The interaction between two objects that are not touching (via "interaction at a distance") has been subject to intense scrutiny from physicists and philosophers. In physics, a model in which "radiation" travels between the interacting objects is often used to explain interaction at a distance. In this Demonstration, the candle and detector are separated by a very large distance. The candle is lit, and the detector reads "10". You can explore the interaction between the candle and the detector with the controls. Can a radiation model explain this interaction? In particular, can you find the path along which the radiation must travel?
Imagine that something invisible is traveling from the flame to the detector. Call it "radiation"—an invisible, but "real", intermediary traveling between two objects that interact without direct contact.
This model of invisible radiation leads us to seek additional evidence for the radiation. In particular, we expect that the radiation must travel along a continuous physical path from source to receiver. By experimenting with the controls, you can find a path for the radiation from the candle to the detector. However, make sure that the path is consistent with all the different ways that the system responds.
This Demonstration is patterned after Heinrich Hertz's wonderful experiments of 1886–88. Hertz astounded the world with incontrovertible evidence for what are now known as radio waves (or, more generally, electromagnetic waves). Hertz's scientific brilliance matched his ineptitude or disinterest in commercial affairs; he said after his discovery that radio waves had no practical use! Others were more prescient, and by 1901, Marconi had transmitted a message via "wireless" radio across the Atlantic, leading to an avalanche of commercial applications for Hertz's electromagnetic waves. Some lovely drawings showing Hertz's equipment and experiments are at the website of the Institute of Chemistry, Hebrew University of Jerusalem: http://chem.ch.huji.ac.il/history/hertz.htm.
Hertz's experimental apparatus was arranged in exactly the same way as shown in this Demonstration. His source was sparks jumping across a narrow gap between two electrodes, driven by an electrical circuit. His detector was a ring of wire with a small gap; visible sparks jumping across the detector gap demonstrated that the radiation from the source was reaching the detector. His mirrors were made from curved pieces of metal and other materials, and they were placed in the same locations as the mirrors in this Demonstration.
Hertz mastered how to control the frequency (the number of times per second) with which the source sparks jumped across the gap by changing the shape and size of the electrodes. He found that sparks appeared most strongly in the detector when the source was operating at certain discrete frequencies. This seemed very similar to the familiar "sympathetic vibration" (or "resonance") observed when one tuning fork, initially motionless, begins vibrating in response to the sound from another tuning fork that has exactly the same pitch. Hertz was able to demonstrate that his experiment was indeed the result of resonance between the source and receiver. This was the origin of our ability to "tune" a radio or TV to receive only one station and not others.
This kind of experiment can also be carried out with a variety of different types of radiation by simply using an appropriate source and detector and selecting an appropriate surface for the mirrors to reflect the radiation. For example, sources of visible light are a flame, the Sun, or an incandescent or fluorescent bulb, and the detector can be photographic film, a photographer's light meter, or even the human eye. The experiment also works well with a type of radiation similar to light, called infrared radiation, which is invisible to our eyes. It is called infrared because it is "below" red in the light spectrum. A hot iron or a flame produces infrared radiation. There are meters (as well as digital sensors and film) sensitive to infrared light that can be used as detectors. The energy of infrared radiation radiation is readily transferred to an object as heat, so a sensitive thermometer can also be used as a detector. The rise in temperature shown on the thermometer can be related to the intensity of the radiation.
By means of similar experiments, scientists have discovered other types of electromagnetic radiation, including ultraviolet, x-ray, and gamma radiation. Sound also acts as an intermediary for interaction at a distance, as can be shown by using a speaker or whistle as the source and a microphone or the human ear as the detector. Concert halls and musical instruments must be designed with careful attention to the behavior of sound radiation, and sonograms of the human body are a valuable use of sound radiation.
Finally, gravitational radiation is an interesting special case in which efforts at detection have not (yet) succeeded. Such radiation has been predicted theoretically, and there are no obvious reasons it should not exist. Gravitational radiation would be the intermediary, for example, between the Sun's mass and the gravitational force that keeps Earth in its orbit. In the same way that the visible light (as electromagnetic radiation) from a distant star takes many years to reach Earth, there should be a "delay" between the time that an event occurs (for example a stellar collision) and an observer at a distance "feels" the gravitational effects. However, in spite of many valiant efforts and construction of several "gravitational antennas" designed to detect such radiation, unambiguous evidence for a "signal" has not been found. In fact, in a celebrated case of how science works to correct errors, one scientist's claims for the discovery of gravitational radiation were initially acclaimed; but, after a number of years, other investigators found that his results could not be duplicated. After investigation and intense debate, his methodology was found to have been invalid (though probably not fradulent), and his claims have been rejected.
This Demonstration is based on an example from Chapter 3 of Introductory Physics: A Model Approach, Second Edition, by Robert Karplus (Captains Engineering Services, 2003). Additional interactive Mathematica Demonstrations are posted at www.fernwig.com.