Virtual Reality 虚拟现实 Virtual reality is a system that enables one or more users to move and react in a computer simulated environment. Various types of devices allow users to sense and manipulate virtual objects much as they would real objects [1] . This natural style of interaction gives participants the feeling of being immersed in the simulated world. Virtual worlds are created by mathematical models and computer programs. Interface Devices Virtual reality simulations differ from other computer simulations in that they require special interface devices that transmit the sights, sounds, and sensations of the simulated world to the user [2] . These devices also record and send the speech and movements of the participants to the simulation program. To see in the virtual world, the user wears a head mounted display (HMD) with screens directed at each eye. The HMD also contains a position tracker to monitor the location of the user's head and the direction in which the user is looking. Using this information, a computer recalculates images of the virtual world a slightly different view for each eye to match the direction in which the user is looking, and displays these images on the HMD. The computer must generate these new views at least ten times a second in order to prevent the user's view from appearing halting and jerky and from lagging behind the user's movements. Virtual world scenes must be kept relatively simple so that the computer can update the visual imagery quickly enough. Because of these simplifications and other shortcomings of current visual displays and computer graphics, VR participants can easily distinguish a simulation from physical reality. The User of Virtual Reality Users hear sounds in the virtual world through earphones in the HMD. The information reported by the position tracker on the HMD can also be used to update audio signals. When a sound source in virtual space is not directly in front of or behind the user, the computer transmits sounds to arrive at one ear a little earlier or later than at the other and to be a little louder or softer and slightly different in pitch. However, as with visual imagery [3] , there are currently scientific and engineering challenges that must be overcome in order to simulate accurately all the sounds heard in the physical world. The haptic interface, which relays the sense of touch and other physical sensations in the virtual world, is the least developed and perhaps the most challenging to create. Currently, with the use of a glove and position tracker, the computer locates the user's hand and measures finger movements. The user can reach into the virtual world and handle objects but cannot actually feel them. It is particularly difficult to generate the sensations that are felt when a person taps a hard surface, picks up an object, or runs a finger across a textured surface.  To simulate these sensations, a set of computer controlled motors, faster and more accurate than any presently available, would have to generate force feedback by physically pushing against the user. Another problem is determining how a user would wear these motors and the wiring needed to control them. Touch sensations would also have to be synchronized with the sights and sounds users experienced in their HMDs. A current solution to the haptic challenge is the use of desktop devices that can apply small forces, through a mechanical linkage, to a stylus held in the user's hand. Users can feel when the point of the stylus encounters a virtual object, and they can drag the stylus across the surface to feel its texture and surface geometry. Development and Uses Researchers have been working on virtual reality devices for many years. In the 1960s Raymond Goertz at Argonne National  Laboratory [4] in Argonne, Illinois, and Ivan Sutherland at the Massachusetts Institute of Technology [5] in Cambridge [6] , Massachusetts, demonstrated early versions of HMDs. Goertz, and later Michael Noll of Bell Laboratories [7] , also developed prototype force feedback devices. In recent years, virtual reality devices have improved dramatically as the result of various technological advances. Computers now are more powerful, have a higher memory capacity, are smaller, and cost less than in the past. These developments, along with the advent of small liquid crystal displays (LCDs) that can be used in HMDs, have made it possible for scientists to develop virtual reality simulations. Virtual reality is currently used to explore and manipulate experimental data in ways