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Propagation

Propagation

Propagation [chatgpt]

Propagation in physics generally refers to how something spreads or travels through space over time. The specific meaning depends on what is propagating: waves, fields, particles, etc. Since you mentioned \(1/r^2 \), I’ll focus on physical phenomena that decay with distance squared.

1. Wave Propagation

In the case of sound or electromagnetic waves, wavefronts spread outward in spherical shapes (like ripples on a pond but in 3D).
The intensity (power per unit area) of the wave decreases as \( 1/r^2 \) because the wave’s energy is spread over an expanding spherical surface of area \( 4\pi r^2 \).
Examples: Sound from a speaker, light from a bulb.

2. Field Propagation (Inverse Square Law)

This \( 1/r^2 \) rule also applies to physical fields like gravity or electric fields.
Gravitational Field: The strength of gravity from a point mass decreases with \( 1/r^2 \).
Electric Field: For a point charge, the electric field strength behaves similarly.

3. Particle Propagation

When particles are emitted from a source (e.g., radiation from a radioactive material), the flux—the number of particles per unit area—also follows \( 1/r^2 \).

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Let me know if you want to focus on wave propagation, fields, or something else specifically!

Notes

The strength of gravity
1. Does gravity have strength?
2. What does this mean?
3. Does it mean that the Newtonian supernatural cause called force reduces its strength with distance?

Newtonian gravity

So, classical Newtonian gravity is not a wave and it is not a field and it is not a propagation so is it a 1/r² phenonenon?

Yes, Newtonian gravity is absolutely a \( 1/r^2 \) phenomenon! But it’s not a propagation in the sense of traveling through space like a wave. Instead, Newton described gravity as an instantaneous action-at-a-distance. This means that, in his view, the gravitational force existed everywhere at once, without any delay between cause and effect—no propagation involved.

So, Newtonian gravity isn’t a wave or a propagating field. But the strength of the gravitational force around a point mass still decreases with distance squared:

\[ F = G \frac{m_1 m_2}{r^2} \]

This \( 1/r^2 \) relationship comes from the geometry of space: the gravitational “influence” spreads out over an ever-increasing spherical area as the distance \( r \) grows, and the surface area of a sphere is \( 4\pi r^2 \).

If you’re curious, in modern physics (Einstein’s general relativity), gravity is no longer instantaneous—it’s a curvature of spacetime, and changes in gravity propagate as gravitational waves at the speed of light. But Newton’s original law was purely static, with no time delay!

Propagation and emission

So, for propagation we need something to be emitted from a source? Is this correct? Is propagation and radiation the same thing?

“Propagation” and “radiation” are related concepts but they’re not exactly the same. Here’s how to distinguish them:

Propagation
- Definition: Propagation refers to the way in which any disturbance, wave, or influence travels through space or a medium over time.
- Source Involvement: Often, propagation does involve something being emitted from a source. For example, when you turn on a light, the electromagnetic wave propagates outward from the source. However, propagation can also refer to the spread of any effect (like a mechanical disturbance in a medium or even a change in a field) without necessarily emphasizing an “emission” event.
Radiation
- Definition: Radiation specifically refers to energy that is emitted from a source in the form of waves or particles. Common examples include electromagnetic radiation (like light, X-rays, or radio waves) and particle radiation (like alpha or beta particles from radioactive decay).
- Relationship to Propagation: Once the energy is emitted, it propagates—that is, it travels through space. So, radiation is a particular case of propagation where energy is being radiated away from a source.

In Summary

Propagation is the general concept of something (a wave, a disturbance, or energy) moving through space or a medium.
Radiation is the process of energy being emitted from a source, which then propagates outward.

So while radiation is a form of propagation (specifically, the outward travel of energy emitted from a source), not every instance of propagation is called radiation. For example, the way a sound wave travels through air is propagation, but we typically reserve the term “radiation” for electromagnetic waves or particle emissions.

Does that help clarify the distinction?

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