Discussion Overview
The discussion revolves around the "brightness" of man-made electromagnetic (EM) signals, particularly radio waves, and how they compare to visible light sources. Participants explore the energy that can be harvested from various EM signals, including radio, television, cellular, and WiFi signals.
Discussion Character
- Exploratory
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- One participant questions how "bright" man-made EM signals would appear if they were in the visible range, asking whether they would be barely visible or as bright as the daytime sky.
- Another participant notes that the power of radio waves varies significantly depending on the source, indicating that while some sources can be very powerful, most ambient radio waves are weak and yield little energy for harvesting.
- A participant provides a comparison of power density, stating that sunlight has a power density of about 1500 watts per square meter, while a UHF TV transmitter 10 km away has a power density of about 0.0008 watts per square meter.
- One participant suggests that the perceived color of EM signals could change based on the observer's speed, hinting at relativistic effects.
- Another participant clarifies that brightness is related to how emitted power is distributed in angle, using the example of a laser versus an incandescent bulb to illustrate this point.
- A later reply discusses the effective radiated power (ERP) of TV transmitters, explaining how antenna design affects the distribution of power and signal strength.
- One participant raises a question about the definition of the Solar Constant and whether radiant intensity is being considered in the discussion.
- A question is posed regarding the limits of intensity for electromagnetic radiation.
- Another participant affirms that there is indeed a limit to how intense electromagnetic radiation can be.
- A request for clarification on the units of "brightness" and "intensity" is made, emphasizing the need to define these terms in the context of the discussion.
Areas of Agreement / Disagreement
Participants express varying views on the concepts of brightness and intensity, with some clarifications and challenges to definitions. The discussion remains unresolved regarding the specific definitions and implications of brightness in relation to EM signals.
Contextual Notes
There are limitations regarding the assumptions made about the definitions of brightness and intensity, as well as the context in which power densities are compared. The discussion does not resolve these ambiguities.