that were not possible before. Therapists use VR to treat sufferers of child abuse and people who are afraid of heights. Muscular dystrophy patients can learn to use a wheelchair through virtual reality. In the future, surgeons may use VR to plan and practice an operation on a virtual patient rather than a real person. Architects could take clients on a virtual tour of a new house before it was built. VR could be used to train the operators of aircraft and other complicated machinery. Network VR simulations could enable people in many different locations to participate together in teleconferences, virtual surgical operations, or simulated military training exercises. Virtual Reality Programs Virtual reality programs give users three essential capabilities—immersion, navigation, and manipulation. People must be immersed in the alternate reality, not merely feel as if they are viewing it on a screen. To this end, some programs require people to wear headphones, use special controllers or foot pedals, or wear 3D glasses. The most sophisticated means of immersing users in a virtual reality program is through the use of head mounted displays, helmets that feed slightly different images to either eye and that actually move the computer image in the direction that the user moves his or her head. Virtual reality programs also create a world that is completely consistent internally. Thus, one can navigate one's way through that world as "realistically" as in the real world. For example, a street scene will always show the same doors and windows, which, though their perspective [8] may change, is always absolutely consistent internally. The most important aspect of a virtual reality program is its ability to let people manipulate objects in that world. Pressing a button may fire a gun, holding down a key may increase a plane's speed, clicking a mouse may open a door, or pressing arrow keys may rotate an object. Flight Simulator A flight simulator [9] is a computer generated recreation of the experience of flying. Sophisticated flight simulators, costing hundreds of thousands [10] of dollars, can provide pilot training, simulating emergency situations without putting human crews and planes at risk [11] . Flight simulator software running on personal computers simulates flight in a less realistic fashion; it provides entertainment and practice in navigation and instrument reading. A flight simulator is a perfect example of programs that create a virtual reality (or cyberspace [12] ), or a computer generated "reality" in which the user does not merely watch but is able to actually participate. The user supplies input to the system by pushing buttons or moving a yoke or joy stick, and the computer uses real world data to determine the results of those actions. For example, if the user pulls back on the flight simulators yoke, the computer translates the action according to built-in rules derived from the performance of a real airplane. The monitor will show exactly what an airplane's viewscreen would show as it begins to climb. If the user continues to climb without increasing the throttle, the "virtual plane" will stall (as would a real plane) and the "pilot" will lose control. Thus the user's physical actions are immediately and realistically reflected on the computer' s display. For all intents and purposes [13] , the user is flying, that is, the "plane" obeys the same laws of nature, has the same mechanical capabilities, and responds to the same commands as a real airplane. Notes [1] Various types... as they would real objects. as在这里为连词，意为“如同”，引出方式状语从句，因实际情况并非如此，故从句用了虚拟语气；另因该从句的谓语动词与主句中不定式动词sense and manipulate相同，所以也省略了。 [2] in that they... devices that: 句中第一个that引出介词in的宾语从句，与in在一起表示原因；第二个that引出的是定语从句，修饰devices。 [3] as with visual imagery... as (challenges) with visual imagery (that have been overcome)：正像战胜视觉形象的挑战一样。 [4] Argonne National Laboratory：阿贡国家实验室，美国核能及高能物理研究中心。该中心位于芝加哥附近，始建于1946年，现有职员约5,000人，拥有强脉冲中子源和质子加速器等先进设备。 [5] the Massachusetts Institute of Technology: 庥省理工学院。 [6] Cambridge: 坎布里奇市(旧译剑桥)，在美国麻省境内，是世界著名的教育和研究中心，哈佛大学、麻省理工学院及其他一些著名大学位于此地。 [7] Bell Laboratories：实验室美国电话电报公司(AT&T)的分支，以其在通讯和计算机方面的成就著名。它是晶体管、C语言和UNIX操作系统的发祥地。 [8] perspective：事物相互关系的外观。 [9] a flight simulator:飞行模拟器。 [10] hundreds of thousands:几十万。 [11] to put... at risk：拿……去冒险。 [12] cyberspace: 计算机控制空间。 [13] For all intents and purposes：实际上，实质上。 Proper Names Raymond Goertz雷蒙德·哥尔兹(人名) Ivan Sutherland伊凡·苏萨兰(人名) Michael Noll迈克尔·诺尔(人名) Argonne阿贡(地名) Illinois(美国)伊利诺伊州 Cambridge坎布里奇(旧译“剑桥”，美国城市) Massachusetts(美国)马萨诸塞州(旧译麻省)