4   [2] 
       
     
     where 
     K is a constant; and 
     R is the distance between the particle and the surface 
     As used in this Specification, the term “near closest approach” or “near minimum distance,” “near minimum approach” or “during interaction,” when used in conjunction with a transition to or from the ground state, means that the particle is within a few micrometers (e.g., less than 5 micrometers), and advantageously within a few hundred angstroms of closest approach. 
     Note that the efficiency of the apparatus  100  is at its maximum when the transition occurs at closest approach. To the extent the transition occurs “near” closest approach, some amount of efficiency is lost. This is true whether the transition occurs near but before closest approach or near but after closest approach. 
     This result—this increase in speed—is not in conflict with the principle of conservation of energy. In particular, the energy of the electromagnetic radiation emitted by particle  105  as it decays is actually less than it originally absorbed. And the difference between the emitted energy and the absorbed energy is exactly equal to the increase in kinetic energy of the particle after its closest approach to surface  116 . 
     With minor modifications that are described below in conjunction with FIG. 2, the apparatus and method described above is used to decelerate a particle. FIG. 2 depicts a particle decelerator in accordance with the present teachings. 
     Like particle accelerator  100 , particle decelerator  200  comprises particle source  102 , collimator  106 , particle exciter  114  and surface  116 , interrelated as shown. In decelerator  200 , however, particle exciter  114  excites particles following but near closest approach  118 , rather than before closest approach as in particle accelerator  100 . 
     As in particle accelerator  100 , ground state particles  101  exit particle source  102  through orifice  104  and are collimated by collimator  106 . Collimated ground state particles  103  approach surface  116 . After but near closest approach  118  to surface  116 , at least some of particles  103  are excited by particle exciter  114 . In some embodiments, particle exciter  114  is a source of electromagnetic radiation, such as, without limitation, a tunable laser that is advantageously tuned to the transition energy of the particles  103 . 
     The van der Waals force between excited particles  105  and surface  116  is greater than the van der Waals force between inbound ground state particles  103  and surface  116 . As a consequence, outbound excited particles  105  are moving substantially more slowly than inbound particles  103 . 
     Excited particles  105  decay, at some point, back to their ground state with the emission of photon γ. Since the decay occurs far from closest approach, such decay has a de minimis effect on particle speed. 
     FIG. 3 depicts an illustrative method  300  for accelerating particles, either positively (i.e., increasing speed) or negatively (i.e., decreasing speed), in accordance with the present teachings. In accordance with operation  302 , particles are directed toward a surface, such as surface  116  (see FIGS.  1  and  2 ). In operation  304 , at least some of the particles are excited to a higher energy level. 
     To function as a particle accelerator, operation  304  (i.e., exciting particles) is performed before closest approach with the surface. To cause deceleration, operation  304  is performed after but near closest approach with the surface. 
     In some embodiments, method  300  includes operation  306  wherein excited particles are de-excited, such as by injecting more energy into the particles. 
     FIG. 4 depicts an alternate embodiment of a method  400  for accelerating particles, either positively (i.e., increasing speed) or negatively (i.e., decreasing speed), in accordance with the present invention. In accordance with operation  402 , particles are directed toward a surface, such as surface  116  (see FIGS.  1  and  2 ). In operation  404 , at least some of the particles are prompted into atomic transitions during their interaction (i.e., near closest approach) with the surface. 
     When accelerating particles, operation  404  comprises, in some embodiments, exciting articles and then de-exciting particles, such as by the addition of energy. The step of de-exciting occurs near closest approach. When decelerating particles, operation  404  comprises exciting the articles after but near closest approach. 
     Further embodiments of a method  500  for particle acceleration are described below in conjunction with FIG.  5 . 
     In accordance with operation  502  of method  500 , particles are directed to closest approach with a surface. In operation  504 , when the particle is near closest approach to the surface, a physical factor of surface that affects a Casimir force between the particle and the surface is altered. Physical factors affecting the Casimir force include, without limitation, the concentration of free charge carrier in the surface and the dielectric properties of the material comprising the surface. See applicant&#39;s co-pending application Ser. No. 09/578,638 entitled “Method and Apparatus for Energy Extraction.” 
     The material(s) comprising the surface are suitably selected as a function of the altered physical factor. For example, in an embodiment wherein the altered physical factor is the concentration of free charge carriers, the material is advantageously a semiconductor or compound semiconductor. Illustrative semiconductors (for embodiments wherein the altered physical factor is the concentration of free charge carriers) include, without limitation, silicon (Si), germanium (Ge), and compound semiconductors such as, without limitation, gallium arsenide (GaAs), indium gallium arsenide (InGaAs) and indium antimonide (InSb). 
     Moreover, in some embodiments, the surface comprises doped semiconductors and doped compound semiconductors, including, without limitation, phosphorus-doped silicon and indium antimonide that includes naturally occurring impurities. In some embodiments, dopants are selected based on their relative ease of ionization. 
     In embodiments in which the altered physical factor is the concentration of free charge carriers, alteration is effected in a variety of ways, including, without limitation, illuminating the surface with a laser, heating the surface, and injecting charge into the surface. 
     An example of the operation of method  500  is now described in an embodiment in which the altered physical factor is the concentration of free charge carriers and the manner in which the concentration is alter ed is by illuminating the surface with a laser. 
     In an embodiment of method  500  in which particles are accelerated, the surface is illuminated by laser before particles are near closest approach. As the particle is near closest approach, it is subjected to a relatively higher force of attraction due to a Casimir force than if the surface was not illuminated. This is due, in this embodiment, to the increase in the concentration of free charge carriers. At closest approach, or after but still near closest approach, illumination is decreased or terminated. This decreases the Casimir force so that particle departs at increased velocity. 
     To decelerate particles, the timing of illumination is simply reversed from the acceleration case. That is, the laser is turned on at closest approach, or after, but near closest approach. The Casimir force will therefore be greater between the particle and the surface when it is outbound from closest approach such that the particle is decelerated. 
     It will be appreciated that due to the timing issues involved in method  500 , particles must be delivered to the surface in a pulsed fashion. In other words, illustrative method  500  can not be practiced using a continuous stream of particles. 
     In further embodiments, method  300  and method  500  can be used in conjunction with one another. In other words, in addition to affecting the transition near closest approach (i.e., method  300 ), the surface can be illuminated, etc. to use the Casimir force to accelerate or decelerate the particle, as well. 
     The present methods and apparatuses find utility in a variety of applications including, for example, particle heating and cooling devices, micro-propulsive systems, sensors and energy conversion devices. 
     It is to be understood that the above-described embodiments are merely illustrative of the invention and that many variations may be devised by those skilled in the art without departing from the scope of the invention and from the principles disclosed herein. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.